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

Abstract

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 that it naturally explains the complex magnetic switching behavior reported recently in 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][7][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...

Anomalous transport in half-metallic ferromagnetic CrO2

“…The only report in the literature on PHE measurements for CrO 2 films showed that at intermediate thickness (100 nm), films can develop biaxial magnetic anisotropy in which two magnetic easy axes occur, one in between the c and the b axes, and one mirrored around the c axis to lie in between the c and the b axes. 16 Moreover, they also predict that their films are in a single magnetic domain structure. We also probed PHE using the L structure of a 100-nm-thick film at 4.2 K in the shielded cryostat with a magnetic field applied in a parallel configuration (H I ) but our film was exhibiting uniaxial magnetic anisotropy, for details see Ref.…”

“…New in our MR data is the strongly nonmonotonous behavior around the crossover temperature of 100 K. We also study the low-field magnetoresistance behavior and come to a similar conclusion as König et al, that intergrain tunneling magnetoresistance (ITMR) takes over from anomalous magnetoresistance (AMR) when the temperature decreases to below 100 K. 15 Data on the planar Hall effect (PHE) confirm that the magnetization does not switch in single-domain fashion in these films, different from one particular case reported by Gönnenwein. 16 The article consists of two parts. First, the measurements of the temperature-dependent resistance R(T ) of the high-field MR and of the Hall effect are presented and discussed.…”

Long-range supercurrents through half-metallic ferromagneticCrO2

“…1b. In the case of TiO 2 , it was due to the peculiar circumstance that strain relaxation in the film can lead to a change in the easy-axis direction, with biaxial behavior occurring around a film thickness of 100 nm [16]. Apart from the experiments we report here, we have grown a number of films on TiO 2 , and we find that growth conditions (including substrate cleaning prior to the growth) crucially determine whether biaxial behavior occurs.…”

Observation of uniaxial anisotropy along the [100] direction in crystalline Fe film

“…In this paper, we report the observation of a new type of uniaxial anisotropy, associated with the in-plane crystallographic direction in the Fe film. To investigate magnetic anisotropy of the Fe film, we make use of the planar Hall effect (PHE), which arises from the tensor character of the anisotropic magnetoresistance 26 27 . Detection of this uniaxial anisotropy is possible due to the fact that in PHE measurements even very small contributions of a uniaxial anisotropy to the reorientation of magnetization between magnetic easy axes lead to conspicuous asymmetric behavior of the hysteresis loops observed by planar Hall resistance (PHR), as discussed below.…”