Crossover In Critical Behaviour Research Articles

Anisotropic microwave response in Cr1∕3TaS2 hosting chiral magnetic soliton

Chromium-intercalated hexagonal Cr1∕3TaS2 possesses exotic properties, such as high-temperature magnetic soliton, crossover of critical behavior, and nontrivial magnetism. In this study, we employ the electron spin resonance (ESR) technique to modulate and study the chiral magnetic soliton lattice (CSL) in Cr1∕3TaS2 using microwave at X-band. The experimental results show that Cr1∕3TaS2 has a particularly strong microwave response when H⊥c but very weak feedback when H//c, revealing an anisotropic microwave response. When H⊥c, the resonance peak at lower fields results from the surface barrier induced by the competition between Dzyaloshinsky–Moriya interaction, Zeeman energy, and exchange energy. The dominant resonance is caused by the ferromagnetic mode with the twisted region as the boundary condition, which transforms into a paramagnetic resonance above the phase transition temperature (TC). Furthermore, an unusual temperature dependence of ESR spectra is revealed. The crystalline electric field (CEF) excitation owing to large strong spin–orbit coupling results in the widening of the ESR linewidth above TC. Furthermore, an abrupt shift in the resonance field Hr2 and ESR linewidth ΔHPP2 is observed around TC, which is attributed to a sudden change in spin-exchange interactions and magnetic anisotropy around TC, as Cr1∕3TaS2 was previously observed for spin interaction transition from strong anisotropic 3D-Ising type to isotropic 3D-Heisenberg magnetic interaction below TC. This work demonstrates that the ESR can detect the CSL state and investigate materials with nontrivial magnetism.

Crossover of critical behavior and nontrivial magnetism in the chiral soliton lattice host Cr1/3TaS2

The chiral magnetic soliton, a topological kinklike spin texture, has significant applications in spintronic components. In this work, a crossover of critical behavior is found in ${\mathrm{Cr}}_{1/3}{\mathrm{TaS}}_{2}$, a chiral magnetic soliton host with the highest ${T}_{C}$ to date. Angular-dependent magnetization reveals that ${\mathrm{Cr}}_{1/3}{\mathrm{TaS}}_{2}$ exhibits an easy orientation within the isotropic $ab$ plane, but displays anisotropy with the $c$ axis. By using a modified iterative method, two distinct sets of critical exponents, including ${\ensuremath{\beta}}_{\ensuremath{-}}=0.3190(1)$ and ${\ensuremath{\gamma}}_{\ensuremath{-}}=1.263(8)$ for $T\ensuremath{\le}{T}_{C}$, and ${\ensuremath{\beta}}_{+}=0.3475(2)$ and ${\ensuremath{\gamma}}_{+}=1.385(5)$ for $T\ensuremath{\ge}{T}_{C}$, are acquired on both sides of the transition. Analysis of the exponents indicates a crossover of the magnetic interaction from a three-dimensional Ising type below ${T}_{C}$ to a three-dimensional Heisenberg type above ${T}_{C}$, implying nontrivial magnetism in this system. Based on universality scaling, a detailed $H\ensuremath{-}T$ phase diagram around ${T}_{C}$ is constructed for $H\ensuremath{\perp}c$. The crossover of the critical behavior in ${\mathrm{Cr}}_{1/3}{\mathrm{TaS}}_{2}$ is peculiar to chiral magnetic soliton hosts, suggesting that the three-dimensional magnetic coupling is replaced by a one-dimensional one in the chiral magnetic soliton phase via a phase transition.

Field induced crossover in critical behaviour and direct measurement of the magnetocaloric properties of La0.4Pr0.3Ca0.1Sr0.2MnO3

La0.4Pr0.3Ca0.1Sr0.2MnO3 has been investigated as a potential candidate for room temperature magnetic refrigeration. Results from X-ray powder diffraction reveal an orthorhombic structure with Pnma space group. The electronic and chemical properties have been confirmed by X-ray photoelectron spectroscopy and ion-beam analysis. A second-order paramagnetic to ferromagnetic transition was observed near room temperature (289 K), with a mean-field like critical behaviour at low field and a tricritical mean-field like behaviour at high field. The field induced crossover in critical behaviour is a consequence of the system being close to a first-order magnetic transition in combination with a magnetic field induced suppression of local lattice distortions. The lattice distortions consist of interconnected and weakly distorted pairs of Mn-ions, where each pair shares an electron and a hole, dispersed by large Jahn–Teller distortions at Mn3+ lattice sites. A comparatively high value of the isothermal entropy-change (3.08 J/kg-K at 2 T) is observed and the direct measurements of the adiabatic temperature change reveal a temperature change of 1.5 K for a magnetic field change of 1.9 T.

The dimensional crossover in critical behavior of layered XY-model

The study of critical properties and size transition in a diluted layered XY-model was carried out for the first time. The dimensional dependence of the temperature TBKT(p, N) of the Berezinskii-Kosterlitz-Thouless phase transition on the system thickness N is obtained for a diluted layered XY-model for a wide range of spin concentrations p. The temperature T, concentration p, and dimensional N dependencies of the vortex density v(p, N, T) in a layered XY-model are obtained.

Dimensionality crossover in critical behaviour of ultrathin ferromagnetic films

We propose the model which takes account of magnetocrystalline anisotropy effects in thin magnetic films. The dimensionality crossover from two-dimensional monolayer to three-dimensional system in multilayer magnetic films is studied using a Monte Carlo technique. Finite-size scaling is applied for the determination of the critical characteristics as a function of film thickness. The transition to intermediate planar phase is discussed.

