Field-induced magnetic phase transitions in the Yafet-Kittel model

The Yafet-Kittel model for a two-sublattice ferrimagnet with an antiferromagnetic exchange interaction in one of the sublattices was developed to describe magnetic-field-induced phase transitions in the isotropic and Ising cases. Depending on the relative values of the exchange parameters of the inter-sublattice interaction and the intra-sublattice interaction in the isotropic case, two types of magnetic phase diagrams with two types of second-order phase transitions are possible: to the noncollinear phase and to the spin-flip phase, and, in the Ising case, three types of magnetic phase diagrams with first-order phase transitions are possible. Fiz. Tverd. Tela (St. Petersburg) 41, 1797–1799 (October 1999)     

Field-induced spin-reorientation transitions in magnetic superlattices with uniaxial anisotropy and biquadratic exchange

Phase transitions induced by an external field are investigated in magnetic multilayer systems with uniaxial anisotropy and biquadratic exchange. A magnetic field directed perpendicular to the plane of the layers changes the effective anisotropy and exchange constants, determining the orientation of the magnetization in the plane of the layers, and can give rise to spin-reorientation transitions. All possible types of such transitions are investigated for the case of uniaxial anisotropy, which differs substantially from the case of cubic anisotropy by the different renormalization of the effective anisotropy constants. Fiz. Tverd. Tela (St. Petersburg) 41, 461–463 (March 1999)     

Field-induced magnetic transitions in Pr2(Co,Fe)17

A study of the different magnetic field-induced transitions has been performed for Pr₂Co_17−xFe_x compounds in the temperature range 78–300 K. By using the singular point detection technique, it has been shown that all compounds with x>0 exhibit at least one field-induced transition, which has been identified as a first-order magnetization process (FOMP). Different types of FOMP (A1, A2, P1) are observed, and the change of the type can be induced by varying either temperature or composition. Intermediate Co–Fe compositions show a P1-type FOMP which persists even above room temperature, while a double transition has been found in iron-rich compounds with x⩾13.6. It has also been noticed that, for the particular values of x when a temperature-induced spin reorientation is present, the change from A1 to P1 type FOMP (or vice versa) exactly occurs at the spin reorientation temperature, with a vanishing critical field value. A comparison with the behavior of Y₂Co_17−xFe_x compounds leads to the conclusion that the contributions of the Pr sublattice are essential for the development of these field-induced transitions (some of which are still present at room temperature), even if it is inferred that the overall anisotropy is mainly determined by the 3d sublattice.     

Quantum phase transitions in CePdAl probed by ultrasonic and thermoelectric measurements

CePdAl has been recently recognized as a frustrated antiferromagnetic heavy-fermion compound with a pressure- or field-tuned, extended quantum critical phase at zero temperature. Identifying characteristic signatures of the emerging quantum critical phase, which are expected to be distinct from those near a quantum critical point, remains challenging. In this work, by performing ultrasonic and thermoelectric measurements down to very low temperatures in a ³He-⁴He dilution refrigerator in the presence of magnetic field, we are able to obtain some crucial thermodynamic and thermal transport features of the quantum critical phase, including a frustration-related elastic softening detected by ultrasound and a Fermi-surface change probed by thermoelectric effect.     

Field-induced phase transitions in antiferroelectric liquid crystals

A theoretical study is made of the process by which an antiferroelectric smectic liquid crystal undergoes a field-induced transition to ferroelectric alignment. We find that for cells of moderate thickness the initial departure from antiferroelectric alignment occurs as a continuous Fréedericksz transition. The following transition from partial alignment to complete ferroelectric ordering may occur as either a first-order or continuous transition, depending on the relative strength of some of the model parameters. The case where the transition is continuous provides a possible mechanism for some recently observed thresholdless transitions in these systems.     

