P. LeClair

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.

Magnetic and structural properties of MnBi multilayered thin films

Magnetic and structural properties of MnBi films with thicknesses up to 50 nm were investigated. Thin films of the MnBi LTP (Low Temperature Phase) were fabricated onto silica-glass substrates by sputter-deposition of Bi/Mn multilayer, followed by a subsequent annealing at about 550 °C for 30 min. Coercivity of such thin films is higher than 15 kOe, even though the film thickness is about 10 nm. These thin films show the preferential growth of c-axis of the LTP along the film normal. Moreover, high resolution transmission electron microscopy indicates that the LTP regions of 30–50 nm in size are physically isolated by Bi. The magnetization reversal mechanism of such a LTP region is mainly governed by a coherent rotation mode based on the δM curve measurement.

Avalanche breakdown in microscale VO2 structures

The mechanism of triggering the metal-insulator transition (MIT) by an external electric field in small scale VO2 structures has been attributed to excess carrier injection, rather than Joule heating. This is in part due to the fact that the delay time for MIT to be induced by Joule heating seems much longer than what is observed. However, modeling the resistivity as a function of temperature, explicitly considering phase coexistence of metallic and insulating states near the MIT, and considering thermal dissipation in realistic structures, we demonstrate that Joule heating can exhibit a self-accelerating, avalanche-like behavior, in which the time scale for thermally driven breakdown can be in the nanosecond regime if the device is small enough. This model matches experimental results of our micrometer scale device quite well. Over-threshold voltages, temperature, and size effects on switching delay time and threshold voltage are discussed.

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 ...

Tailoring exchange bias through chemical order in epitaxial FePt3 films

Intentional introduction of chemical disorder into mono-stoichiometric epitaxial FePt3 films allows to create a ferro-/antiferromagnetic two-phase system, which shows a pronounced and controllable exchange bias effect. In contrast to conventional exchange bias systems, granular magnetic interfaces are created within the same crystallographic structure by local variation of chemical order. The amount of the exchange bias can be controlled by the relative amount and size of ferromagnetic and antiferromagnetic volume fractions and the interface between them. The tailoring of the magnetic composition alone, without affecting the chemical and structural compositions, opens the way to study granular magnetic exchange bias concepts separated from structural artifacts.

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.

Thermal stability of synthetic antiferromagnet and hard magnet coupled spin valves

The magnetic properties of current-in-plane (CIP) giant magnetoresistive (GMR) spin valves employing synthetic antiferromagnet (SAF) pinning have been investigated. The conventional spin valve structure, with a ferromagnetic (FM) layer pinned by an antiferromagnet (AFM) layer, exhibits high electrical resistance, the AFM typically being a high resistivity material. We have investigated pinning with a Co∕Ru∕Co SAF trilayer only, with no additional AFM pinning. We have also investigated spin valves employing a hard magnet layer in three different configurations as the pinning/pinned layer. Elimination of the AFM-induced parasitic resistance has the potential for yielding a higher GMR ratio in current-perpendicular-to-the-plane (CPP) structures. The full-film properties have been optimized by using vibrating sample magnetometry and CIP magnetotransport measurements. The thermal stability of SAF-pinned spin valves and hard magnet-pinned spin valves has been characterized through magnetotransport measurements ...

Investigation of the tunnel magnetoresistance in junctions with a strontium stannate barrier

We experimentally investigate the structural, magnetic, and electrical transport properties of La0.67 Sr0.33MnO3 based magnetic tunnel junctions with a SrSnO3 barrier. Our results show that despite the high density of defects in the strontium stannate barrier, due to the large lattice mismatch, the observed tunnel magnetoresistance (TMR) is comparable to tunnel junctions with a better lattice matched SrTiO3 barrier, reaching values of up to 350% at T=5 K. Further analysis of the current-voltage characteristics of the junction and the bias voltage dependence of the observed tunnel magnetoresistance show a decrease of the TMR with increasing bias voltage. In addition, the observed TMR vanishes for T>200 K. Our results suggest that by employing a better lattice matched ferromagnetic electrode, and thus reducing the structural defects in the strontium stannate barrier, even larger TMR ratios might be possible in the future.