Anne Reilly

MAGNETORESISTANCE OF NIMNSB-BASED MULTILAYERS AND SPIN VALVES

We report magnetic and magnetoresistance (MR) measurements in the current-in-plane (CIP) and current-perpendicular-to-the-plane (CPP) geometries of multilayers and spin-valve structures based on the predicted half-metallic (100% spin-polarized) ferromagnetic Heusler alloy NiMnSb. Multilayers of the form [NiMnSb/Cu]×10, as well as a trilayer spin valve of the form [NiMnSb/Cu/Y] (Y=Ni80Fe20 and NiMnSb/FeMn) have been grown using a low-rate, low-pressure sputtering deposition technique. The multilayers do not show evidence of antiferromagnetic coupling between the NiMnSb layers, and the measured CPP magnetoresistance is 4.5% at 4.2 K. NiMnSb/Cu/NiFe spin valves and NiMnSb/Cu/NiMnSb/FeMn quasispin-valves adopt more closely antiparallel alignments of adjacent magnetic layer magnetizations, and CPP MR ratios as high as 7.2% are measured at 4.2 K. Although these CPP MRs are larger than the CIP MRs for similar samples, the largest CPP MR observed is still smaller than expected for a 100% spin-polarized material.

Effect of deposition parameters on the CPP-GMR of NiMnSb-based spin-valve structures

Abstract We present measurements of current perpendicular magnetoresistance of NiMnSb/Cu/NiMnSb spin-valve structures based upon the predicted half-metallic (100% spin polarized) ferromagnetic alloy NiMnSb. The observed effect of 5–10% is much smaller than the complete spin-valve effect expected from a 100% spin-polarized system. The dependence of the magnetoresistance on film thickness and deposition parameters are explored to understand the factors limiting giant magnetoresistance in these structures.

Giant magnetoresistance of current-perpendicular exchange-biased spin-valves of Co/Cu

We present the first measurements of the specific resistance change, A/spl Delta/R, with increasing Co layer thickness, t/sub Co/, of current-perpendicular (CPP) exchange biased spin-valves (EBSVs) of Co and Cu. A/spl Delta/R grows about as predicted for a long spin diffusion length in Co until about t/sub Co/=20 nm, after which it grows more slowly than predicted. Possible reasons for this deviation from prediction are considered, including a short spin diffusion length in Co.

Co layer thickness dependence of exchange biasing for IrMn/Co and FeMn/Co

We present a high resolution study of the ferromagnetic layer thickness dependence of exchange bias field (HEB) and coercivity (HC) in IrMn/Co and FeMn/Co bilayers using the magneto-optical Kerr effect. Samples are sputtered wedges on silicon with Co thicknesses ranging from 1 to 17 nm. The IrMn/Co (with exchange bias interface energy of ∼0.14 erg/cm2) shows square loops, a smooth increase in HEB with inverse thickness, and a complicated behavior for coercivity, perhaps due to competition with thickness dependent coercive mechanisms. The FeMn/Co (with exchange bias interface energy of ∼0.059 erg/cm2) shows more rounded loops, a plateau of HEB with decreasing thickness, and a smooth increase in coercivity with inverse thickness.

Pulsed laser deposition with a high average power free electron laser: Benefits of subpicosecond pulses with high repetition rate

We have conducted experiments exploring pulsed laser deposition of thin films using the high average power Thomas Jefferson National Accelerator Facility Free Electron Laser. The combination of parameters of this laser, including subpicosecond pulses, high average power, high repetition rate, and tunability, makes it a unique tool for the study of the effects of laser characteristics on thin-film quality. When compared to ablation and deposition with an ultrafast, high energy per pulse, low repetition rate laser (amplified Ti:sapphire), we find that the lower energy per pulse with high repetition rate of the free electron laser leads to very different plasma emission and produces films with high quality with the potential of very high deposition rates. This is demonstrated in the optical spectroscopy of plasma emission from Ti and the growth of Ni80Fe20 thin films.

Enhancing current-perpendicular magnetoresistance in Permalloy-based exchange-biased spin valves by increasing spin-memory loss

Inserting a thin (t*=0.5 or 1 nm) layer of the antiferromagnet FeMn into the “free” Permalloy (Py) layer of a sputtered, current-perpendicular exchange-biased spin valve, Nb/FeMn/Py (pinned)/Cu/Py (free)/Nb, is shown to enhance AΔR, the difference in specific resistance between the states where the magnetizations of the two Py layers are parallel and antiparallel to each other. Such an increase is taken as evidence that spin-memory loss (spin relaxation) due to the FeMn is strong, and that judicious insertion of a source of spin relaxation into a multilayer with high specific resistance contacts can enhance AΔR, the numerator of the magnetoresistance.

Ultrafast laser excitation of spin waves and the permanent modification of the exchange bias interaction in IrMn∕Co

Single mode coherent magnetization oscillations are observed in exchange biased IrMn∕Co thin films by the pump-probe time-resolved magneto-optical Kerr effect, using moderate intensity pump pulses. Unlike past experiments on exchange biased systems, the oscillations produced can be excited when the Co magnetization is saturated along the pinning axis. The oscillation frequencies correspond to ferromagnetic resonance measurements and can be described using a FMR equation. It is also demonstrated that ultrafast laser pulses can induce pinning along the direction of the applied magnetic field.

Temperature, wavelength, and polarization dependent time resolved optical spectroscopy of half-metallic CrO2

CrO2 is an important transition metal oxide due to its half-metallic behavior. We have performed ultrafast laser pump-probe differential transmission experiments on epitaxial CrO2(110). Polarizations parallel and perpendicular to the c axis were used, at wavelengths corresponding to important magnetic excitations: 1300 (1eV), 800 (1.5eV), and 650nm(2eV). Anisotropy is observed in the polarization-dependent measurements and is attributed to the electronic orbital anisotropy. A critical change of transient transmission is also observed at the Curie temperature of 386K. This behavior reveals the close relationship between the optical absorption and the spin dynamics of this material.