On the intensity of shock-initiated magnetoelastic oscillations arising in iron borate single crystals during pulsed magnetizing or magnetization-switching processes

The intensities of magnetoelastic oscillations accompanying pulsed 180° and 90° magnetization switching of iron borate single crystals as well as pulsed magnetization of the crystals from a demagnetized state (with zero total magnetic moment) are compared for the first time. The amplitude A ₁ of the oscillations of the signal obtained from the experimental sample by the induction method is adopted as a measure of the intensity of the magnetoelastic oscillations. It is found that for the same pulse heights of the magnetic field H exciting the magnetization-switching or magnetizing process, the amplitudes A ₁ of the oscillations observed in 90° magnetization-switching and initial-magnetization processes have practically the same value, which is $\sqrt 2 $ times smaller than the amplitude of the oscillations obtained in 180° magnetization switching (reversal). It is concluded on the basis of the result obtained that the intensity of the magnetoelastic oscillations is virtually independent of the initial state of the single crystal and is determined mainly by the energy density ΔM·H acquired by the magnetic subsystem of the crystal from the external field (ΔM is the change in magnetization). Hence it follows that when iron borate is used in fast modulators for Mössbauer γ rays it is preferable to use the 90° magnetization-switching regime rather than the magnetization regime as has been done until very recently.     

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)     

Effect of a magnetoelastic interaction on the rate of transient processes in iron borate single crystals

A semiempirical relation between the rate of transient processes in iron borate single crystals and the amplitude of the magnetic field and intensity of the magnetoelastic oscillations is proposed. Experimental data confirming the applicability of the expression to real pulsed magnetization-reversal curves are presented. Fiz. Tverd. Tela (St. Petersburg) 41, 654–655 (April 1999)     

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)     

Inhomogeneous magnetostriction states in uniaxial ferromagnetic films

Surface magnetoelastic Love waves and nonuniform distributions of the magnetization and elastic strains are investigated in a uniaxial ferromagnetic film on a massive nonmagnetic substrate in a tangential external magnetic field. A new inhomogeneous phase is predicted having spatial modulation of the order parameter, arising from magnetostrictive coupling of the magnetization with lattice strains near the interface of the magnetoelastic and elastic media. It is shown that, at some critical magnetic field H _c, different from the orientational transition field in an isolated sample, a magnetoelastic Love wave propagating parallel to the magnetization vector in the film plane becomes unstable. The frequency and group velocity of the wave vanish at wave number k=k _c≠0 and the wave freezes, forming a domain structure localized in the film and adjoining substrate. Fiz. Tverd. Tela (St. Petersburg) 41, 665–671 (April 1999)     

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)     

Modeling magnetization oscillations in ferromagnetic cobalt disks exposed to picosecond laser pulses

Magnetic reversal of a ferromagnetic single-domain nanoparticle in the form of a disk contained in a multilayer nanostructure irradiated by linearly and circularly polarized picosecond laser pulses is modeled. It is shown that heating of the disk by the laser light causes a change in its direction of magnetization. This is accompanied by oscillations in the magnetization that decay over 1–16 ns. The frequencies of these oscillations lie in the range of 0.5–26 GHz. The main reason for the magnetic reversal of the disk is a change in the magnetic anisotropy energy during heating. The laser pulses also create a spin-polarized current and an inverse Faraday magneto-optical field in the structure, which influence the duration, amplitude, and frequency of the magnetization oscillations.     

Investigation of metamagnetic transitions in the itinerant d subsystem of the intermetallics RCo2 in superstrong magnetic fields up to 300 T

The time derivatives of the magnetization of intermetallic compounds RCo₂ (R = Y, Tm, Er, Ho, or Dy) with a metamagnetic subsystem of itinerant d electrons have been measured in pulsed magnetic fields up to 300 T generated by an explosive method. Peaks associated with abrupt demagnetization or magnetization of the d subsystem are found in the field dependences of dM/dH. The results obtained are compared with theoretical estimates and the values derived for the critical fields from measurements performed on substituted compounds. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 188–192 (10 August 1996)     

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)     

Photoinduced changes in the dynamic magnetoelastic interaction in yttrium-iron garnet

The effect of light on the excitation efficiency and resonant frequency for the fundamental mode of contour elastic oscillations of a circular ferrite-garnet slab is investigated. A reduction in the dimensionless parameter for the dynamic magnetoelastic coupling is observed. It is shown that illumination produces changes in the effective magnetostriction field and in the magnetic anisotropy. Fiz. Tverd. Tela (St. Petersburg) 39, 1081–1083 (June 1997)     

Effect of easy-plane anisotropy on the transient process time in magnetic films and plates

The effect of easy-plane anisotropy on the damping of the nonlinear magnetization oscillations that accompany 90° pulsed magnetization of magnetic films and plates is analyzed numerically. It is shown that the magnetization time can be decreased to ∼0.5 ns at an effective anisotropy field H _{K p} ≥ 6 kOe and that magnetization oscillations are fully damped at H _{K p} ≥ 20–40 kOe. The magnetization time can be ∼0.15–0.20 ns at a magnetizing pulse amplitude H _m ∼ 20–40 Oe.     

