University of California, Davisfadley.physics.ucdavis.edu/Nanotech.Revs.Fischer.Fadley.2012.Repri… · inventions such as AC power systems by Nikola Tesla. John Kerr and again Michael

Nanotechnol Rev 1 (2012): 5–15 © 2012 by Walter de Gruyter • Berlin • Boston. DOI 10.1515/ntrev-2011-0001

Review

Probing nanoscale behavior of magnetic materials with soft X-ray spectromicroscopy

Peter Fischer 1, * and Charles S. Fadley 2,3

1 Center for X-ray Optics , Lawrence Berkeley National Laboratory, Berkeley, CA 94270 , USA , e-mail: [email protected] 2 Department of Physics , University of California, Davis, CA 95616 , USA 3 Material Sciences Division , Lawrence Berkeley National Laboratory, Berkeley, CA 94270 , USA

* Corresponding author

Abstract

The magnetic properties of matter continue to be a vibrant research area driven both by scientifi c curiosity to unravel the basic physical processes which govern magnetism and the vast and diverse utilization of magnetic materi-als in current and future devices, e.g., in information and sensor technologies. Relevant length and time scales approach fundamental limits of magnetism and with state-of-the-art synthesis approaches we are able to create and tailor unprecedented properties. Novel analytical tools are required to match these advances and soft X-ray probes are among the most promising ones. Strong and element-spe-cifi c magnetic X-ray dichroism effects as well as the nano-meter wavelength of photons and the availability of fsec short and intense X-ray pulses at upcoming X-ray sources enable unique experimental opportunities for the study of magnetic behavior. This article provides an overview of recent achievements and future perspectives in magnetic soft X-ray spectromicroscopies which permit us to gain spa-tially resolved insight into the ultrafast spin dynamics and the magnetic properties of buried interfaces of advanced magnetic nanostructures.

Keywords: interfaces; nanomagnetism; spin dynamics; X-ray microscopy; X-ray spectroscopy.

1. Introduction

The magnetic properties of matter have attracted our curi-osity since ancient times. Even though we have no direct sense for magnetism, magnetic materials were technologi-cally used, e.g., for navigation already in ancient China.

Electric power, which since the industrial revolution in the 19th century has been one of the major energy resources, relies heavily on the implementation of magnetic materials in essential components of, e.g., electric motors and trans-formers. And the concept of storing or handling informa-tion in current information technologies is again based on magnetic materials, e.g., in magnetic hard disks or magnetic sensor devices.

Scientists have always been “ mesmerized ” to unravel the very fundamental origin of magnetism. During the classical period of physics James Clark Maxwell and Andr é -Marie Amp è re established the connection between electricity and magnetism, Hans Christian Oersted discovered that electric currents create magnetic fi elds and Michael Faraday dis-covered electromagnetic induction, diamagnetism and the law of electrolysis, which laid foundation for technological inventions such as AC power systems by Nikola Tesla. John Kerr and again Michael Faraday studied the interaction of light polarization with magnetic fi elds. With the beginning of the quantum era of physics at the beginning of the 20th century Wolfgang Pauli postulated the existence of the electron spin [1] , which was soon discovered by George Uhlenbeck and Samuel Goudsmit [2] , and formulated the exclusion principle for spins, which are the two key fea-tures in our current understanding of the origin of mag-netic phenomena. Werner Heisenberg developed a model to explain ferromagnetism using Pauli ’ s exclusion principle. It was realized that since the spin of the electron exhibits the property of angular momentum, there are two differ-ent magnetic moments, the spin and the orbital part, where the latter relates to magnetic anisotropies, which in turn plays an important role in the technological functionality of many modern magnetic devices. Finally, it is the spin-orbit interaction which establishes the link between these two quantities.

Today, we are witnessing the emerging of a new era in elec-tronics, spin electronics or spintronics [3] , where in addition to the charge of the electrons the spin of the individual elec-tron will be taken into account. The importance of spintron-ics was recognized by the Nobel Prize in physics in 2007 to Albert Fert [4] and Peter Gruenberg [5] for their discovery of Giant magnetoresistance (GMR), which can be seen as one of the fi rst spintronic effects. Very soon after its discovery GMR was already utilized in read head technology of magnetic hard disks and has enabled the rapid increase in storage density since then.

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6 P. Fischer and C.S. Fadley: Probing nanoscale behavior of magnetic materials with soft X-ray spectromicroscopy

The key to further successful developments in spintronics is a fundamental understanding of how to control spins on a nanoscale. Among the immediate applications of such know-ledge would be a further increase in storage density, but also a much faster writing process and a signifi cantly lower energy consumption of processing information.

Nanomagnetism research as one branch of nanoscience deals with the magnetic properties of materials at fundamen-tal length and time scale [6] . These are related to the exchange interaction of individual spins, which is the strongest inter-action in magnetic materials. The magnetic exchange length, which is largely determined by characteristic, i.e., material specifi c constants for exchange and anisotropy energies are typically in the sub-10 nm range. The corresponding funda-mental time scale for the exchange interaction, the exchange time, which can be derived from the Heisenberg uncertainty principle between time and energy yields several tens of fs. For comparison, typical time scales for the spin-orbit inter-action have values in the ps regime, which are relevant for understanding, e.g., the dynamics of anisotropies.

