P.J.H. Bloemen

Magnetic anisotropy of multilayers

Abstract The effect of growth conditions on the magnetic anisotropy of Co/Pd multilayers was studied. Epitaxial [111] multilayers grown at elevated substrate temperature show perpendicular magnetization up to 16 A Co layer thickness, due to a large interface anisotropy K s . The results indicate that K s is mainly of the Neel-type. Also Co/X multilayers show perpendicular anisotropy with X = Pt, Au and Ir, but not for X = Cu, Ag and Mo. The possible contribution of lattice misfit strain to interface anisotropy is discussed. A test for the existence of this type of anisotropy in Ni/Pd and NiFe/Pd multilayers indicates that it may not be as strong as predicted by a recent theory.

An STM study of Fe3O4 (100) grown by Molecular Beam Epitaxy

Abstract STM imaging of MBE-grown pseudomorphic (100) Fe 3 O 4 surfaces reveals terrace widths that are typically a few hundred angstroms broad, and can be as broad as 1000 A. These terraces are separated by steps that are 1 4 of the spinel lattice constant high, corresponding to the distance (2.1 A) between planes of oxygen (or equivalent iron) atoms. The images show that the p(1 × 1) surface reconstruction is caused by a clustering of atoms in the unit cell. These clusters are aligned along a [110] direction, and change direction on alternate terraces. The reconstruction is driven by the tetrahedral iron atoms, which have dangling bonds that rotate by 90° from one atomic plane to the next. Some regions of the surface also show a high-symmetry close-packed structure with 3 A spacing between atoms. The presence of stacking faults is revealed by the orientation of the unit cells. In one image, the two possible orientations of the unit cells are present on the same terrace, separated by a disordered band, which must contain a stacking fault. In another case, the unit cells are oriented in the same direction on two terraces separated by a 2.1 A step. Again a disordered region appears at the boundary between the two terraces. Single-domain regions are as large as a few hundred angstroms wide, which indicates that the surface diffusion length of the iron atoms during the initiation of growth on the higher symmetry MgO substrate is of this same order.

Magnetic anisotropies in Co/Ni(111) multilayers

The magnetic anisotropy and the saturation magnetization have been investigated in several series of Co/Ni multilayers, with relatively thick Co and Ni layers, grown in ultrahigh vacuum (UHV) and high vacuum (HV) by vapor deposition. The volume and interface contributions to the magnetic anisotropy were determined from magnetization measurements. The Co/Ni interface contribution was found to favor perpendicular anisotropy. A series of 2 A Co/4 A Ni multilayers, grown in HV, exhibited an easy axis perpendicular to the layer planes, as evidenced both by polar Kerr rotation and ferromagnetic resonance (FMR) measurements. The first‐ and second‐order anisotropy constants were determined from FMR. Indications were found that the presence of FMR signals in (perpendicular easy) samples with low magnetic sublayer thicknesses depends on the magnitude of the surface anisotropy and on the amplitude of fluctuations in the magnetic layer thicknesses.

Short period oscillation of the interlayer exchange coupling in the ferromagnetic regime in Co/Cu/Co(100)

A unique sample was prepared on a Cu(100) single crystal, consisting of three Co layers separated by two Cu layers in the form of wedges oriented perpendicular to each other: Cu(100)/80 A Co/Cu wedge A/30 A Co/Cu wedge B/30 A Co/7 A Cu/30 A Au. Position‐sensitive magneto‐optical Kerr effect measurements along Cu wedge B, at a fixed position on Cu wedge A corresponding to maximum antiferromagnetic (AF) coupling, enabled us to investigate not only the AF but also the ferromagnetic (F) coupling between the two 30 A Co layers as a function of the Cu thickness. The measurements confirmed both long and short period coupling oscillations in the AF regime, and revealed the predicted extension of the short period through the F regime.

Polarized neutron reflectometry studies of magnetic oxidic Fe3O4/NiO and Fe3O4/CoO multilayers

The magnetic properties of [1 0 0] oriented Fe3O4/NiO and Fe3O4/CoO multilayers, MBE-grown on MgO(0 0 1) substrates, have been studied by polarized neutron reflectometry. In both samples, the Fe3O4 layer exhibits a depth-dependent magnetic profile characterized by a reduction in the magnetization near the interfaces. The possible origins of this behaviour, such as domain wall formation in the ferrimagnetic layer and deviations in stoichiometry, are discussed.

Antiferromagnetic coupling between Co/Ni multilayers with perpendicular magnetization across MBE-grown Cu(111) layers

A cleat oscillatory exchange coupling has been observed for an epitaxial (111) M1/Cu/M2 structure, grown by Molecular Beam Epitaxy (MBE) on a single crystal Cu(111) substrate. The magnetic layers M1 and M2 were designed to have perpendicular easy axis by using Co/Ni multilayers. The Cu layer was deposited in the form of a wedge. The magneto-optically measured hysteresis loops show, in the case of antiferromagnetic (AF) coupling, clear first-order spin-flip transitions between the saturated and antiparallel states. Signatures of a ferromagnetic or biquadratic component, observed earlier in loops of an AF coupled Co/Cu/Co(111) sandwich, were absent in the present loops. The absence of these signatures is believed to be a consequence of the relatively strong (perpendicular) uniaxial anisotropy of the present samples.

Interlayer coupling in Co/Os multilayers

Abstract Antiferromagnetic interlayer coupling is determined from hysteresis loop measurements for Co/Os multilayers with Os thickness varying from 4 to 36A. We found two maxima of antiferromagnetic coupling, which is consistent with an oscillatory behavior with a period of approximately 15A: this is significantly larger than the period observed for interlayers of Ru, which has a very similar band structure and a topologically equivalent Fermi surface.

Interlayer coupling in Co/Pt/Co trilayers investigated by ferromagnetic resonance

Co/Pt/Co trilayers with variable Pt interlayer thicknesses between 4 to 100 A were investigated by ferromagnetic resonance. The lowering of the resonance fields with decreasing Pt thickness showed that the Co layers are coupled ferromagnetically for all Pt thicknesses investigated. No oscillations in the coupling were observed.

Oscillatory interlayer coupling in Co/Au(111)/Co

Abstract The antiferromagnetic and ferromagnetic interlayer coupling across Au(111) has been investigated employing a 2 A Co/Au/7 A Co trilayer with the Au spacer layer in the form of a wedge. The sample was deposited by Molecular Beam Epitaxy on a Cu single crystal substrate covered by a Au base layer. A highly localized magneto-optical Kerr effect hysteresis loop study revealed an oscillatory coupling extending to Au thicknesses of 40 A. The period was found to increase markedly with increasing Au thickness, whereas a more than quadratic decrease of the coupling strength with increasing Au thickness was observed.

Structural dependence of the magnetic anisotropy of Co films

In order to study the relation between structure and magnetic anisotropy, the anisotropy of thin Co films with different structural compositions was measured. The results agree with predictions based on a compositional averaging of the bulk anisotropies. The structure of the Co films was manipulated using different growth temperatures.