S. Nadolski

Surface-induced phase separation in manganites: A microscopic origin for powder magnetoresistance

Through the analysis of the magnetic properties and of the nuclear magnetic resonance response of La2/3Ca1/3MnO3 ceramics with different grain sizes, we have found that poorly conducting regions, some ferromagnetic and some weakly magnetic, are located at the surface of the grains. We state that these regions constitute the tunnel barrier responsible for the low-field magnetoresistance usually observed in powders of half-metallic oxides. In addition, the spin disorder accompanying the coexistence of phases with different magnetoelectronic character could contribute to the large high-field magnetoresistance also typical of such ceramic samples. From a more general perspective, these findings can be of relevance to understand the microscopic origin of phase separation in manganites.

Inhomogeneous structure and magnetic properties of granularCo10Cu90alloys

P. Panissod, M. Malinowska, E. Jedryka, M. Wojcik, S. Nadolski, M. Knobel, and J. E. Schmidt Institut de Physique et Chimie des Materiaux de Strasbourg, 67 037 Strasbourg, France Institute of Physics, Polish Academy of Sciences, 02 688 Warszawa, Poland Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas 13083-970, Campinas, SP, Brazil Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, RS, Brazil ~Received 2 November 1999; revised manuscript received 12 May 2000; published 11 December 2000!

Very low chemical disorder in epitaxial NiMnSb films on GaAs(111)B

Single-crystalline NiMnSb(111) films with negligibly low defect levels have been grown epitaxially on GaAs(111)B using molecular beam epitaxy and characterized by nuclear magnetic resonance. In a film with only 1% deviation from stoichiometry, 1.1% of all Mn atoms is involved in planar defects, ∼0.5% of all Sb sites is occupied by AsSb substitutional atoms, and ∼0.2% of all Sb atoms has a modified environment. Both the average concentration of defects and the interface orientation are compatible with maintaining a half-metallic band structure at the ferromagnet/semiconductor interface, making these films a good candidate for spin injection into a semiconductor.

Thickness dependence of surface roughness and transport properties of La2/3Ca1/3MnO3 epitaxial thin films

In this work, we report on the impact of distinct growth parameters that affect the roughness and surface morphology of La2/3Ca1/3MnO3 epitaxial thin films grown by rf sputtering, namely, the film thickness and the deposition temperature. Data for films with thicknesses ranging from 2.4 to 108 nm will be presented. A correlation with transport measurements is also reported: whereas films thicker than 6 nm show the typical metal-to-insulator transition, the thinner film is insulating. The resistivity is strongly enhanced when decreasing film thickness. Nuclear magnetic resonance measurements have been used to monitor the relative concentration of the localized Mn4+ and delocalized Mn3+/4+ states. It is found that the relative intensity of the delocalized Mn3+/4+ configuration (I3+/4+) progressively lowers when reducing film thickness. Of significance could be the observation that I3+/4+ remains finite for the thicknesses corresponding to insulating films, thus suggesting that an electrically inhomogeneous ...

Structural study by NMR in Co/Cu multilayers at second antiferromagnetic maximum

A 59Co NMR study has been carried out in a series of Co/Cu multilayers with tCu = 20 A and 2 A < tCo < 30 A. The Co layer is found to have a strained fcc structure with hcp stacking faults above 15 A Co. The lateral size of columnar grains is estimated to be 45 A. A transformation from continuous to granular Co layer is evidenced below 10 A Co. Below 3 A all deposited Co exists in form of a diffused CoCu alloy and Co clusters disappear. These structural changes, together with the decrease of a number of domain walls inferred from the rapid drop of NMR enhancement factor are to be related with the previously observed disappearance of magnetoresistive hysteresis.

Magnetoresistive oxides: new developments and applications

Abstract We review progress on development of applications of manganese perovskites and double perovskites based on polycrystalline films. We will show that grain boundaries and interfaces, either in polycrystalline materials or epitaxial films are far from being chemically or electronically homogeneous and this may represent a serious handicap for spin polarized tunneling transport across interfaces. We will report on successful strategies of further raising T C .

Identification of magnetic phases in granular Co10Cu90 alloy using 59Co NMR method

A structural analysis in the Co 10 Cu 90 granular alloy prepared by the melt spinning technique has been carried out using 59 Co NMR method. Two magnetic phases have been found in the as quenched sample. A prevailing phase (involving 90% of the observed Co atoms) is a population of small, strained FCC Co precipitates. A second phase is a soft magnetic FCC CoCu alloy with average Cu concentration of about 10%.

Structural study of nanometric electrodeposited Co films using 59Co NMR

A 59Co NMR experiment has been used to investigate the structure of two series of Co films with thicknesses varying from 5 to 400 nm which were electrodeposited on Cu at the electrolyte pH value of 2.1 and 3.7, respectively. It was shown that the overall structure of studied Co films consists of a very good quality fcc phase and a heavily faulted hcp phase in about equal proportions. The exceptions are very thin Co layers (below 20 nm) where the hcp structure was stabilized at pH 3.7 and overpotential of 0.9 V. This effect is attributed to the formation of hexagonal cobalt hydroxide in the early stage of deposition, which acts as a buffer layer stabilizing Co hcp structure.

Automated pulsed NMR spectrometer for modern magnetic materials

Abstract A coherent, computer controlled, frequency swept spin echo spectrometer designed for fast automatic spectrum acquisition in modern ferromagnetic materials is described.

Heat-induced nanocluster formation in codeposited Ag1−xCox thin films: Nuclear magnetic resonance study

59Co nuclear magnetic resonance has been used to study the formation of Co nanoclusters in several series of thin-film Ag1−xCox alloys (0.08⩽x⩽0.4) prepared by coevaporation and subjected to different heat treatments. Co particles originally formed during the deposition process were found to be single-domain fcc Co clusters, having the shape of truncated octahedra of about 4.5 nm in diameter. Annealing at 200 °C resulted in a size increase to about 7 nm, but after annealing at 350 °C, the particles grew to about 26 nm while remaining single magnetic domain. Upon further annealing, the onset of large, multidomain Co clusters is evidenced. The Co concentration in the original alloy influenced the density of Co particles, but not their size, which was essentially determined by the annealing conditions.