A new quasi-Ising magnet

We have performed measurements of the magnetization and differential magnetic susceptibility of Dy_0.62Y_2.38Fe₅O₁₂ single crystals in pulsed magnetic fields up to 45 T at liquid-helium temperature for three orientations of the external field: H‖[100], H‖[110], and H‖[111]. It was found that the magnetization reversal in the rare-earth magnetic subsystem occurs via several phase transitions, whose number depends on the direction of the external field, as is characteristic for Ising magnets. The anomalies in the field dependences of the magnetization are interpreted on the assumption of quasi-Ising ordering of the rare-earth ions. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 552–556 (25 April 1998)     

Investigation of micromagnetism and magnetic reversal of Ni nanoparticles using a magnetic force microscope

Isolated Ni nanoparticles were studied in situ by atomic and magnetic force microscopy in the presence of an additional external field up to 300 Oe. By comparing topographic and magnetic images, and also by computer modeling of magnetic images, it was established that particles smaller than 100 nm are single-domain and easily undergo magnetic reversal in the direction of the applied external magnetic field. For large magnetic particles, the external magnetic field enhances the magnetization uniformity and the direction of total magnetization of these particles is determined by their shape anisotropy. Characteristics of the magnetic images and magnetic reversal of particles larger than 150 nm are attributed to the formation of a vortex magnetization structure in these particles. Fiz. Tverd. Tela (St. Petersburg) 40, 1277–1283 (July 1998)     

Macroscopic magnetic fields of antiferromagnets

The macroscopic magnetic fields arising in the space outside or in the bulk of an antiferromagnet in the absence of external currents are a result of surface magnetization. The general problem of determining these fields is formulated. It is shown that the field distribution near special lines on the surface is monopolar. An experimental study of the field makes it possible to determine the surface magnetization on the faces of an antiferromagnetic crystal. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 724–728 (10 May 1996)     

Dynamic magnetization of <Emphasis Type="Bold">γ</Emphasis>- nanoparticles isolated in an amorphous matrix

We have studied the magnetization of a system of γ-Fe₂O₃ (0.68 vol.%) nanoparticles isolated in an SiO₂ amorphous matrix placed in an alternating magnetic field with a frequency of 640 Hz and in the temperature range of (77-300) K. Compared to temperatures closer to 300 K (where the system has a superparamagnetic behaviour), at lower temperatures, the magnetization has a dynamic hysteresis loop due to the magnetization's phase shift between the field and the magnetization. The delay of the magnetization (attributed to the Néel relaxation processes) increases with the decrease of temperature. It has been shown that the relaxation time resulting from the Néel theory is determined by an effective anisotropy constant ( K ) that takes into account the magnetocrystalline anisotropy, as well as the shape, surface and strain anisotropies. In the following we will show that the surface and strain anisotropy components have the most significant influence. When the temperature decreases from 300 to 77 K, the relative increase of the saturation magnetization of the nanoparticles is much higher than that of the (spontaneous) saturation magnetization of bulk γ-Fe₂O₃. This increase is due to the increase of the mean magnetic diameter of the particles attached to the core of aligned spins, from 10.16 nm to 11.70 nm, as a result of the modification of the superexchange interaction in the surface layer. Received 25 April 2002 / Received in final form 11 August 2002 Published online 14 February 2003 RID="a" ID="a"e-mail: ccaizer@physics.uvt.ro     

Longitudinal dynamic susceptibility of superparamagnetic particles with cubic anisotropy

We study the linear response of a system of single-domain ferromagnetic particles with cubic magnetic anisotropy to a weak external a.c. magnetic field. By averaging the Gilbert equation with a fluctuating field for the magnetization of an individual particle we derive a system of recurrence equations for the spectra of equilibrium correlation functions describing the longitudinal relaxation of the system. We find the solution of this system by using matrix continued fractions. We also evaluate the longitudinal relaxation time and the spectrum of the complex-valued magnetic susceptibility. Finally, we show that the nature of susceptibility dispersion is determined by the anisotropy and dissipation parameters. Zh. éksp. Teor. Fiz. 115, 101–114 (January 1999)     

