Spin state ordering of strongly correlatingLaCoO3induced at ultrahigh magnetic fields

Abstract

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 * = 32 ± 5 K which is characterized by its transition field increasing with increasing temperature. At T < T * , the previously reported transition at B ∼ 65 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. PACS numbers: 75.30.Wx, 75.25.Dk, 75.47.Lx, 75.30.Cr Due to the strong correlations between the electrons, the transition metal oxide serves as a vast field hosting rich electronic phases represented by high-temperature superconductivity, colossal magnetoresistance and magnetic-field-induced ferroelectorics [1,2]. Among them, cobalt oxides are unique for their spin state degrees of freedom which not only bring about a magnetic crossover but also a metal-insulator transition (MIT) [3] in the thermal evolution. Perovskite cobalt oxide, LaCoO 3 , has attracted significant attention for more than five decades for its unusual magnetic and transport properties, namely, the crossover from a diamagnet to a Curie paramagnet at 100 K and the transition from a paramagnetic insulator to a paramagnetic metal at 500 K with increasing temperature [4]. Within the ionic picture, possible spin states of Co g , S = 1) is also argued to be stabilized due to the strong hybridization with the O 2p state [5]. Representative ideas describing the spin states of LaCoO 3 in the temperature range above 100 K are (i) the LS-HS mixture state [6][7][8][9][10][11] and (ii) the IS state [5,12,13]. However, they are still controversial. It is notable that recent theoretical studies on the two-orbital Hubbard model have qualitatively reproduced the thermally induced spin crossover and MIT with paramagnetic local moments [14][15][16]. On the other hand, they are inclined to predict the ordering of different spin states which is not found experimentally except for a few studies [17].The validity of the models on spin states should be well judged by their field effects. One can uncover magnetic excited states using high magnetic fields at low temperatures, eliminating the thermal effect. Thermodynamical properties of the magnetic phase can also be revealed by observing its temperature and magnetic field dependence [18,19] and possibly the orbitals are spatially ordered [22,23] and further, the following two magnetization jumps at B > 100 T are predicted by the Ising type SSC model [22]. With the explosive magnetic flux compression techni...

Excitonic dispersion of the intermediate spin state in LaCoO3 revealed by resonant inelastic x-ray scattering

“…Recent LDA+U calculations [21] find LaCoO 3 to be close to the condensation instability. The metamagnetic transition observed in high fields [20] has the temperature dependence consistent with exciton condensation, but not with HS-LS spin-state order [2]. The properties of the low-temperature phase of related (Pr 1−y R y ) x Ca 1−x CoO 3 have been consistently explained by exciton condensation [22,23].…”

Role of entropy and structural parameters in the spin-state transition of LaCoO3

“…[30][31][32][33][34][35][36] Additionally, there have been reports of the emergence of a striped phase in thin films (with alternating LS and HS/IS regions) 37 , as well as the presence of low temperature ferromagnetism in experiments performed on LaCoO 3 nanoparticles 38 . Furthermore, there have also been experiments conducted on single crystals of LaCoO 3 in the presence of a strong magnetic field [39][40][41][42] which have reported the presence of multiple metamagnetic transitions, which have been attempted to be explained by spin-state superlattices as well as excitonic condensation. 43 Hence, we can see that there is great interest in understanding the complex physical nature of this material.…”

Magnetostriction studies up to megagauss fields using fiber Bragg grating technique

“…Many electron configurations of these spin states are quite different in the electron occupation number of the e g orbitals, resulting in the large difference in their ionic radius. We have found magnetic field induced spin- crossover in cobaltites in megagauss regions [13], [20], which accompanies very large lattice changes [21]. As for heavy fermion system, the c-f hybridization between the localized f electrons and the conduction electrons promotes the formation of Kondo lattice at low temperatures.…”

“…We previously reported an experimental observation of unusual spin-state ordering at megagauss fields as shown on the B-T phase diagram in Fig. 6(c) using magnetization measurements [13]. Later it is argued by several theoretical groups that the B1 and B2 phases correspond to the LS-HS solid phase and the excitonic condensation phase, respectively [44].…”