Antiferromagnetic-paramagnetic Phase Transition Research Articles

Quantum phase transitions in the triangular coupled-top model

We study the coupled-top model with three large spins located on a triangle. Depending on the coupling strength, there exist three phases: disordered paramagnetic phase, ferromagnetic phase, and frustrated antiferromagnetic phase, which can be distinguished by the mean-field approach. The paramagnetic-ferromagnetic phase transition is accompanied by the breaking of the global ${Z}_{2}$ symmetry, whereas the paramagnetic-antiferromagnetic phase transition is accompanied by the breaking of both the global ${Z}_{2}$ symmetry and the translational symmetry. Exact analytical results of higher-order quantum effects beyond the mean-field contribution, such as the excitation energy, quantum fluctuation, and von Neumann entropy, can be achieved by the Holstein-Primakoff transformation and symplectic transformation in the thermodynamic limit. Near the quantum critical point, the energy gap closes, along with the divergence of the quantum fluctuation in certain quadrature and von Neumann entropy. Particular attention should be paid to the antiferromagnetic phase, where geometric frustration takes effect. The critical behaviors in the antiferromagnetic phase are quite different from those in the paramagnetic and ferromagnetic phases, which highlight the importance of geometric frustration. The triangular coupled-top model provides a simple and feasible platform to study the quantum phase transition and the novel critical behaviors induced by geometric frustration.

Exact and many-body perturbation solutions of the Hubbard model applied to linear chains

This study reports on the accuracy of the GW approximation for the treatment of the Hubbard model compared to exact diagonalization (ED) results. We consider not only global quantities, such as the total energy and the density of states, but also the spatial and spin symmetry of wavefunctions via the analysis of the local density of states. GW is part of the more general Green’s function approach used to develop many-body approximations. We show that, for small linear chains, the GW approximation corrects the mean-field (MF) approach by reducing the total energy and the magnetization obtained from the MF approximation. The GW energy gap is in better agreement with ED, especially in systems of an even number of atoms where, in contrast to the MF approximation, no plateau is observed below the artificial predicted phase transition. In terms of density of states, the GW approximation induces quasi-particles and side satellite peaks via a splitting process of MF peaks. At the same time, GW slightly modifies the localization (e.g., edges or center) of states. We also use the GW approximation results in the context of Löwdin’s symmetry dilemma and show that GW predicts an artificial paramagnetic–antiferromagnetic phase transition at a higher Hubbard parameter than MF does.

The antiferromagnetic phase transition in the layered Cu0.15Fe0.85PS3 semiconductor: experiment and DFT modelling

The experimental studies of the paramagnetic-antiferromagnetic phase transition through Mössbauer spectroscopy and measurements of temperature and field dependencies of magnetic susceptibility in the layered Cu0.15Fe0.85PS3 crystal are presented. The peculiar behavior of the magnetization - field dependence at low-temperature region gives evidence of a weak ferromagnetism in the studied alloy. By the ab initio simulation of electronic and spin subsystems, in the framework of electron density functional theory, the peculiarities of spin ordering at low temperature as well as changes in interatomic interactions in the vicinity of the Cu substitutional atoms are analyzed. The calculated components of the electric field gradient tensor and asymmetry parameter for Fe ions are close to the ones found from Mössbauer spectra values. The Mulliken populations show that the main contribution to the ferromagnetic spin density is originated from 3d-copper and 3p-sulfur orbitals. The estimated total magnetic moment of the unit cell (8.543 emu/mol) is in reasonable agreement with the measured experimental value of ∼9 emu/mol.

Are the thermodynamic properties of natural and synthetic Mg2SiO4-Fe2SiO4 olivines the same?

