Stochastic resonance in low-disperse magnets: Mechanism of subbarrier magnetization reversal

The phenomenon of stochastic resonance is studied theoretically in a system of single-domain particles with an “easy-axis”-type magnetic anisotropy at temperatures close to absolute zero, for which the tunneling of the magnetic moment vector between stable states dominates. Calculations are made on the basis of the master equation in the model of discrete orientations in the quasi-adiabatic approximation. The dynamic magnetic susceptibility components of the system under the action of a weak rf field are calculated. The critical temperature at which a transition from the above-the-barrier mechanism of magnetization reversal to the subbarrier mechanism occurs is estimated.     

Noise suppression in an ensemble of easy-axis superparamagnetic particles caused by radio-frequency-field modulation

Stochastic resonance is investigated theoretically in a thermally activated system of small magnetic particles with easy-axis anisotropy. Calculations are performed within a discrete-orientation model using a quasi-adiabatic approximation. The output power of thermal noise of superparamagnetic particles is calculated and the magnitude of the noise suppression effect in the presence of a radio-frequency field is estimated at different temperatures. Estimates for actual iron samples are made with allowance for a spread in the particle sizes and easy-axis orientations.     

Magnetic phase transitions in Nd1 ? xDyxFe3(BO3)4 ferroborates

The magnetic properties of ferroborate single crystals with substituted compositions Nd_{1 − x} Dy _x Fe₃(BO₃)₄ (x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization curves for the spin-flop transition induced by a magnetic field B | c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B | c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions.     

Stochastic Resonance in Finely Dispersed Magnetics: a Comparison of Discrete and Continuous Models

The phenomenon of a stochastic resonance in a system of single-domain particles with easy-axis magnetic anisotropy is treated theoretically for the thermally activated system. The results of calculations for the discrete model based on the control equation for the Kramers transition rates of the magnetic moment vector and for the continuous model based on numerical solution of the Fokker–Planck equation with a periodic drift term are analyzed. The phase shifts between input and output signals and the values of the signal-to-noise ratio calculated for iron superparamagnetic particles in the context of these two models are compared.     

Effect of noncrystallographic magnetic anisotropy on magnetization reversal in multilayer magnetic films

A study has been carried out of the magnetization of a layered system consisting of two exchange-coupled easy-axis ferromagnetic layers separated by a nonmagnetic spacer and having mutually perpendicular anisotropy axes. It is shown that the magnetization of such a system can undergo stepwise decrease with increasing magnetic field. The field dependence of the magnetization has a bifurcation point. The magnetization orientation can exhibit jumps in excess of 90° depending on the angle of magnetic-field orientation. Fiz. Tverd. Tela (St. Petersburg) 41, 660–664 (April 1999)     

Magnetic-field-induced singularity in the tunneling current through an InAs quantum dot

The tunneling transport through a GaAs/(AlGa)As/GaAs single-barrier heterostructure with self-assembled InAs quantum dots is studied experimentally at low temperatures. An anomalous increase in the tunneling current through the quantum dots is observed in magnetic fields both parallel and perpendicular to the current. This result cannot be understood in the framework of the single-electron approximation. The proposed explanation of the phenomenon is based on the modified Matveev-Larkin theory, which predicts the appearance of a singularity in the tunneling current through the zero-dimensional state in a magnetic field because of the interaction between the tunneling electron and the spin-polarized three-dimensional electron gas in the emitter. The absence of spin splitting in the experimental resonance peaks is caused by the complete spin polarization of the emitter in relatively weak magnetic fields.     

Magnetic properties of magnetoactive spin clusters

A simple model is proposed for describing magnetic properties of magnetoactive nanoclusters, which permits exact analytic solution. Exact expressions are obtained for thermodynamic characteristics of the model, which hold in the entire range of temperatures, magnetic fields, and interaction parameters. It is found that in the case of easy-axis anisotropy, the field dependence of magnetization of a nanocluster consisting of N particles with a spin of 1/2 has [N/2] fractional plateaus ([…] is the integer part) corresponding to polarized phases with ruptures singlet pairs. A nonmonotonic behavior observed for the magnetic susceptibility of an easy-plane cluster is typical of gap magnets. The spin gap between the ground state and excited states is proportional to the anisotropy parameter.     

