CARRIER CONCENTRATION INDUCEDFERROMAGNETISM IN SEMICONDUCTORS

摘要

In semiconductor spintronics ferromagnetic semiconductor materials with newmagnetically controlled electrical or optical functionalities are needed to fa-cilitate an integration of standard electronic functions of semiconductors withmagnetic memory function. All classical semiconductors, like GaAs, are non-magnetic materials, in which ferromagnetic properties can be induced by sub-stitutional incorporation of magnetic ions (usually Mn~(2+)ions) and heavy dop-ing with conducting carriers up to 10~(20)-10~(21)cm~(-3). The experimental observa-tions of carrier concentration induced ferromagnetism are discussed for threegroups of semiconductor materials: p-Sn_(1-x)Mn_xTe, p-Pb_(1–x–y)Sn_yMn_xTe,and p-Ge_(1–x)Mn_xTe (diluted magnetic semiconductors of IV-VI group, inwhich paramagnet - ferromagnet and ferromagnet - spin glass transitions arefound for very high hole concentration), p-Ga_(1–x)Mn_xAs, p-In_(1-x)andp-In_(1-x)Mn_xSb (diluted magnetic semiconductors of III-V group, in which fer-romagnetism appears due to Mn ions providing both local magnetic momentsand acting as acceptor centers), and n-Eu_(1-x)Gd_xTe (magnetic mixed crystals,in which the substitution of Gd~(3+) ions for Eu~(2+) creates very high electronconcentration and transforms antiferromagnetic, insulating EuTe compoundinto ferromagnetic n-type semiconductor alloy). For each of these materialssystems the key physical features are discussed concerning: local magnetic mo-ments formation, magnetic phase diagram as a function of magnetic ions andcarrier concentration as well as Curie temperature engineering. Various theoret-ical models proposed to explain the effect of carrier concentration induced fer-romagnetism in semiconductors are presented involving mean field approachesbased on Zener and Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanisms.