Studying the internal structure of granular magnetic nanocomposites by ferromagnetic resonance

A method for estimating the form of magnetic nanoparticles in composite film structures based on the observation of ferromagnetic resonance phenomenon is offered. Within the model of the effective medium, an explanation is given for experimentally observed concentration and temperature dependences of resonant fields for composite nanosystem (Co₄₅Fe₄₅Z₁₀) _f +(Al₂O₃)_100−f .     

Nuclear magnetic resonance in ferromagnetic terbium

The temperature dependence of the magnetic dipole and electric quadrupole hyperfine fields has been measured by NMR in ferromagnetic terbium metal. The results are in excellent agreement with a general theoretical relation connecting moments of the magnetization. This relation allows us to separate ionic and crystalline contributions to the nuclear electric field gradient.     

Local ferromagnetic resonance imaging with magnetic resonance force microscopy

We report nanoscale scanned probe ferromagnetic resonance force microscopy (FMRFM) imaging of individual ferromagnetic microstructures. This reveals the mechanism for high spatial resolution in FMRFM imaging: the strongly inhomogeneous local magnetic field of the cantilever mounted micromagnetic probe magnet used in FMRFM enables selective, local excitation of ferromagnetic resonance (FMR). This approach, demonstrated here in individual permalloy disks, is straightforwardly extended to excitation of localized FMR modes, and hence imaging in extended films.     

铁磁共振法测磁各向异性

采用铁磁共振方法,研究了铁磁/反铁磁双层膜系统中,因交换耦合以及磁晶各向异性而产生的有效各向异性场.结果表明:被测系统有无交换偏置场以及其正负号性质等均能在共振谱中得到辨析.结果还显示:沿着不同结晶方向施加外磁场,共振场的行为与磁晶各向异性以及铁磁/反铁磁交换耦合作用而诱发的单向各向异性等密切相关.将共振频率的变化看成外磁场(包括其方向和大小)的函数,研究得到了单向各向异性,立方各向异性等对共振频率的影响,并同实验结果做了很好的比较. 关键词： 铁磁/反铁磁双层膜 交换耦合 铁磁共振 单向各向异性     

Stray field magnetic resonance tomography using ferromagnetic spheres

The methodology for obtaining two- and three-dimensional magnetic resonance images by using azimuthally symmetric dipolar magnetic fields from ferromagnetic spheres is described. We utilize the symmetric property of a geometric sphere in the presence of a large externally applied magnetic field to demonstrate that a complete two- or three-dimensional structured rendering of a sample can be obtained without the motion of the sample relative to the sphere. Sequential positioning of the integrated sample-sphere system in an external magnetic field at various angular orientations provides all the required imaging slices for successful computerized tomographic image reconstruction. The elimination of the requirement to scan the sample relative to the ferromagnetic tip in this imaging protocol is a potentially valuable simplification compared to previous scanning probe magnetic resonance imaging proposals.     

Ground states for lattices of ferromagnetic granules with dipolar magnetic interaction

The magnetic phase diagrams of 2D and 3D regular lattices formed by nonspherical single-domain ferromagnetic granules featuring a dipolar magnetic interaction are studied. The energy of a magnetic state of such systems is calculated using an approximate expression for the pair interaction of nonspherical granules. The character of the magnetic ground state of the system is determined by three geometric parameters: (i) the eccentricity of granules; (ii) the ratio of periods of the rectangular (2D) or tetragonal (3D) lattice; and (iii) the ratio of a lattice period to a granule size. In contrast to the case of lattices formed by point (or spherical) magnetic moments, in which the ground state is always antiferromagnetic or frustrated (for triangular lattices), the ground state of a 2D lattice composed of nonspherical granules can be ferromagnetic. The magnetic phase diagrams of the systems studied are constructed in the space of the above geometric parameters.     

Unidirectional magnetic interaction in two-layer antiferromagnetic - ferromagnetic films

A study has been made of the directional stability of the unidirectional anisotropy in two-layer films of manganese and Permalloy 82 with respect to temperature and magnetic fields up to 30 kOe. The unidirectional interaction rotates into the direction of the applied magnetic field only after the antiferromagnetic layer is heated above the Neel temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 6, pp. 16–20, June, 1969.In conclusion the authors thank A. A. Glazer and R. I. Tagirov for furnishing us the opportunity to become acquainted with the procedure for producing films having such interesting magnetic properties.     