Ferroelectricity and pressure-induced phase transitions in HgTiO3

Using ab initio density functional theory, the ground state of mercury titanate is determined and phase transitions occurring in it at pressures P ?? 210 kbar are analyzed. It is shown that the R3c structure experimentally observed in HgTiO₃ is metastable at P = 0. The ground state structure at T = 0 varies according to the scheme $R3c \to R\bar 3c \to Pbnm$ with increasing pressure in agreement with available experimental data. It is shown that the appearance of ferroelectricity in HgTiO₃ at P = 0 is associated with an unstable soft mode. Some properties of crystals in the $R\bar 3c$ phase are calculated, in particular, the band gap in the GW approximation (E _g = 2.43 eV), which is in better agreement with experimental data than the value obtained in the LDA approximation (1.49 eV). An analysis of the thermodynamic stability explains why the synthesis of mercury titanate is possible only at high pressures.     

Field-induced anisotropic magnetic phase transitions and tricritical phenomena in GdCr6Ge6

The interaction between complex magnetic structures and non-trivial band structures in ternary rare-earth GdCr₆Ge₆ induces exotic and abundant electro-magnetic phenomena. In this work, we perform a systematical investigation on critical behaviors and magnetic properties of the single-crystal GdCr₆Ge₆. The temperature, field, and angle dependence of magnetization unveils strong magnetic anisotropy along the c-axis and isotropic characteristic in the ab-...     

Field-induced transitions and spatial chaos in the classical XY spin chain

We study the lowest-lying excitation of a classical ferromagnetic XY spin chain, in the presence of a symmetry breaking magnetic field. Extremizing the energy of this system leads to a two-dimensional nonlinear map, whose allowed phase space shrinks with increasing field in a nontrivial manner. The orbits of the map represent the set of extremum energy spin configurations. For each field, we compute the energy of the members of this set and find the lowest energy among them, excluding the obvious ground state configuration with all spins parallel along the field direction. This state turns out to be the unstable fixed point of the map. We show that up to a certain (primary) critical field, a separatrixlike 2pi soliton configuration is the lowest-energy excitation, with an energy very close to the ground state energy. For any field beyond this critical field, the soliton disappears and lowest excitation is a librational configuration corresponding to the outermost orbit in the phase plot at that field. Further, its energy is found to be much higher than the ground state energy, leading to a sharp jump in the difference in energy between the former and the latter at this field. With further increase in the field, sharp jumps in the excitation energy arise at certain secondary critical fields as well. We show that these appear when the corresponding spin configurations become commensurate. This complex behavior of the energy is interpreted and its effect on the magnetization and static susceptibility of the system is also studied.     

Field-induced spin-flip and spin-flip transitions in PrGa2

A detailed neutron diffraction study of a single crystal of hexagonal PrGa₂ under an applied field is presented. The field is applied along the easy [100] direction of the hexagonal plane. The compound exhibits several field-induced magnetic structures and, in particular, an uncompensated long period commensurate antiphase structure defined by the propagation vector (4/27, 4/27, 0) at zero field and below T_t = 3.5 K. Concerning the intermediate magnetic structures below T_t, two types of magnetic phase transitions are observed: a spin-flip transition where one moment per magnetic cell turns over, and a spin-slip transition from the low field commensurate propagation vector (4/27, 4/27, 0) to another commensurate vector (1/7, 1/7, 0). Between T_t and T_N = 7.2 K, the structure becomes apmlitude-modulated with a propagation vector (0.148, 0.148, 0.023). Important and original magnetic domain effects are also observed due to the high degeneracy of the easy direction of the hexagonal plane in both T<T_t and T_t<T<T_N regions.     

Field-induced magnetic form factor of lutetium

The conduction electron field-induced magnetic form factor of Lu, a rare earth metal with a closed 4f shell, has been measured using the polarized neutron technique. The measurements demonstrate that the conduction electron density is more spatially extended than in the free atom. The experimental results are in good agreement with band calculations of the $\text{spin}$ magnetic form factor.     

