C. L. Platt

Magneto‐impedance effect in NiFe plated wire

The voltage V(t) induced by a sinusoidal drive current traversing a 125‐μm‐diameter BeCu wire plated with 1 μm NiFe is observed to be a very strong function of axial dc magnetic field. The basic physics of this phenomenon is explained classically in terms of Faraday’s law of induction and Stoner–Wohlfarth magnetization reversal, which yield theoretical predictions that are in good agreement with experiment. For drive current amplitudes of the order of 100 mA, and frequencies of the order of 5 MHz, the field sensitivity dV/dH can be as large as 1 V/Oe (per cm of wire), which offers the potential for application in relatively simple, very high sensitivity magnetic field sensors.

Spin‐dependent tunneling in HfO2 tunnel junctions

We identified reactively sputtered HfO2 as a particularly good material for making thin insulating barriers for spin‐dependent tunnel junctions. This material allows one to form pinhole‐free tunnel barriers with good transmission of the spin polarization of the tunneling electrons. Magnetic tunnel junctions consisting of a thin layer of HfO2 sandwiched between transition metal electrodes (Co and Fe, for instance) exhibit changes of tunnel resistance up to 30% at low temperature as a function of applied field. This effect can be used in a variety of magnetic field sensing applications or in magnetic random access memory.

Spin polarized tunneling in reactively sputtered tunnel junctions

In order to study spin-polarized electron tunneling, we have grown MOM junctions where M is a magnetic transition metal (Co, Fe or CoFe) and O a reactively sputtered oxide (CoO, NiO, Ta2O5, MgO, HfO2). The oxide thickness ranged between 20 and 300 A. The structural, magnetic, and transport properties of these junctions have been investigated. Among these various oxides, large magnetoresistance due to spin-polarized tunneling effects have been observed in MgO on the order of 20% at 77 K, and a maximum of 31% has been observed in HfO2 based junctions at 30 K. The other oxides had a much higher tendency to form pinholes and had smaller dielectric breakdown thresholds.

Correlation of coercivity and microstructure of thin CoFe films

The magnetic and structural properties of sputtered Co50Fe50(CoFe) films were examined. CoFe films 300 A thick deposited on Si substrates at room temperature showed large coercive fields of 140 Oe. When similar thickness films were deposited at 100 °C, the coercivity dropped to 90 Oe, and when they were deposited on CoO, the coercivity was reduced to 12 Oe. Cross-sectional imaging with transmission electron microscopy revealed that the CoO underlayer had induced a columnar grain structure in the CoFe, with grain diameters ranging from 50 to 150 A. CoFe films grown on Si contained larger grains of 200–350 A in diameter with fewer distinct vertical grain boundaries. Lorentz microscopy showed that domain walls in the hard CoFe film formed complex, fixed patterns in fields less than the coercivity, whereas walls in the CoFe/CoO sample were more conformal and mobile in response to changing fields. Possible structural origins for the wide variation in coercivity obtained with different substrates, deposition te...

Sputter deposited Co/CoO composite materials

Magnetic composites consisting of ultrafine particles of Co and CoO were prepared by dc reactive magnetron sputtering in a Ar+O2 gas mixture. The films were composed of 60–140 A particles of hcp Co and fcc CoO. The magnetization, coercivity, and particle size were found to be dependent on the O2 partial pressure, such that the magnetization decreased and the coercivity increased to a maximum value of 1000 Oe with increasing O2 partial pressure. A shift in the hysteresis loop was observed for several samples, indicating exchange coupling between the Co and CoO.

Reversal modes of exchange-spring magnets revealed by torque magnetometry

Torque magnetometry measurements and related micromagnetic simulations are reported for SmCo/Fe exchange-spring films with uniaxial in-plane easy axis of magnetization. Rotating an applied magnetic field in plane, away from the easy direction, gives rise to an irreversible jump in the torque curves at a critical angle. The jump is associated with a switch in chirality of the magnetization spiral structure that develops in the magnetically soft Fe layer, pinned at the interface with the hard SmCo layer. Two reversal modes of the chirality are identified, one at low fields due to an in-plane untwisting of the spiral, and the other at high fields, due to an out-of-plane fanning of the spiral.

NMR studies of sputtered CoFe alloy thin films

The inclusion of a Au layer in the magnetic tunnel junction structure Fe/MgO/CoFe substantially alters the magnetic properties. In this work we use zero field /sup 59/Co NMR to see if these changes in magnetic properties are correlated with changes in the local atomic environment. The technique of spin-echo NMR allows the effective magnetic field (hyperfine field) at the nucleus to be determined and so provides information on the local atomic environments including strain effects and magnetic moments. The results show that the local atomic environment of Co does not change significantly, however the r.f. power required to obtain the maximum signal is substantially different when the Au layer is included showing that the r.f. electronic enhancement factor changes appreciably.

Layer-resolved magnetometry of a magnetic bilayer using the magneto-optical Kerr effect with varying angle of incidence

We determined the hysteresis loop of each layer in a magnetic bilayer of NiFe–SiO2–CoFe using the magneto-optical Kerr effect. Our method utilizes the fact that varying the angle of incidence of the optical beam changes the magnitude and sign of the polarization rotation signal contributed by each layer to the total signal. The magneto-optical signal of each layer varies differently with the angle of incidence, enabling us to determine the hysteresis loop of each. We measured the hysteresis loop of the bilayer for a range of angles of incidence, and from these loops determined the magnetization of the individual layers. We found that the magnetization of the soft NiFe layer coupled strongly to the CoFe layer for SiO2 interlayer thicknesses of <2 nm. The NiFe loop shears with decreasing interlayer thickness, increases in coercivity, and develops a two-stage switching behavior at an interlayer thickness of 1 nm.

Superconducting tunneling as a probe of sputtered oxide barriers

The tunneling properties of sputtered oxide barriers were studied in Pb/oxide/ferromagnet junctions. The initial oxide/ferromagnet bilayer was made without breaking vacuum. The bilayer was exposed to atmosphere before the deposition of a Pb counterelectrode in a separate vacuum chamber. I–V curves and conductance measurements at 1.5 K confirmed the presence of single-step, elastic tunneling in these structures. Separate experiments involving gold bottom electrodes, variation of exposure times in air between bilayer and Pb depositions, and plasma oxidation proved that the oxide layer is permeable, allowing for oxidation of the ferromagnetic base electrode. This revealed that the tunneling channels were due to the contribution of small-area junctions at thermally oxidized “pinhole” sites on the bottom electrode.

Rotational hysteresis of exchange-spring magnets.

We highlight our experimental studies and micromagnetic simulations of the rotational hysteresis in exchange-spring magnets. Magneto-optical imaging and torque magnetometry measurements for Sm–Co/Fe exchange-spring films with uniaxial in-plane anisotropy show that the magnetization rotation created in the magnetically soft Fe layer by a rotating magnetic field is hysteretic. The rotational hysteresis is due to the reversal of the chirality of the spin spiral structure. Micromagnetic simulations reveal two reversal modes of the chirality, one at low fields due to an in-plane untwisting of the spiral, and the other, at high fields, due to an out-of-plane fanning of the spiral.