Magnetocaloric effect in the intermetallic compound DyNi

Magnetic and heat capacity measurements have been carried out on the polycrystalline sample of DyNi, which crystallizes in the orthorhombic FeB structure (space group Pnma). This compound is ferromagnetic with a Curie temperature of 59 K. Magnetization-field isotherms at low temperatures show a multi-step behavior characteristic of metamagnetic transitions. The magnetocaloric effect has been measured both in terms of isothermal magnetic entropy change and adiabatic temperature change for various applied magnetic fields. The maximum values of the entropy change and the temperature change are found to be 19 J kg⁻¹ K⁻¹ and 4.5 K, respectively, for a field of 60 kOe. The large magnetocaloric effect is attributed to the field-induced spin-flop metamagnetism occurring in this compound, which has a noncollinear magnetic structure at low fields.     

Spin-glass behavior and magnetocaloric effect in Tb-based bulk metallic glass

A noncollinear-ferromagnetic spin-glass-like state was observed in Tb₅₅Co₂₀Al₂₅ bulk metallic glass due to the strong random magnetic anisotropy. Associated with this behavior, we observed a comparatively large magnetic entropy change (ΔS_m is 9.75 J K⁻¹ kg⁻¹) in a field change of 7 T and a correspondingly high value of the magnetic refrigeration capacity (RC is 540 J kg⁻¹) with almost no hysteresis loss in the vicinity of the so-called Curie temperature. This opens the possibility of using this material for magnetic cooling purposes.     

Effect of Mn-site vacancies on the magnetic entropy change and the Curie temperature of La0.67Ca0.33Mn1−xO3 perovskite

Single-phase polycrystalline samples of La_0.67Ca_0.33Mn_1−xO₃ (x=0.00, 0.02, 0.04, 0.06) have been prepared using the sol-gel method. The structure, magnetocaloric properties and the Curie temperature of the samples with different Mn vacancy concentrations have been investigated. The experimental results show that vacancy doping at the Mn-sites has a significant influence on the magnetic properties of La_0.67Ca_0.33Mn_1−xO₃. The Curie temperature decreases monotonically with increasing the Mn-site vacancy concentration x. A remarkable enhancement of the magnetic entropy change has been obtained in the La_0.67Ca_0.33Mn_0.98O₃ sample. The entropy change reaches |ΔS_M|=3.10 J kg⁻¹ K⁻¹ at its Curie temperature (264 K) under an applied magnetic field H=10 kOe, which is almost the same value as that of pure Gd.     

Carbon-doping effects on the metamagnetic transition and magnetocaloric effect in MnAsCx

Carbon doping effects in MnAs alloys have been investigated. More carbon doping in MnAs alloys leads to lower Curie temperature, larger thermal hysteresis and sharper slope of the dependence of critical field on reduced temperature due to severe lattice distortion. The obtained maximum of magnetic entropy change for a field change of 5 T is about 12.8 J kg⁻¹ K⁻¹ near room temperature, and increases with more doping carbon content to about 22.4 J kg⁻¹ K⁻¹ in MnAsC_0.03 and 13.2 J kg⁻¹K⁻¹ in MnAsC_0.05.     

Anisotropic and large low-field magnetic entropy change in a La4/3Sr5/3Mn2O7 single crystal

We have studied the magnetocaloric effect (MCE) in a bilayered La_4/3Sr_5/3Mn₂O₇ single crystal with applied field along both ab-plane and c-direction. Due to the quasi-two-dimensional structure, the crystal exhibits a strong anisotropy in the MCE. The difference of magnetic entropy change between two crystallographic directions depends on external magnetic fields and has a maximum of 2 J/kg K. A large low-field magnetic entropy change, reaching 3.2 J/kg K for a magnetic field change of 15 kOe, is observed when the applied field is along ab-plane. This large low-field magnetic entropy change is attributed to the rapid change of magnetization in response to external magnetic fields in the easy magnetizing plane.     

