Atom-by-atom substitution of Mn in GaAs and visualization of their hole-mediated interactions

Spintronics at the atomic level The addition of metals as 'dopants to semiconductors is used to adjust the electronic properties of transistors and diodes. A new study uses atomically precise substitution of individual dopants to measure their interactions on the nanometre scale. The discovery of ferromagnetism in manganese-doped gallium arsenide sparked interest in semiconductors based on electron spin, or spintronics. This study uses scanning tunnelling microscopy to visualize the GaAs electronic states involved in Mn–Mn interactions. A strong dependence of ferromagnetic interaction on crystal orientation is seen, a property that might be exploited by growing oriented structures with ferromagnetic transition temperatures beyond those of randomly doped samples. This could also lead to coupled quantum bits for memory or information processing. Cover graphic: Mn hole states visualized on a GaAs surface mediate magnetic interactions between spin states. The discovery of ferromagnetism in Mn-doped GaAs 1 has ignited interest in the development of semiconductor technologies based on electron spin and has led to several proof-of-concept spintronic devices 2 , 3 , 4 . A major hurdle for realistic applications of Ga 1- x Mn x As, or other dilute magnetic semiconductors, remains that their ferromagnetic transition temperature is below room temperature. Enhancing ferromagnetism in semiconductors requires us to understand the mechanisms for interaction between magnetic dopants, such as Mn, and identify the circumstances in which ferromagnetic interactions are maximized 5 . Here we describe an atom-by-atom substitution technique using a scanning tunnelling microscope (STM) and apply it to perform a controlled study at the atomic scale of the interactions between isolated Mn acceptors, which are mediated by holes in GaAs. High-resolution STM measurements are used to visualize the GaAs electronic states that participate in the Mn–Mn interaction and to quantify the interaction strengths as a function of relative position and orientation. Our experimental findings, which can be explained using tight-binding model calculations, reveal a strong dependence of ferromagnetic interaction on crystallographic orientation. This anisotropic interaction can potentially be exploited by growing oriented Ga 1- x Mn x As structures to enhance the ferromagnetic transition temperature beyond that achieved in randomly doped samples.