Reduction of hysteresis loss in LaFe11.7Si1.3Hx hydrides with significant magnetocaloric effects

Magnetic properties and magnetocaloric effects (MCEs) have been investigated in hydrogenated LaFe_11.7 Si_1.3H _x (x=0,1.37, and 2.07) compounds. It is found that the Curie temperature, T _C, can be tuned from 192 to 338 K by adjusting the hydrogen content from 0 to 2.07. It is attractive that both thermal and magnetic hysteresis are remarkably reduced because of the weakness of the itinerant-electron metamagnetic transition after hydrogenation. The maximal hysteresis loss at T _C decreases from 33.4 to 8.8 J/kg as x increases from 0 to 2.07. For the samples with x=0,1.37, and 2.07, the maximal values of the isothermal magnetic entropy change, ΔS _M, are 20.9, 15.1, and 15.83 J/kg K for the increasing field and 20.76 J/kg K, 14.53 J/kg K and 15.61 J/kg K for the decreasing field at T _C, with efficient refrigeration capacities of 439, 330, and 304 J/kg for a field change of 0–5 T, respectively. Large reversible MCE and small hysteresis with considerable refrigeration capacity indicate the potential of LaFe_11.7Si_1.3H _x hydride as a candidate magnetic refrigerant around room temperature.     

Large low-field inverse magnetocaloric effect near room temperature in Ni50−x Mn37+x In13 Heusler alloys

Low field inverse magnetocaloric effect (IMCE) associated with first-order martensitic transition in Ni_50−x Mn_37+x In₁₃ (x=3,4,5) alloys was investigated. By tuning the composition of Ni/Mn, large change in the magnetization occurring between martensite and austenite phases in a narrow temperature interval was achieved, which results in large IMCE. Under low magnetic field change of 2 T, a large positive magnetic entropy change (ΔS _M ) of 23.5 J/kg K with a net refrigeration capacity of 53 J/kg was obtained near room temperature (308 K) in the x=3 alloy. The results show that a small variation in Ni/Mn ratio significantly influences the martensitic transition temperature and the associated magnetic and magnetocaloric properties.     

Giant enhancement of magnetocaloric effect in metallic glass matrix composite

The magnetocaloric effect (MCE) has made great success in very low temperature refrigeration, which is highly desirable for application to the extended higher temperature range. Here we report the giant enhancement of MCE in the metallic glass composite. The large magnetic refrigerant capacity (RC) up to 103 J·kg⁻¹ is more than double the RC of the well-known crystalline magnetic refrigerant compound Gd₅Si₂Ge_1.9Fe_0.1 (357 J·kg⁻¹) and MnFeP_0.45As_0.55 (390 J·kg⁻¹)(containing either exorbitant-cost Ge or poisonous As). The full width at half maximum of the magnetic entropy change (ΔS _m) peak almost spreads over the whole low-temperature range (from 303 to 30 K), which is five times wider than that of the Gd₅Si₂Ge_1.9Fe_0.1 and pure Gd. The maximum ΔSm approaches a nearly constant value in a wide temperature span over 100 K, and however, such a broad table-like region near room temperature has seldom been found in alloys and compounds. In combination with the intrinsic amorphous nature, the metallic glass composite may be potential for the ideal Ericsson-cycle magnetic refrigeration over a broad temperature range near room temperature. Supported by the National Natural Science Foundation of China (grant Nos. 50621061 and 50731008) and the National Basic Research Program of China (973 Program) (Grant No. 2007CB613904)     

Inverse magnetocaloric effect in sol–gel derived nanosized cobalt ferrite

The magnetocaloric properties of cobalt ferrite nanoparticles were investigated to evaluate the potential of these materials as magnetic refrigerants. Nanosized cobalt ferrites were synthesized by the method of sol–gel combustion. The nanoparticles were found to be spherical with an average crystallite size of 14 nm. The magnetic entropy change (ΔS _m) calculated indirectly from magnetization isotherms in the temperature region 170–320 K was found to be negative, signifying an inverse magnetocaloric effect in the nanoparticles. The magnitudes of the ΔS _m values were found to be larger when compared to the reported values in the literature for the corresponding ferrite materials in the nanoregime.     

