A. F. Lozenko

Growth-induced perpendicular anisotropy of grains in Co-Al-O nanogranular ferromagnetic films

This paper reports on the results of the magnetostatic measurements for Co-Al-O nanogranular films over a wide range of concentrations of the ferromagnetic component x. It has been revealed that grains in the films are characterized by the growth-induced anisotropy with easy axes directed perpendicular to the film plane. The maximum field of the single-grain perpendicular anisotropy reaches ∼2.5 kOe for samples in the vicinity of the percolation threshold (x ≈ 61 at % Co). It has been established that the characteristic features of the superparamagnetic behavior of an ensemble of oriented Stoner-Wohlfarth particles are retained for the sample with x ≈ 61 at % Co in the presence of the demagnetization field associated with the net magnetization of the film. The influence of the demagnetization field of the film on the shape of the magnetization reversal curves, the coercivity, and the blocking temperature has been investigated and simulated. The results of the simulation are consistent with the experimental data.

Intergranular interactions in nanogranular (CoFeB)x–(SiO2)1−x films with temperature and angular variations in coercivity

The temperature (4.5–550K) and angular (in the film plane) dependences of the coercivity field for physically nonpercolated nanogranular (CoFeB)x–(SiO2)1−x films with an oriented intraplane granule anisotropy have some unusual properties compared to those owing to blocking of thermally activated reorientation of the magnetic moments of the granules. At temperatures above the blocking temperature Tb≈350–370K, the dependence of the coercivity on the measurement duration is weak compared to that below Tb. Its angular (in the film plane) dependence differs qualitatively from that expected with blocking of superparamagnetic particles. This behavior is explained by the appearance of superferromagnetic ordering of the granule magnetic moments at T>Tb owing to interactions among them. As the temperature is reduced below 100K, the coercive field Hc for magnetization in the easy direction increases significantly more than expected for a blocked superparamagnetic ensemble. Here Hc≠0 for magnetization along the hard ...

Rotatable magnetic anisotropy in Si/SiO2/(Co2Fe)xGe1?x Heusler alloy films

Polycrystalline (Co2Fe)(x)Ge(1-x) Heusler alloy films are fabricated by sputtering on amorphous substrates and shown to possess three types of magnetic anisotropy. The nearly stoichiometric composition of x = 50 m.f.% shows a rectangular hysteresis loop and isotropic coercive and ferromagnetic resonance fields when the film is field-magnetized along any in-plane direction, thus predominantly possessing rotatable in-plane magnetic anisotropy. Higher-x compositions show evidence of two- and fourfold in-plane anisotropy superposed on the rotatable one. A qualitative model of the observed anisotropic magnetic properties is proposed. The model explains the rotatable anisotropy by taking into account dry friction for the in-plane rotation of the magnetization direction in a fine-grained polycrystalline film with the magnetic grain size smaller than the correlation length of the inter-grain exchange interaction. The observed two- and fourfold magnetic anisotropy contributions are attributed to partial texturing of the fine-grained films, even though the films are grown on amorphous SiO2 substrates. These results should be valuable for understanding and controlling the magnetic behaviour of highly spin-polarized Heusler alloy films used in various magnetic nanodevices.

The role of defects in the formation of the multidomain state of easy-plane antiferromagnets with magnetoelastic interaction

We analyzed the field dependences of forced magnetostriction in the multidomain state of the easy-plane antiferromagnet CoCl2 obtained in the following cycles: the introduction-removal of a magnetic field lying in the easy plane, the introduction-removal of a magnetic field lying in the easy plane and directed normally to that introduced earlier, etc. The magnetostriction of the crystal in the multidomain state was shown to contain two components. First, the component reversible in the cycle magnetic field introduction-removal, which makes the major contribution in the crystal under consideration, and, second, a comparatively small irreversible component, that is, the contribution retained after magnetic field removal. In low fields, the reversible magnetostriction component was proportional to the square of the applied magnetic field. Field-induced rearrangement of the multidomain antiferromagnetic state was found to be responsible for singularities of the field dependence of crystal magnetization. In particular, in a near-zero field that lay in the easy plane, the transverse susceptibility decreased twofold compared with its value in fields in which the crystal is already in the monodomain state. At the same time, close to the “monodomainization” field, transverse magnetic susceptibility was maximum. Defects were shown to favor the formation of the reversible multidomain state. Determining factors in this process were elastic and magnetoelastic interactions. The multidomain state of antiferromagnets was described using the domain distribution function over the orientations of domain antiferromagnetic vectors with respect to the magnetic field direction and the magnetic field dependence of this function. The results of our analysis were in close agreement with the experimental data on CoCl2.