Oscillations of the thermodynamic properties of a one-dimensional mesoscopic ring caused by Zeeman splitting

It is shown that the interrelationship of spin splitting in a magnetic field and spatial quantization in a one-dimensional ballistic ring coupled with a reservoir results in mesoscopic oscillations of a new type which vanish with increasing temperature. The period of such oscillations is inversely proportional to the density of states in the spin subsystem in the ring. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 595–600 (10 November 1999)     

Time reversal during magnetization of gas atoms by a resonant light pulse

The optically induced magnetic moment of a stationary atom is calculated as a function of time and the resonance detuning ω-ω _ba to within a constant factor having the dimensions of a magnetic moment based on the symmetry of an atom in the field of a resonant light pulse and symmetry with respect to time reversal including the initial conditions. The even dependence of the optically induced magnetic moment on ω-ω _ba for an elliptically polarized pulse with an isotropic initial state of the atom and its odd dependence on ω-ω _ba in the case of a linearly polarized pulsed with an anisotropic initial state in the atom’s alignment are shown to be consequences of symmetry with respect to time reversal and the initial conditions. This behavior is retained even after passage of the light pulse, when the resulting relaxation destroys the time reversal symmetry. The optically induced magnetization of an atomic gas is found to have analogous properties. Zh. éksp. Teor. Fiz. 112, 453–469 (August 1997)     

Exchange mechanism for localization of phonons near the surface of a magnetically ordered crystal

It is shown that first-principles inclusion of nonuniform exchange interaction leads to the creation of a new type of generalized shear acoustic wave propagating near the mechanically free surface of a magnet. Criteria are formulated that can be used to specify conditions under which an “exchange” type of surface acoustic wave can exist at the boundary between a magnetic and a nonmagnetic medium once the spectrum of magnetoelastic oscillations of the unbounded magnetic crystal is known. Fiz. Tverd. Tela (St. Petersburg) 40, 299–304 (February 1998)     

Influence of antiferromagnetic ordering on the de Haas-van Alphen effect in a semimetal

The distinctive characteristics of the de Haas-van Alphen effect in semimetals with antiferromagnetic long-range order are investigated theoretically. It is shown that the transition of the subsystem of localized spins from the canted antiferromagnetic phase to the ferromagnetic phase is accompanied by an abrupt change in the “frequency” of the magnetization oscillations of band carriers M _∼. In the below-critical range of magnetic fields, M _∼ is not a function periodic in 1/H. Significantly, the additional contribution to the phase of the oscillatory factors is proportional to H ² and is determined entirely by quantum fluctuations in the antiferromagnetic subsystem. Fiz. Tverd. Tela (St. Petersburg) 39, 204–210 (February 1997)     

Thermally activated transformation of magnetization-reversal modes in ultrathin nanoparticles

The influence of thermal fluctuations on magnetization-reversal processes in ultrathin magnetic particles is investigated on the basis of a numerical solution of the Landau-Lifshitz-Gilbert equations taking account the thermal-activation fluctuation field. It is shown that for nanoparticles there exists a region of magnetic and geometric parameters where a strong jump-like instability of the critical field for magnetization reversal arises. This instability is due to the thermally activated transformation of magnetization configurations far from the switching threshold. The thermal-instability mechanism described is important for particles of much larger sizes than for the single-mode Néel-Brown instability. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 555–560 (25 April 1999)     

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

On the anomalous H dependence of the amplitude of de Haas-van Alphen oscillations in CeCu2Si2

The de Haas-van Alphen effect is studied in heavy-fermion antiferromagnets near the spin-flip transition is studied. It is shown that the strong increase occurring in the amplitude of oscillations near the spin-flip point in an increasing magnetic field, as observed experimentally in CeCu₂Si₂, can be explained by strong single-site correlations and magnetic ordering in the subsystem of localized electronic states. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 270–275 (25 February 1998)     

The de Haas-van Alphen effect in quasi-two-dimensional materials

We calculate the amplitude of magnetization oscillations for a quasi-two-dimensional electron system. In the two-dimensional case the behavior of this amplitude as a function of magnetic field and temperature differ completely from the conventional Lifshitz-Kosevich formula valid for three-dimensional metals. Previously only the ideal two-dimensional case has been considered, and the difference of the shape of the Fermi surface from cylindrical has not been taken into account. We obtain the general formula for the envelope of magnetization oscillations as a function of magnetic field, temperature, and the strength of the warping of the Fermi surface. This problem is important because of the great amount of interest in heterostructures and quasi-two-dimensional organic metals which has arisen in recent years. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 2, 139–144 (25 January 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Direct experimental investigation of spin-reorientation phase transitions in an antiferromagnetic CoNiCu/Cu superlattice

The characteristic features of phase transitions induced by an external magnetic field and of the corresponding changes in the relative orientations of the spins in the ferromagnetic CoNiCu layers of a multilayer film, which are coupled by an antiferromagnetic exchange interaction via nonmagnetic Cu interlayers, are studied using a magnetooptic method for visualizing the fringing fields. It is established that the magnetization reversal process in this nanocomposite material proceeds by a spin-flop orientational phase transition on account of the formation and motion of specific domain walls as well as by incoherent rotation of the spins toward the applied field. It is shown that, depending on the direction of the external magnetic field with respect to the easy axis, asymmetric canted phases also arise as a result of such transitions. Pis’ma Zh. Tekh. Fiz. 64, No. 11, 778–782 (10 December 1996)