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Transport measurements on lateral MgB2=Fe=MgB2 junctions

Savio Fabretti,a) Markus Schafers, Oliver Schebaum, Patrick Thomas, and Andy Thomasb)

Thin Films and Physics of Nanostructures, Bielefeld University, Bielefeld NRW 33615, Germany

(Presented 3 November 2011; received 23 September 2011; accepted 20 October 2011; publishedonline 15 February 2012)

The magnetic anisotropy and transport properties of superconducting MgB2 thin films on MgO(100) substrates were studied. The films were prepared by rf=dc-magnetron cosputtering and within situ annealing temperatures of 650 !C. The film orientation was measured by X-ray diffractome-try, which revealed a c-axis orientation of the MgB2 films. The critical onset temperature withoutfield cooling is 15.5 K. We found a critical field of 14.73 T parallel to the film plane and 10.79 Tperpendicular to the film plane from transport measurements of the dependence of the appliedmagnetic field. Differential conductance measurements of a lateral MgB2=Fe=MgB2 junction showthe Dp gap and the Dr gap. VC 2012 American Institute of Physics. [doi:10.1063/1.3671792]

I. INTRODUCTION

Even ten years after the discovery of the superconduct-ing properties of magnesium diboride (MgB2),1 it remains atopic of interest for both basic research and technical appli-cations. A high transition temperature of 40 K and a large co-herence length makes the BCS-superconductor MgB2

interesting for spintronic devices.2,3 Many thin film prepara-tion techniques have been developed since the discovery ofthe superconductivity of MgB2, but sputtering is the onlysuitable method for large-scale production.4 Until now, onlya few technical devices with MgB2 thin films have been fab-ricated by sputtering.

Many ferromagnets, in particular Heusler compoundswith their cubic L21-structure, fit MgO (100) substrates witha mismatch of only a few percent.5 The magnetic characteri-zation and investigation of the transport properties of sput-tered MgB2 thin films on MgO (100) substrates is thereforerequired to use MgB2 as a superconductor-ferromagnet-superconductor (p-)junction. Furthermore, lateral junctionsare also of interest for creating SQUIDs based on ferromag-netic weak links.6 A further advantage of MgO substratesover Al2O3 substrates is that no reaction is observed withMgB2 at annealing temperatures up to 800 !C.7 This alsomakes MgO valuable for thin film device applications.

In this work, we will show that MgB2 thin films pre-pared by sputtering on MgO (100) substrates have magneticanisotropic properties that are comparable to films on hexag-onal Al2O3 substrates. Furthermore, we created an iron-based lateral junction that shows tunneling-like behaviorwith respect to its I-V and dI=dV characteristics.

II. SAMPLE PREPARATION

We used MgO (100) substrates for our samples. First, anMgO buffer layer with a thickness of 5 nm was deposited byrf-magnetron sputtering to create a clean surface. Next, a

30 nm thick and 300 lm wide iron strip was sputteredthrough a shadow mask. Then, the sample was placed on aheated substrate holder, which was rotating at 5 rpm. TheMgB2 was deposited by magnetron rf and dc co-sputteringon the heated substrate. Subsequently, the MgB2 layer wascovered with a 3.5 nm thick Boron layer. The substrate tem-perature (Ts) was kept constant at 290 !C during the sputter-ing process. Finally, the samples were in situ annealed at650 !C for 20 min. A 300 lm wide and 7 mm long strip witha thickness of 30 6 3 nm was created by lithography and ionbeam etching for the magnetic anisotropy characterization ofthe MgB2. The values were corrected for an offset of 3.8 Xfrom 2-point measurements of the resistance.

For the I-V and dI=dV measurements, an MgB2 cross-strip with a length of approximately 1000 lm was prepared.A sketch of the sample geometry is given in Fig. 1. TheMgB2 strip was disconnected by a groove of approximately5 lm. With this alignment, we obtain a lateral superconduc-tor ferromagnet superconductor double ‘barrier’ junction. Aquasiparticle current thus flows on the junction surface inboth the c-axis direction and the a-b plane direction of theMgB2 thin film. At the superconducting state, the resistivityof the junction is equal to 5 X as measured by a standardfour-point probe. The transport measurements were taken ina closed-cycle 4He-cryostat with a temperature range of1.6–300 K and a magnetic field of up to 4 T.

III. EXPERIMENTS AND DISCUSSION

First, we determined the transition temperature withmagnetic field applied in both directions. In Fig. 2, the transi-tion temperatures for the magnetic field perpendicular to thefilm plane are shown. Similarly, the in-plane transition tem-perature is shown in Fig. 3. We define the critical onset tem-perature at a resistivity of 90% with regard to the resistivityin the non-superconducting state. This is marked by thedashed lines in Fig. 2 and Fig. 3. The critical onset tempera-ture is 15.5 K at 0 T. This small transition temperature iscaused by the in situ annealing process and the non-epitaxialgrowth of the MgB2 due to the sputtering process. The Tc

a)Author to whom correspondence should be addressed. Electronic mail: [email protected].

b)URL: www.spinelectronics.de.

