Magneto-thermally activated spin-state transition in La_0.95Ca_0.05CoO₃: magnetically-tunable dipolar glass and giant magneto-electricity

Abstract

The magneto-dielectric spectroscopy of La0.95Ca0.05CoO3 covering the crossover of spin states reveals the strong coupling of its spin and dipolar degrees of freedom. The signature of the spin-state transition at 30 K clearly manifests in the magnetization data at a 1 Tesla optimal field. Our Co L3,2-edge X-ray absorption spectrum on the doped specimen is consistent with its suppressed low-to-intermediate spin-state transition temperature at ∼30 K compared to ∼150 K, documented for pure LaCoO3. The dispersive activation step in the dielectric constant with the associated relaxation peak in imaginary permittivity characterize the allied influence of coexistent spin-states on the dielectric character. Dipolar relaxation in the low-spin regime below the transition temperature is partly segmental (Vogel-Fulcher-Tamman (VFT) kinetics) and features magnetic-field tunability, whereas in the low/intermediate-spin disordered state above ∼30 K, it is uncorrelated (Arrhenic kinetics) and almost impervious to the magnetic field H. Kinetics-switchover defines the dipolar-glass transition temperature Tg(H) (=27 K|0T), below which their magneto-thermally-activated cooperative relaxations freeze out by the VFT temperature T0(H) (=15 K|0T). An applied magnetic field facilitates thermal activation in toggling the low spins up into the intermediate states. Consequently, the downsized dipolar-glass segments in the low-spin state and the independent dipoles in the intermediate state exhibit accelerated dynamics. A critical 5 Tesla field collapses the entire relaxation kinetics into a single Arrhenic behaviour, signaling that the dipolar glass is completely devitrified under all higher fields. The magneto-electricity (ME) spanning sizeable thermo-spectral range registers diverse signatures here in kinetic, spectral, and field behaviors, in contrast to the static/perturbative ME observed close to the spin-ordering in typical multiferroics. Intrinsic magneto-dielectricity (50%) along with vanishing magneto-loss is obtained at (27 K/50 kHz)9T. The sub-linear deviant and field-hysteretic split seen in above 4 Tesla suggests the emergence of robust dipoles organized into nano-clusters, induced by the internally-generated high magneto-electric field. An elaborate ω-T multi-dispersions diagram maps the rich variety of phase/response patterns, revealing highly-interacting magnetic and electric moments in the system.

Nanosize effect: Enhanced compensation temperature and existence of magnetodielectric coupling in SmFeO3

“…49 This circumstance is due to the inverse relation between the characteristic frequency-and length-scales. 50,51 In turn, this also explains the detection (absence) of (dε′/dT)-discontinuity at T SR in SFO-1 (SFO-2) over the RF-range (supplementary Fig. S2); 31 in SFO-2, the relatively large-valued activation/relaxation parameters E a and τ, which determine the mostly polaronic ε′(T), seem insensitive to their small steps ΔE a (T SR ) and Δτ(T SR ).…”

Harmonic magneto-dielectric study in doped-, double-, and layered-perovskites

“…Across the IS-LS spin state transition, study of the stretching modes via Raman spectra, corresponding to different spin states, confirmed the gradual change from one spin state to another, and their coexistence over a temperature window corresponds to an order of magnitude dispersive change in the dielectric properties [16]. This vast change is traced to a mixed electrical state, comprising of phase-coexistent dipoles affiliated to the respective spin-states [17]. Thus, the system qualifies for the dielectric study of the bulk electrical-interface, as the inter-domain region between two simpler electrical mediums is well-acknowledged for device applications, because it cohosts the properties of both the media [18].…”

“…Ca-doped La 0.95 Ca 0.05 CoO 3 (LCCO) is chosen for the present study. This system has been studied in literature with signatures of SST in the dielectric properties (with giant magneto-dielectricity of up to 80%); featuring a magneto-thermally activated SST to Vogel-Fulcher/vitreous behavior of dipolar glass below ~30K [17].…”