Abstract It is not known if the thermodynamic behavior of some minerals and their synthetic analogues are quantitatively the same. Olivine is an important rock-forming substitutional solid solution consisting of the two end-members forsterite, Mg2SiO4, and fayalite, Fe2SiO4. We undertook the first heat capacity, CP, measurements on two natural olivines between 2 and 300 K; nearly end-member fayalite and a forsterite-rich crystal Fo0.904Fa0.096. Their CP(T) behavior is compared to that of synthetic crystals of similar composition, as found in the literature. The two natural olivines are characterized by X-ray powder diffraction and 57Fe Mössbauer spectroscopy. The X-ray results show that the crystals are well crystalline. The Mössbauer hyperfine parameters, obtained from a fit with two Fe2+ quadrupole split doublets, are similar to published values measured on synthetic olivines. There are slight differences in the absorption line widths (i.e., FWHM) between the natural and synthetic crystals. CP (2 to 300 K) is measured by relaxation calorimetry. The CP results of the natural nearly end-member fayalite and published values for two different synthetic Fa100 samples are in excellent agreement. Even CP resulting from a Schottky anomaly and a paramagnetic-antiferromagnetic phase transition with both arising from Fe2+ are similar. There are slight differences in the Néel temperature between the natural 63 K and synthetic ~65 K fayalites. This is probably related to the presence of certain minor elements (e.g., Mn2+) in the natural crystal. The third-law entropy, S°, value is 151.6 ± 1.1 J/(mol·K). CP behavior of the natural forsterite, Fo0.904Fa0.096, and a synthetic olivine, Fo90Fa10, are in excellent agreement between about 7 and 300 K. The only difference lies at T < 7 K, as the former does not show Debye T3 behavior, but, instead, a plateauing of CP values. The S° value for the natural forsterite is 99.1 ± 0.7 J/(mol·K).

Nanowire reconstruction under external magnetic fields.

We consider the different structures that a magnetic nanowire adsorbed on a surface may adopt under the influence of external magnetic or electric fields. First, we propose a theoretical framework based on an Ising-like extension of the 1D Frenkel-Kontorova model, which is analyzed in detail using the transfer matrix formalism, determining a rich phase diagram displaying structural reconstructions at finite fields and an antiferromagnetic-paramagnetic phase transition of second order. Our conclusions are validated using ab initio calculations with density functional theory, paving the way for the search of actual materials where this complex phenomenon can be observed in the laboratory.

Surface-induced linear magnetoresistance in the antiferromagnetic topological insulator MnBi2Te4

Through a thorough magneto-transport study of antiferromagnetic topological insulator MnBi2Te4 (MBT) thick films, a positive linear magnetoresistance (LMR) with a two-dimensional (2D) character is found in high perpendicular magnetic fields and temperatures up to at least 260 K. The nonlinear Hall effect further reveals the existence of high-mobility surface states in addition to the bulk states in MBT. We ascribe the 2D LMR to the high-mobility surface states of MBT, thus unveiling a transport signature of surface states in thick MBT films. A suppression of LMR near the Neel temperature of MBT is also noticed, which might suggest the gap opening of surface states due to the paramagnetic-antiferromagnetic phase transition of MBT. Besides these, the failure of the disorder and quantum LMR model in explaining the observed LMR indicates new physics must be invoked to understand this phenomenon.

Raman spectroscopy of alpha-FeOOH (goethite) near antiferromagnetic to paramagnetic phase transition

Synthetic powder, ore samples, and mineral single crystals of goethite (α-FeOOH) were investigated with polarized Raman spectroscopy at temperatures from 293 K to 473 K. The symmetry of the vibrational modes, observed in different scattering configurations, was determined unequivocally. The assignment of the Raman-active modes to definite atomic vibrations is supported by two types of lattice-dynamical calculations: empirical shell model and ab initio density functional theory. The temperature dependencies of the line shape parameters of some Raman-active vibrations at temperatures near to the antiferromagnetic–paramagnetic phase transition infers for a significant spin–lattice coupling in this compound. The most informative in this aspect is the B3g phonon at 387cm−1, which overlays a broad scattering background and displays a pronounced asymmetric Fano-line shape. The asymmetry increases in the paramagnetic state above the Néel temperature (TN=393K), indicating a strong interaction of this mode with the underlying excitation continuum. The origin of the excitation background is discussed in light of our experimental results and the existing data for the magnetic structure and transport properties of α-FeOOH. By using the molecular-orbital dimer approach, we calculate the spin–phonon coupling constants for the B3g mode as J′=−0.2eV/Å and J″=+0.18eV/Å2, respectively. Thus, we rationalize that, most probably, the scattering background stems from magnetic excitations, and the asymmetric shape of the B3g band is a result of a linear spin–phonon coupling of this mode with the Fe–O1–Fe spin dimers. Another mechanism, a phonon interaction with thermally activated charge carriers above TN, is also considered.

