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Department of Physics525 Northwestern Avenue
West Lafayette, IN
47907-2036
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Igor Roshchin
Texas A&M University
"New Magnetic State and Intrinsic Exchange Bias"
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Friday February 22, 2013
3:30pm PHYS 203
Refreshments are served at 3:00 p.m. in Physics room 242.
http://people.physics.tamu.edu/roshchin/cv.htmlDespite more than 50 years since the discovery of exchange bias, proximity effect between a ferromagnet (FM) and an antiferromagnet (AF), its microscopic mechanism remains a puzzle. Most models rely on the uncompensated magnetization (UM) in AF, and its existence has been proven experimentally. The origin of the UM in AF is one of the most challenging pieces of the exchange bias puzzle.
In our quest for the origin of the UM in AF we observe a new magnetic state of matter in AF-only, (110)-FeF2 grown on MgF2, samples. One of the peculiarities of this magnetic state is that the zero magnetic remanence cannot be achieved isothermally, i.e. without heating the sample above its magnetic ordering temperature, TN. This state is enabled by the intrinsic exchange bias. Recently discovered, intrinsic exchange bias is similar to the traditional exchange bias but exists in a single-layer material (AF). This phenomenon is observed by SQUID magnetometry as a shift of the hysteresis loop of the UM in FeF2. The spatial distribution of the UM, revealed by polarized neutron reflectivity (PNR) measurements, and other unexpected properties of the UM will be presented.
Below TN, the UM in FeF2 is coupled to the bulk antiferromagnetic order parameter, as supported by experimental results. We claim that the surface is responsible for the macroscopically broken time-reversal symmetry, uncompensated magnetization in a nominally compensated antiferromagnet, and, ultimately, for the new magnetic state. This is an equilibrium state. The proposed scenario is consistent with our DFT calculations.
The proposed origin of the UM in the AF has significant implication on the mechanism of exchange bias: We propose a new classification of exchange bias systems that allows explaining the differences in the observed behavior of different AF-FM systems, based on the origin of UM.
Work is supported by Texas A&M University, Texas A&M University – CONACyT Collaborative Research Grant Program, and NSF-9976899. Texas Advanced Computing Center (TACC) at the University of Texas at Austin is acknowledged for providing HPC resources.