Joao Edgar Schmidt

Anion effect in Co/Au(111) electrodeposition: structure and magnetic behavior

In-situ scanning tunneling microscopy STM and in-situ alternating gradient field magnetometry AGFM are used in . combination to characterize the structure and magnetic behavior of CorAu 111 layers electrodeposited from CoSO4 solutions, with anions X s Cl y and SCN y added in amount traces. In the Cl y solution, the 2-D growth process is briefly y . recalled. In the thiocyanate SCN solution, preliminary observations show that nanometer clusters accumulate on top of a . . Co underpotential deposition UPD monolayer. Both structures solutionrCorAu 111 exhibit perpendicular magnetic . )) yy anisotropy PMA for a Co thickness t F t , with t ; 2 ML in the Cl solution and ; 4-5 ML in the SCN one. Results are briefly discussed in light of classical models for PMA in ultrathin films. q 2000 Elsevier Science B.V. All rights reserved.

CORRELATION BETWEEN MAGNETIC INTERACTIONS AND GIANT MAGNETORESISTANCE IN MELT-SPUN CO10CU90 GRANULAR ALLOYS

The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spun Co10Cu90 granular alloys were investigated. The interaction effects were studied from both remanent magnetization and magnetotransport data, using two different methods to reach the demagnetized state, ac and dc demagnetization. The analysis of the structural evolution and interaction strength between the magnetic clusters clearly shows the role of some structural parameters (particle size and density, interparticle distance) and the degree of magnetic correlation in the magnetic field response of the resistance in these inhomogeneous systems.

Negative remanent magnetization of fine particles with competing cubic and uniaxial anisotropies

The magnetic properties of noninteracting single-domain particles whose anisotropy is made up of a cubic magnetocrystalline and a uniaxial components were investigated. Various directions of the uniaxial anisotropy were considered and the dependencies of the reduced remanence as a function of the ratio between the two anisotropies were obtained. It was found that for sufficiently strong uniaxial anisotropy and random arrangement of the particle orientations, reduced remanence lower than 0.5 is, in general, an intrinsic property of the system due to the negative remanent magnetization of some of the particles.

Co/Ag multilayer film: Role of annealing on the magnetic properties

Co(15 A)/Ag(60 A) multilayers produced by electron beam deposition on a 50 A chromium buffer layer over a Si(111) wafer have been studied by magnetoresistance, saturation magnetization, coercivity, and anisotropy. Annealing at various temperatures produces striking effects on the observed physical properties, such as an improvement on the value of the magnetoresistance associated to a ‘‘back‐diffusion’’ process in the Co/Ag interfaces. This leads one to believe that interface roughness is indeed most important to understanding the connection between giant magnetoresistance and antiferromagnetic coupling, as well as the behavior of coercivity vs interface anisotropy in these materials.

Dependence of the magnetization and remanence of single-domain particles on the second cubic anisotropy constant

Magnetization and remanent magnetization curves for noninteracting single-domain particles with cubic magnetocrystalline anisotropy have been calculated, taking into account the first two anisotropy constants. The dependencies of the saturation remanence, coercivity, remanence coercivity, and δM plots on the anisotropy constants ratio have been discussed. It has been found that the calculated saturation remanence shows maxima in the vicinity of the spin-reorientation transition points for both negative and positive first cubic anisotropy constants. The remanent magnetization for the case when the face diagonals 〈110〉 are the easiest magnetization orientations has been analytically determined as well. It has been shown that when more than one type of easiest directions coexist, the remanence of the system can only be obtained numerically because of the history dependence of the remanent magnetization for some particles’ configurations. The predicted remanence behavior for systems representing spin-reorient...

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!

Remanent magnetization plots of fine particles with competing cubic and uniaxial anisotropies

Magnetization and remanent magnetization curves for noninteracting single-domain particles whose anisotropy is made up of a cubic magnetocrystalline and an uniaxial components were investigated. For a given cubic anisotropy, the saturation remanence, coercivity, remanence coercivity, and coercivity factor values have been obtained for several different uniaxial anisotropy directions. The corresponding δM plots have been constructed for initially thermal or ac demagnetized systems, and a great variety of their shapes has been obtained when the uniaxial anisotropy does not dominate. Coercivity factor values rather higher than those for pure uniaxial or cubic anisotropy particles have been obtained as well, even for already dominating uniaxial anisotropy for some uniaxial anisotropy directions. Thus, it has been shown that one may not safely draw conclusions about interactions from deviations from the zero line in the δM plots or coercivity factor values for particles with competing anisotropies without spec...

In-situ magnetic measurements of electrodeposited ultrathin Co, Ni and Fe/Au(111) layers

This work presents preliminary in-situ alternating gradient field magnetometry measurements at the electrochemical interface. The technique is used for the first time for real time determination of the magnetic state of ultrathin layers during their electrochemical growth. Electrochemical measurements complement this study and structural information, using in-situ STM, is presented to discuss the magnetic behavior of the different films. Particular attention is given to magnetization anisotropy, and the results give evidence for strong differences between M/Au(111) interfaces, with M = Co, Ni and Fe. The easy axis of magnetization is in-plane for nickel, regardless of the film thickness, whereas the magnetization is out-of-plane for cobalt and iron below a critical thickness t*≈1–2 and 2–3 ML respectively. Above t* the magnetization rotates to take the in-plane orientation.

Controlled rotation of the exchange-bias direction in IrMn∕Cu∕Co via ion irradiation

Co∕Cu∕IrMn films were irradiated with 40keV He+ ions varying the fluence and the current, with magnetic field applied at 120° with respect to the original exchange-bias direction. The angular variations of the exchange-bias field of the irradiated samples were compared with those of the as-made and the thermally annealed films. Gradual deviation of the exchange-bias direction with the fluence increase was observed. Complete reorientation of the easy axes of both ferromagnet and antiferromagnet toward that of the field applied during irradiation was achieved for fluences higher than 1×1015ions∕cm2, accompanied with a significant enhancement of the exchange-bias field.

Bias dependence of magnetoresistance in Fe–Al2O3 granular thin films

This paper reports on the magnetotransport behavior of Fe–Al2O3 granular thin films when the injected dc current is varied. The electric resistance as a function of temperature, magnetoresistance, and the current vs applied bias potential measurements were used to characterize the samples. It was found that the transport mechanism which best describes the electronic properties of these samples is variable range hopping. Non-Ohmic behavior was observed and is claimed as responsible for the great modification of the electronic characteristics of the system as a function of the applied bias potential. Inversion of the tunneling magnetoresistance is observed for applied bias potential greater than 3 V. Such inverted magnetoresistance comes from the activation of low resistivity tunneling paths that are promoted by increasing the bias potential. An expression is proposed to describe the magnetoresistance behavior.