Thickness dependences of structural and magnetic properties of Ni(Co)MnSn/MgO(001) thin films
Introduction
The NiMn-based Heusler magnetic shape memory alloys (MSMAs) are under intensive investigation due to their unusual physical properties and potential applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. In particular Mn-rich NiMn(Sn,In,Sb) shape memory alloys referred in the literature as metamagnetic shape memory alloys (MetaMSMAs), exhibit magnetostructural phase transformation from a ferromagnetic austenite to a weakly magnetic or "non-magnetic" martensite which is accompanied by a huge and steep change of magnetization and entropy (see Ref. [9] and references therein). As a result, they show a giant inverse magnetocaloric effect (MCE) at the martensitic transformation (MT), i.e. the adiabatic application of a magnetic field transforms the martensite into austenite with a decrease of the sample temperature. The inverse MCE is considered as highly promising for the magnetic refrigeration [2], [4], [11], [12]. First principles calculations (see, e. g., [13], [14] and references therein) demonstrated that there is a strong interplay of the antiferromagnetic and ferromagnetic exchange interactions between the magnetic moments of Mn atoms in different sites of the crystal lattice of MetaMSMAs, which results in the dramatic changes of the magnetic properties during MT. In particular, the martensitic transformation leads in some cases to the suppression of ferromagnetism. Recently it has been shown that in such alloys the twin structure produces a nontrivial magnetic state, where ferromagnetic exchange inside the twin components coexists with antiferromagnetic one across the twin boundaries [15].
Generally, structural and magnetic properties of NiMn-based magnetic shape memory thin films can dramatically differ from those in the bulk, particularly, due to the interaction between the film and the substrate [16]. This interaction manifests itself via an internal stress that appears as a result of the misfit between the lattice parameters of the film and substrate for epitaxial films, and/or due to the surface roughness or thermo-elastic mismatch [17], whereas in the bulk state these alloys can be formed practically free of any internal stress. The constrains from the substrate cause a considerable qualitative and quantitative modification of the magnetic anisotropy in the films (see, e.g., [18], [19], [20]). Martensitic transformation (from the cubic austenite phase to the lower symmetry martensitic structure) in thin films cannot occur without twinning, because the surface area of the film on rigid substrate should remain the same during the transformation (a surface area conservation requirement). In the bulk state, owing to a tendency to form a self-accommodated structure for minimization of the elastic energy, all twin variants can be realized. This is not the case for the thin films. Types of the twin variants, that can be formed in films, their size and concentration are governed by the physical parameters of the film and the substrate [21], [22], which, in turn, strongly depend on the previous history of the sample.
It is worth noting, that in epitaxial films MT can be accompanied by formation of submicron periodic twin structures [19], [23], [24]. It was shown experimentally [23], [24] that the period of twin stripe structure in MSMAs obeys universal Landau-Lifshitz-Kittel scaling law and is proportional to square root of the film thickness. This was explained in terms of interplay between elastic volume and interface energies, that favor large twinning periods, and elastic deformation with respect to the substrate, which tends to decrease this period. The rigorous consideration was presented in [25], where using the fictitious dislocations approach it was shown that in lamellar twin structure the highly inhomogeneous strains are mostly concentrated near film/substrate and twin boundaries. A decrease of the period, L, results in the reduction of the stresses on the film interfaces but at the same time it leads to the increase of twin boundaries concentration and their corresponding elastic energy (see the sketch in Fig. 1). The interplay of these two factors results in scaling law which is similar to Landau-Lifshitz-Kittel scaling law as it was stated in [23], [24]. Appearing of submicron twinning structures in MSMA films can lead to the formation of periodic magnetic structures, which is a signature of the magnonic crystals [26]. In this case, the periodicity at submicron level can be achieved without any lithography, only by adjusting the film composition and its thickness, as well as a proper selection of the substrate.
It is worth noting that the stresses arising near the surfaces at MT can modify transition behavior of films in comparison with bulks. The influence of these stresses is especially critical for thin films, where the contribution of the surface elastic energy to the total energy cannot be neglected. This results in the change the total energy of the martensite as illustrated in Fig. 2, leading to the MT temperature shift in films with respect to the bulk (see, e.g., [27] and references therein) or even to the blocking of MT.
Whereas the thickness evolution of the microstructure and properties of FSMA films have been the subject of intensive investigations (see, e.g., [20], [22], [23], [24]), this is not the case for the MetaMSMA films. One can find only two reports about some aspects of the thickness-dependent behavior of NiMnSn/MgO and NiMnSn/W films with thickness either below 100 nm [28] or above 2.9 µm [29], respectively.
In the present work, the role of the thickness, varying from 20 to 1000 nm, in tailoring the twinning structure, transformation and magnetoelastic properties of MetaMSMA epitaxial films is systematically investigated by ferromagnetic resonance spectroscopy (FMR), alongside with synchrotron X-ray diffraction and standard magnetic measurements. Ni(Co)MnSn alloy, a typical MetaMSMA, was selected for the study.
Section snippets
Experimental
Ni-Co-Mn-Sn films with thicknesses of 20, 50, 100, 300, 500 and 1000 nm were deposited by DC magnetron sputtering of the target Ni46.1Co4.9Mn38.9Sn10.1 onto six heated (500 °C) single crystalline MgO (001) substrates using shuttering system, at argon pressure of 1.1·10-2 mbar and the power of 150 W. The 1000 nm-thick film was selected for the most accurate determination of the alloy composition with the energy-dispersive X-ray spectroscopy (EDX). EDX analysis provided the composition of Ni49.0Co
Results and discussion
X-ray diffraction patterns in Fig. 3 show sharp (002) and (004) reflections of the cubic phase for the films with thicknesses down to 100 nm. For the 50 nm-thick film, these reflections are strongly broadened (Fig. 3). For the 20 nm-thick film, they can be hardly discerned on the irregular background scattering, therefore XRD pattern for this film is not shown in Fig. 3. No other peaks of the cubic phase are visible in Fig. 3 indicating a highly textured film growth in the [001] direction.
Conclusions
In the present work, the evolution of the microstructure, transformation behavior and magnetic resonance properties of the epitaxial Ni(Co)MnSn MetaMSMA films have been investigated as a function of the film thickness, in the range 20 – 1000 nm. It has been shown that constraints from the film/substrate interface block the martensitic transformation in the 20 nm thick film. The increase of the films thickness results in a progressive stress relaxation and the martensitic transformation becomes
CRediT authorship contribution statement
V. Golub: Conceptualization, Investigation, Writing - original draft. I.R. Aseguinolaza: Investigation, Visualization. O. Salyuk: Investigation, Visualization. D. Popadiuk: Investigation. I. Sharay: Investigation, Visualization. R. Fernández: Investigation, Formal analysis. V. Alexandrakis: Investigation, Formal analysis. S. A. Bunyaev: Investigation. G. N. Kakazei: Investigation, Formal analysis, Writing - review & editing. J. M. Barandiarán: Writing - review & editing, Supervision. V. A.
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.
Acknowledgements
The financial supports from Ministry of Science, Innovations and Universities (project RTI2018–094683-B-C53-55) and from the Basque Government Department of Education (project IT1245-19) are greatly acknowledged. The work is partially supported by the NAS of Ukraine through the project “Nanostructured magnetic composites for the systems of thermoelectronic control and thermostabilization” (No. 0120U100457). VG and DP are grateful for the support from grant 02.2020/0261 provided National
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