Curie Transition Research Articles

NiFe2O4 nanospheres with size-tunable magnetic and electrochemical properties for superior supercapacitor electrode performance

Hereby, we present our efforts to understand the influence of ferric ions in the preparation of NiFe2O4 magnetic nanoparticles (MNPs) by facile chemical oxidation method. The variation in the Ferric ions content provoked size tunability of the MNPs (particle size reduced with increasing ferric ions content). The effect of ferric ions content on the prepared MNPs were analyzed for phase, morphological and magnetic characteristics by using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Vibrating Sample Magnetometer (VSM) and Thermomagnetic Analysis (TMA) measurements. The saturation magnetization and coercivity values were observed to be 45 emu/g and 90 Oe for the nanoparticles prepared without ferric ions. The size depended reduction in magnetization and coercivity values were evident for increased ferric ion concentration. The curie transition temperature was observed to be 584 °C from TMA analysis. The cyclic voltammogram, galvanostatic charge/discharge and electrochemical impedance measurements were carried out to evaluate the electrochemical characteristics of prepared NiFe2O4 nanoparticles for supercapacitor application. The highest specific capacitance of 277 F/g was observed for the NiFe2O4 nanoparticles that prepared with 50 % of ferric ions (NF50). From the cyclic stability study, 101 % of capacitance retention was demonstrated up to 5000 cycles. An asymmetric two-electrode cell that fabricated with NF50 exhibited a maximum specific capacitance of 56 F/g (with an applied current density of 1 A/g) and higher energy density of 22.5 Wh/ Kg (with a power density of 0.85 kW/Kg). The cyclic stability study of the cell revealed the increased capacity retention (126 % up to 5000 cycles) due to the particle size reduction. The observed enhancements in the magnetic and electrochemical properties with the size reduction are discussed in-detail. The facile strategy presented by this work would give insights for high-performance supercapacitor electrode material.

Correlation of magnetocaloric effect through magnetic and electrical resistivity on Si doped Ni–Mn–In Heusler melt spun ribbon

The effect of chemical substitution of Si on Mn site in Ni47Mn40-xSixIn13 (x = 1, 2 & 3) melt spun ribbons on the structure, electrical, magnetic and magnetocaloric studies are investigated. All the synthesized ribbons exhibits austenite phase at room temperature. It is found that the increase of Si content under applied magnetic fields of 0.05 T and 5 T shifts both the Curie transition temperature of the austenitic phase (TCA) and martensitic phase transition temperature (TM) towards lower temperatures. The splitting of ZFC and FC curve during magnetic measurements exposes exchange bias phenomenon attributed to irreversibility nature of magnetization due to inhomogeneous magnetic anisotropy below the blocking bias temperature. The large magnetic entropy change (ΔSM) is observed across the martensite transformation and a maximum magnetic entropy change (ΔSM)max of 10 Jkg−1K−1 is obtained for x = 1 ribbon at 273 K with a magnetic field change of 5 T. Upon further increase of Si content, the (ΔSM)max values are found to decrease from 6.3 Jkg−1K−1 (x = 2) to 5.54 Jkg−1K−1 (x = 3) and the same trend is observed for ΔSMm-ρ values.

Intraband Lifshitz transition and Stoner ferromagnetism in Janus PA2As ( A=Si,Ge,Sn , and Pb) monolayers