Progress in spintronics is also intimately connected with novel and advanced materials, which exhibit desired proper-ties to enable new functionalities. Materials and chemical sci-ences have developed and are hence now capable to design and tailor by numerous state-of the-art synthesis approaches materials, which often do not exist in nature. Bottom-up approaches, such as molecular beam epitaxy (MBE) growth or top-down strategies, such as e-beam lithography allows fabricated nanostructures, where interfaces, structural and chemical composition can be designed on purpose.

There are several ways to control spins on a nanoscale. Based on the Oersted discovery that a compass needle follows the earth magnetic fi eld, the reversal of a magnetic moment by application of an external magnetic fi eld pointing in oppo-site direction is still the method of choice to write information in a magnetic hard disk. However, serious limitations arise with this concept of fi eld driven manipulation of spins, when it comes down to small length and fast time scales. Dipolar fi elds extending over large distances will impact neighboring bits and the fact that at T = 0 K the application of an antiparal-lel fi eld constitutes a metastable confi guration which there-fore only works for magnetic reversal at room temperature due to the local fl uctuation of the magnetic moments, slows down signifi cantly the reversal speed.

Considering the local and much stronger spin torque cre-ated by the spin of an electron fl owing in a current through a ferromagnetic wire onto a non-collinear spin confi guration has triggered enormous scientifi c interest in spin torque phe-nomena and technological applications for magnetic devices [7 – 9] . The racetrack memory proposed by Parkin et al. is just one of those revolutionary concepts of applications of the spin torque effect [10] .

Alternative concepts follow the idea of coupling various degrees of freedom to gain control on the nanoscale spins. Multiferroic materials exhibit, e.g., a coupling of ferromag-netic and ferroelectric properties which is a promising route to switch magnetization by simply applying an electric fi eld [11 – 13] . However, as nature provides only very few materials

showing these interesting properties the challenge is again for the materials science community to discover and develop new materials.

The question as to how fast one can manipulate spins on the nanoscale can be addressed by involving the fastest man-made events, which are ultrafast optical laser pulses. If the photon itself carries suffi cient angular momentum, a photo-nic magnetization switching should occur. Indeed, this topic is currently intensely studied by numerous groups and fi rst pioneering experiments by Rasing et al. have successfully demonstrated such effects, which could form the concept to increase the writing speed in data storage by many orders of magnitude in the future [14 – 16] .

There are three components, which build the fundamentals for nanomagnetism research: synthesis – analysis – theory/modeling, which have to be intimately connected for success-ful research projects. In particular, state-of-the-art tools to characterize magnetic nanoscale materials have to meet the following requirements:

spatial resolution below 10 nm, • temporal resolution down to the fs regime, • chemical and magnetic sensitivity with elemental • specifi city.

Soft X-rays are among the most promising probes and X-ray spectromicroscopic techniques have therefore fl our-ished recently. In the following sections, recent achieve-ments in soft X-ray microscopy will be briefl y reviewed and the potential for future developments is outlined. A few specifi c examples of imaging nanoscale magnetic structures, their fast dynamics and buried interfaces will illustrate these techniques.

2. Polarized soft X-rays as a unique tool to study

magnetism

Soft X-rays are a part of the electromagnetic radiation spec-trum covering the energy range from approximately 250 eV up to several keV, which corresponds to a wavelength regime from approximately 5 nm to 0.5 nm [17] . The dominant inter-action process of soft X-rays with material is through pho-toabsorption. Very importantly, in the soft X-ray spectral regime there are primary atomic resonances and absorption edges of most low and intermediate Z elements, such as the K edges of C, N, O, up to Si, P and S, the L edges of the magnetically most relevant 3d transition elements such as Fe, Co, Ni and Cu and the M edges of rare earth elements includ-ing Ce, Nd, Sm, Gd and Tb. These X-ray absorption edges, i.e., the resonant enhancement of the photoabsorption cross-section refl ect the situation, where the energy of the photon matches the binding energy of an inner core electronic level. As the atomic electronic confi guration and therefore their energy levels are uniquely element-specifi c, any technique which detects photoabsorption shows this element specifi c-ity and especially serves as a highly accurate fi ngerprint of the chemical specifi cs of the material. The penetration depth of soft X-rays, which is also characterized by the absorption



P. Fischer and C.S. Fadley: Probing nanoscale behavior of magnetic materials with soft X-ray spectromicroscopy 7

length, has typical values up to a few hundred nm. A special situation occurs in the so-called water window around 2.4 nm, where the absorption in water drops signifi cantly to allow for large penetration depth of aqueous samples.

This short wavelength of soft X-rays has an immediate consequence for soft X-ray imaging. As, in general, the dif-fraction limited spatial resolution in microscopies scales with the wavelength of the probe, any X-ray based microscopy technique is in principal capable of providing nanometer spa-tial resolution.

