Spin-liquid state with precursor ferromagnetic clusters interacting antiferromagnetically in frustrated glassy tetragonal spinel Zn0.8Cu0.2FeMnO4.

Spinels (AB2O4) with magnetic ions occupying only the octahedralBsites have inherent magnetic frustration which inhibits magnetic long-range order (LRO) but may lead to exotic states. Here we report on the magnetic properties of the tetragonal spinel Zn0.8Cu0.2FeMnO4, the tetragonality resulting from the Jahn-Teller active Mn3+ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition (Zn0.82+Cu0.22+)A[Fe0.42+Fe0.63+Mn3+]BO4‒δ. Analysis of the temperature dependence of magnetization (M), ac magnetic susceptibilities (χ'andχ''), dc susceptibility (χ), heat capacityCp, and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data ofχ vs. Tfits the Curie-Weiss law:χ = C/(T ‒ θ) fromT= 250 K to 400 K withθ≃ 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constantJ/kB= 17 K, andC= 3.29 emu K mol‒1Oe‒1yielding an effective magnetic momentµeff= 5.13µBresulting from the high-spin states of Cu2+(Asite) and Fe2+(Bsite), while theBsite trivalent ions Mn3+and Fe3+are in their low-spin states. The extrapolated saturation magnetization obtained from theM vs. Hdata atT= 2 K is explained using the spin arrangement (Cu2+↓)A[Fe2+↑, Fe3+↓, Mn3+↑]Bleading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d(χT)/dTshows the onset of ferrimagnetism below ∼100 K and peaks near 47 K and 24 K. The relaxation timeτobtained from temperature and frequency dependence ofχ″when fit to the power law and Vogel-Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperatureTSGHfollows the equation:TSGH=TSG01-AH2/ϕwithTSG(0) = 46.6 K,A= 8.6 × 10‒3Oe‒0.593andϕ= 3.37. The temperature dependence of hysteresis loops yields coercivityHC∼ 3.8 kOe at 2 K without exchange-bias, butHCdecreases with increase inTbecoming zero above 24 K, theTSG(H) forH= 800 Oe. Variations ofCpvs. Tfrom 2 K to 200 K inH=0 andH=90 kOe do not show any peak characteristic of LRO. However, after correcting for the lattice contribution, a broad weak peak typically of SRO becomes evident centered around 40 K. ForT< 9 K,Cpvaries asT2; a typical signature of spin-liquids (SLs). Comparison of the ND measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermo-remanent magnetizationMTRM(t) studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn0.8Cu0.2FeMnO4, ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster SG state atTSG(0) = 46.6 K, followed by SL behavior below 9 K.