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Abstract's Details

Origin of Anomalous Magnetic Behavior in NiMn2O4 Thin Films
Abstract IDMAT-12 
PresenterBrittany  Nelson-Cheeseman
Presentation TypePoster
Full Author ListB. B. Nelson-Cheeseman (1), R. V. Chopdekar (1,2), M. F. Toney (3), J. S. Bettinger (1), E. Arenholz (4), Y. Suzuki (1)
Affiliations(1) Dept. of Materials Science and Engr, Univeristy of California, Berkeley, CA
(2) School of Applied Physics, Cornell University, Ithaca, NY
(3) Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA
(4) Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA
CategoryMaterials Science
AbstractMagnetic thin films often exhibit properties markedly different from the bulk due to the effects of surfaces or interfaces, strain and non-equilibrium synthesis. We have investigated thin films of the ferrimagnetic spinel NiMn2O4 (NMO) that exhibit anomalous magnetic behavior. While NMO exhibits two magnetic transitions in the bulk (a collinear moment configuration below 100K and a canted moment configuration below 60K), epitaxial thin films grown on SrTiO3 and MgAl2O4 substrates exhibit a single magnetic transition around 60K. As the magnetism in this spinel is determined by the ferrimagnetic interaction among the transition metal cations with octahedral and tetrahedral coordination, an understanding of the cation valences and site occupancies is necessary. Resonant X-ray diffraction and X-ray absorption spectroscopy have been used to investigate the chemical structure of NMO thin films and bulk powder. Site-specific resonant X-ray diffraction performed at SSRL indicates that the cation site occupancy of the thin films does not differ markedly from bulk samples, but that there are differences in the relative amounts of Mn2+, Mn3+ & Mn4+. This difference in Mn valence for thin films likely plays a key role in the difference in magnetic properties, since the collinear magnetic state of bulk NMO is dictated by interactions between the octahedral Mn3+ and Mn4+ cations. High resolution X-ray diffraction of NMO thin films shows an anomaly in the NMO lattice parameters below 60K, demonstrating a structural transformation linked to the magnetic transformation. Finally, bulk magnetization measurements and AC susceptibility measurements indicate a canted ferrimagnetic moment arrangement. Together these results suggest that the collinear ferrimagnetic state experienced by bulk NMO at intermediate temperatures is suppressed in NMO thin films due to a difference in the relative amounts of Mn2+, Mn3+ & Mn4+, and the canted ferrimagnetism of the film dominates below 60K.
Footnotes 
Funding AcknowledgementDOE-BES, NSF-IGERT