First-order magneto-structural transition and magnetocaloric effect in Mn(Co_0.96Fe_0.04)Ge

Highlights

• Magnetic and structural properties of Mn(Co_0.96Fe_0.04)Ge investigated.

• Neutron diffraction revealed the occurrence of a magneto-structural transition.

• Effects of chemical pressure on phase transition temperatures discussed.

• Large magnetocaloric effect obtained in Mn(Co_0.96Fe_0.04)Ge.

• The magneto-structural transition shown to be first-order.

Abstract

The magnetic properties and magnetic structure of an as-prepared Mn(Co_0.96Fe_0.04)Ge sample has been investigated by powder neutron diffraction as well as X-ray diffraction and magnetisation measurements. The sample has a ferromagnetic structure in the low-temperature orthorhombic phase and a magneto-structural transition at 299 (1) K to the high-temperature paramagnetic hexagonal phase. This transition occurs at a higher temperature than for as-prepared (Mn_0.96Fe_0.04)CoGe (T_M = 239 (1) K). Increased occupancy by Fe of the Co (4c) site rather than the Mn (4c) site results in this smaller suppression of the structural transition temperature away from that of undoped MnCoGe. It was found that chemical pressure increased the Curie temperature $T_{\text{C}}^{\text{orth}}$ in the orthorhombic phase from 355 (5) K in Mn(Co_0.96Fe_0.04)Ge to 379 (6) K in MnCoGe. Mn(Co_0.96Fe_0.04)Ge exhibits a large magnetocaloric effect around the magneto-structural transition, $-\Delta S_{\text{m}}^{\text{peak}}$ = 11 (2) J kg⁻¹ K⁻¹ and RC = 187 (30) J kg⁻¹ with μ₀ΔH = 5 T. The magneto-structural transition is a first order transition as demonstrated by master curve analysis.

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: LaFe_11.5Si_1.5H_x [5], MnFeP_1−xAs_x [6], Mn_1−xFe_xAs 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 T_M, from a low-temperature orthorhombic phase (TiNiSi-type structure, Pnma) to a high-temperature hexagonal (Ni₂In-type structure, P6₃/mmc) phase between T_M ∼398 K and T_M ∼458 K [10]. The orthorhombic phase has a ferromagnetic structure below a Curie temperature near 350 K (e.g. $T_{\text{C}}^{\text{orth}}$ ∼345 K [11]; $T_{\text{C}}^{\text{orth}}$ ∼355 K [10]). The structural transition temperature at T_M 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 T_M 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 $T_{\text{C}}^{\text{hex}}$ ∼275 K [23]. For a case that the resulting T_M is engineered to lie within the temperature range between $T_{\text{C}}^{\text{hex}}$ and $T_{\text{C}}^{\text{orth}}$, 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 T_M 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 T_M = 304 (1) K was obtained for Mn(Co_0.96Fe_0.04)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(Co_0.96Fe_0.04)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].