V. Yu. Yakovchuk

Mössbauer investigation of temperature transformations in bacterial ferrihydrite

Ferrihydrite nanoparticles formed as a result of the microorganism activity have been studied using Mössbauer spectroscopy, X-ray powder diffraction analysis, and X-ray fluorescence analysis. Three positions of trivalent iron with nonoverlapping ranges of quadrupole splittings have been revealed in bacterial ferrihydrite: QS{Fe3+(1)} = 0.49–0.83 mm/s, QS{Fe3+(2)} = 0.84–1.10 mm/s, and QS{Fe3+(3)} = 1.25–1.73 mm/s. It has been experimentally demonstrated that the Fe3+(3) positions are the centers of nucleation of the hematite phase in the course of heat treatment.

Spin-wave resonance in Co/Pd magnetic multilayers and NiFe/Cu/NiFe three-layered films

The spectrum of standing spin waves has been detected by the ferromagnetic resonance method in NiFe(740 Å)/Cu/NiFe(740 Å) three-layered film structure in the perpendicular configuration for the copper thickness dCu ≤ 30 Å. At thicknesses dCu > 30 Å, the resonance absorption curve is a superposition of two spinwave resonance spectra from individual ferromagnetic NiFe layers. For Co/Pd multilayer films, united spinwave responance spectra have also been observed at thicknesses of the paramagnetic palladium layer up to dPd < 30 Å. The partial exchange stiffness has been calculated for a spin wave propagating across the Pd layer (APd = 0.1 × 10−6 erg/cm). This value is always positive (up to the critical thickness of the palladium interlayer dPd < dc) or equal to zero (dPd > dc).

Structural and magnetic features of solid-phase transformations in Mn/Bi and Bi/Mn films

Solid-phase transformations at different annealing temperatures in Mn/Bi (Mn on Bi) and Bi/Mn (Bi on Mn) films have been studied using X-ray diffraction, electron microscopy, and magnetic measurements. It has been shown that the synthesis of the α-MnBi phase in polycrystalline Mn/Bi films begins at a temperature of ~120°C and the Mn and Bi layers react completely at 300°C. The resulting α-MnBi(001) samples have a large perpendicular magnetic anisotropy (Ku ≃ 1.5 × 107 erg/cm3) and a coercive force H > HC ~ 3 kOe. In contrast to Mn/Bi, the ferromagnetic α-MnBi phase in Bi/Mn films is not formed even at annealing processes up to 400°C and Mn clusters are formed in a Bi melt. This asymmetry in phase transformations occurs because chemosorbed oxygen existing on the surface of the Mn film in Bi/Mn films suppresses a solid-phase reaction between Mn and Bi. The analysis of the results obtained implies the existence of new low-temperature (~120°C) structural transformation in the Mn–Bi system.

Studying the mechanism of exchange coupling in ferro/ferrimagnet NiFe/DyCo film structures

Dependence of the magnetic and magnetooptical properties of an exchange-coupled NiFe/DyCo bilayer system on the thickness (tDyCo) of a magnetically hard layer has been studied. It is established that the unidirectional anisotropy vanishes at tDyCo ∼ 400 Å, while the coercive field in the magnetically soft layer becomes comparable to the exchange-induced field shift. In this case, the DyCo layer magnetization is almost parallel to the film plane, whereas a reference DyCo film exhibits a perpendicular anisotropy. A model of the magnetic state of layers in the ferro/ferrimagnetic layer structure under consideration is proposed, which assumes that a 180° domain wall is formed at the interface upon magnetization reversal in the magnetically soft layer.

Unidirectional anisotropy in ferromagnetic-ferrimagnetic film structures

A mechanism of unidirectional anisotropy formation in an exchange-coupled ferromagnetic-ferrimagnetic film structure with orthogonal effective magnetizations in the layers is investigated. The reason for unidirectional anisotropy is the magnetic heterogeneity of the ferrimagnetic layer in the compensation range. Magnetization reversal in the magnetically soft layer of an (REE–transition metal)/NiFe film structure is discussed based on a model of uniform rotation of magnetization. It is found that unidirectional anisotropy sharply decreases the magnetic noise level in the magnetically soft layer. The field of application of these materials is outlined.

Effects of exchange interaction in bilayer DyxCo1−x/NiFe films in the vicinity of compensation compositions of amorphous DyCo alloys

The displacement field of the hysteresis loop due to exchange anisotropy in planar DyCo/NiFe systems is studied experimentally as a function of the concentration of the rare-earth element. The bilayer DyCo/NiFe film system is characterized by an orthogonal arrangement of the effective magnetizations of separate layers under the condition that the amorphous DyCo layer is prepared in the region of magnetic compensation. An analysis of the dependence of the displacement field on the Dy concentration has led to an understanding of the physical mechanism of the formation of the exchange anisotropy in these planar systems.

High rotatable magnetic anisotropy in MnBi thin films

The variations of the structural and magnetic properties of Bi/Mn/Bi and Mn/Bi/Mn trilayer film systems of equiatomic composition in the process of vacuum annealing are studied. The annealing of Bi/Mn/Bi films at a temperature of 270°C for an hour results in the synthesis of the well-studied highly oriented low-temperature LT-MnBi(001) phase with the perpendicular magnetic anisotropy Ku ∼ 1.1 × 107 erg/cm3 and coercivity HC ∼ 1.5 kOe. In contrast to Bi/Mn/Bi, polycrystalline LT-MnBi nanoclusters are formed in Mn/Bi/Mn films under the same annealing conditions. A high rotatable magnetic anisotropy exceeding the shape anisotropy is detected in the films under consideration: the easy axis of anisotropy with the inclusion of the delay angle in magnetic fields above the coercivity H > HC = 9.0 kOe can be oriented in any spatial direction. It is shown that the nature of rotatable magnetic anisotropy is due to the structural coexistence of epitaxially coupled LT-MnBi and QHTP-Mn1.08Bi phases. The reported experimental results indicate the existence of a new class of ferromagnetic film media with the spatially tunable easy axis.

Magnetic resonance in FeNi/Bi/FeNi films

The magnetic resonance in FeNi/Bi/FeNi trilayer films with nonmagnetic semimetal spacer has been experimentally studied. It is found that the microwave absorption spectrum of samples has a complicated shape dependent on the nonmagnetic spacer thickness. In the interval of Bi layer thicknesses within 3–15 nm, the interlayer coupling has an antiferromagnetic character.

Parameters of the transition layer in the exchange-biased NiFe/DyCo film structure

The parameters of the transition layer in exchange-biased film structures are necessary agents to understand the mechanism of formation of unidirectional anisotropy. The layer thickness in NiFe/DyCo films has been determined by comparison of signals of the polar magneto-optical Kerr effect from a reference DyCo film and a hard magnetic layer of the exchange-biased structure. The layer thickness obtained is one order of magnitude larger than that characteristic of ferromagnet-antiferromagnet bilayer films. The mechanism of magnetization reversal of the structure under study has been explained within the model suggesting the formation of 180° boundaries in the interface.

Specific features of unidirectional anisotropy in exchange-coupled DyCo/NiFe film structures

The dependence of the unidirectional anisotropy in a DyCo/NiFe bilayer system on the thickness of the magnetically soft layer has been studied using dynamic and static techniques. The magnitude of the exchange interaction between layers evaluated using the ferromagnetic resonance method is two times that determined from an analysis of the hysteresis loops. It is established that this difference is related to features of the magnetic microstructure of the DyCo layer.