Ferromagnetic-paramagnetic Transition Research Articles

Separability Transitions in Topological States Induced by Local Decoherence.

We study states with intrinsic topological order subjected to local decoherence from the perspective of separability, i.e., whether a decohered mixed state can be expressed as an ensemble of short-range entangled pure states. We focus on toric codes and the X-cube fracton state and provide evidence for the existence of decoherence-induced separability transitions that precisely coincide with the threshold for the feasibility of active error correction. A key insight is that local decoherence acting on the "parent" cluster states of these models results in a Gibbs state. As an example, for the 2D (3D) toric code subjected to bit-flip errors, we show that the decohered density matrix can be written as a convex sum of short-range entangled states for p>p_{c}, where p_{c} is related to the paramagnetic-ferromagnetic transition in the 2D (3D) random bond Ising model along the Nishimori line.

Magnetic transition of ytterbium atoms confined in optical superlattice with local ferromagnetic interaction

We used the density matrix renormalization group to study the ground state of ytterbium atoms (171Yb) for the Hund lattice model, where the delocalized atoms are confined in a onc-dimensional optical superlattice and his number is one third of the lattice sites. We found a paramagnetic-ferromagnetic quantum phase transition for any value of the potential strength. The local critical ferromagnetic coupling decreases as the superlattice potential increases.

Thermal and thermoelectric properties of potassium-doped Pr0.6Sr0.4-xKxMnO3 (x = 0.05 and x = 0.1)

The temperature dependence of thermal conductivity and thermopower was studied for a series of monovalent doped Pr0.6Sr0.4-xKxMnO3 (with x = 0.05 and x = 0.1). The influence of the external magnetic field up to 2 T was investigated with a focus on the vicinity of the ferromagnetic–paramagnetic transition. The analysis of κ(T) data suggests that thermal transport is governed by phonon scattering processes, as both the electronic and magnon contribution to total thermal conductivity is well below 1%. The temperature behavior corresponds to typical curves observed in other manganites, both in shape and value. Thermopower investigation S(T) showed a negative value in all studied temperatures, and for further analysis was divided into ferromagnetic and paramagnetic regions. In the first one, the experimental data followed a standard empirical equation S0+S3/2T3/2+S4T4, bringing information about the prevailing electron–magnon scattering processes. For the latter one, the polaron hopping model was used.

Effect of Zn substitution on magnetic properties and magnetic entropy change in La0.7Ba0.25Nd0.05Mn1-xZnxO3 (x = 0.03 and 0.05) synthesized by using sol-gel method

This investigation explored the structural, magnetic, and magnetic entropy change characteristics of La0.7Ba0.25Nd0.05Mn1-xZnxO3 (x = 0.03 and 0.05) synthesized through the sol-gel method. The structural analysis revealed the crystallization of both samples in a Rhombohedral structure with the R-3c space group. The examination of magnetic properties indicated a suppression in magnetic behavior, with the ferromagnetic-paramagnetic transition temperature (TC) decreasing from 285 K to approximately 260 K with an increase in Zn content (x) from 0.03 to 0.05, respectively. The maximum value of magnetic entropy change (−ΔSM) under an applied magnetic field of 50 kOe was determined to be 3.96 J/kg·K for x = 0.03 and 2.27 J/kg·K for x = 0.05. Analysis of Arrot plots and the universal curve of magnetic entropy change confirmed that the ferromagnetic-paramagnetic phase transition observed in both samples exhibits a second-order phase transition (SOPT) transformation.

The effect of the gelation temperature on the structural, magnetic and magnetocaloric properties of perovskite nanoparticles manufactured using the sol-gel method.

This study presents a comprehensive investigation of the structural and magnetic properties of La0.8Sr0.2Mn0.8Co0.2O3 (LS1, LS2 and LS3) compounds synthesized via the sol-gel method at different gelation temperatures through X-ray diffraction and different magnetic measurement techniques. The Rietveld refinement demonstrated that all samples exhibit a rhombohedral perovskite structure with the R3̄C space group. Their magnetic behavior, characterized through magnetization measurements, hysteresis loops, and Arrot plots, demonstrates a ferromagnetic-paramagnetic transition with notable soft ferromagnetic characteristics. The samples also demonstrate second-order magnetic transitions, short-range magnetic order and the presence of both ferromagnetic and antiferromagnetic contributions. AC magnetic susceptibility measurements, allowing the investigation of the magnetic dynamics of the samples, shows that the Vogel-Fulcher and the Conventional Critical Slowing Down models are the most appropriate for describing the dynamic behavior, confirming the spin-glass nature of the compounds and the presence of medium to strong interaction between magnetic nanoparticles. The influence of gelation temperature in the magnetocaloric effect of the compounds was proven and LS1, synthesized at the lowest gelation temperature (70 °C), exhibits the higher magnetic entropy change (|ΔSmax| = 3.25 J kg-1 K-1 and RCP = 209.83 J kg-1 at 5 T). For a better evaluation of the magnetocaloric efficiency, the temperature average entropy change (TEC) parameter was calculated for all three samples and LS1 showed the highest value (TEC (LS1) ∼3.2 J kg-1 K-1 for ΔTH-C = 10 K and ΔH = 5 T).

