Ferromagnetic-paramagnetic Transition Research Articles

Emergence of Griffiths phase and exploiting magnetic ordering state in the intermetallic LaCeCo7

We have studied the evolution of Griffiths phase and the magnetic ordering state in the intermetallic compound LaCeCo7. A ferromagnetic-like ordering phase is built up at 260 K but an anomalous magnetic susceptibility related to the Griffiths phase occurs in 300 K. The studies of critical behavior confirm that LaCeCo7 exhibits a second-order paramagnetic-ferromagnetic phase transitions. The critical exponents of β = 0.665, γ = 1.151, and δ = 2.728 determined by the magnetic field dependence on magnetic entropy change method deviate a little from the theoretical estimated long-range mean-field model (β = 0.5, γ = 1.0, and δ = 3.0). This result implies that the disorder effect in the intermetallic LaCeCo7 possibly plays a key role on the anomalous critical behavior and the establishment of magnetic ordering phase.

Electrical and magnetic properties of La0.7Ca0.3MnO3 nanoparticle prepared by reactive milling method

La0.7Ca0.3MnO3 nanoparticle in an orthorhombic structure with space group of Pnma was prepared by a combination of the reactive milling method and annealing at 900 °C for 1 h in air. The mean crystal size determined through the powder x-ray diffraction data is 18 ± 2 nm. Based on the dependences of the resistivity and the magnetisation on temperature and applied magnetic field, the electrical-magnetic properties, the magnetoresistance and the magnetocaloric effects have been investigated. It shows La0.7Ca0.3MnO3 nanoparticle undergoes a metal-insulator and a ferromagnetic-paramagnetic phase transitions at T MI = 252 K and T C = 253 K, respectively. The temperature dependences of resistivity of material in the absence and presence of 4 kOe in a range of 30–300 K were fitted to the phase segregation model. The magnetoresistance and the magnetocaloric effects related to the double-exchange interactions taking place around T MI and T C were observed, corresponding to the values of the magnetoresistance MR = 9.29% and the maximum magnetic entropy change ∣ΔS max∣ = 0.18 J/kg.K under an applied magnetic field of 4 kOe. Besides, the low field magnetoresistance value at low temperature region and the change of the specific heat of La0.7Ca0.3MnO3 nanoparticle have also been concerned.

Onsager reaction field theory applied to the phase diagram of Heisenberg chain with ferromagnetic long-range interaction and antiferromagnetic nearest-neighbor interaction

Onsager reaction field theory is used to investigate the one-dimensional ferromagnetic long-range interacting spin chain with the antiferromagnetic nearest-neighbor interaction (NNI) [Formula: see text]. The ferromagnetic long-range interactions considered in this paper decay as [Formula: see text] with the distance [Formula: see text] between lattice sites. It is found that both the zero temperature and finite-temperature phase diagrams of the system are strongly affected by the interplay between ferromagnetic long-range and antiferromagnetic NNIs. The critical temperature and the uniform susceptibility are obtained as a function of [Formula: see text] and [Formula: see text]. At finite temperatures and in the region [Formula: see text] in which [Formula: see text] is dependent of [Formula: see text], the ferromagnetic-paramagnetic phase transition survives for [Formula: see text] and no phase transition exists for [Formula: see text]. At [Formula: see text], the ferromagnetic-antiferromagnetic phase transition happens at zero temperature for [Formula: see text]. The ground state of the system keeps ferromagnetic when [Formula: see text]. But for [Formula: see text], the system becomes antiferromagnetic at all temperatures.

Structural characterization, magnetic and optical properties of perovskite (La1−xLnx)0.67Ca0.33MnO3 (Ln = Nd and Sm; x = 0.0–0.5) nanoparticles synthesized via the sol-gel process

