Second-order Magnetic Phase Transition Research Articles

Modeling of Magnetic and Magnetocaloric Properties by the Molecular Mean Field Theory in La0.6Sr0.4Mn0.9V0.1O3 Oxide

Based on the mean-field theory, the magnetic and magnetocaloric behavior of La0.6Sr0.4Mn0.9V0.1O3 manganese oxide has been analyzed. The Bean-Rodbell equation of state presents a second-order magnetic phase transition with a η factor of η = 0.71. Using the experimental data of magnetization M (H, T), the molecular mean-field parameter is found to be λ1 = 2.19 T g emu−1. The Brillouin function makes it possible to determine the total angular moment J, the saturation magnetization MS, and the Lande factor g for La0.6Sr0.4Mn0.9V0.1O3 sample. We simulated the magnetization as a function of the magnetic field and the temperature by applying the mean-field theory, as well as the variation of the magnetic entropy change ΔSM(T). As observed, the simulated results close with experimental data. The magnetic entropy changes for the samples were also estimated by using the mean-field scaling theory, and the results show a difference between the theoretical and experimental values.

Influence of spin and orbital fluctuations on Mott-Hubbard exciton dynamics in LaVO3 thin films

Recent optical conductivity measurements reveal the presence of Hubbard excitons in certain Mott insulators. In light of these results, it is important to revisit the dynamics of these materials to account for excitonic correlations. We investigate time-resolved excitation and relaxation dynamics as a function of temperature in perovskite-type LaVO${}_{3}$ thin films using ultrafast optical pump-probe spectroscopy. LaVO${}_{3}$ undergoes a series of phase transitions at roughly the same critical temperature $T_C\cong 140\ K$, including a second-order magnetic phase transition (PM $\xrightarrow{}$ AFM) and a first-order structural phase transition, accompanied by \textit{C}-type spin order (SO) and \textit{G}-type orbital order (OO). Ultrafast optical pump-probe spectroscopy at 1.6 eV monitors changes in the spectral weight of the Hubbard exciton resonance which serves as a sensitive reporter of spin and orbital fluctuation dynamics. We observe dramatic slowing down of the spin, and orbital dynamics in the vicinity of $T_C\cong 140$ K, reminiscent of a second-order phase transition, despite the (weakly) first-order nature of the transition. We emphasize that since it is spectral weight changes that are probed, the measured dynamics are not reflective of conventional exciton generation and recombination, but are related to the dynamics of Hubbard exciton formation in the presence of a fluctuating many-body environment.

Magnetic and magnetothermal properties of Gd75Cu25 prepared by melt-spinning

Magnetic and magnetothermal properties of melt-spun Gd75Cu25 alloy were studied. A second-order magnetic phase transition is observed at the Curie temperature of 144 K. The peak value of the magnetic entropy change −ΔSmpk = 7.0 J kg−1 K−1 is observed near the Curie point under the magnetic field change 0–5 T, while the refrigerant capacity is found to be 594 J kg−1. The specific heat exhibits a wide transition near the Curie temperature, and the non-magnetic contributions to the total specific heat are described using the Debye temperature ΘD = 155 K and the electronic specific heat coefficient γ = 65 mJ mol−1 K−2.

Effect of Ga substitution on structural and magnetic properties of Fe50Mn25Al25-xGax Heusler alloys

Heusler alloys possess the structure of an ordered compound while displaying most of the properties of metals. Numerous magnetic exchange mechanisms may be operating in such systems. Herein, the structural and magnetic properties have been investigated as a function of substitution of Al by Ga in Fe50Mn25Al25-xGax (x = 0, 2.5, 5.0, 7.5, 10.0) quaternary (pseudo-ternary) full Heusler alloys. The X-ray diffraction analysis confirms the substituting Al by Ga promoting the lattice expansion in the L21 crystal structure in Fe50Mn25Al25-xGax alloys. The line scanning profile using energy dispersive X-ray analysis investigations suggests the homogeneous distribution of Ga in all the alloys. The magnetic moment of the alloys was found to be affected by Ga as the antiferromagnetic counterpart increases with Ga. A decrease in the value of Curie temperature was found with increase in Ga content. A second-order magnetic phase transition was observed in the alloys above room temperature.

