Kichizo Asai

Two spin-state transitions in LaCoO3

Neutron-diffraction measurements have revealed a new anomalous thermal lattice expansion of LaCoO 3 near 500 K that indicates the existence of a second spin-state transition, in addition to the one previously established near 100 K. The anomalous expansion and the temperature dependence of the Co magnetic moments are successfully interpreted in a wide temperature range based on a simple model assuming low-spin (LS, S =0), intermediate-spin (IS, S =1), and high-spin (HS, S =2) states of Co atoms. The first spin transition, near 100 K, is from LS to IS, and the second, near 500 K, is from IS to a mixed state of IS and HS. The fitted model parameters indicate that the initially-large energy difference between IS and HS states decreases towards zero as the second transition proceeds. The large drop in resistivity associated with the latter transition appears to be correlated with the population of the HS state.

Pressure dependence of the 100 K spin-state transition in LaCoO3

The pressure dependence of the 100 K spin-state transition in LaCoO 3 is investigated through magnetization measurement under pressures of up to 18.2 kbar. The energy gap, Δ, between the low-spin ground state and the excited magnetic state (intermediate spin) increases remarkably with increasing pressure. The linear part of the pressure dependence of Δ, which is dominant below 5 kbar, is interpreted consistently by the volume expansion due to the spin-state transition under ambient pressure. In the higher pressure region, a quadratic pressure dependence is dominant, which suggests a very steep volume dependence of the energy of the excited magnetic state under the highly compressed condition.

Magnetization Measurement and 55Mn NMR Study of La(Ni1-xMgx)0.5Mn0.5O3

Magnetization measurement and 55 Mn NMR study on a ferromagnet system La(Ni 2+ 1- x Mg x 2+ ) 0.5 Mn 0.5 4+ O 3 are made. The divalent and the tetravalent ions are found to be ordered to a considerable extent in the whole compositional range, and the ferromagnetic superexchange interactions between neighboring Mn 4+ and Ni 2+ ions are estimated to be 26 K in magnitude. 55 Mn NMR signals are observed for samples with x ≤0.6 in a frequency range between 270 and 310 MHz. The spectra composed of several equidistantly separated peaks can be successfully interpreted as each peak being the signal from Mn 4+ surrounded by a specific number of Ni 2+ ions. The hyperfine magnetic field at 55 Mn supertransferred from a neighboring Ni 2+ ion is determined to be 6.3 kOe, and the field originating from the own 3d magnetic moment of Mn 4+ ion is estimated to be -295 kOe, being a typical value for Mn 4+ ions in oxides.

Magnetic, electronic transport, and calorimetric investigations of La1-xCaxCoO3 in comparison with La1-xSrxCoO3

The magnetic and electronic transport properties and the specific heat of La 1- x Ca x CoO 3 (0.05≤ x ≤0.5) have been investigated. In the whole composition range, the Co atoms are in an intermedia...

NMR Study on the Supertransferred Hyperfine Magnetic Field at 55Mn in Ferromagnetic Perovskites La(Co1-xMgx)0.5Mn0.5O3

Magnetic properties and the hyperfine magnetic fields at 55 Mn in ferromagnetic oxides La(Co 2+ 1- x Mg x 2+ ) 0.5 Mn 0.5 4+ O 3 (0≤ x ≤0.4) are studied. The exchange integral, J , for the superexchange interaction between neighboring Mn 4+ and Co 2+ ions is found to be +15.3 K in magnitude. 55 Mn NMR spectra, composed of several equidistantly separated peaks, are interpreted as each peak being the signal associated with Mn 4+ surrounded by a specific number of Co 2+ ions. The supertransferred hyperfine magnetic field, H STHF , at 55 Mn from each neighboring Co 2+ ion is found to be +5.0 kOe, which is considerably smaller than that (+6.3 kOe) from Ni 2+ in a similar oxide system La(Ni 1- x Mg x ) 0.5 Mn 0.5 O 3 . On the other hand, the hyperfine magnetic field (-296 kOe) originating from 3 d magnetic moment of Mn 4+ is virtually the same in both systems. The values of J and H STHF are discussed in relation with the spin density in 2 p σ orbitals of the oxygen ions.

Structural Phase Transition from Rhombohedral to Cubic in LaCoO3.

