Numerical study of the one-dimensional quantum compass model

The ground state magnetic phase diagram of the one-dimensional quantum compass model (QCM) is studied using the numerical Lanczos method. A detailed numerical analysis of the low energy excitation spectrum is presented. The energy gap and the spin-spin correlation functions are calculated for finite chains. Two kind of the magnetic long-range orders, the Néel and a type of the stripe-antiferromagnet, in the ground state phase diagram are identified. Based on the numerical analysis, the first and second order quantum phase transitions in the ground state phase diagram are identified.     

Ground state phase diagram of the 1D XXZ model in a transverse magnetic field

The Ground state phase diagram of the 1D spin-1/2 XXZ model in a transverse magnetic field is studied by the numerical Lanczos method. Also by computing the energy gap and different spin-spin structure factors, quantum phase transitions in whole range of the anisotropy parameter are determined.     

Scaling approach to the magnetic phase diagram of nanosized systems

Based on the discovery of a simple scaling relation, a new technique for the investigation of the phase diagram of nanosized magnetic systems is proposed. By scaling the exchange interaction between magnetic moments, the magnetic phase diagram of currently lithographically produced particles can be obtained from those corresponding to much smaller systems. Such a technique reduces the computation time by several orders of magnitude, and provides a new approach to the investigation of the relative stability of distinct internal magnetic configurations of nanosized systems. The technique is illustrated by the determination of the phase diagram of cylindrically shaped Co particles.     

Phase transformations of the magnetic structure in film nanobridges

The magnetic structure of a plane nanobridge consisting of two ferromagnetic film electrodes connected by a nanosized crossbar of the same material is studied. Due to their magnetoresistive properties, such bridges are of considerable interest for microelectronics. Using a numerical micromagnetics method, it is shown that a domain wall is displaced from the center of the bridge crossbar as the anisotropy constant of the system decreases and reaches a critical value. A phase diagram is constructed, which makes it possible to determine the possible magnetic states of real nanobridges. The mechanism of the phase transformation is described in terms of an analytical model. This model explains the shape of the phase diagram of the nanobridge. Formally, the transformations of the magnetic structure of the nanocontact can be described in terms of the Landau theory of phase transitions in a certain range of parameters of the system.     

Phase diagrams of magnetic nanotubes

Analytical expressions for the total magnetic energy of two characteristic internal configurations of nanometric tubes are calculated. A magnetic phase diagram with respect to the aspect ratio of the tubes is obtained which allows a discussion about the possibility of getting ensembles of nanotubes with low coercive fields. A comparison with recently reported coercive fields of three different cobalt nanotube arrays agrees well with the phase diagram derived here.     

Magnetic properties of the spin S = 1/2 Heisenberg chain with hexamer modulation of exchange

We consider the spin-1/2 Heisenberg chain with alternating spin exchange in the presence of additional modulation of exchange on odd bonds with period 3. We study the ground state magnetic phase diagram of this hexamer spin chain in the limit of very strong antiferromagnetic (AF) exchange on odd bonds using the numerical Lanczos method and bosonization approach. In the limit of strong magnetic field commensurate with the dominating AF exchange, the model is mapped onto an effective XXZ Heisenberg chain in the presence of uniform and spatially modulated fields, which is studied using the standard continuum-limit bosonization approach. In the absence of additional hexamer modulation, the model undergoes a quantum phase transition from a gapped phase into the only one gapless Lüttinger liquid (LL) phase by increasing the magnetic field. In the presence of hexamer modulation, two new gapped phases are identified in the ground state at magnetization equal to [Formula: see text] and [Formula: see text] of the saturation value. These phases reveal themselves also in the magnetization curve as plateaus at corresponding values of magnetization. As a result, the magnetic phase diagram of the hexamer chain shows seven different quantum phases, four gapped and three gapless, and the system is characterized by six critical fields which mark quantum phase transitions between the ordered gapped and the LL gapless phases.     

Toward the theory of quantum phase transitions in DTN-type van Vleck antiferromagnets

A magnetic field-induced singlet-to-antiferromagnetic quantum magnetic phase transition has been considered. It has been shown that the phenomenon can be described within an approach similar to the Landau theory of phase transitions. The case of an oblique magnetic field has been studied and the critical field-angle phase diagram has been determined. The phase diagram agrees with the experimental one observed in DTN.     

Quantum tunneling of the magnetic moment in a free nanoparticle

We study tunneling of the magnetic moment in a particle that has full rotational freedom. Exact energy levels are obtained and the ground-state magnetic moment is computed for a symmetric rotor. The effect of mechanical freedom on spin tunneling manifests itself in a strong dependence of the magnetic moment on the moments of inertia of the rotor. The energy of the particle exhibits quantum phase transitions between states with different values of the magnetic moment. Particles of various shapes are investigated and the quantum phase diagram is obtained.     

