The efficient coupling between lattice degrees of freedom and spin degrees of freedom in magnetic materials can be used for refrigeration and energy conversion. This coupling is enhanced in materials exhibiting the giant magnetocaloric effect. First principle electronic structure calculations on hexagonal MnFe(P, Si) reveal a new form of magnetism: the coexistence of strong and weak magnetism in alternate atomic layers. The weak magnetism of Fe layers (disappearance of local magnetic moments at the Curie temperature) is responsible for a strong coupling with the crystal lattice while the strong magnetism in adjacent Mn-layers ensures Curie temperatures high enough to enable operation at and above room temperature. Varying the composition on these magnetic sublattices gives a handle to tune the working temperature and to achieve a strong reduction of the undesired thermal hysteresis. In this way we design novel materials based on abundantly available elements with properties matched to the requirements of an efficient refrigeration or energy-conversion cycle.Comment: 6 pages, 6 figure

Using high-resolution neutron diffraction measurements for Mn-rich hexagonal Mn-Fe-P-Si compounds, we show that the substitution of Mn for Fe on the 3f sites results in a linear decrease of the Fe/Mn(3f ) magnetic moments, while the Mn(3g) magnetic moments remain constant. With increasing temperature, the Mn(3g) magnetic moments show almost no change, while the Fe/Mn(3f ) moments decrease quickly when the transition temperature is approached. The reduction of the magnetic moments at the transition temperature and in the high-temperature range is discussed based on changes in interatomic distances and lattice parameters and high-temperature magnetic-susceptibility measurement.

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...

Structural, magnetic and magnetocaloric properties of Fe 2 P-based Mn-Fe-P-Si compounds were investigated. The study reveals a large magneto-elastic coupling that starts to develop in the paramagnetic state and grows when the ferromagnetic transition temperature is approached. Based on the behavior of the magneto-elastic coupling, we show the thermal evolution of the magnetic moments. On cooling, magnetic moments on the tetrahedral site form and gradually increase in the paramagnetic state. At the magnetic ordering temperature the moments attain a much larger value in a discontinuous step. We also find that the hysteresis and magnetic entropy change are correlated with discontinuous changes in the lattice parameters at the transition temperature. Small hysteresis can be obtained while maintaining giant magnetocaloric effect.PACS number(s): 75.30. Sg, 75.30.Kz, 75.50.Bb Recently, mixed strong and weak magnetism in adjacent Mn(3g) and Fe(3f) lattice planes was proposed from ab initio

Neutron-diffraction experiments on the (Mn,Fe) 2 (P,Si)-type compounds have shown a site preference of Si atoms in the hexagonal structure. The degree of ordering of Si depends on the Si/P ratio, while it is independent of the Mn/Fe ratio. The ferromagnetic-paramagnetic magnetoelastic transition is closely related to the size of the magnetic moment on the 3f site. A preferred occupation of Si atoms on the 2c site stabilizes and decreases the magnetic moment on the 3f and 3g site, respectively, which is supported by our first-principles density functional theory calculations. This effect, together with the contribution from the Si substitution-induced changes in the interatomic distances, leads to a phase transition that is tunable in temperature and degree of first order in Mn 1.25 Fe 0.70 P 1−x Si x compounds. These results provide us with further insight into the relationship between the magnetoelastic phase transition and the local atomic coordination.