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic,differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La_0.45Pr_0.25Ca_0.3MnO₃ by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with T_C=200 K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔS_m=−7.2 J kg⁻¹ K⁻¹ at T=212.5 K and refrigeration capacity of 228 J kg⁻¹ are found for a field change of ΔH=5 T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature in nanocrystalline La0.7Te0.3MnO3

Sol-gel prepared nanocrystalline La_0.7Te_0.3MnO₃ has rhombohedral crystal structure (space group R3¯C) at room temperature and orders ferromagnetically at ∼280 K (T_C). A large magnetic entropy change of ∼12.5 J kg⁻¹ K⁻¹ is obtained near T_C for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal-insulator transition at T_C and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La_0.7Te_0.3MnO₃ a promising material for magnetic refrigeration and spintronic device applications.     

Hydrothermal synthesis and Magnetocaloric effect of La0.5Ca0.3Sr0.2MnO3

The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La_0.5Ca_0.3Sr_0.2MnO₃ have been studied. The La_0.5Ca_0.3Sr_0.2MnO₃ can be produced as phase-pure, crystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 513 K in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped particles in typical dimension of 4.0×2.5×1.6 μm. Heat treatment can improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change ΔS_M for the as-prepared sample is 0.88 J kg⁻¹ K⁻¹ at 315 K for a magnetic field change of 2.0 T. It increases to 1.52 J kg⁻¹ K⁻¹, near its Curie temperature (317 K) by annealing the sample at 1473 K for 6 h. The hydrothermal synthesis method is a feasible route to prepare high-quality perovskite material for magnetic refrigeration application.     

Time resolved magnetic ordering and photo-induced gyrotropy at room temperature in CdMgTe-CdMnTe semiconductor micro-cavities

Using a picosecond pump and probe time-resolved technique we evidence a single pump pulse photo-induced magnetic ordering in a Mn-doped semiconductor magneto-photonic microcavity operating in the strong coupling regime at room temperature. This nanosecond duration magnetization is attributed to a magnetic ordering of the Mn-impurities mediated through photo-generated holes and enhanced through the confinement. It is distinct from the preceding short lived photo-induced spin orientation of carriers also evidenced by our technique for circularly polarized pump beams. The photo-generated magnetic flux density amounts to a 1 kG for beam fluences of few tens μJ cm⁻² and effective Mn concentrations of 5 nm⁻³; large photo-induced magneto-optic Kerr rotations are also evidenced.     

Structures, magnetic properties, and magnetocaloric effect in MnCo1−xGe (0.02?x?0.2) compounds

The crystalline structures, magnetic properties and magnetocaloric effect (MCE) of MnCo_1−xGe alloys (0.02?x?0.2) have been reported. The crystalline structures of MnCo_1−xGe (x?0.06) alloys are mainly of TiNiSi-type phase, and Ni₂In-type structure dominates for x>0.06. With decreasing Co concentrations the saturated magnetization of these compounds decreases. Large low-field magnetic entropy change −ΔS_M of about 2.3 J/kg K in MnCo_0.94Ge alloy has been obtained for a magnetic field change of 1 T. Moreover, it is found that TiNiSi-type phase exhibits larger −ΔS_M than Ni₂In-type one. For MnCo_0.94Ge alloy, considerable low-field refrigerant capacity (RC) (∼460 mJ/cm³), low coercivity and easy synthesis make these alloys potential candidates for near-room temperature magnetic refrigerants.     

Magnetic ordering in CeMnCuSi2

Temperature and field-dependent magnetization measurements on polycrystalline CeMnCuSi₂ reveal that the Mn moments in this compound exhibit ordering with a ferromagnetic (FM) component ordered instead of the previously reported purely antiferromagnetic (AFM) ordering. The FM ordering temperature, T_c, is about 120 K and almost unchanged with external fields up to 50 kOe. Furthermore, an AFM component (such as in a canted spin structure) is observed to be present in this phase, and its orientation is modified rapidly by the external magnetic field. The Ce L₃-edge X-ray absorption result shows that the Ce ions in this compound are nearly trivalent, very similar to that in the heavy fermion system CeCu₂Si₂. Large thermomagnetic irreversibility is observed between the zero-field-cooled (ZFC) and field-cooled (FC) M(T) curves below T_c indicating strong magnetocrystalline anisotropy in the ordered phase. At 5 K, a metamagnetic-type transition is observed to occur at a critical field of about 8 kOe, and this critical field decreases with increasing temperature. The FM ordering of the Mn moments in CeMnCuSi₂ is consistent with the value of the intralayer Mn–Mn distance R^a_Mn–Mn=2.890 Å, which is greater than the critical value 2.865 Å for FM ordering. Finally, a magnetic phase diagram is constructed for CeMnCuSi₂.     

