From first-order magneto-elastic to magneto-structural transition in (Mn,Fe)1.95P0.50Si0.50 compounds

Abstract

We report on structural, magnetic, and magnetocaloric properties of Mn x Fe 1.95Àx P 0.50 Si 0.50 (x ! 1.10) compounds. With increasing the Mn:Fe ratio, a first-order magneto-elastic transition gradually changes into a first-order magneto-structural transition via a second-order magnetic transition. The study also shows that thermal hysteresis can be tuned by varying the Mn:Fe ratio. Small thermal hysteresis (less than 1 K) can be obtained while maintaining a giant magnetocaloric effect. This achievement paves the way for real refrigeration applications using magnetic refrigerants. Nowadays, advanced magnetocaloric materials often undergo a first-order magnetic transition (FOMT), [1][2][3][4] because the FOMT is associated with an abrupt change in crystal lattice which enhances magnetocaloric effects (MCEs) via a spin-lattice coupling. The FOMT can be divided into first-order magneto-structural transition (FOMST), which exhibits a structure change coupled with a magnetic transition as observed for Gd 5 (Ge x Si 1Àx ) 4 , 5,6 Ni 0.50 Mn 0.50Àx Sn x (Ref. 7), and MnCoGeB x , 8 or first-order magneto-elastic transition (FOMET) for which the crystal structure remains unchanged but the lattice constants suddenly change at the magnetic transition, as observed for MnFeP 1Àx As x (Ref. 9) and La(Fe 1Àx Si x ) 13 . 10,11Fe 2 P-based compounds are known as giant magnetocaloric materials with a FOMET. Most studies have recently focused on (Mn,Fe) 2 (P,As,Ge) compounds.1-3,9,12,13 However, the limited availability of Ge and toxicity of As hold these materials back from real refrigeration applications. Substitution of Si for As and Ge becomes one of the most prominent studies towards making a high performance room-temperature magnetic refrigerant. Some efforts have been made to reduce thermal hysteresis (DT hys ) which was found to be very large (DT hys ¼ 35 K) in the MnFeP 0.50 Si 0.50 alloy.14 Here, we report on (Mn,Fe) 1.95 P 0.50 Si 0.50 compounds when changing the Mn:Fe ratio with emphasis on the behavior of magnetic and structural transitions. We observe a previously unknown FOMST and a modified FOMET favorable for real refrigeration applications.The (Mn,Fe) 1.95 P 0.50 Si 0.50 alloys were prepared by ballmilling. Proper amounts of Mn (99.9%), Si (99.999%) chips, binary Fe 2 P (99.5%), and red-P (99.7%) powder were mixed and ball-milled for 10 h. The fine powder was then pressed into small tablets and sealed in quartz ampoules in an Ar atmosphere of 200 mbar. The samples were sintered at 1373 K for 2 h and then annealed at 1123 K for 20 h before being oven cooled to room temperature. Magnetic measurements were carried out using the reciprocating sample option (RSO) mode in a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS 5XL). X-ray diffraction patterns were obtained by a PANalytical X-pert Pro diffractometer with Cu Ka radiation, secondary flat crystal monochromator, and X'celerator real time multiple strip (RTMS) detector system.The room-temperature x-ray diffraction measurement...

Study of the first paramagnetic to ferromagnetic transition in as prepared samples of Mn–Fe–P–Si magnetocaloric compounds prepared by different synthesis routes

“…The lattice parameters are estimated from the XRD measurements using Rietveld refinement. According to the determined lattice parameters (a = 610 ± 0.5 pm and c = 344 ± 1.5 pm) and literature data [1], all the samples are expected to be in the paramagnetic state at room temperature.…”

“…Hexagonal Mn-Fe-P-Si compounds are known to represent a promising rare-earth-free road to room temperature magnetic refrigeration [1]. The giant magnetocaloric effect (MCE) has been studied in this system with particular emphasis on the tuning of the critical temperature and magnitude of the entropy change as a function of the composition [2].…”

Kinetic-arrest-induced phase coexistence and metastability in(Mn,Fe)2(P,Si)

“…Our previous studies reveal that the T C and the thermal hysteresis of the (Mn,Fe) 2 (P,Si)-type compounds can be tuned directly by varying the Mn/Fe ratio [39]. With an increase in Mn/Fe ratio from 1.00/0.95 to 1.30/0.65, a rapid decrease in T C and a reduction in thermal hysteresis is observed in Fig.…”

High/low-moment phase transition in hexagonal Mn-Fe-P-Si compounds

“…11. To make the samples more homogeneous, they were reannealed at 1373 K for 20 h before being quenched in cold water.…”