NMR spectra under skin-effect conditions at ultralow temperatures

An investigation based on the coupled Maxwell-Bloch equations for a system of equivalent exchange-coupled spins is performed in order to explain a number of features of NMR spectra obtained in metals by Fourier-transforming of the free-induction decay at ultralow temperatures. Small angles of tilting of the nuclear magnetization by the exciting rf field are considered. It is shown that the free precession inherits the nonuniformity in the distribution of the rf field and the magnetization produced at the excitation stage inside the sample on account of the skin effect. As a result, the NMR spectrum is found to consist of a set of peaks—signals due to standing spin waves. However, such a spectrum can be observed only when the detuning of the exciting rf field is sufficiently large relative to the Larmor frequency of the spins. Otherwise, the rf field does not penetrate into the sample because of strong absorption by the spins. If the detuning is large, the dispersion signal and part of the NMR absorption signal are proportional to the equilibrium magnetization to the power 3/2. Such behavior is expected at low temperatures so that the coupling of the magnetization with the rf field is strong. The results obtained qualitatively explain the experimentally observed characteristics of the NMR spectra: the presence of kinks and structure of the NMR lines, the dependence of the shape and intensity of the spectrum on the detuning of the exciting rf field, and the nonlinear dependence of the nuclear susceptibility on the reciprocal of the sample temperature. Zh. éksp. Teor. Fiz. 114, 1836–1847 (November 1998)     

Effective and accurate single-shot NMR diffusion experiments based on magnetization grating

Current single-shot diffusion methods based on magnetization gratings suffer from low sensitivity due to small rf tip angles and, consequently, from inefficient use of the total equilibrium magnetization. Here, we propose and illustrate the use of a slightly modified form of the magnetization encoding scheme OUFIS for single-shot diffusion experiments. In a detailed theoretical and experimental analysis, we compare the performance of the proposed method to other encoding schemes such as the one-phase or two-phase DANTE and conclude that the OUFIS-based experiment is a superior one. The primary reason is that this scheme allows one to use a larger total pulse area. Hence, one can encode a far larger portion of the initial magnetization into a frequency grating before the onset of various nonlinear effects. In the experimental illustration, we present a single-shot measurement of multicomponent diffusion.     

Magnetically induced spin-rotation transition in iron-garnet films with canted-phase anisotropy

Epitaxial iron-garnet films exhibiting canted-phase and easy-plane anisotropies, with crystallographic (111) surface orientation are investigated by magnetooptical and inductive frequency methods. It is shown that four types of domain structure exist in the films. When the magnetization of the films is reversed by a static magnetic field oriented at various angles to the normal to the plane, anomalous behavior of the magnetic susceptibility is observed in the interval of magnetic-field orientation angles from 0.3° to 1.5°. Fiz. Tverd. Tela (St. Petersburg) 39, 1415–1420 (August 1997)     

Reversible magnetization of a thin superconducting film with radiation defects

The thermodynamic potential of a system of Peierls vortices in a thin superconducting film, containing radiation defects, in a perpendicular external magnetic field is calculated. The equilibrium temperature dependences of the densities of free vortices and vortices trapped by defects are found in the mean-field approximation for various magnitudes of the external field. It is shown that the equilibrium magnetization of a thin superconducting film exhibits the same features that were observed experimentally in the reversible magnetization of high-temperature superconductors. An asymptotic expression is obtained for the difference of the magnetizations of perfect and irradiated films. According to this expression, the difference depends on the pinning energy of a vortex on a defect and the density of defects.     

Dynamical regimes of conversion of magnetogyrotropic waveguide modes in the region of ferromagnetic resonance

The dynamical conversion of magnetogyrotropic waveguide modes in the regime of uniform spin precession at large angles is studied. It is shown that the nutation of the magnetization vector due to the precession frequency doubling results in dependence of the light modulation amplitude on the polarization of the microwave field. The contribution of harmonics with frequencies that are multiples of the fundamental precession frequency of ferromagnetic resonance to the total mode-conversion efficiency is analyzed for different lengths of the waveguide and different equilibrium orientations of the magnetization. State University, Ulianovsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 5, pp. 651–663, May, 1998.     

Realignment capability of the nCPMG sequence

The nCPMG sequence is based on a particular phase modulation of the refocusing pulse train, and was originally designed for rendering the spin echo amplitude insensitive to the initial magnetization phase. This pulse sequence has the peculiarity of being easily invertible, which enables perfect driven equilibrium experiments, in the absence of relaxation. This magnetization 'realignment' is effective for all three components. Hence the overall operation is transparent. Supporting theory is presented here, together with the first direct experimental proof of the claim. The experiment shows that, with the present stabilization sequence, perfect realignment is indeed made possible for a range of refocusing pulse nutation angles from 130° to 230°.     