The large low-field magnetic entropy changes in Ni43Mn46Sn11−xSbx alloys

A series of Ni₄₃Mn₄₆Sn_11−xSb_x (x=0, 1, and 3) alloys were prepared by an arc melting method. The martensitic transition shifts to higher temperature with the increasing Sb content. The isothermal magnetization curves and Arrott plots around martensitic transition temperatures show a typical metamagnetic behavior. Under a low applied magnetic field of 10 kOe, large magnetic entropy changes around the martensitic transition temperature are 10.4, 8.9, and 7.3 J/kg K, for x=0, 1, and 3, respectively. The origin of the large magnetic entropy changes and potential application for Ni₄₃Mn₄₆Sn_11−xSb_x alloys as working substances in magnetic refrigeration are discussed.     

Large vertical loop shifts in mechanically synthesized (Mn,Fe)2O3−t nanograins

Magnetic properties of zero field cooled (ZFC) and field cooled (FC) sample of (Mn,Fe)₂O_3−t nanograins have been investigated by magnetometry (up to 70 kOe) and Mössbauer spectroscopy (up to 60 kOe) in the temperature interval 4.2–300 K. Large horizontal (up to 0.8 kOe) and vertical (up to 80%) shifts of the magnetization hysteresis loops are observed in the FC regime. The obtained results are discussed in terms of exchange interaction between an antiferromagnetic core and a spin-glass-like state of the nanograins boundaries. It is shown that hysteresis loop shifts (horizontal and vertical) depend on the field cooling magnitude, an effect that can be understood by the change of the boundary magnetic structure induced by the external magnetic field. The vertical magnetization shift is described by a phenomenological model, which takes into account the magnetic interaction between the spin-glass like boundary spins and the applied field.     

Anomaly in the field dependence of the anisotropic magnetic susceptiblity of a single-grained α-Mn

A kink was observed near 20 kOe in the field dependence of the magnetization in a single-grained α-Mn without a magnetic hysteresis below the Neel temperature in an accurate magnetization measurement below 80 kOe. It was observed along [1 0 0], [1 1 0] and [1 1 1] crystallographic directions. The field dependence of the magnetization above 40 kOe extrapolates to positive finite magnetization at null field. The kink suggests a weak metamagnetism induced by the external magnetic field. A small but clear anisotropy was observed between the weak-field susceptibilities along [1 0 0] and [1 1 0]/[1 1 1] directions. A previously reported large susceptibility anomaly was not affirmed between 90 and 270 kOe.     

Heat capacities of the electron acceptor 7,7,8,8-tetracyano- quinodimethane (TCNQ) and its radical-ion salt NH4(TCNQ) showing spin-Peierls transition

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH₄-TCNQ have been measured at temperatures in the 12-350 K range by adiabatic calorimetry. A λ-type heat capacity anomaly arising from a spin-Peierls (SP) transition was found at 301.3 K in NH₄-TCNQ. The enthalpy and entropy of transition are Δ_trsH=(667±7) J mol⁻¹ and Δ_trsS=(2.19±0.02) J K⁻¹ mol⁻¹, respectively. The SP transition is characterized by a cooperative coupling between the spin and the phonon systems. By assuming a uniform one-dimensional antiferromagnetic (AF) Heisenberg chains consisting of quantum spin (S=1/2) in the high-temperature phase and an alternating AF nonuniform chains in the low-temperature phase, we estimated the magnetic contribution to the entropy as Δ_trsS_mag=0.61 J K⁻¹ mol⁻¹ and the lattice contribution as Δ_trsS_lat=1.58 J K⁻¹ mol⁻¹. Although the total magnetic entropy expected for the present compound is R ln 2 (=5.76 J K⁻¹ mol⁻¹), a majority of the magnetic entropy (∼4.6 J K⁻¹ mol⁻¹) persists in the high-temperature phase as a short-range-order effect. The present thermodynamic investigation quantitatively revealed the roles played by the spin and the phonon at the SP transition. Standard thermodynamic functions of both compounds have also been determined.     

