B. Y. Wang

Hydrogenation induced reversible modulation of perpendicular magnetic coercivity in Pd/Co/Pd films

In perpendicularly magnetized Pd/Co/Pd trilayers, the hydrogenation not only increased Kerr signal but also significantly enhanced the magnetic coercivity (H C ) by 17%. The reversibility was demonstrated by cyclic H2 exposure. The time constants of hydrogen absorption and desorption effect on H C range from tens to hundreds seconds, depending on the H2 gas pressure. The magneto-optical Kerr signal and magnetic coercivity was simultaneously recorded during H2 absorption and desorption. These multifarious signals respond differently and provide a detailed understanding of hydrogenation effect on the functional Pd/Co/Pd trilayers.

Effect of geometry on the magnetic properties of CoFe2O4–PbTiO3 multiferroic composites

In this study, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD) and element- and site-specific magnetic hysteresis (ESMH) are used to elucidate the effect of geometry (0-3- and 2-2-type) on the magnetic properties of CoFe2O4–PbTiO3 (CFO–PTO) multiferroic composites by comparison with those of the reference CFO and PTO powders. Magnetic Co ions in CFO have been confirmed to be located at both the tetrahedral (A)- and octahedral (B)-sites. CFO retains its mixed-spinel structure as verified by the EXAFS, XMCD and ESMH measurements. ESMH measurements further demonstrate that the magnetic moments of Co2+ and Fe3+/Fe2+ cations at both the A- and B-sites in the composites are smaller than those of the CFO powder. The reduction of the magnetic moments in the 2-2-type composite was larger than that in the 0-3-type composite. The reduction of the magnetic moments in the composites was attributable to the formation of anti-phase boundaries owing to the compressive strain in CFO, which is the largest strain in the 2-2-type composite. Based on the Ti L3,2-edge XMCD measurements of the CFO–PTO composites, no induced magnetic moment was observed at the Ti sites in the PTO matrix, excluding the possibility that the Ti ions in the PTO matrix affect the magnetic properties of these CFO–PTO composites.

Uniaxial magnetic anisotropy in Pd/Fe bilayers on Al2O3 (0001) induced by oblique deposition

This study reports the preparation of self-organized 1-dimensional magnetic structures of Fe on Al2O3 (0001) by oblique deposition. The x-ray diffraction (XRD) results in this study show the preferred (110) texture of the Fe films. XRD and extended x-ray adsorption fine structure measurements indicate larger oblique deposition angle (65°) leads to more disorder in the Fe crystalline structure. After capping with a Pd overlayer, the Pd/Fe/Al2O3 (0001) still exhibits uniaxial magnetic anisotropy induced by the underlying 1-dimensional Fe nanostructure. This uniaxial magnetic anisotropy changes with the variation in Fe thickness and oblique deposition angle. These results clearly indicate the feasibility of manipulating uniaxial magnetic anisotropy and crystalline order through the oblique deposition of magnetic materials.

Hydrogen-mediated long-range magnetic ordering in Pd-rich alloy film

The effect of hydrogenation on a 14 nm Co14Pd86/Al2O3(0001) thin film was investigated on the basis of the magnetooptical Kerr effect. After exposure to H2 gas, the squareness of the hysteresis loop showed a large transition from approximately 10% to 100% and the saturation Kerr signal was reduced to nearly 30% of the pristine value. The reversibility of the transition was verified and the response time was within 2–3 s. These observations indicate that the hydride formation transformed the short-range coupled and disordered magnetic state of the Co14Pd86 film to a long-range-ordered ferromagnetic state and induced appreciable decrease in the magnetic moment. The enhanced long-range-ordering and the reduction of the magnetic moment were attributed to the change of electronic structure in Co14Pd86 with hydrogen uptake.

Magnetic anisotropic properties of Pd/Co/Pd trilayer films studied by X-ray absorption spectroscopy and magnetic circular dichroism

We study the magnetic anisotropic properties of as-grown and annealed Pd/Co/Pd trilayer films based on their atomic and electronic structures using extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) spectroscopy and magnetic circular dichroism (XMCD) measurements. The annealed film exhibits interesting perpendicular magnetic anisotropy (PMA) whereas the as-grown film exhibits in-plane anisotropy. Cross-sectional transmission electron microscopic analysis together with the Co K-edge EXAFS results confirm the formation of an ordered-alloy CoPd phase in the annealed film, whereas the as-grown film has an hcp Co-like phase. Co L3,2-edge XMCD measurements reveal an enhanced ratio of the Co 3d orbital to spin moments of the annealed film providing evidence of the observed PMA upon annealing.

Enhanced perpendicular magnetic anisotropy in Fe/Mn bilayers by incorporating ultrathin ferromagnetic underlayer through magnetic proximity effect

The perpendicular magnetic anisotropy(PMA) was shown to be established in ferromagnetic (FM)/fcc-Mn bilayers through the FM-antiferromagnetic (AFM) exchange coupling. We demonstrate here that such PMA can be further enhanced by incorporating an ultrathin Fe film as an underlayer. In a series of Fe/Mn bilayers, hysteresis loop measurement shows that the thickness of top Fe layer with PMA can be extended to a thicker range while an ultrathin Fe underlayer is inserted. Such enhancement of PMA is attributed to an increase of AFM ordering on the Mn film originated from the magnetic proximity effect with the Fe underlayer.

Growth, structure, and magnetism of γ-phase Mn ultrathin films on Cu3Au(100)

Ultrathin γ-phase (face-center cubic) Mn films were prepared by epitaxial growth on Cu3Au(100). Kinematic analysis of low energy electron diffraction I∕V showed a structure transformation of Mn films from nearly face-center cubic to face-center tetragonal with increasing coverage. No ferromagnetic signal in Mn∕Cu3Au(100) was observed. For 21 ML capping film of Fe on Mn films, the hysteresis loop of Fe was biased. The bias field for 21MLFe∕15MLMn∕Cu3Au(100) was ∼200Oe at 110 K with the blocking temperature Tb∼300K. This observation of exchange bias substantiates the theoretical prediction of antiferromagnetism in γ-phase Mn.

Nanopatterning of magnetic domains: Fe coverage of self-assembled alumina nanostructure

Nanosized ultrathin magnetic films were prepared by controlling the deposition of Fe onto an oxidized NiAl(001) surface with an alumina nanostructure on it. Because the ultrathin ferromagnetic Fe films on the bare NiAl(001) surface are separated by paramagnetic Fe nanoparticles on the alumina stripes, as determined by scanning electron microscopy with spin analysis, they form rectangular domains with sizes ranging from tens of nanometer to larger than a micrometer. Magnetic domain patterning can thus be achieved by controlling the Fe coverage and nanostructured template.