J. J. Krebs

Epitaxial growth and magnetic properties of single-crystal Co2MnGe Heusler alloy films on GaAs (001)

Single-crystal Co2MnGe Heusler alloy films were epitaxially grown on GaAs (001) substrates by molecular beam epitaxy. In situ reflection high-energy electron diffraction patterns and Auger spectroscopy confirmed the high-quality growth and stoichiometry. At 5 K, a saturation magnetization of 1000 emu/cm3 was measured. In-plane ferromagnetic resonance shows narrow linewidths and four-fold plus uniaxial anisotropy. A room-temperature resistivity of 115 μΩu200acm has also been determined. The temperature dependence of the resistivity shows metallic behavior down to low temperatures.

Magnetic properties of single crystal Co2MnGe Heusler alloy films

Single crystal Co2MnGe Heusler alloy films were epitaxially grown on GaAs substrates by molecular beam epitaxy. The films have the bulk L21 crystal structure as determined by reflection high energy electron diffraction patterns and x-ray diffraction. A high saturation moment of 1006 emu/cm3 at room temperature has been measured. Ferromagnetic resonance of these ordered alloy films shows narrow linewidths and a small anisotropy. A room temperature resistivity of 115 μΩu200acm and a 0.11% anisotropic magnetoresistance has also been determined.

Magnetoresistance origin for nonresonant microwave absorption in antiferromagnetically coupled epitaxial Fe/Cr/Fe(001) sandwiches

Nonresonant microwave‐absorption signals have been observed in a series of molecular‐beam‐epitaxy‐grown Fe/Cr/Fe (001) sandwiches which were previously shown to exhibit an antiferromagnetic coupling of the two Fe layers. Using field‐modulation detection, one observes signals which display an unusual magnetic‐field dependence containing sharp features in the range 0–2 kOe. The magnetic‐field locations of these features are independent of the microwave frequency, but their intensity depends strongly on the field‐modulation amplitude. The features also have a strong in‐plane angular dependence. Comparison with earlier magnetization and magnetoresistance data on the same samples shows that a sharp feature occurs when the magnetization orientations in the Fe films change abruptly, and that one can explain the microwave‐absorption signals in terms of magnetoresistance effects in the sandwich. Thus, microwave absorption provides a contactless way to investigate the magnetoresistance in such samples.

Perpendicular current magnetoresistance in Co/Cu/NiFeCo/Cu multilayered microstructures

An approach is demonstrated for lithographic fabrication of perpendicular current magnetoresistance devices which avoids the necessity of postprocess ‘‘trimming’’ of electrical leads. Magnetoresistance properties are investigated as a function of device size, magnetic layer thickness, and temperature. It is found that in some cases the current distribution in the microstructures change substantially with temperature resulting in a negative temperature coefficient of resistivity. It is also shown that the devices’ own magnetic fields have a dramatic effect on magnetoresistance properties.

Magnetotransport properties of single crystal Co2MnGe/NM/Co2MnGe trilayers epitaxially grown on GaAs (001)

The magnetotransport properties of single crystal trilayers 60 A Co2MnGe/NM/30u200aA Co2MnGe where NM is a nonmagnetic spacer material has been studied. The samples were grown by molecular beam epitaxy on GaAs (001) substrates. The 2 to 1 ratios in thickness of the Co2MnGe layers allows for the easy determination of which ferromagnetic layers have switched during magnetometry measurements. A weak 90° coupling has been observed in trilayers with either a Mn or a V spacer layer. In these coupled films a giant magnetoresistance type magnetoresistance of less than 1% has been measured at room temperature.

