J. Leib

Magnetization reversal in CoFeHfO films

Magnetization reversals of a thick CoFeHfO film (800 mn), and a thin CoFeHfO (10 nm) film overcoated with CrSi (10 nm) have been studied by magnetic force microscopy (MFM) with in situ applied field capability. The CoFeHfO layers were deposited by RF sputtering and annealed in a magnetic field to induce an in-plane uniaxial anisotropy. For easy-axis applied fields both samples showed significant switching of MFM image contrast for fields approximately equal to the coercivity. This is consistent with the high easy-axis coercive squareness of the films. For hard-axis applied fields the thin CoFeHfO film displayed a distinctive reversal process: a ripple-like domain pattern formed and rotated gradually as the reversed field was increased. The image contrast increased, reached a maximum when the ripple-like features aligned perpendicular to the field and then diminished as the sample approached saturation. The observed magnetization reversal processes appear to be consistent with the formation of magnetization ripples which rotate toward the hard-axis applied field.

Magnetic force microscopy study of magnetization reversal in sputtered FeSiAl(N) films

The magnetization reversal in a series of rf-sputtered FeSiAl(N) films has been studied using magnetic force microscopy. A system has been developed which has the capability to image domain structure while an in-plane magnetic field is applied in situ. All films exhibited a stripe domain structure in zero applied field which was indicative of a perpendicular component of domain magnetization which alternates in sign. All films showed a similar sequence of magnetization processes: on reducing the applied field from saturation a fine stripe domain structure nucleated and then coarsened as the field was decreased to zero. Local switching of domain contrast was observed along the steepest part of the hysteresis loop as the perpendicular component reversed. As the reverse field was increased toward saturation, the stripe domains disintegrated into smaller regions. This observation is consistent with an interpretation that the domain magnetization rotated locally into the sample plane. The saturation field and the film stress exhibited similar trends with nitrogen partial pressure. The results suggest that the perpendicular anisotropy that caused the formation of the stripe domain structure could be induced by the film stress via magnetoelastic coupling.

Thermal expansion studies on the unusual first order transition of Gd5Si2.09Ge1.91: effects of purity of Gd

Two polycrystalline samples were made by using high purity Gd and commercial Gd, respectively, but with Si and Ge starting materials of the same purity in both cases. Thermal expansion results showed that both samples exhibited a first order phase transformation, with a discontinuity in thermally-induced strain and with hysteresis in the Curie temperature. Magnetic force microscopy has been used to demonstrate the magnetic phase transformation process from paramagnetic to ferromagnetic upon cooling. It was found that the Curie temperature was lower and the thermally-induced strain higher, in the sample made from lower purity level Gd starting materials compared with the sample made from high purity Gd metal. These results indicate that the impurities (mainly C, O, N, and F) in the Gd starting material can significantly alter the strain and Curie temperature of Gd5(SixGe1−x)4 alloys.