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Bachagha, T.;  Suñol, J. All-d-Metal Heusler Alloys. Encyclopedia. Available online: https://encyclopedia.pub/entry/41025 (accessed on 02 May 2024).
Bachagha T,  Suñol J. All-d-Metal Heusler Alloys. Encyclopedia. Available at: https://encyclopedia.pub/entry/41025. Accessed May 02, 2024.
Bachagha, Tarek, Joan-Josep Suñol. "All-d-Metal Heusler Alloys" Encyclopedia, https://encyclopedia.pub/entry/41025 (accessed May 02, 2024).
Bachagha, T., & Suñol, J. (2023, February 09). All-d-Metal Heusler Alloys. In Encyclopedia. https://encyclopedia.pub/entry/41025
Bachagha, Tarek and Joan-Josep Suñol. "All-d-Metal Heusler Alloys." Encyclopedia. Web. 09 February, 2023.
All-d-Metal Heusler Alloys
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This entry is adapted from the peer-reviewed paper 10.3390/met13010111

A promising strategy, resulting in novel compounds with better mechanical properties and substantial magnetocaloric effects, is favoring the d–d hybridization with transition-metal elements to replace p–d hybridization. The term given to these materials is “all-d-metal”. 

Heusler alloys hybridization magnetocaloric effect

1. Background of These Kinds of Alloys

Recently, Wei et al. [1] suggested the notion of an all-d-metal Heusler based on d–d orbital hybridization. The authors of this seminal work established the term “all-d-metal” after discovering that the Heusler phase could be formed without the p-group atom. Experiments on the crystal structure of Zn2AuAg and Zn2CuAg compounds [2][3] may be traced all the way back to the 1960s. Both compounds have L21 organized and B2 disordered structures, according to these ancient publications. The Zn2AuAg, in particular, shows a B2 to L21 order-disorder transition, as evidenced by changes in structural order characteristics. On the other hand, their applications as FSMAs are limited due to the absence of FM ordering in these alloys, and no further research on these alloys has been conducted. Both alloys now belong within the category of Heusler alloys since the notion of all-d-metal Heusler is widely known.

2. Crystalline Structure

In the recently found Ni2Mn2-yTiy and Ni2−yMn2Tiy systems, Ti atoms have the fewest valence electrons (3d24s2) compared to Ni (3d84s2) and Mn (3d54s2), and Wei et al. [1] predicted that Ti would occupy the D site. Both systems crystallize in a B2-type disordered structure, with the Mn excess atoms sharing the D site with Ti atoms in the Ni2Mn2-yTiy system, resulting in a strong AFM coupling due to the Mn(B)–Mn(D) interaction (Figure 1). The main difference with conventional Heusler alloys is the competence between the L21 and the inverse XA phases.
Metals 13 00111 g005 550
Figure 1. Schema of the L21 Heusler crystallographic structure (left) indicating with red lines the cell of the L10 tetragonal martensite (right) for stoichiometric Ni50Mn25Ti25 alloy.
The Ni2−xCoxMn1.4Ti0.6 quaternary series was discovered by Wei et al. [4], who employed the strategy of introducing Co atoms at Ni sites to impose FM long-range ordering on the Ni–Mn–Ti system, resulting in the first FSMAs among all-d-metal Heusler alloys. Partially replacing Co atoms in Ni2MnZ systems has previously been investigated [5], resulting in a strong local Mn(B)–Co(A/C)–Mn(D) exchange coupling with FM ordering [6], overcoming the Mn(B)–Mn(D) AFM coupling inherent in the B2-type disordered lattice. Moreover, some experimental results observed that strong ferromagnetism provides direct evidence of this probable atomic configuration and of the ferromagnetic activation effect in the Ni(Co,Fe)–Mn–Ti all-d-metal Heusler alloys. For example, in Ni50−xCoxMn35Ti15 alloys, Co atoms that have been substituted for Ni atoms will also share the (A,C) sites with Ni atoms, leaving Mn and Ti with fewer valence electrons at the B/D sites. With the aid of the strong FM exchange interactions between nearest-neighbor Co-Mn atoms, the original AFM exchange coupling between Mn-Mn atoms in Ni-Mn-Ti alloys is converted into FM one, resulting in parallel alignment of the Mn-Co-Mn moments [4]. This phenomenon is known as the “ferromagnetic activation effect of the Co atom” [6]. FM ordering in Ni2−xFexMn1.4Ti0.6 alloys [7], in which Fe substitutes Co in the exchange coupling, is generated via a similar mechanism. Co (3d74s2) and Fe (3d64s2) atoms are considered to share the (A/C) sites with Ni (3d84s2) atoms in both series since their valence numbers are greater than those of Mn and Ti. Feng [8] used theoretical calculations on the X2MnTi (X = Pt and Pd) series to study the impact of Ti as a p-group atom replacement. The findings show that the L21 crystallographic structure is energetically stable for both compositions, with high valence Pd (4d85s2) and Pt (5d86s2) filling the (A/C) sites and Mn (3d54s2) occupy the (B) site, respectively. Ti prefers to remain in the D site. Han et al. [9] developed research for all-d-metal Heusler alloys X2−xMn1+xV (X = Pd, Ni, Pt, Ag, Au, Ir, Co; x = 1, 0). They looked at the atomic occupancy of these alloys in the cubic phase and discovered that the well-known site preference criterion does not apply to all of them. Han et al. [10][11] and Wang et al. [12] studied other Zinc-based all-d-metal Heusler ZnCdTMn combinations by theoretical calculations on the Zn2YMn series. As a result of its complete 3d occupied state, the Zn atoms behave as a major group element, and Zn atoms prefer to occupy the D site rather than replacing Pd atoms at site A. The phenomenon of this process is the whole 3d shell of the Zn atom.
In Figure 1, as the content of Ti was usually lower than 25 at.%, some Mn atoms are located on D sites by substituting Ti atoms. However, the martensitic crystallographic structure can be more complex, including modulation, as shown in Figure 2.
Metals 13 00111 g006 550
Figure 2. (a) Cell of the seven-layer modulated martensite phase (14M) of an arbitrary composition sample seen in 3D and laterally. (b) Cell of the five-layer modulated martensite phase (10M) of an arbitrary composition sample seen in 3D and laterally (generated with MAUD-free software, version 2.08, Luca Lutterotti, Trento, Italy).

