Magnetic phase transitions and magnetocaloric effect in the Fe-doped MnNiGe alloys

The magnetic phase transition and magnetocaloric effects in Fe-doped MnNiGe alloys are investigated. The substitution of Fe for Ni decreases the structural transition temperature remarkably, resulting in the magnetostructural transition occurring between antiferromagnetic and ferromagnetic states in MnNi_{1 - x}Fe_xGe alloy. Owing to the enhanced ferromagnetic coupling induced by the substitution of Fe, metamagnetic behaviour is also observed in TiNiSi-type phase of MnNi_{1 - x}Fe_xGe alloys at temperature below the structural transition temperature.     

Large inverse and normal magnetocaloric effects in HoBi compound with nonhysteretic first-order phase transition

HoBi single crystal and polycrystalline compounds with NaCl-type structure are successfully obtained, and their magnetic and magnetocaloric properties are studied in detail. With temperature increasing, HoBi compound undergoes two magnetic transitions at 3.7 K and 6 K, respectively. The transition temperature at 6 K is recognized as an antiferromagnetic-to-paramagnetic (AFM-PM) transition, which belongs to the first-order magnetic phase transition (FOMT). It is interesting that the HoBi compound with FOMT exhibits good thermal and magnetic reversibility. Furthermore, a large inverse and normal magnetocaloric effect (MCE) is found in HoBi single crystal in the $H|| [100]$ direction, and the positive $\Delta S_{\rm M}$ peak reaches 13.1 J/kg$\cdot$K under a low field change of 2 T and the negative $\Delta S_{\rm M}$ peak arrives at $-18 $ J/kg$\cdot$K under a field change of 5 T. These excellent properties are expected to be applied to some magnetic refrigerators with special designs and functions.     

Metamagnetic transition and magnetocaloric effect in antiferromagnetic TbPdAl compound

Magnetic properties and the magnetocaloric effect of the compound TbPdAl are investigated. The compound exhibits a weak antiferromagnetic (AFM) coupling, and undergoes two successive AFM transitions at T_N=43 K and T_t=22 K. A field-induced metamagnetic transition from AFM to ferromagnetic (FM) state is observed below T_N, and a small magnetic field can destroy the AFM structure of TbPdAl, inducing an FM-like state. The maximal value of magnetic entropy change is −11.4 J/kg K with a refrigerant capacity of 350 J/kg around T_N for a field change of 0-5 T. Good magnetocaloric properties of TbPdAl result from the high saturation magnetization caused by the field-induced AFM-FM transition.     

Magnetocaloric effect in ErCo2 compound

The ErCo2 compound is prepared by arc-melting and its entropy changes are calculated using Maxwell relation. Its entropy change reaches 38 J/（kg·K） and its refrigerant capacity achieves 291 J/kg at 0-5 T. The mean field approximation is used to calculate the magnetic entropy of ErCo2 compound. Results estimated by using the Maxwell relation deviate from mean field approximation calculations in ferrimagnetic state; however, the data obtained by the two ways are consistent in the vicinity of phase transition or at higher temperatures. This indicates that entropy changes are mainly derived from magnetic degree of freedom, and the lattice has almost no contribution to the entropy change in the vicinity of phase transition but its influence is obvious in the ferrimagnetic state below TC.     

Large reversible magnetocaloric effect in HoMn2O5

Magnetocaloric effect (MCE) in polycrystalline HoMn2O5 was investigated by isothermal magnetization curves from 2 K to 50 K. A relatively large magnetic entropy change, △SM = 7.8 J/(kg · K), was achieved with the magnetic field up to 70 kOe (1 Oe = 79.5775 A · m-1). The magnetic entropy change is reversible in the whole range of temperature. The contributions of elastic and magnetoelastic energy to the changing of the magnetic entropy are discussed in terms of the Landau theory. The reversibility of MCE with maximal refrigerant capacity RC = 216.7 J/kg makes polycrystalline HoMn2O5 promising as a magnetic refrigerant.     

Magnetic properties and magnetocaloric effect in TbCo2-xFex compounds

Magnetic properties and magnetocaloric effect in TbCo2-xFex compounds are studied by DC magnetic measurement. With increasing content of Fe, the entropy changes decrease slightly, though the Curie temperature is tuned from 231 K （x = 0） to 303 K （x = 0.1）. Magnetic entropies of TbCo2 compound are calculated by using mean field approximation （MFA）. Results estimated by using Maxwell relation are consistent with that of MFA calculation. It is shown that the entropy changes are mainly derived from the magnetic entropy changes. The lattice has almost no contribution to the entropy change in the vicinity of phase transition.     

