M. Mikhov

Magnetic properties of nanophase CoFe2O4 particles

Abstract Fine CoFe 2 O 4 particles with average size of 50 A have been produced by coprecipitation from Fe(II) and Co(II) solutions followed by a low-temperature calcination. The ac and dc magnetic susceptibilities, as well as magnetization, were studied between 5 and 300 K. The disordered system of such particles behaves superparamagnetically above T B = 250 K and ferrimagnetically below this temperature. T B can be considered as mean blocking temperature at zero field in the timescale of ac (111.1 Hz) magnetic susceptibility measurements. The anisotropy field exceeds 50 kOe below 150 K. The maximal coercivity measured at 5 K is 14.5 kOe and the maximal measured magnetization is 14.6 emu/g, which is much lower than the saturation magnetization of bulk CoFe 2 O 4 .

Hopkinson effect in an assembly of single domain particles- thermonagnetic curves of Nd2Fe14B-type ribbons

Abstract We examine the Hopkinson effect in a disordered system due to the rotation of magnetization of noninteracting uniaxial single domain particles, (the Stoner-Wohlfarth model). The experimentally observed Hopkinson effect in rapidly quenched Nd-Fe-B and Mm-Fe-B (Mm denotes Mishmetal) ribbons is explained on the basis of the proposed calculation procedure. There is a good qualitative agreement between theory and experiment.

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...

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...

Experimental study of the Hopkinson effect in fine BaFe12O19 particles

Abstract The Hopkinson effect has been experimentally observed and theoretically investigated in a disordered system of fine (single domain) BaFe 12 O 19 particles. An explanation based on the Stoner-Wohlfarth model which accounts for thermal activation is proposed.

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.

Fourier analysis of hysteresis loops and initial magnetization curves: application to the singular-point-detection method

Abstract Hysteresis loops and initial magnetization curves, obtained within the framework of the Stone-Wohlfarth model, and experimental ones for a system of single-domain barium ferrite particles, have been studied by Fourier analysis. A simple, but significant relation between the Fourier coefficients of the initial and hysteresis curves has been derived. The correctness of the Fourier description of magnetization curves permits many of the magnetization quantities to be easily calculated with high accuracy. It has been demonstrated that the anisotropy field can be obtained from the Fourier-described hysteresis loops, following the SPD method.

Thermal activation and the Hopkinson effect

Abstract We explore the thermal activation contribution to the initial thermomagnetic curves with a Hopkinson maximum. The thermal activation is incorporated into the Stoner-Wohlfarth model of magnetization of noninteracting uniaxial single-do-main particles of random orientation.

Interaction effects detected by Fourier analysis of hysteresis loops and initial magnetization curves

A new technique for the estimation of interparticle interaction effects in some magnetic systems is developed only on the basis of Fourier description of experimental hysteresis loops and initial magnetization curves, avoiding remanence measurements. This technique is applied for Ba-ferrite powders and for several currently used magnetic recording media. It is found that the results coincide fairly well with these obtained by the commonly accepted /spl Delta/I technique, using remanence curves. >