Effect of partial Nd-substitution on the magnetic and magnetocaloric properties in spin-reorientation PrCo<Subscript>4</Subscript>Al alloy

Partial Nd-substitution effects on the magnetic and magnetocaloric properties in spin-reorientation Pr_1−x Nd _x Co₄Al (x = 0, 0.2, 0.4, 0.6, 0.8) alloys are investigated. All these alloys undergo two successive spin-reorientation transitions. Accordingly, the successive positive and negative magnetic entropy changes (Δ S _M ) for all these alloys are obtained. With the increase of Nd content, the spin reorientation transition temperatures increase from about 170 and 186 K for x = 0 to room temperature (276 and 294 K) for x = 0.8, covering a wide temperature interval. More interestingly, the values of positive Δ S _M values increase gradually from 0.1 to 0.9 J/kg K⁻¹, while the negative Δ S _M decrease from 1.3 to 0.2 J/kg K⁻¹ in the field change of 10 kOe. In addition, the series has an appreciable relative cooling power, which is therefore suitable to be used in a magnetic refrigerator.     

Magnetic properties and magnetic entropy change in Heusler alloys Ni50Mn35?xCuxSn15

The influence of Cu substitution for Mn on magnetic properties and magnetic entropy change has been investigated in Heusler alloys, Ni₅₀Mn_35−x Cu _x Sn₁₅ (x=2,5 and 10). With increasing Cu content from x=2 to x=5, the martensitic transition temperature, T _M , decreases from 220 K to 120 K. Further increasing Cu up to x=10 results in the disappearance of T _M . For samples Ni₅₀Mn₃₃Cu₂Sn₁₅ and Ni₅₀Mn₃₀Cu₅Sn₁₅, both martensitic and austenitic states exhibit ferromagnetic characteristics, but the magnetization of martensitic phase is notably lower than that of austenitic phase. The magnetization difference, ΔM, across the martensitic transition leads to a considerably large Zeeman energy, μ ₀ΔM⋅H, which drives a field-induced metamagnetic transition. Associated with the metamagnetic behavior, a large positive magnetic entropy change ΔS takes place around T _M . For the sample Ni₅₀Mn₃₃Cu₂Sn₁₅,ΔS reaches 13.5 J/kg⋅K under a magnetic field change from 0 to 5 T.     

Magnetic and transport properties of monocrystalline<Emphasis Type="Italic">Fe</Emphasis><Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript>

A monocrystal ofFe ₃ O ₄ is characterized by resistance, magnetoresistance and magnetic measurements in a temperature range from 4.2 K to 350 K and magnetic field-cycling from −9 T to 9 T. The resistance measurements revealed a metal-insulator Verwey transition (VT) atT _v =123.76 K with activation energy E=92.5 meV at T >T _v and temperature-substitute for the activation energy below the VT,T ₀=E/k _B ≈3800 K within 70 K–110K. The magnetotransport results independently verified the VT at 123.70 K, with discontinuous change in the magnetic moment ΔM≈0.21 ΔM≈0.21μ _B and resistance hysteresis, dependent on the magnetic field in a narrow temperature range of 0.4° around theT _v . The magnetic characterization established self consistentlyT _v as ≈123.67 K, the jump in the magnetization at the VT≈0.25μ _B and confirmed, that the magnetocrystalline anisotropy is the main microscopic mechanism responsible for the magnetization of the monocrystal (88%) with additional natural and imposed defects contributing as 12%.     

Heat capacity and magnetocaloric properties of La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites

The magnetocaloric effect and the heat capacity of La_{1 − x} K _x MnO₃ (x = 0.1, 0.15, 0.175) ceramic samples have been studied at temperatures in the range 77–350 K and in magnetic fields of up to 27 kOe. The technique for preparing the samples has been described. The heat capacity anomalies related to the ferromagnetic-paramagnetic magnetic phase transition have been revealed and interpreted. It has been demonstrated that the change in the magnetic entropy ΔS calculated from the data on the heat capacity C _p and direct measurements of the magnetocaloric effect ΔT reaches values that are of practical interest.     