Magnetic critical behavior in the intermetallic compound (Ce0.65Pr0.35)2Co7

Clarification of magnetic phase transition nature and critical behavior is an essential step in the development of new magnetic functional materials. In this work, we studied critical properties of intermetallic compound (Ce0.65Pr0.35)2Co7 by bulk dc magnetization around the paramagnetic to ferromagnetic phase transition. It exhibits a second-order magnetic phase transition with TC = 214 K. Critical exponents β = 0.546(5) and γ = 1.054(5) are obtained by the modified Arrott plot, whereas δ = 2.843 is obtained by a critical isotherm analysis at TC. These critical exponents not only obey the Widom relation δ = 1 + γ/β, but also collapse the isothermal magnetization M-H-T data into two branches below and above TC following the single scaling equation M(H, ε) = εβf±(H/εβ+γ). Although the obtained exponents are close to the theoretical values of mean-field model, a noticeable deviation unable to be ignored, indicating the existence of localized magnetic interaction which competes with a long-range ferromagnetic state. Our results will establish a sound theoretical basis for understanding the complex magnetic interaction in the light rare earths ternary alloy.

Extraordinary magnetic properties of double perovskite Eu2CoMnO6 wide band gap semiconductor

Some novel magnetic behaviours in double perovskite Eu2CoMnO6 (ECMO) have been reported. The x-ray photoemission spectroscopy study shows the presence of mixed valence states of transition metal ions. The UV–visible absorption spectroscopic study suggests that the ECMO has a direct wide band gap. A second-order magnetic phase transition as a sudden jump in the magnetization curve has been observed around 124.5 K. The large bifurcation between the zero field cooling and field cooling, suggests existence of strong spin frustration in the system. The inverse DC susceptibility confirms the presence of the Griffiths like phase. Sharp steps in magnetization have been observed in the M–H curve at 2 K, which vanishes on increasing temperature. The AC susceptibility study demonstrates the Hopkinson like effect as well as the presence of volume spin-glass-like behaviour. The temperature dependent Raman spectrum shows the presence of spin–phonon coupling.

Caloric effects around phase transitions in magnetic materials described by ab initio theory: The electronic glue and fluctuating local moments

We describe magneto-, baro-, and elastocaloric effects (MCEs, BCEs, and eCEs) in materials, which possess both discontinuous (first-order) and continuous (second-order) magnetic phase transitions. Our ab initio theory of the interacting electrons of materials in terms of disordered local moments has produced explicit mechanisms for the drivers of these transitions, and here, we study associated caloric effects in three case studies where both types of transition are evident. Our earlier work had described FeRh’s magnetic phase diagram and large MCE. Here, we present calculations of its substantial BCE and eCE. We describe the MCE of dysprosium and find very good agreement with experimental values for isothermal entropy (ΔSiso) and adiabatic temperature (ΔTad) changes over a large temperature span and different applied magnetic field values. We examine the conditions for optimal values of both ΔSiso and ΔTad that comply with a Clausius–Clapeyron analysis, which we use to propose a promising elastocaloric cooling cycle arising from the unusual dependence of the entropy on temperature and biaxial strain found in our third case study—the Mn3GaN antiperovskite. We explain how both ΔSiso and ΔTad can be kept large by exploiting the complex tensile strain–temperature magnetic phase diagram, which we had earlier predicted for this material and also propose that hysteresis effects will be absent from half of the caloric cycle. This rich and complex behavior stems from the frustrated nature of the interactions among the Mn local moments.

Tuning Magnetic Entropy Change and Relative Cooling Power in La0.7Ca0.23Sr0.07MnO3 Electrospun Nanofibers.

We present experimental evidence about the magnetocaloric tuning effect in one-dimensional nanostructure fibers mixed-valence manganite as synthesized by electrospinning techniques and under heat treatments of 973, 1073 and 1173 K. The stoichiometry obtained is La0.7Ca0.23Sr0.07MnO3 and Rietveld refinement indicates a single-phase with an orthorhombic (Pnma) crystal structure. Scanning and transmission electron microscopy observations indicate coalescence in granular colonies of La0.7Ca0.23Sr0.07MnO3 nanoparticles to conform nanofibers. Magnetic entropy change is tuned due to heat treatments at 1173 K with maximum values of 1, 1.82 and 2.51 J/kgK for applied external magnetic fields of μ0H = 1, 2 and 3T, respectively, with a maximum magnetic entropy difference at a Curie temperature of 293 K (furthermore, second-order magnetic phase transition was observed). Additionally, for a magnetic field, ~μ0H = 3 T values of 49, 95 and 143 J/kg for 973, 1073 and 1173 K heat-treated samples were obtained.

