Manipulation of magnetization states of ferromagnetic nanorings by an appliedazimuthal Oersted fieldT Yang, Nihar R. Pradhan, Abby Goldman, Abigail S. Licht, Yihan Li, M. Kemei, Mark T. Tuominen, andKatherine E. Aidala Citation: Applied Physics Letters 98, 242505 (2011); doi: 10.1063/1.3599714 View online: http://dx.doi.org/10.1063/1.3599714 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/98/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The creation of 360° domain walls in ferromagnetic nanorings by circular applied magnetic fields J. Appl. Phys. 115, 17D135 (2014); 10.1063/1.4864441 Micromagnetic analysis of the magnetization dynamics driven by the Oersted field in permalloy nanorings J. Appl. Phys. 111, 07D103 (2012); 10.1063/1.3671433 Magnetization states and switching in narrow-gapped ferromagnetic nanorings AIP Advances 2, 012136 (2012); 10.1063/1.3685590 Effect of localized magnetic field on the uniform ferromagnetic resonance mode in a thin film Appl. Phys. Lett. 94, 172508 (2009); 10.1063/1.3123264 Manipulation of magnetic nanoparticles by the strayfield of magnetically patterned ferromagnetic layers J. Appl. Phys. 102, 013910 (2007); 10.1063/1.2752146

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Manipulation of magnetization states of ferromagnetic nanoringsby an applied azimuthal Oersted field

T Yang,2 Nihar R. Pradhan,1 Abby Goldman,1 Abigail S. Licht,1 Yihan Li,1 M. Kemei,1

Mark T. Tuominen,2 and Katherine E. Aidala1,a�

1Department of Physics, Mount Holyoke College, South Hadley, Massachusetts 01075, USA2Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, USA

�Received 3 March 2011; accepted 23 May 2011; published online 16 June 2011�

We manipulate the magnetic states of ferromagnetic nanorings with an azimuthal Oersted fielddirected along the ring circumference. The circular field is generated by passing current through anatomic force microscope tip positioned at the center of the ring, and can directly control the chiralityof the vortex state. We demonstrate switching from an onion state to a vortex state and between twovortex states, using magnetic force microscopy to image the resulting magnetic states. Theunderstanding of the magnetization switching behavior in an azimuthal Oersted field could improvepractical magnetic data storage devices. © 2011 American Institute of Physics.�doi:10.1063/1.3599714�

Ferromagnetic nanorings have received increasing atten-tion over the past few years.1–4 The ring shape is of particu-lar interest due to its closed-flux “vortex” state, in which thecircular field lines are entirely contained within the structure.Ferromagnetic rings have been proposed as a basis for prac-tical nonvolatile data storage devices.5,8 The vortex chiralitycan be used to encode binary information, and the lack ofstray fields suggests that the bits could be packed denselywithout coupling between neighbors. Much work has beendevoted to the investigation of the orientations of the vortexin rings under the influence of a uniform magnetic field. Insymmetric rings, the vortex chirality cannot be controlled bya homogeneous external field,10 while in asymmetric rings,the chirality can be manipulated with a uniform in-plane fieldin a predictable way.10–13 A metastable onion state was re-ported as an intermediate remanent state during the switch-ing process of these rings. Direct vortex-to-vortex switchingis not achievable with a uniform in-plane magnetic field. It ispossible to use the nonuniform field created from the strayfields of a magnetic force microscope �MFM� probe to con-trol the chirality of the vortex in a disk.14

Our work uses a perpendicular current through the centerof a ring to generate a local circular magnetic field, andachieves direct switching and control over the magnetic vor-tex chirality of ferromagnetic nanorings. Creating this fieldand measuring the resulting vortex chirality presents experi-mental challenges. Limited experimental efforts15,16 havedemonstrated multilayered structures that use spin transfertorque and the accompanying Oersted field to switch twofree and reference magnetic rings back and forth betweenparallel and antiparallel onion states by passing currentthrough the ring structure itself. Recent theoretical work hasbegun exploring the switching mechanism3,8,17–19 resultingfrom a local circular field applied to the ring center, to pre-dict magnetic states.

We present a method to manipulate the magnetic statesof ferromagnetic nanorings using an azimuthal Oersted mag-netic field directed along the ring arm circumference. Thistype of circular field can select the desired vortex magneti-

zation. The direction and magnitude of the field are deter-mined by passing a current through a solid metal atomicforce microscope probe into a metal underlayer �Fig. 1�a��.We demonstrate control over switching from an onion stateto a vortex state �O-V� and the direct switching between twovortex states �V-V�, using MFM to image the resulting mag-netic states. Domain wall �DW� types �head-to-head or tail-to-tail� and vortex chiralities �clockwise �CW� or counter-clockwise �CCW�� can be predetermined by an in-planesaturation field, and confirmed by MFM contrast.