Nonlinear low-temperature magnetization of the quasi-one-dimensional charge-density-wave conductor K0.3 MoO3

A study is made of the temperature and field dependences of the anisotropic magnetization of a quasi-one-dimensional charge-density-wave conductor — the blue bronze K_0.3 MoO₃. Nonlinearity of the magnetization curves is observed at temperatures below 100 K. The temperature and field dependences of the magnetic moment show the effect to be of a collective nature. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 281–286 (25 August 1998)     

Mesoscopic oscillations of the magnetization near Ising impurity centers in strong magnetic fields

The frequency of orientational quantum oscillations of the magnetization near impurity-ion clusters with Ising properties in a saturated magnetic crystal is calculated. It is noted that in compounds of the type Ho_xY_3−x Fe₅O₁₂, where magnetic phase transitions are observed, additional magnetization reversal and magnetic resonance features due to mesoscopic oscillations of the magnetization can be observed at low concentrations x<0.001 and cryogenic temperatures in fields comparable to the intersublattice exchange interaction field. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 445–448 (25 March 1997)     

Influence of spin-orbit interaction of two-dimensional electrons on the magnetization of nanotubes

Calculations are made of the magnetization of a nanotube in a longitudinal magnetic field. It is shown that the spin-orbit interaction of two-dimensional electrons located at the surface of the nanotube causes a qualitative change in the magnetization. Depending on the parameters of the system, either diamagnetism or paramagnetism can occur and the dynamic susceptibility is characterized by anomalous dispersion at low frequencies. Zh. éksp. Teor. Fiz. 115, 1478–1483 (April 1999)     

Longitudinal critical current in a ferrite-type-II superconductor structure

The effect of the interaction of Abrikosov vortices with the magnetization on the longitudinal vortical instability in a layered ferromagnet-type-II superconductor structure is analyzed. It is shown that in the vicinity of the orientational phase transition in the magnet, where the transverse magnetic susceptibility is large, the magnitude of the longitudinal critical current in the structure can be almost 1.5 times smaller than in the isolated superconductor. The reason for this is compensation of stray field sources outside the superconductor by “magnetic charges” arising from a jump in the transverse magnetization on the surface of the magnet. A structure is considered in which the thickness of the superconductor significantly exceeds the London penetration depth of the magnetic field and the wavelength of the critical mode. For this reason (in light of the absence of high-quality bulk high-temperature superconductors), to experimentally study the described phenomenon it is necessary to use conventional low-temperature superconductors. Fiz. Tverd. Tela (St. Petersburg) 39, 231–235 (February 1997)     

Exact quantum mechanical description of the motion of spin-1/2 particles and of spin motion in a uniform magnetic field

The Dirac-Pauli equation is used to obtain the exact equation of spin motion for spin-1/2 particles with an anomalous magnetic moment in a constant and uniform magnetic field. Exact formulas are established for the angular velocity of the revolution of such particles along circular orbits and the rotation of the particle spin with respect to momentum. Finally, a quantum mechanical equation for the motion of the particles in a strong magnetic field is derived. Zh. éksp. Teor. Fiz. 114, 448–457 (August 1998)     

Dynamic properties of moderately concentrated magnetic liquids

On the basis of statistical analysis, we derive expressions for the dynamic susceptibility, magnetization relaxation times, and the effective rheological characteristics of a moderately concentrated homogeneous ferrocolloid consisting of identical spherical ferroparticles suspended in a Newtonian liquid. The magnetic moment of a particle is assumed constant and rigidly “frozen” into the body of the particle. We also estimate how the magnetodipole and hydrodynamic interactions of the particles influence the effective dynamic properties of the ferrocolloid. Zh. éksp. Teor. Fiz. 114, 892–909 (September 1998)     