Long-range ferromagnetism of Mn12 acetate single-molecule magnets under a transverse magnetic field

We use neutron diffraction to probe the magnetization components of a crystal of Mn12 single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the crystallographic c axis. The application of a magnetic field H(perpendicular) perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. For T approximately < 0.9(1) K, this equilibrium state shows spontaneous magnetization, indicating the onset of ferromagnetism. These long-range magnetic correlations nearly disappear for mu0H(perpendicular) approximately > 5.5 T, possibly suggesting the existence of a quantum critical point.     

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.     

Hydrodynamic modes in a gas of magnons

Density waves analogous to second sound are studied in a gas of magnons. Quasiparticle interaction is considered for both equilibrium and non equilibrium thermodynamics. The non equilibrium theory is based on a Boltzmann equation for magnon-magnon scattering. Contrary to the total energy and magnetization, (quasi)-momentum is not strictly conserved. In the hydrodynamic regime, the transport equation is reduced to a set of two coupled equations for the magnetization and the local temperature. For low temperatures these have diffusive and propagating solutions while for high temperatures, where momentum is dissipated by Umklapp processes, the solutions are only diffusive. The magnetization response function and the corresponding spectral function are discussed for various wavenumbers and temperatures.     

Movement of liquid gallium dispersing low concentration of temperature sensitive magnetic particles under magnetic field

In this work, a temperature sensitive functional fluid was synthesized, and then its movement under the influence of magnetic field was investigated. Silica coated FeNbVB particles, prepared by chemical synthesis, were dispersed into liquid gallium, because they have a relatively high magnetization and a high temperature dependency. The synthesized functional fluid (solid fraction of 0.3 mass%) showed temperature dependence for magnetization within the testing temperature range between 298 and 353 K. The movement of gallium based fluid under the influence of the magnetic field with a magnetic field gradient was observed at various temperatures. We found that at 318 K, fluid displacement of the synthesized functional fluid is better when compared with the fluid displacement at 348 K.     

On the observation of negative magnetization under zero-field-cooled process

We have addressed problems associated with the measurement of zero-field-cooled (ZFC) magnetization and its interpretation in the light of negative magnetization reported in certain ferrimagnetic materials such as CoCr₂O₄. We demonstrate that a small negative trapped field in the sample space as well as large coercive fields are responsible for the observed negative magnetization. The problem is commonly encountered while working with magnetometers and a superconducting magnet where the sign of the trapped field can be positive or negative depending on the way the field is reduced to zero.     

Magnetization plateaus of the frustrated Ising Shastry–Sutherland system: Wang–Landau simulation

The Wang–Landau algorithm is used to study the magnetic properties of the Ising model on the Shastry–Sutherland lattice in order to understand the interesting magnetization plateaus observed in TmB₄. The simulated results demonstrate that the equilibrium state of the model produces only the 1/3 and 1/2 magnetization plateaus at low temperatures even when the random-exchange term or the long-range interactions are taken into account. This confirms our earlier conclusion (Huang et al., 2013) [20] that those fractional plateaus observed in experiments may be due to the magnetization dynamics.     

Temperature dependence of nuclear magnetization and relaxation

The temperature dependences of nuclear magnetization and relaxation rates are reviewed theoretically and experimentally in order to quantify the effects of temperature on NMR signals acquired by common imaging techniques. Using common sequences, the temperature dependences of the equilibrium nuclear magnetization and relaxation times must each be considered to fully understand the effects of temperature on NMR images. The temperature dependence of the equilibrium nuclear magnetization is negative because of Boltzmann's distribution for all substances at all temperatures, but the combined temperature dependences of the equilibrium magnetization and relaxation can be negative, weak or positive depending on the temperature (T), echo time (T(E)), repetition time (T(R)), and the temperature dependences of the relaxation times T(1)(T) and T(2)(T) in a pulse sequence. As a result, the magnitude of the NMR signal from a given substance can decrease, increase or stay somewhat constant with increasing temperature. Nuclear thermal coefficients are defined and predictions for spin echo and other simple sequences are verified experimentally using a number of substances representing various thermal and NMR properties.     

