Metal-insulator Transition and Magnetic Domains in (Ga,Mn)As Epilayers
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0941-Q07-11
Metal-insulator Transition and Magnetic Domains in (Ga,Mn)As Epilayers Alexandre Dourlat1, Catherine Gourdon1, Vincent Jeudy1, Frédéric Bernardot1, Christophe Testelin1, Emmanuelle Lacaze1, Laura Thevenard2, Olivia Mauguin2, Ludovic Largeau2, Gilles Patriarche2, and Aristide Lemaître2 1 Institut des Nanosciences de Paris, Universities Paris 6 and Paris 7, CNRS, 140 rue de Lourmel, PARIS, 75015, France 2 Laboratoire de Photonique et Nanostructures, CNRS, Route de Nozay, MARCOUSSIS, 91460, France
ABSTRACT We investigate the magnetic domain pattern in (Ga,Mn)As epilayers with perpendicular magnetic anisotropy. We show that post-growth annealing, besides improving the magnetic and transport properties, also drastically changes the domain pattern. Strong pinning of domain walls along the directions is suppressed and large domains are observed.
INTRODUCTION In the diluted magnetic semiconductor (Ga,Mn)As, the ferromagnetic alignment of Mn spins originates from the sp-d exchange coupling of Mn spins with carriers [1] . A remarkable feature of the magnetic properties of (Ga,Mn)As layers is the persistence of the ferromagnetic phase even below the metal-insulator transition [1]. However, strong modifications of the magnetic properties between these two regimes are expected, in particular the magnetic domain patterns. It has been shown that annealing of (Ga,Mn)As epilayers with in-plane magnetic easy axis improves their transport properties, increases their Curie temperature [2] and changes the magnetic domain structure [3]. Here we investigate the effect of annealing in (Ga,Mn)As layers with perpendicular anisotropy. SAMPLES (Ga,Mn)As epilayers were grown by molecular beam epitaxy on a thick relaxed (Ga,In)As buffer deposited on GaAs (001) in order to favor perpendicular magnetic easy-axis. The (Ga,In)As buffer is designed to minimize the amount of threading dislocations emerging in the magnetic layer. It contains a graded part in order to prevent the formation of a too large amount of misfit dislocations, which act as sources of threading dislocations propagating along directions towards the surface, as usually observed in abrupt mismatched interfaces. For sample 1, a 0.5 µm graded (Ga,In)As buffer with indium concentration increasing from 0 to 16% was first grown. It was followed by a 1 µm Ga0.84In0.16As layer and a 200 nm (Ga,Mn)As layer with a 5% nominal content of Mn. No post-growth treatment was done. For sample 2, the buffer layer consists first of a 0.5 µm thick layer with In content increasing from y = 0 to 9.8 %. Then
2-3 µm of Ga0.902In0.098As were grown on top of it, before depositing a 50 nm thick Ga1−xMnxAs layer. The Mn concentration is 7%. The substrate temperature was set to 400 °C during the growth of the buffer in order to avoid the formation of three-dimensional strain-induced islands. The Ga1−xMnxAs layer was deposited at 250 °C. A part of the sample was annealed under nitrogen atmosphere for 1 hour in a tube furnace at 250°C to out-diffuse the interstitial Mn atoms, in order to improve
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