Symmetric and asymmetric nanorings were fabricatedusing standard electron-beam lithography, using electron-beam evaporation to deposit 25 nm of permalloy on a gold-coated silicon wafer, capped with 4 nm Pt to protect thepermalloy from oxidization. MFM images were obtainedwith an Asylum Research MFP3D atomic force microscope,and circular fields were created by passing a current througha solid Pt tip �Rocky Mountain Nanotechnology�. Figure 1�a�depicts a schematic of how we generated the circular field.After obtaining a topographic image of the rings, the Pt solidtip was placed at the center of the ring. We ramped a voltageand measured the resulting current across a 100 � resistor.Figure 1�b� is a typical I-V curve of the current. The initialtip diameter is nominally 40 nm, though the high currentdensities �3.18�109 A /cm2� will likely deform the tip, cre-ating a blunter tip and, subsequently, lower current densities.We were generally able to use the same tip to locate the ringstopographically and pass current through the ring centerabout ten times. After applying current, we switched back to

a�Electronic mail: kaidala@mtholyoke.edu.

Pt solid AFM tip

Au coated Si

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20

30

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FIG. 1. �Color� �a� Schematic of the experiment setup. �b� A current-voltageplot generated directly from the experimental setup in �a�.

APPLIED PHYSICS LETTERS 98, 242505 �2011�

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a magnetically coated Asylum high coercivity MFM probe toimage the resulting magnetic state.

Figure 2 shows the magnetic switching of symmetricnanorings, with Figs. 2�a� and 2�h� showing the topographi-cal AFM images. Before the MFM characterization, an initialin-plane, uniform magnetic field of 5 kOe was applied. InFigs. 2�b� and 2�i�, remanent onion states in all six ringswere observed at zero field with MFM, shown schematicallyin Figs. 2�c�, 2�d�, 2�j�, and 2�l�. We can identify the whitecontrast with head-to-head DWs, and dark as tail-to-tail,given the initialization. We passed 40 and 30 mA of currentthrough two identical rings indicated by the double andsingle red dashed circles. The corresponding Oersted fieldsin the vicinity of the ring center are around 178 and 133 Oerespectively, as calculated according to Ampere’s Law, as-suming an infinite wire approximation. Two different mag-netic states, shown by MFM contrast and schematically inFigs. 2�e�–2�g� resulted from the different circular fields. Inring 4, with the smaller applied current, we observed a meta-stable state, with a 360° DW,6–9,20 resulting from the propa-gation of two 180° DWs. In ring 2, with the larger appliedcurrent, the two DWs moved toward each other and annihi-lated, entering the CW vortex. We can directly observe thechirality in the 360° DW or so called “twisted onion” statefrom the light/dark MFM contrast, while for ring 2, we as-sumed the vortex direction based on the external CW Oer-sted field. When the circular field is applied, one semicirculardomain in the onion state is more energetically favored dueto the Zeeman energy, and the two DWs slide toward theminimum energy configuration, leading to the vortex state ofdesired chirality. A stronger field is required to annihilate theDWs than to move them.6

Figures 2�i�–2�n� show the switching of another sym-metric ring set from the initialized onion state �Figs. 2�i� and

2�j�� to the opposite, CCW vortex state, achieved by passinga current in the opposite direction at the center of ring 6. Asmall negative current of �25 mA generated an intermediatestate �Figs. 2�k� and 2�l��, with a 360° DW. The light/darkcontrast of this DW is opposite to the one created with thepositive current in ring 4 �Fig. 2�e��, revealing the underlyingCCW circulation. Passing a larger current of �35 mA anni-hilated the 360° DW, forming the CCW vortex state �Figs.2�m� and 2�n��, as seen by the lack of MFM contrast. Minorchanges in magnetic contrast for nominally identical states�the small rings with no applied Oersted fields� can be ex-pected due to the wear on the tip from the substantial topo-graphic imaging required to find the identical ring after ap-plying the circular field with the solid metal tip.

To directly measure the direction of vortex chirality, weintroduced asymmetry into the ferromagnetic permalloynanoring design. Since asymmetric ferromagnetic nanoringshave incomplete vortex magnetization circulation, there isstray magnetic field around the edge of the decentered hole.Therefore, MFM images will show contrast that distin-guishes the two directions of vortex, as shown in the MFMsimulations of Figs. 3�t� and 3�u�, in which the light/darkcontrast is reversed for the opposite chiralities. Figures3�a�–3�g� show the evolution of magnetic states in threerings, initialized in onion states �Figs. 3�b�–3�d��. We