Nonequilibrium critical dynamics of the two-dimensional ±J Ising model.

The ±J Ising model is a simple frustrated spin model, where the exchange couplings independently take the discrete value -J with probability p and +J with probability 1-p. It is especially appealing due to its connection to quantum error correcting codes. Here, we investigate the nonequilibrium critical behavior of the two-dimensional ±J Ising model, after a quench from different initial conditions to a critical point T_{c}(p) on the paramagnetic-ferromagnetic (PF) transition line, especially above, below, and at the multicritical Nishimori point (NP). The dynamical critical exponent z_{c} seems to exhibit nonuniversal behavior for quenches above and below the NP, which is identified as a preasymptotic feature due to the repulsive fixed point at the NP, whereas for a quench directly to the NP, the dynamics reaches the asymptotic regime with z_{c}≃6.02(6). We also consider the geometrical spin clusters (of like spin signs) during the critical dynamics. Each universality class on the PF line is uniquely characterized by the stochastic Loewner evolution with corresponding parameter κ. Moreover, for the critical quenches from the paramagnetic phase, the model, irrespective of the frustration, exhibits an emergent critical percolation topology at the large length scales.

Traveling/non-traveling phase transition and non-ergodic properties in the random transverse-field Ising model on the Cayley tree

We study the random transverse field Ising model on a finite Cayley tree. This enables us to probe key questions arising in other important disordered quantum systems, in particular the Anderson transition and the problem of dirty bosons on the Cayley tree, or the emergence of non-ergodic properties in such systems. We numerically investigate this problem building on the cavity mean-field method complemented by state-of-the art finite-size scaling analysis. Our numerics agree very well with analytical results based on an analogy with the traveling wave problem of a branching random walk in the presence of an absorbing wall. Critical properties and finite-size corrections for the zero-temperature paramagnetic-ferromagnetic transition are studied both for constant (independent of the system volume) and algebraically vanishing (scaling as an inverse power law with the system volume) boundary conditions. In the later case, we reveal a regime which is reminiscent of the non-ergodic delocalized phase observed in other systems, thus shedding some light on critical issues in the context of disordered quantum systems, such as Anderson transitions, the many-body localization or disordered bosons in infinite dimensions.

Highly oriented crystallization, and tunable structural transformation and magnetic transition in Ni-Co-Mn-Al shape memory alloys

Alloy ribbons of Ni50-xCoxMn50-yAly with x = 5–10 and y = 16–17 were prepared by rapidly solidified technique. Highly oriented column-shaped crystalline grains are observed in the alloy ribbons. The formation of martensitic (L10) and austenitic (B2) phases and their structural transformation are strongly dependent on the concentration correlation of Co and Al. Co tends to enhance the fraction and the ferromagnetic interaction of the austenitic phase leading to a greatly increase of the saturation magnetization and Curie temperature of the alloy. The martensitic-austenitic (M−A) structural transformation process can be tuned to occur entirely above room temperature by appropriate concentrations of Co and Al. An irregular variation of the M−A transformation temperature happens in a narrow range, 8–8.5 at%, of Co concentration, while the ferromagnetic–paramagnetic (FM-PM) phase transition temperature monotonically increases. The external magnetic field favors the appearance of the austenitic phase and induces shifts to low and high temperatures for the M−A transformation and the FM-PM transition, respectively.

