M. Ziese

Magnetoresistance of magnetite

The resistivity and magnetoresistance of a magnetite single crystal and Fe3 O4 films of various thicknesses were measured in the temperature range 70 K<T <300 K and in magnetic fields in the range -1 T µ0 H 1 T. The magnetoresistance depends on both current and magnetic field direction. The anisotropic magnetoresistance is determined as the difference of the magnetoresistances in longitudinal and transverse geometry. The data were analysed within a phenomenological model above the Verwey temperature. The anisotropic magnetoresistance for currents along [100] was found to show a sign change simultaneously with that of the crystalline anisotropy constant K 1 . Whereas the magnetoresistance of the single crystal saturates above the anisotropy field, the Fe3 O4 films show a significant high-field magnetoresistance depending linearly on the applied field. This behaviour was attributed to carrier transport across antiphase boundaries. A simple model was proposed that is in good qualitative agreement with the data. The single crystal shows a significant decrease of the Verwey transition in magnetic fields applied along [110]; this leads to a magnetoresistance of 70% in an external field of 1 T.

Anisotropic magnetoresistance of thin La0.7Ca0.3MnO3 films

The low-field magnetoresistance of films deposited on different substrates has been measured. Whereas post-annealed films on and exhibit a clear anisotropic magnetoresistance (AMR), the low-field magnetoresistance of as-deposited films on and Si is dominated by grain-boundary magnetoresistance. At low temperatures the anisotropic magnetoresistance is temperature independent with a value of about . A simple atomic d-state model can explain the sign of the anisotropic magnetoresistance.

STEP-EDGE MAGNETORESISTANCE IN LA0.7CA0.3MNO3 FILMS

The magnetoresistance of step-edge structures in La0.7Ca0.3MnO3 films was investigated. Step-edge arrays with 200 steps of height 140–200 nm and step separation 20 μm along [110] were fabricated on LaAlO3 substrates by chemically assisted ion-beam etching. Thin La0.7Ca0.3MnO3 films were deposited on the structured substrates by pulsed-laser deposition. Measurements of the large low-field magnetoresistance, the dynamic conductance, and the anisotropic magnetoresistance lead to the proposal of a model of spin-polarized tunneling in a ferromagnet/spin-glass/ferromagnet geometry.

Magnetoresistance of mechanically induced grain boundaries in La0.7Ca0.3MnO3 films

The magnetoresistance of mechanically induced grain boundaries in La0.7Ca0.3MnO3 thin films is investigated. The grain boundaries are fabricated by a mechanical deformation of the LaAlO3 substrate prior to film deposition. During film deposition, the deformed substrate region induces some growth disorder in the film leading to a wide grain boundary. The resulting structures show a reproducible, substantial magnetoresistance in magnetic fields below 2 kG. We discuss the temperature and direction dependence of the magnetoresistance and argue that two contributions due to spin-polarized electron tunneling and the suppression of magnetic frustration by a magnetic field can be identified.

Extrinsic magnetoresistance and resistance relaxation in and films and heterostructures

The magnetoresistance and resistance relaxation after a discontinuous field change of (LCMO) and thin film structures was investigated. We compare the magnetoresistance and resistance relaxation of epitaxial films, polycrystalline films, mechanically induced grain boundaries and heterostructures in current-perpendicular-to-plane geometry. We show that the low-field magnetoresistance of various LCMO structures containing strongly scattering interfaces is significantly larger than the intrinsic magnetoresistance of epitaxial LCMO films. On the other hand, the corresponding structures made from magnetite show only a small enhancement of the magnetoresistance compared with epitaxial films. The resistance relaxation of the LCMO heterostructures is found to increase with increasing magnetoresistance, whereas no relaxation was observed in the heterostructures. We propose that the extrinsic magnetoresistance in LCMO structures is due to the formation of a spin-glass-like state at highly resistive interfaces.

Dynamical response of vibrating ferromagnets

The resonance frequency of vibrating ferromagnetic reeds in a homogeneous magnetic field can be substantially modified by intrinsic and extrinsic field-related contributions. Searching for the physical reasons of the field-induced resonance frequency change and to study the influence of the spin glass state on it, we have measured the low-temperature magnetoelastic behavior and the dynamical response of vibrating amorphous and polycrystalline ferromagnetic ribbons. We show that the magnetoelastic properties depend strongly on the direction of the applied magnetic field. The influence of the re-entrant spin glass transition on these properties is discussed. We present clear experimental evidence that for applied fields perpendicular to the main area of the samples the behavior of ferromagnetic reeds is rather independent of the material composition and magnetic state, exhibiting a large decrease of the resonance frequency. This effect can be very well explained with a model based on the dynamical response of the reed and the magnetomechanical pole effect within a domain rotation model and is not related to magnetoelasticity.