|Depth Profile of Uncompensated Spins in an Exchange-Bias System|
The phenomenon known as exchange bias at the interface between a ferromagnet and an antiferromagnet is currently a subject of intense research because of its applications in the magnetic recording and read-head industries. An international collaboration headed by researchers from the University of California, San Diego, has used resonant x-ray scattering and polarized-neutron reflectometry to determine the depth-dependent magnetization in an exchange-biased sample. These results provide atomic-level insights into the mechanism of exchange bias, specifically the involvement of mutual interactions between two kinds of uncompensated spins in the antiferromagnet and spins in the ferromagnet.
In modern nanolayer magnetic devices, the shifted hysteresis loop centered on a nonzero magnetic field that characterizes exchange bias makes the ferromagnet an excellent magnetic reference layer because it is difficult to reverse the magnetization. Although exchange bias has been extensively studied over the years, fundamental questions remain unanswered or inadequately answered. For example, photoemission electron microscope (PEEM) images and x-ray circular magnetic dichroism (XMCD) studies have revealed the existence of uncompensated moments (spins in one direction not matched by an opposite spin) at the antiferromagnetic interface. What is the nature of the uncompensated spins? What are their lateral and depth distributions? How do they interact with the spins in the ferromagnet?
To address these issues, the collaboration used circularly polarized soft x-ray and spin-polarized neutron beams in small-angle reflection geometry with a sample consisting of a trilayer of antiferromagnetic FeF2, ferromagnetic cobalt, and aluminum epitaxially grown onto a single-crystal MgF2 substrate. Resonant x-ray measurements at the L edges of the magnetic atoms provided the variation of the element-specific magnetization, while neutron measurements yielded the variation of the vector magnetization. With the two techniques, the group was able to determine, in an element-specific way, the depth dependence of the magnetic density in absolute units.
Soft x-ray resonant magnetic reflectivity at ALS Beamline 4.0.2 was used to measure hysteresis loops at the L3 edges of cobalt and iron in a 1-tesla field at 20 K. The measurements showed that both elements exhibited hysteresis, indicating that some cobalt and some iron spins were unpinned or free to rotate with the applied field. Both loops were shifted along the positive-field axis, with a similar bias field resulting from the shifted loops. The spin-density profiles obtained from analyzing the reflectivity as a function of wave vector transfer revealed a thin interfacial layer (about 17 Å thick) in FeF2 with unpinned iron spins that were aligned opposite to cobalt spins in the bulk cobalt. Pinned, uncompensated iron spins were also found throughout the depth of the antiferromagnet.
To probe the depth profile of the pinned magnetization, the group turned to the Manuel Lujan Jr. Neutron Scattering Center at Los Alamos National Laboratory for polarized-neutron reflectometry measurements in a cooled sample in a 0.7-T field. The oscillations of the reflectivity with wave vector transfer indicated the presence of pinned moments in the bulk of the FeF2 at distances greater than about 3.5 nm from the interface. Further analysis showed that near the Co/FeF2 interface, the angular dependence of the magnetization relative to the applied field is consistent with an antiferromagnetic coupling across an interface.
From these results, the collaborators concluded that the antiferromagnet has a net magnetization made up of two types of spins: those in a thin (about 2-nm) interfacial layer at the interface, which are strongly coupled to the magnetic spins of the ferromagnetic cobalt (and respond with them to an applied field) and those in the bulk of the FeF2, which do not respond to an external field. It is the coupling of these spins to the ferromagnetic spins via the interfacial spins that gives rise to the exchange bias.
Research conducted by S. Roy, M. Dorn, Z.-P.Li, I.V. Roshchin, and I.K. Schuller (University of California, San Diego); O. Petracic (UCSD and Universität Duisburg-Essen, Germany); X. Batlle (UCSD and Universitat de Barcelona, Spain); R. Morales (UCSD and Universidad de Olviedo, Spain), K. Chesnel (ALS); J.B. Kortright (Berkeley Lab); S. Park, M.R. Fitzsimmons, A. Mishra, and X. Zhang (Los Alamos National Laboratory); and S.K. Sinha (UCSD and Los Alamos National Laboratory).
Research funding: U.S. Department of Energy, Office of Basic Energy Sciences; National Science Foundation; University of California; Alexander von Humboldt Foundation; Spanish MECD; Fulbright Commission; Catalan Dursi; and Swiss National Science Foundation. Operation of the ALS and of LANSCE is supported by BES.
Publication about this research: S. Roy, M.R. Fitzsimmons, S. Park, M. Dorn, O. Petracic, I.V. Roshchin, Z.-P. Li, X. Batlle, R. Morales, A. Mishra, X. Zhang, K. Chesnel, J.B. Kortright, S.K. Sinha, and I.K. Schuller,, “Depth Profile of Uncompensated Spins in an Exchange Bias System,” Phys. Rev. Lett. 95, 047201 (2005).