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• Mg doping enlarges temperature span and reduces hysteresis. • Impurity phase and the order of magnetic phase transition control hysteresis. • Covalent bond length adjust the Curie temperature. Magnetocaloric materials with wide working temperature span and reversible magnetic transition are beneficial for the magnetic refrigerator. Besides of giant magnetocaloric effect, (Mn,Fe) 2 (P,Si) alloys exhibit significant thermal/magnetic hysteresis and a narrow working temperature span. In this work, the impact of Mg doping on the phase structure and magnetocaloric effect of Mn 1.05 Fe 0.9 P 0.5 Si 0.5- x Mg x alloys has been studied. The results show that all samples crystallize into Fe 2 P-type main phase with little impurity phase. After Mg doping, the lattice parameter a of the main phase increases from 6.027(0) Å to 6.049(0) Å and c decreases from 3.488(6) Å to 3.480(6) Å, which leads to the increase of Curie temperature. Density functional theory calculation results confirm that the Mg atom located on the 2c site can increase the covalent bond length, and then enhance the ferromagnetic coupling between atoms leading to the increase of Curie temperature. Moreover, the first-order magnetic phase transition of Mn 1.05 Fe 0.9 P 0.5 Si 0.5- x Mg x alloys is inhibited with the increase of Mg content, which results in the decrease of thermal/magnetic hysteresis and the increase of the effective working temperature span. For Mn 1.05 Fe 0.9 P 0.5 Si 0.48 Mg 0.02 alloy, it has a Curie temperature of about room temperature, effective refrigeration capacity of 291.7 J·kg−1, working temperature window of 46 K and magnetic entropy change of 9.4 J·kg−1·K−1. The alloy can be an excellent candidate in room temperature magnetic refrigeration devices with a wide temperature span. [ABSTRACT FROM AUTHOR]

The materials combining high-entropy alloys with good mechanical properties and magnetic materials with a significant magnetocaloric effect are among the best candidates for magnetic refrigeration. For this purpose, the high-entropy alloys (MnNiSi) 1- x (FeCoGe) x (x = 0.4, 0.42, 0.45, and 0.47) were designed in this study. The effects of the Si/Ge ratio on the magnetocaloric effect and mechanical properties were also investigated. The results show that high-entropy alloy (MnNi) 0.6 Si 0.62 (FeCo) 0.4 Ge 0.38 possesses a Vickers hardness value of 580.6HV2 and compressive strength of 267 MPa with respect to a significant entropy change of 48.5 J·kg−1K−1 under a magnetic field of 5 T near the Curie temperature of 308.8 K. In order to clearly understand the origin of this excellent comprehensive performance, the influence mechanism of crystal structure and configurational entropy were explored. Furthermore, the first-principles calculations indicated that the magnetic properties mainly originate from Mn atoms located in 3 d sites and are influenced by the interaction between the Mn and Fe atoms. Additionally, the total magnetic moment and electron density are taken into account to reveal their influence on magnetocaloric properties. [Display omitted] • An MnNiSi-based magnetocaloric high-entropy alloys with excellent comprehensive properties were obtained. • The MnNiSi-based alloys have good mechanical properties (the Vickers hardness of 580.6 HV2 and compressive strength of 267 MPa). • The magnetocaloric effects of the MnNiSi-based alloys is 48.5 J·kg−1K−1 under 0–5 T at T C = 309 K. • The physical mechanism about good mechanical properties and magnetocaloric effects is revealed from the crystal structure, configurational entropy and electronic structure. [ABSTRACT FROM AUTHOR]

• A custom built device to directly measure Δ T was designed, and its running frequency is up to 9 Hz and temperature span between about 235 K and 313 K. • The relationship between the temperature change of Gadolinium and alternating frequency of magnetic field under the near working condition can be directly obtained. • The theoretical model was built to investigate the effects of different influencing factors on the temperature change for MCE materials. In magnetic refrigerator, the magnetic induction and its alternating frequency have great influence on the magnetocaloric effects of materials. In order to relationship between the magnetocaloric effect and alternating frequency of magnetic field, the direct measurements of temperature change for Gadolinium near working conditions in alternating magnetic fields up to 9 Hz are carried out. In this device, the magnetic field is alternately changed by rotating the magnet while sample holder keeps stationary to defend the measurement signal interference. Results shows that the temperature change of Gd decreases from 1.5 K to 0.7 K with increasing field alternating frequency from 2.2 Hz to 9 Hz under magnetic field of 0.72T. Furthermore, based on the heat transfer process between sample, thermocouple and environment, we have established a theoretical model to investigate the effects of different influencing factors on the temperature change of Gadolinium. It reveals that near work conditions, there are lots of influencing factors to affect the temperature change, such as sample mass, applied magnetic field mode, adiabatic conditions, cycle frequency, and so on. The experimental and simulation results provide the guidance for screening the best candidate of magnetocaloric materials and developing the magnetic refrigerator. [ABSTRACT FROM AUTHOR]