Finite block spin phenomenology of spin glass

We herein explain the phase of spin glass by reference to finite block spin theory, in which the phase of the spin glass may be considered as being a ferromagnetic ordering between block spins comprised of random spins that have a majority of individual spins in a given sense. By making use of the Curie law of block spins, we obtained the magnetization, susceptibility, and specific heat for the lower and higher temperature approximations of the Brillouin function. Both the susceptibility and the specific heat thus obtained are in good agreement with existing experimental data, although in the latter case the agreement is less convincing near absolute zero temperature.     

A finite block spin model for permanent magnets

We propose a finite block spin phenomenology for permanent magnets in which we consider an average domain as a block spin. The permanent ferromagnetism arises in two ways: (1) the ferromagnetism that occurs inside a big block spin, i.e. the intrablock ferromagnetism, and the ferromagnetism between small block spins (SBSs) in a big block spin (BBS) which can be induced by collective ferromagnetic pairing between two SBSs mediated by temperature-irreversible bosonic strains; and (2) the ferromagnetism between BBSs, i.e. the interblock ferromagnetism. The coercivity originates from temperature-irreversible strains treated as external phonons.     

Delay of Spatial Quantum Correlations in Magnetic Nanostructures

Simultaneous quantum correlations between two spins in magnetic nanostructures are considered in the model of a linear chain of a finite number of atoms with exchange interaction between electron spins of neighboring atoms in the framework of the Heisenberg ferromagnetism theory. We assume that in the initial state, the spins of all chain atoms except the first two are oriented along the same direction. The spins of the first two atoms are flipped. Due to the exchange interaction, this initial state generates a spin flip wave along the chain. The expressions obtained for nonstationary quantum amplitudes of the flip probability waves for an even number of spins can be used for calculating quantum correlations between two spins separated by a large distance in a chain. Numerical calculations of the spin correlator reveal that the correlation between two spins in the chain occurs with a delay on the order of the time of propagation of the exchange interaction along the spin chain. After the delay, the spin correlation amplitude abruptly increases followed by subsequent oscillatory temporal behavior.     

Spin-selective Kondo insulator: cooperation of ferromagnetism and the Kondo effect

We propose the notion of a spin-selective Kondo insulator, which provides a fundamental mechanism to describe the ferromagnetic phase of the Kondo lattice model with antiferromagnetic coupling. This unveils a remarkable feature of the ferromagnetic metallic phase: the majority-spin conduction electrons show metallic while the minority-spin electrons show insulating behavior. The resulting Kondo gap in the minority-spin sector, which is due to the cooperation of ferromagnetism and partial Kondo screening, evidences a dynamically induced commensurability for a combination of minority-spin electrons and parts of localized spins. Furthermore, this mechanism predicts a nontrivial relation between the macroscopic quantities such as electron magnetization, spin polarization, and electron filling.     

Interaction of the ground state of quarter-filled one-dimensional strongly correlated electronic system with localized spins

We study numerically the ground-state properties of the one-dimensional quarter-filled strongly correlated electronic system interacting antiferromagnetically with localized S = 1/2 spins. It is shown that the charge-ordered state is significantly stabilized by the introduction of relatively small coupling with the localized spins. When the coupling becomes large the spin and charge degrees of freedom behave quite independently and the ferromagnetism is realized. Moreover, the coexistence of ferromagnetism with charge order is seen in the strongly interacting region. The present theoretical results are to be compared with the experiments on phthalocyanine compounds.     

Spin dynamics in (III,Mn)V ferromagnetic semiconductors: the role of correlations

We address the role of correlations between spin and charge degrees of freedom on the dynamical properties of ferromagnetic systems governed by the magnetic exchange interaction between itinerant and localized spins. For this we introduce a general theory that treats quantum fluctuations beyond the random phase approximation based on a correlation expansion of the Green's function equations of motion. We calculate the spin susceptibility, spin-wave excitation spectrum, and magnetization precession damping. We find that correlations strongly affect the magnitude and carrier concentration dependence of the spin stiffness and magnetization Gilbert damping.     

Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors

We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the quantum dots and the itinerant holes in the semiconductor valence band makes possible hole transfer between the DMS quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.Received: 17 Mars 2003, Published online: 30 January 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.30.Ds Spin waves - 75.50.Dd Nonmetallic ferromagnetic materials - 75.50.Pp Magnetic semiconductors     

Simulation of coexisting ferromagnetic order and disorder of geometrically frustrated Co2Cl(OH)3

The ground state and phase transition of Co₂Cl(OH)₃ were investigated by Monte Carlo simulation. This compound is a magnet, with a pyrochlore structure distorted along one axis. The magnetic structure at low temperatures consists of coexisting ferromagnetism and random spin, according to experiments. However, the formation mechanism of the coexistence and the interaction between the spins were unclear. We assumed an anisotropic Ising model and examined the ground state by multicanonical Monte Carlo simulation. In a nearest neighbor model, the ground states were highly degenerated. Almost all of the states were spin glass states, but a few of the states were ferromagnetic. The latter magnetic states were ferromagnetic at triangular layers and two in-one out random state at Kagome layers. The latter states should be stabilized if weak ferromagnetic interactions exist between second nearest neighbor spins and correspond to the states reported by the experiments. This expectation was confirmed by simulation.     

Room-temperature ferromagnetism in hydrothermally treated glassy carbon

Extraordinary room-temperature ferromagnetism was observed in our hydrothermally treated glassy carbon systems, taken to comprise nearly graphitized regions. Strong correlation between the ferromagnetic response and the diamagnetic susceptibility, superposed together, suggests the ferromagnetic long-range order in the graphitic phase to be of itinerant nature. Besides, the graphitization process associated with the itinerant spins and the ferromagnetic order revealed a systematic correlation with the average graphitic cluster size.     

Ising ferromagnetism and domain morphology in the fractional quantum Hall regime

The density driven quantum phase transition between the unpolarized and fully spin polarized nu = 2/3 fractional quantum Hall state is accompanied by hysteresis in accord with 2D Ising ferromagnetism and domain formation. The temporal behavior is reminiscent of the Barkhausen and time-logarithmic magnetic after-effects ubiquitous in familiar ferromagnets. It too suggests domain morphology and, in conjunction with NMR, intricate domain dynamics, which is partly mediated by the contact hyperfine interaction with nuclear spins of the host semiconductor.     

Ferromagnetic ground state in the Kondo lattice model

We present a series of rigorous examples of the Kondo lattice model that exhibit full ferromagnetism in the ground state. The models are defined in one-, two- and three-dimensional lattices, and are characterized by a range of hopping terms, specific electron filling, and large ferromagnetic coupling. Our examples show that a sufficient strong but finite exchange coupling between conduction electrons and localized spins could overcome the competition from mobility of a finite density of electrons and drive the system from a paramagnetic phase to a ferromagnetic phase. We also establish a relation of ferromagnetism between the Hubbard model and Kondo lattice model. Meanwhile some rigorous results on ferromagnetism in the corresponding Hubbard model are presented. Received: 10 September 1997 / Revised: 15 October 1997 / Accepted: 17 October 1997     

Exchange bias effect in multiferroic Eu0.75Y0.25MnO3

The exchange bias phenomenon has been investigated in multiferroic Eu_0.75Y_0.25MnO₃. The material shows a weak ferromagnetism with cone spin configuration induced by external magnetic field below 30 K. Consequently, the electric polarization coming from the cycloid spin order below 30 K can be suppressed by external magnetic fields. The magnetic hysteresis loops after cooling in a magnetic field exhibit characteristics of exchange bias below the spin glassy freezing temperature (T_g)∼16 K. The exchange bias field, coercivity field, and remanent magnetization increase with increasing cooling magnetic field. The exchange bias effect is ascribed to the frozen uncompensated spins at the antiferromagnetism/weak ferromagnetism interfaces in the spin-glass like phase.     

Magnetic susceptibility of dilute magnetic alloys

The magnetic susceptibility of dilute magnetic alloys is calculated using the Nagoaka approximation to the Kondo problem. We use the exact solutions of the Nagaoka equations, or equivalently Suhl's dispersion relations, as obtained recently. Our result is represented by a universal function of a certain temperature parameter. In the case of ferromagnetic coupling no appreciable change of the free spin susceptibility is found over the whole temperature range. In the case of antiferromagnetic coupling we find that the free spin susceptibility is greatly reduced. In fact, for spin 1/2, the result indicates the breakdown of the expansion in terms of the impurity concentration and suggests the onset of impurity ferromagnetism.     

