Complex magnetic phases in LuFe2O4

DC magnetization and AC susceptibility measurements on LuFe₂O₄ single crystals reveal a ferrimagnetic transition at 240 K followed by additional magnetic transitions at 225 K and 170 K, separating cluster glass phases, and a kinetically arrested state below 55 K. The origin of giant magnetic coercivity is attributed to the collective freezing of ferrimagnetic clusters and enhanced domain wall pinning associated with a structural transition at 170 K. Magnetocaloric effect measurements provide additional vital information about the multiple magnetic transitions and the glassy states. Our results lead to the emergence of a complex magnetic phase diagram in LuFe₂O₄.     

Three-dimensional magnetic correlations in multiferroic LuFe2O4

We present single crystal neutron diffraction measurements on multiferroic LuFe(2)O(4). Magnetic reflections are observed below transitions at 240 and 175 K indicating that the magnetic interactions in LuFe(2)O(4) are three-dimensional in character. The magnetic structure is refined as a ferrimagnetic spin configuration below the 240 K transition. Below 175 K a significant broadening of the magnetic peaks is observed along with the buildup of a diffuse component to the magnetic scattering.     

Charge order and the origin of giant magnetocapacitance in LuFe2O4

The nature of the charge order in the charge frustrated compound LuFe(2)O(4) and its effect on magnetocapacitance were examined on the basis of first-principles electronic structure calculations and Monte Carlo simulations of electrostatic energy. Our work shows that two different types of charge order of almost equal stability (i.e., square root of 3 x square root of 3 and chain types) occur in the Fe(2)O(4) layers of LuFe(2)O(4), and that the ground state of LuFe(2)O(4) has a ferrielectric arrangement of the Fe(2)O(4) layers with square root of 3 x square root of 3 charge order. The giant magnetocapacitance effect of LuFe(2)O(4) at room temperature is accounted for in terms of charge fluctuations arising from the interconversion between the two types of charge order, that becomes hindered by an applied magnetic field.     

Charge ordering of LuFe2O4 electronic multiferroic

Charge ordering of iron ions in LuFe2O4 frustrated electronic ferroelectric with variable valence is considered. Charge degrees of freedom are described in frameworks of Ising model with Coulomb interaction between sites. Mean field approximation is used to find free energies of the most probable structures. Phase diagrams of a single triangular bilayer and a system of triangular bilayers are plotted. A partially disordered dimer structure is proposed as a 2D high temperature phase. The transition from 2D to 3D ordering associated with a drop in temperature and the formation of spontaneous dipole moments is discussed.     

Charge-stripe order in the electronic ferroelectric LuFe2O4

The structural features of the charge ordering states in LuFe(2)O(4) are characterized by in situ cooling transmission electron microscopy observations from 300 K down to 20 K. Two distinctive structural modulations, a major q1=(1/3,1/3,2) and a weak q2 = q1/10+(0,0,3/2), have been well determined at the temperature of approximately 20 K. Systematic analysis demonstrates that the charges at low temperatures are well crystallized in a charge-stripe phase, in which the charge-density wave behavior in a nonsinusoidal fashion results in elemental electric dipoles for ferroelectricity. It is also noted that the charge ordering and ferroelectric domains often change markedly with lowering temperatures and yield a rich variety of structural phenomena.     

Competing ferri- and antiferromagnetic phases in geometrically frustrated LuFe2O4

We present a detailed study of magnetism in LuFe(2)O(4), combining magnetization measurements with neutron and soft x-ray diffraction. The magnetic phase diagram in the vicinity of T(N) involves a metamagnetic transition separating an antiferro- and a ferrimagnetic phase. For both phases the spin structure is refined by neutron diffraction. Observed diffuse magnetic scattering far above T(N) is explained in terms of near degeneracy of the magnetic phases.     

Formation and evolution of giant dynamic domains in an ac magnetic field

Magnetization reversal in ferrite-garnet films placed in an ac magnetic field, bringing about the formation of metastable dynamic domains with sizes exceeding those of quasi-static domains by an order of magnitude or greater, was studied using a stroboscopic method. The formation of giant dynamic domains (GDDs) is due to the finite domain wall velocity and depends on the density of domain nucleation centers. It was shown that the GDD comblike boundary forms during the part of a field period near the moment of field polarity change. GDDs arise when the dynamic hysteresis loop shape changes from a triangle to an ellipse.     