Order of magnetic magnetocaloric transition and large effect in Er3Co

We have studied the magnetic and magnetocaloric properties of the Er3Co compound, which undergoes ferromagnetic ordering below the Curie temperature Tc = 13 K. It is found by fitting the isothermal magnetization curves that the Landau model is appropriate to describe the Er3Co compound. The giant magnetocaloric effect （MCE） without hysteresis loss around Tc is found to result from the second-order ferromagnetic-to-paramagnetic transition. The max- imal value of magnetic entropy change is 24.5 J/kg.K with a refrigerant capacity （RC） value of 476 J/kg for a field change of 0-5 T. Large reversible MEC and RC indicate the potentiality of Er3Co as a candidate magnetic refrigerant at low temperatures.     

Magnetic transition and large reversible magnetocaloric effect in EuCu1.75P2 compound

The magnetocaloric effect(MCE) in EuCu1.75P2 compound is studied by the magnetization and heat capacity measurements.Magnetization and modified Arrott plots indicate that the compound undergoes a second-order phase transition at TC ～ 51 K.A large reversible MCE is observed around TC.The values of maximum magnetic entropy change(-△SxMma) reach 5.6 J·kg-1·K-1 and 13.3 J·kg-1·K-1 for the field change of 2 T and 7 T,respectively,with no obvious hysteresis loss in the vicinity of Curie temperature.The corresponding maximum adiabatic temperature changes(△Tadmax) are evaluated to be 2.1 K and 5.0 K.The magnetic transition and the origin of large MCE in EuCu1.75P2 are also discussed.     

Magnetic phase transition and magnetocaloric effect in Mn_(1-x)Zn_xCoGe alloys

The magnetic phase transition and magnetocaloric effect are studied in a series of Mn1-xZnxCoGe(x = 0.01, 0.02,0.04, and 0.08) alloys. By introducing a small quantity of Zn element, the structural transformation temperature of the MnCoGe alloy is greatly reduced and a first-order magnetostructural transition is observed. Further increasing the Zn concentration results in a second-order ferromagnetic transition. Large room-temperature magnetocaloric effects with small magnetic hysteresis are obtained in alloys with x = 0.01 and 0.02, which suggests their potential application in magnetic refrigeration.     

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.     

Order of magnetic transition and large magnetocaloric effect in Er3Co*

We have studied the magnetic and magnetocaloric properties of the Er3 Co compound,which undergoes ferromagnetic ordering below the Curie temperature TC = 13 K.It is found by fitting the isothermal magnetization curves that the Landau model is appropriate to describe the Er3 Co compound.The giant magnetocaloric effect(MCE) without hysteresis loss around T C is found to result from the second-order ferromagnetic-to-paramagnetic transition.The maximal value of magnetic entropy change is 24.5 J/kg.K with a refrigerant capacity(RC) value of 476 J/kg for a field change of 0-5 T.Large reversible MEC and RC indicate the potentiality of Er3 Co as a candidate magnetic refrigerant at low temperatures.     

Large reversible magnetocaloric effect in spinel MnV2O4 with minimal Al substitution

Magnetocaloric effect of MnV_1.95Al_0.05O₄ was studied by the magnetization and heat capacity measurements. MnV_1.95Al_0.05O₄ is a cubic spinel structure with ferromagnetism of second order in nature and performs reversible magnetic entropy around the magnetic transition temperature. The large magnetic entropy changes −ΔS_M∼5.2 and 8.2 J/kg K and the adiabatic temperature changes ΔT_ad∼1.5 and 2.6 K are revealed for the magnetic field changes of 2 and 4 T near the Curie temperature (T_C) of 59.6 K, respectively. The relative cooling power (RCP) are about 82.2 and 177.2 J/kg K for magnetic field changes 2 and 4 T, respectively. Compared with the parent compound, although the −ΔS_M and ΔT_ad become smaller, the refrigeration working temperature span and the RCP have been improved.     

Determination of the magnetocaloric effect associated with martensitic transition in Ni46CunMn38Sn12 and Ni50CoMn34In15 Heusler alloys

This paper presents a study of the inverse magnetocaloric effect （MCE） corresponding to martensitic transition using various experimental approaches for Ni46Cu4Mn38Sn12 and NisoCoMn34In]5 Heusler alloy. Through heat capacity measurements, it is found that the ＂giant inverse MCE＂ upon martensitic transition evaluated by the Maxwell relation in these alloys are unphysical results. This is due to the coexistence of both martensitic and austenitic phases, as well as thermal hysteresis during martensitic transition. However, careful study indicates that the spurious results during martensitic transition can be removed using a Clausius Clapeyron equation based on magnetization measurements.     