Synchrotron radiation (SR), which is currently generated at numerous dedicated large-scale facilities around the world such as the Advanced Light Source in Berkeley, CA, is the pri-mary source for soft X-rays of high intensity, broad and tun-able wavelength regime and controllable polarization. These multiuser facilities provide a large portfolio of instrumenta-tion, where up to hundreds of experiments can be performed simultaneously using the unique properties of SR. The light is generated by electrons when they are accelerated in magnetic fi elds, which either keeps them on the circular track in the storage ring (bending magnet radiation) or which are deliber-ately wiggled in arrays of linearly arranged permanent mag-nets. The latter devices, so-called undulators or wigglers, are the characteristic sources for third generation SR facilities. They produce much enhanced photon intensities compared to a bending magnet source and they also provide largely coher-ent X-rays with controllable polarization. The electrons in the storage ring with a typical energy of several GeV, which makes them run close to the speed of light, are packaged in short bunches, which means that the X-rays emitted also have an inherent time structure. These X-ray fl ashes are typically around 100 ps long and can be used for stroboscopic X-ray studies. The next generation of X-ray sources will be X-ray free electron lasers. They will provide not only fully coherent light, both spatially and temporally, but also single bursts of X-rays at fs time scales and very high peak intensities. They will allow performing unprecedented X-ray studies, such as single shot fs dynamics of solid state materials.

For the study of magnetic properties the polarization of X-rays is very important. The magneto-optical Kerr or Faraday effect, which detects the interaction of polarized optical light with the magnetization in a ferromagnetic sample as a rota-tion of the light polarization after refl ection or transmission, serves as a magnetic contrast mechanism. Soft X-ray mag-netic circular dichroism (XMCD), which is a similar effect in the X-ray regime, has been discovered at the nickel L 2,3 edges in the late 1980s [18] . It describes the observation that the absorption of circularly polarized X-rays depends strongly on the relative orientation between the photon helicity and the photon propagation direction. The XMCD effect occurs pre-dominantly in the vicinity of resonant X-ray absorption edges, such as the spin-orbit coupled L 2 and L 3 edges, which refl ect the element specifi c binding energies of inner core electrons. Large XMCD effects with values up to 25 % occur particu-larly for 3d transition metals such as Fe, Co, Ni, which are the most prominent materials for magnetic specimens.

Magnetic X-ray spectroscopies provide a wealth of impor-tant information particularly for magnetic systems. Owing to

angular momentum conservation in the X-ray absorption pro-cess, the photoelectron carries spin and orbital momentum, which is different for the spin-orbit split p 3/2 and p 1/2 electronic levels, i.e., for the L 3 and L 2 absorption edges. Using the so-called magneto-optical sum rules, XMCD spectroscopy can be used to determine quantitatively spin and orbital moments in ferro- and ferromagnetic systems [19, 20] .

The magnetization direction particularly in antiferromag-nets can be studied using linearly polarized X-rays. The effect of X-ray magnetic linear dichroism (XMLD) detects the dif-ferent absorption of the electric fi eld vector of the photons for parallel and orthogonal orientation of the spin axis [21] .

There are various ways to experimentally detect photoab-sorption. The most direct measurement is the transmission method, which basically counts the incoming and the trans-mitted photon intensity and relates them through an exponen-tial law. However, owing to the limited penetration length of soft X-rays this method can only be applied for X-ray trans-parent specimens, such as thin fi lms on X-ray transparent substrates. During the absorption process secondary electrons are generated and detecting the electron yield, i.e., measur-ing the current associated with that is another commonly used method. Another alternative is to record the fl uorescence yield which occurs after photoabsorption, which is a very sen-sitive detection method; however, it also exhibits a dichroism effect, which therefore convolutes with the original XMCD effects of interest.

The basic idea of magnetic X-ray microscopies is to record the spectroscopic magnetic response, such as XMCD, in a lat-erally resolving microscopic technique. So far, both transmis-sion [22, 23] and photoemission [24] approaches have been realized and are widely used, whereas fl uorescence mode detection for magnetic imaging has not yet been demonstrated experimentally.

The concept for transmission soft X-ray microscopy follows the optical design of optical microscopes [17] . However, as the refractive index of X-rays is close to one in the soft X-ray regime, conventional lenses cannot be used. The lack of appro-priate optics has prevented soft X-ray microscopes until the mid-1980s when Fresnel zone plates (FZPs), which are circular gratings with a radially increasing line density have been estab-lished as high resolution optical elements [25, 26] . The fabrica-tion of the FZP for the soft X-ray regime was enabled by the maturity of nanotechnology tools such as e-beam lithography which has then enabled the fabrication of high quality diffractive X-ray lenses. FZPs can be designed and customized for specifi c purposes and applications. Varying a few parameters, such as ∆ r , which is the outermost ring diameter, N , the number of zones, and λ , the photon wavelength at which the FZP is operating, one obtains a spatial resolution which is proportional to ∆ r , a focal length, which is ∼ 4 N ( ∆ r ) 2 / λ and a spectral bandwidth which is ∼ 1/ N . The most advanced FZPs for soft X-ray microscopy have achieved a spatial resolution better than 10 nm [27] (W. Chao et al. 2011, private communication), and current develop-ments seem to make the single digit nanometer spatial resolu-tion regime become feasible in the near future. The scheme of the optical setup of the full-fi eld soft X-ray microscope XM-1, located at the Advanced Light Source in Berkeley, CA, where




the data presented in this review have been obtained, is shown in Figure 1 [28] . Its main components are (i) a light source, which is the bending magnet source at a third generation X-ray synchrotron; (ii) a condenser part, which is the fi rst FZP, the condenser zone plate (CZP), and acts as both monochromator and illuminating optic; (iii) a high resolution objective lens, the microzone plate (MZP); and (iv) a two-dimensional detector, which is a commercially available charge-coupled device (CCD) system.

