V. M. Kalita

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 ...

Influence of the demagnetizing factor on the magnetization of an ensemble of Stoner-Wohlfarth particles

The influence of the demagnetizing factor on the coercivity of a sample of oriented uniaxial Stoner-Wohlfarth particles is investigated in the approximation of a relaxation model with a self-consistent determination of the demagnetization field. The demagnetizing field is shown to change the particle magnetization reversal conditions and to reduce the blocking temperature and coercivity, for which, despite allowance for the demagnetizing field, the Neel-Brown relation holds. It is also shown that under the demagnetizing field, the internal magnetic field in which the particles are located is not stationary but varies with time concordantly with the time sweep of the temperature during magnetization in a constant external field after zero-field cooling (ZFC). Nonstationarity of the internal field in the ZFC regime causes the interval of transition temperatures of the ensemble particles from a blocked state to a state with equilibrium magnetization to increase.

Isostructural magnetic phase transitions and the magnetocaloric effect in Ising ferromagnets

It is shown that the first-order isostructural magnetic phase transition between antiferromagnetic phases with different magnitudes of the antiferromagnetism vector induced by an external magnetic field in Ising antiferromagnets can be related to entropy. It is found that, depending on the temperature, the entropy jump and the corresponding heat release can change their signs at the phase transition point. In the low-temperature region of the metamagnetic first-order phase transition the entropy jump is positive, while the entropy jump is negative near the triple point for isostructural magnetic phase transitions.

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.

Temperature-induced magnetic phase transitions in crystals with competing single-ion and interionic magnetic anisotropies

Temperature-induced phase transitions in a uniaxial ferromagnetic system of spins S = 1 with competing one-particle and two-particle anisotropies are studied. It is shown that, in the case where easy-plane single-ion anisotropy dominates over easy-axis two-particle anisotropy, the transition from the paramagnetic state to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is a second-order displacive magnetic phase transition. In the opposite case, where two-particle anisotropy dominates over single-particle anisotropy, the transition to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is also continuous but of the order-disorder type. In a system with competing second-order one-and two-particle anisotropies, the orientational first-order phase transition can occur to a state with the magnetization directed along or perpendicular to the anisotropy axis.

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.

Critical magnetization and hysteresis of nanogranular films with perpendicular anisotropy

The magnetic-field dependences of the stability boundaries of the nonequilibrium magnetic states that exist in a nanogranular film with perpendicular anisotropy in tilted magnetic fields are theoretically described, and the corresponding critical magnetization is calculated. The field dependences of the critical magnetization of the film are analyzed at various ratios of the anisotropy field of particles to the maximum possible demagnetizing field of the film. In a tilted magnetic field, the magnetization reversal curves, which include hysteresis loops, are shown to consist of segments of the following three types: equilibrium stable magnetization, nonequilibrium stable magnetization, and critical type of magnetization.

Theory of quantum phase transitions in dimerized antiferromagnets

The quantum phase transition induced by an external magnetic field in a dimerized spin crystal from its singlet ground state to an antiferromagnetic ordered state is examined under the assumption that the exchange interaction is antiferromagnetic, both within dimers and between them. It is shown that during this transition the magnetic polarization of the ions within a dimer and, therefore, of the dimers as a whole, is critical. The orientations and projections of the spin vectors in the magnetically ordered phase depend on the applied magnetic field, while the magnetic susceptibility of this essentially quantum phase remains (similarly to the case of a classical Neel antiferromagnet) constant as the spins of the magnetic sublattices undergo reorientation.