M. Pathak

Spin pumping in Co56Fe24B20 multilayer systems

Broadband in-plane ferromagnetic resonance measurements were performed over a frequency range from 7 to 40 GHz on various Co56Fe24B20 systems with adjacent thin non-magnetic layers of Ru, Ta and Cu. Co56Fe24B20 samples bounded by either Ru or Ta layers exhibit a contribution to the Gilbert damping constant inversely proportional to the thickness of the Co56Fe24B20 layer, consistent with spin-pumping theory. In contrast, samples with 20 nm thick Cu bounding layers did not show a significant dependence of the Gilbert damping constant on the Co56Fe24B20 thickness, which can be understood based on the far larger spin diffusion length of Cu in comparison with Ru or Ta.

Structural and magnetic properties of (100)- and (110)-oriented epitaxial CrO2 films

We report the successful growth of epitaxial CrO2 (100) and CrO2 (110) films by chemical vapor deposition on TiO2 (100) and TiO2 (110) substrates, respectively. Films on TiO2 (100) follow a layer-by-layer growth mode, with smooth surfaces but significant out-of-plane compressive stress. In contrast, films on TiO2 (110) follow an islandlike growth mode and are found to be essentially strain free for even the thinnest films studied (∼35 nm). The substrate-induced stress for (100) films plays a dominant role in the evolution of the magnetic anisotropy with increasing film thickness, while (110) films show little variation in anisotropy with film thickness. As a result, the in-plane angular dependence of the saturation fields for (110) films can be understood by presuming domain wall nucleation and motion for small angles with respect to the easy axis and by coherent rotation for angles approaching the hard axis.

Substrate-induced strain and its effect in CrO2 thin films

We report a study of substrate-induced strain and its effect in (100) and (110) CrO2 thin films deposited on TiO2 substrates of respective orientations. While the (110) CrO2 films grow essentially strain-free, the (100) CrO2 films were found to be strained in all lattice directions—out of plane direction was compressively strained while in-plane directions were under tensile strain. Crystal lattice parameters were determined in strained (100) and strain-free (110) CrO2 films together with the amount of strain in the three lattice directions. We found substrate-induced strain to significantly affect the magnetic moment in the (100) CrO2 films at room temperature—reducing the magnetic moment with increasing strain in the (100) films while strain-free (110) CrO2 thin films have higher moments for all thicknesses. Qualitative macroscopic conductance behavior in the strained (100) and strain-free (110) CrO2 films were found to be comparable for temperatures in the range of 5–400 K, showing similar behavior at ...

Barrier height and tunneling aspects in (110) CrO2 with its natural barrier

We have investigated (110) CrO2/natural barrier/Co magnetic tunnel junctions for their barrier and magneto-transport properties. A negative tunnel magnetoresistance (TMR) of over 5% was observed in micro-fabricated devices at 4.2 K, which is comparable to TMR values obtained with (100) CrO2. Both transport and cross-sectional transmission electron microscopy analysis reveal a natural barrier thickness 3.5 ± 0.5 nm. However, we obtain a low effective barrier height of 0.4 eV from transport measurements. The inelastic electron tunneling spectroscopy showed significant bias dependence with peak positions showing vibrational modes, which deviate from stoichiometric Cr2O3. We conclude that the transport characteristics are controlled by defects within the natural barrier, consistent with recent theoretical reports.

Comparing magnetotransport and surface magnetic properties of half-metallic CrO2 films grown by low pressure and atmospheric pressure chemical vapor deposition

CrO2 films prepared by low pressure chemical vapor deposition (LPCVD) using Cr(CO)6 precursor have been investigated and compared with epitaxial half metallic CrO2 films prepared at atmospheric pressure (APCVD) using CrO3 precursor for their magnetotransport and surface magnetic properties. LPCVD films showed higher resistivity than APCVD epitaxial (100) CrO2 films prepared on (100) TiO2 substrates. Magnetoresistance of LPCVD films is comparable to that of APCVD films. X-ray magnetic circular dichroism suggests a reduced surface magnetic moment for LPCVD films. This reduced magnetic moment is attributed to antiferromagnetic alignment of the uncompensated Cr spins in the Cr2O3 surface layer.

Anomalous Hall effect behavior in (100) and (110) CrO2 thin films

First and second order magnetic anisotropy constants have been determined in (110) and (100) CrO2 films using magnetometry and anomalous Hall effect measurements. Higher in-plane anisotropy is observed in strain-free (110) CrO2 films as compared to strained (100) CrO2 films, while out-of-plane magnetic anisotropy (OPMA) is stronger in (100) films. Temperature-dependent OPMA is particularly striking for (110) films with a sharp drop below 200 K, whereas for (100) films the anisotropy increases as the temperature decreases. These results are consistent with changes in the magnetization orientation with decreasing temperature, possibly caused by differences in the thermal expansion coefficient between the substrate and film.