Optical study on the possible Slater insulator SrIr0.8Sn0.2O3

The orthorhombic perovskite iridate ${\mathrm{SrIr}}_{0.8}{\mathrm{Sn}}_{0.2}{\mathrm{O}}_{3}$ is reported to be a new candidate of Slater insulator at low temperatures. Here, we present optical spectroscopy measurements on ${\mathrm{SrIr}}_{0.8}{\mathrm{Sn}}_{0.2}{\mathrm{O}}_{3}$ at different temperatures ($T$) across the paramagnetic-antiferromagnetic phase transition. The low energy reflectivity decreases with reducing $T$. Six phonon peaks in the spectrum at low $T$ are visible at room temperature, indicating the absence of crystal structure phase transition from 300 K to 10 K. The real part of conductivity spectra ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ are obtained by employing the standard Kramers-Kronig transformation. In the paramagnetic state at room temperature, ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ reaches a limiting value as $\ensuremath{\omega}\ensuremath{\rightarrow}0$, implying an electrical conducting state. Upon decreasing $T$, the low frequency ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ decreases and approaches zero with a concomitant opening of a direct gap. Detailed analysis of the low energy data shows that the gap opening is continuous and ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ at the low energy gap edge follows an unusual ${\ensuremath{\omega}}^{1/2}$ dependence at low temperatures which is remarkably similar to other Slater insulators, such as ${\mathrm{NaOsO}}_{3}$.

Structural, electronic, magnetic, and magnetocaloric properties in intermetallic compound TbCu2Si2

Abstract The electronic, magnetic, and magnetocaloric properties of the intermetallic TbCu2Si2compound are studied. From Ab-initio calculations, we found that the magnetic moment and the exchange coupling interactions values are 5.93 μ B and J1 = 0.58 meV, J2 = -0.69 meV and J3 = 0.41 meV, respectively. On the other, the magnetic and magnetocaloric properties are calculated using Monte Carlo simulation based on the Ising model. The intermetallic TbCu2Si2compound exhibits a second-order antiferromagnetic-paramagnetic phase transition around TN 8K, as well as a large MCE has been obtained for different external magnetic field values. Besides, the maximum values of the magnetic entropy change (ΔSmag) and the relative cooling power (RCP) are found to be 40.78 J/Kg.K, 71.58 J/kg, respectively, at h = 2 T. Additionally, by increasing the magnetic field, it is shown that the maximum of ΔSmag increase widely and TN shifts to lower temperature.

Влияние фазового перехода второго рода на электропроводность структуры металл/полупроводник

AbstractThe properties of the Cr– p -Si potential barrier near the temperature of the antiferromagnetic–paramagnetic phase transition in chromium are investigated. A significant change in the potential barrier and an anomalous increase in the conductivity in the Cr– p -Si–Au structure are observed near the temperature of the second-order antiferromagnetic–paramagnetic phase transition in chromium. It is established that current fluctuations in the structure are enhanced when approaching the phase-transition point. The experimental results are interpreted based on the assumption that the observed change in the electron-transport properties of the Cr–Si interface is due to a shift of the Fermi quasi-level in chromium as a result of the second-order phase transition.

