Nuclear magnetic resonance in inhomogeneous magnetic fields

The response of the spin system has been investigated by numerical simulations in the case of a nuclear magnetic resonance (NMR) experiment performed in inhomogeneous static and radiofrequency fields. The particular case of the NMR-MOUSE was considered. The static field and the component of the radiofrequency field perpendicular to the static field were evaluated as well as the spatial distribution of the maximum NMR signal detected by the surface coil. The NMR response to various pulse sequences was evaluated numerically for the case of an ensemble of isolated spins (1/2). The behavior of the echo train in Carr-Purcell-like pulse sequences used for measurements of transverse relaxation and self-diffusion was simulated and compared with the experiment. The echo train is shown to behave qualitatively differently depending on the particular phase schemes used in these pulse sequences. Different echo trains are obtained, because of the different superposition of Hahn and stimulated echoes forming mixed echoes as a result of the spatial distribution of pulse flip angles. The superposition of Hahn and stimulated echoes originating from different spatial regions leads to distortions of the mixed echoes in intensity, shape, and phase. The volume selection produced by Carr-Purcell-like pulse sequences is also investigated for the NMR-MOUSE. The developed numerical simulation procedure is useful for understanding a variety of experiments performed with the NMR-MOUSE and for improving its performance. Copyright 2000 Academic Press.     

Radial magnetic field in magnetic confinement device

The intrinsic radial magnetic field(B r) in a tokamak is explored by the solution of the Grad–Shafranov equation in axisymmetric configurations through an expansion of the four terms of the magnetic surfaces. It can be inferred from the simulation results that at the core of the device, the tokamak should possess a three-dimensional magnetic field configuration, which could be reduced to a two-dimensional one when the radial position is greater than 0.6a. The radial magnetic field and the amzimuthal magnetic field have the same order of magnitude at the core of the device. These results can offer a reference for the analysis of the plasma instability, the property of the core plasma, and the magnetic field measurement.     

Giant magnetic moments in dilute magnetic semiconductors

The concentration dependence of the specific magnetic moment value at room temperature in dilute semiconductor titanium oxides doped with either Co or Fe has been investigated. This value was found to increase sharply at small concentrations of magnetic impurity. The magnetic moment of 22.9 μ_B per impurity atom has been revealed for TiO₂ doped with 0.15 at% Co, not yet reported in any semiconductor oxide systems. We conclude the observed giant magnetic moments are caused by the crystal lattice polarization at small impurity concentrations. The comparison with published data point to different types of the magnetization concentration dependence for various semiconductor matrixes that is probably related to the dielectric permittivity of the environment.     

Tunable magnetic relaxation mechanism in magnetic nanoparticles

We investigate theoretically the magnetization dynamics of a conducting magnetic nanoparticle weakly coupled to source and drain electrodes, under the assumption that all relaxation comes from exchange of electrons with the electrodes. In the regime of sequential tunneling, the magnetization dynamics is characterized by a relaxation time t(1), which strongly depends on temperature, bias voltage, and gate voltage. While a direct measure of a nanoparticle magnetization might be difficult, we find that t(1) can be determined through a time resolved transport measurement. For a suitable choice of gate voltage and bias voltage, the magnetization performs a bias-driven Brownian motion regardless of the presence of anisotropy.     

In situ magnetic hysteresis measurement of magnetic tips in magnetic force microscope

In situ magnetic hysteresis measurements of magnetic tips in a magnetic force microscope (MFM) are demonstrated using alternating gradient force magnetometry. The measured magnetic moments of MFM tips are estimated in the range from 10⁻⁶ to 10⁻⁵ emu by this technique and the whole MFM tips in cantilevers are considered to be measured from the value of measured magnetic moments. The relationship between the magnetic hysteresis loops of MFM tips and those of coated magnetic films is discussed.     