Perovskite (La1−xLnx)0.67Ca0.33MnO3 (Ln = Nd and Sm; x = 0.0–0.5) manganite nanoparticles were synthesized via the sol-gel process. Their structural, magnetic and optical properties of were investigated as a function of the Ln-doping concentration. Crystal structural Rietveld refinements on the x-ray diffraction data verified that all the nanoparticles had the orthorhombic structure with space group Pnma. The lattice parameters a, b/√2, and c are almost equal for the respective nanoparticles. With increasing the Ln-doping concentration unit cell volume contracts in a faster rate in the case of Sm3+-doping than the Nd-doping. Nanoparticles also became more agglomerated each other and their morphologies evolved from spherical shape to irregular one. Their single-crystalline nature was confirmed by HRTEM images. Infrared spectra identified the stretching vibration of the Mn–O bond with a frequency of ~600 cm−1, whose intensity was reduced as increasing the Ln-doping concentration whereas its frequency retained. Mn 2p XPS spectra revealed both Mn3+ and Mn4+ ions existed in the nanoparticles with the Mn3+/Mn4+ ratio = 2:1, and O 1 s XPS spectra confirmed the presence of lattice oxygen and chemically absorbed oxygen. The presence of Ca2+, Nd3+(or Sm3+) and La3+ ions was also confirmed by the Ca 2p, Nd 3d (or Sm 3d) and La 3d XPS spectra. The Ln-doped nanoparticles exhibit smaller Ms and higher Hc than the pristine nanoparticles due to the existence of antiferromagnetic (AFM) interactions. At the low Nd-doping level (0.1 ≤ x ≤ 0.2) the MZFC (T) and MFC (T) curves are similar to that of the pristine nanoparticles, whereas at high Nd-doping level (0.3 ≤ x ≤ 0.5) the MZFC begins to reach and even exceed the MFC at low temperature. Such phenomenon can be understood from a hierarchy of energy barriers existing between the ferromagnetic and charged-ordered AFM phases. All the nanoparticles undergo a paramagnetic-ferromagnetic phase transition with decreasing the temperature. Their Curie temperature, freezing temperature and irreversibility temperature moved to low temperature as the Ln-doping concentration increased. Optical band gaps of all the nanoparticles are in the range of 1.50–1.66 eV, which is ascribed to the electronic charge transfer between two eg bands with up- and down-spins that are separated by Hund’s coupling energy.

Room Temperature Synthesis of Colossal Magneto-Resistance of La2/3Ca1/3MnO3: Ag0.10 Composite

Rare-earth manganite-based perovskite has great potential as a promising material for spintronics and ferroelectromagnets. Herein, we have synthesized La2/3Ca1/3MnO3:Silverx (LCMO-Agx; where x = 0.00 and 0.10) composite using a standard solid-state reaction route. Their structural and physical properties have been investigated. Pristine LCMO and LCMO-Ag composite are crystallized in an orthorhombic structure, which is in a single-phase and has a space group of Pbnm. Pristine LCMO and LCMO-Ag composite’s structural analysis showed better grain connectivity in ferromagnetic domains of LCMO-Ag composite compared to pristine LCMO. Ag doping enhances the paramagnetic-ferromagnetic transition Tc (Curie temperature) to 277 K, which is 8 K higher than that of pristine LCMO (Tc = 269 K). Additionally, the magneto-resistance (MR) of LCMO-Ag composite was improved by ∼10% with Ag doping even at room temperature (RT), which is due to improved connectivity and grain size with Ag doping. Thus, the enhanced value of MR at RT may efficiently open up the possible use of LCMO-Ag composite as ferroelectromagnets and spintronics applications. Additionally, LCMO thin films can be useful in artificial planar junctions, vertical tunnel junctions, and sensing applications.

ПЕРВОПРИНЦИПНЫЕ ИССЛЕДОВАНИЯ ФАЗОВЫХ ПРЕВРАЩЕНИЙ В СПЛАВАХ Fe-Si

In this paper, the structural and magnetic properties of Fe100-xSix alloys (10 ≤ x ≤ 25,0 at. %) were calculated. The structural phase transition temperatures for the crystal structures A2, B2, and D03 were estimated from the geometry optimization. The Curie temperatures were calculated in a molecular-field approximation using the constants of magnetic exchange interaction calculated ab initio. For all the considered concentrations, with the temperature increase, we observed the structural transitions from the ordered cubic phase to a disordered structure, with the intermediate stage of a partially disordered state. The ferromagnet–paramagnet transition was observed for all the compositions, though in various crystal phases.

Two-dimensional magnetic interplay in the tensile-strained LaCoO3 thin films.

