Lattice Distortion Effects on the Magnetostructural Phase Transition of MnAs

Abstract

We present a systematic experimental and theoretical study of the first-order phase transition of epitaxially grown MnAs thin films under biaxial tensile stress. Our results give direct information on the dependence of the phase-transition temperature of MnAs films on the lattice parameters. We demonstrate that an increase of the lattice constant in the hexagonal plane raises the phase-transition temperature (T p ), while an increase of the perpendicular lattice constant lowers T p . The results of calculations based on density functional theory are in good agreement with the experimental ones. Our findings open exciting prospects for magneto-mechanical devices, where the critical temperature for ferromagnetism can be engineered by external stress. DOI: 10.1103/PhysRevLett.95.077203 PACS numbers: 75.70.Ak, 61.50.Ks, 68.60.2p MnAs presents a first-order phase transition at 40 C, changing from ferromagnetic/hexagonal ( -phase NiAs structure) to paramagnetic/orthorhombic ( -phase MnP structure) [1]. This magneto-structural phase transition has important implications for technological applications. The magneto-elastic effects are useful for transducers [2], while their magneto-caloric properties are interesting for developing refrigeration devices [3]. In recent years, the attention given to MnAs has been strongly amplified by the possibility of epitaxial growth on GaAs substrates [4]. The integration of ferromagnetic materials with semiconductors is a subject of great interest for spintronics, and MnAs grown on GaAs is a strong candidate for spin injection devices [5].From the theoretical point of view, the treatment of the first-order phase transition of materials with magnetoelastic properties is a rather complex issue. Early simple phenomenological thermodynamic treatments based on the localized Heisenberg model [1] have been used to explain the properties of MnAs under an external hydrostatic pressure and magnetic field. Sophisticated band structure calculations are required for a precise quantitative analysis, although in this case it is difficult to introduce a statistical treatment to describe a first-order phase transition [6].We present an experimental and theoretical investigation of the magneto-structural phase transition of MnAs films grown on GaAs. Those films present a nonabrupt phase transition with the coexistence [7] of the two phases in form of periodically alternating stripes [8,9] for a large temperature range ( 20 C) [7][8][9][10]. As a result of this phase coexistence a considerable fraction of the volume of the MnAs epitaxial films is usually in the paramagnetic phase at 30 C, which is a strong limitation for room temperature spintronic devices. The growth of MnAs films on different crystal orientations has been suggested as an alternative that can provide higher phase-transition temperatures [10]. The detailed mechanism that associates the crystal distortion (lattice parameter variation) with the phase-transition temperature is, however, still unclear, This issue was addressed in the ear...

Ab initiostudy of the magnetostructural properties of MnAs

“…The results for K a and K b differ slightly from the ones given in Ref. 8, since in that case we did not constrain K a to be equal to K b . If the cell changes only its volume, the expansion corresponds to the one of Eq.…”

“…This is mainly attributed to the induced strain. 1,3,7,8 The aim of this paper is to use ab initio density functional theory ͑DFT͒ calculations to develop an understanding of the phase transitions of MnAs, which can be compared with experiments and with existing phenomenological models.…”

Semiclassical electronic transport in MnAs thin films

“…MnAs, which is a ferromagnetic metal at room temperature, is on the focus of attention because it can be successfully integrated with semiconductors such as GaAs [1], Si [2] and ZnSe [3]. Several magnetic and structural characterizations as a function of temperature have been done on MnAs [4][5][6], but the magneto-electrical transport properties remain still unexplained.…”

Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature

“…Kaganer et al [11] have shown that the epitaxial strain leads to the α − β phase coexistence to minimize the elastic energy. Strain dependent magnetic properties were analyzed by Das et al [12] and Iikawa et al [13] in the case of MnAs epitaxied on GaAs(001). The unit cell of the ferromagnetic α-phase was found to be orthorhombically distorted, as the hexagonal plane of MnAs lies out of the substrate surface.…”

“…We anticipate that larger T (2) C critical temperature may be obtained by increasing the tensile strain in the film plane. This could be achieved by applying external strain to the substrate similarly to the experiments performed by Iikawa et al [13]. For example, if we apply a tensile strain of 0.5% in the film plane, it would be possible to increase the critical temperature to T (2) C =373 K [19].…”