PHYSICAL REVIEW B VOLUME 48, NUMBER 4 15 JULY 1993-II Transfer hyperfine interaction in Cd, „Mn„Te studied by NMR Kebede Beshah, * Peter Becla, and Robert G. Griffin Francis Bitter ¹tional Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 David Zamir Solid State Physics Department, Soreq Nuclear Research Center, Yaune, Israel 70660 (Received 6 February 1992; revised manuscript received 21 October 1992) NMR measurements of ' 'Te and '"Cd have been performed on a series of samples of Cd, „Mn„Te (x =0. 01, 0. 015, 0. 02, 0. 029, 0. 055). The nuclear spin-lattice relaxation rates of the two nuclei are enhanced by two orders of magnitude compared to the relaxation rates of the same nuclei in the non- magnetic II-VI semiconductor alloys. Well-resolved " Cd and ' Te spectra were obtained using the magic-angle-spinning technique. The line shifts are temperature dependent and follow the Curie-Weiss law, indicating that the predominant interaction responsible for the NMR behavior is the transfer hyperfine interaction of the Mn + ion with the Cd or Te nuclei. All the lines in the Te spectra and all but two of the lines in the Cd spectra are shifted to higher frequencies with reference to the NMR line in CdTe. A full spectral assignment has been obtained based on two assumptions: (i) the Mn ions are ran- domly distributed in the lattice, and (ii) the transfer hyperfine interaction that causes the line shifts de- pends on the number of bonds between the Mn ion and the Cd (or Te) ion. According to the spectral as- signment, the spin density transferred to the Cd sites in the lattice decreases tenfold from one Cd shell to the next. I. INTRODUCTION Cd& Mn Te belongs to the 2 &' B C family of semiconductor alloys, where 2 and C are group-II and -VI elements, respectively, and B is a magnetic ion, e.g. , Mn, Fe, and Co. At relatively low concentrations of the magnetic ion (x (0. 1), such alloys are referred to as di- luted magnetic semiconductors (DMS). The most exten- sively studied alloys in this group contain Mn as the mag- netic ion. In addition to properties in common with the II-VI semiconductor alloys, DMS materials exhibit unique magnetic and magneto-optic properties (for re- views, see Refs. 1 and 2). As shown in recent theoretical and experimental ' studies, the predominant mechanism responsible for the unique magnetic properties of DMS is the superexchange interaction, i.e. , the interaction of Mn-Mn spins via the intervening Te and Cd ions. These studies also showed that both the nearest-neighbor exchange constant J& and the next-nearest-neighbor (NNN) exchange constant Jz were antiferromagnetic and Ji )) J2 ~ Spalek et al. have introduced bonding features in order to interpret the magnetic properties of DMS materials and showed that bond angles, covalency, and orbital hybridization were the major parameters affecting the superexchange in- teraction. These properties are very sensitive to the dis- tance between the ions and a strong correlation was indeed found between the magnetic exchange constants and the cation-anion distances for a large number of DMS. Recent calculations have shown that the chemi- cal bond path between the two interacting Mn ions also plays an important role in the superexchange interaction. Another interaction showing similarity to the superex- change interaction is the transfer hyperfine interaction (THFI) between the unpaired electron spins of Mn + and the nuclear spins of the nonmagnetic ions in the lattice. Similar to the superexchange interaction, THFI is sensi- tive to the bonding properties, especially the degree of co- valency in the lattice. Because the spin transfer mecha- nism is very much the same in both interactions, ' one expects that measurements of THFI might contribute to the detailed understanding of the superexchange mecha- nism in DMS. THFI is usually detected by electron paramagnetic res- onance" (EPR) or NMR (Ref. 12) techniques. Reason- ably resolved NMR spectra are obtained for short ( — 10 ' s) electron spin-relaxation times r. For longer ~, the EPR technique provides better resolved spectra. THFI in II-VI semiconductor alloys with paramagnetic impurities has been observed using EPR and electron nu- clear double resonance techniques. ' ' Recently, Marshall et al. ' have measured the superhyperfine struc- ture in the EPR spectra of Mn in ZnSe and noted an in- teraction with three NNN shells. But, due to the large broadening of EPR signals with increasing Mn concen- trations, the detection of THFI by EPR is limited to ma- terials with low Mn concentrations, x (0. 005. However, it is possible to use NMR to study THFI for up to x =0. 10 with reasonable line resolution. In addition, the sign of the THFI can be obtained only by NMR. In the present paper we report on a NMR study of Cd& Mn Te. The presence of the paramagnetic ion in the lattice has been found to have a large effect on spin- relaxation times and line shifts. These results lead us to conclude that the transfer hyperfine interaction is the predominant factor in affecting NMR parameters. The 0163-1829/93/48(4)/2183(8)/$06. 00 2183 1993 The American Physical Society