A strategy of optimizing magnetism and hysteresis simultaneously in Ni–Mn-based metamagnetic shape memory alloys

Highlights

• A strategy to strengthen magnetism and reduce hysteresis simultaneously is proposed.

• Replacement of Cu for the p-block element can effectively reduce thermal hysteresis.

• The proposed alloying manner of Cu doesn't lead to a degradation of magnetism.

• An effective refrigeration capacity of 270.2 J·kg⁻¹ at 5 T is realized.

• The reported refrigeration capacity belongs to the highest one in Ni–Mn-based alloys.

Abstract

Due to the existence of the magnetic-field-induced reverse martensitic transformation, the Ni–Mn–Z alloys where Z is the p-block element of In, Sn or Sb exhibit several state-of-the-art magnetoresponsive properties. From the perspective of practical application, a major challenge of this compound is to strengthen the ferromagnetism and reduce the transitional hysteresis ΔT_Hys simultaneously. In this work, we report an alloying strategy to this issue, i.e., the partial substitutions of Co and Cu for Ni and the p-block element, respectively, by a combined study of experiment and ab-initio calculation in the Ni₄₅Co₅Mn₃₆In_14–xCu_x (x = 0–1.5) alloys. Results show that the Cu replacement for the p-block element (In) can effectively reduce ΔT_Hys owing to the improved geometrical compatibility between austenite and martensite. Moreover, the adopted alloying strategy of Cu hardly weakens the strong ferromagnetism of Ni(Co)–Mn–In, since the magnetism exhibits a weak dependence on the p-block element compared with Mn and Ni. The maximum reversible magnetic entropy change $\Delta S_M^{Max}$ of the Ni₄₅Co₅Mn₃₆In_13.3Cu_0.7 directionally solidified sample at a magnetic field of 5 T is 13.8 J·kg⁻¹·K⁻¹. The refrigeration capacity RC and effective refrigeration capacity RC_eff equal to 473.7 J·kg⁻¹ and 270.2 J·kg⁻¹, respectively. Both values belong to the highest one in the Ni–Mn-based alloys. Additionally, the working temperature window of the magnetocaloric effect is as wide as ~40 K. This work is expected to promote the design of advanced metamagnetic shape memory alloys.

Introduction

Ni–Mn–Z metamagnetic shape memory alloys where Z is the p-block elements of In, Sn, and Sb [[1], [2], [3], [4], [5], [6], [7], [8], [9]] are attracting considerable interest since this compound possesses several magnetoresponsive properties, such as metamagnetic shape memory [1], magnetocaloric [5], and magnetoresistance effects [10,11]. These fantastic properties make the Ni–Mn-based alloys as promising candidates in the fields of smart sensor and solid-state refrigerant. For the Ni–Mn-based alloys, the magnetoresponsive behaviors are mostly originated from the magnetic-field-induced first-order magnetostructural transition [1,5,12,13]. From the perspective of practical application, a common issue for various functional properties of the Ni–Mn-based alloys is the relatively higher critical magnetic field μ₀H_cr needed to drive a reversible magnetostructural transition. Based on the Clausius-Clapeyron relation, the critical magnetic field μ₀H_cr can be expressed as (ΔT_Hys+ΔT_Int)/(ΔM/ΔS) [14,15] where ΔT_Hys, ΔT_Int, ΔM, and ΔS represent the thermal hysteresis, the transformation interval, the magnetization change, and the transformation entropy change, respectively. Among these factors, ΔM and ΔT_Hys are the crucial and adjustable ones in materials design. A low μ₀H_cr requires that the compound possesses a considerable ΔM and a small ΔT_Hys at the same time. Therefore, the strategy enabling to elevate ΔM and reduce ΔT_Hys simultaneously should be significantly meaningful to improve the magnetoresponsive performances of the Ni–Mn-based alloys.

For the Ni–Mn-based alloys, the low-temperature phase (martensite) usually exhibits a weak-magnetism (paramagnetism or antiferromagnetism) [1,12]. The ΔM value is majorly decided by the magnetization of the high-temperature phase (austenite). Thus, improving the magnetism of austenite should be the most effective way to elevate ΔM when we focus on the magnetostructural transition with the transformation temperature near room temperature. To date, the most successful way of strengthening the magnetism in Ni–Mn-based alloys is to add some amount of Co for replacing Ni [1]. Unfortunately, the alloying of Co generally leads to the broadening of ΔT_Hys. For example, the ternary Ni–Mn–In alloys exhibit a ΔT_Hys of 5–12 K [13,16], while the ΔT_Hys value of Ni(Co)–Mn–In alloys is typically increased to 10–30 K [17,18].

Aiming to reduce ΔT_Hys, several approaches have been proposed, such as chemical composition modification [19], microstructure design [16], application of external stimuli [20], and operating in minor loops of thermal hysteresis [21]. Among them, alloying is the most effective approach. It is because ΔT_Hys is related to the geometrical compatibility between austenite and martensite, which is intrinsically decided by the composition. Recently, the alloying with Cu was reported to be an effective way to reduce ΔT_Hys in several ternary (or quaternary) Ni–Mn-based alloys, such as Ni–Mn(Cu)–In Refs. [[22], [23], [24]], Ni–Mn(Cu)–Ga [[25], [26], [27]], and Ni(Co)–Mn(Cu)–In Ref. [28]. In these studies, Cu was mostly added by substituting Mn. This alloying method is beneficial to prevent the precipitation of the second phase, as Cu (3d¹⁰4s¹) possesses a similar valence electron configuration with Mn (3d⁵4s²). However, the Cu replacement for Mn generally results in a sharp decrease of magnetism since the magnetic moment of Ni–Mn-based alloys is mainly provided by Mn (~85%) [9,29]. As mentioned earlier, the decreased magnetism is harmful to the magnetoresponsive performance of Ni–Mn-based alloys.

In this work, by a combined study of experiment and ab-initio calculation, we propose a multielement alloying strategy, i.e., the partial substitutions of Co and Cu for Ni and the p-block element, respectively (Fig. 1), aiming to strengthen magnetism and reduce ΔT_Hys simultaneously. Here, the Ni₅₀Mn₃₆In₁₄ alloy is chosen as the parent system. First, following the pioneering work of R. Kainuma et al. [1], we dope 5% (at.) Co to replace Ni for strengthening magnetism. To decrease ΔT_Hys, with inspirations from the previous work [[22], [23], [24], [25], [26], [27], [28]], we also add some amount of Cu but by substituting the non-magnetic p-block element of In. This alloying manner of Cu is to avoid the degradation of magnetism that occurs when Cu replaces Mn [[22], [23], [24], [25], [26], [27], [28]].