Ferromagnetic MnAs is a promising candidate for electrical spin injection into GaAs and Si based semiconductors, since it exhibits a large carrier spin polarisation, small coercive field and relatively high saturation magnetisation. Bulk MnAs is ferromagnetic at RT (α-phase) and shows near 40°C a second order transition to the paramagnetic β-phase. On contrary, epitaxial MnAs films on GaAs substrate show at RT the coexistence of both phases due to the large and anisotropic lattice mismatch. Elastic domains of both phases form a periodic stripes pattern in a self organised way [1]. The nature of the nonmagnetic phase, which coexists in thin films is of great interest[2]. Until recently β-MnAs has generally been considered to be paramagnetic but the effects just mentioned and first principles calculations[5] suggest antiferromagnetism.

Stimulated by this controversy, we have recently reexamined our XMCD-PEEM [6],[7] results and have complemented them with corresponding linear dichroism (XMLD) studies. We present the results of this comparison and will demonstrate that β-MnAs is antiferromagnetic. These important results end a more than 50 year old scientific controversy and allow to give a new revisited description of the magnetic behaviour in striped MnAs thin films.

XMCDPEEM image (left), showing the magnetic domain structure in the ferromagnetic phase and XMLDPEEM image (right), showing the coexisting antiferromagnetic phase, of a 300 nm thick MnAs film on GaAs (100). Diameter of field of view 5 μm.

[1] T. Plake et al, Appl. Phys. Lett. 80, 2523 (2002).

[2] L. Däweritz, Rep. Prog. Phys. 69, 2581 (2006)

[3] K.-S. Ryu et al, Phs. Rev. B 71, 155308 (2005)

[4] J. Mira et al, Phys. Rev. Lett. 90, 097203 (2003)

[5] M. K. Niranjan, B.R. Sahu and L. Kleinman, Phys. Rev. B 70, 180406 (2004)

[6] E. Bauer et al. J. Vacuum Sci. Technol. B 20, 2539 (2002)

[7] L. Daeweritz et al, J. Vacuum Sci. Technol. B 23, 1759 (2005)

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1. Magnetic frustration in “Spin-Ice” nanostructures networks.