0021-8979/2012/111(7)/07E112/3/$30.00 VC 2012 American Institute of Physics111, 07E112-1

JOURNAL OF APPLIED PHYSICS 111, 07E112 (2012)

Downloaded 01 Oct 2012 to 129.70.124.89. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

differs with the preparation method and the choice of thesubstrates.8–12 Our Tc is in good agreement with the investi-gation of Mori et al. if we compare our results with theirsamples that were in situ annealed at 600 !C for 30 min.4 Weachieve transition onset temperatures of up to 31 K for ex-situ annealed MgB2 samples with a thickness of 60 nm onsapphire substrates. However, the in situ annealed samplesare of greater interest for further investigations with respectto applications and devices. We observed a (0002) MgB2

peak at a 2 H angle of 51.8! by X-ray diffraction measure-ments with an average wavelength of k " 1:54184A emittedby a copper anode. This indicates an out-of-plane orientationof the c-axis with a lattice constant of c " 3:5298A. Due tothe different crystal structures of MgB2 and MgO, a non-epitaxial growth is most certain. However, it was reported byvan Erven et al. that the MgB2 film is rotated by approxi-mately 45! in the plane with respect to MgO. This leads to alattice mismatch of # 3% for two unit cells of MgB2 on oneMgO unit cell.13,14

Figure 4 shows the dependence on the upper criticalfield of the normalized transition temperature. We estimatedour upper critical field with the formula

HC2 T$ % " HC2 0$ % 1& T=Tc$ %2%h i

;

which was also used by Shimakage et al.15 The anisotropymeasurements of our films lead to a critical magnetic field of

HkC2 " 14:73 T and H?C2 " 10:8 T. The coherence length forthe a-b plane is nab" 4.06 nm and for the c-plane is nc" 3.1nm as calculated by the Ginzburg-Landau theory.16 Withthese values, we achieve a magnetic anisotropy ratio ofc" 1.36, which is an increase of 11.8% over the resultsof Shimakage et al.15 They measured an anisotropy ratio ofc" 1.25 for sputtered thin films on hexagonal-oriented sap-phire substrates.

Finally, we conducted transport measurements of lat-eral junctions. Figure 5 shows the I-V characteristic of thejunction at 2 K. The I-V measurement shows the expectedohmic behavior at room temperature (not shown), whereasthe gap in the density of states of the superconductor isobserved in the superconducting state. The dI=dV character-istics are also shown in Fig. 5. These characteristics showthe weakly coupled three-dimensional Dp gap at approxi-mately 0.5 meV and the strongly coupled two dimensionalDr gap at 2.25 meV. The small values of both gaps arecaused by the low transition temperature of 15.5 K. Withrespect to this low Tc, the gaps at 0 K can be theoreticallyestimated as Dr" 2.67 meV and Dp" 0.86 meV by usingthe formulas of Liu et al.17 The zero bias conductance peak(ZBCP) and a decrease in the normalized differential con-ductance down to 0.36 are caused by proximity effects at

FIG. 2. (Color online) Magnetic field dependence of the electrical resistivitywith the magnetic field perpendicular to the film plane. The dashed linedefines the critical onset temperature.

FIG. 3. (Color online) Magnetic field dependence of the resistivity with themagnetic field parallel to the film plane.

FIG. 4. (Color online) The magnetic field in dependence of the normalizedtemperatures. The solid line shows the fit of the upper critical field (HC2(0))perpendicular and in plane to the a-b axis of MgB2.

FIG. 1. (Color online) The schematic sample geometry of the iron based lat-eral junction. The MgB2 strip is separated by a groove of 5 lm.

07E112-2 Fabretti et al. J. Appl. Phys. 111, 07E112 (2012)


the ferromagnet-superconductor interface. A magnetic fieldof 1 T parallel to the a-b plane destroys the gap in the pband, which was reported by Gonnelli et al. in point contactstudies of MgB2 single crystals.18 The p-gap vanishes at0.125 T parallel to the film plane in our measurements. Thiscould be caused by a small contribution of the quasiparticlecurrent to the a-b plane due to the small transition tempera-ture. Figure 5 shows the dI=dV characteristic with anapplied magnetic field of 0.125 T parallel to the surface.

IV. SUMMARY

In summary, we show that the magnetic properties of sput-tered MgB2 films on MgO (100) substrates provide results thatare comparable to MgB2 films prepared on sapphire substrates.Furthermore, we observed the density of states of MgB2 in lat-eral junctions. These devices are easily prepared and allow theobservation of both gaps on the (001) MgO substrates. Furtherinvestigations in this direction could lead to sputtered MgB2

films that are suitable for technical applications.

ACKNOWLEDGMENTS

We would like to acknowledge the MIWF of the NRWstate government and the German Research Foundation DFGfor financial support. We are very grateful to J. S. Mooderaand G. Reiss for encouraging us to start this project.

1J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, and J. Akimitsu,Nature 410, 63 (2001).

2X. X. Xi, Supercond. Sci. Tech. 22, 043001 (2009).3R. Richter, H. Boeve, L. Bar, J. Bangert, U. K. Klostermann, J. Wecker,and G. Reiss, J. Magn. Magn. Mater. 240, 127 (2002).

4Z. Mori, T. Doi, K. Eitoku, Y. Ishizaki, H. Kitaguchi, M. Okada,K. Saitoh, and Y. Hakuraku, Supercond. Sci. Technol. 17, 47(2004).

5D. Ebke, P. Thomas, and O. Schebaum, J. Magn. 322, 996 (2010).6J.-B. Laloe, T. H. Kim, and J. S. Moodera, Adv. Condens. Matter Phys.2011, 1 (2011).

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FIG. 5. (Color online) dI=dV spectra. The red curve shows the spectra at 0T. The blue curve an applied field of 125 mT. The black curve shows the I-Vcurve.

07E112-3 Fabretti et al. J. Appl. Phys. 111, 07E112 (2012)


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