Using first-principles calculations, a class of Janus two-dimensional materials, $\mathrm{P}{A}_{2}\mathrm{As}$ ($A=\mathrm{Si}$, Ge, Sn, and Pb) monolayers, is predicted to be dynamically and thermally stable. The band gap of $\mathrm{P}{A}_{2}\mathrm{As}$ can be linearly modulated by external strain and electric field. The presence of the peculiar sombrero-type band in strained $\mathrm{P}{A}_{2}\mathrm{As}$ results in the intraband Lifshitz transition near the valence band top when including Rashba spin-orbit coupling. The energy range of intraband Lifshitz transition is linearly dependent on electric field, indicating potential electric control of the Fermi surface properties. In particular, hole doping leads to the ferromagnetic transition in $\mathrm{P}{A}_{2}\mathrm{As}$ due to Van Hove singularity and a divergence with $1/\sqrt{E}$ of the density of states. This ferromagnetism can be understood in terms of the Stoner criterion $D$(${E}_{\mathrm{F}}$)${I}_{\mathrm{S}}>1$. A saturated magnetic moment of $1\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{hole}$ is obtained in hole-doped ${\mathrm{PSn}}_{2}\mathrm{As}$ and ${\mathrm{PPb}}_{2}\mathrm{As}$ with a wide range of hole densities. The highest Curie transition temperature ${T}_{\mathrm{c}}$ is estimated to be 93.5 K at the mean field level. We demonstrate that hole density and tensile strain are two effective approaches to control the Stoner ferromagnetism in practical applications. Moreover, various electronic transitions, such as semiconductor to half-metal, half-metal to metal, and semiconductor to metal, are revealed in hole-doped $\mathrm{P}{A}_{2}\mathrm{As}$. Finally, we explore the feasibility of realizing ferromagnetism by introducing $p$-type dopants and defects. The experimentally controlled intraband Lifshitz transition and Stoner ferromagnetism demonstrate that Janus $\mathrm{P}{A}_{2}\mathrm{As}$ monolayers have a wide application potential in future electronic and spintronic devices.

Impact of the Co Doping on the Structure, Magnetic Properties, and Phase Transition in Ni50-xCoxMn38Sn12 Ribbons

Three Heusler alloys, Ni50-xCoxMn38Sn12 (x = 1, 2, and 3), were elaborated by rapid solidification. The impact of the Co doping on the structure, magnetic properties, and phase transition in these alloys was studied. The structure of the Ni49Co1Mn38Sn12 and Ni48Co2Mn38Sn12 ribbons was martensite 14M monoclinic structure, while the Ni47Co3Mn38Sn12 sample structure was austenite cubic L21. The thermal analysis showed the impact of the substitution Ni by Co. It was noted that the temperatures of martensitic transition moved lower, and a decreases progressively of enthalpy and entropy changed. Likewise, an obvious increase in the temperature of Curie transition for austenite phase (TAC) was observed and a jump of magnetization change (ΔM) was detected, with increasing Cobalt content.

Magnetic fabric of the Parashi stock and related dyke swarm, Alta Guajira (Colombia): The Caribbean-South American plates oblique convergence

Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) were used to evaluate the emplacement history of the Parashi stock and related dyke swarm situated in NW Colombia. The average magnetic susceptibility of 4.5×10-2 SI, in conjunction with low-coercivity components provided by the isothermal remanence and thermomagnetic curves with net Verwey and Curie transitions, indicates that multidomain magnetite records the anisotropy directions. The similar orientation and shape of the AMS and AAR ellipsoids indicate the absence of very fine magnetite with an inverse fabric. The magnetic foliation is the best-defined fabric element in these rocks and outlines a concentric structure, elongated parallel to the NE-SW direction of the pluton. Crystallisation age of the stock and dykes (51-47 Ma), along with pressure of emplacement determination indicate that the stock and the dyke swarm probably formed simultaneously, and they were emplaced in the shallow crust (

Deconvolution of overlapping first and second order phase transitions in a NiMnIn Heusler alloy using the scaling laws of the magnetocaloric effect

Magnetocaloric materials with a first order phase transition may exhibit additional thermomagnetic phase transitions in the temperature range of interest due to the second-order magnetic phase transitions of the phases. In the case of Heusler alloys exhibiting magneto-structural transformation (martensitic type), this first-order phase transition is usually accompanied by a temperature dependence of magnetization of the different magnetic structures. The temperature of the martensitic transition in this type of materials can be tuned by altering the composition, making possible a close proximity between this transition and the Curie transitions (second-order type) of the structures. This can result in a tight overlap between the two types of magnetocaloric effects, usually of different signs. In this work, the magnetocaloric response of overlapping first and second-order phase transitions of a Ni48.1Mn36.5In15.4 Heusler alloy has been deconvoluted by applying the scaling laws of the magnetocaloric effect. This deconvolution allows the prediction of the magnetocaloric response of the pure first-order phase transition, helping to gauge if it is worth synthesizing other alloys of the same family for which the Curie transition would be much more distant. The deconvoluted signal confirms that the anomalous experimental decrease of ΔSiso with increasing the field above ~ 3 T is due to the competing contribution of the overlapping second-order phase transition.

