F. B. Mancoff

Control of magnetization dynamics in Ni/sub 81/Fe/sub 19/ thin films through the use of rare-earth dopants

We show that the magnetization dynamics of soft ferromagnetic thin films can be tuned using rare-earth (RE) dopants. Low concentrations (2 to 10%) of Tb in 50 nm Ni/sub 81/Fe/sub 19/ films are found to increase the Gilbert magnetic damping parameter /spl alpha/ over two orders of magnitude without great effect on easy axis coercivity or saturation magnetization. Comparison with Gd dopants indicates that the orbital character of the Tb moment is important for transferring magnetic energy to the lattice. Structural transformations from the crystalline to the amorphous state, observed over the first 2%-10% of RE doping, may play a contributing but not sufficient role in damping in these films. The approach demonstrated here shows promise for adjusting the dynamical response, from underdamped to critically damped, in thin film materials for magnetic devices.

A giant magnetoresistance sensor for high magnetic field measurements

We used giant magnetoresistance in a magnetic field sensor able to measure large fields of up to several kG. We deposit spin valves with a magnetic multilayer with perpendicular anisotropy as one ferromagnet and a material with in-plane anisotropy as the other ferromagnet. For magnetic fields along the film normal, the multilayer’s magnetization is fixed perpendicular while the magnetic layer with in-plane anisotropy is rotated towards out-of-plane magnetization. The device response is nearly linear with applied field and determines both the magnitude and sign of the field, making it attractive for measuring large magnetic fields.

Spin-Dependent Tunneling Junctions with Hard Magnetic Layer Pinning

The successful fabrication of high quality A1203 tunnel barriers has proven to be a crucial step in obtaining high junction magnetoresistance (JMR) in magnetic tunnel junctions. Values up to 2 7 8 at 77 K are routinely obtained, yet the effect is generally reduced at higher temperature (typically to below 20% at room temperature), or upon application of bias voltages of a few tenths of volts. Though this may partially be ascribed to the magnetic properties of the device, spin scattering due to imperfections in the oxide barrier cannot a priori be ruled out as a responsible mechanism. In order to gain insight into this phenomenon, we have studied Co/A1203/NiFe junctions containing submonolayer amounts of foreign elements (Cu, CO, Pd, Ag, Au) incorporated in the insulating oxide. We find a monotonic decay of the JMR with increasing impurity content, that depends sensitively on the type of impurity. In addition, the voltage and temperature dependence of both JMR and junction resistance are altered. The results will be correlated and interpreted with help of a simple model. Implications for spin-polarized tunneling in planar junctions, granular alloys and related systems will also be discussed.

SPIN-DEPENDENT TUNNELING JUNCTIONS WITH HARD MAGNETIC LAYER PINNING

We have fabricated ferromagnet-insulator-ferromagnet tunnel junctions with Co and NiFe electrodes, where the Co electrodes are pinned with a hard magnetic Co81Pt19 alloy layer. This approach gives a coercivity of about 300 Oe for the Co layer, while that of the NiFe is about 80 Oe, so we obtain antiparallel magnetization over a wide field range. The Al2O3 tunneling barrier layers were formed by in situ plasma oxidation of elemental Al layers with thicknesses from 10 to 25 A. For the junctions, we find room temperature magnetoresistance ratios as high as 13% and nonlinear current–voltage curves that are well fit by the Simmons tunneling theory. Depth profiling x-ray photoelectron spectroscopy of oxidized Al barrier layers on Co underlayers reveals a stoichiometry of nearly Al2O3.

Study of DC plasma oxidized Al/sub 2/O/sub 3/ barriers in spin dependent tunneling junctions using high resolution transmission electron microscopy

We used cross sectional high resolution transmission electron microscopy (HRTEM) to observe DC plasma oxidized Al/sub 2/O/sub 3/ barriers directly. We measured average, minimum, and maximum thicknesses for a variety of barriers. We studied the effects of plasma oxidation time, precursor Al thickness, and oxygen plasma conditions on barrier thickness and thickness spread. The barrier thickness spread is important in the transport behavior of these junctions because of the exponential dependence of tunneling current on barrier thickness. The thickness spread initially increases with time and then appears to remain constant. The relative spread initially remains constant with increasing thickness. The precursor Al thickness does not affect the barrier thickness distribution significantly, although there may be some oxidation of the bottom electrode for thin (/spl ap/10 /spl Aring/) precursors. As expected, the barrier oxidation rate and final barrier thickness can be reduced significantly (/spl ap/30%) by changing the plasma parameters.

Structural and Magnetic Properties of XMnSb/PtMnSb Clb Heusler Alloy Superlattices (X=Ni,Cu)

We have grown superlattices based on the Cl b Heusler alloys PtMnSb, CuMnSb and NiMnSb between 200–500°C on A1 2 O 3 (0001). X-ray diffraction (XRD) indicates (111) oriented ordered structures for growth at 300°C. Higher deposition temperature leads to interdiffusion, loss of the multilayer structure and appearance of extra phases. Growth at 200°C slightly reduces the intermixing but also reduces the quality of the crystal structure. For PtMnSb/CuMnSb, we found an enhancement of the saturation magnetization compared to equivalent PtMnSb single layer films and a CuMnSb spacer thickness dependence of the squareness of the M(H) 100ps suggestive of interlayer coupling. Short periodicity NiMnSb/PtMnSb superlattices show an in-plane magnetic easy axis, but correction for shape anisotropy indicates a tendency for perpendicular anisotropy.

Epitaxial Growth of Atomically Flat Spin Dependent Tunneling Junctions

Spin dependent tunneling junctions with epitaxially grown underlayers have been investigated to examine the possibility of achieving very flat and uniform barrier layers. Pt/Ni 80 Fe 20 /Fe 50 Mn 50 /Ni 80 Fe 20 layers were deposited on sapphire (0001) substrates at different temperatures and monitored by in-situ reflection high energy electron diffraction (RHEED). The surface morphology has been found to depend strongly on the growth temperature. X-ray diffraction and magnetic hysteresis loop measurements were also performed to characterize the film structures