(An) investigation of ferromagnetic precipitations observed in diluted magnetic semicondcutors by nuclear magnetic resonance

Abstract

In establishing spintronics technologies, where electronic spins as well as the charges are utilized in a single device, ferromagnetic semiconductors are required for injection of spin-polarized currents to semiconductors to control the spin states of carriers in a device. A particular interesting ferromagnetic semiconductor is diluted magnetic semiconductor (DMS), a non-magnetic semiconductor containing several percents of transition metal atoms such as Mn, Co, Fe, etc. to become ferromagnetic. In DMS, carrier-mediate ferromagnetism has been widely accepted. Transition metal atoms incorporated into a semiconductor generate hole carriers by occupying substitutional sites and the hole carriers induce ferromagnetism in a whole sample through a p-d exchange interaction between the holes and magnetic ions. However, we found an evidence supporting extrinsic ferromagnetism of DMSs originating from ferromagnetic precipitations in zero-field nuclear magnetic resonance experiments. The samples we studied are (Zn,Mn)GeP₂ poly-crystals and (Ga,Mn)As epitaxial films. For the case of (Zn,Mn)GeP₂ poly-crystals, room temperature ferromagnetic semiconductors, about 90 % of total Mn atoms incorporated into ZnGeP₂ participates in forming MnP precipitations, which determine the macroscopic magnetization of the samples. The size of the precipitation is an order of tens of nanometers. In (Ga,Mn)As epilayers, MnAs precipitations having hexagonal NiAs structure were observed. About 1/7 of total Mn atoms incorporated into a sample participates in forming the precipitations whose average size is as small to form single magnetic domains. No evidences of intrinsic ferromagnetic properties of a sample is observed at the frequency ranges we investigated. Temperature dependent magnetic moments of MnAs precipitations estimated by NMR spectral frequencies measured below 12 K indicates that its magnetic phase transition should take place at the temperature much lower than that of b...