Planar Hall effect and magnetic anisotropy in epitaxially strained chromium dioxide thin films

S. T. B. Goennenwein, R. S. Keizer, S. W. Schink, I. van Dijk, T. M. Klapwijk, G. X. Miao, G. Xiao, A. Gupta
2007 Applied Physics Letters  
We have measured the in-plane anisotropic magnetoresistance of 100 nm thick CrO 2 thin films at liquid He temperatures. In low magnetic fields H, both the longitudinal and the transverse ͑planar Hall͒ resistance show abrupt switches, which characteristically depend on the orientation of H. All the experimental findings consistently demonstrate that the magnetic anisotropy in these CrO 2 thin films is biaxial. We show that the biaxial magnetic anisotropy is due to epitaxial coherency strain, and
more » ... herency strain, and that it naturally explains the complex magnetic switching behavior reported recently in CrO 2 films with thicknesses of 50 nmഛ d ഛ 250 nm. Half-metallic ferromagnets ͑HMFs͒ such as chromium dioxide ͑CrO 2 ͒ are intriguing materials. 1 Their density of states is finite at the Fermi energy E F for one spin direction, while an insulating gap exists for the other. Accordingly, HMFs are 100% spin polarized at E F . This makes them very attractive for the study of spin-related transport phenomena, e.g., in magnetic tunnel junctions, 2 for the investigation of the spin-dependent decay of superconducting correlations, 3,4 or for current-induced magnetization reversal. 5 Bulk CrO 2 is usually considered as a uniaxial ferromagnet with sizeable second order uniaxial contributions, the c axis being the easy direction. 6-8 In contrast, the magnetic properties of CrO 2 thin films are more diverse. In CrO 2 layers grown epitaxially on ͑100͒ TiO 2 substrates, the strain resulting from the lattice mismatch makes the b axis magnetically easy for layer thicknesses d Ͻ 50 nm. 9,10 For films with 50 nmഛ d ഛ 250 nm, a more complex magnetic behavior was observed and attributed to the superposition of different easy directions because of an inhomogeneous strain distribution across the film thickness. 10 In this letter, we present anisotropic magnetoresistance ͑AMR͒ experiments, which show that the complex magnetic behavior in 100 nm thick CrO 2 films can be understood in terms of biaxial magnetic symmetry. We observe abrupt switches in the AMR, which sensitively depend on the angle enclosed between the current direction and the external, in-plane magnetic field. From the fields at which these switches take place, we conclude that the magnetic anisotropy in the film plane is biaxial. Upon taking into account the magnetic anisotropy contributions due to epitaxial coherency strain, the experimentally observed switching fields can be modeled quantitatively. This shows that crystalline strain qualitatively alters the magnetic anisotropy of CrO 2 thin films. The 100 nm thick, single crystalline CrO 2 films are grown by chemical vapor deposition on ͑100͒-oriented TiO 2 substrates. 9,10 The films are patterned into 50ϫ 200 m 2 Hall bar structures using optical lithography and wet chemical etching. For the magnetotransport experiments, the samples are mounted in a superconducting magnet system and the sample temperature is stabilized using a variable temperature insert. The magnetic field H is applied in the film plane. The angle between H and the current density j can be freely adjusted by means of a rotateable sample stage. We simultaneously record both the resistance parallel to the current direction, long , and the resistance perpendicular to j, trans . Figure 1 shows the magnetoresistance of a CrO 2 Hall bar at 5 K, for an angle = 62°between H and the a͒ Electronic
doi:10.1063/1.2715442 fatcat:sen5stgwiffj5eyyrg65t2j2ze