Neutron diffraction study on the magnetic structure of Fe2P-based Mn0.66Fe1.29P1−xSix melt-spun ribbons

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Highlights

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    The magnetic structure is determined using neutron diffraction.

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    A magnetic moment of up to 4.57 μB/f.u. is observed.

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    Alignment of the magnetic moment is canted when varying the Si content.

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    The 3g site disorder shows a stabilizing effect on the hexagonal crystal structure.

Abstract

We report on the magnetic and structural properties of Mn0.66Fe1.29P1−xSix melt-spun ribbons with 0.34≤x≤0.42 that are promising candidates for high-temperature magnetocaloric applications. A magnetic moment of up to 4.57 μB/f.u. for x=0.34 indicates high magnetic density in the system, which is certainly advantageous for the magnetocaloric effects. Introducing site disorder at the 3g site by replacing 1/3 of Fe with Mn appears to enhance the magnetic interaction, while the strong magnetoelastic coupling is maintained. This site disorder also shows a stabilizing effect on the hexagonal crystal structure, which is maintained to a high Si content. The moment alignment within the crystallographic unit cell is also affected when the Si content is increased from x=0.34 to 0.42 in the Mn0.66Fe1.29P1−xSix compounds as the canting angle with respect to the c-direction increases.

Introduction

The giant magnetocaloric effect (GMCE), that is associated with a first-order magnetic transition (FOMT), makes near room-temperature magnetic refrigeration attractive as a highly efficient and eco-benign technology [1], [2], [3], [4], [5], [6], [7]. We discovered a GMCE in the Fe2P-type hexagonal (Mn,Fe)2(P,Si) compounds. Initially a large thermal hysteresis (>20 K) associated with the FOMT in MnFe(P,Si) hampered the use of this material for cyclic applications [8]. By varying the Mn/Fe ratio well away from 1, the thermal hysteresis has been significantly reduced to about 1 K with a certain P/Si ratio while the GMCE is well preserved [9]. The discovery of high-performance and low cost (Mn,Fe)2(P,Si) compounds paves the way for commercial applications.

The earlier results show that Mn and Fe preferentially occupy different alternating layers in the Fe2P structure. A high local magnetic moment is associated with the Mn layer and a lower moment with the Fe layer. For a Mn/Fe ratio that is not equal to 1, the excess Fe or Mn atoms will inevitably occupy the other layer. Consequently, the site disorder will affect the magnetic interactions. Meanwhile, first principle electronic structure calculations suggest that the origin of the GMCE is the coexistence of strong and weak magnetism in alternate atomic layers. The weak magnetism (disappearance of local magnetic moments at the Curie temperature) is responsible for a strong coupling with the crystal lattice [9]. Knowing how the site disorder affects the local magnetic moment of each layer will help understanding the property-tuning mechanism. With neutron diffraction, the magnetic structure can be determined through the ordered and disordered states. The distribution of Fe and Mn atoms can also be resolved by neutron diffraction, while they can hardly be distinguished by X-ray diffraction. The insight obtained from these experiments will provide a handle to further improve this material for applications.

Section snippets

Experimental details

The Fe-rich compounds Mn0.66Fe1.29P1−xSix (x=0.34, 0.37 and 0.42) were prepared by the melt-spinning technique. The melt-spun ribbons were produced under argon atmosphere at a 40 m/s surface speed of the copper wheel. The as-spun ribbons were subsequently quenched into water after annealing at 1100 °C for 2 h. The X-ray diffraction patterns were collected at various temperatures in zero field using a PANalytical X-pert Pro diffractometer equipped with an Anton Paar TTK450 low-temperature chamber

Results and discussion

X-ray diffraction measurements indicate that all the compounds Mn0.66Fe1.29P1−xSix (x=0.34, 0.37 and 0.42) crystallize in the hexagonal Fe2P-type structure (space group of P-62m). As shown in Fig. 1, the contour plots display the thermal evolution of the X-ray diffraction patterns of all the compounds. For the compounds with x=0.34 and 0.37, a discontinuity of the diffraction peak positions at the transition temperatures indicates a step change in lattice constants while the compound with the

Conclusions

Fe-rich Mn–Fe–P–Si compounds are promising materials for high-temperature magnetocaloric applications. A magnetic moment of up to 4.57 μB/f.u. for x=0.34 indicates high magnetic density in the system, which certainly benefits the GMCEs. Introducing site disorder at the 3g site by replacing 1/3 of Fe with Mn appears to enhance the magnetic interaction, while the strong magnetoelastic coupling is maintained. The site disorder also shows a stabilizing effect on the hexagonal crystal structure,

Acknowledgments

The authors would like to thank Anton J. E. Lefering, Michel P. Steenvoorde, and Bert Zwart (Delft University of Technology) for their help in magnetic and structural measurement and sample preparation. This work is financially supported by the Foundation for Fundamental Research on Matter (FOM), the Netherlands, via the Industrial Partnership Program IPP I18 and co-financed by BASF Future Business.

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