Field-induced magnetic transition in a Cu-Mn spin-glass alloy

The time-independent magnetization of a Cu-Mn alloy (9 at% Mn) as an isothermal function of field is found to describe an S-shaped curve indicative of a field-induced transition. The critical field for the transition decreases to zero as the temperature is raised to that of the susceptibility cusp.     

Field-induced phase transitions of repulsive spin-1 bosons in optical lattices

We study the phase diagram of repulsively interacting spin-1 bosons in optical lattices at unit filling, showing that an externally induced quadratic Zeeman effect may lead to a rich physics characterized by various phases and phase transitions. We find that the main properties of the system may be described by an effective field model, which provides the precise location of the phase boundaries for any dimension, in excellent agreement with our numerical calculations for one-dimensional (1D) systems. Thus, our work provides a quantitative guide for the experimental analysis of various types of field-induced quantum phase transitions in spin-1 lattice bosons. These transitions, which are precluded in spin-1/2 systems, may be realized by using an externally modified quadratic Zeeman coupling, similar to recent experiments with spinor condensates in the continuum.     

Structural transitions in N-isopropylcarbazole single crystal monitored by pressure-induced luminescence

Abstract

Phase diagram for solid-solid transitions in N-isopropyl-carbazole single crystal is determined by monitroing pressure-induced-luminescence (PIL) pulses appearing at transition points. A comparison of the data obtained from PIL and from microcalorimetric measurements shows remarkable difference in the transition temperature at ambient pressure and enormous discrepancy in the transition enthalpy. These discrepances are discussed in terms of local strains influence on parameters of phase transitions.     

Monte Carlo simulation of pressure-induced phase transitions in spin-crossover materials

Pressure-induced phase transitions of spin-crossover materials are studied in a microscopic model taking into account the elastic interaction among distortions of lattice due to the difference of the molecular sizes between the high-spin state and the low-spin state. We perform Monte Carlo simulations in the constant pressure ensemble and reproduce several important properties of the pressure effect in a unified way with a microscopic mechanism for the first time. The simulation newly reveals how the temperature dependence of the ordering process changes with the pressure.     

Spectroscopic investigation of pressure-induced phase transitions in TCNQ complex salts

In most of the TCNQ complex salts, conduction electrons are localized on specific TCNQ sites, so that these salts show nonmetalic behavior. The caesium salt, Cs₂(TCNQ)₃, is one of the 2:3 complex salts. In the crystal, TCNQ molecules form trimeric units, which consist of two TCNQ radical anion sandwiching a neutral TCNQ along the column. The rubidium salt, Rb₂(TCNQ)₃, also has a similar crystal structure to Cs₂(TCNQ)₃. We measured infrared absorption (IR) and Raman spectra for these salts under high pressure by using a diamond anvil cell. In the case of IR spectra, Cs₂(TCNQ)₃ showed a spectral change probably due to a pressure-induced phase transition. Similar feature was not clearly observed in the Rb₂(TCNQ)₃. On the other hand, the Raman spectra, Cs₂(TCNQ)₃ showed two phase transition at 2.5 and 4.1 GPa in the compression stage. The change from localization phase to delocalization phase occurred at latter transition with large hysteresis. Similar phase transition occurred at 3.2 GPa in the Rb₂(TCNQ)₃. The reason for the difference in transition pressure is that the ion radius of Rb⁺ is smaller than that of Cs⁺, because a small ion radius of the counter ion probably favors the charge localization-delocalization transition of the TCNQ column.     

Field-induced magnetic transition in CrCo3

The action of a magnetic field on a single crystal of the compound ErCo₃ has been investigated by both magnetization and neutron diffraction measurements. ErCo₃ is ferrimagnetic with an easy magnetization axis parallel to the c axis of the crystal. When the field is applied perpendicular to the c axis a transition occurs: the moment of one of the two Er sites remains nearly oriented along the c axis while this of the other Er site rotates through an angle of 55°. From the value of the threshold field, the difference between the anisotropy energies of the two sites has been evaluated.