Structure and magnetic entropy change of suck-cast Gd60Co26Al6Ge8 alloy

The Gd₆₀Co₂₆Al₆Ge₈ alloy has been prepared by the copper-mold suck-casting and its phase component has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). It is shown that this alloy consists of primary crystalline Gd₅Ge₃ phase and amorphous matrix. The glass transition temperature (T_g) and crystallization temperatures (T_x) occur at 292 and 320 °C, respectively. The maximal magnetic entropy change (ΔS_M) under 0-5 T field is about 7.6 J (kg⁻¹ K⁻¹) at 155 K and the refrigeration capacity (RC) is about 768 J kg⁻¹, which makes Gd₆₀Co₂₆Al₆Ge₈ bulk metallic glass matrix composite a promising candidate for magnetic refrigerant.     

Room temperature magnetic entropy change and magnetoresistance in La0.70(Ca0.30−xSrx)MnO3:Ag 10% (x=0.0−0.10)

The magnetic and magnetocaloric properties of polycrystalline La_0.70(Ca_0.30−xSr_x)MnO₃:Ag 10% manganite have been investigated. All compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the insulator–metal transition temperature (T^IM) and Curie temperature (T_c) are observed at 298 K for x=0.10 composition. Though both T^IM and T_c are nearly unchanged with Ag addition, the MR is increased. The MR at 300 K is found to be as large as 31% with magnetic field change of 1 T, whereas it reaches up to 49% at magnetic field of 3 T for the La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample. The maximum entropy change (ΔS_Mmax) at near its T_c (300.5 K) is 7.6 J kg⁻¹ K⁻¹ upon the magnetic field change of 5 T. The La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample having good MR (31%^1 T, 49%^3 T) and reasonable change in magnetic entropy (7.6 J kg⁻¹.K⁻¹, 5 T) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.     

Hard magnetic properties of ErCo7−xCux [0.3⩽x⩽1] system

We report the observation of excellent hard magnetic properties on purely single phase ErCo_7−xCu_x compounds with x=0.3, 0.5, 0.8 and 1. Cu substitution leads to a decrease in the saturation magnetization, but enhances the uniaxial anisotropy in this system. The large anisotropy field (∼100 kOe) is attributed to the Er and the Co sublattices. Domain wall pinning effect seems to play a crucial role in determining the temperature and field dependences of magnetization in these compounds. The hard magnetic properties obtained at room temperature (RT) are comparable to the best results obtained in other RCo₇ based materials.     

Spin-glass behavior and magnetocaloric effect in melt-spun TbCuAl alloys

Magnetic properties and magnetocaloric effects of amorphous and crystalline TbCuAl ribbons are investigated by measuring their ac susceptibilities including a nonlinear term and dc magnetizations. The in-phase third harmonic ac susceptibility is found to be negative. It can be well fitted by the expression at high temperatures, indicating a spin-glass behavior in amorphous TbCuAl alloy. ΔT_f(ω)/[T_f(ω)Δlog₁₀ω], a possible distinguishing criterion to judge the presence of a spin-glass behavior is ∼0.011. The frequency-dependent data can be well fitted by the conventional critical slowing down law and the spin-glass transition temperature is obtained to be 20.1 K. The maximum of magnetic entropy change reaches 4.5 J kg⁻¹ K⁻¹ for a field change of 0-50 000 Oe, while the crystalline TbCuAl compound experiences a simple ferromagnetic-to-paramagnetic phase transition. The peak value of magnetic entropy change is obtained at the Curie temperature and reaches 14.4 J kg⁻¹ K⁻¹ for the same field change, which is much larger than that of amorphous TbCuAl alloy.