Large magnetocaloric effect in van der Waals crystal CrBr3

We study the magnetocaloric effect (MCE) in van der Waals (vdW) crystal CrBr₃. Bulk CrBr₃ exhibits a second-order paramagnetic-ferromagnetic phase transition with T_C = 33 K. The maximum magnetic entropy change −ΔS_M near T_C is about 7.2 J·kg⁻¹·K⁻¹ with the maximum adiabatic temperature change ΔT^max_ad = 2.37 K and the relative cooling power RCP= 191.5 J·kg⁻¹ at μ₀H = 5 T, all of which are remarkably larger than those in CrI₃. These results suggest that the vdW crystal CrBr₃ is a promising candidate for the low-dimensional magnetic refrigeration in low temperature region.     

Martensitic phase transition with magnetocaloric effect and double-shifted exchange bias in the martensitic phase for Ni48Mn39Sn13-xGex alloys

Ni₄₈Mn₃₉Sn_13-xGe_x (x?=?1, 2) Heusler alloys have been prepared, and the martensitic phase transition (MPT), magnetocaloric effect and exchange bias (EB) have been explored. At room temperature, the structure of both samples presents L2₁ type, and the MPT shifts to a higher temperature, while the Curie temperature (T_C) of the austenitic phase decreases with the increase of the Ge content. The maximum magnetic entropy change of Ni₄₈Mn₃₉Sn_13-xGe_x with x?=?2 reaches about 14.67?J/kg?K under the magnetic field of 5?T during reverse MPT. In addition, an interesting phenomenon is the enhancement of EB with the increase of the Ge content, especially the abnormal presence of the double-shifted hysteresis loop has been realized in Ni₄₈Mn₃₉Sn_13-xGe_x with x?=?2, which can be interpreted by the fact that the antiferromagnetic (AFM) regions couple with proximal ferromagnetic (FM) regions in the opposite way at low temperature.     

Large magnetoresistance in metamagnetic CoMnSi0.88Ge0.12 alloy

The magneto-transport properties are investigated in metamagnetic CoMnSi0.88Ge0.12 alloy.By applying a magnetic field or increasing temperature,a metamagnetic phase transition from antiferromagnetic to ferromagnetic is observed in this alloy.Around the metamagnetic phase transition,CoMnSi0.88Ge0.12 alloy exhibits a large and negative magnetoresistance effect(～32%) under a magnetic field of 20 kOe(1 Oe = 79.5775 A/m),which is ascribed to the spin-dependent scattering of conduction electrons.     

Giant magnetoresistance in Y0.9La0.1Mn6Sn6 compound

Magnetic transitions and magnetoresistance effect of the HfFe_6Ge_6-type Y_{0.9}La_{0.1}Mn_6Sn_6 compound have been investigated in the temperature range of 5－380K. The sample displays antiferromagnetic behaviour in the whole temperature range below Néel temperature T_N=309K. The metamagnetic transition from antiferromagnetism to ferromagnetism can be induced by an applied field. The metamagnetic transition field decreases monotonically from 2T at 5K to 0.4T at 300K. The giant magnetoresistance effect is observed with the metamagnetic behaviour, such as -10.4% at 245K under a field of 5T.     

Magnetotransport properties and magnetocaloric effects of Mn1.95Cr0.05Sb0.95Ga0.05 compound

The magnetotransport properties and magnetocaloric effects of the compound Mn_{1.95}Cr_{0.05}Sb_{0.95}Ga_{0.05} have been studied. With decreasing temperature, a spontaneous first-order magnetic phase transition from ferrimagnetic (FI) to antiferromagnetic (AF) state takes place at T_s=200K. A metamagnetic transition from the AF to FI state can be induced by an external field, accompanied by a giant magnetoresistance effect of 57%. The magnetic entropy changes are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation. Mn_{1.95}Cr_{0.05}Sb_{0.95}Ga_{0.05} exhibits a negative magnetocaloric effect, and the absolute values of ΔS_M^{max}(T,ΔH) are 4.4, 4.1, 3.6, 2.8 and 1.5 J/(kg·K) for magnetic field changes of 0－5T, 0－4T, 0－3T, 0－2T and 0－1T, respectively.     

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)     

Determination of the magnetocaloric effect associated with martensitic transition in Ni46Cu4Mn38Sn12 and Ni50CoMn34In15 Heusler alloys

This paper presents a study of the inverse magnetocaloric effect (MCE) corresponding to martensitic transition using various experimental approaches for Ni46Cu4Mn38Sn12 and Ni50CoMn34In15 Heusler alloy. Through heat capacity measurements,it is found that the "giant inverse MCE" upon martensitic transition evaluated by the Maxwell relation in these alloys are unphysical results. This is due to the coexistence of both martensitic and austenitic phases,as well as thermal hysteresis during martensitic transition. However,careful study indicates that the spurious results during martensitic transition can be removed using a Clausius-Clapeyron equation based on magnetization measurements.