S. Hope

Suppression of 90° spin switching in Co nanomagnets grown on the Cu(110)-{2×1}O surface

We report on the effect of oxygen on the CO-induced 90° spin switching in the Co/Cu(110) system [Hope et al., Phys. Rev. B 57, 7454 (1998)]. The epitaxial fcc Co films were grown on the unsaturated Cu(110)-{2×1}O surface and their magnetic properties have been studied with in situ magneto-optic Kerr effect. The easy axis switch of the Co(110) films is suppressed when Co is grown on O exposed Cu surfaces. Scanning tunneling microscopy images of the film surface reveals the growth of elongated nanostructures preferentially oriented along the [001] direction, providing step-like edges for gas adsorption. The density of steps is similar to the density developing during growth on the clean Cu(110) surface but the step edges now run in the perpendicular direction. The suppression of the easy axis switch can be attributed to subtle changes of the number and adsorption probability of the available atop adsorption sites along the step edges for CO chemisorption. Our experiments highlight the remarkable chemical se...

Two-dimensional percolation phase transition in ultrathin Co/Cu(110)

Near the critical region for the onset of ferromagnetic order, the magnetic susceptibility χ follows a power law with a critical exponent γ=2.39±0.08, in excellent agreement with the theoretical value for a perfect two-dimensional geometric percolation phase transition. Scanning tunneling microscopy studies reveal that the Co grows via the formation of three-dimensional elongated island structures, forming an array of Co “nanomagnets.” The ordering transition is a consequence of direct exchange developing between Co islands. The in-plane uniaxial anisotropy in the ferromagnetic phase is predominantly attributed to the internal dipolar field of the Co islands.

A direct test of x-ray magnetic circular dichroism sum rules for strained Ni films using polarized neutron reflection

We have directly compared the values of the magnetic moment obtained from x-ray magnetic circular dichroism (XMCD) measurements with those obtained from polarized neutron reflection (PNR) measurements on strained Ni films grown on Cu(001)/Si(001). The PNR measurements show that the absolute magnetic moments differ from that of bulk Ni. We find agreement within experimental errors between the two magnetometry measurements, confirming that the XMCD sum rules are applicable to this strained low-symmetric system.

Magnetic anisotropy engineering in in-plane magnetized ultrathin ferromagnetic films (invited)

We have studied the effect of depositing submonolayer quantities of Cu onto the CO exposed Co/Cu(110) system at room temperature using the magneto-optic Kerr effect. Cu overlayers are found to completely reverse the in-plane 90° easy axis switch caused by the CO adsorption, for all Co thicknesses studied up to 40 ML. The Cu reverses the sign of the effective in-plane uniaxial anisotropy KUeff thereby switching the easy axis from the [1-10] to the [001] direction. Two modes of switching are observed depending on the magnitude of the cubic magnetocrystalline anisotropy constant K1 which is in turn dependent on the thickness of the Co films. For sufficiently thick Co films (dCo>15 ML), the easy axis is found to shift gradually from the [1-10] to the [001] direction due to the competition between the cubic and effective uniaxial anisotropy contributions. Therefore, we are able to controllably engineer the direction of the easy axis in this system as a function of Cu overlayer thickness. For thin Co films (dCu...

Variation of the magnetic moment and strain in epitaxial Cu/Ni/Cu sandwiches

We have studied the variation of the strain and the magnetic moment in epitaxial fct Ni(001) film structures using a combination of reflection high energy electron diffraction (RHEED) and x-ray magnetic circular dichroism. A 30 A Cu/Ni step-wedge (30, 60, 90, and 150 A)/600 A Cu buffer/Si(001) structure has been used to study the Ni thickness-dependent strain and magnetic moment. A decreased magnetic moment with decreasing Ni thickness is observed. A 30 A Cu/50 A Ni/step-wedged (600, 1000, 1500, and 2000 A) Cu buffer/Si(001) was used to vary the degree of strain as determined by in situ RHEED measurements in the same Ni film by varying the Cu buffer layer thickness. Our study shows a possible correlation between the magnetic moment and strain.

Atomic scale evolution of magnetic anisotropies

The effect of non-magnetic overlayers on the magnetic anisotropy behaviour of FCC Co(1 1 0) epitaxial films are discussed. It is shown that uniaxial magnetic anisotropies are the generic signature of the presence of symmetry breaking atomic and nm scale structures, e.g. steps, clusters, islands in ultrathin films. Surprisingly, strong changes in anisotropy behaviour occur with sub-monolayer depositions of non-magnetic overlayers.