Effect of hydrostatic pressure on martensitic transformation and low-temperature magnetic properties in Ni45Cu5Mn35In15 Heusler alloy

The effect of hydrostatic pressure on martensitic transformation (MT), superparamagnetic (SPM) and exchange bias (EB) behaviors in polycrystalline Ni45Cu5Mn35In15 has been intensively investigated by various experimental methods. The experimental results show that the studied alloy experiences a MT from the austenitic state with a L21-type cubic structure to the martensitic state with a seven-layer (7 M) modulated orthorhombic structure with decreasing temperature, accompanied by a significant change in magnetization. It is found that applying hydrostatic pressure can induce the MT to occur in the higher temperature, while the Curie transition almost remains unchanged due to the suppression of ferromagnetic (FM) coupling. In addition, the applied hydrostatic pressure can suppress the EB field and coercivity of the studied alloy due to the reduction of the competition between FM and AFM interactions. This phenomenon can be attributed to the fact that the application of hydrostatic pressure reduces the size of the SPM cluster and its average magnetic moment.

Multi-scale characterisation of a ferroelectric polymer reveals the emergence of a morphological phase transition driven by temperature

Ferroelectric materials exhibit a phase transition to a paraelectric state driven by temperature - called the Curie transition. In conventional ferroelectrics, the Curie transition is caused by a change in crystal symmetry, while the material itself remains a continuous three-dimensional solid crystal. However, ferroelectric polymers behave differently. Polymeric materials are typically of semi-crystalline nature, meaning that they are an intermixture of crystalline and amorphous regions. Here, we demonstrate that the semi-crystalline morphology of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) strongly affects its Curie transition, as not only a change in crystal symmetry but also in morphology occurs. We demonstrate, by high-resolution nanomechanical measurements, that the semi-crystalline microstructure in the paraelectric state is formed by crystalline domains embedded into a softer amorphous phase. Using in situ X-ray diffraction measurements, we show that the local electromechanical response of the crystalline domains is counterbalanced by the amorphous phase, effectively masking its macroscopic effect. Our quantitative multi-scale characterisations unite the nano- and macroscopic material properties of the ferroelectric polymer P(VDF-TrFE) through its semi-crystalline nature.

Two-dimensional uranium halide monolayers UX3 (X[dbnd]Cl, Br) with high Curie temperatures

The successful fabrication of two-dimensional ferromagnetic materials, such as monolayer CrI3 and bilayer Cr2Ge2Te6, has drawn much attention in recent years. Here, we found that UX3 (with XCl, Br) monolayers are ferromagnetic materials with high Curie temperatures. In this study we explored the electronic structures and magnetic properties of UX3 by using density functional theory. UX3 has a ferromagnetic order with in-plane magnetic easy axis (i.e., along x direction), and its magnetic anisotropy is 31.2∼34.2 meV per U atom. We performed Monte Carlo calculations to predict the Curie transition temperatures. They are as high as 160 and 125 K for UCl3 and UBr3 monolayers, respectively, which are higher than that of the UI3 found before. Therefore, UX3 monolayers offer new platforms for the study of spintronic devices.

Identification and comparison of peculiarities in physical properties of multiferroic morphotrophic phase boundary sintered BiFeO3-xPbTiO3 nano-ceramics

In this paper a comprehensive study that includes structural, magnetic, ferroelectric, dielectric and electrical characterizations of as-prepared and quenched MPB 1-x(BiFeO3)-x(PbTiO3)(BF-xPT) solid solutions prepared by wet chemical method has been presented. Qualitative and quantitively structural analysis revealed coexistence of rhombohedral/monoclinic, tetragonal and orthorhombic phases. Sensitivity to the experimental variables such as composition, synthesis conditions, sintering temperature and particle size, confirms the existence of a complex energy phase diagram where minima associated with various polymorphs lies almost at same level for MPB compositions. Magnetic characterizations reveal the presence of three anomalies, i.e., originating from the Néel transition, iso-structural transition and ferroelectric phase transition. Second anomaly detected to be the most sensitive to the quenching. Enhancement of magnetic moment in quenched compositions is a manifestation of strong interaction between spins of Fe2+ ions with the pinned point defect dipoles. Interestingly, thermal quenching enhanced the ferroelectric properties by removing the domain wall pinning. Saturated ferroelectric loops were observed only for quenched samples which is a consequence of strong domain wall de-pinning. Low temperature dielectric relaxation has been observed while the high temperature dielectric data of as-prepared composition exhibited Maxwell-Wagner and iso-structural phase transition. In contrast, along with these two features, quenched samples showed additional Curie transition. Enhancement in large polaron hoping in the low frequency region for the quenched composition has been confirmed via frequency dependent profile of the ac conductivity. Low value of activation energy of charge carriers for quenched compositions indicates large polaron hoping characteristics in BF-xPT solid solution.