High-quality epitaxial LaCoO3 (LCO) thin films have been deposited on SrTiO3 (STO) substrates with pulsed laser deposition (PLD). We find that the LCO films undergo a typical ferromagnetic-paramagnetic (FM-PM) phase transition at ∼80 K. To understand the nature of magnetic phase transition, various methods, including the modified Arrott plot and critical isotherm analysis, were used to determine the critical exponents, which are β = 0.754(1) with a Curie temperature TC = 79.8(8) K and γ = 1.52(2) with TC = 79.9(2) K. The reliability of these critical exponents was confirmed using the Widom scaling relation and the scaling hypothesis. Further analysis revealed that the spin coupling within the LCO films exhibits two-dimensional (2D) long-range magnetic interaction and the magnetic exchange distance decays as J(r) ∼r-(3.46). Our critical behavior analysis may shed new light on the further understanding of the origin of FM and the relatively fixed TC in LCO thin films.

Switching the magnetostructural coupling in MnCoGe-based magnetocaloric materials

We performed neutron-diffraction experiments and density functional theory calculations to study the magnetostructural coupling in MnCoGeBx (x=0, 0.01, and 0.05) alloys. By varying the amount of boron addition, we are able to freely switch the magnetostructural coupling on and off in the MnCoGe alloys. It is found that the boron addition stabilizes the high-temperature hexagonal phase due to the reduced interatomic distances and the enhanced covalent bonding. The hexagonal-orthorhombic structural transition shifts to low temperatures with the boron addition and coincides with the paramagnetic-ferromagnetic (PM-FM) transition in the MnCoGeB0.01 alloy. With a further increase in the boron addition, the structural and magnetic transitions are decoupled again. The hexagonal-orthorhombic structural transition is significantly suppressed in the MnCoGeB0.05 alloy, although subtle distortions in the hexagonal structure are evidenced by a canted spin arrangement below 75 K. The MnCoGe and MnCoGeB0.01 alloys show a collinear FM structure, having a much larger Mn moment than the MnCoGeB0.05 alloy. The relatively small Mn moment in the MnCoGeB0.05 alloy can be attributed to the shortened Mn-Mn distance and the enhanced overlap of the 3d orbitals between the neighboring Mn atoms. The uncovered relationship between the structural evolution and the sizable magnetic moment in the present work offers more insight into the magnetostructural coupling in the MnCoGe-based alloys.

Structural and magnetic characterisation of Co substituted Ni2MnSb Heusler alloy: effect of cobalt substitution on magnetism and Curie temperature

ABSTRACT In this study, Ni2-x Co x MnSb full-Heusler alloys (1.00 ≤ x ≤ 1.75) were prepared by arc melting process, and examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and SQUID. The L21 crystal structure of the synthesised Heusler alloy was confirmed by XRD patterns, and it was observed that the use of Co instead of Ni caused a decrease in the interatomic distance. The magnetisation results revealed that all samples show ferromagnetic (FM) behaviour, and substitution Co for Ni leads to increasing FM interactions in the samples. The calculated magnetic moment per formula unit increase from 4.38 µB(x = 1) to 5.17 µB (x = 1.75). On the other hand, the increase in FM interaction caused an increase in ferromagnetic-paramagnetic (PM) phase transition (T C) temperatures from 513 (x = 1) to 568 K (x = 1.75). The ability to control the Curie temperature with Co substitution makes these alloys suitable candidates for future spintronics and magnetoelectronic applications.

Magnetocaloric effect and microstructure of amorphous/nanocrystalline HoErFe melt-extracted microwires

High-entropy alloys (HEA) represent potentially disruptive materials across multiple industries, especially for refrigeration technologies. Building metal components layer-by-layer increases design freedom and manufacturing flexibility, thereby enabling high magnetic thermal properties of the multicomponent alloy. However, excessive alloying elements could limit the mass production potential, while prolonging the time to market. Here we demonstrate that a high performance magnetocaloric material can be synthesized from only three elements, HoErFe, by taking advantage of the combination of rare earth and transition elements. Novel medium-entropy alloys (MEA) are prepared by melt-extraction and exhibit excellent magnetocaloric properties. The amorphous/nanocrystalline structure of the microwires, which is confirmed by both transmission electron microscopy (TEM) and X-ray diffraction (XRD), gives the primary contribution to the MCE. The microwires undergo a ferromagnetic-paramagnetic (FM-PM) transition near the Curie temperature (TC = ~44 K), and a spin-glass (SG) behavior could be observed below the TC. The maximum magnetic entropy (-ΔSMmax) was 9.5 J kg−1 K−1 under a field change of 5 T. Meanwhile, the refrigerant capacity (RC) and the relative cooling power (RCP) of the alloy microwires were 450 J·kg−1and 588 J kg−1, respectively. The high refrigeration efficiency and high magnetocaloric effect that is reversible make these novel metallic microwires attractive working materials for low-temperature magnetic refrigeration applications.

