First magnetic and structural microscopy study of a striped phase transition in a thin ferromagnetic film | ||
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E. Bauer1,2, S. Cherifi2, L. Daeweritz3, M. Kaestner3, S. Heun2 and A. Locatelli2
The desire to combine ferromagnetic and conventional semiconductor materials into novel devices that make not only use of the charge but also of the spin of the electron has stimulated in recent years intensive search for ferromagnetic layers suitable for spin injection into semiconductors. One of these ferromagnetic/semiconductor systems of current interest is epitaxial MnAs on GaAs. Although the ferromagnetism of hexagonal α-MnAs has been known for a long time [1], detailed studies of epitaxial MnAs layers on GaAs started only recently [2-6]. These studies have revealed that the phase transition near room temperature from the paramagnetic orthorhombic β phase above 40°C to the ferromagnetic hexagonal α phase below 40°C is rather complex due to the epitaxial strain that leads to the coexistence of the two phases over a temperature range of nearly 30°C. This phase transition has been studied up to now only with a laterally averaging technique (X-ray diffraction) [6]. Laterally resolving studies (AFM and MFM) of the intermediate state at room temperature [2, 6] and of the temperature-dependent surface corrugation [7] revealed an interesting striped phase that stimulated the present study. In this study we have used XMCDPEEM, LEEM and LEED in order to understand how structure and magnetic order change with temperature in a closely coupled two-phase system. The samples were prepared as described in Ref. [2]. After desorption of the As capping layer LEED showed a (1x2) diffraction pattern. The phase transition studied was not influenced noticeably by further heating to higher temperatures that produced a (1x1) and a (2x1) LEED pattern. Fig. 1 shows the evolution of the magnetic structure as imaged with XMCDPEEM (a-d) and of the crystal structure as imaged with LEEM (e-g) with increasing temperature. In the fully magnetized single-domain state no magnetic and structural contrast is seen. In the early phase of the transition discontinuous stripes of the paramagnetic β phase form (bright regions in a and e). With increasing temperature most of these stripe segments grow together into continuous stripes and broaden at the same time, separating the ferromagnetic α phase into stripes and thinning these stripes. At a width of about 120 nm domains with opposite magnetization nucleate in these stripes (b), multiply and grow in width (c). The width of the ferromagnetic stripes decreases further until, at about 80 nm width, the stripes break up into individual pieces. Simultaneously the magnetization in the domains decreases as seen in the decreasing signal to noise ratio (d). At slightly higher temperature all magnetic contrast is lost. The LEEM images taken at similar temperatures (e-h) clearly show that the magnetic transition is intimately connected with the structural transition. This transition is strongly influenced by the misfit strain [4] that causes the phase separation into the striped phase, known from many other strained systems capable of reducing total energy by phase separation. A quantitative analysis of the images is expected to provide valuable information of the structural and magnetic energetics of this system.
References
Correspondence and requests for materials should be addressed to E. Bauer
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