Theoretical study on the origin of anomalous temperature-dependent electric resistivity of ferromagnetic semiconductor

Employing Korringa–Kohn–Rostoker Green’s function methodology, our investigation elucidates the previously obscure origins of the anomalous temperature-dependent electrical resistivity behavior of (Ga,Mn)As ferromagnetic semiconductors. Phonon and magnon excitations induced by temperature effects are addressed via the coherent potential approximation, while the Kubo–Greenwood formula is employed to compute transport properties. Consequently, the anomalous temperature-dependent electrical resistivity arising from the ferromagnetic–paramagnetic transition is successfully replicated. Our examination of electronic structures and magnetic interactions reveals pivotal roles played by antisite defects and interstitial Mn atoms in governing this behavior. As this approach enables both the estimation of temperature-dependent transport properties and the assessment of underlying mechanisms from a microscopic standpoint, it holds significant potential as a versatile tool across diverse fields.

Thermodynamic Properties and Magnetocaloric Effect of Spins - 1/2 Square Lattice

This work aims to investigate the thermodynamic properties and magnetocaloric effect (MCE) of spins - 1/2 square lattice using Monte Carlo simulation. The considered Ising model is parameterized by the couplings J, [Formula: see text] and [Formula: see text]; where J is the ferromagnetic exchange interaction between first nearest-neighbor spins, a is the anisotropic parameter and [Formula: see text] is the ferromagnetic exchange interaction between second nearest-neighbor spins. Monte Carlo simulation under Metropolis algorithm allows us to study some thermodynamic properties and MCE of spins - 1/2 square lattice. The discussion of the obtained results allowed us to specify the conditions which can enrich the MCE in spins - 1/2 square lattice. The two-dimensional anisotropic Ising model presents clarifying the critical point of the ferromagnetic–paramagnetic transition, and we have shown that the maximum entropy variation increases when it occurs at the ferromagnetic–paramagnetic transition temperature regardless of the intensity of the applied magnetic field and the values of “a” and “b”.

Study of AC Susceptibility of La0.8Sr0.2MnO3 at Different Frequencies

In the present work, the structural and ac magnetic susceptibility as a function of temperature at different frequencies of perovskite type La08Sr0.2MnO3 (LSMO) compound have been discussed. Conventional solid-state route was used to synthesize LSMO compound. Orthorhombic crystal structure with Pnma space group of the compound was witnessed from the analysis of X-ray diffractograms by Rietveld refinement. The real (χꞌ) and imaginary (χꞌꞌ) part of the ac susceptibility of the compound have been studied. The real part of magnetic susceptibility (χꞌ) of La0.8Sr0.2MnO3 sample increases as temperature decrease. This indicates that the paramagnetic-ferromagnetic phase transition occurs. At different frequencies, i.e. 100Hz and 500Hz the Curie temperature Tc are 326K and 336K respectively. Peaks observed in the imaginary part χꞌꞌ) of magnetic susceptibility plot indicate the presence of sample inhomogeneity.

Correlation between critical magnetic behavior and griffiths phase in ferromagnetic La0.7Sr0.3Mn1−xFexO3 (x = 0.05) nanoparticle

The magnetic behavior of the La0.7Sr0.3Mn0.95Fe0.05O3 nanoparticles was investigated. X-ray diffraction analysis revealed that this sample exhibited rhombohedral symmetry with space group. The sample underwent a ferromagnetic-paramagnetic transition at approximately 290 K, characterized by a second-order phase transition. Interestingly, the state above the transition temperature is not purely paramagnetic but rather exhibits a short-range ferromagnetic order or Griffiths phase, which becomes more pronounced as the temperature increases up to the Griffiths transition temperature (345 K). The values of the critical exponents obtained in this study, β = 0.671, γ = 1.087, and δ = 2.612, with T C = 290.09 K, are distinct from those of any known universality class, and are confirmed by a single scaling equation of state. This unusual behavior is ascribed to the existence of the Griffiths phase, which reflects the unconventional critical behavior of magnetic susceptibility. In addition to conventional magnetic measurements, detailed studies of isothermal magnetization and magnetocaloric effects have been performed, and theoretical analyses have been utilized to further explore the Griffiths phase. Furthermore, a correlation between the Griffiths phase and the anomalous critical parameters was established.

A DFT and Monte Carlo studies of physical properties of the Fe2N compound

In this paper, we studied the magnetism of Fe2N, using the ab initio and Monte Carlo studies. The magnetic moment and the exchange couplings are deduced by ab initio calculations. The obtained density of states is not symmetrical with respect to the axis of energy proving the ferromagnetic character of this compound. Any gap energy, at the Fermi level, is found confirming the metallic character of this compound. The Hamiltonian of this system consists of several physical parameters. Both the magnetic entropy and the maximum magnetic entropy are obtained. The relative cooling power and the magnetic hysteresis cycle are illustrated. The transition temperature TC value has been deduced and found to be about 252.01 K. This value is in good agreement with the experimental results presented. Also, we found that the magnetizations decrease rapidly to zero with increasing temperature. This is due to the ferromagnetic-paramagnetic phase transition caused by thermal energy.

