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Effect anion substitution on ferromagnetism in GaMn(As,P)


Because of their potential as both injectors and filters of spin-polarized carriers, ferromagnetic semiconductors may play an important role in spin-based electronics, or spintronics .  Ferromagnetic semiconductors are formed by the substitution of a relatively small fraction of host atoms with a magnetic species.  Ga 1-x Mn x As has been the most thoroughly studied material among these, and ferromagnetism in it arises from hole-mediated inter-Mn exchange. The Curie temperature T C in Ga 1-x Mn x As has been shown to increase with increasing concentration of substitutional Mn acceptors.  However, room temperature ferromagnetism in this canonical system has been elusive in large part due to challenges in materials synthesis—namely, raising x while avoiding the formation of second phases or compensating defects.  Even the relatively low alloying levels necessary for ferromagnetism require the application of non-equilibrium growth strategies, in particular low-temperature molecular beam epitaxy (LT-MBE).  An alternative, less explored route by which T C is expected to rise is via anion substitution in the host semiconductor (e.g., replacing As with P in GaAs) in order to enhance p-d exchange between the holes and Mn acceptors.  At Berkeley we developed a simple, versatile process to synthesize ferromagnetic semiconductors.  Using a combination of ion implantation and pulsed-laser melting, we have produced epitaxial, single crystalline films of ferromagnetic Ga-Mn-pnictide alloys that display the magnetic and electrical properties observed in films grown by LT-MBE and have realized new materials that enable us to explore the effect anion substitution on ferromagnetism.  I will describe our synthesis process and present results on these novel semiconductors that reveal an intimate relationship among ferromagnetism, electrical transport, and composition.