Magnetic and cryogenic magnetocaloric properties of GdPO4 nanorods

A sample of GdPO4 nanorods (NRs) with average diameter about 20 nm and length 150 nm has been prepared by a hydrothermal method. The analyses of the XRD pattern and transmission electron microscopy images have proved that the NRs are single crystals with a hexagonal structure. The variations of the magnetization versus temperature and magnetic field have confirmed magnetic ordering at low temperatures. The assessment of the magnetocaloric effect from the magnetization isotherms, M(H), in the temperature range T = 2 – 25 K has revealed a strong increase of the magnetic entropy change (|ΔSm|) below T = 15 K with increasing H. The magnetic entropy change reaches maximum (|ΔSmmax|) at temperatures T = 2 – 2.7 K, around a ferromagnetic–paramagnetic transition. It has been found that the relative cooling power (RCP) and refrigerant cooling (RC) increase with increasing H by the broadened linewidth of the |ΔSm(T)| curve. At H = 60 kOe, the values of ΔSmmax and RCP (or RC) are about 56.2 J/kg⋅K and 415.9 (296.5) J/kg, respectively. Our analyses also have inferred that the GdPO4 NRs exhibit a short-range magnetic order with coexisting ferromagnetic and antiferromagnetic interactions.

Critical behavior of polycrystalline Pr0.7Ca0.1Sr0.2MnO3 exhibiting the crossover of first and second order magnetic phase transitions

A polycrystalline ceramic of Pr0.7Ca0.1Sr0.2MnO3 was prosperously synthesized by the solid-state reaction method. The M(T, H) data measured at different temperatures in applied magnetic fields H = 0–30 kOe showed that the material has a ferromagnetic-paramagnetic phase transition and exhibits the crossover of the first and second order magnetic phase transitions. Above Curie temperature (TC), the χ−1(T) curve witnesses a downturn in the paramagnetic region, suggesting an appearance of the Griffiths phase. Using the modified Arrott plots and the Kouvel-Fisher methods, we have identified a set of the critical parameters (TC, β, γ, and δ) for Pr0.7Ca0.1Sr0.2MnO3 compound. The results are found to be TC = 205.5 ± 0.1 K, β = 0.191 ± 0.010, γ = 0.856 ± 0.016 and δ = 5.649 ± 0.030. It can be seen that the values of the exponents β, γ, and δ are quite close to the exponents of the tricritical mean field model. Additionally, they ​​satisfy the scaling theory well, proving their reliability. Based on obtained results, we suggest that the ferromagnetic-paramagnetic transition in Pr0.7Ca0.1Sr0.2MnO3 compound is located at a crossover of the first and second order transformations.

Structural, magnetic, magnetocaloric effect and critical behaviour study of NiCeFeO4 spinel ferrite

We report the structural, magnetic, magnetocaloric properties, and the critical behavior of the NiCeFeO4 spinel ferrite synthesized via the sol–gel method. The XRD analysis has revealed that NiCeFeO4 belongs to the Fd-3m space group of the cubic spinel structure. Furthermore, magnetic properties demonstrate that the prepared sample undergoes a second-order ferromagnetic-paramagnetic (FM-PM) phase transitions at TC = 640 K. Moreover, it was found that the maximum magnetic entropy change(−ΔSMmax) at an applied magnetic field of 5 T is about 0.43 J kg−1K−1, corresponding to relative cooling powers (RCP) of 70.4 J kg−1. Finally, employing various experimental and theoretical techniques, the critical exponents deduced (β, γ and δ) in this study were observed to follow the Heisenberg theory in three dimensions, signifying short-range ferromagnetic interactions.