Coexistence of normal and inverse magnetocaloric effect in inhomogeneous Ni95Cr5 layers

Understanding of the coexistence of normal and inverse magnetocaloric effects (MCE) makes it promising for use in magnetic cooling applications. This study discusses the selection of Ni95Cr5 layers and their expected magnetocaloric effects for an extended temperature range (200–800 K). These layers offer a wide range of suitable magnetic ordering temperatures as a function of thicknesses. A modified phenomenological model was used to estimate the entropy changes () and relative cooling power (RCP). Under adiabatic magnetic field variation, normal and inverse MCE are computed near the ferromagnetic-paramagnetic transition temperatures, T C and spin−glass transition temperatures, T G , respectively. Based on their values (= −0.06 to 0.11 J kg−1-K−1) and RCP( = 7.6 to 15.5 J kg−1) at the smallest magnetic field of 0.5 kOe, which are comparable to other MCE layer materials. We provide evidence of a strong correlation between the change in magnetic anisotropy (measured in terms of MR/MS and HC) below T G , which subsequently leads to the reversal of However, after annealing these effects somewhat vanish. The current study demonstrates that adjusting thickness, in addition to Cr doping, can externally regulate soft magnetic and magnetocaloric properties to create diverse magnetic ground states for future MCE and inverse MCE functionalities.

Gaussian-state Ansatz for the non-equilibrium dynamics of quantum spin lattices

The study of non-equilibrium dynamics is one of the most important challenges of modern quantum many-body physics, in relationship with fundamental questions in quantum statistical mechanics, as well as with the fields of quantum simulation and computing. In this work we propose a Gaussian Ansatz for the study of the nonequilibrium dynamics of quantum spin systems. Within our approach, the quantum spins are mapped onto Holstein-Primakoff bosons, such that a coherent spin state - chosen as the initial state of the dynamics - represents the bosonic vacuum. The state of the system is then postulated to remain a bosonic Gaussian state at all times, an assumption which is exact when the bosonic Hamiltonian is quadratic; and which is justified in the case of a nonlinear Hamiltonian if the boson density remains moderate. We test the accuracy of such an Ansatz in the paradigmatic case of the S=1/2S=1/2 transverse-field Ising model, in one and two dimensions, initialized in a state aligned with the applied field. We show that the Gaussian Ansatz, when applied to the bosonic Hamiltonian with nonlinearities truncated to quartic order, is able to reproduce faithfully the evolution of the state, including its relaxation to the equilibrium regime, for fields larger than the critical field for the paramagnetic-ferromagnetic transition in the ground state. In particular the spatio-temporal pattern of correlations reconstructed via the Gaussian Ansatz reveals the dispersion relation of quasiparticle excitations, exhibiting the softening of the excitation gap upon approaching the critical field. Our results suggest that the Gaussian Ansatz correctly captures the essential effects of nonlinearities in quantum spin dynamics; and that it can be applied to the study of fundamental phenomena such as quantum thermalization and its breakdown.

Structural and critical properties of Pr0.5Sr0.5MnO3 nanoparticle

In this work, a sample of Pr0.5Sr0.5MnO3 nanoparticles with an average crystalline size of <D> = 58 ± 2 nm was prepared by a combination of reactive milling method for 6 h at room temperature and heat treatment at the 1100 °C for 0.5 h. The x-ray diffraction analysis revealed the existence of a Pr0.5Sr0.5MnO3 single phase with the tetragonal structure (I4/mcm space group). Temperature and magnetic field dependences of magnetisation measurements indicated a coexistence of two magnetic phase transitions. One is the antiferromagnetic-ferromagnetic transition at T N = 150 K. The other is the second-order ferromagnetic-paramagnetic phase transition at T C = 273.5 K. Using the modified Arrott plots and the Kouvel-Fisher methods, the critical isotherm analysis, and the scaling relation, the magnetic order in Pr0.5Sr0.5MnO3 nanoparticle sample has been pointed out. Accordingly, the critical exponents were found to be β = 0.486, γ = 1.181, and δ = 3.249. These values are quite close to the allowable exponents of the mean field model, suggesting an existence of the long-range ferromagnetic order. A slight deviation from the mean field model has been explained by the formation of the core/shell structure in Pr0.5Sr0.5MnO3 nanoparticle.

