Magnetic Phase Transition Point Research Articles

Effect of Fe-doping on magnetic structures and “spin-lattice-charge” strong correlation properties in Mn3Sn1-xFexC compounds

The influences of Fe-doping on the magnetic structure, correlated lattice variation, and electronic transport of the antiperovskite compounds Mn3Sn1-xFexC have been reported for the first time. The results indicate that Fe-doping is effective in altering the magnetic behaviors in the Mn3Sn1-xFexC compounds. The magnetic phase transition point (TC) initially shifts to lower temperatures at low Fe-doping concentrations and then increases to higher temperatures at higher Fe-doping levels. This change is accompanied by the development of macroscopic ferromagnetism in the compound, which is essentially due to the emergence of a new canted ferrimagnetic phase named MCF, revealed by neutron powder diffraction (NPD). By virtue of the Fe-doping effect on the magnetism, the regulation of the negative thermal expansion (NTE) behavior and the abrupt change of resistivity in Mn3SnC are also realized. Based on these results, a magnetic phase diagram for the Mn3Sn1-xFexC series is established. This work not only provides a method for effective regulation of magnetic ordering but also strengthens our understanding of strongly correlated physical properties.

Dimension reduction induced anisotropic magnetic thermal conductivity in hematite nanowires

The thermophysical properties near the magnetic phase transition point is of great importance in the study of critical phenomenon. Low-dimensional materials are suggested to hold different thermophysical properties comparing to their bulk counterpart due to the dimension induced quantum confinement and anisotropy. In this work, we measured the thermal conductivity of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanowires along the [110] direction (growing direction) with temperature from 100 to 150 K and found a dip of thermal conductivity near the Morin temperature. We found that the thermal conductivity near Morin temperature varies with the angle between magnetic field and the [110] direction of nanowire. More specifically, an angular-dependent thermal conductivity is observed, due to the magnetic field induced movement of the magnetic domain wall. The angle corresponding to the maximum of thermal conductivity varies near the Morin transition temperature, due to the different magnetic easy axis as suggested by our calculation based on magnetic anisotropy energy. This angular dependence of thermal conductivity indicates that the easy axis of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanowires is different from bulk $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ due to the geometric anisotropy.

Assessing the Size of Phase Inclusions in Ferrochrome Alloys by Means of Ultrasound Resonance

In the strategic research program on closing the nuclear fuel cycle with fast neutron reactors, nonstandard equipment must be introduced in the fuel chambers for nondestructive monitoring of materials that are critical to fast neutron reactors. In particular, ferrochromium alloys are regarded as promising for the fuel-element casings of fast neutron reactors. An unexpected effect is observed in ultrasonic spectroscopy of cylindrical ferrochromium-alloy samples by the internal-friction method, in a narrow temperature range close to 550 K on cooling at around 0.2 K/s: coupled oscillations are seen in samples with a nonuniform temperature distribution over the radius if the temperature range includes the material’s point of magnetic phase transition (Curie point). Such a sample may be regarded as a complex oscillatory system consisting of a peripheral (cooler) region and a central region in different magnetic states. Mechanical stress is present at the pulsating boundary of those regions. Such anomalous oscillation is associated with the influence of dynamic oscillatory stress on the formation of carbonitride inclusions in heat treatment and on the magnetic phase transitions in those inclusions. A theoretical description of this effect is proposed. By recording the parameters of the coupled oscillations, the size of the phase inclusions that form may be estimated. By means of these resonant oscillations, in combination with the traditional internal-friction method, metastable phase inclusions appearing in intermediate stages of structure formation in the material may be detected, and their dimensions may be estimated. This will be of great value in the primary nondestructive testing of highly irradiated alloy samples in the fuel chambers when optimizing the alloy composition for the fuel-element casings in fast neutron reactors.

Local quantum criticality of a one-dimensional Kondo insulator model

The continuous quantum phase transition and the nature of quantum critical point (QCP) in a modified Kondo lattice model with Ising anisotropic exchange interactions is studied within the density-matrix renormalization group algorithm. We investigate the effect of quantum fluctuations on critical Kondo destruction QCP, by probing static and dynamic properties of the magnetic order and the Kondo effect. In particular, we identify that local Kondo physics itself becomes critical at the magnetic phase transition point, providing unbiased evidences for local quantum criticality between two insulators without resorting to the change of Fermi surface.

