磁性颗粒复合体磁渗流区矫顽力异常的研究

制备了MnZn铁氧体/SiO₂颗粒复合体.研究了磁性颗粒复合体的有效磁导率μ、 比磁化强度σ以及矫顽力H_c随磁性颗粒含量的变化.研究发现，在MnZn铁氧体体积百分含 量为90%—98%的区域，复合体的有效磁导率μ的变化速率发生突变，出现磁渗流现象，从实验得到的体系磁渗流阈值V_c=97.9%.在磁渗流区，矫顽力表现出异常行为.结果表明 ，这种异常行为与复合体微观结构有着密切关系.在磁渗流前，矫顽力H_c的变化主要来 源于磁 关键词： 颗粒复合体 磁渗流 矫顽力     

磁性纳米颗粒膜的微磁学模拟

用微磁学方法对磁性纳米颗粒膜的磁特性进行了模拟，采用的模型是由122个磁性纳米颗粒组成的面心立方(fcc)结构体系.结果表明：在该体系中，偶极相互作用对体系的静态磁结构的影响显著，而交换相互作用的影响表现不明显.在此基础上，本文还采用有效媒质理论计算分析了磁性合金颗粒不同体积比时颗粒膜的磁谱和表征电磁参量发生显著变化的逾渗现象和逾渗阈值.并完成了对高磁损耗磁性纳米颗粒膜的材料设计. 关键词： 微磁学 纳米颗粒膜 逾渗阈值 磁导率 材料设计     

Study of the magnetic microstructure of Ni/NiO nanogranular samples above the electric percolation threshold by magnetoresistance measurements

Magnetoresistance measurements have been exploited to gain information on the magnetic microstructure of two Ni/NiO nanogranular materials consisting of Ni nanocrystallites (mean size of the order of 10 nm) embedded in a NiO matrix and differing in the amount of metallic Ni, ~33 and ~61 vol%. The overall conductance of both samples is metallic in character, indicating that the Ni content is above the percolation threshold for electric conductivity; the electric resistivity is two orders of magnitude smaller in the sample with higher Ni fraction (10(-5) Ωm against 10(-3) Ωm). An isotropic, spin-dependent magnetoresistance has been measured in the sample with lower Ni content, whereas both isotropic and anisotropic magnetoresistance phenomena coexist in the other material. This study, associated with magnetization loop measurements and the comparison with the exchange bias effect, allows one to conclude that in the sample with lower Ni content neither the physical percolation of the Ni nanocrystallites nor the magnetic percolation (i.e., formation of a homogeneous ferromagnetic network) are achieved; in the other sample physical percolation is reached while magnetic percolation is still absent. In both behaviors, a key role is played by the NiO matrix, which brings about a magnetic nanocrystallite/matrix interface exchange energy term and rules both the direct exchange interaction among Ni nanocrystallites and the magnetotransport properties of these nanogranular materials.     

Effects of vibrational intercentre interaction in a trigonal two-centre system with twofold electronic degeneracy at each centre

The effect of intercentre interaction (via vibrations and electron-electron coupling) on the magnetic and magnetic resonance characteristics of a pair of Jahn-Teller centres is investigated. In the limiting case of strong vibronic coupling when the splitting of degenerate electronic terms is sufficiently large the exchange Hamiltonian for the pair has the Heisenberg form. The energy spectrum of spin states of the pair system is obtained and the temperature dependence of the effective magnetic moment is investigated. It is shown that under some conditions this temperature dependence has an anomalous nonmonotonic character. The g factors of the two-centre Jahn-Teller system are obtained and the changes of the EPR spectrum with the exchange and vibrational coupling constants is analysed. With inclusion of only the exchange interaction, the pair spectrum possesses an additional isotropic resonance placed in the middle of the known broadened EPR spectrum of single centres with the Jahn-Teller effect for an orbital E term. The vibrational interaction between the centres results in a radical transformation of the spectrum, which in this case contains only an isotropic resonance line.     

Threshold effect of microwave power on ferromagnetic resonance in K<Subscript>0.4</Subscript>[Cr(CN)<Subscript>6</Subscript>][Mn(S)-pn](S)-pnH<Subscript>0.6</Subscript> single crystals

A stepwise change in the ferromagnetic resonance spectrum in a K_0.4[Cr(CN)₆][Mn(S)-pn](S)-pnH_0.6 chiral magnet has been observed in a critical microwave magnetic field of 1.8 Oe. The threshold changes in the ferromagnetic resonance spectra are caused by Suhl instability leading to the generation of defects of the magnetic structure, chiral spin solitons. The threshold effect is not observed in the same microwave range in similar chiral crystals [Cr(CN)₆][Mn(S)-pnH(H₂O)](H₂O) with a stronger Heisenberg exchange interaction.     