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FIG. 2. �Color� �a� Topographic AFM image of two identically designed setsof Ni81Fe19 symmetric rings. Ring 2 and 4 have 300 nm arm width and 1200nm outer diameter, while 1 and 3 have 300 nm arm width and 800 nm outerdiameter. �b� MFM image of the initial remanent states of rings in �a�,revealing the onion state as indicated schematically in �c� and �d� for rings 2and 4. The double red dashed line indicates that we apply a stronger currentand CW field �40 mA and 178 Oe� through the ring center, while the singlered dashed line indicates a smaller current �30 mA, 133 Oe�. �e� MFMimage after applying currents, revealing ring 2 is in a vortex state while ring4 remains in an intermediate state, and rings 1 and 3 are unchanged. �f�Schematic of the state of ring 2, and �g� ring 4. �h� Topographic AFM imageof a third set of symmetric rings. �i� MFM image of initial remanent onionstates, indicated schematically in �j� for ring 6. Blue circles indicate currentdirection opposite to red circles. �k� MFM image after applying a �25 mA�111 Oe� CCW current, revealing a 360° DW, shown schematically in �l�.�m� MFM image after applying a stronger current ��35 mA and 156 Oe�,revealing the vortex state, indicated schematically in �n�.

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FIG. 3. �Color� �a� Topographic AFM image of a set of three Ni81Fe19

asymmetric rings. The offset from the center of the ring is 50 nm and outerdiameters of three rings are 770 nm, 870 nm, and 1050 nm, respectively. �b�MFM images of initial onion states of the rings at zero field, indicatedschematically for rings 2 and 3 in �c� and �d�. Blue and red dashed linesindicate out-of-plane and into-plane applied currents, respectively. �e� MFMimage after applying �18 mA �97 Oe, CCW� to ring 2 and 19 mA �90 OeCW� to ring 3, showing different light/dark contrast indicating oppositevortex chiralities, as shown schematically in �f� and �g�. �h� MFM image ofring 1 and 2 with initial CW vortex states at zero field, shown schematicallyin �i�. �j� MFM images of rings in �h� after �30 mA �191 Oe, CCW� isapplied to ring 1, revealing switched vortex chirality, as indicated schemati-cally in �k�. �l� MFM images of ring 1 and 2 with initial CCW vortex statesat zero field, shown schematically in �m�. �n� MFM image after applying 30mA �162 Oe, CW� to ring 2, revealing switched chirality, as shown sche-matically in �o�. �p� MFM image of ring 3 with initial CW vortex states atzero field, shown schematically in �q�. �r� MFM image of ring 3 after �18mA �85 Oe CCW� is applied, revealing an intermediate state with a 360°DW, indicated schematically in �s�. �t� Simulated MFM image of a permal-loy asymmetric ring in CW vortex state. �u� Simulated MFM image of apermalloy asymmetric ring in CCW vortex state, revealing reversed light/dark contrast.

242505-2 Yang et al. Appl. Phys. Lett. 98, 242505 �2011�

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applied-18 mA �blue circle, ring 2� and 19 mA �red circle,ring 3� to generate the CCW and CW vortex states in Fig.3�e�, schematically shown in Figs. 3�f� and 3�g�. Comparedto the strong MFM signal of the DWs in onion states in Fig.3�b�, the MFM contrast of the vortex shows soft light anddark patterns �Fig. 3�e�� that indicate stray field distributionof the incomplete vortex loops due to the decentered ringgeometry �Figs. 3�t� and 3�u��. In our MFM images, top-bright-bottom-dark represents CCW vortex and top-dark-bottom-bright represents CW vortex.

We are able to switch directly between the stable vortexstates, as shown in Figs. 3�h�–3�o�. Figure 3�h� shows tworings, both initialized in the CW vortex states, indicatedschematically in Fig. 3�i�, shown by dark-top/light-bottomcontrast. After passing �30 mA of current through bluecircled ring 1, we see that the contrast has reversed, indicat-ing the vortex state has switched to CCW, while ring 2 re-mains unchanged. In Fig. 3�l�, two rings are initialized in theCCW state �Fig. 3�m��, and +30 mA is applied to red circledring 2, which switches into the CW vortex state in Figs. 3�n�and 3�o�, while ring 1 remains unchanged.

After multiple trials, we observed that the required cur-rent for V-V switching was greater than the current requiredfor O-V switching for identical asymmetric rings. V-Vswitching includes DW nucleation, DW motion and annihi-lation, while O-V switching only includes the last two parts,hence requiring less energy.3 We were able to observe anintermediate state showing DW nucleation in the process ofthe V-V switching. In Figs. 3�p� and 3�r�, a 360° DW wasnucleated after passing �18 mA of current through ring 3initialized in CW vortex, shown schematically in Figs. 3�q�and 3�s�.

In summary, we demonstrate a method to manipulate themagnetic configurations in ferromagnetic nanorings throughan azimuthal field. Controlled O-V and V-V switching areachieved by our technique and confirmed by MFM measure-ments. This experimental study could improve the explora-

tion of magnetic states and practical designs of magnetic datastorage devices.

This work was supported by NSF Grant No. DMR-0906832, the Research Corporation for Science Advance-ment Grant No. 7889, and the NSF Center for HierarchicalManufacturing �Grant No. CMMI-0531171�.

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