Magnetization of ballistic quantum dots induced by a linear-polarized microwave field

On a basis of extensive analytical and numerical studies we show that a linear-polarized microwave field creates a stationary magnetization in mesoscopic ballistic quantum dots with two-dimensional electron gas being at a thermal equilibrium. The magnetization is proportional to a number of electrons in a dot and to a microwave power. Microwave fields of moderate strength create in a one dot of few micron size a magnetization which is by few orders of magnitude larger than a magnetization produced by persistent currents. The effect is weakly dependent on temperature and can be observed with existing experimental techniques. The parallels between this effect and ratchets in asymmetric nanostructures are also discussed.     

Master Equation in Phase Space for a Uniaxial Spin System

A master equation, for the time evolution of the quasi-probability density function of spin orientations in the phase space representation of the polar and azimuthal angles is derived for a uniaxial spin system subject to a magnetic field parallel to the axis of symmetry. This equation is obtained from the reduced density matrix evolution equation (assuming that the spin-bath coupling is weak and that the correlation time of the bath is so short that the stochastic process resulting from it is Markovian) by expressing it in terms of the inverse Wigner-Stratonovich transformation and evaluating the various commutators via the properties of polarization operators and spherical harmonics. The properties of this phase space master equation, resembling the Fokker-Planck equation, are investigated, leading to a finite series (in terms of the spherical harmonics) for its stationary solution, which is the equilibrium quasi-probability density function of spin “orientations” corresponding to the canonical density matrix and which may be expressed in closed form for a given spin number. Moreover, in the large spin limit, the master equation transforms to the classical Fokker-Planck equation describing the magnetization dynamics of a uniaxial paramagnet.     

Isotropic vector Preisach particle

A Preisach-type pseudoparticle is proposed, to describe macroscopic vector magnetic behaviour of isotropic materials. The magnitude of magnetization is computed by means of the classical Preisach model with the projection of field strength on its direction as input. The orientation of the magnetization vector is governed by the equilibrium between its vector product with the field strength and a friction-like opposing torque proportional to the square of its magnitude. The behaviour of the model is studied in various conditions and the results compared to experiments reported in literature.     

Large planar Hall effect in perpendicularly magnetized W/CoFeB/MgO structures

The planar Hall effect (PHE) in W/CoFeB/MgO structure with perpendicular magnetic anisotropy was investigated as a function of CoFeB thickness (t_CoFeB). The PHE is measured by sweeping the in-plane magnetic field at various azimuthal angles as well as by rotating strong magnetic field which is enough to saturate the magnetization. We observed a huge PHE in the W/CoFeB/MgO sample, which is even larger than anomalous Hall effect (AHE). This is distinct from the results in Ta/CoFeB/MgO samples showing a much smaller PHE than AHE. Since the PHE is insensitive to the t_CoFeB while the AHE is proportional to the t_CoFeB, the unprecedented PHE can be attributed to the W layer with a large spin-orbit coupling.     

Increase in the Saturation Magnetization in a Fe<Subscript>3</Subscript>Al Superstructure under Large Plastic Deformations

The effect of large plastic deformations at room temperature in a Bridgman anvil cell on the specific saturation magnetization of the DO₃-type ordered (Fe–24 at % Al) alloy has been revealed. It has been found that torsion at high pressure and certain deformation parameters results in the complete suppression of long-range order in the alloy and in the corresponding increase in the specific saturation magnetization by 11% with respect to the equilibrium state. A theoretical model qualitatively describing the revealed phenomenon has been proposed.     

Equilibrium states and quasi-static magnetization switching of a multilayer structure

The equilibrium orientations of magnetic moments that correspond to various values and directions of the biasing field are found in a set of magnetic films with cubic crystalline anisotropy and uniaxial induced anisotropy. The films are coupled by exchange interaction of the antiferromagnetic type. Field intervals are established where noncollinear and bistability states causing orientational phase transitions and hysteresis exist. Ninety degree magnetization switching (per switching cycle) of the magnetic moments of the films, as well as an orientational phase transition of bifurcation character, is discovered. Hysteresis loops for 180° in-plane magnetization switching are constructed.     

Observation of fractional harmonics in the NMR signal from superfluid 3He-B

Unusual spin-precession states of ³He-B in which the magnetization is half the equilibrium value are investigated by continuous-wave NMR methods. Signals at frequencies equal to 1/2 and 3/2 of the magnetization precession frequency are observed in two such states. Such signals exist because the order parameter of superfluid ³He in these states precesses with frequency equal to half the magnetization precession frequency. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 200–205 (10 February 1999)