Non-magnetic Co3 Research Articles

Influence of a Strong Magnetic Field on Magnetic Order in Polycrystalline Layered Cobaltites EuBaCo2 –xO5.5 – δ

The magnetic properties of polycrystalline EuBaCo2 –xO5.5 – δ (x ≤ 0.1, δ ≤ 0.14) are studied in the temperature range 2 K < T < 300 K in magnetic fields H ≤ 90 kOe. A comparison is made of the dependencies of magnetization M on temperature T and magnetic field H for polycrystalline and single crystal EuBaCo1.90O5.36, in which only Cu(III) ions are present. It is shown that in the magnetically ordered temperature range of magnetic order, the samples are ferrimagnetic, in which antiferromagnetic interactions between the nearest Co3+ ions with the same spins compete with the ferromagnetic exchange. With a decrease in temperature, a strong magnetic field affects the magnetic order. At T = 2 K in fields H = 50–90 kOe, all samples exhibit the properties of mictomagnetism. There are a shift in the hysteresis loops M(H) and a large difference in the values of the magnetization of the sample cooled in a magnetic field M (FC) and without a field M (ZFC) due to unidirectional magnetic anisotropy. In this case, there is no saturation of magnetization. The field dependences of magnetization taken in the cooling and heating in unequal fields (CHUF) mode indicate a metastable magnetic state. Near T = 20–30 K, an orientational transition characteristic of micromagnets takes place and a complex ferrimagnetic structure is formed. The main reasons for the manifestation of mictomagnetism properties in layered cobaltites are the presence of magnetoactive Co3+ ions in the states with intermediate and high spins at low temperatures and structural defects that affect the exchange interaction between the nearest neighbors. Nonmagnetic Co3+ ions in the low-spin state can also act as defects.

Magnetic and thermodynamic studies on the charge and spin ordering in the highly-doped La2− xSrxCoO4

We report magnetic studies on the charge and spin ordering in La2− xSrxCoO4 for x=1/3, 0.60, and 0.75. The magnetic susceptibility displays a clear cusp which we attribute to a spin glass freezing transition in all three compositions stduied. The behaviors of the evaluated effective magnetic moment and Curie-Weiss temperature indicate that the antiferromagnetic (AFM) interaction among Co2+ ions surrounded by the non-magnetic Co3+ weakens with increasing the doping concentration x. In addition, we have found that the incommensurate AFM short range order is frozen at Tg which is further below the onset temperature TIC of the neutron intensity of the incommensurate AFM correlation.

Magnetic Properties of Electron-Doped LaCoO3

We studied electron-doped \(\text{LaCo}_{1 - y}\text{Te}_{y}^{6 + }\text{O}_{3}\) by magnetization measurements and neutron scattering. The effective Bohr magneton, estimated by Curie–Weiss fitting around room temperature, is independent of y. This suggests that magnetic Co3+(HS), not nonmagnetic Co3+(LS), is mainly replaced by doped magnetic Co2+(HS). At the lowest temperatures, a Brillouin-function-like saturating behavior persists in the magnetization curves even in the high-y samples, and neither a clear magnetic reflection nor magnetic dispersion is observed by neutron scattering. These findings indicate that the magnetic correlation is very weak, in contrast to the well-known hole-doped LaCoO3 accompanied by a drastic transition to a ferromagnetic metal. However, we also found that the low-y samples exhibit nonnegligible enhancement of the saturated magnetization by ∼2μB per a doped electron. All these characteristics are discussed in the light of the activation and inactivation of a spin-state bloc...

Observation of spin glass transition in spinel LiCoMnO4**Project supported by the National Key Basic Research Program of China (Grant No. 2015CB251103), the Development Program of Science and Technology of Jilin Province, China (Grant No. 20140101093JC), and the Program of Science and Technology of Jilin City, China (Grant No. 201434006).

Spinel LiCoMnO4 is prepared by solid-state reaction and its magnetic properties are comprehensively studied by direct current (DC) and alternating current (AC) susceptibilities, isothermal remanent magnetizations, and magnetic hysteresis. Fitting to the Curie–Weiss law by using high-temperature zero-field-cooled susceptibility confirms a low-spin state of Co3+ with S = 0. Both the fitting parameters first increase and then tend to be saturated at high magnetic fields through using isothermal remanent magnetizations, which suggests a spin glass transition at low temperature. AC susceptibility study also supports this conclusion since the frequency dependence of peak position and intensity follows the tendency of a spin glass transition. The origin of the spin-glass transition in LiCoMnO4 might be attributed to a spatial segregation between non-magnetic Co3+ regions and spin glass ordered regions of Mn4+ ions.