Weak ferromagnetism and possible non-Fermi-liquid behavior in the itinerant electronic material Co3SnC

We report the observation of weak ferromagnetism and possible non-Fermi-liquid behavior in itinerant electronic material ${\mathrm{Co}}_{3}\mathrm{SnC}$ via magnetic properties, electrical transport, and specific heat under various magnetic fields. The analysis of magnetic properties suggests that ${\mathrm{Co}}_{3}\mathrm{SnC}$ undergoes a noncollinear itinerant ferromagnetic-paramagnetic phase transition around 3.6 K. With this transition, non-Fermi-liquid (NFL) behavior appears at low temperature covering a wide temperature range. The features of NFL are revealed by the power-law temperature dependence of resistivity and also proved by the $--T\mathrm{log}T$ dependence upturn in specific heat and the ${T}^{4/3}$ dependence of inverse susceptibility. With increasing the fields, the temperature coefficient of the ${T}^{2}$ term in the resistivity data shows a ${(H\phantom{\rule{0.16em}{0ex}}--\phantom{\rule{0.16em}{0ex}}{H}_{\mathrm{c}})}^{\ensuremath{-}0.5}$ divergence approaching the ${H}_{\mathrm{c}}$ and reduces monotonously with increasing the field further, apart from the usual $1/(H\phantom{\rule{0.16em}{0ex}}--\phantom{\rule{0.16em}{0ex}}{H}_{\mathrm{c}})$ dependence with the whole Fermi surface under the singular scattering. The exponent $n$ in the temperature dependence of resistivity shows an increase from $n=1.0$ to 2.0 with increasing the field as the evidence for the field-induced crossover from NFL to FL behavior; it is thought that field dependence of magnetic ground states should be responsible for this crossover; the relative mass enhancement factor $\ensuremath{\lambda}(H)/\ensuremath{\lambda}(0)$ reduces monotonically to nearly 60% at 9.0 T, indicating the gradual suppression of magnetic fluctuations and electron-electron scattering. Our results indicate that ${\mathrm{Co}}_{3}\mathrm{SnC}$ is a candidate for exploring itinerant quantum materials.

Anisotropic Effect on Local Magnetic Properties of 3D Extended Ising Model

The magnetic properties of anisotropic 3D Ising model on a cubic lattice are studied by Monte Carlo simulation. In particular, we have considered an extended 3D Ising model with spatially uniaxial anisotropic bond randomness on the simple cubic lattice parameterized by exchange interaction parameter [Formula: see text], anisotropy parameter [Formula: see text] and external longitudinal magnetic field [Formula: see text]. The obtained numerical data clearly point out a second-order ferromagnetic-paramagnetic phase transition belonging in the same universality class with the 3D random Ising model at critical temperature [Formula: see text] that is strongly correlated to [Formula: see text] and [Formula: see text]. Especially, in the limit, [Formula: see text], the spin ½ cubic lattice becomes a collection of noncorrelated Ising chains, whereas in the other limit, [Formula: see text], the system becomes a stack of noncorrelated Ising square lattice.

Experimental isothermal section phase diagram of Ho–Fe–In at 773 K and magnetic properties of Ho12Fe2.08In2.92 alloy

The isothermal section of the Ho–Fe–In system at 773 K has been constructed by X-ray powder diffraction. One known structure ternary compound Er12Fe2In3-type Ho12Fe2In3 has been confirmed. At the same time, solid solutions are not detected in Ho–Fe–In system at 773 K. The magnetic transition and magnetocaloric effect of Ho12Fe2.08In2.92 alloy with Er12Fe2In3-type structure were investigated by magnetic susceptibility and isothermal magnetization measurements. One normal antiferromagnetic–paramagnetic transition and another abnormal one are discovered at 18 and 76 K in ground state, respectively. Owing to a first-order field-induced metamagnetic transition (antiferromagnetic–ferromagnetic) at/below the Neel temperature of 18 K), the negative entropy changes are observed at corresponding temperature. There is only a second-order ferromagnetic–paramagnetic transition near Curie temperature (TC), the maximum entropy change ( $$ \Delta S_{ {\rm max} } $$ ) values are − 6.14 J·kg−1·K−1 at 3 K and 7.88 J·kg−1·K−1 at 28 K in a field range of 0–7 T. The reversible relative cooling power corresponding to negative entropy change can reach about 600 J·kg−1 in an wide operating temperature region $$ \Delta T_{\text{cycl}} $$ = 74 K from 16 to 90 K, which suggests that Ho12Fe2.08In2.92 could be a potential material for magnetic refrigeration in the corresponding temperature range.