Observation of magnetic droplets in magnetic tunnel junctions

Magnetic droplets,a class of highly nonlinear magnetodynamic solitons,can be nucleated and stabilized in nanocontact spintorque nano-oscillators.Here we experimentally demonstrate magnetic droplets in magnetic tunnel junctions(MTJs).The droplet nucleation is accompanied by power enhancement compared with its ferromagnetic resonance modes.The nucleation and stabilization of droplets are ascribed to the double-Co Fe B free-layer structure in the all-perpendicular MTJ,which provides a low Zhang-Li torque and a high pinning field.Our results enable better electrical sensitivity in fundamental studies of droplets and show that the droplets can be utilized in MTJ-based applications and materials science.     

Low-frequency magnetic noise in micron-scale magnetic tunnel junctions

We have observed low-frequency noise due to quasiequilibrium thermal magnetization fluctuations in micron-scale magnetic tunnel junctions (MTJs). This strongly field-dependent magnetic noise occurs within the magnetic hysteresis loops, either as 1/f or Lorentzian (random telegraph) noise. We attribute it to the thermally excited hopping of magnetic domain walls between pinning sites. Our results show that magnetic stability is a crucial factor in reducing the low-frequency noise in small MTJs.     

High magnetic field effects in some metallic magnetic materials

An account is given of the following magnetic properties in high magnetic fields: (1) Magnetic isotherms of pure nickel and Ni-Pd alloys near the Curie points; the exchange splitting energies deduced on this basis are discussed. (2) Metamagnetic transitions in internal fields for the rare earth-cobalt compounds R Co₂. (3) Metamagnetic transitions in external fields for Co(S, Se)₂ and ordered FePt₃.     

Spin-wave contributions to nuclear magnetic relaxation in magnetic metals

The longitudinal and transverse nuclear magnetic relaxation rates 1/T ₁(T) and 1/T ₂(T) are calculated for three- and two-dimensional (3D and 2D) metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic moments in the spin-wave temperature region. The contribution of the one-magnon decay processes is strongly enhanced in comparison with the standard T-linear Korringa term, especially for the FM case. For the 3D AFM case this contribution diverges logarithmically, the divergence being cut at the magnon gap ω due to magnetic anisotropy, and for the 2D AFM case as ω^-1. The electron-magnon scattering processes yield T ²ln(T/ω) and T ²/ω^1/2-terms in 1/T ₁ for the 3D AFM and 2D FM cases, respectively. The two-magnon (“Raman”) contributions are investigated and demonstrated to be large in the 2D FM case. Peculiarities of the isotropic 2D limit (where the correlation length is very large) are analyzed. Received 29 November 1999 and Received in final form 6 June 2000     

Site-directed research of magnetic nanoparticles in magnetic drug targeting

The targeting of ferrofluids composed of 20 nm magnetic particles was studied through simulation and animal experiment. The results showed that some magnetic particles were concentrated in the target area depending on the applied magnetic field. Through theoretical analysis, the retention of the magnetic nanoparticles in a target area is due to large magnetic liquid beads formed by the magnetic field.     

磁性液体在磁场中产生光的双折射效应机理

磁性液体是一种特殊的高分子稳定胶体,在磁场中会产生光的双折射效应,对磁性液体在胶体学科方面展开研究,发现磁性液体在磁场中的弱絮凝行为表现异常明显,显示出特有的方向性,且又不至胶体系统失稳,证明了磁性液体中的磁性微粒在磁场中聚集成方向性的链状而又不失稳的临界状态存在。从而揭示了方向性弱絮凝是磁性液体在磁场中产生光的双折射效应的机理。     

Nonlinear surface magnetic polaritons in a nonuniform magnetic field

High-frequency surface magnetic polaritons of finite amplitude propagating along the interface between a ferrite and a nonlinear insulator in a weakly nonuniform, shaft-shaped external magnetic field are investigated theoretically. The analysis is based on employment of the variational method together with bilinear relations having the form of Lorentz’s lemma. It is shown that the wave dispersion and the transverse profile of a wave along the field nonuniformity depend significantly on the amplitude of the wave. Zh. Tekh. Fiz. 68, 92–95 (September 1998)     

Resonance double magnetic bremsstrahlung in a strong magnetic field

The possibility of resonance double magnetic bremsstrahlung in the approximation of weakly excited electron states in a strong external magnetic field is analyzed. The differential probability of this process in the Breit-Wigner form is obtained. The probability of double magnetic bremsstrahlung (second-order process of perturbation theory) is compared with the probability of magnetic bremsstrahlung (first-order process of perturbation theory).     

