Magneto-thermally Activated Spin-state Transition in La0.95Ca0.05CoO3: Magnetically-tunable Dipolar Glass and Giant Magneto-electricity

Abstract

Magneto-dielectric spectra of La0.95Ca0.05CoO3 covering the crossover of spin states reveals strong coupling of its spin and dipolar degrees of freedom. Signature of spin-state transition at 30K clearly manifests in magnetization, supported by Co L_3,2-edge XAS data on the doped-specimen as consistent with its suppressed T_SST vs. ~150K for pure LaCoO3. Dispersive activation-step {\Delta}{\epsilon}'(T_{\omega})~O(10^2) and relaxation-peak {\epsilon}"(T_{\omega}) reflect the allied influence of coexistent spin-states on the dielectric character. Dipolar relaxation in the LS regime below T_SST is partly segmental (VFT kinetics) featuring magnetic-field tunability, whereas in the LS/IS-spin disordered state above 30K, it is uncorrelated (Arrhenic kinetics) and almost impervious to the H-field. Kinetics-switchover defines the dipolar-glass transition temperature Tg(H), below which the magneto-thermally-activated cooperative relaxations freeze-out by the VFT temperature T_0(H). Applied H-field facilitates thermally-activated SST and accelerates the dipolar relaxations; a critical 5T field collapsing the entire kinetics into a single Arrhenic behavior. Magneto-electricity (ME) spanning sizable thermo-spectral range registers diverse signatures here in the 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 (27K/50kHz)_9T. Sub-linear deviant field-hysteretic split seen in {\epsilon}'(H)|_>4T suggests the emergence of robust dipoles organized into nano-clusters, realized by the internally-generated high magneto-electric field. An elaborate {\omega}-T multi-dispersions diagram maps the rich variety of phase/response patterns, revealing the highly-interacting magnetic and electric moments in the system.