Chalcopyrite Magnetic Semiconductors: An AB-Initio Study of Their Structural, Electronic and Magnetic Properties
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CHALCOPYRITE MAGNETIC SEMICONDUCTORS: AN AB{INITIO STUDY OF THEIR STRUCTURAL, ELECTRONIC AND MAGNETIC PROPERTIES S. PICOZZIy, A. CONTINENZAy , W. T. GENGx, Y. J. ZHAOx and A. J. FREEMANx y INFM { Dip. Fisica, Univ. L'Aquila, 67010 Coppito (L'Aquila), Italy x Dept. of Phys. and Astron. and Materials Research Center1 , Northwestern University, Evanston, IL 60208 (U.S.A.) ABSTRACT Stimulated by recent experimental observations of room{temperature ferromagnetism of MnxCd1;x GeP2 , we investigate the structural, electronic and magnetic properties of these systems as a function of Mn concentration by means of rst{principles density-functionaltheory-based codes. Moreover, we investigate the e ect of the anion substitution (P vs As) in Mn-rich chalcopyirites. Our calculations indicate that the antiferromagnetic alignment is the most stable ordering for all the systems studied, at variance with that experimentally reported. Moreover, we nd that there is a slight reduction of the total magnetic moment per Mn atom from 5 B in all the Cd-rich P-based chalcopyrites to 4 B in the Mn{rich MnGeP2 and MnGeAs2 systems. INTRODUCTION AND COMPUTATIONAL DETAILS A great e ort is being devoted to explore new, advanced magnetic semiconductors for spintronics applications[1], based on crystal structures that transcend the limitations (defect formation and too low operating temperatures) of the magnetic zinc-blende systems explored so far (e.g. Mnx Ga1;xAs). Within this framework, the chalcopyrites seem to be good candidates; the expected advantage of these systems is that Mn can readily substitute for the type{II cations, as has been demonstrated for II1;xMnxVI alloys with x up to 1.0 without defect formation, due to the natural tendency of Mn to adopt a +2 valence state. Recently, Medvedkin et al. [2] deposited a Mn overlayer in vacuum on the surface region of a CdGeP2 crystal, followed by a solid phase reaction at elevated temperatures. A relevant nding of Medvedkin et al. is the room temperature ferromagnetism in highly doped MnxCd1;x GeP2 - which constitutes a tremendous improvement from the TC of 110 K found in MnxGa1;xAs at x = 5.3 %. Moreover, their photoluminescence spectrum at 20 K shows a peak around 3.2 eV, from which they proposed a Mn induced enhancement of the energy gap of CdGeP2 (Eg = 1.83 eV). This is quite di erent from the III-V alloys, in which Mn is known to reduce the band{gap [7]. In order to understand the magnetic interaction in chalcopyrite semiconductors, we investigated Mnx Cd1;xGeP2 for various x and MnGe(V)2 where V = As, P with the full-potential linearized augmented plane wave (FLAPW)[3] method. This is one of the most accurate ab{ initio methods, in which there is no arti cial shape approximation for the wave functions, charge density and potential. For all atoms, the core and valence states are treated fully{ and semi{relativistically (i.e. without spin{orbit coupling), respectively. The mun|tin 1
Supported by the US National Science Foundation T6.9.1
(MT) radii for Cd, Ge, Mn are set to 2.3 a.u.; 2.
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