Influence of Dynamic Three Point Bending on the Work Hardening Capacity of T105Mn120 Manganese Steel

The paper analyzes work hardening behavior of T105Mn120 Hadfield steel under dynamic conditions. The specimens were investigated under two states: (a) untreated (as cast) and (b) solution treated to 1100 °C. Dynamic flexural behavior was examined by means of three-point-bending tests performed with a dynamic mechanical analyser (DMA), and structural analysis was done by x-ray diffraction, optical and scanning electron microscopy; DMA tests were performed under two variants: (a) temperature scans, between − 150 and 400 °C and (b) isothermal strain sweeps, up to 0.15% strain amplitudes. The former emphasized the critical temperatures of thermally induced reversible martensitic transformation and antiferromagnetic–paramagnetic phase transition, while the latter enabled to monitor the storage modulus increase due to the work hardening caused by dynamic bending. Strain sweeps tests revealed the effects of both dynamic bending frequency and number of cycles. The largest work hardening effect, obtained after five strain sweep cycles applied at the frequency of 5 Hz, was associated with the finest distribution of precipitated carbides, observed by differential scanning calorimetry and the formation of slip micro-bands, illustrated on OM micrographs and SEM energy dispersion spectroscopy maps.

Spin transport across antiferromagnets induced by the spin Seebeck effect

For prospective spintronics devices based on the propagation of pure spin currents, antiferromagnets are an interesting class of materials that potentially entail a number of advantages as compared to ferromagnets. Here, we present a detailed theoretical study of magnonic spin current transport in ferromagnetic-antiferromagnetic multilayers by using atomistic spin dynamics simulations. The relevant length scales of magnonic spin transport in antiferromagnets are determined. We demonstrate the transfer of angular momentum from a ferromagnet into an antiferromagnet due to the excitation of only one magnon branch in the antiferromagnet. As an experimental system, we ascertain the transport across an antiferromagnet in Y3Fe5O12 |Ir20Mn80|Pt heterostructures. We determine the spin transport signals for spin currents generated in the Y3Fe5O12 by the spin Seebeck effect and compare to measurements of the spin Hall magnetoresistance in the heterostructure stack. By means of temperature-dependent and thickness-dependent measurements, we deduce conclusions on the spin transport mechanism across Ir20Mn80 and furthermore correlate it to its paramagnetic-antiferromagnetic phase transition.

Effect of Gd substitution on the structural, magnetic, and magnetocaloric properties of HoCrO3

Rare-earth chromites are a new type of magnetoelectric multiferroics. In this work, a Ho0.33Gd0.67CrO3 powder sample was synthesized via a citrate route, and the structural properties were characterized by X-ray diffraction, scanning electron microscopy, and the Raman technique. The UV-Visible optical absorbance spectra were also measured in the wavelength range of 200–800 nm. The valence state of Cr was found to be purely 3+ according to the X-ray photoelectron spectroscopy. The temperature-dependent dielectric constant and loss tangent data measured between the frequencies of 1 kHz and 1 MHz show no anomalies around the magnetic transition temperature of the material. The dc magnetization measurements show that the ordering temperature of Cr3+ (TNCr) is 155 K for Ho0.33Gd0.67CrO3, which is larger than 140 K for HoCrO3. The positive slope of the Arrott plots from 0 T to 7 T reveals that the antiferromagnetic-paramagnetic phase transition is second-order in nature. At a field of 7 T, the Ho0.33Gd0.67CrO3 sample showed a giant magnetocaloric entropy change, −ΔS, of ∼23.3 J/kg K at 5 K, and a refrigeration capacity of ∼481.2 J/kg, which are much higher than those of pure bulk HoCrO3. This renders this material prospective for magnetic refrigeration in the low temperature (<30 K) range.

