Structural phase transition induced in Fe50Rh50 alloys by high pressure

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Abstract

The equiatomic composition of the FeRh alloy was studied by in situ high pressure energy dispersive X-ray experiments. Pressures as high as 19 GPa were generated using a diamond anvil cell. An incomplete bcc → fcc phase transition was evidenced, both bcc and fcc phases coexisting above 10.0 GPa. The transition is accompanied by a decrease of the lattice parameter of the remaining bcc phase by approx. 1.5%. A substantial low lattice parameter was observed for the newly formed fcc phase, reflecting a rather compressed structure.

Introduction

The Fe50Rh50 alloy with body-centred-cubic (bcc) CsCl-type structure was found to undergo a first order magnetic phase transition by heating at a temperature Tt ≈ 360 K, from an antiferromagnetic (AF) to a ferromagnetic (FM) order [1], [2]. This transition is accompanied by an abrupt lattice expansion (0.3–1.0% in volume) as in an anti-invar alloy [3], an increase of the iron magnetic moment [4], a large increase in the entropy [5] and a remarkable fall of the electrical resitivity [6]. A new magnetic transition from the FM state to a paramagnetic (PM) one appears at the Curie temperature, Tc ≈ 650 K [6]. Both transition temperatures are very sensitive to small changes in concentration around the equiatomic composition. The above properties promote the binary Fe–Rh alloy as a potential material for various kind of sensors.

Since the discovery of these transformations, the equiatomic composition Fe50Rh50 has been intensively studied both theoretically and experimentally. Quantitative spin polarised band calculations have been performed in order to study the electrical properties as well as the energy state of the CsCl-type FeRh alloys [7], [8]. First principle band structure calculations based on the augmented-spherical-wave method have been used by Moruzzi and Marcus in order to determine the volume dependence of the total energy and of the magnetic moment in the ordered FeRh alloys [9]. Their calculations reveal the coexistence of the AF and FM solutions over a wide range of volumes but the zero-pressure equilibrium state at room temperature was found to be the AF one. The metastable FM state lies a few mRy above the AF state and has a minimum energy at a lattice parameter only 0.5% larger. Therefore, the FM state can be reached by magnetic or thermal excitation, e.g. at higher temperatures where also an expanded cell volume is stabilised.

A disordered fcc metastable variant of the FeRh alloy prepared by plastic deformation by Lommel and Kouvel [10] was paramagnetic at room temperature. Recently, Hernando et al. [11] and Rosenberg et al. [12] have analysed disordered fcc FeRh nanophases obtained by ball milling and showed that these behave as spin glasses (SG) with a freezing temperature of about 80 K. A new transition from a PM to a FM state accompanied by a fcc → bcc transformation was evidenced at 450 K [13].

Clearly, all the above results indicate a strong correlation between the micro-structural and the magnetic/electric properties of the mentioned alloys. In particularly, the magnetic transitions could be controlled via induced modifications of the lattice parameters. One thermodynamic parameter allowing the fine control of the structural behaviour is the pressure. This work reports on a X-ray diffraction study of the Fe50Rh50 alloys at high pressure. A partial structural transition was evidenced above 10.0 GPa and the evolution of the lattice parameters of the involved phases were analysed versus the applied pressure.

Section snippets

Experimental

The sample with the equiatomic composition Fe50Rh50 was prepared as ingot by induction melting of the components under Ar atmosphere, followed by spin melting giving rise to ribbons with an average cross section of 3 mm × 5 μm. High energy resolution diffraction experiments were performed at the B2 beamline of the DESY-HASYLAB synchrotron radiation laboratory (Hamburg, Germany) in order to check the initial CsCl-type structure. The “in situ” high pressure X-ray diffraction experiments were

Results and discussion

Fig. 1 shows the EDX spectrum at ambient conditions with the sample fixed in the gasket hole. Fluorescence doublets of Rh are observed around 20 and 23 keV. The first three diffraction lines of the gasket material (g-1 1 1, g-2 0 0 and g-2 2 0) are in agreement with the inconel fcc-structure with a lattice parameter of 0.3545(5) nm. The other diffraction lines correspond to a simple cubic structure assigned to the equiatomic FeRh alloy in the ordered bcc-CsCl-like phase. A lattice parameter of

Conclusions

The structural data of Fe50Rh50 alloys under high pressure point out to an incomplete transformation from a bcc- to an fcc-type structure above 10.0 GPa and are refuting the assumption of a continuous deformation of the CsCl-type structure along the martensitic Bain path. Following the structural transition, both the bcc and fcc phases are coexisting.

A decrease of the lattice parameter of the bcc phase by approx. 1.5% was evidenced during the transition. The new fcc phase appearing above 10 GPa

Acknowledgements

This work was supported by the DFG (Contract BU 547/6-1). The authors wish to thank Prof. Antonio Hernando (Instituto de Magnetismo Aplicado, Las Rozas, Madrid 28230, Spain) for providing us with Fe50Rh50 ribbons.

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