Phase transitions and magnetic entropy changes are studied in ${\text{Mn}}_{1.9-x}{\text{Ni}}_x\text{Ge}$ ($x=0.85$ , 0.855) alloys. In these off-stoichiometric alloys, the crystallographic transition temperature decreases remarkably due to the deficiency of transition-metal atoms, and, consequently, a magnetostructural transition from the antiferromagnetic TiNiSi-type structure to the ferromagnetic ${\text{Ni}}_2\text{In}$ -type structure is observed. Owing to the abrupt change in magnetization, large magnetic entropy changes are obtained. The effect of transition-metal vacancies on the phase transition temperature is discussed.

In this paper, we propose a tunable, multi-band, selective absorber composed of multiple layers. Each layer consisted of SiO₂/graphene/SiC, and a layer of silver was used as the ground plane of the entire structure. Simulation results show that we can passively and actively coordinate the resonant frequency of the perfect absorption peak by changing the geometric parameters of the array and the Fermi level of the graphene. The absorber is not sensitive to the angle of incidence and the direction of polarization. We propose a theoretical basis for the formation of multiple absorption peaks. The theoretical calculations are in good agreement with the simulation results. In addition, we simulated the three- and four-layer structures. The results show that in the terahertz (THz) band, composite structures of three and four layers can obtain three and four perfect absorption peaks, respectively. Our results provide new insights into the THz band of harmonizable multi-band absorbers that can be applied to THz imaging to coordinate sensors and other optoelectronic devices.

A series of MnNi_1−x Co _x Ge_1.05 (x = 0, 0.03, 0.05, 0.07, 0.09, and 0.11) alloys were prepared by the arc-melting method. With increasing content of Co, a first-order magnetostructural transformation between the antiferromagnetic TiNiSi-type phase and the ferromagnetic Ni₂In-type phase was observed. A magnetic and crystallographic phase diagram for MnNi_1−x Co _x Ge_1.05 alloys was proposed in this paper. Owing to the abrupt and large jump of magnetization around the magnetostructural transformation, MnNi_1−x Co _x Ge_1.05 (x = 0.07, 0.09, 0.11) alloys show large and positive magnetic entropy changes at relatively low field.

The tuning of magnetostructural transition was demonstrated for MnNiSi-FeNiGe system. The MnNiSi alloy undergoes a structural transition around 1200 K in the paramagnetic region. By chemically alloying MnNiSi with FeNiGe, the structural transition temperature was remarkably decreased by nearly 1000 K. Magnetostructural transformation from the paramagnetic hexagonal phase to the ferromagnetic orthorhombic phases, which can be induced by applying magnetic field, were realized near room temperature, enabling the MnNiSi-FeNiGe system to become a promising candidate for the magnetoresponsive applications.

A series of magnetic amorphous ribbons with different Gd/Co ratios was prepared by melt-spun method. With the decrease in Gd/Co ratio, Curie temperatures increase gradually from 166 to 193 K. The maximum values of magnetic entropy changes under a magnetic field of 10 kOe are $-3.1$ , $-3.0$ , $-2.9$ , and $-2.8\text{J}/\text{kg}\text{K}$ for ${\text{Gd}}_{71}{\text{Co}}_{29}$ , ${\text{Gd}}_{68}{\text{Co}}_{32}$ , ${\text{Gd}}_{65}{\text{Co}}_{35}$ , and ${\text{Gd}}_{62}{\text{Co}}_{38}$ samples, respectively. The approximately constant peak values of $\Delta S_M$ at different working temperatures indicate that they are advantageous for an Ericsson refrigeration cycle. In addition, these samples have large resistivity and greatly reduced magnetic hysteresis losses, which could increase the refrigeration efficiency. These advantages make the Gd–Co amorphous ribbons good candidates for the practical magnetic refrigeration.