Magneto Transport Anomaly of Bismuth Substituted (La, Na) MnO3

The structural and magneto-transport properties of polycrystalline La0.85–xBixNa0.15MnO3, (x = 0, 0.05, 0.1, 0.2) manganites are reported. The polycrystalline samples were prepared using solid-state synthesis route. Rietveld refinement of X-ray data conclude the samples crystallized in a rhombohedrally distorted structure belonging to $$ R\overline{3}C $$ space group with no prominent structural changes upon bismuth substitution. Scanning electron microscopy has revealed improvement of microstructure from loosely bound system to densely packed array of grain. Resistivity measurements disclose metal–insulator transition for all the samples. A double metal–insulator transition is observed in samples with low concentrations (x = 0.05, 0.1). All samples have shown second-order magnetic phase transition.

Structural, Magnetic and Magnetocaloric Properties of Co2Y1−xCux (x = 0.00, 0.05, and 0.10) Compounds

We investigate the structural, magnetic, and magnetocaloric properties of the intermetallic Co2Y1−xCux (x = 0.00, 0.05, and 0.10) compounds prepared by arc melting method. The X-ray diffraction measurements show that all compounds crystallize in a cubic MgCu2-type structure with Fd-3m space group, in which the lattice parameter decreases with increasing Cu concentration. The magnetic properties of Co2Y were found to strongly depend on Cu substitution. The Curie temperature increases with increasing Cu content. In addition, the magnetic entropy change (−ΔSM) and the relative cooling power (RCP) increase with increasing Cu content. Our results highlight that Co2Y0.9Cu0.1 exhibits a second-order magnetic phase transition around room temperature with only a small magnetocaloric effect.

Magnetic Phase Transitions to an Incommensurate Structure in LiMn2O4 Compound

Possible second-order magnetic phase transitions to the incommensurate magnetic structure in the orthorhombic phase of LiMn2O4 compound have been investigated. It is shown that “weak Lifshitz condition” holds for this compound (i.e., Lifshits invariants are immaterial in all transitions being considered), and only the incommensurate phase, in which the wavevector near the transition point varies continuously with temperature and pressure, can be formed. A transition is considered both in the exchange approximation occurring in three irreducible representations forming an exchange multiplet as well as in one and two irreducible representations. Expressions have been derived for the mean density of the magnetic moment emerging because of such transitions. It is found that structures of the types of a longitudinal spin wave, a transverse spin wave with polarization along the crystallographic axes perpendicular to the wavevector of the structure, as well as certain superpositions of these waves can be formed in the system.

Magnetic, magnetocaloric properties and phenomenological model in amorphous Fe40Ni38Mo4B18 alloy

We investigate in detail the magnetic, magnetocaloric properties and phenomenological model of amorphous Fe40Ni38Mo4B18 alloy. The melt spinning method has been used to synthesize the amorphous alloy. With a Curie temperature of 610 K, the magnetic transition nature undergoes a second-order magnetic phase transition from ferromagnetic to paramagnetic states. The magnetic entropy change (-ΔSM) is determined using the thermodynamic Maxwell’s relation. Moreover, we report a theoretical investigation of the magnetocaloric effect using a phenomenological model. The best model parameters and their variation with temperature and the magnetic field were obtained. The theoretical predictions were found to be in line with the experimental results.

Magnetic Properties and Magnetocaloric Effect of Binary Compound NdPd

The crystal structure, magnetic and magnetocaloric properties of the binary NdPd compound were investigated by X-ray diffraction and the physical property measurement system in detail. The NdPd compound with a CrB structure exhibits a second-order magnetic phase transition from ferromagnetic to paramagnetic states at the Curie temperature TC = 15.5 K. For a magnetic field change of 0–5 T, the peak value of magnetic entropy change (− ΔSM) is evaluated to be 12.2 J/kg K, and the corresponding refrigerant capacity (RC) reaches its maximal value of 94 J/kg. The relatively small thermal hysteresis and magnetic hysteresis with the considerable magnetocaloric effect indicate that NdPd is an anticipated material for the low-temperature magnetic refrigeration.

Intermetallic RE6T5Al7 Phases (RE = Sc, Y, Ce-Nd, Sm, Gd-Lu; T = Ru, Ir): Diversity in their Magnetic, Magnetocaloric, and Critical Properties.