Depth-dependent ordering, two-length-scale phenomena, and crossover behavior in a crystal featuring a skin layer with defects

Structural defects in a crystal are responsible for the "two length-scale" behavior, in which a sharp central peak is superimposed over a broad peak in critical diffuse X-ray scattering. We have previously measured the scaling behavior of the central peak by scattering from a near-surface region of a V2H crystal, which has a first-order transition in the bulk. As the temperature is lowered toward the critical temperature, a crossover in critical behavior is seen, with the temperature range nearest to the critical point being characterized by mean field exponents. Near the transition, a small two-phase coexistence region is observed. The values of transition and crossover temperatures decay with depth. An explanation of these experimental results is here proposed by means of a theory in which edge dislocations in the near-surface region occur in walls oriented in the two directions normal to the surface. The strain caused by the dislocation lines causes the ordering in the crystal to occur as growth of roughly cylindrically shaped regions. After the regions have reached a certain size, the crossover in the critical behavior occurs, and mean field behavior prevails. At a still lower temperature, the rest of the material between the cylindrical regions orders via a weak first-order transition.

Dynamic Crossover of Critical Behavior in Polymer Blend Solutions

By the use of quasi-elastic light scattering, dynamic critical behavior was studied for symmetrical blend solutions of polystyrene/poly(methyl methacrylate)/deuterated benzene and polystyrene/poly(2-chlorostyrene)/deuterated benzene having different polymeric indices N and polymer concentrations c. The decay rate Γ I of polymer-composition fluctuations showed the dynamic crossover from the diffusive to the mode-mode-coupled hydrodynamic behavior in all solutions. The dynamic-crossover number τ*, which represents the distance between the stability limit and the location of the dynamic crossover, was comparable to that theoretically expected for the dynamically-unentangled system in the present ranges ofN and c including a chain-entangled region. The blend solutions with entanglements exhibited a wider mode-mode-coupled regime than theoretical prediction. The dynamic-crossover number τ* was larger than the Ginzburg number Gi, which characterizes the location of the static crossover from the mean-field to the nonclassical Ising behavior, and the mode-mode-coupled dynamics covered a wider region than the statical Ising behavior.

Abrupt but continuous antiferromagnetic transition in nearly stoichiometric La2CuO4+ delta.

Zero-field $^{139}\mathrm{La}$ nuclear quadrupole resonance (NQR) has been used to characterize the antiferromagnetic transition in nearly stoichiometric single crystals of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ (N\'eel temperatures ${\mathit{T}}_{\mathit{N}}$ between 275 and 318 K). The onset of the NQR Zeeman splitting at ${\mathit{T}}_{\mathit{N}}$ is abrupt but continuous, indicative of a second-order phase transition with either a crossover in critical behavior just below ${\mathit{T}}_{\mathit{N}}$ or a very small critical exponent \ensuremath{\beta}\ensuremath{\lesssim}0.1. The absence of any anomaly at ${\mathit{T}}_{\mathit{N}}$ in the $^{139}\mathrm{La}$ electric field gradient indicates no accompanying structural distortion.

Phase transitions and critical phenomena in alloys with random anisotropy.

The effects of random magnetic anisotropy on ferromagnetic critical behavior in four rare-earth-based amorphous alloys, ${\mathrm{Gd}}_{65}$${\mathrm{Co}}_{35}$, ${\mathrm{Nd}}_{50}$${\mathrm{Co}}_{50}$, ${\mathrm{Nd}}_{65}$${\mathrm{Co}}_{35}$, and ${\mathrm{Tb}}_{65}$${\mathrm{Co}}_{35}$, are reported. The ratio of anisotropy to exchange, D/${\mathit{J}}_{0}$, ranges from 0.004(1) to 0.323(35) in these alloys. The zero-field state of the anisotropic alloys (Nd and Tb based) is spin-glass-like below ${\mathit{T}}_{\mathit{c}}$. In order to determine the extent of the critical regime as a function of reduced temperature, \ensuremath{\epsilon}=(T-${\mathit{T}}_{\mathit{c}}$)/${\mathit{T}}_{\mathit{c}}$, and to determine if the asymptotic critical regime is probed, a wide range of \ensuremath{\epsilon} is covered. Evidence for crossovers as a function of applied field and as a function of D/${\mathit{J}}_{0}$ are observed. As a function of applied field, the alloys containing anisotropic rare-earth atoms cross over with increasing field from a state that shows the characteristics of a speromagnet to a state where the magnetic isotherms follow a ferromagnetic-scaling equation of state. The field above which good ferromagnetic scaling is observed increases monotonically with D/${\mathit{J}}_{0}$. No deviations from scaling were found at small \ensuremath{\epsilon} giving confidence that the asymptotic critical regime is probed. Deviations from scaling were seen at large values of \ensuremath{\epsilon} (\ensuremath{\epsilon}>0.06) which gives the limit of the critical regime. We find systematic variations in the measured ferromagnetic critical exponents \ensuremath{\gamma} and \ensuremath{\delta} with D/${\mathit{J}}_{0}$ indicating a crossover in critical behavior presumably from an isotropic to a random anisotropy class with increasing D/${\mathit{J}}_{0}$. Finally, we point out that there are some theoretical hints of a ferromagneticlike state below ${\mathit{T}}_{\mathit{c}}$ in random-anisotropy models.

Experimental evidence of a crossover in critical behaviour of water-in-oil microemulsions

An extensive static and dynamic light-scattering experiment on a water-in-oil microemulsion system (water/n-decane/AOT) showed that the critical dynamics of this three-component microemulsion is completely governed by a Debye cut-off wavenumber qD, which is equal to the inverse diameter of the microemulsion droplet. Furthermore, the authors found occurrence of a crossover phenomenon for the statics and dynamics, at a correlation length xi c such that qD xi c=1, the crossover temperature being at about 10 degrees C below the critical temperature.