Electric Polarization Induced by Spin Ordering under Magnetic Fields in Distorted Triangular Lattice Antiferromagnet RbCoBr3

Magnetization and electric polarization are measured for RbCoBr3 in the presence of an applied high magnetic field. The saturation of magnetization is recognized in the magnetization curve. The g-value of pseudospin and the nearest-neighbor intrachain exchange interaction of RbCoBr3, which has the properties of a quasi-one-dimensional Ising antiferromagnet, are evaluated. The electric polarization parallel to the c-axis under a magnetic field alone and also under the simultaneous application of electric and magnetic fields along the c-axis is observed to increase around the magnetic phase transition point from the ferrimagnetic low-temperature phase to the partially disordered high-temperature phase. Experimental results indicate that the electric polarization is induced through the rearrangement of the spin structure accompanied by the magnetic phase transition under an applied magnetic field. A probable reason for the enhancement of electric polarization is given from the viewpoint of the interplay betw...

Entanglement detection in the mixed-spin Ising–X Y model

In the present work, we initially verify anisotropy effect on the heat capacity of a mixed-three-spin (1/2,1,1/2) system (where spins (1/2,1/2) have XY interaction and spins (1,1/2) have Ising interaction together) at finite temperatures, then, the pairwise entanglement for spins (1/2,1/2), by means of negativity (as a measure of entanglement) as a function of the temperature T, homogeneous magnetic field B, and anisotropy parameter γ is investigated. In addition, we show that one can find magnetic phase transition points for the spins (1/2,1/2) at finite temperatures and understand properly their behavior with respect to the magnetic field and the anisotropy parameter, via the negativity function. An interval of the magnetic field from the negativity diagram of the spins (1/2,1/2) is presented in which quantum phase transition occurs for the tripartite mixed-three-spin system. Finally, some new interesting entanglement witnesses are introduced by using non-degenerate perturbation theory for the mixed-three-spin system.

Anisotropic magnetic hyperfine interactions in phosphide FeP

The results from a Mossbauer study of 57Fe Mossbauer nuclei in binary phosphide FeP, which has an unusual magnetic structure, are presented. Model decoding of the Mossbauer spectra is performed with allowance for features of the crystal structure of this compound, using data on its spin-modulated magnetic ordering. A proposed model expansion of the spectra allows us to describe consistently Mossbauer data obtained over a wide range of temperatures that includes the magnetic phase transition point. Possible explanations are offered for the low magnitude and spatial anisotropy of hyperfine magnetic field Hhf on 57Fe nuclei.

Ferromagnetic half-metallic characteristic and phase transition in rare-earth Yb doped SiC: A GGA+U study

The electronic, magnetic and structural properties of rare earth Yb doped on SiC are investigated theoretically based on the density functional theory (DFT). Both zincblende (ZB) and rocksalt (RS) structures of SiC have been calculated. We found that Si3YbC4 with ZB exhibits a complete half-metallic characteristic with a wide gap more than 1.8eV using GGA and GGA+U methods. However, the half-metallicity is destroyed with RS. The sensitivity of magnetic moments of Si3YbC4 as a function of pressure is also discussed, there are two magnetic phase transition points with increased pressure. The exchange interaction between local Yb-4f electrons and conduction electrons plays an important role in their heavy fermion characters. The exchange splitting of the conduction band is confirmed to be much larger than that of the valence band in Si3YbC4, which makes the holes-mediated ferromagnetism in this material.

