Research Article
Observation of magnetic domain evolution in constrained epitaxial Ni–Mn–Ga thin films on MgO(0 0 1) substrate

https://doi.org/10.1016/j.jmst.2021.06.029Get rights and content

Highlights

  • The evolutions of the magnetic domain of a typical epitaxial Ni–Mn–Ga thin film were investigated through wide-field magneto-optical Kerr-microscopy.

  • The abrupt magnetization changes in the hysteresis loops can be observed in the Type-Y area with the applied magentic field parallel to the twin interfaces.

  • The abrupt magnetization changes in the hysteresis loops should be attributed to the magnetic domain evolution instead of the magnetically induced variant reorientation.

Abstract

Epitaxial Ni–Mn–Ga thin films have promising application potential in micro-electro-mechanical sensing and actuation systems. To date, large abrupt magnetization changes have been observed in some epitaxial Ni–Mn–Ga thin films, but their origin - either from magnetically induced martensite variant reorientation (MIR) or magnetic domain evolution - has been discussed controversially. In the present work, we investigated the evolutions of the magnetic domain and microstructure of a typical epitaxial Ni–Mn–Ga thin film through wide-field magneto-optical Kerr-microscopy. It is demonstrated that the abrupt magnetization changes in the hysteresis loops should be attributed to the magnetic domain evolution instead of the MIR.

Introduction

Ni–Mn-based ferromagnetic shape memory alloys are conceived as promising candidate materials for micro-sensors and actuators [1], [2], [3], [4], solid-state cooling [5], [6], [7], [8], [9], thermal energy harvesting [10] and thermomagnetic generator systems [11], owing to their multifunctional behaviors under external thermal-, magnetic-, and/or stress-field [12], [13], [14], [15], [16]. Especially, the functional property of large magnetic field-induced strain with a fast dynamic response, being resulted from the magnetically induced reorientation (MIR) of martensite variants, has been extensively explored on Ni–Mn–Ga alloys [17], [18], [19]. For instance, giant magnetic field-induced strains as high as 6%-12% can be achieved in the single-crystalline alloys [17], [18], [19]. Such giant strains have also attracted great interest in epitaxial Ni–Mn–Ga thin films were grown on MgO(0 0 1) substrate [20], [21], [22], [23], as they are of single-crystalline in the austenite state during deposition at elevated temperatures and transform to the self-accommodated martensite upon cooling.

Similar to the case of bulk Ni–Mn–Ga alloys, an initial austenite orientation in epitaxial thin films can produce 24 modulated martensite variants with different crystallographic orientations that are self-organized into 6 martensite groups, each having 4 twin-related variants in a plate shape [24], [25]. However, due to the specific crystallographic orientation of austenite in the epitaxial Ni–Mn–Ga thin films, the martensite groups demonstrate two characteristic organization manners, denoted as the so-called Type-X and Type-Y patterns [23, 25]. In the former, the twin interfaces are inclined to the substrate, and the easy magnetization axes of two variants are in-plane and those of the other two are out-of-plane. In the latter, the twin interfaces are oriented perpendicular to the substrate, and the easy magnetization axes of the variants are parallel to the film plane [23]. In all cases, individual variants are very fine with widths of several tens of nanometers, which is lower than the resolution of the optical microscope. The only way to validate the MIR assessment is to provide direct microstructural evidence. However, the ultrafine microstructural constituents in the thin film make the direct observation of MIR almost impossible. Thus, it represents a challenging task to characterize the occurrences of variant reorientation under a magnetic field in the epitaxial Ni–Mn–Ga thin films.

The pioneer efforts were made by examining the features of magnetization hysteresis (M-H) loops [20, 23, 26]. It was found that when the magnetic field was applied in the film plane and along the traces of twining interface in the Type-Y pattern (the film contained 80% Type-Y regions), an abrupt magnetization change as large as 55% appeared in the M-H loop [23]. As the MIR can bring a collective rotation of the atomic magnetic moments of the reoriented variants, the abrupt magnetization change was naturally interpreted as a result of MIR. It should be noted that the value of magnetization M for each H in the M-H loop is a sum of the individual magnetization value of each martensite variant. The abrupt magnetization change could also be a result of magnetic domain evolution. Recently, a theoretical micromagnetic simulation has been conducted on the epitaxial Ni–Mn–Ga thin films [27]. It was demonstrated that the abrupt magnetization changes in the M-H loops might result from the evolution of magnetic domains rather than the MIR of martensite variants [27].