Variation in the magnetic structure of a single-domain particle ensemble and its response to an rf pulse

A set of equations in the Gilbert form that describes the motion of the magnetization vector in an ensemble of interacting magnetic nanoparticles is numerically solved for the case of high-amplitude rf pulses. Based on the numerical solution, the magnetic structure of spherical particle ensembles showing cubic anisotropy at certain parameters of the variable field is studied. This phenomenon is shown to have a threshold. The dependence of the field threshold amplitude on the acting pulse repetition rate and amount of magnetic interaction is determined. It is demonstrated that a change in the magnetic structure of the interacting particle ensembles causes a change in the spectrum of the response. This fact can be used for pulsed rf writing and readout of information based on ferromagnetic resonance.     

Induced Resonance Processes Accompanying Interaction of the Field of a TEM Trap with Oscillating Charges

We theoretically analyze resonance processes in an electromagnetic trap (TEM trap) formed by a circularly polarized high-frequency standing field of homogeneous plane waves and a uniform static magnetic field aligned with the direction of wave propagation. The regime of resonance amplification of the trap field by an ensemble of initially nonphased oscillators in the absence of a static magnetic field is described. The regime of resonance acceleration of charges from thermal to relativistic velocities for a bounded particle motion in the presence of a static magnetic field is considered. It is shown that charge oscillations in the trap are similar to flutter in mechanical systems. The efficient energy exchange is stipulated by an M-type interaction mechanism.     

Magnetic phase transitions in amorphous systems with competing exchange interactions

A method is proposed for analyzing magnetic phase transitions within the Ising model under the conditions of competing direct and indirect exchange interactions. It is demonstrated that the competition of exchange interactions leads to a reentrant phase transition between the ferromagnet and spin glass near the percolation threshold below the Curie temperature.     

H-particle in a chiral quark model

In this paper we discuss the binding energy of the H-particle using a chiral quark model, where pion exchange plays an important role to reproduce the mass difference between the nucleon and Δ resonance. Since the main source for the bound H-particle is believed to be the color magnetic interaction, which gives the nucleon and Δ mass difference, it is very interesting to investigate whether the chiral quark model gives rise to the bound H-particle or not. We employ an extended resonating group method in order to take into account the possibility of a change of baryon wave functions when two baryons interact with each other. We found that a change of baryon size together with the Hamiltonian which consists of gluon, pseudoscalar meson and sigma meson exchange potentials gives rise to the bound H-particle. The binding energy is found to be about 25 MeV in a hybrid chiral quark model. Differences between the ordinary gluon dominant model and chiral quark models are also investigated. It is found that a pure chiral model has no bound state when the widely used sigma-quark coupling strength is employed.     

Quantum phase transition of the randomly diluted heisenberg antiferromagnet on a square lattice

Ground-state magnetic properties of the diluted Heisenberg antiferromagnet on a square lattice are investigated by means of the quantum Monte Carlo method with the continuous-time loop algorithm. It is found that the critical concentration of magnetic sites is independent of the spin size S, and equal to the two-dimensional percolation threshold. However, the existence of quantum fluctuations makes the critical exponents deviate from those of the classical percolation transition. Furthermore, we found that the transition is not universal, i.e., the critical exponents significantly depend on S.     

Magnetic properties of Fe–Ti–O composite films obtained via solid-phase synthesis

The results from investigating the magnetic and magneto-optical properties of Fe–Ti–O composite films with compositions above the percolation threshold, prepared via a solid-phase reaction with oxygen exchange in layered FeO/Ti structures, are presented. Features of the magneto-optical spectra of prepared films are compared to the spectra of continuous metal films.     

High coercivity in large exchange-bias Co/CoO-MgO nano-granular films

We present a detailed study on the magnetic coercivity of Co/CoO-MgO core-shell systems, which exhibits a large exchange bias due to an increase of the uncompensated spin density at the interface between the CoO shell and the metallic Co core by replacing Co by Mg within the CoO shell. We find a large magnetic coercivity of 7120 Oe around the electrical percolation threshold of the Co/CoO core/shell particles, while samples with a smaller or larger Co metal volume fraction show a considerably smaller coercivity. Thus, this study may lead to a route to improving the magnetic properties of artificial magnetic material in view of potential applications.     

The density of states in two-dimensional dilute Heisenberg ferromagnets near the percolation threshold

Two-dimensional dilute Heidenberg ferromagnets with the magnetic atom concentration near the percolation threshold are considered. The density of magnetic excitation states at zero temperature is obtained by the scaling theory for the percolation problem.     