Milling effects on the distribution of Curie temperatures and magnetic properties of Ni-doped La0·7Ca0·3MnO3 compounds

A systematic study of the magnetic and magnetocaloric properties around the ferromagnetic-paramagnetic phase transition temperature has been carried out on La0·7Ca0·3Mn1-xNixO3 (x = 0, 0.02, 0.07, 0.10) compounds synthesized by ball milling from oxides and pure Ni. The order of the transition, analyzed by the Banerjee’s criterion and the field dependence of the magnetic entropy change, was found to be of second order for all the studied compositions. The existence of small traces of β-MnO2 phase and the distortions produced during the milling process may induce a competition character of the ferromagnetic and the antiferromagnetic interactions at low fields and motivate the existence of a distribution of Curie temperatures. Once the antiferromagnetic contributions are avoided, the parameters of the distribution, the average Curie temperature TC¯ and the broadening of the distribution ΔTC, have been obtained from the analysis of the approach to saturation curves and the magnetocaloric effect.

Sintering temperature effect on the magnetic interactions in Pr0.67Sr0.33MnO3 manganite

In this work, we report the effect of sintering temperature (TS) on the magnetic interactions and the critical behavior of Pr0.67Sr0.33MnO3 manganite prepared via sol–gel method (700 °C ≤ TS ≤ 1200 °C). The critical exponents study revealed that the 3D-Ising model is the best model to describe the magnetic interactions in the P1000 (TS = 1000 °C) and P1200 (TS = 1000 °C) samples. Consequently, a strong magnetic anisotropy characterizes these samples near the paramagnetic-ferromagnetic transition temperature, such anisotropy can be linked to the magnetic moment of Pr3+ ions (µ = 3.58 µB). For TS ≤ 850 °C, it becomes nearly impossible to speak about universality class due to the presence of great magnetic disorder induced by several factors like spin disordered shell in nanoparticles, magnetic frustration and Griffiths phase.

Pseudo-Yang-Lee Edge Singularity Critical Behavior in a Non-Hermitian Ising Model.

The quantum phase transition of a one-dimensional transverse field Ising model in an imaginary longitudinal field is studied. A new order parameter M is introduced to describe the critical behaviors in the Yang-Lee edge singularity (YLES). The M does not diverge at the YLES point, a behavior different from other usual parameters. We term this unusual critical behavior around YLES as the pseudo-YLES. To investigate the static and driven dynamics of M, the (1+1) dimensional ferromagnetic-paramagnetic phase transition ((1+1) D FPPT) critical region, (0+1) D YLES critical region and the (1+1) D YLES critical region of the model are selected. Our numerical study shows that the (1+1) D FPPT scaling theory, the (0+1) D YLES scaling theory and (1+1) D YLES scaling theory are applicable to describe the critical behaviors of M, demonstrating that M could be a good indicator to detect the phase transition around YLES. Since M has finite value around YLES, it is expected that M could be quantitatively measured in experiments.

Magnetic, Magnetocaloric, and Critical Properties of Fe84-xCr2+xB2Co2Zr10 Melt-Spun Ribbons

Magnetic properties, magnetocaloric effect, and critical behavior of Fe84-xCr2+xB2Co2Zr10 (x = 1, 2, 3, 4, 5, and 6) rapidly quenched alloy ribbons prepared by melt-spinning method have been investigated. X-ray diffraction analysis shows that the ribbons are almost amorphous. All the ribbons exhibit soft magnetic behavior with a low coercivity, Hc 0.8 J.kg−1.K−1 (under a magnetic field change of 12 kOe), and the wide working temperature range, δT > 90 K, around room temperature, have been achieved on these alloy ribbons. The obtained results reveal that Fe84-xCr2+xB2Co2Zr10 alloys are potential candidates for the magnetic refrigerants at room temperature region. Using the Arrott-Noakes method, critical analyses around the ferromagnetic-paramagnetic phase transition elucidated the magnetic orders in the alloys. The critical parameters determined for Fe84-xCr2+xB2Co2Zr10 ribbons are close to those of the mean-field theory applied for the long-range ferromagnetic orders.