The quantum compass chain in a transverse magnetic field

We study the magnetic behaviors of a spin-1/2 quantum compass chain (QCC) in a transverse magnetic field, by means of the analytical spinless fermion approach and numerical Lanczos method. In the absence of the magnetic field, the phase diagram is divided into four gapped regions. To determine what happens by applying a transverse magnetic field, using the spinless fermion approach, critical fields are obtained as a function of exchanges. Our analytical results show, the field-induced effects depend on in which one of the four regions the system is. In two regions of the phase diagram, the Ising-type phase transition happens in a finite field. In another region, we have identified two quantum phase transitions (QPT)s in the ground state magnetic phase diagram. These quantum phase transitions belong to the universality class of the commensurate-incommensurate phase transition. We also present a detailed numerical analysis of the low energy spectrum and the ground state magnetic phase diagram. In particular, we show that the intermediate state (h _c1 < h < h _c2) is gapful, describing the spin-flop phase.     

Phase field simulation of microstructure evolution in Fe-Cr-Co alloy during thermal magnetic treatment and step aging

The evolution of modulated structures in Fe-Cr-Co alloys during isothermal aging under an external magnetic field and multiple step aging was simulated based on a phase field method. In this simulation, the magnetic configuration during the decomposition was calculated by a micromagnetic method, and the chemical Gibbs energy function was calculated by the CALPHAD approach based on the experimental equilibrium phase diagram. The calculation results provide a quantitative microstructure change directly linked to the phase diagram and demonstrate obvious microstructure difference between isothermal aging and multiple aging. The ferromagnetic precipitates elongate along the direction of the external magnetic field. The simulated evolution and microstructure are in good agreement with the experimental results.     

Phase diagram of spin-1 bosons on one-dimensional lattices

Spinor Bose condensates loaded in optical lattices have a rich phase diagram characterized by different magnetic order. Here we apply the density matrix renormalization group to accurately determine the phase diagram for spin-1 bosons loaded on a one-dimensional lattice. The Mott lobes present an even or odd asymmetry associated to the boson filling. We show that for odd fillings the insulating phase is always in a dimerized state. The results obtained in this work are also relevant for the determination of the ground state phase diagram of the S = 1 Heisenberg model with biquadratic interaction.     

Experimental and micromagnetic first-order reversal curves analysis in NdFeB-based bulk “exchange spring”-type permanent magnets

In this paper we present the results of applying the first-order reversal curves (FORC) diagram experimental method to the analysis of the magnetization processes of NdFeB-based permanents magnets. The FORC diagrams for this kind of exchange spring magnets show the existence of two magnetic phases—a soft magnetic phase and a hard magnetic one. Micromagnetic modeling is used for validating the hypotheses regarding the origin of the different features of the experimental FORC diagrams.     

On the magnetic phase diagram of (Mn,Fe)WO4 wolframite

The magnetic phase diagram of mixed crystals of type (Mn, Fe)WO₄ is discusses within molecular field approximation. The two species of magnetic ions tend to align in different orderings. Depending on the interaction strengths the two ordered phases are separated either by a phase in which both orderings are superimposed or by a first order transition.     

Effects of the Copper and Oxygen atoms of the CuO-plane on magnetic properties of the YBCO by using the effective-field theory

We investigate the effects the Copper and Oxigen atoms of the CuO-plane on magnetic properties in the YBCO within the frame work of the Ising model by using the effective field theory. In particular, we study the thermal behaviors of magnetizations of CuO-plane, YBCO and their components with taking the antiferromagnetic spin orientations. We find that the system undergoes a second-order phase transition and only the magnetization of CuO-plane illustrates the N-type compensation behavior. We also investigate the hysteresis loops behavior of CuO-plane, YBCO and their components and present the superconducting phase diagram. The phase diagram contains two different Meissner and four different vortex and a normal states.     

On the mean-field Ising model in a random external field

We use a method developed by van Hemmen to obtain the free energy of the mean-field Ising model in a random external magnetic field. Some results of previous mean-field calculations are confirmed and generalized. The tricritical point in the global phase diagram is discussed in detail. We also consider different probability distributions of the random fields and provide some proofs regarding the conditions for the existence of a tricritical point.     

First order phase transition in the frustrated triangular antiferromagnet CsNiCl3

By means of high-resolution ultrasonic velocity measurements, as a function of temperature and magnetic field, the nature of the different low temperatures magnetic phase transitions observed for the quasi-one-dimensional compound CsNiCl3 is established. Special attention has been devoted to the field-induced 120 degrees phase transition above the multicritical point in the H-T phase diagram where the elastic constant C44 reveals a steplike variation and hysteresis effects. These results represent the first experimental evidence that the 120 degrees phase transition is weakly first order and contradict the popular notion of new universality classes for chiral systems.     

From antiferromagnetic order to static magnetic stripes: the phase diagram of (La,Eu)2-xSrxCuO4

The magnetic order of (La,Eu)2-xSrxCuO4 ( x相似文献   

Spin-orbit scattering in the quantum Hall effect

We study dynamics of electrons in a magnetic field using a network model with two channels per link with random mixing, while the intrachannel potential is periodic (non-random); the channels represent two spin states. We consider channel mixing as function of the energy separation of the two extended states, and show that the phase diagram is different from the standard quantum Hall diagram for random intrachannel potential.     

Quantum criticality and the phase diagram of the cuprates

I discuss a proposed phase diagram of the cuprate superconductors as a function of temperature, carrier concentration, and a strong magnetic field perpendicular to the layers. I show how the phase diagram gives a unified interpretation of a number of recent experiments.