Charge correlations in cobaltates La2–xSrx CoO4

We report on an elastic neutron scattering study of the charge correlations in La2–xSrx CoO4 with x = 1/3, 0.4 and 0.5. We found that the checkerboard charge ordering correlations present in the x = 0.5 sample persist in the x = 0.4 and 1/3 materials. These checkerboard charge ordering correlations are robust and explain the occurrence of nano‐phase separation in layered cobaltates for Sr‐concentrations away from half‐doping. The half‐integer reflections then arise from the nanometer‐sized hole‐rich regions (blue areas in title figure) instead of the undoped ones (red areas in title figure). The appearance of nano‐phase separation is an important ingredient for understanding the formation of hour‐glass shaped magnetic excitation spectra in La2–xSrx CoO4. Nano‐phase separation in La2–xSrx CoO4 (schematically). Red areas: undoped La2CoO4 islands, blue areas: checkerboard charge ordered regions; black, green and blue balls represent nonmagnetic Co3+ ions, magnetic Co2+ ions and oxygen ions, respectively; green arrows indicate Co2+ spins [1, 2].magnified imageNano‐phase separation in La2–xSrx CoO4 (schematically). Red areas: undoped La2CoO4 islands, blue areas: checkerboard charge ordered regions; black, green and blue balls represent nonmagnetic Co3+ ions, magnetic Co2+ ions and oxygen ions, respectively; green arrows indicate Co2+ spins [1, 2].(© 2015 WILEY‐VCH Verlag GmbH &amp;Co. KGaA, Weinheim)

The structural and electronic properties of spinel MnCo2O4 bulk and low-index surfaces: From first principles studies

The structural and electronic properties of the bulk and low-index (001) and (110) surfaces of the MnCo2O4 spinel have been studied from the spin-polarized GGA+U calculations. Results show that Mn prefers to occupy the octahedral sites and exhibits as high-spin Mn3+ while half Co takes the tetrahedral sites as high-spin Co2+ and the other half takes the octahedral sites as nonmagnetic Co3+. The electronic structure of the MnCo2O4 bulk is insulating with very small gap of about 0.03eV. The surface atomic relaxations and surface energies are calculated with different terminations and stable MnCo2O4 (001) and (110) surface models are obtained. The electronic structures of the Mn and the tetrahedral site Co atoms at the MnCo2O4 (001) and (110) surfaces resemble their bulk characters. However, due to the lowered Co–O coordination, the electronic structure of the octahedral Co atoms changes substantially from the bulk state t32g(↑)t32g(↓) into the surface state t32g(↑)e1g(↓) t22g(↓) at the MnCo2O4 (001) surface (5-coordinated Co) and t32g(↓)e2g(↓)t12g(↓)e1g(↓) at the MnCo2O4 (110) surface (4-coordinated Co).

Na order and Co charge disproportionation in NaxCoO2

We have synthesized and characterized different stable phases of sodium cobaltates NaxCoO2 with sodium content 0.65<x<0.80. We demonstrate that 23Na NMR allows us to determine the difference in the susceptibility of the phases and reveals the presence of Na order in each phase. 59Co NMR experiments give clear evidence that Co charge disproportionation is a dominant feature of Na cobaltates. Only a small fraction (≈25%) of cobalt are in a non-magnetic Co3+ charge state whereas electrons delocalize on the other cobalt. The magnetic and charge properties of the different Co sites are highly correlated with each other as their magnetic shift KZZ scales linearly with their quadrupolar frequency νQ. This reflects the fact that the hole content on the Co orbitals varies from site to site. The unusual charge differentiation found in this system calls for better theoretical understanding of the incidence of the Na atomic order on the electronic structures of these compounds.

Structural studies of a mixed-valence state in the incommensurate composite crystal Sr1.261CoO3

The incommensurate modulated crystal structure of the hexagonal cobalt oxide Sr1.261CoO3 has been studied using a four-dimensional (4D) superspace profile analysis of neutron powder diffraction data. Sr1.261CoO3 is a composite crystal that consists of the [CoO3] and [2Sr] subsystems. The [CoO3] subsystem forms 1D chains that run parallel to the c-axis and consist of face-sharing CoO6 polyhedra with octahedral (Oh) and trigonal prismatic (TP) coordinations. The structure analysis reveals that the [CoO3] chains contain 73.9% Oh and 26.1% TP sites, and that the TP sites have longer Co–O bonds than the Oh sites:dav. =2.039(4) Å (TP) and 1.895(3) Å (Oh). The averaged Co bond valences are Co3.56(3)+ in the Oh sites and Co2.45(3)+ in the TP sites, suggesting that a considerable amount of Co3+ ions are mixed with Co4+ions in the Oh sites and with Co2+ ions in the TP sites. The observed magnetic susceptibility can be well explained assuming that the compound has the Co mixed-valence state with the spin configurations of S=0 low-spin state for Co3+(dε6), S=1/2 low-spin state for Co4+(dε5) and S=3/2 high-spin state for Co2+(dε5dγ2). The Weiss temperature, approximately 0.8 K, implies that Sr1.261CoO3 naturally assumes a Curie paramagnetic state, probably owing to the obstruction of the intrachain magnetic interaction by the nonmagnetic Co3+ ions. These results suggest that the nonmagnetic Co3+ ions play an essential role in the magnetism of Sr2γCoO3 systems.