Spin orientation in solid solution hematite-ilmenite

The spin orientation in synthetic hematite-ilmenite samples and in a sample of natural hematite was studied from room temperature to above the antiferromagnetic-paramagnetic phase transition (the Néel temperature; TN ≈ 600–950 K) by neutron powder diffraction and at room temperature by Mössbauer spectroscopy. The usually assumed magnetic structure of hematite within this temperature range is antiferromagnetic with the spins confined to the basal plane of the hexagonal structure; however, an out-of-plane spin component is allowed by the symmetry of the system and has been observed in recent studies of synthetic hematite samples. We find the spins in the antiferromagnetic sublattices to be rotated out of the basal plane by an angle between 11(2)° and 22.7(5)° in both synthetic hematite-ilmenite samples and in the natural hematite sample. The spin angle remains tilted out of the basal plane in the entire temperature range below the Néel temperature and does not depend systematically on Ti-content. The results indicate that the out-of-plane spin component is an intrinsic feature of hematite itself, with an origin not yet fully understood, but consistent with group theory. This represents a major shift in understanding of one of the two main mineral systems responsible for rock magnetism.

“Entropics”: Science and engineering of caloric phenomena related to itinerant-electron magnetism and spin fluctuations

In this paper, the relationship between giant caloric phenomena and itinerant-electron magnetism is examined in order to construct of a newly proposed concept “Entropics”, which is a fusion of science and technology with the objective of solving and controlling entropic phenomena. An anomalous hall resistivity is present in the paramagnetic state of the La(Fe0.88Si0.12)13 magnetocaloric compound. Further, its coefficient exhibits a Curie-Weiss type temperature dependence, indicating the existence of disordered local moment, even though the Rhodes-Wohlfarth (RW) ratio reveals that the magnetic feature in the system is an itinerant-electron type. In addition, the correlation between the magnitude of the transition entropy change of the itinerant-electron metamagnetic transition and the RW ratio was observed. In the Mn3GaN barocaloric compound, the transition entropy of the first-order antiferromagnetic-paramagnetic phase transition marginally depends on the external pressure, in contrast to the data for Gd5Ge2Si2. The origin of this tendency is phase stability against the pressure, as opposed to large volume change at the transition temperature, which results in an enhancement of the barocaloric effect. The influence of topological frustration is also distinguished by comparing it with that of other Mn-based antiperovskite compounds.

Magneto-elastic coupling across the first-order transition in the distorted kagome lattice antiferromagnet Dy3Ru4Al12

Structural changes through the first-order paramagnetic–antiferromagnetic phase transition of Dy3Ru4Al12 at 7K have been studied by means of X-ray diffraction and thermal expansion measurements. The compound crystallizes in a hexagonal crystal structure of Gd3Ru4Al12 type (P63/mmc space group), and no structural phase transition has been found in the temperature interval between 2.5 and 300K. Nevertheless, due to the spin-lattice coupling the crystal volume undergoes a small orthorhombic distortion of the order of 2×10−5 as the compound enters the antiferromagnetic state. We propose that the first-order phase transition is not driven by the structural changes but rather by the exchange interactions present in the system.

Magnetic Phase Transitions in Zn1−x Mn x O

Thermal diffusivity measurements have been used to study the magnetic phase transitions in ZnO doped with Mn. The temperature dependencies of the thermal diffusivity in the wide temperature range from 70 to 300 K reveal an inverted-lambda-shaped peak at a temperature of 75 K for Zn0.95Mn0.05O doped with nitrogen and additional peaks at 117 K and 122 K for the nitrogen-doped and the undoped Zn0.95Mn0.05O samples, respectively. The lambda-shaped peak demonstrates the existence of a second order phase transition in these samples. The thermal diffusivity peak at 75 K was attributed to a ferromagnetic-paramagnetic phase transition in the Zn0.95Mn0.05O doped with nitrogen, and the peaks at 117 K and 122 K were attributed to an antiferromagnetic-paramagnetic phase transition in the MnO nanoclusters.

Characteristic temperatures of PbFe1/2Nb1/2O3 ferroelectrics crystals seen via acoustic emission

PbFe1/2Nb1/2O3 ferroelectrics crystals have been investigated using an acoustic emission. All the characteristic points have been detected: the Néel antiferromagnetic–paramagnetic phase transition at 158.6 K, both the rhombohedral–tetragonal at 363–348 K and tetragonal–cubic at 382–365 K structural phase transitions, the intermediate temperature T* at 431–427.5 K and the Burns temperature Td, extending through 564–603 K. It is shown that a dielectric response is unable to locate the Td correctly, but both the thermal expansion and acoustic emission are able. Acoustic emission is found to be more powerful for T* than for Td, which is usually observed in some well known relaxor ferroelectrics. Such a phenomenon is discussed from a viewpoint of dynamics of polar nanoregions.

