DOE OSTI2011
We identified and investigated some of the scientific and technically most challenging issues in thin film magnetism focusing on epitaxially grown layers of specific L1 0 ordered, intermetallic, heterostructures with well-controlled crystallography and interface structures. Specifically, we addressed antiferromagnetic/ferromagnetic heterostructures, exhibiting exchange bias (EB) in both in-plane (MnPd/Fe) and perpendicular (IrMn/(Co/Pt) n ) geometries, and ferromagnetic/ferrimagnetic (Co/Y 3 Fe 5 O 12 ) bilayers with strong interlayer exchange coupling and exhibiting spin reorientation transitions. In the former case, the work included experimental and theoretical studies to gain more fundamental insight into the origin and magnitude of EB, as well as to address important aspects of EB such as the asymmetry in the magnetic reversal mechanism, the role of interfacial structure, including compensated or uncompensated spins, AF domains, competing anisotropies and the angular dependence of the magnetization reversal process. Exchange bias is central to many magnetic technologies and driven by the fast development of nanotechnology, there is much interest in understanding the phenomenon of exchange bias on the nanoscale. By patterning, as the FM domain size reaches a lateral scale comparable to the AF domain size (~100nm), each nano-element can be treated as a separate and isolated exchange bias system that behaves independently. Therefore, the non-averaged, intrinsic, exchange bias, in all its complexity, can be studied. Such size and dimensionality effects, particularly in structures with lateral dimension of the order of their domain sizes, were studied by developing and implementing a novel Nano-imprint as well as convention optical lithography/patterning. However, one limitation of the NIL method is that after imprinting the material-deposition or -evaporation has to be done at around room temperature in order to keep the resists structure undisturbed. As a result, the method is unsuitable for epitaxial growth, since the latter often involves growth at elevated temperatures higher than the glass transition temperature of the resist. Therefore, a mask transfer NIL process was developed to grow epitaxial nanostructure arrays at elevated temperatures where organic resists are rendered unstable. In the case of the metal/oxide heterostructures, the domain structure of the metal is carefully modulated by that of the underlying oxide, opening the possibility of carrying out novel experiments to study spin-dependent domain-wall scattering and quantify domain wall resistance in mesoscopic geometries. Utilizing state-of-the-art characterization methods, using synchrotron radiation and electron holography, we addressed the critical role of all aspects of the microstructure, at relevant length scales, in determining these specific magnetic properties. Two significant highlights of this project were the use of photoemission electron microscopy (PEEM) work to elucidate their asymmetric magnetization reversal mechanism and the use of element-specific X-ray magnetic reflectivity and x-ray resonant scattering to probe buried interfaces, both of importance in understanding the fundamental physics of exchange bias. In the latter case, a complex magnetic interfacial configuration in Fe/MnPd, consisting of a 2-monolayer-thick induced ferromagnetic region, and pinned uncompensated Mn moments that reach far deeper (~13 Å), both in the antiferromagnet, were found. Such epitaxial EB samples also show in-plane reorientation transitions, determined by the competition between the interface exchange coupling and the intrinsic uniaxial energies, and is driven by the temperature, as well as the thickness of MnPd and Fe layers. Complementing these results, work on multilayers show that perpendicular EB arise from a complex interplay between unidirectional anisotropy at the terminating FM/AFM interface, the perpendicular anisotropy of the FM/nonmagnet(NM) multilayer stack and the overall magnetostatic energy of the structure. Collaborative work with Prof. R. Stamps (UWA) in modeling and analysis of slow-dynamics, using an inductive ferromagnetic resonance technique, were also carried out. Further details of our research is presented below broadly in five thematic areas. Overall, this research allowed us to obtain a deeper understanding of the range of related magnetic phenomena and establish pathways for potential technological applications of these thin film and patterned heterostructures.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