Calculation of exchange constants of III–V dilute magnetic semiconductors using aquasiparticle method

Abstract

A quasiparticle method based on a multisite s(p)–d mixing model is used to calculate theexchange constants of dilute magnetic semiconductors (DMS). The effective interaction,which is mediated by s(p) electrons, between d electrons, was taken into account inan effective field approximation or a coherent potential approximation (CPA).The equation-of-motion technique was applied to Green functions to calculatethe quasiparticle state density. The exchange constants were calculated ina double-valence-band model, and the microscopic parameter dependences ofthe exchange constants are investigated for fixed valence band parameters inthe model. A ferromagnetic d–d coupling is obtained when the d energy levelEd is in the vicinity of the valence band top, and an antiferromagnetic d–d coupling is obtained whenEd is in the vicinity of the valence band bottom. The maximum of the d–d exchange constantJd d(Ed) increases with the mixingstrength. Jd d increases withdecreasing Fermi energy EF, and the effect is more significant whenEd approaches thevalence band top. Jd d also increases with small magnetic impurity concentrationx, then reaches a maximum, and finally decreases sharply whenx increases further. The quasiparticle state density shows that the ferromagnetismoriginates from the unsymmetrical broadening of the shallow d energy level inferromagnetic alignment and related lowering of the centre of gravity of the d statedensity. Relevant experimental results are discussed on the basis of our calculation.