[en] We demonstrate dipolar correlation in the orientation of magnetization in square arrays of MnAs nanodisks fabricated from epitaxial layers on GaAs(001). The MnAs(1100) layers possess strong in-plane uniaxial magnetocrystalline anisotropy, which enables us to discard microvortex-type ordering from degeneracy while maintaining the disks as circularly shaped with a small diameter. The autocorrelation function reveals antiferromagnetic collinear arrangements of the magnetic moments in the arrays both in the single-domain and flux-closure-state regimes of the disks. The interaction range is deduced to be nearly identical to the period of the arrays

[en] The discontinuous change in the lattice constant that occurs at the first-order phase transition between α- and β-MnAs gives rise to a coexistence of the two phases in MnAs layers grown on GaAs substrates. When the GaAs substrates are oriented in the (111)B direction, the c axis of MnAs is aligned normal to the growth plane. We identify the domain structure of α- and β-MnAs for this crystal orientation by utilizing the different reactivities of the two phases against wet chemical etching. Submicrometer-size islands of α-MnAs are found to be interwoven in a honeycomblike network of β-MnAs. We also show that this domain structure combined with strain effects results in a formation of MnAs lumps by etching

[en] We explore wet and dry etching processes of thin MnAs layers grown on GaAs(001) substrates for microstructuring. Most of the common wet chemical etch solutions for GaAs react with MnAs strongly and in a peculiar manner. Unidirectional cracks are generated when the MnAs layers are thicker than 100nm. We demonstrate that the crack generation can be avoided by choosing a suitable etch solution or etch temperature. We fabricate submicrometer-wide MnAs wires using Ar ion milling. The resistivity of the narrow channels is measured over a temperature range covering the phase transitions in MnAs between the α, β, and γ phases. The resistivity along the MnAs[0001] direction is found to be smaller than that along the MnAs[112-bar 0] direction regardless of the phase. A nearly linear temperature variation of the phase fraction is deduced in the α-β phase coexistence regime. The temperature coefficients of the resistivities are negative for the nonmagnetic phases