Besides, the switching magnetic field (0.08 T) for the abrupt magnetization change in the epitaxial Ni–Mn–Ga thin films was much smaller than that for the MIR in the single-crystalline alloys (0.25 T) [20, 23, 26, 28, 29]. Such a small switching magnetic field may not be large enough for the MIR. Therefore, it needs to be further clarified whether the abrupt magnetization changes in the M-H loops of the <0 0 1>A oriented epitaxial Ni–Mn–Ga thin films was originated from MIR or magnetic domain motion. In this context, the direct observation of the magnetic domain and microstructure evolution under an applied magnetic field is indispensable to unweave the doubt and helpful for the development of Ni–Mn–Ga thin films.

Normally, the magnetic force microscopy is one of the conventional approaches to observe the magnetic domain. However, this technique can hardly distinguish the magnetic domains in the Type-Y regions in the epitaxial Ni–Mn–Ga thin films, since the easy magnetization axes of martensite variants in those areas are oriented in-plane [23, 30], thus providing no stray fields. The advanced wide-field magneto-optical Kerr-microscopy [31], [32], [33], [34] that allows directly observing in-plane magnetic contrast has already been successfully applied to the characterization of magnetic domains in the bulk Ni–Mn–Ga alloys [35], [36], [37]. The wide-field magneto-optical Kerr microscopy (plotting the average image grey level in one selected region as a function of the applied magnetic field) provides an opportunity for local magnetometry with independent analysis of different regions [38], [39]. Thus, it opens a new venue for exploring the interactions between martensite microstructure and magnetic domain structure in multiferroic Ni–Mn–Ga martensite. In the present work, in order to elucidate the nature of the abrupt magnetization changes in the M-H loops of the epitaxial Ni50.3Mn28.2Ga21.5 thin films grown on MgO(0 0 1) substrate, the Kerr microscopy was employed to capture martensite variant reorientation and magnetic domain evolution under external magnetic field.

Section snippets

Sample preparation

Ni50.3Mn28.2Ga21.5 thin films with a thickness of 500 nm were grown on MgO(0 0 1) mono-crystalline substrates at 650°C by DC magnetron sputtering. The composition of the target material is Ni48Mn30Ga22. During the deposition, a Cr layer with a thickness of 50 nm was firstly sputtered on the MgO substrate. The base pressure for the deposition of the Ni–Mn–Ga thin films and the Cr layer was set below 9.0 × 10−5 Pa. To obtain a continuous film, the deposition was conducted under a constant Ar

Structure and microstructure

The phase constituents and crystal structures of the present Ni50.3Mn28.2Ga21.5 thin films were determined with X-ray diffractometer using Cu-Kα radiation (λ = 0.15406 nm). Considering that the thin films may have in-plane texture, the θ-2θ coupled scans were performed between 30° and 80° at tilt angle (Psi) ranging from 0° to 10° with a step size of 1°, under two azimuth angle positions with PHI equal to 0° and 45°. As shown in Fig. 1, X-ray diffraction characterization confirmed that the thin

Conclusions

In summary, the evolutions of magnetic domains and microstructures of epitaxial Ni–Mn–Ga thin films under the external magnetic field have been investigated by means of wide-field magneto-optical Kerr-microscopy. The abrupt magnetization changes in the M-H loops, usually recognized as a sign for the MIR, were proven to be the result of the magnetic domain evolution in differently oriented martensite variants of the Type-Y regions at the film surface, rather than that of MIR. Thus, any analysis

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grants Nos. 52071071), the Liaoning Revitalization Talents Program (Grant No. XLYC1802023) and the Fundamental Research Funds for the Central Universities of China (Grant Nos. N2102006), and the Program of Introducing Talents of Discipline Innovation to Universities 2.0 (the 111 Project of China 2.0, No. BP0719037). Fruitful discussions with Professor Houbin Huang at Beijing Institute of Technology and Professor

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