K. B. Chetry

Inelastic tunneling spectroscopy of magnetic tunnel junctions based on CoFeB∕MgO∕CoFeB with Mg insertion layer

Magnetic tunnel junctions (MTJs) based on textured MgO barriers have thus far shown the highest tunneling magnetoresistance (TMR) at room temperature. In contrast to traditional magnetic tunnel junctions, it appears that the large TMR observed in these systems arises from a type of coherent tunneling in which the symmetry of the Bloch state wave functions plays a critical role. We have fabricated MTJs with artificial asymmetric barriers by depositing a thin layer of Mg of varying thickness (0–10 A) prior to the growth of the MgO barrier into otherwise identical CoFeB∕MgO∕CoFeB MTJs. The inelastic tunnel spectrum shows magnon and phonon excitation peaks similar to traditional Al2O3 barriers, and an additional peak at about 300 meV. The conventional interpretation that this peak corresponds to density of states of the s electrons in the ferromagnetic electrodes, however, does not apply in the MgO system.

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 ...

A comparison of the growth modes of (100)- and (110)-oriented CrO2 films through the calculation of surface and interface energies

The mechanism leading to different growth modes of (100)- and (110)-oriented CrO2 films on a TiO2 substrate has been investigated by using first-principles calculations based on density functional theory (DFT). The surface energies of (100)- and (110)-oriented CrO2 and TiO2 structures were calculated within a three-dimensional slab model. The convergence of the surface energy was studied with respect to the interslab vacuum distance and the thickness of the slab. A sandwich geometry was used to study the interface energy between CrO2 and TiO2. These results shed light on published experimental results on the growth of epitaxially grown CrO2 on (100)- and (110)-oriented TiO2 substrates.

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.