Static and dynamic magnetic properties of normal spinel Co2RuO4 = (Co2+) are reported based on our investigations of the temperature (T), magnetic field (H) and frequency (f) dependence of the ac-magnetic susceptibilities and dc-magnetization (M) covering the temperature range T = 2 K–400 K and H up to 90 kOe. These investigations show that Co2RuO4 exhibits an antiferromagnetic (AFM) transition at TN ∼ 15.2 K, along with a spin-glass state at slightly lower temperature (TSG) near 14.2 K. It is argued that TN is mainly governed by the ordering of the spins of Co2+ ions occupying the A-site, whereas the exchange interaction between the Co2+ ions on the A-site and randomly distributed Ru3+ on the B-site triggers the spin-glass phase, Co3+ ions on the B-site being in the low-spin non-magnetic state. Analysis of measurements of M (H, T) for T < TN are used to construct the H–T phase diagram showing that TSG shifts to lower T varying as H2/3.2 expected for spin-glass state whereas TN is nearly H-independent. For T > TN, analysis of the paramagnetic susceptibility (χ) vs. T data are fit to the modified Curie–Weiss law, χ = χ0 + C/(T + θ), with χ0 = 0.0015 emu mol−1Oe−1 yielding θ = 53 K and C = 2.16 emu-K mol−1Oe−1, the later yielding an effective magnetic moment μeff = 4.16 μB comparable to the expected value of μeff = 4.24 μB per Co2RuO4. Using TN, θ and high temperature series for χ, dominant exchange constant J1/kB ∼ 6 K between the Co2+ on the A-sites is estimated. Analysis of the ac magnetic susceptibilities near TSG yields the dynamical critical exponent zν = 5.2 and microscopic spin relaxation time τ0 ∼ 1.16 × 10−10 sec characteristic of cluster spin-glasses and the observed time-dependence of M(t) is supportive of the spin-glass state. Large M–H loop asymmetry at low temperatures with giant exchange bias effect (HEB ∼ 1.8 kOe) and coercivity (HC ∼ 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed below TN is interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) for T > TN to inverse MCE for T < TN observed in Co2RuO4 (and reported previously in other systems also) is related to the change in sign of (∂M/∂T) vs. T data.

We report a detailed study on the dynamical response of localized electron hopping and dipole relaxation in bulk polycrystals of Zn diluted Cuprospinel (Cu1 − x Zn x Fe2O4). The variations in the dielectric dispersion and ac-resistivity (ρ ac ) were analyzed over a wide temperature (77 K ≤T ≤ 823 K) and frequency (20 Hz ≤ f ≤ 20 MHz) window for a critical composition x c = 0.4. The variation of R (f, T) followed the Maxwell–Wagner type polarization mechanism in-line with the Koops phenomenological theory. Our analysis of ρ ac (T, f) provide strong evidence to the Mott’s variable range hopping of charge transport between the localized states at low temperatures, however, thermally-activated Arrhenius like behaviour was noticed at high temperatures with E A = 656 meV for x c = 0.4. Moreover, electric modulus spectroscopic studies (M *(f, T)) reveals two distinct types of relaxation phenomena: (i) the short-range oscillations of the charge carriers within the potential well of grains and (ii) the long-range movement of charge carriers across the grain boundaries. The depressed semi-circles of the Nyquist plots and lower values of non-exponential parameter extracted from M *(f, T) suggest the non-Debye type relaxation process present in the system with a widespread distribution of relaxation times. The frequency exponent (s(x, T)) study of Jonscher’s power law reveals that the ac-conductivity follows small-polaron tunnelling followed by the correlated-barrier-hopping mechanism for 0.1. However, for x ≥ x c reorientational hopping mechanism is predominant, except for 400 K, where thermally activated Arrhenius-type conduction of charge carriers is prevalent in this spinel system. Furthermore, the tetragonally (I41/amd) distorted systems (x ≤ 0.05) exhibit less activation energy (E A − VRH ) values as compared to those of cubic-spinel symmetry (Fd-3m) which saturates at 130 meV for 0.1 ≤ x ≤ 0.6. Compositional dependent tunability of the above discussed parameters may open a constructive approach to design low energy-loss and high-resistive electromagnetic elements for microwave devices which is the key significance of the present study.

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We report a detailed study on the electronic structure and temperature (1.9K≤T≤900K) dependence of magnetization in Zn diluted cuprospinel [Cu1−xZnxFe2O4 (0≤x≤0.6)]. The electronic structure determined from the x-ray photoelectron spectroscopy and Rietveld analysis of the x-ray diffraction patterns reveals the structure to be (Cu(1−x)/5ZnxFe4(1−x)/5)A [Cu4(1−x)/5Fe2−4(1−x)/5]BO4. Beyond a specific dilution limit (0.05≤xp≤0.1), a sudden phase-change from tetragonal (I41/amd) to cubic (Fd3m) is noticed with an alteration in the A–O–A (3.29%) bond angle and A–O bond length (0.67%). Our analysis shows that all these compounds order ferrimagnetically below the Néel temperature (TFN) due to dissimilar site-specific magnitudes of spins, yet, they undergo a second transition at low temperatures T1∼66K with asymptotic Curie temperature TA(=C/χ0) as high as −547.2K for the undoped case. Dilution with Zn cause quadratic decay (b2x2+b1x+yo) of the ferrimagnetic ordering temperature from 743 K to 370.5 K for x=0 and 0.6, respectively. On the contrary, a significant increase in the saturation magnetization (MS) was observed with increasing x until the critical composition xc∼0.4 beyond which MS decreases continuously (MS=1.64μB and 4.73μB for x=0 and 0.4, respectively). From the temperature dependence of inverse paramagnetic susceptibility [χ−1(T&gt;TFN)] data and Néel’s expression for ferrimagnets, we evaluated the molecular field constants and exchange interactions (J) between the tetrahedral A- and octahedral B-sites. A systematic compositional dependence of this analysis yields that JAB (∼25kB for x=0) is the dominant exchange interaction in comparison to JBB and JAA; however, JAB decreases significantly with increasing the composition (JAB∼−5.5kB for x=0.6). The isothermal magnetization data and law of approach to saturation analysis reveals that the investigated system possesses very high anisotropy field HK ≥5.5kOe with cubic anisotropy constant K1≥1.6×106erg/cc at xc.

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