Charge order in LuFe2O4: an unlikely route to ferroelectricity

We present the refinement of the crystal structure of charge-ordered LuFe2O4, based on single-crystal x-ray diffraction data. The arrangement of the different Fe-valence states, determined with bond-valence-sum analysis, corresponds to a stacking of charged Fe bilayers, in contrast with the polar bilayers previously suggested. This arrangement is supported by an analysis of x-ray magnetic circular dichroism spectra, which also evidences a strong charge-spin coupling. The nonpolar bilayers are inconsistent with charge order based ferroelectricity.     

Temperature dependence of phonon spectra and structural characteristics in multiferroic LuFe2O4 system

The phonon spectra and structural characteristics of the LuFe₂O₄ system were investigated using X‐ray diffraction as well as Raman and infrared (IR) spectroscopic techniques. Two step‐like anomalies of the wavenumbers of the peak in the Raman spectra and the peak in IR spectra, as well as some weak anomalies of the lattice parameters, were found around the ferrimagnetic and ferroelectric transition temperatures. The substitutions of Lu with Yb and Fe with Mn lead to the corresponding wavenumber shifts in the optical spectra, which can be attributed to the changes in the atomic mass and lattice parameters. Furthermore, the Raman intensity was found to be related to the changes in the local structure and electrical conductivity. These results illustrate that the structural fluctuations could effectively influence the phonon modes, and a possible interplay among the structural, magnetic, and charge‐ordering properties exists in this multiferroic system. Copyright © 2011 John Wiley & Sons, Ltd.     

Pressure-induced structural transition in LuFe2O4: towards a new charge ordered state

The electronic ferroelectric lutetium ferrite (LuFe(2)O(4)) was studied by x-ray diffraction as a function of pressure. Pressure is shown to induce an irreversible rhombohedral to orthorhombic transition leading to a supercell determined by the combination of electron and synchrotron x-ray diffraction. This new configuration is proposed to be charge ordered in agreement with the results of resistivity measurements.     

Mechanical stability of ferrimagnetic CoFe2O4 flexible thin films

We fabricated high-quality ferrimagnetic CoFe₂O₄ (CFO) thin films on a mica substrate using a pulsed laser deposition technique. High-resolution X-ray diffraction and transmission electron microscopy revealed that the film grew epitaxially with the relationships of CFO <1-10> || Mica [010] and CFO [111] || Mica [001]. The films were highly flexible in terms of both inward and outward bending, and exhibited clear ferrimagnetic hysteresis with weak anisotropy in both the in-plane and out-of-plane directions. We observed that the magnetization of CFO films was robust against mechanical stimuli without microcracks. The remnant magnetization and coercive field were within 8% and 11% over a strain of ±0.54%. As the number of bending cycles increased, the magnetic easy axis became more closely aligned to the out-of-plane direction, without any noticeable change in domain size.     

Nano-crystallite Fe3C with giant magnetic coercivity in SiO2

⁵⁷Fe and ¹²C ions were implanted at room temperature into single crystal SiO₂ with energies yielding approximately the same calculated ranges. The energies were 60 and 20 keV, respectively for ⁵⁷Fe and ¹²C and their corresponding doses were 5 × 10¹⁶ and 2 × 10¹⁷ at/cm². The cementite Fe₃C was formed after annealing at 650°C with a size of about 7 nm. Magnetic coercive forces of 900 and 800 Oe were obtained at RT and 80 K, respectively. An explanation for such a strong coercivity is suggested.     

A kinetic Monte Carlo study for stripe-like magnetic domains in ferrimagnetic thin films

The topology and dynamics of stripe-like magnetic domains obtained in a ferrimagnetic garnet subjected to a time-dependent external magnetic field is studied experimentally and theoretically. Experiments are performed on a commercially available magnetic bubble apparatus, allowing the observation of the time-evolution of the magnetic domain structure. The system is modeled by a meso-scale Ising-type lattice model. Exchange and dipolar interactions between the spins, and interaction of the spins with the external magnetic field are considered. The model is investigated by kinetic Monte Carlo simulations with time-varying transition rates. In the limit of low temperatures the elaborated model leads to a magnetic domain topology and dynamics that is similar to the ones observed in the experiments. In the highly non-equilibrium limit with a high driving frequency the model reproduces the experimentally recorded hysteresis loops as well.     