Effect of magnetic annealing on phonon behaviors of nanocrystalline BiFeO 3

Influence of magnetic annealing at 823 K up to 10 T (T) on the phonon behaviors of nanocrystalline BiFeO 3 was investigated by Raman spectroscopy. The frequencies of fundamental Raman modes increase obviously with increasing annealing magnetic field, and the intensity of the 1260 cm −1 two-phonon mode decreases. The pronounced anomalies of Raman phonon modes under magnetic annealing are attributed to the change of the spin-phonon coupling due to the modulation of spiral spin order. Furthermore, the temperature dependence of Raman peak positions, for the two prominent modes (147 and 176 cm −1), show no notable anomaly around T N except the sample annealed under 10 T magnetic field; meanwhile, in this sample, another obvious phonon anomaly occurs at ∼150 K (another magnetic phase transition point), which indicate that stronger magnetic annealing with 10 T intensely enhances the spin-phonon coupling, and possibly increases magnetoelectric coupling of nanocrystalline BiFeO 3 due to severely modulation of spiral spin order.

High-Field Magnetization of Distorted Triangular Lattice Antiferromagnet RbCoBr3

High-field magnetization measurements were performed on a distorted triangular-lattice antiferromagnet, RbCoBr 3 . The magnetization curve shows anomalies characterized by two peaks of d M /d H . The temperature dependences of these peak positions approach the magnetic phase transition points at H = 0. The results suggest that an intermediate magnetic phase, which is known as the partial disordered phase, may appear even in a finite magnetic field.

Lattice, magnetic and transport properties in antiperovskite [formula omitted] compounds

The temperature-dependent magnetization, lattice, and transport properties of Mn 3Sn 1− x Ge x C ( 0 ≤ x ≤ 0.5 ) compounds are systematically investigated. The Mn–Mn atomic distance decreases as Ge content is increased, and the transition temperature from ferromagnetic (or ferrimagnetic) to paramagnetic state decreases too. Mn 3SnC has a large magnetovolume effect (MVE). However, Ge-doping in Mn 3SnC gradually reduces the MVE, till the MVE disappears. Whether there is an abnormal lattice change or not, there always exists an anomalous increase in resistivity near the magnetic phase transition point with decreasing temperature.

Magnetic and thermal properties of perovskite YFeO 3 single crystals

In this work, we reported the magnetic and thermal properties of perovskite YFeO 3 crystal grown by the floating zone method. YFeO 3 crystal is a canted antiferromagnet with a weak ferromagnetic behavior. The magnetization varied nonlinearly from 0.204 emu/g to 0.173 emu/g at the temperature range of 2–300 K. Its Néel temperature is around 644.5 K. The specific heat is about 0.56 J/(g K) at room temperature and reaches the maximum value of 0.77 J/(g K) at 644.5 K where the antiferromagnetic–paramagnetic transition occurs. Both the lattice and magnetic contributions to the heat capacity have been calculated. Thermal diffusivity and the thermal conductivity are as large as 3.53 mm 2/s and 11.27 W/(m K) at 298 K, respectively. Both of them show anomalies around the magnetic phase transition point. The average thermal expansion coefficients are −1.72 × 10 −6 K −1 and 2.28 × 10 −6 K −1 before and after Néel temperature, respectively. The high Néel temperature, high thermal conductivity and small expansion coefficient indicate that YFeO 3 crystal is a promising candidate for laser-induced or optical device applications in broad temperature range and high power system.

The thermal expansion coefficient and volume magnetostriction of Heusler Ni 2MnGa alloys

In this work the thermal expansion coefficient of Ni 2MnGa alloy was experimentally determined. It was shown that the thermal expansion coefficient has a peak at the point of magnetic phase transition. The value of thermal expansion coefficient jump at Curie point is Δ κ=1.8×10 −5 K −1. From the phenomenological Ginzburg–Landau theory of a magnetic second-order phase transition the expression for a jump of thermal expansion coefficient in Curie point was obtained. From the comparison of experimental and theoretical jumps of thermal expansion the value of volume magnetostriction for Ni 2MnGa alloy was determined: B 1≈20×10 9 erg/cm 3.