Spatial resolution is largely set by ∆ r of the MZP. The range of photon energies determines the accessible elements. At XM-1 the range from approximately 500 eV to 1.2 keV covers the L-edges of transition metals and the early rare earth elements. The spectral resolution is determined by the source performance and the illuminating part, which for XM-1 is approximately 1 – 2 eV at 1 keV. Time resolution is limited by the time structure of the X-ray source, and therefore progress towards better time resolution requires X-ray sources with shorter time structures.

There are two complementary microscopy techniques, which utilize the FZPs as described above. Similar to the situation with transmission electron or optical microscopes there is a scanning transmission X-ray microscope (STXM), where the FZP is used to focus the X-rays onto the specimen, which is then raster-scanned to build an image. STXM has the advantage to provide good spectral resolution for microspectroscopic studies and is rather fl exible in the choice of point detectors. By contrast, the full-fi eld transmission soft X-ray microscope (TXM) uses the FZP as a high resolution X-ray objective lens and is therefore the tool of choice for spectromicroscopic studies. Recently, both STXM and full-fi eld TXM have shown similar spatial resolu-tion depending on the degree of coherent illumination. Whereas STXM has some advantages as a microspectroscopic tool, the full-fi eld system can easily cover a larger fi eld of view at high-est spatial resolution. State-of-the-art full-fi eld microscopes at undulator sources have recently demonstrated high spectral resolution microspectroscopy capabilities [29] .

The penetrability of soft X-rays through matter, described by the X-ray absorption length, is typically in the few 100-nm

regime. Therefore, the transmission geometry probes the volume of the sample with a thickness up to few 100 nm, which, however, perfectly matches most of the magnetic sys-tems of interest, such as thin fi lms or multilayered structures. The bulk sensitivity of transmission soft X-ray microscopy is complemented by detecting the secondary electrons which are created in the primary photoabsorption process locally within an X-ray photoemission electron microscope. The sur-face sensitivity is provided by the limited escape depth of the electrons to approximately 5 nm. Whereas X-PEEM systems are now commercially available, the latest generation of aber-ration corrected PEEM systems are currently being developed with the promise to provide < 10 nm spatial resolution.

Finally, lensless imaging techniques have started being developed recently [30] . Essentially, these techniques work in reciprocal space and the concept is to record a diffraction pattern from the sample, which is back-transformed into real space via sophisticated phase retrieval algorithms or holo-graphic methods to obtain a real space image. However, in terms of broad applicability and ultimate spatial resolution, these techniques still seem to have a long way to go [31] .

All of the above-mentioned techniques are being used for magnetic imaging with XMCD serving as a magnetic contrast mechanism. The fi rst magnetic images were reported in 1993 using an X-ray photoemission electron microscope (X-PEEM) at the Advanced Light Source in Berkeley, CA [24] , fol-lowed by the fi rst zone plate based magnetic images obtained with full-fi eld magnetic transmission soft X-ray microscopy (M-TXM) in 1996 at the Berlin Electron Storage Ring Society for Synchrotron Radiation (BESSY) in Berlin Germany [22] .

3. Some recent examples

3.1. Spin dynamics of magnetic vortex structures

Vortex structures are commonly found on various length scales extending from cosmological dimensions in galax-ies to large tornadoes across thousands of kilometers to

Figure 1 Schematics of the optical setup of the full-fi eld soft X-ray microscope XM-1 located at beamline 6.1.2 at the Advanced Light Source. Electrons in the storage ring emit X-rays in the bending magnet. The off-orbit emitted polarized X-rays are monochromatized by the condenser zone plate (CZP) – pinhole arrangement before they illuminate the magnetic sample. The microzone plate (MZP) downstream the sample projects a magnifi ed image of the transmitted X-rays onto the 2 dim CCD detector. From Ref. [28] .




superconducting materials, where the vortices play a crucial role to allow superconducting wires to be used in future power grids. Vortex structures and their associate structures such as antivortices are also very interesting topological objects for fundamental studies.

Magnetic vortex structures occur in soft ferromagnetic fi lms and patterned elements, such as thin disks of the soft Ni 80 Fe 20 alloy, as a result of the balance between exchange and dipolar energies [32] . They are characterized by a curl-ing magnetization in the plane of the disk with a vortex core (VC) in the center, where the magnetization points perpendicular to the plane of the disk. Two binary proper-ties are commonly used to describe this structure: the chi-rality, i.e., the counter-clockwise or clockwise curling of the in-plane magnetization, and the polarity, i.e., the up or down direction of the vortex core ’ s magnetization. Both the static and dynamic properties of these objects have recently attracted an increased scientifi c interest both for fundamental and applied reasons [33 – 37] . For example, magnetic vortex structures were suggested as potential future high-density and non-volatile recording systems, as the size of the vortex core is proportional to the magnetic exchange length Λ , which can extend into the sub-10 nm regime, and the magnetic core represents a very stable spin confi guration, in fact protected by topology.