Influence of Thomson effect on the resultant local Seebeck coefficient in thermoelectric composite materials

The resultant local Seebeck coefficient α _R (=α _S−α _T) at the interface of a thermoelement has not yet been measured, although it is an important factor governing the thermoelectric efficiency, where α _S is the local Seebeck coefficient and α _T is the one caused by the Thomson effect. It is shown in this paper that α _S, α _T, and α _R of the p- and n-type Cu/Bi–Te/Cu composites are obtained analytically and experimentally on the assumption that the local temperature of the composite on which the temperature difference ΔT is imposed varies linearly with changes in position along the composite. They were indeed estimated as a function of position from the local experimental data of R,ΔI,ΔT, and V generated by applying an additional current of ±I to the composite, where R is the electrical resistance and ΔI is a current generated by the composite. As a result, it was found that the absolute values of α _S at the hot interface of the p- and n-type composites are approximately 1.5 and 1.4 times higher than their lowest values in the middle region of the composite, respectively, while those of α _T are less than 8% of α _S all over the composite and are so small that the relation α _R≈α _S can be held. We thus succeeded in measuring α _R at the interfaces of the composite.     

Magnetic dipole,electric quadrupole and magnetic octupole moments of the <Emphasis Type="Italic">Δ</Emphasis> baryons in light cone QCD sum rules

Due to the very short life time of the Δ baryons, a direct measurement on the electromagnetic moments of these systems is almost impossible in the experiment and can only be done indirectly. Although only for the magnetic dipole moments of Δ ⁺⁺ and Δ ⁺ systems there are some experimental data, the theoretical, phenomenological and lattice calculations could play crucial role. In the present work, the magnetic dipole (μ _Δ ), electric quadrupole (Q _Δ ) and magnetic octupole (O _Δ ) moments of these baryons are computed within the light cone QCD sum rules. The results are compared with the predictions of the other phenomenological approaches, lattice QCD and existing experimental data.     

Magnetocaloric effect of compounds in Gd<Subscript>5−<Emphasis Type="Italic">x</Emphasis></Subscript>Dy<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>2</Subscript>Ge<Subscript>2</Subscript> system

Magnetization of magnetically ordered Gd_5−x Dy _x Si₂Ge₂ compounds with a partial substitution of Gd atoms by isovalent Dy atoms has been investigated. From temperature and field dependences of Gd_5−x Dy _x Si₂Ge₂ alloys with x = 0−2.0 changes of the magnetic part of entropy (ΔS _M ) of alloys are determined. It is established that ΔS _M achieves its maximum values at different temperatures, which linearly depend on the Dy concentration, and their values are comparable with ΔS _M ^max in Gd₅Si₂Ge₂. The obtained data allow us to conclude that the above-mentioned compounds have high magnetocaloric effect and are promising materials for using as a combined working body of magnetic refrigerators operating in the 200–270 K range of temperatures.     

Large magnetic entropy change in Nd<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>

In order to search for new materials for the application of magnetic refrigeration, the polycrystalline perovskite compound Nd_2/3Sr_1/3MnO₃ was prepared by a solid-state method. The dependence of the magnetization on the applied field and temperature was measured near the Curie temperature. In terms of Maxwells equation, the temperature dependence of the absolute value of the isothermal magnetic entropy change |S_M| at various applied fields from 1 T to 5 T was determined. The results showed that a large magnetic entropy change was observed in this compound. The maximum magnetic entropy change |S_M^max|can reach 3.25 J/kgK with an applied field of 1 T at the Curie temperature of 257.5 K, which equals that of Gd. At 5 T applied field, it is 7.57 J/kgK. Such good magnetocaloric properties make this compound a promising candidate for the application of magnetic refrigeration in the room-temperature range. PACS 74.25.Ha; 75.30.-m; 75.30.Sg; 75.50.-y; 75.60.-d     

Magnetocaloric effect in antiferromagnetic Dy3Co compound

Magnetic properties and magnetocaloric effects (MCEs) of the Dy₃Co compound are studied. Two successive magnetic transitions: the antiferromagnetic (AFM)-to-AFM transition at T _AF =29 K and the AFM-to-paramagnetic (PM) transition with increasing temperature at the Néel temperature T _N =44 K are observed. Dy₃Co undergoes a field-induced metamagnetic transition from the AFM to the ferromagnetic (FM) state below T _N , giving rise to a large MCE. The maximal value of magnetic entropy change ΔS _m is −13.9 J/kg K with a refrigerant capacity (RC) of 498 J/kg around T _N for a field change of 0–5 T. A sign change of MCE in Dy₃Co with magnetic field and temperature is observed near the critical field where the metamagnetic transition occurs.     