Superradiance by a system of highly polarized exchange-coupled nonequivalent spins

We propose a model of radiofrequency (rf) superradiance by a system of interacting nonequivalent spins in a point specimen. In contrast to the rf superradiance observed and described earlier, here spin-spin coupling acts as the interaction with the cavity. To be definite, we examine the spins of two isotopes of a metal that are coupled by the Ruderman-Kittel interaction. The analysis of such a system when the magnetization of one spin species is inverted shows that the system can have one resonance frequency and two different decay times, instead of two resonance frequencies and one decay time in the usual situation. When such “repulsion” of decay times occurs and the absolute values of the spin polarizations are large, transverse magnetization increases and exhibits features characteristic of superradiance. Finally, we calculate the parameters of this superradiance: the voltage across the terminals of an rf pickup coil, the pulse length, the delay time, and the superradiant intensity. Zh. éksp. Teor. Fiz. 112, 551–563 (August 1997)     

Dependence of the Dynamic Susceptibility of Superparamagnetic Particles on the Strength of a Constant Magnetic Field Perpendicular to the Direction of Particle Easy Magnetization

The dynamic magnetic susceptibility of single-domain iron particles with magnetic anisotropy in the form of the direction of easy magnetization is calculated under conditions of stochastic resonance with allowance for a constant magnetic field perpendicular to the direction of particle easy magnetization. Calculations are carried out in the approximation of discrete orientations based on the guiding equation for the Kramers above-the-barrier transition rates of the particle magnetic moment vector. It is demonstrated that changing the constant field strength, the internal noise level in the system can be controlled, for example, to obtain a maximum response to an external radio-frequency perturbation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 64–68, May, 2005.     

Electron spin resonance studies on quantum tunneling in spinel ferrite nanoparticles

The electron spin resonance (ESR) spectrometer, a very sensitive instrument with fast detecting window to explore quantum phase transitions for magnetic nanoparticles, was exploited to study the fascinating interplay between thermal and quantum fluctuations in the vicinity of a quantum critical point. We have measured ESR in ferrofluid samples containing nanosize particles of Fe₂O₃. The evolution of the ESR spectrum with temperature suggests that quantum tunneling of spins occurs in single domain magnetic particles in the low temperature regime. The effects of various microwave fields, particle sizes, and temperatures on the magnetic states of single domain spinel ferrite nanoparticles are investigated. We can consistently explain experimental data assuming that, as the temperature decreases, the spectrum changes from superparamagnetic (SPR) to blocked SPR and finally evolves quantum superparamagnetic behaviour as the temperature lowers down further. A nanoparticle system of a highly anisotropic magnetic material can be qualitatively specified by a simple quantum spin model, or by the Heisenberg model with strong easy-plane anisotropy.Received: 29 August 2003, Published online: 15 October 2003PACS: 76.30.-v Electron paramagnetic resonance and relaxation - 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.) - 05.30.-d Quantum statistical mechanics - 75.50.Dd Nonmetallic ferromagnetic materials     

Simple theory of superparamagnetism and spin-tunneling in Mössbauer spectroscopy

The magnetic relaxation of isolated small (< 100 Å) monodomain magnetic particles is due to superparamagnetic relaxation (predominant at high temperatures) and eventually quantum tunneling of the magnetic moment (at low temperatures). The superparamagnetic relaxation process can be formally described by an (multiple phonon absorption and emission) Orbach process with an anisotropy Hamiltonian due to crystalline or form anisotropy \widehat_Ion = S² _z and a usual dynamical spin-Hamiltonian for the spin--phonon interaction. From this Mössbauer spectra can be calculated using ab-initio or stochastic methods. Phonon-assisted tunneling and its influence on Mössbauer spectra are discussed.     

外磁场中单畴反铁磁颗粒的宏观量子效应

用瞬子方法研究了外加磁场对单畴双轴反铁磁颗粒宏观量子效应的影响. 当外磁场沿易磁化方向时,简并基态中的一个能量抬高,变为亚稳态,其隧穿衰变率随外磁场增大;当外磁场沿中间磁化方向时,能级的隧穿劈裂随外磁场的变化而振荡.从而提出一种观察单畴反铁磁颗粒中宏观量子相干和隧穿现象的实验方法.     

Angular dependence of shot noise in the presence of Rashba spin–orbit coupling in semiconductor spintronics junctions

We investigate spin-dependent current and shot noise, taking into account the Rashba spin–orbit coupling (RSOC) effect in double diluted magnetic semiconductor (DMS) barrier resonant tunneling diodes. The calculation is based on an effective mass approach. The magnetization of DMS is calculated by the mean-field approximation in low magnetic field. The spin-splitting of DMS depends on the sp–d exchange interaction. We also examine the dependence of transport properties of CdTe/CdMnTe heterostructures on applied voltage and relative angle between the magnetization of two DMS layers. It is found that the RSOC has great different influence on the transport properties of tunneling electrons with spin-up and spin-down, which have different contributions to the current and the shot noise. Also, we can see that the RSOC enhances the spin polarization of the system, which makes the nanostructure a good candidate for new spin filter devices. Thus, these numerical results may shed light on the next applications of quantum multilayer systems and make them a good choice for future spintronics devices.     