The fi rst experimental images of magnetic vortex cores have been obtained by magnetic force microscopy (MFM) [33] (Figure 2 A) and Lorentz transmission electron micros-copy (L-TEM) [38] (Figure 2B). The static internal struc-ture of a vortex has been studied at almost atomic spatial resolution by spin-polarized scanning tunneling microscopy (SP-STM) (Figure 2C) [39] . Figure 2D shows a typical image of magnetic vortex cores in the center of permalloy (PY) disks obtained with M-TXM [40] . The difference to, e.g., MFM or L-TEM imaging is that M-TXM images directly component of the element-specifi c magnetization along the photon prop-agation direction instead of imaging the stray fi eld emanating from the VC as in MFM or an increased/decreased electron intensity due to the Lorentz force when the electrons travel through the object in L-TEM. M-TXM images can therefore be used to measure the actual M z profi le of the vortex core limited only by the spatial resolution of the instruments. Fine details such as a small negative dip close to the vortex core, which originates from the dipolar fi eld of the VC closing the fi eld lines back into the PY disks could be observed in a recent study and were in full agreement with 3 dim micromagnetic simulations [40] .

To understand the functionality of potential technologi-cal devices based on magnetic vortex structures, studies of the vortex dynamics are of paramount importance. Ultrafast dynamics of the vortex structure has been investigated by time-resolved Kerr microscopy [41] , revealing the rich spec-trum of excitations and eigen modes of the vortex [41 – 43] . However, owing to the diffraction limits of spatial resolution in optical microscopies these techniques are insuffi cient for smaller structures.

Time-resolved studies of spin dynamics with soft X-ray microscopy combine the high spatial resolution with the

inherent time structure of current synchrotron storage rings which is given by the length of the electron bunches circulat-ing the storage ring corresponding to < 100 ps. However, the fact that the number of photons per electron bunch at current third generation synchrotrons is rather low, typically only a few tens of photons, a stroboscopic pump-probe scheme has to be used. To accumulate suffi cient photons, e.g., for a single image, approximately 10 10 photons are required; therefore, 10 8 – 9 pump-probe cycles are needed per image. As a conse-quence, only fully reproducible processes can be studied or, in other words, only the fully reproducible part of the magne-tization dynamics can be investigated.

The results shown in Figure 3 were obtained at the Advanced Light Source in Berkeley, CA, operating in the so-called two-bunch mode operation. Two electron bunches, each 70 ps in length circulate at a 3-MHz frequency, i.e., separated by 328 ns. The clock signal of the synchrotron triggers a fast electronic pulser, which launches fast electronic pulses into a waveguide structure. These pump pulses create either a local Oersted fi eld pulse or, if the current is sent directly through the magnetic element creates a spin torque onto the magnetic spin structure. To follow the time development of the excited magnetic domain pattern the pump pulses are delayed relative to the X-ray probing pulse. Time zero, i.e., the time when the X-rays arrive at the sample is monitored by a fast avalanche photodiode close to the sample.

Figure 3 shows a sequence of images taken at varying delay times of several ns in steps of 100 ps between the pump and the probe-pulse [44] . One can clearly see that the vortex center, i.e., the vortex core performs a gyrotropic motion. The high spatial resolution allows to measure the gyration radius as a function of excitation frequency. Solving the Thiele equa-tion with these measured radii as experimental input, one can derive quantitatively the polarization P of the currents in PY, which is an important quantity describing the spin-torque strength. These studies revealed a value of p = 0.67, which was the fi rst direct and unambiguous determination.

3.2. Depth resolved magnetic soft X-ray microscopy

In addition to elemental composition new functionalities in nanoscale magnetic devices can also be tailored through confi nement and proximity effects. Whereas the former can be achieved, e.g., by artifi cial nanopatterning, typical exam-ples for the latter are the properties of interfaces, which occur, e.g., in magnetic multilayered systems. The study of magnetism at surfaces is a particular case and numerous experimental techniques with enhanced surface sensitivities have been developed and are widely available. Scanning probe techniques sensing various forces in the proximity of surfaces or techniques which detect electrons emanat-ing from the topological structure of the surfaces are just a few examples. However, rather often, interfaces, which are deeply buried in the bulk underneath a stack of several other magnetic and non-magnetic layers are of greater tech-nological importance and a depth resolved analysis of mag-netic properties, e.g., in multilayered structures is therefore highly required.




A B

C D

10 nm

1 �m

Figure 2 Images of magnetic vortex cores recorded with various imaging techniques. (A) Magnetic force microscopy (from Ref. [33] ), (B) Lorentz-TEM (from Ref. [38] ), (C) spin-polarized scanning tunneling microscopy (SP-STM) (from Ref. [39] ), and (D) M-TXM (from Ref. [40] ).