Active ferromagnetic shimming of the permanent magnet for magnetic resonance imaging scanner

This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging (MRI) magnet.It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for MRI applications.An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications.This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards.Simulation and experimental results verify that the method is valid and efficient.     

Nonlinear effects of magnetization precession near ferromagnetic resonance

The features of precession of the magnetic moment of a film near ferromagnetic resonance are investigated which are due to the magnetic-moment nutation in the effective field and to the frequency-doubling effect. The contribution to this precession from harmonics with frequencies which are multiples of the basic resonance frequency is analyzed for a garnet ferrite film of the (111) type in a perpendicular bias magnetic field.     

Tilted spin torque-driven ferromagnetic resonance in a perpendicular-analyzer magnetic trilayer

A theoretical study is presented on the current-driven ferromagnetic resonance in the magnetic trilayers. On the basis of the Landau-Lifshitz-Gilbert-Slonczewski equation, we derive the output dc voltage for arbitrary anisotropy in the free and pinned layers by the linearization method. As an example, the resonance spectra of the tilted-polarizer and perpendicular-analyzer trilayer show that the equilibrium position, the resonant linewidth and the resonant location can be tuned by changing the magnitude and the direction of spin torque. The effective damping can be minimized through adjusting the current and the pinned-layer magnetization direction.     

Influence of inhomogeneities in magnetic crystal on ferromagnetic resonance line shape

The general theory of the preceding paper is applied to a concrete case. The spin wave interaction operator, ^s , is derived by quantizing a phenomenological expression, which expresses the contribution to the energy of the magnetic sample, caused by fluctuations in the anisotropic and exchange interaction. The influence of thermally excited spin waves on the absorption of energy from a high-frequency field and the justification for the application of the general results of the preceding paper are discussed on the basis of the form of the operator ^s . The interaction operator of the magnetic sample with the h-f field is also determined, and it is used to give a definite form to the expression for the resonance curve of the imaginary part of the susceptibility '. Using the matrix elements of the operator ^s we calculate the quantitiy, which formally corresponds to the width of the resonance -curve. The magnitude of is estimated numerically and our results are compared with those obtained by A. M. Clogston et al. and E. Schlömann for the line width in monocrystals and the shape of the resonance curve in polycrystals respectively. The expression obtained on the basis of the theory of the preceding paper for the frequency shift diverges in our case. The way in which to avoid this divergence is indicated.

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. , ^s , , , . ^s , , . , . ^s , . . . , , . .

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The author would like to thank S. Krupika for advice and remarks during the work, J. Duek, V. Syneek, and J. ternberk for valuable discussions and F. Zounová for help in the numerical calculations.     

Electron magnetic resonance investigation of gadolinium diffusion in zircon powders

The electron magnetic resonance (EMR) technique was used to investigate the diffusion of gadolinium in zircon (ZrSiO₄) powders. The EMR absorption intensity was measured for several annealing times and three different temperatures of isothermal annealing: 1273, 1323 and 1373 K. The activation energy for diffusion, calculated from the experimental data using a theoretical model based on the Fick equation, was found to be E_A=506±5 kJ mol⁻¹. This value is close to the ones for the diffusion of Gd in UO₂ and CeO₂, but much larger than for the diffusion of gadolinium in a compound with the same crystal structure as zircon, YVO₄. This is attributed to a difference in the relative sizes of the ions involved in the diffusion process.     

Influence of dipolar interaction on magnetic properties of ultrafine ferromagnetic particles

We use Monte Carlo simulations to study the influence of dipolar interaction and polydispersity on the magnetic properties of single-domain ultrafine ferromagnetic particles. From the zero field cooling (ZFC)/field cooling (FC) simulations we observe that the blocking temperature T(B) clearly increases with increasing strength of interaction, but it is almost not effected by a broadening of the distribution of particle sizes. While the dependence of the ZFC/FC curves on interaction and cooling rate are reminiscent of a spin glass transition at T(B), the relaxational behavior of the magnetic moments below T(B) is not in accordance with the picture of cooperative freezing.     

Lattices of nonspherical ferromagnetic grains with magnetic dipole interaction: Theory and experimental examples

The magnetic properties (ground state, magnetic phase diagram, and phase transitions in a magnetic field) of two-and three-dimensional lattices of ferromagnetic grains with the intergrain dipole interaction are studied. The main attention is paid to the lattices formed by nonspherical grains (prolate and oblate ellipsoids of revolution) and their extreme forms (rodlike and disc-shaped grains). An analysis shows that the conclusions of the theory are in good agreement with the results of experiments.