Effect of B-Site Ordering on the Magnetic Order in Multifunctional La2NiMnO6 Double Perovskite**Supported by the National Natural Science Foundation of China (Grant Nos. 51702289, 21801054 and 11604067), and the Natural Science Foundation of Zhejiang Province (Grant No. LQ20E020006).

To obtain various Ni/Mn orderings, we use a low-temperature synthesized method to modulate the Ni/Mn ordering of the ferromagnetic-ferroelastic La2NiMnO6 compound, and the Ni/Mn ordering is estimated by the low-temperature saturation magnetism. The microstructures, crystal structures and magnetic properties are investigated, and the Landau theory are used to describe the form and magnitude of the coupling effects between Ni/Mn ordering and magnetic order parameters. It is predicted that the Ni/Mn ordering would be a strong coupling effect with the Curie transition temperatures if the La2NiMnO6 sample stoichiometry is close.

Multicaloric Effects in (MnNiSi)1−x (Fe₂Ge) x Alloys

MnNiSi-based alloys, substituted with isostructural Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ge as (MnNiSi) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ge) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , were prepared by arc-melting to examine their structural, magnetocaloric, and barocaloric properties. A simultaneous (coupled) first-order magnetic and structural, that is, magnetostructural, transition from a low-temperature ferromagnetic (FM) (TiNiSi-type) orthorhombic phase to a high-temperature paramagnetic (PM) (Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> In-type) hexagonal phase was observed in all compositions, where 0.33 ≤ x ≤ 0.35, by magnetothermal and structural analyses. The magnetostructural transition temperature, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> , decreased from 350 to 256 K (for μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> H = 0.1 T, on heating) by providing chemical pressure used by increasing the compositional variable, x, to 0.35. However, application of hydrostatic pressure overall decreased both T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> at dT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> /dP ~7.3 K/kbar and thermal hysteresis, ΔT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> , significantly. Indeed, in the x = 0.33 composition, ΔT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> was drastically reduced by >80% from 30 K at ambient pressure to 5 K at P = 7.6 kbar. A maximum magnetic entropy change, ΔS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mag</sub> (μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> H = 2 T), corresponding to 30 J/kg · K is noted at 270 K in (MnNiSi) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.67</sub> (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ge) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.33</sub> at an applied pressure of 7.9 kbar. For the x = 0.34 and 0.35 compositions, an application of P ~ 8 kbar resulted in the partial, and complete, decoupling of the magnetic (FM → PM) and structural (orthogonal → hexagonal) transitions, respectively, and consequently, a large drop in ΔS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mag</sub> . Application of P = 7.9 kbar on the x = 0.35 composition converted the first-order T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> to the second-order Curie transition, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , and therefore loss of the structural transition indicating the stabilization of the high-temperature hexagonal structure. Overall, these features emphasize strong coupling between the magnetic spins and the lattice in MnNiSi-based alloys. Further fundamental and applied insight is obtained concerning pathways for optimizing the multicaloric response of MnNiSi-based alloys with isostructural substitution by Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ge for potential solid-state cooling applications.

