MUONS AND MUONIUM IN ZN-SPINELS

We have studied the magnetic spinel (Zn)[Fe-2]O-4 (T-N approximate to 10.5 K) and the non-magnetic spinels (Zn)[Al-2]O-4, (Zn)[Ga-2]O-4, (Zn )[ZnTi]O-4 and (Zn)[ZnSn]O-4, both with surface and decay channel muon s. In (Zn)[Fe-2]O-4 the relaxation rate increases monotonically from r oom temperature down, typical for a paramagnet. Around 30 K, an additi onal, stronger damped signal appears which is the signature of short-r ange ordered (SRO) regions. Their total volume fraction increases dras tically towards T-N (reaching 75%) and astonishingly, continues to be present also below T-N where the rest of the material has become long- range ordered. Longitudinal field mu SR proves the SRO to be dynamic. In (Zn)[Al-2]O-4 and (Zn)[Ga-2]O-4 muon depolarization is caused solel y by Al-27 Or Ga-69,Ga-71 nuclear dipoles. In the inverse spinel (Zn)[ ZnTi]O-4, half of the implanted muons depolarize rapidly (lambda appro ximate to 3 mu s(-1) at room temperature). This, together with repolar ization behavior in longitudinal fields indicates that the muon in (Zn )[ZnTi]O-4 undergoes a chemical reaction after implantation forming mu onium. The fact that no such muonium formation occurred in another inv erse spinel ((Zn)[ZnSn]O-4) means that the presence of muonium is not connected to the inverse structure but rather due to the presence of T i which offers two d-electrons to participate in the chemical bonding. Additional evidence for d-electron participation is provided by Zn-67 -Mossbauer data which indicate unusual electron densities at the Zn-67 nuclei only in (Zn)[ZnTi]O-4.