A. Reinders

Orientational dependence of the exchange biasing in molecular‐beam‐epitaxy‐grown Ni80Fe20/Fe50Mn50 bilayers (invited)

The exchange biasing field (Heb) and coercive field (Hc) of molecular‐beam‐epitaxy‐grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001], and [110] orientations have been investigated by longitudinal Kerr effect measurements. Ni80Fe20 and Fe50Mn50 were deposited as orthogonal wedge‐shaped layers on single‐crystal Cu substrates in a magnetic field, enabling the study of the thickness dependence of Heb and Hc on a single sample for each orientation. A strong dependence of Heb and Hc on the growth orientation is observed. The results are interpreted in terms of the observed noncollinear spin structure of the antiferromagnet and a comparison is given with the predictions from recent theoretical models.

Orientational and structural dependence of magnetic anisotropy of Cu/Ni/Cu sandwiches: Misfit interface anisotropy

Magnetic anisotropies and misfit strain relaxations have been investigated in Cu/Ni‐wedge/Cu (100) and (111) sandwiches deposited by molecular beam epitaxy on single‐crystal Cu substrates. Our results reveal a clear distinction in the nature of the measured anisotropy at Ni thicknesses below and above the critical value tc, where the growth becomes incoherent. Below tc, coherent lattice strain modifies only the volume anisotropy, while interface anisotropy is Neel type; above tc, magnetoelastic effects are found to contribute to the interface anisotropy.

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.

Perpendicular giant magnetoresistance of Co/Cu multilayers deposited under an angle on grooved substrates

We propose a novel experimental technique for investigating the giant magnetoresistance effect measured with the current perpendicular to the layer plane (the so‐called CPP geometry). Using holographic laser interference nanofabrication techniques and anisotropic etching the surface of semi‐insulating InP substrates is patterned into V‐shaped grooves of 0.2 μm width. Subsequently, a magnetic multilayer can be evaporated under an angle with the substrate normal, naturally resulting in a CPP‐like magnetoresistance configuration. The technique is illustrated for Co/Cu multilayers, for which we present magnetization and magnetoresistance experiments.

Exchange biasing in MBE-grown Ni80Fe20/Fe50Mn50 bilayers

The exchange biasing field (Heb) and coercive field (Hc) of molecular-beam-epitaxy (MBE) grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001] and [110] orientations were investigated by Kerr effect measurements. A strong dependence of Heb and Hc on the growth orientation is observed. A strong uniaxial in-plane anisotropy introduced by the Fe50Mn50 layer was found for the [110]-oriented sample. Conversion electron Mossbauer studies revealed a roughness of 2–3 A and no significant moment reduction at the Ni80Fe20/Fe50Mn50 interface.

Brillouin light scattering investigations of exchange biased (110)-oriented NiFe/FeMn bilayers

All contributing magnetic anisotropies in (110)-oriented exchange biased Ni80Fe20/Fe50Mn50 double layers prepared by molecular beam epitaxy on Cu(110) single crystals have been determined by means of Brillouin light scattering. Upon covering the Ni80Fe20 films by Fe50Mn50, a unidirectional anisotropy contribution appears, which is consistent with the measured exchange bias field. The uniaxial and fourfold in-plane anisotropy contributions are largely modified by an amount, which scales with the Ni80Fe20 thickness, indicating an interface effect. The strong uniaxial anisotropy contribution shows an in-plane switching of the easy axis from [110] to [001] with increasing Ni80Fe20-layer thickness. The large mode width of the spin wave excitations, which exceeds the linewidth of uncovered Ni80Fe20 films by a factor of more than six, indicates large spatial variations of the exchange coupling constant.

Structural and magnetic analysis of MBE-grown Fe/Si/Fe and Fe/Ge/Fe sandwiches

Abstract The structure and the interlayer exchange coupling in MBE-grown nominal Fe/Ge1−xFex/Fe(x=0.0, 0.25, 0.33, 0.5) and Fe/Si/Fe sandwiches with wedge-shaped spacer layers were investigated. In the case of Fe/Si a sample with a wedge-shaped magnetic layer was also studied. From a structural analysis with LEED and from a magnetic analysis with the magneto-optical Kerr effect, it is concluded that the Si spacer transforms into an SiFe alloy with the deposition of Fe. A strong antiferromagnetic coupling is found in the case of Fe/Si, but no antiferromagnetic coupling is found for Fe/Ge1−xFex. It appears that the coupling behavior in Fe/Si(SiFe)/Fe sandwiches as a function of spacer thickness is monotonous rather than oscillatory.

Perpendicular giant magnetoresistance using microlithography and substrate patterning techniques

Abstract We present experimental results on the giant magnetoresistance effect in magnetic multilayers measured with the current perpendicular to the multilayer plane. Two different experimental techniques are used, which both allow the study of the perpendicular magnetoresistance from 4 to 300 K. The first technique is based on the fabrication of pillar-like microstructures of Fe/Cr and Co/Cu multilayers using microlithography and reactive ion etching. In the second technique we use holographic laser interference nanofabrication and wet anisotropic etching to pattern V-shaped grooves of 0.2 μm width into semi-insulating InP substrates. Subsequently, a Co/Cu multilayer is evaporated at an angle with the substrate normal, naturally resulting in a perpendicular magnetoresistance configuration. Both experimental techniques demonstrate that the perpendicular magnetoresistance is a factor 2 to 4 larger than the conventional current-in-plane magnetoresistance.

In-plane anomalies of the exchange bias field in Ni80Fe20/Fe50Mn50 bilayers on Cu(110)

We report on the exchange bias effect as a function of the in-plane direction of the applied field in twofold symmetric, epitaxial Ni80Fe20/Fe50Mn50 bilayers grown on Cu(110) single-crystal substrates. An enhancement of the exchange bias field, Heb, up to a factor of 2 is observed if the external field is nearly, but not fully aligned perpendicular to the symmetry direction of the exchange bias field. From the measurement of the exchange bias field as a function of the in-plane angle of the applied field, the unidirectional, uniaxial and fourfold anisotropy contributions are determined with high precision. The symmetry direction of the unidirectional anisotropy switches with increasing NiFe thickness from [110] to [001].

Perpendicular giant magnetoresistance in magnetic multilayers deposited on grooved substrates

Abstract We have deposited Co/Cu magnetic multilayers at an angle onto grooved InP substrates with 0.2 μm groove period and measured the giant magnetoresistance with the current perpendicular to the layer plane. We have determined as a function of temperature the spin-dependent scattering parameters due to magnetic bulk and interface scattering, using the two channel model for electron transport. We find that the decrease of the magnetoresistance from 4.2 K to room temperature is mainly due to an increase of the Co and Cu bulk resistivities, while the temperature dependence of the interface resistance and of the spin-asymmetry scattering parameters is very small.