Ferromagnetic GaMnAs is currently being used to investigate prototype spintronics devices. The progress in material fabrication in recent years led to a remarkable increase of Curie temperature (Tc) and improved conductivity of GaMnAs. This made it possible, e.g. to observe the Coulomb blockade anisotropic magnetoresistance effects in ferromagnetic GaMnAs yielding an interesting material system combining nanoelectronic and spintronic properties. Because of large carrier density, typically in the range of 10²⁰ to 10²¹ cm^-3 the depletion of GaMnAs via the field effect is only possible in nanoscale structures. It is therefore desirable to investigate the ferromagnetism of GaMnAs towards ultrasmall thicknesses. It was already reported that the thickness limit for ferromagnetism in single GaMnAs layers is close to 5 nm. So far it is unclear which factors cause the ferromagnetism in very thin GaMnAs layers to vanish. To address this problem we have investigated single GaMnAs layers with thicknesses between 2 and 5 nm. Samples were grown by MBE on GaAs(100) substrates. Magnetic properties of the samples were studied by SQUID magnetometry and transport experiments employing standard Hall effect measurements. Magnetisation and transport measurements revealed that the lower limit for the FM phase transition in a single Ga_0.95Mn_0.05As layer is close to 3 nm. Samples with thicknesses between 3 and 5 nm exhibited normal behaviour both in transport and in magnetisation measurements. This required however the insertion of suitable buffer layers which needed to be deposited prior to GaMnAs growth and a suitable post-growth annealing treatment.

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