Slow spin dynamics of cluster spin-glass spinel Zn(Fe1-xRu)O: role of Jahn-Teller active spin-1/2 Cuions at B-sites.

We report the slow spin dynamics of cluster spin-glass (SG) spinel Zn(Fe1-xRu)Oby means of detaileddc-magnetization andac-susceptibility studies combined with the heat capacity analysis. Two specific compositions ( = 0.5, 0.75) have been investigated in detail along with the substitution of Jahn-Teller (JT) active spin-1/2 Cuions at B-sites. Measurements based on the frequency and temperature dependence ofac-susceptibility (χac(f,T)) and the subsequent analysis using the empirical scaling laws such as: (a) Vogel-Fulcher law and (b) Power law reveal the presence of cluster SG state below the characteristic freezing temperatureTSG(17.77 K ( = 0.5) and 14 K ( = 0.75)). Relaxation dynamics of both the compositions follow the non-mean field de Almeida-Thouless (AT)-line approach(TSG(H)=TSG(0)(1-AH2/ϕ)), with an ideal value of = 3. Nevertheless, the analysis of temperature dependent high fielddc-susceptibility,χhf(2kOe ⩽ HDC ⩽ 20kOe,) provides evidence for Gabay-Toulouse type mixed-phase (coexistence of SG and ferrimagnetic (FiM)) behaviour. Further, in the case of CuZnFeRuOsystem, slowly fluctuating magnetic clusters persist even above the short-range FiM ordering temperature (TFiM) and their volume fraction vanishes completely across ∼6TFiM. This particular feature of the dynamics has been very well supported by the time decay of the thermoremanent magnetization and heat-capacity studies. We employed the high temperature series expansion technique to determine the symmetric exchange coupling (JS) between the spins which yieldsJS=-3.02×10-5 eV for CuZnFeRuOrepresenting the dominant intra-sublattice ferromagnetic interactions due to the dilute incorporation of the JT active Cuions. However, the antiferromagnetic coupling is predominant in ZnFeRuOand CuZnFeRuOsystems. Finally, we deduced the magnetic phase diagram in theHDC-Tplane using the characteristic parameters obtained from the field variations of bothac- anddc-magnetization measurements.