Domain structure in LaFeO 3 thin films and its role on exchange coupling
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Domain structure in LaFeO3 thin films and its role on exchange coupling Jin Won Seo1,2, Jean Fompeyrine2, Heinz Siegwart2 and Jean-Pierre Locquet2 1 Institut de Physique, Université de Neuchâtel, CH-2000 Neuchâtel, Switzerland 2 IBM Research, Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland ABSTRACT The origin of the exchange bias between a ferro- and an antiferromagnet still is not fully resolved. Many structural parameters such as film thickness, roughness, domain size, domain walls, strain, dislocations, and surface defects play a role. To disentangle their individual contributions, one must correlate each of these structural effects with the exchange bias. Here we report our first such attempts using thin films of the antiferromagnetic orthoferrite LaFeO3 grown by molecular beam epitaxy. We investigate how the epitaxial relationship and strain affect the domain configuration as well as their size and orientation. Next, we explore how these different domains can pin ferromagnetic domains in exchange-coupled systems. We conclude that decreasing the domain size of the antiferromagnet leads to a higher exchange bias. The highest exchange bias was obtained for LaFeO3 films on MgO substrates. INTRODUCTION Antiferromagnets (AF) are essential elements in the read heads of current hard-disk drives, where their purpose is to pin the nearest ferromagnetic (FM) layer into one preferred orientation. This coupling between the spins in the AF and FM layers, called exchange bias, largely determines the efficiency and the long-term stability of the giant magnetoresistance ratio. Although discovered a long time ago [1], this phenomenon still is poorly understood. A number of experiments and theoretical models have suggested that the exchange bias is correlated with the presence and morphology of AF domains [2,3]. The AF orthoferrite LaFeO3 (LFO) is an interesting model system to explore this correlation, as the AF axis A is uniquely defined along the a-axis of the lattice. The orthorhombicity of this crystal is large enough so that its structural recognition on a local scale leads to a direct determination of A. Recently, we reported that it is possible to observe the local magnetic contrast related to the AF domains in LFO films grown on SrTiO3 (STO) by means of photoemission electron microscopy (PEEM) [4]. These AF domains could be directly correlated with the structural domains, as a detailed comparison between the magnetic sensitive PEEM and the structural measurements using transmission electron microscopy (TEM) has shown [4]. This illustrates the strong anisotropy in this system. Next, after growing a thin polycrystalline Co layer on top of LFO, the magnetic correlation between the AF and FM layer was studied. The results suggest that the FM spins are aligned in-plane, parallel to the projected spin directions of the underlying AF layer [5]. The local exchange coupling is stronger than the anisotropy in Co and takes place on an individual AF domain basis. In this paper, we investigate how parameters, such as the
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