Structural and Magnetic Properties of Cobalt(II) Complexes of Triphenylphosphine

This work is related to the structural and mag- netic properties of the Co(PPh3)Cl2 metal complex for dif- ferent sintering times. The structural and physical proper- ties of the complex have been studied by using SEM, EDX, and magnetization techniques. SEM results clearly demon- strate that the grains of the complex are very well connected to each other and tightly packed. Magnetization measure- ments with respect to magnetic field and temperature show a paramagnetic behavior above 20 K for the all samples. Fur- thermore, the susceptibility (χ) as a function of temperature indicates a paramagnetic-antiferromagnetic phase transition with a negative value of the Curie-Weiss temperature, θ .I n addition, increasing sintering time appears to also enhance the effective magnetic moment, μeff.

Almandine: Lattice and non-lattice heat capacity behavior and standard thermodynamic properties

The heat capacity of three synthetic polycrystalline almandine garnets (ideal formula Fe3Al2Si3O12) and one natural almandine-rich single crystal was measured. The samples were characterized by optical microscopy, electron microprobe analysis, X-ray powder diffraction, and Mossbauer spectroscopy. Measurements were performed in the temperature range 3 to 300 K using relaxation calorimetry and between 282 and 764 K using DSC methods. All garnets show a prominent λ-type heat-capacity anomaly at low temperatures resulting from a paramagnetic-antiferromagnetic phase transition. For two Fe3+-free or nearly Fe3+-free synthetic almandines, the phase transition is sharp and occurs at 9.2 K. Almandine samples that have ~3% Fe3+ show a λ-type peak that is less sharp and that occurs at 8.0 ± 0.2 K. The low- T C P data were adjusted slightly using the DSC results to improve the experimental accuracy. Integration of the low- T C P data yields calorimetric standard entropy, S∘ , values between 336.7 ± 0.8 and 337.8 ± 0.8 J/(mol·K). The smaller value is recommended as the best S∘ for end-member stoichiometric almandine, because it derives from the “best” Fe3+-free synthetic sample. The lattice (vibrational) heat capacity of almandine was calculated using the single-parameter phonon dispersion model of Komada and Westrum (1997), which allows the non-lattice heat capacity ( C ex) behavior to be modeled. An analysis shows the presence of an electronic heat-capacity contribution ( C el, Schottky anomaly) superimposed on a larger magnetic heat-capacity effect ( C mag) around 17 K. The calculated lattice entropy at 298.15 K is S vib = 303.3 J/(mol·K) and it contributes about 90% to the total standard entropy at 298 K. The non-lattice entropy is S ex = 33.4 J/(mol·K) and consists of S mag = 32.1 J/(mol·K) and S el = 1.3 J/(mol·K) contributions. The C P behavior for almandine above 298 K is given by the polynomial [in J/(mol·K)]: C P = 649.06 ( ± 4 ) - 3837.57 ( ± 122 ) · T - 0.5 - 1.44682 ( ± 0.06 ) · 10 7 · T - 2 + 1.94834 ( ± 0.09 ) · 10 9 · T - 3 which is calculated using the measured DSC data together with one published heat-content datum determined by transposed-drop calorimetry along with a new determination in this work that gives H 1181K − H 302K = 415.0 ± 3.2 kJ/mol. Using our S∘ value and the C P polynomial for almandine, we derived the enthalpy of formation, Δ H °f, from an analysis of experimental phase equilibrium results on the reactions almandine + 3rutile = 3ilmenite + sillimanite + 2quartz and 2ilmenite = 2Fe + 2rutile + O2. A Δ H °f = −5269.63 kJ/mol was obtained.