Phase diagrams of a nonequilibrium mixed spin-1/2 and spin-2 Ising ferrimagnetic system under a time-dependent oscillating magnetic field

We present phase diagrams for a nonequilibrium mixed spin-1/2 and spin-2 Ising ferrimagnetic system on a square lattice in the presence of a time dependent oscillating external magnetic field. We employ the Glauber transition rates to construct the mean-field dynamical equations. The time variation of the average magnetizations and the thermal behavior of the dynamic magnetizations are investigated, extensively. The nature (continuous or discontinuous) of the transitions is characterized by studying the thermal behaviors of the dynamic magnetizations. The dynamic phase transition points are obtained and the phase diagrams are presented in two different planes. Phase diagrams contain paramagnetic (p) and ferrimagnetic (i) phases, and one coexistence or mixed phase region, namely the i+p, that strongly depend on interaction parameters. The system exhibits the dynamic tricritical point and the reentrant behaviors.     

Random crystal-field effects in a mixed spin-1/2 and spin-2 ferrimagnetic Ising system

The magnetic properties of a mixed spin ferrimagnetic system σ?=?1/2 and S?=?2 at the sites of a square lattice with a random crystal field are studied by the mean field approximation. The ground state of the system is determined and the total magnetization is plotted according to the model parameters. Different behaviors for the thermal dependence of the magnetization are highlighted. In particular, one sees the appearance of one or two compensating points and many types of phase diagrams with first and second order phase transition lines as well as isolated critical and tricritical points.     

Kinetics of a mixed spin-1/2 and spin-3/2 Ising ferrimagnetic model

We present a study, within a mean-field approach, of the kinetics of a mixed ferrimagnetic model on a square lattice in which two interpenetrating square sublattices have spins that can take two values, , alternated with spins that can take the four values, . We use the Glauber-type stochastic dynamics to describe the time evolution of the system with a crystal-field interaction in the presence of a time-dependent oscillating external magnetic field. The nature (continuous and discontinuous) of transition is characterized by studying the thermal behaviors of average order parameters in a period. The dynamic phase transition points are obtained and the phase diagrams are presented in the reduced magnetic field amplitude (h) and reduced temperature (T) plane, and in the reduced temperature and interaction parameter planes, namely in the (h, T) and (d, T) planes, d is the reduced crystal-field interaction. The phase diagrams always exhibit a tricritical point in (h, T) plane, but do not exhibit in the (d, T) plane for low values of h. The dynamic multicritical point or dynamic critical end point exist in the (d, T) plane for low values of h. Moreover, phase diagrams contain paramagnetic (p), ferromagnetic (f), ferrimagnetic (i) phases, two coexistence or mixed phase regions, (f+p) and (i+p), that strongly depend on interaction parameters.     

Phase relations of hexagonal ferrimagnetic oxides I. The material subsystem BaFe2O4-Fe2O3-ZnFe2O4 at 1200°C

Reaction products of BaO, Fe₂O₃ and ZnO were obtained after prolonged firings at 1200°C, by the air quenching technique. The region of Fe₂O₃ solid solution in zinc spinel and the U phase solubility in the Z phase determine three pseudoternary regions of phase equilibria. The occurence of the M, X, W and Y phases delimitates nine additional three-phase regions. On the basis of the results obtained by X-ray diffractometry and optical microscopy, the 1200°C isothermal section for the subsystem BaO · Fe₂O₃-Fe₂O₃-ZnO · Fe₂O₃ was constructed.     

NMR study of the intermetallic Laves alloys LuFe2 and LuFe1.96Al0.04

NMR spectra of ⁵⁷Fe and ²⁷Al in LuFe₂ and LuFe_0.96Al_0.04 alloys were measured by the spin echo method at 4.2 K. The main contributions to the hyperfine fields on ⁵⁷Fe nuclei are estimated, along with the magnetic moments on the Fe and Lu atoms in LuFe₂. Fiz. Tverd. Tela (St. Petersburg) 41, 1142–1144 (July 1999)     

CoFe2O4和MnFe2O4纳米复合介质的制备及其磁性研究

采用热分解法制备了分散程度高且平均晶粒尺寸为20 nm的CoFe₂O₄和MnFe₂O₄复合介质.低温磁化曲线测量显示,制备的复合介质具有软-硬磁交换弹性耦合效应,且合成温度以及软磁和硬磁相的成分比例对磁交换弹性耦合的强度有很大的影响.变温磁测量显示,温度为20K时,复合纳米介质的表面自旋冻结效应导致饱和磁化强度显著增加.Henkel测量显示,对分散的CoFe₂O₄和MnFe₂O₄复合介质,磁偶极相互作用占主导作用.