First-order magneto-structural transition and magnetocaloric effect in Mn(Co0.96Fe0.04)Ge
Introduction
Magnetic refrigeration based on the magnetocaloric effect (MCE) has attracted increasing attention in recent decades due to its environmental friendly and energy-saving advantages [1], [2], [3]. With practical applications in mind, relatively inexpensive materials that exhibit a large MCE are required. Following on from the discovery of a giant magnetocaloric effect in Gd5Si2Ge2 [4], at least six types of materials that exhibit coupling of magnetic and structural transitions – a magneto-structural transition - have been explored for their potentially high magnetocaloric performance [4], [5], [6], [7], [8], [9]. The list of these six groups of materials include: LaFe11.5Si1.5Hx [5], MnFeP1−xAsx [6], Mn1−xFexAs alloys [7], Ni–Mn–based Heusler alloys [8], MnCoGe-based compounds [9] as well as Gd5Si2Ge2 related materials [4]. In cases involving a magneto-structural transition, a change in magnetic field can induce simultaneously changes in both the magnetic and lattice entropies in materials, thereby bringing about a large magnetocaloric effect [8].
MnCoGe-based compounds are a family of promising materials with a large magnetocaloric effect; they are relatively low cost compared with rare earth compounds and exhibit magneto-structural transitions over the important temperature region around room temperature (∼275 K–345 K) [9]. MnCoGe-based compounds commonly undergo a change in structure at the martensitic reverse transformation temperature TM, from a low-temperature orthorhombic phase (TiNiSi-type structure, Pnma) to a high-temperature hexagonal (Ni2In-type structure, P63/mmc) phase between TM ∼398 K and TM ∼458 K [10]. The orthorhombic phase has a ferromagnetic structure below a Curie temperature near 350 K (e.g. ∼345 K [11]; ∼355 K [10]). The structural transition temperature at TM is sensitive to external pressure [12], vacancies in the Co and Mn sites [13], [14], as well as variation in chemical environment resulting from introduction of interstitial atoms [9] or element substitution for Mn, Co or Ge [15], [16], [17], [18], [19], [20], [21], [22]. All of these factors can drive TM towards lower temperatures, e.g. a suitable partial substitution for Mn or Co favours stabilisation to lower temperature of the hexagonal phase which has a ferromagnetic ordering temperature of ∼275 K [23]. For a case that the resulting TM is engineered to lie within the temperature range between and , a magneto-structural transition from the ferromagnetic orthorhombic structure to the paramagnetic hexagonal structure is created, thereby offering scope for a large magnetocaloric effect at the transition [14].
Recent studies have established that Fe is an effective substitute for Mn in MnCoGe in driving TM towards lower temperatures [24], [25], [26], [27], [28]. At the same time, Li et al. [24] also reported that substitution of Fe for Co can bring about coincidence of the magnetic and structural transitions. A martensitic reverse transformation temperature of TM = 304 (1) K was obtained for Mn(Co0.96Fe0.04)Ge(57Fe) in the as-prepared state by X-ray diffraction measurements in an initial investigation of Fe dopant occupation using 57Fe Mössbauer spectroscopy [29]. Here we present a comprehensive investigation of the magnetic properties and magnetocaloric behaviour of Mn(Co0.96Fe0.04)Ge using X-ray diffraction, neutron diffraction and magnetisation measurements. The resulting magnetic transitions are also evaluated in terms of magnetocaloric entropy and refrigeration capacity (RC), and the nature of the transition is investigated using master curve analysis [30], [31].
Section snippets
Experimental
The polycrystalline Mn(Co0.96Fe0.04)Ge sample was prepared by arc melting stoichiometric amounts of Mn, Co, Ge and Fe (>99.95 wt%) in an argon arc furnace with 3% excess of Mn added to compensate for the mass loss of Mn during sample preparation. The ingot was re-melted five times to improve sample homogeneity. The quality of the sample and its crystallographic structure were studied by X-ray powder diffraction measurements at room temperature with Cu-Kα radiation. The orthorhombic and
Magnetisation
Magnetisation curves collected in a magnetic field of 0.01 T are shown in Fig. 1: the data were collected as follows - on heating after zero-field cooling (ZFC), on cooling (FC) and on heating (FH) in a field. The sample exhibits a transition around 300 K from a low temperature ferromagnetic state to a high temperature paramagnetic state. The magnetic state change temperatures are found to be = 305 (4) K and = 295 (4) K on heating and cooling respectively, as determined from the
Conclusions
The effect of substitution of Fe for Co in Mn(Co0.96Fe0.04)Ge has been investigated by variable temperature X-ray diffraction, neutron diffraction and magnetisation measurements. Irreducible representation analysis and Rietveld refinements of the neutron data indicated that the as-prepared Mn(Co0.96Fe0.04)Ge sample has a ferromagnetic structure with magnetic moments on the Mn sublattice in the orthorhombic phase. In addition, the neutron diffraction experiments demonstrated directly the
Acknowledgements
This work was supported in part by grants from the Australian Research Council: (Discovery project DP110102386) and LIEF grant LE1001000177. QYR is grateful to the UNSW Canberra for a Research Training Scholarship and Research Publication Fellowship.
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