3. Influence of d–d Hybridization

Theoretical calculations revealed that stoichiometric Ni2MnTi displays mechanical properties superior to those of conventional Heusler systems. Yan et al. [13] later discovered that the Ni2.0Mn1.27Ti0.73 composition has a significant eCE (under 600 MPa of uniaxial stress, adiabatic temperature change ΔTad = −20.4 K). Many Ni–Mn-based systems, such as Ni1.80Mn1.76Sn0.44 (ΔTad = −11.6 K unloading 600 MPa of uniaxial stress) [14] and Ni2.0Mn1.11Ga0.89 (unloading 100 MPa of uniaxial stress, ΔTad = −6.1 K) [15], have lower values. Changing the chemical bonding character of the Ni2.0Mn1.27Ti0.73 all-d-metal Heusler alloy by substituting high p–d hybridization with somewhat weaker d–d hybridization among transition metals increases its ductility, according to further examination of the electron localization function. Pugh’s ratio [16] is a popular metric for determining solid ductility. It is defined as the ratio of the bulk modulus B to the shear modulus G of a material. Values greater than 1.75 indicate ductile behavior. Researchers see that Ni2MnTi is more ductile than Ni2MnGa [17] and Ni2MnIn [18], two of the most promising FSMAs among Heusler systems, according to the results. Furthermore, Ni2MnTi [19] also has the highest Cauchy pressure, indicating that chemical bandings are weakly covalent. As a consequence, reducing covalent p–d hybridization in Ni2MnZ Heusler alloys is connected to enhanced ductility.

