S. Nayak

Peculiarities of the temperature dependence of electron spin resonance and Raman studies of Zn1−xNixO/NiO two-phase nanocomposites

A meticulous investigation of electron-spin-resonance (ESR) and Raman spectroscopy of the two-phase nanocomposites of Zn1−xNixO/NiO is reported. The temperature variation of X-band ESR parameters viz., resonance field HR(T) and line-width ΔHPP(T) follows the power-law variation (δHR = (ΔHPP)n) of Nagata and Ishihara model, which was used to understand the orientation of statistical ensemble of particles with respect to a given direction of the anisotropy axis. This analysis yields the exponent “n” ≃ 2.13 and 2.85 for the composite system Zn1−xNixO/NiO and pure NiO suggesting the presence of partial and randomly oriented ellipsoidal nanocrystallites, respectively. The Raikher and Stepanov model has been employed to probe the role of amorphous Ni3+ clusters on the observed ESR spectra. Interestingly, after Ni substitution, a new zone boundary phonon mode was noticed at 129 cm−1 for all the samples, which is usually forbidden in the first-order Raman scattering for wurtzite ZnO. In addition to the 2M magnon ...

Reentrant spin-glass behavior and bipolar exchange-bias effect in “Sn” substituted cobalt-orthotitanate

We report the co-existence of longitudinal ferrimagnetic behavior with Neel temperature TN ∼ 46.1 K and reentrant transverse spin-glass state at 44.05 K in Tin (Sn) doped cobalt-orthotitanate (Co2TiO4). The ferrimagnetic ordering is resulting from different magnetic moments of Co2+ on the A-sites (3.87 μB) and B-sites (5.069 μB). The magnetic compensation temperature (TCOMP) shifts from 31.74 K to 27.1 K when 40 at. % of “Sn4+” substitutes “Ti4+” at B-sites where the bulk-magnetization of two-sublattices balance each other. For T > TN, the dc-magnetic susceptibility (χdc = M/Hdc) fits well with the Neels expression for the two-sublattice model with antiferromagnetic molecular field constants NBB ∼ 15.44, NAB ∼ 32.01, and NAA ∼ 20.88. The frequency dependence of ac-magnetic susceptibility χac data follows the Vogel-Fulcher law, and the power-law of critical slowing-down with “zν” = 6.01 suggests the existence of spin-clusters (where “z” and “ν” being dynamic critical-exponent and correlation length of cri...

Low-temperature anomalous magnetic behavior of Co2TiO4 and Co2SnO4

We report the low-temperature anomalous magnetic behavior of ferrimagnetic spinels cobalt orthotitanate (Co2TiO4), which exhibits magnetic compensation behavior across 31.74 K, and cobalt orthostannate (Co2SnO4) exhibiting two sequential magnetic transitions, namely (i) ferrimagnetic to paramagnetic transition with Neel temperature TN ∼ 41 K and reentrant spin-glass behavior with glass transition temperature TSG ∼ 39 K. The Arrott plot (H/M versus M2) criterion has been used to extricate the order of sequential magnetic transitions occurring below TN. Negative slopes of the Arrott plots below 32 K, metamagnetic-like character of the M-H isotherms, anomalies in the specific-heat (CP T−1 versus T) below 15 K, and a zero-crossover of isothermal magnetic-entropy-change (ΔS) signify the presence of pseudo first-order discontinuous magnetic phase transition in the low-temperature regime 5 K ≤ T ≤ 32 K. The dc- and ac-susceptibilities of both Co2TiO4 and Co2SnO4 are interpreted in terms of frozen-spin-clusters, ...

The ac-magnetic susceptibility and dielectric response of complex spin ordering processes in Mn3O4

We report a meticulous study of the ac-magnetization dynamics (χac(T)), relative dielectric permittivity er(T), and magneto-dielectric (Δer/er(H)) response of various complex magnetic transitions that occur below the ferrimagnetic Neel temperature TN of Mn3O4. Besides the known sequence of transitions at TN ∼ 42.75 K, T1 ∼ 39 K, and T2 ∼ 34 K, the existence of a new anomaly reported recently at 38 K (T*) has been successfully probed by χac(T) and er(T) measurements. The effect of external dc-bias fields (HDC) and driving frequency (f) on the above mentioned transitions has been investigated in consonance with the er(T) and Δer/er(T,H) results. For the first time, we observed a clear hysteresis of about 5.15 K in the zero-field er(T) across the incommensurate-to-commensurate transition T2 ∼ 34 K, which provides evidence to the first-order nature of this transition. The Arrott plot (H/M vs. M2) criterion has been used to distinguish the nature of all the sequential transitions that take place below TN.

Finite-size scaling and exchange-bias in SrRuO3/LaNiO3/SrRuO3 trilayers

We report a detailed magnetization study of the coherently strained trilayer heterostructures of metallic ferromagnet SrRuO3 (SRO)/paramagnetic-LaNiO3/SRO grown on a (001) oriented SrTiO3 single crystal substrate using the pulsed laser deposition technique. A tunable positive exchange bias (μoHEB ∼ 0.2 T, at 10 K) across the interface has been observed. Upon varying the SRO layer thickness (2 nm ≤ tSRO ≤ 18 nm), the saturation magnetization (MS ∼ 0.46 μB/Ru) increases significantly owing to electronic-reconstruction across the interfaces and finite size effects. The magnitude of μoHEB increases continuously with increasing tSRO and saturates above 16 nm; however, the loop-asymmetry increases rapidly as the temperature is lowered below the ferromagnetic Curie temperature (TC) of SRO. Notable reductions in both TC (125.6 K ≤ TC ≤ 146.4 K) and blocking temperature TB = T* (90.6 K ≤ T∗ ≤ 119.2 K) are observed due to surface effects that comply with the finite-size scaling law: TC(tSRO) = TC (∞) [1 − (ξo/tSRO)...