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Register a new userTwo Spin-State Crystallizations in LaCoO$_{3}$
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We report a magnetostriction study of a perovskite $rm{LaCoO}_{3}$ above 100 T using our state-of-the-art strain gauge to investigate an interplay between electron correlations and spin crossover. There has been a controversy regarding whether two novel phases in $rm{LaCoO}_{3}$ at high magnetic fields result from crystallizations or Bose-Einstein condensation during spin crossover as manifestations of localization and delocalization in spin states, respectively. We show that both phases are crystallizations rather than condensations, and the two crystallizations are different, based on the observations that the two phases exhibit as magnetostriction plateaux with distinct heights. The crystallizations of spin states have emerged manifesting the localizations and interactions in spin crossover with large and cooperative lattice changes.
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Magnetization measurements of LaCoO$_{3}$ have been carried out up to 133 T generated with a destructive pulse magnet at a wide temperature range from 2 to 120 K. A novel magnetic transition was found at $B>100$ T and $T>T^{*}=32pm 5$ K which is characterized by its transition field increasing with increasing temperature. At $T<T^{*}$, the previously reported transition at $Bsim65$ T was observed. Based on the obtained $B$-$T$ phase diagram and the Clausius-Clapeyron relation, the entropy of the high-field phase at 80 K is found to be smaller for about $1.5$ J K$^{-1}$ mol$^{-1}$ than that of the low-field phase. We suggest that the observed two high-field phases may originate in different spatial orders of the spin states and possibly other degrees of freedom such as orbitals. An inherent strong correlation of spin states among cobalt sites should have triggered the emergence of the ordered phases in LaCoO$_{3}$ at high magnetic fields.
Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-$L_{2,3}$ edge we reveal that the spin state transition in LaCoO$_{3}$ can be well described by a low-spin ground state and a triply-degenerate high-spin first excited state. From the temperature dependence of the spectral lineshapes we find that LaCoO$_{3}$ at finite temperatures is an inhomogeneous mixed-spin-state system. Crucial is that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low-/intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance and inelastic neutron data.
We studied the spin-state responses to light impurity doping in low-spin perovskite LaCoO$_{3}$ (Co^3+: d^6) through magnetization and X-ray fluorescence measurements of single-crystal LaCo$_{0.99}$$M_{0.01}$O$_{3}$ ($M$ = Cr, Mn, Fe, Ni). In the magnetization curves measured at 1.8 K, a change in the spin-state was not observed for Cr, Mn, or Fe doping but was observed for Ni doping. Strong magnetic anisotropy along the [100] easy axis was also found in the Ni-doped sample. The fluorescence measurements revealed that the valences were roughly estimated to be Cr^3+, Mn^4+, Fe^(3+delta)+, and Ni^3+. From the observed chemical trends, we propose that the chemical potential is a key factor in inducing the change of the low-spin state. By expanding a model of the ferromagnetic spin-state heptamer generated by hole doping, we discuss the emergence of highly anisotropic ferromagnetic spin-state clusters induced by low-spin Ni^3+ with Jahn-Teller activity. We also discuss applicability of the present results to mantle materials and impurity-doped pyrites with Fe (d^6).
Spin crossover is expected to enrich unusual physical states in various types of condensed matter. Through inelastic neutron scattering, we study the spin-state excitations in the canonical and advanced platform, LaCoO$_3$, and reveal that the spatial correlation robustly maintains the seven-Co-site size below 300 K and the internal Co-$d$ electrons are spatially delocalized. By combining theoretical calculations, this dynamical short-range order is identified as a new collective unit for describing spin-state with dual spin-state nature beyond the conventional one-Co-site classification.
We report a comprehensive investigation of the magnetism of the $S$ = 3/2 triangular-lattice antiferromagnet, $alpha$-CrOOH(D) (delafossites green-grey powder). The nearly Heisenberg antiferromagnetic Hamiltonian ($J_1$ $sim$ 23.5 K) with a weak single-ion anisotropy of $|D|$/$J_1$ $sim$ 4.6% is quantitatively determined by fitting to the electron spin resonance (ESR) linewidth and susceptibility measured at high temperatures. The weak single-ion anisotropy interactions, possibly along with other perturbations, e.g. next-nearest-neighbor interactions, suppress the long-range magnetic order and render the system disordered, as evidenced by both the absence of any clear magnetic reflections in neutron diffraction and the presence of the dominant paramagnetic ESR signal down to 2 K ($sim$ 0.04$J_1$$S^2$), where the magnetic entropy is almost zero. The power-law behavior of specific heat ($C_m$ $sim$ $T^{2.2}$) observed below the freezing temperature of $T_f$ = 25 K in $alpha$-CrOOH or below $T_f$ = 22 K in $alpha$-CrOOD is insensitive to the external magnetic field, and thus is consistent with the theoretical prediction of a gapless U(1) Dirac quantum spin liquid (QSL) ground state. At low temperatures, the spectral weight of the low-energy continuous spin excitations accumulates at the K points of the Brillouin zone, e.g. $|mathbf{Q}|$ = 4$pi$/(3$a$), and the putative Dirac cones are clearly visible. Our work is a first step towards the understanding of the possible Dirac QSL ground state in this triangular-lattice magnet with $S$ = 3/2.
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