Crossover of polaron conductivity and the inhomogeneous state of lanthanum manganites in the magnetic phase transition region

The transport properties of lanthanum manganites over a wide range of temperatures below the magnetic phase transition point are discussed within the model of a two-phase composite whose phases differ in the magnetic order and charge carrier concentration. The volume ratio of the phases depends on the temperature and the magnetic field. The magnetoelastic polarons are charge carriers in both phases, and the metal-dielectric transition occurs as a percolation transition accompanied by the crossover of the polaron conductivity. The results obtained by numerical simulation of the resistivity, magnetoresistance, and thermopower are compared with the experimental data for La0.7Mn1.3O3−δ thin films. The theoretical and experimental data are in good agreement.

Hall Effect through Magnetic Phase Transitions inNd2Fe14B

We have observed large variations of the Hall effect in the vicinity of magnetic phase transition points. Sharp peaks near the spin-reorientation and Curie temperatures are observed in Hall effect data from ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ single crystals. The effect is very sensitive to the applied magnetic field direction near the spin-reorientation temperature. Away from these critical regions the Hall resistivity is proportional to the square of the total resistivity. Different scattering mechanisms dominate in different regions.

Magnetic phase transitions in erbium

Magnetic properties of a high-purity single crystal of erbium have been investigated in magnetic fields of Oe and kOe ranges and temperature region from 4.2 to 300 K. Considerable temperature hystereses of erbium magnetization were found near each of the magnetic phase transition points Θ 1, Θ 2 and Θ B. The data obtained are used for the analysis of magnetic structure transformations in erbium.

Low-frequency magnetic properties of CsDy(MoO4)2 in region of magnetic and structural phase transitions

The dynamic magnetic susceptibility of single crystal CsDy(MoO4)2 is studied in the frequency region of 4-2 · 107 Hz at 0.5-300 K. Using spin-lattice relaxation data, the energies of lower excited Dy3+ levels are found in the region of the structural phase transition (Tc = 38 K), which is responsible for the transition in the low symmetry phases. At temperatures above the magnetic phase transition point, (Tm= 1.3 K), some absorption strongly dependent on frequency (up to 4 Hz) is observed. Possible mechanisms of this phenomenon are discussed.

A SQUID Magnetometer for Simultaneous Measurements of Spontaneous Magnetization and AC Susceptibility around the Magnetic Phase Transition Point

A cryostat for simultaneous measurements of spontaneous magnetization and AC susceptibility by using a SQUID with a temperature control system was constructed. The temperature of samples can be controlled to an accuracy of 1.2×10-5 at temperatures from 1.6 K to 6.5 K during measurements. This accuracy is obtained by good thermal isolation from their surroundings and use of an isolation amplifier which breaks ground loop currents interfering with the detection of temperature. The SQUID magnetometer consists of a flux locked loop followed by another phase-sensitive detector referenced to the frequency of AC susceptibility. The frequency range of AC susceptibility is from 1 Hz to about 200 Hz and the sensitivity is 2×10-10 emu at 10 Hz with a time constant of 3 seconds, which corresponds to 10-4 of the flux quantum at a SQUID.

Dynamic behavior of a uniaxial ferromagnet near a magnetic phase-transition point

It is shown in this paper that in the sense of the dynamic behavior a uniaxial ferromagnet magnetized along the “difficult” direction is equivalent near a magnetic phase-transition point to some nonlinear oscillator, described by the Duffing differential equation. The frequency dependence of absorption and dispersion signals is studied and the existence of a crunode point of the dispersion curve is shown.

Ultrasonic Attenuation in the Simple Cubic Antiferromagnet near the Néel Temperature

Ultrasonic attenuation in an insulating simple cubic Heisenberg antiferromagnet near the magnetic phase transition point is investigated on the basis of Mori's theory of irreversible processes. The sound wave is assumed to couple mainly to the spin-energy-density fluctuation which decays via a fast non-diffusive relaxation process arising from the spin-lattice interaction. Firstly, the relaxation time of the spin-energy-density fluctuation, τ, and then the sound attenuation coefficient, α q , is calculated. The calculations of τ and α q applied to RbMnF 3 agree quantitatively with the experimental results. The contribution of the longitudinal thermal-phonons to 1/τ is found to be 1/7 of that of the transverse thermal-phonons. The present calculations for RbMnF 3 involve none of adjustable parameters. The sound attenuation coefficient of MnF 2 also is discussed.