Asymmetric line broadening in the electron resonance spectra of biradicals

At high temperatures the dominant relaxation process which determines the linewidths in the electron resonance spectra of flexible biradicals is modulation of the scalar electron-electron exchange interaction. In systems of high viscosity, the modulation of the exchange interaction is often quenched, and the rotational modulation of the anisotropic magnetic interactions now constitutes the principal relaxation mechanism. In this paper we derive a theoretical expression for the broadening which results from this relaxation process. The applications of the theory to the determination of molecular configurations, electron-electron separations and the sign of the exchange interaction are illustrated by comparison with the electron resonance spectrum of bis(2,2,6,6-tetramethyl-piperidinol-1-oxyl)carbonate. The theory is also of value in understanding the spectra of partially immobilized biradical spin labels.     

The mass of a bound Δ-isobar

The properties of the Δ -isobar when bound in the nuclear medium are studied. The pionnucleon resonance nature of the Δ -isobar yields self-energy corrections, i.e., an energy- and medium-dependent mass and, above pion-production threshold, an energy- and medium-dependent width. Their importance on nucleon-nucleon scattering, the three-nucleon bound state and nuclear matter are discussed. Other effects arising from the resonance nature of the Δ -isobar are the retardation of the two-body interaction, three-nucleon forces and e.m. exchange currents.     

Topological destruction of the phase transition to the spin-glass state in amorphous alloys with an asymmetric distribution of exchange interactions

It is shown that an increase in the asymmetry in the distribution of exchange interactions reduces the real magnetic dimension of bulk samples of amorphous spin glasses FeNi and FeMn, which attains its lowest critical value D _L =2.51±0.12 near the percolation threshold of infinite ferromagnetic clusters. The experimentally obtained result is in good agreement with calculations made using computer experiments.     

Micromagnetism in a planar system with a random magnetic anisotropy and two-dimensional magnetic correlations

The hysteresis loops and the micromagnetic structure of a ferromagnetic nanolayer with a randomly oriented local easy magnetization axis and two-dimensional magnetization correlations are studied using a micromagnetic simulation. The properties and the micromagnetic structure of the nanolayer are determined by the competition between the anisotropy and exchange energies and by the dipole–dipole interaction energy. The magnetic microstructure can be described as an ensemble of stochastic magnetic domains and topological magnetization defects. Dipole–dipole interaction suppresses the formation of topological magnetization defects. The topological defects in the magnetic microstructure can cause a sharper change in the coercive force with the crystallite size than that predicted by the random magnetic anisotropy model.     

Evidence of a critical Mn-Mn distance for the onset of ferromagnetism in NiAs type compounds

Recently, the technique of spin wave resonance has been used to measure the exchange coupling constants as a function of composition in several solid solutions of MnSb. From an examination of the results we show that in those alloys in which the linear chains of magnetic atoms are not disturbed, the exchange coupling constants become positive leading to ferromagnetic coupling when the distance between Mn atoms becomes equal to 2.83 Å or more. This experimental value of the Mn-Mn distance for the onset of ferromagnetism is equal to the critical distance derived by Forrer after examining a large number of materials which owe their ferromagnetism predominantly to the direct exchange interaction.     

Phase diagram of the disordered RKKY model in dilute magnetic semiconductors

We consider ferromagnetism in spatially randomly located magnetic moments, as in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect exchange RKKY interaction. We obtain, via Monte Carlo calculations, the magnetic phase diagram as a function of the impurity moment density n(i) and the relative carrier concentration n(c)/n(i). As evidenced by the diverging correlation length and magnetic susceptibility, the boundary between ferromagnetic and nonferromagnetic phases constitutes a line of zero temperature critical points which can be viewed as a magnetic percolation transition. In the dilute limit, we find that bulk ferromagnetism vanishes for n(c)/n(i) >0.1. We also incorporate the local antiferromagnetic direct superexchange interaction between nearest neighbor impurities and examine the impact of a damping factor in the RKKY range function.     

Evidence of domain wall scattering in thin films of granular CoFe-AgCu

Thin films of the giant magnetoresistive granular CoFe-AgCu system prepared by rf sputtering displayed a great variety of domain-like microstructures with a net out-of-plane component of the magnetization for ferromagnetic volume concentrations above about 0.25. Therefore, magnetic percolation takes place at ferromagnetic concentrations much lower than the physical percolation threshold. The out-of-plane structure of the as-deposited samples in magnetic virgin state consisted of a distribution of both quasi-circular domains and short stripes depending on the ferromagnetic content. Furthermore, these samples present high metastability and a variety of remanent in-plane and out-of-plane microstructures can be achieved as a function of the magnetic history. Besides, the evolution of the magnetic microstructure yields strong training effects on magnetotransport properties, due to the extra contribution of the electron scattering at the domain walls. All in all, the observed behavior is the result of a subtle correlation between perpendicular anisotropy produced by residual stresses, exchange interparticle interactions due to CoFe alloyed in the matrix, and dipolar interactions. Thus, as high structural evolution occurs through annealing, the features of randomly distributed ferromagnetic particles are recovered and, the out-of-plane domain structures and the training effects disappear. Received 30 September 1999