R. von Helmolt

Giant magnetoresistance in melt spun Cu‐Co alloys

Cu1−xCox alloys with x=0.1 and x=0.2 have been prepared by conventional melt spinning. The rapid solidification process results in an extended solubility of Co in Cu although some Co precipitates already during quenching. In the as‐quenched ribbons, the magnetoresistance (MR) is only of the order of 1.5%. It increases dramatically with the controlled nucleation and growth of Co precipitates from the supersaturated Cu matrix. The highest MR of 11% at 300 K occurs for Cu90Co10 after an aging at about 440 °C when the Co clusters are superparamagnetic. Saturation is possible only after a higher annealing or at lower measuring temperatures. For the optimally annealed samples the MR increases to 36% at 30 K.

Intrinsic giant magnetoresistance of mixed valence La‐A‐Mn oxide (A=Ca,Sr,Ba) (invited)

A large intrinsic magnetoresistance has been found near the ferromagnetic transition of metallic manganese oxides with perovskite‐type crystal structure. The magnetic and transport properties were measured on bulk and thin‐film La1−xAxMnO3+δ with A=Ca,Sr,Ba. Assuming the double‐exchange model proposed by Zener [Phys. Rev. 81, 440 (1951); 82, 403 (1951)], the strong dependence of the transport properties on the magnetic field and also on the chemical composition is attributed to the mixed Mn3+/Mn4+ valence.

Magnetoresistance in La2/3Ca1/3MnO3+δ : Dependence on magnetic history

We investigated the magnetoresistance of La2/3Ca1/3MnO3+δ magnetic thin films. Under certain conditions, the magnetoresistance curves show a remarkable hysteresis and the resistance becomes dependent on the magnetic history. A relaxation behavior of the resistance has also been observed. Hysteresis and relaxation of the resistance are very similar to the phenomena of magnetoviscosity of the magnetic moment in spin glasses and the magnetic aftereffect in hard magnets.

Transport properties of manganates with giant magnetoresistance

Abstract A short review of transport properties related to the giant magnetoresistance in perovskite-type double-exchange ferromagnets is presented. The paper deals with the essential experimental results for the understanding of the large MR effect, which is based on the correlation of electric and magnetic properties. These experiments are mainly the dependence of the resistivity on temperature and magnetic field, and also on the chemical composition. In the region of the highest magnetoresistance, which usually occurs close to a magnetic transition, a peak in resistivity indicates the strong spin disorder scattering. Even localization of charge carriers and thus a metal-insulator transition occurs in some cases.

Magnetoresistance in Au‐Co‐B heterogeneous alloys

Au‐Co and Au‐Co‐B alloys have been prepared by rapid solidification, followed by heat treatment. The precipitation of the ferromagnetic phases Co and Co2B in the Au matrix phase has been observed during annealing. At room temperature, both magnetization and magnetoresistance are determined by the superparamagnetic behavior of the ferromagnetic particles. The magnetoresistance is believed to have the same origin as that in multilayers and granular films. Both Co particles and the precipitates of the Co2B intermetallic phase give rise to a high magnetoresistance. The magnetoresistance effect was less than 1% in the as‐quenched samples and increased to about 3% after an appropriate heat treatment.

Magnetic properties of melt-spun Cu97.5Co2.5 ribbons

Abstract Magnetisation and AC susceptibility measurements have been performed to characterize the melt-spun Cu97.5Co2.5 alloy. The magnetisation curve, at room temperature, shows two contributions, one superparamagnetic and the other nearly paramagnetic. The temperature dependence of the in-phase AC susceptibility shows two maxima with different dynamical character. These two anomalies are ascribed to be superparamagnetic blocking and spin-glass freezing and agree with previous magnetic and structural studies of the same alloy system.

Magnetoresistance of Cu90FexCo10-x heterogeneous alloys and the effect of boron addition

Abstract We investigated heterogeneous alloys of the quasi-binary series Cu 90 Fe x Co 10− x (0≤ x ≤10). Two magnetic phases appear in the ternary alloy, and the magnetoresistance decreases with increasing Fe content. The effect of B substitution was investigated for several Cu-Co-B samples, where intermetallic CoB and Co 2 B precipitates appeared. The magnetoresistance is strongly depressed even for only small additions of B.