4. MCE in All-d-Metal Heusler Alloys

Until recently, only a few investigations have focused on Heusler alloys of this type. Wei et al. [1] recently discussed the creation of all-d-metal alloys that exclusively include 3d transition metal components. They found that adding Ti to Ni–Mn aids in the B2 phase creation and stability. Cong et al. [20] achieved a massive elastocaloric impact in Ni–Mn–Ti alloys, with a ΔTad = 31.5 K and ΔSM = 45 Jkg−1K−1, at a pressure of 700 MPa. Yan et al. [11] also discovered that the bulk Ni50Mn31.75Ti18.25 alloy has outstanding mechanical characteristics, with a stress of 1.1 GPa and a convincing compressive strain of 13%, respectively. Despite its exceptional elastocaloric sway, the Ni–Mn–Ti combination has a modest MCE when compared to Heusler alloys. The absence of magnetic contrast between the austenite and martensite phases is largely responsible for this. Yan et al. [13] examined the austenite phase’s antiferromagnetic condition in Ni–Mn–Ti alloys. As a result, some thought has been given to doping the elements to improve the magnetocaloric impact. Furthermore, according to Wei et al. [4], cobalt doping in the Ni50Mn35Ti15 alloy affects the transition from AFM to FM in the austenite phase. They created the Mn–Co–Mn configuration by replacing Co for Ni sites in the austenite phase, resulting in the ferromagnetic activation effect. Additionally, the AFM state of austenite in Ni–Mn–Ti alloys was confirmed by Yan et al. [13] and Wei et al. [4]. Therefore, some researchers have attempted to improve its MCE by means of doping elements. Li et al. [21] indicated that under a magnetic field of 3 T, Fe and Co doping in the Ni–Mn–Ti alloy could produce the refrigeration capacity (RC) of 79.5 Jkg−1 and ΔSM of 8.4 Jkg−1K−1. In addition, taking into account the magnetostructural coupling in the Ni2−xFexMnTi [15] and Ni–Co–Mn–Ti [7] Heusler alloys, a giant MCE was observed. Recently, Aznar et al. [22][23] revealed the B-doped Ni50Mn31.5Ti18.5 all-d-metal Heusler alloy. These alloys created an excellent BCE with an RC up to 1100 Jkg−1 and ΔSM of 74 Jkg−1K−1 under the pressure of 3.8 kbar, undergoing MT with an enormous volume change (ΔV) in its PM. Due to the eCE and MCE coexisting near RT, these alloys must be candidates for magnetic refrigeration.

5. Perspectives

Recently, experimental efforts targeted at increasing the MCE of all-d-metal Heusler alloys are worth emphasizing. One option is to add more elements to analyze multicomponent specimens and the effect of the combination of four or fifth elements in the functional response in a similar pathway to the applied to conventional Heusler alloys. Taubel et al. [24] revealed that an ideal annealing technique sharpened the phase transformation in the quaternary Ni2−xCoxMn2−yTiy series, while the Co content governs the transition’s sensitivity to the external magnetic field. The direct MCE is particularly strong in the Ni1.40Co0.60Mn1.48Ti0.25 Heusler alloy (ΔT = 4.0 K and ΔSiso = −20.0 Jkg−1K−1 under an applied magnetic field of 20 kOe). Li and coworkers [25] made a significant addition to the field by investigating the Ni1.4Co0.6−xFexMn1.4Ti0.6 series experimentally. The partial replacement of Fe atoms with Co atoms decreases the d–d hybridizations, in this case, changing the Curie temperature and MT temperatures of the system. The maximum direct MCE value (ΔSiso = −20.0 Jkg−1K−1 under a field of 50 kOe) was found for the x = 0.024 composition. By applying hydrostatic pressure (0.35 GPa), the functional response is improved, achieving values of ΔSiso = −24.20 Jkg−1K−1 and RC = 347.26 JK−1 (under a field of 50kOe) [19]. Because this system has both direct and inverse MCE, both demagnetization and magnetization processes may be investigated for solid-state refrigeration.
It should be remarked that these materials are also under study for applications such as catalysis with chemically diverse surface configurations [26]. These materials can also have high magnetoresistance [27] and spintronic properties [28].
Researchers advocate focusing future research on the atomic site occupancy rules of these kinds of alloys. First principles and density functional theory (DFT) studies will be useful in understanding atomic site influence on the total magnetic moment per formula unit [24][25][26][27][28][29]. It is known that a large reversible magnetocaloric effect and high magnetoresistance were achieved by improving the crystallographic compatibility between the austenitic and martensitic phases [30]. Likewise, the influence of d–d hybridizations on atomic ordering may be shown by evaluating structural and magnetic properties when the quantity of the p-atom group decreases. Furthermore, the impact of thermal annealing, applied pressure as well as several synthesis methods on the creation of the L21 phase should be further investigated. Finally, because of the increased mechanical ductility and high induced strain values observed, the shape memory impact of ternary Ni2xMn1+xTixand Mn2xNi1+xTix series should be further investigated.

References

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  2. Murakami, Y.; Watanabe, Y.; Kachi, S. An X-ray Study of the Heusler-type Ordering in AuAgZn2 Alloy. Trans. Jpn. Inst. Met. 1980, 21, 708–713.
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  26. Samanta, S.; Ghosh, S.; Chatterjee, M.K. Large magnetocaloric effect and magnetoresistance in Fe-Co doped Ni50-x(FeCo)xMn37Sn13 all-d-metal Heusler alloys. J. Alloys Compd. 2022, 910, 164929.
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