Theory of a weakly nonideal Bose gas in a magnetic field

This paper investigates Bose-Einstein condensation of an ideal gas of finite-spin bosons in an external magnetic field. We generalize Bogolyubov’s theory of a weakly nonideal Bose gas to the case where the gas of finite-spin bosons is located in an external magnetic field. We find the corresponding quasiparticle spectrum and formulate the superfluidity criterion for the boson gas. The magnetization of the weakly nonideal Bose gas is also determined. Finally, we specify a method of studying kinetic processes that take place in a weakly nonideal Bose gas. Zh. éksp. Teor. Fiz. 113, 918–929 (March 1998)     

Collective effects accompanying magnetization of two-dimensional lattices of nanosize magnetic particles

Collective effects arising in a two-dimensional lattice of nanosize magnetic particles as a result of the dipole interparticle interaction are investigated by Hall magnetometry. The experimental system consists of 10⁵ permalloy particles having a diameter of ∼40 nm and a height of ∼40 nm and forming a lattice with a rectangular unit cell (90 nm×180 nm). We attribute the characteristic features observed in the magnetization curves to quasi-one-dimensionality of the experimental lattice of particles and to the formation of solitons in chains of dipoles. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 475–479 (25 September 1998)     

Weak disorder in a two-dimensional dipole magnet

The effect of “random-field” disorder and “random-anisotropy-axis” disorder on a two-dimensional dipole ferromagnet is investigated. It is shown that disorder results in instability of the ferromagnetic phase. The correlation function of the magnetization is calculated with the aid of a self-consistent harmonic approximation. It is found that in the presence of a random field the correlation function is a power-law function of the distance. In the presence of random anisotropy the correlation function decreases logarithmically slowly as a function of distance. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 108–112 (10 January 1997)     

Spin vortices and stationary spin flows in a normal Fermi liquid

We solve the equations for the collisionless spin dynamics of a normal Fermi liquid, which describe structures resembling spin vortices coherently precessing in a uniform magnetic field. We examine their stability and relaxation, and consider various regimes of stationary magnetization transport along a channel. Zh. éksp. Teor. Fiz. 114, 1266–1283 (October 1998)     

Theory of the de Haas-van Alphen effect in doped semiconductors

The field dependence of the magnetization in doped semiconductors of the type InSb, InAs, GaAs, etc. is studied in high magnetic fields. The standard theory of the de Haas-van Alphen effect, mostly applicable to metals, is modified to include the long-range fluctuations of charge carriers. The experimental investigation of this effect can shed light on some open questions in semiconductor physics, e.g., the problem of tails in the electronic density of states. It is shown that in such systems the mean magnetization is sensitive to magnetic interactions. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 181–186 (10 February 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Collective effects in a system of Pearl vortices and the magnetization of a thin superconducting film

The magnetization curves of a thin superconducting monolayer are calculated. It is found that for sufficiently high fields and temperatures the magnetization of a monolayer exhibits the same features as the reversible magnetization of layered superconductors in high fields. It is shown that these features are due to instability with respect to the dissociation of pairs of Pearl vortices into a gas of free vortices and to collective effects in a system of free vortices. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 460–465 (25 September 1998)     

Quantum oscillations of resistance and magnetization in the degenerate semiconductor n-HgCr2Se4

In the magnetic field range ΔH=8–60 kOe we observed and studied the anomalous oscillations in the magnetic field dependence of the resistance and magnetization of single crystals of n-HgCr₂Se₄. The absence of periodicity in 1/H in the ΔH=8–20 kOe range can be explained by the non-Fermi-liquid behavior of the electron subsystem and agrees with the theory of the de Haas-van Alphen in systems with intermediate valence. In stronger fields, ΔH=20–60 kOe, the amplitude of the fundamental harmonic decreases, with the number and amplitude of the higher-order harmonics increasing. As a result, noise is superimposed on the signal as magnetic field strength grows. The temperature dependence of the magnetization is the sum of the monotonic spin-wave contribution and the oscillating part. Zh. éksp. Teor. Fiz. 113, 1877–1882 (May 1998)