Elsevier

Journal of Solid State Chemistry

Volume 221, January 2015, Pages 240-246
Journal of Solid State Chemistry

Detailed study of the magnetic ordering in FeMnP0.75Si0.25

https://doi.org/10.1016/j.jssc.2014.10.013Get rights and content

Highlights

  • Magnetic and crystallographic properties of FeMnP0.75Si0.25 have been investigated.

  • Co-existing ferro- and antiferromagnetic ordering arise from two phases of Fe2P-type.

  • A low temperature incommensurate antiferromagnetic structure is revealed.

Abstract

Magnetic and crystallographic properties of FeMnP0.75Si0.25 in the hexagonal Fe2P-type structure have been investigated by X-ray powder diffraction, neutron powder diffraction and magnetic measurements. The room temperature diffractograms reveal co-existence of two distinct structural phases in the samples with small, but significant, differences only in the unit cell dimensions. The volume ratio between the two phases is governed by the annealing conditions. One of the phases orders ferromagnetically (TC=250 K) and the other in an incommensurate antiferromagnetic structure at low temperatures (qx=0.363(1), TN=150 K).

Graphical abstract

The ferromagnetic structure of sample I (a) and the antiferromagnetic and incommensurate (qx=0.363(1)) low temperature structure of sample II (b). The magnetic moments of the Mn and Fe atoms in (b) are aligned in the basal plane along the a- and the b-axis, respectively, and the amplitude of the moments propagates sinusoidally along the a-axis.

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Introduction

Investigations of the structural and magnetic properties of the (Fe,Mn)2(P,Si)-system and related Fe2P-based compounds have been subject of large interest in recent years [1]. Selected compositions of these alloy systems are strong candidates to become the cooling media in commercial magnetic refrigeration systems [1], [2]. This is due to their cheap and non-toxic raw materials combined with magnetic properties providing good magnetocaloric characteristics (e.g. Curie temperature (TC) tunable to room temperature, low thermal hysteresis and sharp magnetic transitions).

A main focus in reports on these systems has been on compounds with equal amounts of Fe and Mn [3], [4], [5], [6], [7]. Moreover, recent studies have revealed non-stoichiometric compounds ((Fe,Mn)1.95(P,Si)) with properties suitable for magnetocaloric applications [8], [9], [10]. Nevertheless, the (Fe,Mn)2(P,Si)-system is complex involving several structural and magnetic phases. FeMnP1−xSix crystallizes in the orthorhombic Co2P-type structure for x0.24 and the hexagonal Fe2P-type structure for 0.25x0.65. A ternary phase region is reached for x>0.65 [3], [11].

In magnetocaloric materials with a first-order phase transition an understanding of the driving force behind the phase transition is crucial. In this context, one can distinguish transitions due to magnetostriction effects from crystalline phase transitions. The first-order phase transition in Fe2P is a result of large magnetostriction [12], by elemental substitutions this transition can be tuned to a phase transition with large magnetocaloric effect and weak thermal hysteresis. A strong spin–lattice coupling is advantageous since it may enhance the magnetocaloric effect [13].

The hexagonal/orthorhombic phase border (x~0.24) is of particular importance since it involves interesting magnetic properties. Previous studies of FeMnP0.75Si0.25 have shown that small differences in the structural ordering of Fe and Mn on the two independent metal sites alter the magnetic properties significantly [4]. A slow cooled sample resulted in an antiferromagnetic ordering while a quenched sample showed ferromagnetic ordering. Furthermore, ferromagnetic ordering has been associated with the hexagonal Fe2P-type structure whereas antiferromagnetic ordering occurs in the orthorhombic Co2P-type structure.

This work reports neutron diffraction studies on FeMnP0.75Si0.25 samples in the hexagonal Fe2P-type structure. A homologous structural phase transition governed by the annealing conditions is discovered within the hexagonal structure; one phase being ferromagnetic and the other antiferromagnetic.

Section snippets

Sample preparation

The FeMnP0.75Si0.25 samples were prepared by the drop synthesis method [14] using a high-frequency induction furnace at 1623–1673 K in an Ar atmosphere of 40 kPa. Stoichiometric amounts of iron (Leico Industries, purity 99.995%. Surface oxides were reduced in H2-gas.), manganese (Institute of Physics, Polish Academy of Sciences, purity 99.999%), phosphorus (Cerac, purity 99.999%) and silicon (Highways International, purity 99.999%) were used as raw materials. The sample preparation process showed

X-ray powder diffraction

Phase analysis and crystal structure characterizations were performed using X-ray powder diffraction (XRD) with a Bruker D8 diffractometer equipped with a Våntec position sensitive detector (PSD, 4° opening) using Cu1 radiation, λ=1.540598 Å. The measurements were made using a 2θ-range of 20–90° at 298 K and 16 K.

Neutron powder diffraction

Neutron powder diffraction (NPD) intensities were collected on the PUS diffractometer at the Institute for Energy Technology in Kjeller, Norway. The neutron beam was monochromated by a

Phase analysis

The observed and calculated XRD patterns of samples I and II at 298 K and 16 K (shown in Fig. 2, Fig. 3) confirm that both samples crystallize in the hexagonal Fe2P-type structure (P6¯2m) in this temperature interval. However, detailed studies of the intensity profiles reveal overlapping reflexions over the whole diffractogram, which originate from two Fe2P-type phases with slightly different unit cell dimensions. These phases are denoted A and B. Careful phase analysis estimates the phase ratio A

Summary and conclusions

A detailed study of FeMnP0.75Si0.25 has been performed using X-ray powder diffraction, neutron powder diffraction and SQUID magnetic measurements. The neutron powder diffraction data reveal a low temperature incommensurate antiferromagnetic structure (qx=0.363(1)) not reported before in the FeMnP1−xSix-system. Both samples I and II contain ferromagnetic and antiferromagnetic orders** at low temperatures arising from the two coexisting phases A and B of Fe2P type. In sample II the contribution

Acknowledgments

This work was financed by the Swedish Research Council and the Swedish Energy Agency, which are gratefully acknowledged. Premysl Beran wants to thank for the support from the CANAM infrastructure.

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