Trapping energy of a magnetic monopole in magnetic materials

Analytical methods to investigate the interaction of magnetic monopoles with known magnetic media have been developed. Trapping energies of monopoles inside ferro-magnetic or super onducting materials of size greater than about 10⁻⁶ cm are found to be of the order of several kiloelectron volts. These are two to three orders of magnitude higher than in paramagnetic materials. Thus if stable magnetic monopoles exist at all in the universe, they are perhaps trapped in these magnetic materials. The effect of the finite size of the magnetic bodies is taken into account explicitly in our calculations of the trapping energy.     

Multi-dimensional magnetic resonance imaging in a stray magnetic field

Stray field imaging has been extensively utilized in the last 10 years to perform very high resolution imaging of samples in a single dimension using the massive field gradient present in the fringe of a superconducting magnet. By spinning the sample around the magic-angle, the stray field gradient is successively reoriented along three orthogonal directions in the sample reference frame, allowing the acquisition of a full three-dimensional Fourier image, thereby providing the possibility to perform multi-dimensional very high-resolution imaging with standard nuclear magnetic resonance spectroscopy equipment. Here, we show multi-dimensional images demonstrating the feasibility of this technique.     

Free oscillations of magnetic fluid in strong magnetic field

The paper presents the esults of measuring the elastic parameters of an oscillatory system (coefficient of pondermotive elasticity, damping factor, and oscillation frequency) whose viscous inertial element is represented by a magnetic fluid confined in a tube by magnetic levitation in a strong magnetic field. The role of elasticity is played by the pondermotive force acting on thin layers at the upper and lower ends of the fluid column. It is shown that, by measuring the elastic oscillation frequencies of the magnetic fluid column, it is possible to develop a fundamentally new absolute method for determining the saturation magnetization of a magnetic colloid.     

Spin-glass-like magnetic ordering in Zn substituted magnetite magnetic fluids

Electron spin resonance (ESR) spectra of magnetic fluids involving polydispersed Zn(0.5)Fe(0. 5)Fe(2)O(4) (FZ5) and Zn(0.7)Fe(0. 3)Fe(2)O(4) (FZ7) nanomagnetic particles are scanned from 4.2 to 300K. The FZ7 fluid exhibits certain distinct features below 40K which are different from FZ5 fluid. These include (i) an isotropic shift in resonance field in zero-field-cooled ESR study, (ii) deviation of resonance field from sin(2)theta behavior (where theta is the angle between axis of the particle and field) in field cooled (FC) sample and (iii) abrupt increase in anisotropy field for FC sample. The results are analyzed in light of the core-shell model for nanomagnetic particles.     

Role of magnetic circular dichroism in all-optical magnetic recording

Using magneto-optical microscopy in combination with ellipsometry measurements, we show that all-optical switching with polarized femtosecond laser pulses in ferrimagnetic GdFeCo is subjected to a threshold fluence absorbed in the magnetic layer, independent of either the excitation wavelength or the polarization of the laser pulse. Furthermore, we present a quantitative explanation of the intensity window in which all-optical helicity-dependent switching (AO-HDS) occurs, based on magnetic circular dichroism. This explanation is consistent with all the experimental findings on AO-HDS so far, varying from single- to multiple-shot experiments. The presented results give a solid understanding of the origin of AO-HDS, and give novel insights into the physics of ultrafast, laser controlled magnetism.