PHYSICAL REVIEW 8 VOLUME 47, NUMBER 24 15 JUNE 1993-II Anisotropy of the electronic structure of Fe + in CdS in a magnetic field W. LU Sational Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Academia Sinica, 420 Zhong Shan Bei Yi Road, Shanghai 200083, People's Republic of China Y. M. Mu, J. P. Peng, and S. C. Shen Chinese Center of Aduanced Science and Technology (8'orld Laboratory), P. O. Box 8730, Beijing 100080, People's Republic of China and 1Vational Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Academia Sinica, 420 Zhong Shan Bei Yi Road, Shanghai 200083, People's Republic of China M. von Ortenberg Institut fur Halbleiterphysik und Optik, Technische Universitat Braunschweig, Federal Republic of Germany A. Twardowski Institute of Experimental Physics, Uniuersity of 8'arsaw, Poland (Received 22 October 1992) We report the electronic structure of Fe + in the wurtzite-structure iron-based semimagnetic semicon- ductor Cd& Fe S in the spectral range 10 — 90 cm ' at temperatures of 1. 6 and 4. 2 K. A strong spectral anisotropy is observed. All of the experimental spectra are well explained by crystal-field theory. I. INTRODUCTION While a number of studies have been undertaken for the investigation of the optical-absorption spectra of semimagnetic semiconductors (SMSC's) containing ions with permanent magnetic moments, such as in (CdMn)Te, the SMSC with magnetic ions possessing only field-induced moments in the ground state (e.g. , Fe +) has been investigated only very recently. ' This class of SMSC's exhibits behavior which is closely related to the energy spectrum of the transition-metal ion Fe + in the material. Therefore a comprehensive examination of the electronic structure of the Fe + ion in the Fe +-based SMSC is interesting and necessary. When Fe + ions are substitutionally incorporated for the cation in a cubic crystal, the D free-ion state of the Fe +(3d ) electrons splits into two levels: l 3 orbital doublet and I 3 orbital triplet; furthermore, the spin- orbit coupling splits the lower energy state I 3 into five states with the nondegenerate I, ground level. Many ex- perimental investigations using optical spectra and mag- netic measurements have been reported to support this crystal-field theory. In those cases with wurtzite struc- ture (such as Cd& „Fe„S and Cd, XFe„Se), the addition- al trigonal crystal field splits the energy states of the Fe + ion even further. The scheme of energy levels of the Fe + ion split by spin-orbit interaction and the trigonal crystal field is given in Fig. 1. The effect of the trigonal crystal field has been observed in the magnetic susceptibilities of Cd& „Fe„Se and Zn& Fe„O measured by Mahoney et al. and in Raman-scattering measurements of the low-energy excited states of the Fe + ion in Cd& Fe Se. Mauger et al. have calculated the magnetic-field dependencies of the energy spectra of indi- vidual Fe + ions in CdSe. Their spin-orbit interaction constant and the trigonal distortion parameter v' are different from the values obtained by Mahoney et aI. Therefore comprehensive experimental data and accurate calculations are very important to determine the elec- tronic structure of Fe + in Fe +-based SMSC's. The pur- pose of the present work is to investigate, both experi- mentally and theoretically, the energy of the low-lying ex- cited states of the Fe + ion in Cd, „Fe S and its aniso- tropic property in a magnetic field. The paper is organized as follows. We describe our ex- periment and present the experimental results in Sec. II. In Sec. III, the theoretical calculation is provided, and the spin-orbit coupling constant A, and the trigonal distor- tion parameters b and c are determined by the best fit to experimental data. Good agreement between the theory and experiment is achieved. By using the theoretical re- Cubic Spin — Orbit Trigonal Zeeman I j( il I r, FIG. 1. Splitting of the 'I 3 energy level of the Fe + ion by spin-orbit interaction, trigonal crystal field, and magnetic field. 0163-1829/93/47{24)/16247(5)/$06. 00 47 16 247 1993 The American Physical Society