Magnetocaloric effect in polycrystalline La0.65Ba0.3M0.05MnO3 (M=Na, Ag, K) manganites

The effects of monovalent doping on the crystallographic, magnetic and magnetocaloric properties of La_0.65Ba_0.3M_0.05MnO₃ (M=Na, Ag, K) powder samples, elaborated using the solid state reaction method at high temperature, have been investigated. In our three samples the Mn⁴⁺ amount remains constant equal to 40%. The Rietveld refinement of the X-ray powder diffraction shows that all our synthesized samples are single phase and crystallize in the distorted rhombohedral system with R3¯c space group. All our studied samples undergo a paramagnetic–ferromagnetic transition with decreasing temperature. Using the Arrott plot, the second-order transition Curie temperature T_C for M=Na, Ag and K is found to be 310, 300 and 290 K, respectively. The magnetic entropy change, deduced from isothermal magnetization curves, exhibits a maximum |ΔS_M^Max| of about 2.65, 2.82 and 2.66 J/kg K for M=Na, Ag and K, respectively, in a magnetic applied field change of 5 T. Although these values are modest, the magnetocaloric effect extends over a large temperature range leading to an important value of the relative cooling power (RCP). The RCP values exhibit a nearly linear dependence with the magnetic applied field. The refrigeration capacity in a magnetic applied field of 1 T is found to be 28.8, 27.8 and 25.6 J/kg for M=Na, Ag and K compounds.     

Entanglement entropy fluctuations in quantum Ising chains

The behavior of Ising chains with the spin-spin interaction value λ in a transverse magnetic field of constant intensity (h = 1) is considered. For a chain of infinite length, exact analytical formulas are obtained for the second central moment (dispersion) of the entropy operator Ŝ = -lnρ with reduced density matrix ρ, which corresponds to a semi-infinite part of the model chain occurring in the ground state. In the vicinity of a critical point λ_c = 1, the entanglement entropy fluctuation ΔS (defined as the square root of dispersion) diverges as ΔS ∼ [ln(1/|1 − λ|)]^1/2. For the known behavior of the entanglement entropy S, this divergence results in that the relative fluctuation δS = ΔS/S vanishes at the critical point, that is, a state with almost nonfluctuating entanglement is attained.     

Adjoint representation admixture to weak nonleptonic decays in SU(4) and SU(8)

In order to remove some of the unsatisfactory features of the GIM model, we consider15-admixture in SU(4). ΔC=±ΔS decays remain unaffected. Lee-Sugawara sum-rule is obtained.15-admixture is extended then to63-admixture in SU(8). The most general Hamiltonian (H _w∼63⊕720⊕1232) in SU(8) is found to give Lee-Sugawara relation and Σ₊ ⁺=0 forS-wave decays of uncharmed baryons. Decay amplitude relations for ΔC=−1, ΔS=0 mode are obtained.     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

Andreev spectroscopy of FeSe: Evidence for two-gap superconductivity

Current-voltage characteristics (CVCs) of Andreev superconductor-constriction-superconductor (ScS) contacts in polycrystalline samples of FeSe with the critical temperature T _C = (12 ± 1) K have been measured using the break-junction technique. In Sharvin-type nanocontacts, two sets of subharmonic gap structures were detected due to multiple Andreev reflections, indicating the existence of two nodeless superconducting gaps Δ _L = (2.75 ± 0.3) meV and Δ _S = (0.8 ± 0.2) meV. Well-shaped CVCs for stacks of Andreev contacts with up to five contacts were observed due to the layered structure of FeSe (the intrinsic multiple Andreev reflections effect). An additional fine structure in the CVCs of Andreev ScS nanocontacts is attributed to the existence of a Leggett mode. A linear relation between the superconducting gap Δ _L and the magnetic resonance energy E _magres ≈ 2Δ _L is found to be valid for layered iron pnictides.     

Large relative cooling power of Gd52.5Co16.5Al31 magnetic bulk metallic glass

The magnetocaloric effect and thermal stability have been investigated on the new bulk metallic glass (BMG) Gd_52.5Co_16.5Al₃₁ alloy. The extent of supercooled liquid region is 70 K, which is wider than that of any other Gd-Co-Al ternary BMGs. The magnetic entropy change (ΔS_M) and relative cooling power (RCP) of 9.8 J/kg K and 9.1×10² J/kg are obtained, respectively, under a field change of 5 T. The large ΔS_M and RCP values make Gd_52.5Co_16.5Al₃₁ BMG attractive potential candidate for the magnetic refrigeration application.