J. Geshev

The magnetization of epitaxial nanometric CoFe2O4(001) layers

We have studied the magnetic anisotropy of nanometric CoFe2O4 (CFO) thin films grown on (100)SrTiO3 (STO) substrates. It has been found that epitaxial substrate-induced compressive strain makes the normal-to-film axis harder than the in-plane directions. In agreement with some previous reports, the magnetization loops are found to display a characteristic shrinking at low fields. Detailed structural and microstructural analyses, together with a modeling of the magnetization loops, revealed that the microstructure of the films, namely, the coexistence of a continuous CFO and a distribution of pyramidal CFO huts emerging from the surface, are responsible for this peculiar feature. We argue that this behavior, which significantly impacts the magnetic properties, could be a general trend of spinel films grow on (001)STO substrates.

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.

Raman spectra of R2O3 (R—rare earth) sesquioxides with C-type bixbyite crystal structure: A comparative study

Raman spectra of R2O3 (R—Sc, Er, Y, Ho, Gd, Eu, and Sm) powders with C-type bixbyite crystal structure are measured. With the help of these data and ones, previously published for other oxides from the same structural family, general dependencies of the frequencies of the Raman peaks on the cubic crystal unit cell parameter are constructed. Using these dependencies and knowing the symmetry of the peaks for one of the oxides, determined from previous single-crystal measurements, it is possible to find out the symmetry of the peaks from the spectra of all compounds. It was found that the frequency of the six lowest frequency peaks scales with the square root of the mass of the rare earth showing that mainly R ions take part in these vibrations. These results agree with performed here lattice dynamical calculations. The anomalous softening of the frequency of some peaks in the spectra of Eu2O3 is discussed.

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.

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...

A polycrystalline model for magnetic exchange bias

This work introduces a realistic model for the magnetic behavior of polycrystalline ferromagnet/antiferromagnet (FM/AF) systems with granular interfaces. It considers that, for strong enough interface exchange coupling, the AF layer breaks the adjacent FM into small-sized domains and that at the interface there exist grains with uncompensated spins interacting with the FM magnetizations; the classification of these grains as unstable (rotatable, responsible for a coercivity enhancement) or stable (adding to the bias) depends on both the anisotropy and the magnetic coupling with the adjacent FM. The distinctive characteristic of the model is that the effective rotatable anisotropy changes when the external magnetic field is varied resulting in a non-zero hard-axis coercivity, a feature commonly observed, though little understood and often ignored. The applicability of this model was checked on a typical magnetron-sputtered IrMn/Co bilayer and excellent agreement between experiment and simulation was achieved.

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...

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.

On the magnetic properties of nanosized CoFe2O4

Abstract Thermomagnetic properties of fine CoFeO 4 particles with average sizes of 43, 51, 66, 195 and 465 A were investigated between 5 and 800 K in fields up to 140 kOe. The particles with sizes of 43 and 51 A behave superparamagnetically at room temperature, but the others exhibit mixed superparamagnetic and ferrimagnetic behavior. The concentration of the ferrimagnetic phases and their magnetic properties were evaluated.

Remanence curves for a disordered system of three- and four-axial fine particles. Henkel-type plots

Abstract The isothermal remanent magnetization curve, dc demagnetization remanence curve and Henkel-type plots are studied numerically for a disordered system of non-interacting single-domain spherical particles with cubic anisotropy (three and four easy axes). Remanence coercivities and coercivity factors are also obtained.