Strain dependence and magnetic anisotropy in chromium dioxide thin films
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Strain dependence and magnetic anisotropy in chromium dioxide thin films L. Spinu, H. Srikanth1, X. W. Li2, A. Gupta3 and Gang Xiao2 Advanced Materials Research Institute, University of New Orleans, New Orleans, LA 70148, U.S.A. 1 Department of Physics, University of South Florida, Tampa, FL 33620, U.S.A. 2 Physics Department, Brown University, Providence, Rhode Island 02912 3 IBM T. J. Watson Research Center, Yorktown Heights, New York 10598 ABSTRACT Field-dependent transverse susceptibility measurements based on a novel tunnel-diode oscillator (TDO) method were used to probe the magnetic anisotropy and switching in epitaxially grown CrO2 films on TiO2 substrates. The static field was applied in three different orientations (0°, 45° and 90°) with respect to the magnetic easy axis. Singular peaks in the transverse susceptibility were observed that are associated with the anisotropy and switching fields in CrO2. Theoretical calculations based on a coherent rotation model display remarkable agreement with the experimental data. A new peak emerges at low temperatures and is identified to be a consequence of temperature-dependent magnetoelastic contributions resulting from the in-plane tensile strain in the films due to varying coefficients of thermal expansion in the film and the substrate. INTRODUCTION The ferromagnet chromium dioxide belongs to a technologically important class of magnetic oxides. It is an ideal material for fabricating spin-polarized magnetic tunnel junctions, as it is a half-metallic ferromagnet with almost 100% spin polarization [1]. While structural, transport and magnetic properties of chromium dioxide have been investigated by several groups [2-5], there have been only limited studies on high frequency switching characteristics. It is vital to obtain a good understanding of the dynamic magnetic response in CrO2 for RF electromagnetic fields. In this paper, we present RF transverse susceptibility (χT) measurements on high quality epitaxially grown thin films of CrO2. Singular peaks are observed in χT at the characteristic anisotropy fields that differ by an order of magnitude for static magnetic field (H) applied parallel and perpendicular to the magnetic easy axis. Our experiments also reveal that when the field is applied at a 45° angle to the inplane magnetic easy axis, the peaks are transformed to sharp dips. We observe also evidence for the temperature dependent influence of strain in the films due to varying coefficients of thermal expansion in the film and the substrate. All the observed data, including the peak and dip structures at various field angles, are well described by a generalized theoretical model, based on coherent rotation of the magnetization. EXPERIMENTAL The CrO2 films studied in this work are 5x5 mm2 in size and typically 2000 Å thick grown on single crystal (100) TiO2 substrates by chemical vapor deposition (CVD) at atmospheric pressure [3]. Both, P3.31.1
Figure 1. Hysteresis loops measured with a SQUID magnetometer with the applied field parallel and perpendicular to
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