Over 20 new compounds of the RE6T5Al7 series (RE = Sc, Y, Ce-Nd, Sm, Gd-Lu; T = Ru, Ir; Yb6Ir5Ga7 type structure; superstructure of MgZn2; fully ordered Nb6.4Ir4Al7.6 type) have been synthesized. They crystallize in the hexagonal crystal system with space group P63/mcm. Their lattice parameters are in the ranges of a = 935-963 and c = 851-874 pm for the RE6Ru5Al7 and a = 913-966 and c = 825-865 pm for the RE6Ir5Al7 series. Four structures (Ho6Ru5Al7, Yb6Ru4.68(1)Al7.32(1), Sc6Ir5Al7, and Ho6Ir4.53(1)Al7.47(1)) have been refined from single-crystal data, indicating fully ordered structures for Ho6Ru5Al7 and Sc6Ir5Al7; however, T/Al mixing was observed on one crystallographic site for Ho6Ir4.53(1)Al7.47(1) and on two sites for Yb6Ru4.68(1)Al7.32(1). The Sc-, Y-, and Lu-containing compounds exhibit Pauli paramagnetism in line with a filled d band for Ru and Ir. Ce6Ru5Al7 exhibits mixed-valent behavior, while Yb6Ru5Al7 is solely trivalent. The other compounds exhibit paramagnetism and ferromagnetic phase transitions up to temperatures of TC = 83.4(1) K for Gd6Ru5Al7. In addition to the basic magnetic characterizations and studies of the electrical resistivity and heat capacity, the magnetocaloric properties of Gd6Ru5Al7, Tb6Ru5Al7, and Dy6Ru5Al7 have been investigated, revealing magnetic entropy changes of -ΔSMmax = 6.2(1), 7.7(1), and 5.4(1) J kg-1 K-1 (0 → 5 T) and relative cooling powers RCP = 242, 207, and 135 J kg-1, respectively. For a deeper insight into the second-order magnetic phase transitions, the critical behavior was investigated according to the scaling hypothesis. The critical behavior of Gd6Ru5Al7 is in accordance with the mean-field theory; the critical exponents of Tb6Ru5Al7 and Dy6Ru5Al7, however, deviate strongly from this universality class. For comparison of the structural and magnetic properties, the thus far unstudied members of the RERuAl series (RE = Sc, Y, Sm, Gd-Lu; MgZn2 type) have additionally been prepared and characterized.

Consequences of magnetic ordering in chiral Mn1/3NbS2

We have investigated the structural, magnetic, thermodynamic, and charge transport properties of Mn1/3NbS2 single crystals through x-ray and neutron diffraction, magnetization, specific heat, magnetoresistance, and Hall effect measurements. Mn1/3NbS2 displays a magnetic transition at TC ~ 45 K with highly anisotropic behavior expected for a hexagonal structured material. Below TC, neutron diffraction reveals increased scattering near the structural Bragg peaks having a wider Q-dependence along the c-axis than the nuclear Bragg peaks. This indicates helimagnetism with a long pitch length of ~250 nm (or a wavevector q~0.0025 {\AA}-1) along the c-axis. This q is substantially smaller than that found for the helimagnetic state in isostructural Cr1/3NbS2 (0.015 {\AA}-1). Specific heat capacity measurements confirm a second-order magnetic phase transition with a substantial magnetic contribution that persists to low temperature. The large low-temperature specific heat capacity is consistent with a large density of low-lying magnetic excitations that are likely associated with topologically interesting magnetic modes. Changes to the magnetoresistance, the magnetization, and the magnetic neutron diffraction, which become more apparent below 20 K, imply a modification in the character of the magnetic ordering corresponding to the magnetic contribution to the specific heat capacity. These observations signify a more complex magnetic structure both at zero and finite fields for Mn1/3NbS2 than for the well-investigated Cr1/3NbS2.

Statistical model for the martensitic transformation simulation in Heusler alloys

In this work, the one-dimensional statistical model based on the phenomenological approach for the study of the magnetic and magnetocaloric properties in Heusler alloys is considered. The proposed model is approved for Ni-Co-Mn-In exhibiting both the first-order structural transformation and second-order magnetic phase transition. Temperature dependencies of magnetization and magnetic entropy change under applied magnetic field and external pressure are studied. The results of the simulations are in good agreement with the experimental data.