High permittivity BaTiO3 and BaTiO3-polymer nanocomposites enabled by cold sintering with a new transient chemistry: Ba(OH)2∙8H2O

Cold sintering process (CSP) offers a promising strategy for the fabrication of innovative and advanced high permittivity dielectric nanocomposite materials. Here, we introduce Ba(OH)2∙8H2O hydrated flux as a new transient chemistry that enables the densification of BaTiO3 in a single step at a temperature as low as 150 °C. This remarkably low temperature is near its Curie transition of 125 °C, associated with a displacive phase transition. The cold sintered BaTiO3 shows a relative density of 95 % and a room temperature relative permittivity over 1000. This new hydrated flux permits the fabrication of a unique dense BaTiO3-polymer nanocomposite with a high volume fraction of ceramics ((1-x) BaTiO3 – x PTFE, with x = 0.05). The composite exhibits a relative permittivity of approximately 800, at least an order of magnitude higher than previous reports on polymer composites with BaTiO3 nanoparticle fillers that are typically well below 100. Unique high permittivity dielectric nanocomposites with enhanced resistivities can now be designed using polymers to engineer grain boundaries and CSP as a processing method opening up new possibilities in dielectric materials design.

Multiple magnetic phase transition and short-range ferromagnetic behavior influence on magnetocaloric effect of Sm2NiMnO6 nanoparticles

Structural characteristics and magnetocaloric effect of Sm2NiMnO6 double perovskite oxide nanoparticles synthesized via the solution combustion technique have been investigated. From the X-ray structural study, it is found that the Sm2NiMnO6 compound crystallizes in the monoclinic structure (P21/n space group), where Ni and Mn atoms are ordered at 2c and 2d sites, respectively. Transmission electron microscopic image reveals agglomerated nanoparticles with an average particle size of 60 nm. Two magnetic Curie transition points (TC1 = 141 K, TC2 = 67 K) observed from the isofield magnetization measurement affirmed the short-range ferromagnetic behavior of Sm2NiMnO6 creating the continuous second-order transition followed by the discontinuous first-order transition. A minor cusp at 10 K confirmed the microscopic superparamagnetic behavior at low fields apart from the dual magnetic phase transition. Adiabatic demagnetization of Sm2NiMnO6 nanoparticles was measured in terms of magnetic entropy, and the magnetic refrigerant capacity was calculated to be 0.44 J kg−1 K−1 and ~ 20 J kg−1 respectively.

Tuning thermo-magnetic properties of dilute-ferromagnet multilayers using RKKY interaction

We demonstrate a 20-fold enhancement in the strength of the Ruderman–Kittel–Kasuya–Yosida interlayer exchange in dilute-ferromagnet/normal-metal multilayers by incorporating ultrathin Fe layers at the interfaces. Additionally, the resulting increase in the interface magnetic polarization profoundly affects the finite-size effects, sharpening the Curie transition of the multilayer, while allowing us to separately tune its Curie temperature via intra-layer magnetic dilution. These results should be useful for designing functional materials for applications in magnetocaloric micro-refrigeration and thermally assisted spin-electronics.

Measurement and modelling of the thermodynamic properties of Ni51Fe36Co13 soft magnetic alloy

A soft magnetic alloy, Ni51Fe36Co13 is used as the sensor in temperature sensitive magnetic switch circuit of diverse safety rod drive mechanism of advanced fast breeder reactors. In the present work the magnetic and thermodynamic properties of this indigenously developed soft ferromagnetic alloy were measured using variety of experimental techniques. The alloy has a single phase face centered cubic structure from room temperature up to 1273 K with the Curie transition at 898 ± 1 K. Temperature dependent enthalpy increment, dilatation and thermal diffusivity were measured from 298 to 1273 K and the corresponding values for isobaric heat capacity, coefficient of linear thermal expansion and thermal conductivity were determined. In addition, dynamic elastic moduli and Poisson’s ratio were also estimated using impulse excitation technique. In the course of evaluation of thermodynamic properties, contribution due to magnetic ordering was treated separately using the Calphad approach to indicate the magneto-volume and magneto-caloric effect. To understand the low temperature behavior, calculations based on quasi harmonic theory were carried out using measured thermodynamic quantities. Based on the study, it is observed that magnetic ordering has significant effect on the thermodynamic properties of Ni51Fe36Co13 ferromagnetic alloy.

