van de Rjm René Veerdonk

On the construction of an Fe3O4 based all-oxide spin-valve

Abstract The progress made in constructing an all-oxide spin valve based on the intrinsic, fully spin-polarized electron transport in Fe 3 O 4 is discussed. Two possible oxidic spacer layers, MgO and Mn 3 O 4 , have been investigated. Interlayer coupling studies indicate that MgO is the more suitable spacer layer of the two. Thus far a limited magnetoresistive effect ⩽0.4% is found in the all oxide, Fe 3 O 4 -based, spin-valves which we have made. Possible causes for this low magnetoresistive effect are discussed.

Current distribution effects in magnetoresistive tunnel junctions

The influence of an inhomogeneous current density on the (magneto)resistance of a ferromagnet–insulator–ferromagnet tunnel junction in the cross-strip geometry is analyzed using a finite element approach. The four-probe resistance is smaller than the actual resistance for electrode resistances (in the junction area) comparable to or higher than the junction resistance. Even negative four-probe resistances can be obtained. The apparent resistance change due to the junction magnetoresistive effect also decreases, but always remains positive. This results in unrealistically large apparent magnetoresistance ratios which can even approach infinity, which explains some recent experiments.

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.

1/f noise in anisotropic and giant magnetoresistive elements

Microfabricated magnetoresistive elements based on either the anisotropic or the giant magnetoresistance effect were tested for their frequency dependent resistance noise behavior at room temperature in a dc magnetic field, using a dc sense current. Thermal resistance noise was the dominant noise source above about 10 kHz. At low frequencies the resistance noise was found to be dominated by a 1/f contribution that depends on the applied magnetic field. The 1/f noise is relatively low and field independent when the element is in a saturated state and contains a relatively large and field dependent excess contribution when the magnetic field is in the sensitive field range of the element. The 1/f noise level observed in saturation is comparable to the 1/f noise level found in nonmagnetic metals; the excess noise has a magnetic origin. The variation of the excess noise level with the applied dc magnetic field can be explained qualitatively using a simple model based on thermal excitations of the magnetization direction.

A study of the magneto-optical Kerr spectra of bulk and ultrathin Fe3O4

An analysis of the complete dielectric tensor of bulk magnetite, Fe3O4, and of Fe3O4 substituted with a variable degree of Mg2+ or Al3+ allowed the assignment of the main magneto‐optical (MO) transitions in Fe3O4 between 0.5 and 4.0 eV. This assignment is consistent with our reinterpretation of the MgFe2O4 and Li0.5Fe2.5O4 MO‐Kerr spectra. An analysis of the stoichiometry of a thin Fe3O4 layer is made, based on the Kerr spectrum, which is compared to an analysis based on the Verwey transition. The Kerr spectrum is also used to compare wavelengths for MO‐Kerr effect studies of wedge‐shaped samples.

Asymmetric bias voltage dependence of the magnetoresistance of Co/Al2O3/Co magnetic tunnel junctions: Variation with the barrier oxidation time

Recently it has been observed that the magnetoresistance (MR) of plasma oxidized exchange biased Co/Al2O3/Co tunnel junctions can have a strongly asymmetric bias voltage (Vbias) dependence. In this article we report on the dependence of this phenomenon on barrier oxidation time tox. For junctions based on 1.5 nm Al, tox was varied from 20 to 120 s. For tox=20 s, for which the MR is approximately 20% at Vbias=0, and for tox⩾90 s symmetric MR(Vbias) curves are found, with the MR decreasing monotonically with |Vbias|. A strong asymmetric bias voltage dependence was observed for intermediate oxidation times, which correspond to essentially full oxidation of the Al layer, but almost no formation of stoichiometric CoO at the bottom electrode. Samples with tox=60 s show even an asymmetric double peak in MR(Vbias). Due to its strength, it has an important consequence for device applications: for a series of junctions with variable tox the maximum signal voltage (at a fixed current) is not necessarily obtained for...

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.

Study of the Verwey transition of Fe3O4 films and Fe3O4/MgO multilayers grown by MBE

Thin magnetite (Fe304) films and Fe304/MgO multilayers have been epitaxially grown by Molecular Beam Epitaxy (MBE) on MgO(100) and MgA12O4(100) substrates. The epi taxial growth on MgO(100) substrates, with a slightly larger bulk lattice parameter than that of magnetite, resulted in an in-plane expansion of the magnetite lattice, accompanied by a perpendicular compression. For films grown on MgAlsO4(100), with a smaller lattice parameter, the substrate misfit is relaxed by the incorporation of misfit dislocations at the interface. It is shown that the substrates have a large effect on the magnetic and electronic properties of the films. The characteristic Verwey transition is shifted towards lower temperatures, broadened, and reduced in amplitude, more so for thinner films. This can not be quantitatively ex plained by substrate induced stress alone, but is more likely due to a rigid structural coupling between the magnetite film and the cubic lattice of the substrate. Hereby the orthorhombic deformation accompanying the Verwey transition may be supressed. When growing at reduced oxygen pressure, the length scale for the rigid coupling will be reduced by the introduction of vacancies. This leads to more bulk-like resistivity and Verwey transition characteristics, but also to deviations from stoichiometry, as suggested by magnetization and Ferromagnetic Resonance (FMR) experiments.

Characterization of magnetic tunnel junctions using IETS

In this paper inelastic electron tunneling spectroscopy measurements are presented on tunnel junctions containing both magnetic and nonmagnetic electrodes. Magnon excitations have been found near zero voltage; phonons are observed between 30 and 110 mV. Therefore, a model description of the transport properties of magnetic tunnel junctions at elevated temperatures or non-zero bias voltage should include both inelastic contributions.