Cluster phase transition in insulating spin glass Ga0.4Fe0.6NiAlO4

Mössbauer effect, a.c. susceptibility and magnetisation studies (4.2≦T≦350 K, ?20≦H≦20 kOe) show that in the mixed spined ferrite Ga_0.4Fe_0.6NiAlO₄, spin freezing occurs via cluster formation. As in some other insulating spin glasses, the spin glass phase diagram transitions occur not for individual spins but for clusters whose effective spins’ transverse and longitudinal components successively freeze. The presence of entropic spins and the centre shift of the field cooled hysteresis loop confirm the system’s frustrated nature.     

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.     

Topological estimator of block entanglement for Heisenberg antiferromagnets

We introduce a computable estimator of block entanglement entropy for many-body spin systems admitting total singlet ground states. Building on a simple geometrical interpretation of entanglement entropy of the so-called valence bond states, this estimator is defined as the average number of common singlets to two subsystems of spins. We show that our estimator possesses the characteristic scaling properties of the block entanglement entropy in critical and noncritical one-dimensional Heisenberg systems. We invoke this new measure to examine entanglement scaling in the two-dimensional Heisenberg model on a square lattice revealing an "area law" for the gapped phase and a logarithmic correction to this law in the gapless phase.     

EPR and ferromagnetism in diluted magnetic semiconductor quantum wells

Motivated by recent measurements of electron paramagnetic resonance spectra in modulation-doped CdMnTe quantum wells [Phys. Rev. Lett. 91, 077201 (2003)]], we develop a theory of collective spin excitations in quasi-two-dimensional diluted magnetic semiconductors. Our theory explains the anomalously large Knight shift found in these experiments as a consequence of collective coupling between Mn-ion local moments and itinerant-electron spins. We use this theory to discuss the physics of ferromagnetism in (II,Mn)VI quantum wells and to speculate on the temperature at which it is likely to be observed in n-type modulation-doped systems.     

准一维梯状结构化合物Sr14-xCaxCu24O41磁化率随外磁场的变化性质

采用常规的固相反应方法制备了Sr14-xCaxCu24O41系列样品(x=0.0,3.5,6.0,8.4).X-ray衍射结果显示,所有样品均为单相,并且随着掺杂离子含量的增加,样品的晶格常数a,b,c的值均逐渐减小.在10 K～200 K温度范围内测量了不同磁场强度(H=1 T,3 T,5 T)Ca掺杂样品(x=0,3.5,6,8.4)的磁化率温度曲线.磁化率测量结果表明,磁场的增强导致居里-外斯贡献降低,并且导致自旋能隙变小.进一步拟合结果表明,参与磁化率贡献的自旋数目随Ca掺杂浓度的变化不明显,而随磁场强度的增大而减小.     

嵌入化合物Fe0.95PS3(MV)0.11的合成与磁性能研究

利用嵌入反应合成了有机-无机嵌入化合物Fe_0.95PS₃(MV)_{0.1 1}(MV为1,1′- 二甲基-4,4′-联吡啶阳离子)，对其结构和磁性进行了研究.x射线衍射数据表明，此嵌入化 合物的晶体结构仍为单斜晶系，空间群为C2/m，晶胞参数a=0.879 nm, b=0.944 nm, c=1.07 0 nm, β=114.76°.相对于纯FePS₃, 层间距离增大0.33 nm.磁化率研究表明， 从室温降 到4.2 K 关键词： 嵌入化合物 Mssbauer谱 磁相互作用 分子磁体     

Ising-Model, Block-Spin Distributions by the Markov Property Method

The idea that near the critical point each block of spins behaves just like a single big spin is investigated. The case where a diamond-shaped block of spins is embedded in a (small) sea of spins is studied. Use is made of the Markov property method to make exact computations of the various spin moments needed to test this hypothesis. The residual fluctuation about the mean value of the block spin is seen to tend to a finite fraction of the length of the mean block-spin. This result is in line with previous studies which used different types of boundary conditions.