Effect of Si substitution on the Structural, Magnetic and Magnetocaloric Properties of Ni–Mn–In Heusler alloys

The effect of partial substitution of manganese by silicon in Ni47Mn40−xSixIn13 (x = 1, 2, 3) on the structural (martensitic), magnetic transitions and associated magnetocaloric properties around the martensitic transition of these alloys, was investigated. The alloys exhibited the single austenite structure at the room temperature. The lattice contraction was observed with an increase in Si content, thereby inducing the FM and AFM interactions between manganese atoms, which further influence the magnetic and martensitic transitions. The increase in Si concentration was observed with the decrease in valence electron to atom ratio (e/a ratio) and thus influencing the martensitic transitions. The transition temperatures such as martensitic transition (TM) and the Curie transition (TC) of the austenite phase decrease with increasing Si-content. With the increase in Si content from x = 1 to 3, the TM was found to decrease from 259 to 169 K. A high magnetic entropy change (∆S)M of 12.3, 3.1 and 30.6 J kg−1 K−1 was observed for the x = 1, 2, and 3 alloys at their respective martensitic transitions for a magnetic field of 2 T.

Achieving a broad refrigeration temperature region through the combination of successive caloric effects in a multiferroic Ni50Mn35In15 alloy

Solid-state refrigeration based on the caloric effects has been conceived to be a high-efficient and environmental-friendly alternative to replace the vapor-compression refrigeration technique. The implementation of solid-state refrigeration requires that the refrigerants should possess not only remarkable caloric effect but also wide working temperature region. In this work, we demonstrate that various caloric effects can be achieved successively in a multiferroic Ni50Mn35In15 meta-magnetic shape memory alloy prepared by directional solidification, including inverse magnetocaloric effect around inverse martensitic transformation, conventional magnetocaloric effect around Curie transition and elastocaloric effect above Curie transition. Among them, the elastocaloric effect is particular striking, where a giant adiabatic temperature variation up to –19.7 K is achieved on removing a moderate stress of 350 MPa due to the elimination of negative magnetic contribution, with the specific adiabatic temperature change of 56 K GPa–1. Furthermore, through the combination of these successive caloric effects, a broad refrigeration temperature region covering the temperature range from 270 K to 380 K can be achieved. It is demonstrated that the combination of various caloric effects could be a promising way to extend the working temperature range of solid state refrigeration.

Achievement of Curie transition to 330 K by Ba substitution in LaMnO3

To achieve Curie transition temperature near to room temperature, Ba substituted lanthanum manganite (La0.67Ba0.33MnO3) bulk samples has been synthesized by solid state reaction route which exhibits a Curie transition temperature (Tc) of 330 K. X-ray diffraction pattern shows the formation of single phasic La0.67Ba0.33MnO3 without having any impurity phases. Scanning electron micrograph reflects the polygonalization of a single grain and the compact arrangement of the grains. The grains form trigonal, tetragonal and somewhere pentagonal grain boundary junction. The small grains in between the big grains favors the double exchange interaction for which a long range ferromagnetic ordering is established in La0.67Ba0.33MnO3 sample.

BaSrLaFe12O19 nanorods: optical and magnetic properties

In this work, the Ba0.2Sr0.8−yLayFe12O19 (y = 0.2, 0.4, 0.6 & 0.8) (BSLFO) nanorods were prepared using hydrothermal method. The hexagonal structure and nanorod formation were evidenced by X-ray diffraction patterns and micrographs, respectively. Further, the optical and magnetic properties of the BSLFO nanorods were investigated. Using the UV–Visible spectra of y = 0.2–0.8 contents, the optical bandgap (Eg) was determined. The Eg was increased from 3.502 to 3.714 eV as a function of La content. The Raman modes were detected using the Raman spectra for confirming the hexagonal structure of x = 0.2–0.8. The magnetic hysteresis curve indicated the decreasing trend of saturation magnetization (Ms) from 14.44 ± 0.507 to 4.15 ± 0.162 emu/g with ‘y’. This was determined using the law of approach to saturation (LAS) method. In addition, the magnetic permeability versus temperature (μ′-T) plots showed the decreasing nature of magnetic Curie transition temperature (Tc) from 753 to 703 K with increase of La content. Furthermore, the frequency dependence of magnetic permeability (μ′), magnetic loss (μ″) and relative magnetic loss (rlf: tanδ/μ′) was studied for all samples.