The ε-(Mn1-xFex)3+δGe and Ni-Mn-Sb compounds were prepared by melting in a magneto-controlled arc furnace and aging at a certain temperature in purified argon. Gd-based and Tb-based bulk metallic glasses were prepared by melting in a magneto-controlled arc furnace and re-melting the ingots to suck cast into a Cu mold to get a cylindrical rods. The microstructure of the crystalline compounds and the bulk metallic glasses were investigated by means of X-ray diffraction (XRD). The thermodynamic parameters of metallic glasses were investigated by means of differential scanning calorimetry (DSC). Magnetostriction of Ni-Mn-Sb compounds was investigated by means of a standard strain technique and a laser vibrometer. The magnetic, electronic properties, magnetoresistance (MR) and magnetocaloric effects (MCE) were measured by a superconducting quantum interference device (SQUID).
In ε-(Mn1-xFex)3+δGe system with different Fe substitution (0 ≤ x ≤ 0.3), the temperature dependence of resistivity in a zero and nonzero magnetic field were studied. The change in the conductivity from the metallic conductivity of un-doped compound to the nonmetallic conductivity of doped compounds originates from the special electronic band structures. Small negative MR observed in the compounds with 0 ≤ x < 0.17 was due to the suppression of spin fluctuations by the magnetic field. For the compounds with 0.17 ≤ x ≤ 0.2, the positive MR appeared below Tt (from collinear to triangular antiferromagnetic configuration) was attributed to collinear antiferromagnetic configuration, which confirmed the theoretical conclusion suggested by Yamada et. al. that positive MR can appear in antiferromagnetism system as in our experiments. Meanwhile, the positive MR observed above TN is due to the shrinkage of the orbits in a magnetic field in the variable range hopping (VRH) conductivity.
Nowadays, magnetic refrigeration technology becomes more popular because of environment protection. In antiferromagnet ε-(Mn0.83Fe0.17)3+δGe compound, a giant magnetic entropy change (the maximum ΔSM in a field change of 7 T is 11.6 J kg-1K-1 at 93 K) was obtained around the magnetic transition Tt at the function of a magnetic field. The study of magnetic refrigeration materials is focused on ferromagnetism around TC, while the large MCE obtained in our antiferromagnetic compound may open a new field in searching new magnetic refrigeration materials.
Magnetocaloric effect has been extensively studied in three ferromagnetic Heusler alloys: Ni-Mn-Ga, Ni-Mn-Sn and Ni-Mn-In. We choose a new type of Heusler alloy Ni-Mn-Sb (Ni50Mn50-xSbx) to investigate its martensitic transition, MCE and magnetic field induced strain. In Ni50Mn37Sb13 compound, a large ΔSM of 9.1 J kg-1K-1 was obtained at temperature (287 K) and in a field change of 5 T. This value can be compared with well-known room-temperature magnetic refrigeration material Gd (ΔSM is - 9.8 J kg-1K-1 at
293 K in 5 T). Additionally, the Ni50Mn37Sb13 compound has bidirectional shape memory effect (SME) in a relatively low field (1.2 T). The origin of MCE and SME is a magnetic - field - induced inverse martensitic transition, and the motivation of SME is the large magnetic crystal anisotropy of martensitic phase.
The wider temperature of magnetic transition of ferromagnetic bulk metallic glasses (BMG) due to the disorder structure is an advantage to achieve a lager value of magnetic refrigeration capacity (RC). MCE in two Gd - based BMGs (Gd55Co20Al25 and Gd55Ni25Al20) has been investigated, and a large entropy change (with a ΔSM of
11.2 J kg-1K-1 at 103 K or 10.8 J kg-1K-1 at 82.5 K in 7 T) was obtained. Large RC values of 846 J kg-1 (541 J kg-1 in 5 T) and 920 J kg-1 (640 J kg-1 in 5 T) were achieved for Gd55Co20Al25 and Gd55Ni25Al20, respectively. These RC values exceed all the crystalline magnetic refrigeration materials, while that of Gd55Ni25Al20 exceeds the reported values for all crystalline (Gd5Si2Ge1.9Fe0.1, RC is 360 J kg-1 in 5 T) and amorphous refrigeration materials (Gd53Al24Co20Zr3， RC is 590 J kg-1 in 5 T). Large ΔSM and RC values of the two Gd-based BMGs make them good candidates for application.
Spin-glass like (SGL) behaviors have been observed in Tb-based (Tb55Co20Al25) BMG, and a large MCE (ΔSM is 20 J kg-1K-1 at 4.5 K) at a relatively low field (2 T) is achieved related to SGL. The origin of SGL in Tb55Co20Al25 BMG is random caused by disorder in amorphous structure, and spin frustration between ferromagnetic Co-Co couplings and antiferromagnetic Tb-Co couplings. The large magnetic entropy change is due to the magnitude of magnetic moments in this Tb-based BMG and the low applied field is because that the frozen magnetic moments below Tf are easy to align along the magnetic field in a low field. The positive MCE can provide a base for studying thermodynamic behaviors in magnetic refrigeration cycles. MCE discovered in SGL is not restricted by TC, different to paramagnetic salts, and therefore, it is possible to apply for temperatures lower than in the latter.
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