Various approaches using soft X-rays are currently being developed to determine magnetic depth profi les. The basic strategy for obtaining full three-dimensional information of a specimen is to combine a depth resolved analysis with a laterally resolving microscopy technique. If the targeted layer is different in its elemental composition from all other layers, then the elemental specifi city of X-ray photoabsorption can easily be used to identify the specifi cs of the buried layer. One example is shown in Figure 4 where the magnetic domain structure of a bilayer structure consisting of a perpendicu-lar anisotropy [Pt/Co] 50 multilayer and a TbFe ferromag-netic alloy was imaged with M-TXM [44] . Both the Pt/Co multilayer and the TbFe layer exhibit a strong out-of-plane anisotropy on their own; however, the interfacial exchange coupling between the Pt/Co and the TbFe fi lms is dominated by the ferromagnetic Co-Fe exchange interaction, i.e., the Co and Fe moments are parallel aligned. As the Tb sublattice moments dominate the total Tb 30 Fe 70 magnetization, the inter-facial exchange coupling leads to an antiparallel alignment of the TbFe layer relative to the PtCo multilayer in the absence

t0=0 ns1 �m

t1=1 ns t2=2 ns t3=3 ns t4=4 ns

t5=5 ns t6=6 ns t7=7 ns t8=8 ns t9=9 ns

Figure 3 Time-resolved magnetic soft X-ray microscopy of the gyra-tional motion of a magnetic vortex core in a PY disk. From Ref. [44] .

of a magnetic fi eld. By inserting another Pt layer between these two constituents, one can tailor the degree of exchange interaction by varying the thickness of the Pt layer. XMCD spectromicroscopy is able to gain insight into the complex behavior of these coupled systems with elemental, and thus




layer, sensitivity by either the Fe L 3 edge at 707 eV or the Co L 3 edge at 778 eV, respectively. A strong magnetic contrast is observed at both edges, however, with the sign at corres-ponding edges pointing in opposite directions. This immedi-ately confi rms the antiparallel alignment of the Fe and the Co moments on a microscopic length scale.

Another approach to obtain depth resolved information is based on excitation with soft X-ray standing waves (SWs) generated by Bragg refl ection from a multilayer mirror sub-strate. The SW can then be moved vertically through the sam-ple by varying the photon energy around the Bragg condition [45] , or by moving the X-ray spot across a sample in which one layer has a wedge profi le [46] . Photoemission intensi-ties which are recorded as a function of the incoming pho-ton energy or spot position can then be used quantitatively to derive the depth-resolved fi lm structure of the sample by comparing the intensities to X-ray optical theory calculations. Again, if one combines this method of depth resolution, e.g., with lateral information from PEEM, a three-dimensional representation of the magnetic structures inside the speci-men can be obtained. Figure 5 shows an example from a nanostructured system consisting of square arrays of circu-lar magnetic Co nanodots, nominally 4 nm in thickness and 1 µ m in diameter, which were grown on a multilayer substrate of confi guration (23.6 Å -Si/15.8 Å -Mo) × 40 to act as the SW generator. The experiments were conducted at the elliptically polarized soft X-ray undulator beamline UE49-PGM-a at the storage ring BESSY-II in Berlin, Germany. This microfocus beamline is equipped with an Elmitec PEEM-III endstation with an integrated photoelectron energy analyzer (Figure 6 ). The experimental geometry allowed for photon incidence angles between 13.8 ° and 17.8 ° , as measured from the sample surface plane. For the particular multilayer substrate and for the incidence angle range of 13.8 ° – 17.8 ° , the Bragg condi-tion was achieved by using a photon energy range of 663 eV and 516 eV, respectively. As confi rmed from simulations for

Figure 4 (A) Scheme of the (Pt 0.75 nm/Co 0.25 nm) 50/Tb 30 Fe 70 25 nm/Pt 5 nm multilayered fi lm. (B) Magnetic X-ray microscopy images recorded at the Co L 3 (left) and Fe L 3 (right) edge at a mag-netic fi eld of approximately 3 kOe. The two images show the same domain pattern with reversed contrast demonstrating the antiparallel alignment of the Fe and Co moments and their direct coupling. From Ref. [44] .

electron spectra, the combination of a photon energy of 663 eV and a grazing angle of 13.8 ° yields the best resolved Co 3p, Al 2p, Si 2p and C 1s photoemission spectra, unobstructed by any Auger electron features. In conjunction with X-ray opti-cal theoretical modeling, quantitative information about the depth-dependent chemical and magnetic composition of the sample can be extracted from the photoemission data [45] .

Magnetic depth profi les have also been studied with mag-netic resonant X-ray refl ectivity (MRXR) [48] , which is simi-lar to neutron refl ectivity measurements; however, it has the

Figure 5 Schematic diagram of the experimental setup including the sample and the Elmitec PEEM endstation at the elliptically polar-ized soft X-ray undulator beamline UE49-PGM-a at the storage ring BESSY-II and cross-sectional as-grown schematic of the Co micro-dot structure, with the photoemission core peaks used in the element-specifi c study of the constituents of the structure indicated by arrows. From Ref. [47] .

Figure 6 Standing-wave PEEM images captured at h v = 680 eV, for (A) Al 2p, (B) C 1s, (C) Si 2p and (D) Co 3p core levels; 680 eV is the energy for which the Al image is a maximum off the microdots. From Ref. [47] .




advantage of inherent elemental specifi city. In principle, this technique can also be combined with soft X-ray microscopy to add lateral resolution, but so far this approach has not yet been realized. Whereas the depth resolving capabilities would provide interesting insight into interfacial structures, the requirement for transmission experiments to be prepared onto X-ray transparent substrates could be released. This would give access, e.g., to a variety of epitaxially grown systems, which require specifi c, generally non-transparent substrates such as MgO or sapphire. Various alternatives for making non-transparent substrates accessible for soft X-ray transmission spectromicroscopy are

currently being investigated. Options include the development of fabrication techniques to thin down the sample after growing either through chemical processes or through nanopatterning, such as focused ion beam ablation or through ion beam assisted deposition of thin fi lms onto commercially available transparent substrates without sacrifi cing the growth conditions [49] .