Possibility of observing magnetic macroscopic quantum tunneling using a scanning tunneling microscope

A theory describing the detection of magnetic macroscopic quantum tunneling on the basis of current fluctuations in a scanning tunneling microscope is proposed. Zh. éksp. Teor. Fiz. 112, 1011–1020 (September 1997)     

Magnetic properties of solidified ferrocolloids

Experimental and theoretical investigations are made of the magnetization of solidified dielectric colloids prepared using single-domain magnetite particles with randomly oriented axes of easy magnetization. A method is proposed to calculate the magnetization curves with allowance for blocking of the magnetic moments of particles and magnetic-dipole interparticle interactions. It is shown that magnetic colloids with an extremely low remanent magnetization (a few tens of amps per meter) may be obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 1062–1067 (June 1998)     

Mesoscopic mixing of spin orientation phases in particles with Ising ions: Holmium: Yttrium iron garnets in strong magnetic fields

The paper presents a theoretical analysis of macroscopic quantum tunneling phenomena in small particles of a cubic ferromagnet of the Ho_xY_3?xFe₅O₁₂ type with strongly anisotropic (Ising) impurity ions present in a low concentration x ? 1 in the region of strong magnetic fields, at which many orientation phase transitions related to the competition of external and exchange field actions on the spin subsystem are observed. The theory of path integrals for the magnetic subsystem was used to calculate the instanton contributions to interphase tunneling amplitudes in the vicinity of first-order transitions for three principal orientations of an external magnetic field in a cubic crystal. It was shown that low-energy barriers separating angular phases could result in anomalously large mesoscopic volumes at which macroscopic spin tunneling effects could appear in the energy spectra of particles. The special features of spectral splitting caused by the mixing of azimuthally degenerate angular phases and phases with different polar angles of magnetization orientation were revealed.     

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.     

铁磁/半导体/铁磁结构的双量子环自旋输运的特性

研究了与铁磁/半导体/铁磁结构相关的双量子环自旋输运的规律,研究结果表明：总磁通为零条件下,铁磁电极磁化方向反平行时,双量子环与单量子环相比提高了自旋电子透射概率的平均值.铁磁电极磁化方向平行时,双量子环对提高自旋向下电子平均透射概率的效果更明显;双量子环受到Rashba自旋轨道耦合作用影响时,自旋电子的平均透射概率明显高于单量子环,即使再加上外加磁场的影响,透射概率较高这一特征依然存在;双量子环所含的δ势垒具有阻碍自旋电子输运的作用,随δ势垒强度Z的增大透射概率 关键词： 双量子环 Rashba自旋轨道耦合 透射概率 δ势垒')" href="#">δ势垒     

Competition between quantum spin tunneling and Kondo effect

Quantum spin tunneling and Kondo effect are two very different quantum phenomena that produce the same effect on quantized spins, namely, the quenching of their magnetization. However, the nature of this quenching is very different so that quantum spin tunneling and Kondo effect compete with each other. Importantly, both quantum spin tunneling and Kondo effect produce very characteristic features in the spectral function that can be measured by means of single spin scanning tunneling spectroscopy and allows to probe the crossover from one regime to the other. We model this crossover, and the resulting changes in transport, using a non-perturbative treatment of a generalized Anderson model including magnetic anisotropy that leads to quantum spin tunneling. We predict that, at zero magnetic field, integer spins can feature a split-Kondo peak driven by quantum spin tunneling.     

Two-dimensional electron gas in double quantum wells with tilted bands

When a voltage is applied to double quantum wells based on AlGaAs/GaAs heterostructures with contact regions (n-i-n structures), a two-dimensional (2D) electron gas appears in one of the quantum wells. Under illumination which generates electron-hole pairs, the photoexcited holes become localized in a neighboring quantum well and recombine radiatively with the 2D electrons (tunneling recombination through the barrier). The appearance, ground-state energy, and density of the degenerate 2D electron gas are determined from the structure of the Landau levels in the luminescence and luminescence excitation spectra as well as from the oscillations of the radiative recombination intensity in a magnetic field with detection directly at the Fermi level. The electron density is regulated by the voltage between the contact regions and increases with the slope of the bands. For a fixed slope of the bands the 2D-electron density has an upper limit determined by the resonance tunneling of electrons into a neighboring quantum well and subsequent direct recombination with photoexcited holes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 840–845 (10 June 1997)