New Room-temperature Magnetic Refrigerant MnFe(P,Si)

Fe2P-type MnFe(P1-x;Asx) compounds become one of the best candidate materials for room temperature magnetic refrigeration, because of their giant magnetocalotic effect (MCE), low-cost raw materials and simple preparation. In order to avoid using As, lots of work have been done to replace As. The results show that As can be replaced by Ge, and keeped giant MCE and proper working temperature, but Ge is more expensive than As for applications. This work reports on the preparation, structure, magnetic properties and magnetocaloric effects in MnFe(P,Si) compounds, showing Si is a best element for replacing As.Our results show that Mn2-xFexP0.51Si0.49 (x= 0.8,0.85,0.9,0.95,1.0,1.1 and 1.2) compounds mainly retain Fe2P-type hexagonal structure with space group P-62m, a second phase of (Fe,Mn)3Si was detected in all compounds. The Curie temperature ranges from 277 to 358 K with increasing Fe content. It worth to mention that thermal hysteresis of compounds decreases with decreasing the content of Fe and the minimal thermal hysteresis (â–³Thys) of the compounds is about 11 K. The maximal magnetic-entropy change reaches 13.9 J/kgK in a field change from 0 to 1.5 T for the content of Fe reach to 0.85. The maximal magnetic-entropy change of Mn2-xFexP0.51Si0.49 compounds is larger than Gd, Gd5Ge2Si2 and MnFeP0.45Si0.55 compounds in the same magnetic filed changes. A virgin effect, much lower phase transition temperature for first cooling, has been detected, which is a close relationship with internal stress in the MnFe(P,Si) compounds.In order to decrease the thermal hysteresis of MnFe(P,Si) compounds, we also investigated the magnetocaloric effect of Mn2-xFexP0.4Si0.6(x= 0.75,0.7, 0.65,0.6,0.55 and 0.5) compounds. The XRD results show that the compounds retain Fe2P-type hexagonal structure with space group P-62m, two miscellaneous phases of (Fe,Mn)5Si3 and (Fe,Mn)5Si2 were detected. The (Fe,Mn)5Si3 phase appears and the content of (Fe,Mn)5Si2 phase increased with decreasing the content of Fe. The Curie temperature of Mn2-xFexP0.4Si0.6 compounds decreases with decreasing the content of Fe, but the Curie temperature is in the room temperature region. The maximal magnetic-entropy change in Mn1.25Fe0.75P0.4Si0.6 compound reaches 7.2 J/kgK in a field change from 0 to 1.5 T. The thermal hysteresis of all compounds is less than 4 K. It means that the thermal hysteresis can reduced by decreasing the content of Fe in MnFe(P,Si) compounds.Atomic radius of Co is less than Fe. The Curie temperature can be tuned by size effect and the exchange interaction when substitute a part of Fe element by Co. According this, we investigate the structure and the magnetocaloric effect of Mn1.3Fe0.7-xCoxP0.46Si0.54(x= 0,0.025,0.05 and 0.1) compounds. The results show that the adding of Co does not change the formation of Fe2P-type hexagonal structure, a small amount of second phase of (Fe,Mn)3Si was detected in all compounds. The paramagnetic-ferromagnetic transition temperature ranges from 247 to 298 K with decreasing the content of Co. The thermal hysteresis of these compounds is less than 4 K. The maximum magnetic entropy changes of Mn1.3Fe0.65Co0.05P0.46Si0.54 compound reaches 17.3 J/kgK in a field change of 0 to 5 T. The maximum adiabatic temperature change is 2.2 K in 'Mn1.3Fe0.7P0.46Si0.54 and Mn1.3Fe0.65Co0.05P0.46Si0.54 compounds for a field change from 0 to 1.48 T.For further study the effect of Fe atom on the thermal hysteresis in the MnFe(P,Si) compounds, the structural and magnetic properties of non-stoichiometric Mn1.3Fe0.7-xP0.45Si0.55 (x= 0.00,0.02,0.04,0.06,0.08 and 0.10) compounds were investigated by means of x-ray diffraction and magnetization measurements. The Mn1.3Fe0.7-xP0.45Si0.55 compounds crystallize in the Fe2P-type hexagonal structure with space group P-62m symmetry, a small amount of second phase appears in all compounds. The Curie temperature can be turn from 234 to 307 K and non-orderliness with decreasing the content of Fe. The thermal hysteresis in all the compounds is less than 6 K and can be reduced to 1 K. The maximal magnetic-entropy change is 7.9 J/kgK in a field change from 0 to 1.5 T and thermal hysteresis is just 1 K for x= 0.04. It means that the reduced Fe content not only could reduce thermal hysteresis but also retains maximal magnetic-entropy change.In summary, we have replaced As and Ge by Si successfully and obtained giant magnetocalotic effect near room-temperature. The low-cost raw materials, the simple preparation techniques, the smaller thermal hysteresis, the ideal cooling tempreature and the large magnetocaloric effect make MnFe(P,Si) compounds an excellent candidate material for room-temperature magnetic refrigeration applications.