4. Future directions and opportunities

Magnetic X-ray spectromicroscopy has a unique potential for the study of nanoscale magnetic behavior in fundamentally

Figure 7 The full-fi eld soft X-ray microscope XM-1 at the Advanced Light Source in Berkeley, CA. As with other large-scale instrumenta-tions, which are funded by the US Department of Energy, access to this instrument for general users is obtained through a highly competitive peer-review process based on the scientifi c quality of the proposals submitted.

5-Axissamplemanipulator

Sample prep.chamber: LEED,Knudsen cells,electromagnet,...

Chamberrotation

Scientaelectron

spectrometer(hidden)

Multi-techniquespectrometer/

diffractometer (MTSD)

Scientasoft X-rayspectrometer

ALSBL 9.3.1hν=2-5 keV

Figure 8 The multi-technique spectrometer/diffractometer (MTSD) developed by the Fadley group operated at the Advanced Light Source.




interesting and technologically relevant systems. It com-bines inherent elemental specifi city with the capability to image spin structures and their dynamics down to funda-mental magnetic length and time scales. These are the in nm length and the fs temporal range. The spatial resolution, which is inherently limited by the wavelength of soft X-rays has been demonstrated with state-of-the-art zone plate optics to achieve < 10 nm. Novel approaches are currently exploring the feasibility of obtaining three-dimensional images, e.g., by combining depth-resolved techniques with two-dimensional imaging microscopies. In terms of sample environment, soft X-rays can operate at low and high tem-peratures, in infi nite magnetic and electric fi elds, at low and high pressures, which are all relevant parameters for mag-netic behavior. Only a fraction of these parameters is cur-rently available at existing X-ray microscopes, but it can be foreseen, that in the near term future these parameters will become available. The obtainable time resolution is limited by the time structure, i.e., the X-ray pulse length of cur-rent soft X-ray sources at third generation synchrotrons. However, next generation light sources, such as X-ray free electron lasers (XFEL) are showing up on the horizon and they will be capable of delivering fs X-ray pulses with a pulse intensity, which is suffi cient for a single shot mag-netic image. It can thus be foreseen that these facilities will become more widely available as more routine user tools at numerous synchrotron facilities and next generation free-electron lasers around the world.

Current scientifi c activities of the authors

Dr. Fischer and Prof. Fadley are both principal investigators in the Magnetic Materials Program at the Lawrence Berkeley National Laboratory (LBNL). This program aims to explore and create a ba-sic understanding of novel magnetic materials for spintronics ap-plications, where the spin of the electrons is the dominant physical quantity. Particular emphasis in their research is to look for magnet-ic effects occurring on fundamental magnetic length and time scales as well as ways to minimize the energy consumption when used in magnetic devices. As progress in this research requires enhanced and new capabilities of advanced characterization, a major component of their scientifi c activities is to develop instrumentations which are largely based on utilizing the unique properties of X-rays.

Dr. Fischer is in charge of the full-fi eld soft X-ray micro-scope XM-1 (see Figure 7 ) which is jointly operated between the Materials Science Division (MSD) at LBNL and the Advanced Light Source (ALS) at beamline 6.1.2 at the ALS. Worldwide, this instrument is still the only full-fi eld soft X-ray transmis-sion microscope used for magnetic imaging with XMCD as the magnetic contrast mechanism. The key components to run this instrument are the Fresnel zone plate (FZP) optics, which are de-veloped and fabricated at the Center for X-ray Optics (CXRO), a unique research facility within MSD at LBNL. Highest qual-ity FZPs have demonstrated at XM-1 a world record 10 nm spatial resolution with soft X-rays and the current standard spa-tial resolution, enjoyed by a large general and worldwide user community of XM-1, is approximately 20 – 25 nm. The most attractive feature of XM-1 is the combination of high spatiotem-poral resolution and chemical and magnetic sensitivity, which is of utmost importance for nanomagnetism research.

Dr. Fischer ’ s current scientifi c interest is to investigate ways to control spins on the nanoscale by imaging with MTXM, e.g., spin current induced domain wall and vortex core dynamics, as well as to study the relatively simple, but yet unanswered question as to whether magnetic processes on a nanometer length scale exhibit a deterministic behavior or show stochastic character. This question is of both fundamental interest but moreover of high technological relevance.

One out of many of Prof. Fadley ’ s scientifi c research interests is the question how buried interfaces determine the functional behav-ior of complex and multicomponent nanoscale magnetic devices. Also here, new instrumentation utilizing X-rays can provide very valuable and new insight (see Figure 8 ). Using the standing wave approach, which Prof. Fadley and his team has pioneered in the recent past with experiments at various worldwide synchrotron radiation facilities used a high precision and tailored multilayer substrate fabricated also at CXRO. With the advanced instrumen-tation and the sophisticated X-ray optics analysis calculations, his team was able to demonstrate the feasibility to fully gain three-di-mensional information of the magnetism at deep-buried interfaces in technologically relevant systems. Another important scientifi c direction of Fadley ’ s team is to enhance the capabilities of photo-emission spectroscopy, which owing to the limited escape depth of soft X-rays has so far remained a highly surface sensitive tech-nique, into the hard X-ray regime, which allows to probe deep into the bulk of interesting materials. Fadley is also closely col-laborating with theory groups to achieve a solid understanding of the experimental data.

