Research articles
Magnetization enhancement in B-doped Heusler alloys Fe2MnSi1−xBx (x = 0–0.4)

https://doi.org/10.1016/j.jmmm.2020.167161Get rights and content

Highlights

  • New Fe2MnSi1−xBx Heusler alloys have been synthesized.

  • B can enter substitutional or interstitial site, depending on its content.

  • B substitution can enhance ferromagnetism in Fe2MnSi obviously.

  • Mechanism for this enhancement was discussed in detail.

Abstract

A series of Fe2MnSi1−xBx (x = 0–0.4) Heusler alloys was synthesized and investigated. The variation of lattice parameters with B content suggests that B prefers entering the substitutional site of Fe2MnSi when the amount is small. But when B content is high, it enters the interstitial site. A spin reorientation is observed at ~54 K in Fe2MnSi, which moves to low temperatures with increasing B content and disappears when x = 0.3. Both the Curie temperature TC and saturation magnetization Ms increase with the substitution of B for Si. The Ms gets closer to the calculated total spin moment Mt. All this suggests that the dope of B can enhance the ferromagnetic character of Fe2MnSi1−xBx. For the high B content, B-doping expands the lattice and enlarges the distance between magnetic atoms. This leads to the increase of Fe and Mn spin moments. The substitution of B for Si has an ordering-effect for Fe2MnSi lattice by forming a strong covalent bonding and relieves the possible Fe-Mn disorder. This effect also contributes to the increase of Ms.

Introduction

Fe2MnSi is a well-known Heusler alloy with ordered cubic L21 structure. In its lattice two Fe atoms occupy the A (0, 0, 0) and C (0.5, 0.5, 0.5) sites and Mn, Si occupy the B (0.25, 0.25, 0.25) and the D (0.75, 0.75, 0.75) sites, respectively [1]. Fe2MnSi has been predicted to be a half-metal in literature [2], [3], [4]. In the electronic structure of Fe2MnSi, the minority spin channel has an energy gap at the Fermi level EF like a semiconductor but the majority spin channel has non-zero states at EF. This specific electronic structure leads to a 100% spin polarization of the conduction electrons at EF and has potential applications in spintronics [5]. Many experimental and theoretical works on Fe2MnSi and related alloys have been reported now. Fe2MnSi thin films were grown on Ge (1 1 1) using MBE method [6]. A high spin polarization ratio was reported in Co-doped Fe2MnSi [7]. Fe2MnSi1−xGax alloys were also reported to be possible multifunctional Heusler alloys with enhanced magnetocaloric effect, high Curie temperature TC and half-metallicity [8]. To improve its performance, the electronic structure and half-metallicity of doped Fe2MnSi have also been reported [9], [10].

However, one problem for Fe2MnSi is the large difference between its saturation magnetization Ms at 5 K and the theoretical results. It is known that, for half-metallic Heusler alloys, their total spin moments follow the Slater-Pauling curve. In Fe2MnSi, it is M = Z − 24, where M is the total spin moment and Z is the total number of valence electrons [5]. With 27 valence electrons (16 from Fe, 7 from Mn and 4 from Si), Fe2MnSi should have a total moment of 3.0 μB/f.u. However, its Ms at 5 K is only 2.0 μB [11]. This implies that the half-metallicity in Fe2MnSi may be disturbed and more investigations are necessary. One possible reason for the difference is the antiferromagnetic (AFM) component at low temperatures. A spin reorientation is observed at ~60 K in Fe2MnSi and the Mn-sublattice becomes antiferromagnetic coupled below the spin reorientation temperature TR [1], [12]. According to the work of Hiroi et al. in Fe2Mn1−xVxSi, this AFM phase can transform to a ferromagnetic (FM) phase in a high magnetic field, which makes the Ms get close to the expected 3.0 μB/f.u. [13].

Another reason for the deviation is the atomic disorder. Tedesco et al. investigated Fe2MnSi1−xGax by using magnetic measurement and neutron diffraction, and observed the existence of Fe-Mn disorder in Fe2MnSi lattice [14]. The work of Hu et al. [15] also reported the influence of Fe-Mn disorder on the electronic structure and magnetic properties of Fe2MnSi. All these above require possible methods of adjusting the atomic ordering and magnetic structure of Fe2MnSi.

In this paper, we substituted B for Si and prepared a series of Fe2MnSi1−xBx (x = 0–0.4) Heusler alloys. The effect of B doping on the formation, structure, and magnetic properties of Fe2MnSi was investigated. In order to suppress the AFM phase and stabilize the half-metallicity in Fe2MnSi, people have tried Co or Cr doping in Fe2MnSi and obtained interesting results [16], [17]. But the experiments on main group elements doping are still not so much. Here B atom has a much smaller atomic radius compared with other main group elements like Al, Ga and Si. Therefore, when replacing Si, B may enter both substitutional and interstitial sites and has different influence on the magnetic properties [18]. At the same time, B is more electronegative than Si, which can enhance the p-d covalent hybridization and has influence on the atomic ordering in Fe2MnSi1−xBx alloys. These results can help to design more B-based Heusler alloys and explore their applications.

Section snippets

Experimental and theoretical methods

The Fe2MnSi1−xBx (x = 0–0.4) ingots were prepared by arc-melting the high purity raw materials at least four times under the protection of argon atmosphere. The B content was introduced in Fe-B alloy form for homogenization. Then the ingots were annealed at 1123 K for 72 h in an argon atmosphere, finally quenched to water. Crystal structure and lattice parameters of the samples were determined by X-ray powder diffraction (XRD) with Cu-Kα radiation. The magnetic properties of Fe2MnSi1−xBx

Results and discussions

In Fig. 1, we presented the powder XRD patterns of Fe2MnSi1−xBx (x = 0–0.4) samples. When x = 0–0.2, the diffraction peaks can be indexed successfully with powderX software (Dong, 1999 [23]) based on the cubic L21 structure, which suggesting that the samples are fine single Heusler phase. In the supplementary material, some detailed indexing results were presented as examples to give more information. The main diffraction peaks (2 2 0), (4 0 0) and (4 2 2) for the cubic structure are sharp and

Conclusions

In this paper, a series of Fe2MnSi1−xBx (x = 0–0.4) Heusler alloys was synthesized. When x  < 0.1, B enters the substitutional site of Fe2MnSi and leads to the decrease of lattice parameter. But when x = 0.2–0.4, the lattice parameter increases with increasing B content, suggesting that B enters the interstitial site when its content is high. The spin reorientation in undoped Fe2MnSi at 54 K moves to low temperatures with B-doping and disappears when x  ≥ 0.3. The Curie temperature TC increases

CRediT authorship contribution statement

Fanbin Meng: Investigation, Writing - review & editing. Shuang Liu: Investigation, Writing - original draft. Kaichen Sun: Formal analysis. Ruirui Gao: Investigation. Xiangyu Shi: Visualization. Hongzhi Luo: Conceptualization, Methodology.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work is supported by Natural Science Foundation of Hebei Province [Grant No. E2018202097] and [Grant No. E2019202143].

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

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