2896 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012 Effect of Interface Structure on Exchange Biased Heusler Alloy Films H. Endo , A. Hirohata , J. Sagar , L. R. Fleet , T. Nakayama , and K. O’Grady , Fellow, IEEE Nihon University, Koriyama, Fukushima 963-8642, Japan Department of Electronics, The University of York, York, YO10 5DD, U.K. PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan Department of Physics, The University of York, York, YO10 5DD, U.K. Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan We report on the effects of grain size in antiferromagnetic IrMn layers exchange bias to . We also report on an enhanced effect where Mn layers are inserted in the interface. The exchange-biased IrMn/ samples were grown by a HiTUS system, which allows us to control the grain size. The smaller IrMn grains were too small to give a large while an Mn layer 0.5 nm thick dramatically increased . This significant increase is attributed to optimization of the Mn concentration at the interface. This grain- size and interface tuning offers a way to control the exchange bias in such systems. Index Terms— , exchange bias, grain size, heusler alloy, IrMn. I. INTRODUCTION I n spintronic device applications, one of the desired mate- rials is a half-metallic ferromagnet to provide 100% spin polarization. Heusler alloy films have been widely studied due to their potential to exhibit half-metallicity at room temperature [1]. In particular, Co-based full-Heusler alloys possess a large minority-spin band-gap ( 0.8 eV) and a high Curie tempera- ture ( 1000 K) [2], [3]. Recently, has been re- ported to show the largest tunneling magnetoresistance [4] and high spin polarization [5] at room temperature, indicating crys- talline robustness against atomic disordering at the interfaces. Another requirement for applications is a large remanent magnetization which can be achieved by introducing an ex- change bias where the magnetization of the ferromagnetic layer is exchange-coupled to the interfacial moment of a neighboring antiferromagnetic layer. The exchange bias induces a shift in the magnetization curve, , towards negative field when the system is thermally annealed in a positive field. The magnitude of depends strongly on the interfacial properties between the and layers. We have recently reported the growth and annealing con- ditions required to maximize both the spin polarization in a film and the exchange bias induced by IrMn [6]. Post- annealing at 400 for 30 min is found to induce an exchange bias of 200 Oe. In this paper, we extend our investigation into grain size ef- fects in a similar structure. We report on an investigation of the effects of grain size of antiferromagnetic IrMn and Mn insertion on the exchange bias. II. EXPERIMENTAL A series of samples with structure of NiCr (6 nm)/IrMn (grain size was changed)/ (10 nm)/Ta (3 nm) were grown onto Si substrates using an HiTUS system, which allows us to control the grain size precisely by tuning the DC bias [7]. An NiCr seed Manuscript received March 02, 2012; accepted March 21, 2012. Date of current version October 19, 2012. Corresponding author: H. Endo (e-mail: endo@ee.ce.nihon-u.ac.jp). Digital Object Identifier 10.1109/TMAG.2012.2194479 Fig. 1. loops of smaller and larger grain IrMn samples. layer gives strong (111) in-plane texture to the IrMn layer [8]. The grain sizes were controlled via the DC bias voltage during the growth [9]. The IrMn layers were grown directly onto the NiCr seed layer at a DC bias of 500 V. The grain size of this layer is too small ( 7 nm) to induce an exchange bias for a film thickness of less than 7.5 nm; however, thicker films were grown to increase the grain volume, thereby increasing . The Heusler alloy layer was grown at . The samples were capped by a 3 nm thick Ta layer. The samples were grown in an in-plane field of 300 Oe and annealed in a vacuum better than in a field of 1 kOe along the same direction as that applied during the growth. The annealing temperature was varied from 220 to 400 to transform the Heusler alloy into its ordered phase and to set the IrMn layer at the same time. The annealing process was carried out for 30 min, which was found to be the optimum to provide both an exchange bias of 200 Oe and a saturation magnetization of as in our previous study [6]. loops were measured after annealing using a KLA Tencor model 10 VSM to determine the saturation magnetiza- tion and exchange bias . III. RESULTS AND DISCUSSION Fig. 1 shows loops of the smaller and larger grain IrMn samples. The grain sizes of the smaller and larger 0018-9464/$31.00 © 2012 IEEE