Both Dr. Fischer and Prof. Fadley are looking forward to next generation of soft X-ray instrumentation such as a next generation full-fi eld soft X-ray microscope for materials, environmental and energy related research and a new hard X-ray photoemission instru-ment, as well as to the next generation of light sources, such as the upcoming X-ray Free Electron Laser (X-FEL) facilities, which will completely provide new capabilities for X-ray sciences in terms of peak brilliance, coherence and time scales which will become acces-sible to the community.

Acknowledgments

This work was supported by the Director, Offi ce of Science, Offi ce of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under Contract no. DE-AC02-05-CH11231. We thank the many colleagues for longstand-ing and fruitful collaborations, in particular M.-Y. Im, W.L. Chao, E. Anderson, F. Salmassi, E.M. Gullikson i(CXRO), S.-Ch. Shin (KAIST Korea), D.-H. Kim (Chungbuk Natl. U, Korea), G. Meier, L. Bocklage, (U. Hamburg, Germany), S. Mangin, (U. Nancy, France), S.K. Kim, (Seoul Natl. U., Korea), A.X. Gray (UC Davis), F. Kronast (HZB Berlin), Ch. Papp (U. Erlangen), S.-H. Yang (IBM Almaden), S. Cramm, I.P. Krug, C.M. Schneider (Juelich), H.A. D ü rr (SLAC), F. Hellman (UC Berkeley) and J. Kortright (MSD LBNL). We also especially thank the staff of Center for X-ray Optics (CXRO), Advanced Light Source (ALS) and BESSY.

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Received August 17, 2011; accepted September 1, 2011

Dr. Peter Fischer received his PhD in Physics (Dr.rer.nat.) from the Technical University in Munich, Germany in 1993 and his habilitation from the University in W ü rzburg in 2000 based on his pioneering work on magnetic soft X-ray microscopy.

Since 2004, he has been staff scientist and principal investigator at the Center for X-ray Optics within the

Materials Sciences Division at Lawrence Berkeley National Laboratory (LBNL) in Berkeley, CA. His current research program is focused on the use of polarized synchrotron radiation for the study of fundamental problems in nanomag-netism. He has pioneered magnetic soft X-ray transmission microscopy for the imaging of magnetic domain structures and their fast dynamics. He is involved in developing the scientifi c case for a next generation soft X-ray free electron laser at LBNL. Dr. Fischer has published more than 130 peer-reviewed papers and has given more than 180 invited pre-sentations at national and international conferences. For his achievements of “ hitting the 10 nm resolution milestone with soft X-ray microscopy ” he was co-awarded with the Klaus Halbach Award at Advanced Light Source in 2010. He was appointed as Distinguished Lecturer of the IEEE Magnetics Society in 2011.

Dr. Fischer serves the magnetism and X-ray community as a member of various program and review committees world-wide. He was conference chair of the 7th MML in Berkeley in 2010. He is a member of the APS, AVS and senior member of the IEEE Magnetics Society.

Professor Charles S. Fadley received his BS from MIT and his MS and PhD from UC Berkeley, CA. He is Distinguished Professor of Physics at UC Davis, CA and Faculty Scientist at the Materials Sciences Division at Lawrence Berkeley National Laboratory in Berkeley, CA. His general research areas include surfaces, buried solid-solid interfaces, multilayer

nanostructures and complex multicomponent materials that form the active elements in many devices for information stor-age and processing, as well as energy conversion, storage and utilization. Research in the Fadley Group is directed towards developing new X-ray based methods for studying such sur-faces, interfaces, nanostructures and complex materials. The principal technique used by Fadley is photoelectron spectros-copy (photoemission) as excited by synchrotron radiation, and in the future, free-electron lasers. One new direction of interest is the use of X-ray standing waves created by Bragg refl ec-tion from multilayer mirrors to probe the depth dependence of composition, electronic structure and magnetism in spintronic-, complex oxide- and semiconductor-multilayer nanostructures. A second is the use of harder X-rays at 3 – 6 keV or more to excite photoelectrons from deep within nanostructures and bulk materials. The systems studied are of relevance to next-generation magnetic information storage and logic, and other emerging ideas for logic, memory and energy conversion. Fadley and his team carry out experiments with unique facili-ties at the Advanced Light Source in Berkeley, as well as other laboratories in Germany, Japan and Switzerland.

Prof. Fadley has received numerous awards and honors, including the Medard W. Welch Award of the AVS (2005), the Helmholtz-Humboldt Research Award (2006), the Japanese Society for the Promotion of Science, Microbeam Analysis Committee Award (2007) and an Alfred P. Sloan Foundation Fellowship, and has been elected fellow of the APS, AVS and IOP, and a foreign member of the Russian Academy of Sciences and the Royal Society of Sciences in Uppsala, Sweden. He has published close to 300 peer-reviewed papers and given close to 200 invited talks at international venues.



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