Magnetic properties and anisotropic coercivity in nanogranular films of Co/Al2O3 above the percolation limit

Magnetic properties of nanogranular ferromagnetic Co/Al2O3 films with 74.5 at% Co, which is above the percolation limit, are investigated. It is established that the films have perpendicular magnetic anisotropy and a weaker in-plane anisotropy. The magnetization curves show that the film consists of two magnetic components: a dominating contribution from magneto-anisotropic isolated grains with the anisotropy axis perpendicular to the film plane and a weaker contribution from the percolated part of the film. This two-component magnetic composition of the films, with the dominating contribution from the nanograins, is confirmed by transmission electron microscopy as well as by ferromagnetic resonance spectroscopy. It is further established that the coercive field of the film is almost entirely determined by the percolated part of the film. In this, the angular dependence of the coercive force, H-c(theta(H)), is essentially proportional to sin(-1)theta(H), where theta(H) is the angle between the applied field and the films normal. However, for theta(H) -> 0, H-c(theta(H)) there is a narrow minimum with H-c approaching zero. Such non-linear dependence agrees well with our modelling results for a two-component magnetic system of the film, where the non-percolated nanograins have a distinct perpendicular anisotropy. The reported results should be important for in-depth characterization and understanding the magnetism and anisotropy in inhomogeneous systems as well as for applications, specifically in perpendicular magnetic recording.

Sol-gel synthesis and properties of tin-doped lanthanum manganites

The crystallographic, magnetic, electrical, and magnetoresistive properties of samples of La0.775Sr0.225Mn1−xSnxO3 synthesized by the sol-gel method are studied. It is shown that introducing tin atoms, with their large size, into the manganese sublattice produces significant deformations of the crystal lattice and enhances the structural and magnetic inhomogeneity of the samples. It is discovered that increasing the tin content leads to a reduction in magnetization, a lowering of the Curie temperature, and an increase in the electrical resistivity. The way the crystallographic parameters change is found to correlate with the character of the changes in the magnetic parameters. It is shown that the low-temperature magnetoresistance increases with x, while the magnetoresistance around room temperature is a nonmonotonic function of the tin concentration and has a maximum at x=0.015.

Effect of multidomain structure on the field dependences of magnetization and forced striction in easy-plane antiferromagnets

The forced magnetostriction and magnetization are measured in the easy-plane-type two-sublattice NiCl2 antiferromagnet (AFM) in the case where this AFM passes from the multidomain to a single-domain state. It is shown that, in accordance with the magnetoelastic nature of the multidomain state, the field dependences of the forced magnetostriction and magnetization are interrelated and affected by the transition from the multidomain to the single-domain state. The character of these dependences corresponds to the case where the magnetization and striction are proportional to the number of domains with an energetically favored orientation with respect to the external magnetic field.

Phenomenological description of the multidomain state of the easy-plane antiferromagnet NiCl2

An analysis is made of the experimental data on the magnetic-field dependence of the magnetostriction and magnetization of the NiCl2 crystal at its transition from the multidomain antiferromagnetic state to a uniform state. It is shown that their field dependence is determined by the mean orientation of the domains, which is characterized by a domain coalignment parameter. That parameter is used to give a phenomenological description of the multidomain state of the antiferromagnet NiCl2.

Magnetic anisotropy of epitaxial Co2Fe-Ge Heusler alloy films on MgO (100) substrates

Films of Co2Fe-Ge Heusler alloy with variable Ge concentration deposited on monocrystalline MgO (100) substrates by magnetron co-sputtering are investigated using microstructural, morphological, magnetometric, and magnetic resonance methods. The films were found to grow epitaxially, with island-like or continuous-layer morphology depending the Ge-content. The ferromagnetic resonance data versus out-of-plane and in-plane angle indicate the presence of easy plane and 4-fold in-plane anisotropy. The magnetometry data indicate additional weak 2-fold in-plane anisotropy and pronounced at low fields rotatable anisotropy. The observed magnetic anisotropy properties discussed in correlation with the microstructure and morphology of the films.