Temperature dependence of the 90° coupling in Fe/Al/Fe(001) magnetic trilayers

The temperature dependence of the unusual magnetic 90° coupling in epitaxial Fe/Al/Fe(001) trilayers has been determined. For specific values of applied field and temperature, in‐plane anisotropy and the interlayer coupling drive abrupt first‐order changes in the temperature‐dependent moment of these trilayers. The occurrence of such a first‐order transition can be used to determine the value of the interlayer coupling constant, JQ, at the transition temperature. We show that Fe/Al/Fe(001) trilayers also exhibit distinct second‐order transitions in the temperature‐dependent moment, and that the occurrence of a second‐order transition can similarly be used to specify JQ(T). A comparison is made of the values of JQ(T) determined by these two approaches, and the dependence of JQ on T is analyzed.

Spin separation in diluted magnetic semiconductor quantum well systems (invited)

Heterostructures containing diluted magnetic semiconductor (DMS) layers offer the possibility of magnetically tuning the heterojunction band alignment due to the extraordinarily large spin‐splitting of the DMS bands (large effective g‐factor). This field‐dependent band alignment has significant consequences for spin‐dependent carrier confinement as evidenced in magnetooptic or magneto‐transport experiments. We have examined two wide‐gap DMS quantum well systems in which the band alignment is dominated by the DMS spin‐splitting rather than by the more commonly observed effects of differences in bandgap, natural band offset, and strain. Quantum well structures with (Zn,Fe)Se or (Zn,Mn)Se barriers and ZnSe wells have been grown to investigate magnetically tuned, spin‐dependent quantum confinement. In these systems, the band offset appears almost entirely in the conduction band, so that the electrons are confined to the ZnSe wells. However, the hole confinement is continuously tunable by an external magnetic ...

Factors affecting performance of NiO biased giant magnetoresistance structures

We have made spin‐valve structures of Permalloy/Cu/Co by sputtering or electron‐beam deposition onto the antiferromagnetic oxide NiO. The oxides were made either by deposition of the metals and subsequent oxidation or by growing them in situ using reactive sputtering. The magnetic properties of the giant magnetoresistance structures were studied by magnetoresistance, vibrating sample magnetometry, and ferromagnetic resonance methods. The oxides were characterized by x‐ray diffraction and atomic force microscopy. We studied surface roughness and structure as functions of thickness and oxidation temperature and correlated the oxide properties with the magnetic performance. We found that the metal layer roughened during the postdeposition oxidation process and that the resulting oxide layers were very effective in pinning the direction of the magnetic moment of adjacent metal films. Coercive fields over 500 Oe were obtained for Co overlayers on NiO films but the exchange bias field was generally less than 10...

Temperature dependence of ferromagnetic resonance as induced by NiO pinning layers

Ferromagnetic resonance (FMR) experiments have been conducted near 9.5 GHz on permalloy (Py) thin films which are components of spin valves and related structures. These so-called giant magnetoresistance structures often use antiferromagnetic NiO to achieve pinning of one magnetic layer. Magnetic anisotropies acting on these pinned layers were deduced by observing their resonances for fields perpendicular to and in the sample plane. We used data taken from 4 to 600 K to identify potential mechanisms of pinning, anisotropy, and linewidth. The anisotropic exchange pinning and an isotropic downward FMR shift vanish at a blocking temperature well below the bulk Neel temperature of NiO. The strong temperature dependencies of the isotropic shift and linewidth may reflect the presence of different spin pinning subsystems and the different time scales of the FMR and low frequency or static measurements.

Novel absolute linear displacement sensor utilizing giant magnetoresistance elements

A novel concept for an absolute linear displacement sensor utilizing giant magnetoresistance elements is introduced. This device is based on the division of a magnetically soft layer of a spin-valve device into two antiparallel magnetic domains. The total resistance of the spin valve varies linearly with displacement of the domain wall because of different resistances of the aligned and antialigned magnetic states of the spin valve. Domain-wall translation is achieved by the use of a magnetic wall “trap” generated by permanent magnets. Displacement is thus related directly to the device resistance. Position resolution to date is on the order of 10 μm over ranges of approximately 2 cm and on the order of a micron over reduced ranges. Magnetic hysteresis yields backlash on the order of tens of microns.