Electronic origin of half-metal to semiconductor transition and colossal magnetoresistance in spinel HgCr2Se4

Half-metals are ferromagnets hosting spin-polarized conducting carriers and crucial for spintronics applications. The chromium spinel HgCr2Se4 represents a unique type of half-metal, which features a half-metal to semiconductor transition (HMST) and exhibits colossal magnetoresistance (CMR) across the ferromagnetic-paramagnetic (FM-PM) transition. Using angle-resolved photoemission spectroscopy (ARPES), we find that the Fermi surface of n-type HgCr2Se4 (n-HgCr2Se4) consists of a single electron pocket which moves above the Fermi level (EF) upon the FM-PM transition, leading to the HMST. Such a Lifshitz transition manifests a giant band splitting which originates from the exchange interaction unveiled with a specific chemical nonstoichiometry. The exchange band splitting and the chemical nonstoichiometry are two key ingredients to the HMST and CMR, consistent with our ab-initio calculation. Our findings provide spectroscopic evidences of the electronic origin of the anomalous properties of HgCr2Se4, which address the unique phase transition in half-metals.

Quantification of electronic and magnetoelastic mechanisms of first-order magnetic phase transitions from first principles: application to caloric effects in La(FeSi)13

and derived quaternary compounds are well-known for their giant, tunable, magneto- and barocaloric responses around a first-order paramagnetic-ferromagnetic transition near room temperature with low hysteresis. Remarkably, such a transition shows a large spontaneous volume change together with itinerant electron metamagnetic features. While magnetovolume effects are well-established mechanisms driving first-order transitions, purely electronic sources have a long, subtle history and remain poorly understood. Here we apply a disordered local moment picture to quantify electronic and magnetoelastic effects at finite temperature in from first-principles. We obtain results in very good agreement with experiment and demonstrate that the magnetoelastic coupling, rather than purely electronic mechanisms, drives the first-order character and causes at the same time a huge electronic entropy contribution to the caloric response.

Optimization of temperature coefficient of resistivity and magnetoresistance of La0.67Ca0.33Mn0.98Fe0.02O3: Agx ceramics by Ag adjustment

High-quality La0.67Ca0.33Mn0.98Fe0.02O3: Agx ceramics were prepared by sol-gel and solid-phase methods. Ag and Ag+ were found to be present in the system after Ag doping, exhibiting a significant effect on the crystal structure and the concentration of Mn3+/Mn4+. Ag provided the desired conductive pathway, promoted the connectivity between grains, and improved the electrical conductivity of the ceramic. With the increase of Ag doping, the resistivity gradually decreased, while the Curie and the metal-insulator transition temperatures gradually increased. The temperature coefficient of resistivity (TCR) and the magnetoresistance (MR) were greatly improved. At x = 0.15, the peak TCR reached 75.4%·K−1, and the peak MR reached 87.0%. In order to adjust the TCR and MR, the contents of Ag and Ag+ were optimized by controlling the amount of Ag doping. The results demonstrated that Ag doping can regulate the metal-insulator transition and ferromagnetic-paramagnetic transition of the system effectively.

Role of Mn2+ doped in La0.66-xCa0.33-yMn1+x+yO3 porous nanospheres: Structure, critical behavior, magnetocaloric effects and a first-order reversal curve analysis

In this study, the effects of A-site La and Ca defects with Mn2+ doping on the structure, magnetic phase transition, magnetocaloric effect, critical indices, and magnetic interactions of La0.66-xCa0.33-yMn1+x + yO3 porous nanospheres were investigated. The results indicated that the critical parameters of La0·66Ca0·33MnO3 porous nanospheres were similar to those of the short-range ordered Ising model. The A-site La-defect Mn2+ doping makes the critical parameters β and γ tend to the Ising model and mean-field model, respectively. When A-site Ca-defect Mn2+ doping is obtained, β and γ tend to follow the Ising model and 3D-Heisenberg model, respectively. The phase transition exhibited a second-order ferromagnetic-paramagnetic transition for all samples. The A-site defect Mn2+ doping in La0.66-xCa0.33-yMn1+x + yO3 porous nanospheres enhanced the Curie temperature to near room temperature without affecting magnetic entropy change. The effect of A-site defect Mn2+ doping on the magnetic interaction of La0.66-xCa0.33-yMn1+x + yO3 was visualized by the first-order reversal curve. The variation of interaction field distribution at the Curie temperature and 300 K indicated that exchange interaction was dominant at high temperature and dipole interaction was dominant at the Curie temperature.