Calculating the magnetocaloric effect in second-order-type material by micromagnetic simulations: A case study on Co2B

We propose a calculation scheme which integrates micromagnetic simulations and the Arrott–Noakes equation for the investigation of the influence of microstructure and magnetocrystalline anisotropy on the magnetocaloric effect (MCE) in second-order magnetic phase transition materials. Using the Arrott–Noakes equation, the micromagnetic simulation results are extrapolated to temperatures above Curie temperature and applied to MCE calculations. With Co2B as model material, we found that increasing magnetocrystalline anisotropy facilitates higher isothermal entropy changes (ΔST). The grain size reduction results in increased ΔST for constant saturation magnetization (MS), while ΔST decreases for a reduction of MS with reduced grain size.

Rotating magnetocaloric effect in the ferromagnetic Weyl semi-metal Co3Sn2S2

The rotating magnetocaloric effect is of recent interest in the magnetic refrigeration technique, in which the cooling effect is attained by rotating the anisotropic magnetocaloric material from one orientation to the other in a fixed magnetic field. In this work, we report the anisotropic magnetocaloric properties of single crystals of the ferromagnetic Weyl semimetal Co3Sn2S2 for magnetic field H along the c-axis (H∥c) and magnetic field H along the ab-plane (H∥ab). We observed a significant (factor of 2) difference between the magnetocaloric effect measured in both orientations. The rotating magnetocaloric effect has been extracted by taking the difference of the magnetic entropy change (ΔSM) for fields applied in the two crystallographic orientations. Scaling analysis of ΔSM, the rescaled ΔSM(T,H) vs reduced temperature θ curves collapse onto a single universal curve, indicates that the transition from paramagnetic to ferromagnetic phases at 174 K is a second-order magnetic phase transition. Furthermore, using the power law dependence of ΔSM and relative cooling power RCP, the critical exponents β and γ are calculated, which are consistent with the recent critical behavior study on polycrystalline samples of this compound [Yan et al., Solid State Commun. 281, 57 (2018)].

Observation of a ferromagnetic-to-paramagnetic phase transition in Ce0.65Mg0.35Co3

are promising candidates for the permanent magnets since they can show ferromagnetism and large anisotropy by substituting Mg for Ce in a paramagnetic compound. In this work, a room-temperature ferromagnetism with a second-order magnetic phase transition is observed in the (CMC) compound prepared by a cold crucible levitation melting method. Based on the DC magnetization data, the critical phenomenon around the phase transition point () in CMC is investigated and the critical exponents for spontaneous magnetization, for susceptibility and for critical isothermal magnetization are determined independently by three different data processing techniques including the modified Arrott plot, Kouvel-Fisher plot, and critical isotherm analysis. It is noteworthy that the magnetizations obey the scaling equation, indicating that the critical parameters including Tc, , and are reliable and self-consistent. The calculation of spin interaction with the obtained critical exponents further suggests a long-range ferromagnetic coupling with in CMC.

Effect of B site partial Ru substitution on structural magnetic and magnetocaloric properties in La0.7Pb0.3Mn1-xRuxO3 (x = 0.0, 0.1 and 0.2) perovskite system

The La0.7Pb0.3Mn1-xRuxO3 (x = 0.0, 0.1 and 0.2) perovskite manganite compounds have been fabricated by solid-state reaction method to investigate structural, magnetic and magnetocaloric properties. All samples are indexed in the Rhombohedral structure with R3‾c space group by using the Rietveld refinement method. The Curie temperature decreases from 336 K to 313 K when Ru (x = 0.1) is added to the structure and it becomes 335 K for x = 0.2. Maximum magnetic entropy change and relative cooling power values were calculated as 3.17, 3.15, 3.06 J kg−1 K−1 for 5 T magnetic field change and 214.40, 160.20, 128.38 J kg−1 under 4 T magnetic field change for x = 0.0, 0.1 and 0.2 in La0.7Pb0.3Mn1-xRuxO3, respectively. From the H/M vs M2 plots obtained from isothermal magnetization curves, it is found that all samples exhibit a second-order magnetic phase transition, which shows reversible magnetocaloric effect. Under the light of all observed results, La0.7Pb0.3Mn1-xRuxO3 (x = 0.0, 0.1 and 0.2) perovskite manganite compounds can be considered as candidate materials above room temperature range magnetic cooling systems.