Lanthanum cobalt oxide LaCoO3 has a rhombohedrally distorted perovskite structure with the crystal space group of R3c.1-3) Raccah and Goodenough have reported that the crystal space group varies with increasing temperature from R3c to R3 around K. In the high temperature phase, two kinds of inequivalent Co atoms order preferentially on alternate (111) Co 2) They have also reported a first order insulator-to-metal phase transition at 1210K accompanied with an abrupt drop of the rhombohedral angle from 60.4◦ to 60◦ keeping R3 symmetry through the transition. In addition, they reported an endothermic first order transition at 1210K, and higher order phase transitions at around 550 and 900K from the differential thermal analysis. A powder neutron diffraction study has revealed that LaCoO3 has R3c symmetry between 4.2 and 1248K, with possible exception at 668K. According to the study, the rhombohedral angle remains 60.3◦ even at 1248K. Their study does not support the first order phase transition at 1210K. In order to investigate the structural phase transition of LaCoO3, we performed X-ray powder diffraction measurement, and thermogravimetric (TG) and differential thermal analysis (DTA) at high temperatures. The results suggest a second order crystallographic phase transition from rhombohedral to cubic around 1610K, and show that any anomaly previously reported is not intrinsic one in pure LaCoO3. The powder samples of LaCoO3 for X-ray diffraction measurements were obtained from single crystal grown with a lamp-image floating zone furnace. The X-ray powder diffraction measurements were performed using a high-accuracy X-ray powder diffractometer equipped with a parabolic multilayer and parallel slit analyzer between 300 and 1700K in air atmosphere. The thermogravimetric and differential thermal analysis (TG-DTA) was performed between 300 and 1773K for both single crystalline bulk and polycrystalline powder samples with a TG-DTA (Rigaku TG-8120D) coupled with a mass spectrometer (Q-MS, Rigaku Thermo-Mass QA550

Field Induced Spin-State Transition in LaCoO3

The magnetization of LaCoO 3 shows a magnetic transition induced by level crossing at a magnetic field of 60 T below 25 K, which is consistent with the energy diagram determined from electron spin resonance experiments. However, the amount of Co ions with g = 3.35 contributing to the magnetic transition is sample-dependent and at most 14% of the total Co ions in LaCoO 3 . We found that the rest of the Co ions also contribute to the magnetization, which is a maximum at around 100 K. We analyzed the magnetization assuming two kinds of Co species, Co I and Co II , both of which have the nonmagnetic ground state. The energies and the g values of the magnetic excited states are 135 K and 3.35 for Co I , and 250–260 K and 1.9–2.0 for Co II , respectively. Co I ions are suggested to be associated with oxygen vacancies and the surface of LaCoO 3 .

Ultrasonic Measurement of LaCoO3 –Evidence for the Orbital-Order Fluctuation–

The sound velocity propagating along [111] of LaCoO 3 has been measured from 4 to 580 K for longitudinal ( L ) and transverse ( T ) sounds with frequencies, f , of 10 to 100 MHz. A comparison of the data for the single-domain rhombohedral crystal with those for a twinned pseudocubic specimen reveals the anisotropy of the elastic constants with the rhombohedral symmetry. The temperature dependence of the velocity of the L mode with f =10 MHz is analyzed consistently with the anomalous lattice expansion associated with the spin state transition. A frequency dispersion of the sound velocity appears in the L mode above 100 K. The amplitude of the dispersion increases pronouncedly with increasing temperature up to the measured highest temperature. The relaxation rate of the lattice evaluated based on the Debye model shows an activation type of temperature dependence with an attempt frequency of (1/τ) 0 =187×10 6 s -1 and the energy E a =11 meV. The slow lattice relaxation is attributed to a fluctuation of the ...

Inelastic neutron scattering study of phonons and magnetic excitations in LaCoO3

We have investigated the phonon and the magnetic excitations in LaCoO3 by inelastic neutron scattering measurements. The acoustic phonon dispersions show some characteristic features of the folded Brillouin zone (BZ) for the rhombohedrally distorted perovskite structure containing two chemical formula units of LaCoO3 in the unit cell. We observed two transverse optical (TO) phonon branches along (delta, delta, delta), consistent with previously reported Raman active Eg modes which show remarkable softening associated with the spin-state transition [Ishikawa et al., (Phys. Rev. Lett. 93 (2004) 136401.)]. We found that the softening takes place in the TO mode over the whole BZ. In contrast, the acoustic phonons show no anomalous softening associated with the spin-state transition. The low-energy paramagnetic scattering at 8 K is weak, increasing towards a maximum at E > 15 meV, consistent with excitation of the nonmagnetic low-spin to magnetic intermediate-spin state of Co 3+ ions.

Spin-State Transition in La1-xPrxCoO3

We have investigated the crystalline structure and the magnetic properties of La 1- x Pr x CoO 3 single crystals. The crystalline symmetry is rhombohedral R \bar3 c for 0 ≤ x ≤0.3, and orthorhombic...