Description of the pressure-induced insulator-metal transition in BaCoS2 within the LDA + DMFT approach

The pressure-induced insulator-metal transition in paramagnetic sulfide BaCoS₂ at a temperature of 370 K has been described for the first time using the combination of the local electron density approximation and the dynamic mean field theory (LDA + DMFT). Based on the analysis of the spectral functions of Co 3d orbitals, the local magnetic moments of Co, and the frequency dependence of the imaginary part of the self-energy, the existence of the insulator-metal transition for 97% of the BaCoS₂ unit cell volume at normal pressure has been established. Simultaneously, the high-to-low spin magnetic transition of Co²⁺ ions occurs.     

Magnetic structure of geometrically frustrated compound Co2Cl(OH)3 determined by proton NMR

The magnetic structure of a geometrically frustrated system Co₂Cl(OH)₃ is determined by comparing the observed proton NMR spectrum with numerical calculations based on various magnetic models. The best fit is obtained with a model that the magnetic moments of Co²⁺ ions in the triangular plane are parallel to the principal axis of local crystal field and those of Co²⁺ ions in the kagome lattice plane are randomly disordered in the a-b plane, which nearly bisects the angle between the principal axis of the local field and a line pointing towards the body center of the tetrahedron. The coexistence of the ferromagnetic order in the triangular plane and the random disorder in the kagome plane is consistent with the results of measurements by Zheng et al. However, the magnetic moments of Co²⁺ ions are not directed towards the body center of the tetrahedron as characteristic in the “spin ice” magnetic structure. Furthermore, the Co²⁺ ions in the triangular plane have a smaller magnitude of magnetic moment than those in the kagome plane. Thus, our result suggests that the transition metal compound Co₂Cl(OH)₃ is different from the “spin ice” in magnetic structure, although it is similar to rare-earth pyrochlores in crystal structure.     

Magnetic ground state and the spin-state transitions in YBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript>

The crystal and magnetic structure of the perovskite-like, oxygen deficient cobalt oxide YBaCo₂O_5.5 has been studied by means of neutron and X-ray diffraction in the 10–300 K temperature range. The magnetic ground state is characterized by a coexistence of two distinct antiferromagnetic phases. In the first one, the ionic moments of high-spin Co³⁺ ions in the pyramidal sites are ordered in a spiral arrangement, while octahedral sites are non-magnetic due to presence of low-spin Co³⁺ ions. The arrangement in the second phase is collinear of the G-type, with non-zero moments both in pyramidal (high-spin Co³⁺ ions) and octahedral sites (presumably a mixture of the low- and high-spin states). With increasing temperature, at 260–300 K, the system develops a gradual structural transformation, which is associated with appearance of spontaneous magnetic moment. This process is related to a thermally induced reversion of low- and high-spin states at the octahedral sites to the intermediate-spin Co³⁺ states, resulting in an insulator-metal transition at T_C ≈ T_IM ≈ 295 K.     

Spin state and magnetic interaction of cobalt ions in niobium-doped cobaltites

The magnetic properties and electrical conductivity of La_1?x Sr_xCo_1?x/2Nb _x/2O₃ solid solutions with trivalent cobalt ions are studied. These solid solutions are found to be spin glasses with T _f ～ 25 K. The ferromagnetic component is most pronounced in the composition with x = 0.15. The electrical conductivity decreases with increasing strontium content. The results obtained are interpreted within a model according to which cobalt ions located in the vicinity of strontium ions reside in an intermediate-spin state and the Co³⁺-O-Co³⁺ super-exchange interaction is ferromagnetic because of the local dynamic orbital correlations.     

Structure, transport and magnetic properties in La2xSr2−2xCo2xRu2−2xO6

The perovskite solid solutions of the type La_2xSr_2−2xCo_2xRu_2−2xO₆ with 0.25≤x≤0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for x=0.4 and 0.6. Most importantly, low field (50 Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate the presence of Co²⁺/Co³⁺ and Ru⁴⁺/Ru⁵⁺ redox couples in all compositions except x=0.5. Presence of magnetic ions like Ru⁴⁺ and Co³⁺ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.     

High-pressure effects on the crystal and magnetic structure of the Nd0.78Ba0.22CoO3 cobaltite

The crystal and magnetic structure of the Nd_0.78Ba_0.22CoO₃ cobaltite is studied by neutron diffraction at high pressures up to 4.2 GPa in the temperature range 10–300 K. The pressure dependences of structural parameters are obtained. Ferromagnetic ordering of the Co sublattice is observed at normal pressure below T _C ～ 140 K, and ferrimagnetic ordering of the Co and Nd sublattices with an antiparallel direction of magnetic moments appears at T _F ～ 40 K. The magnetic moment of Co and the temperature T _C change slightly under pressure, which points to the stability of the initial intermediate-spin (S = 1) state of Co³⁺ ions. This behavior differs considerably from the characteristic behavior of cobaltites that are close in chemical composition and structure and exhibit ferromagnetic ordering of only the Co sublattice. In these cobaltites, the magnetic moment of Co is substantially suppressed and T _C decreases under pressure, which is related to the change in the state of Co³⁺ ions from the intermediate spin state to the nonmagnetic low-spin state (S = 0). The interplay between the appearance of the magnetic interaction of the R-Co sublattices and the stability of the spin state of Co³⁺ ions in the Nd_0.78Ba_0.22CoO₃ cobaltite is discussed.     

The crystal field and exchange coupling in iron group metal carbonates

The wave functions of Co²⁺ and Fe²⁺ ions near the ground state in the CaCO_3? type lattice have been calculated from EPR data in the Abragam-Pryce approximation. The orbital angular momentum contributions to the anisotropic and antisymmetric parts of exchange coupling are determined assuming that this interaction between the magnetic ions occurring in nonequivalent positions is isotropic with respect to spin orientations. It is shown that, in the given approximation, the exchange coupling components in the basal plane for such Fe²⁺?Fe²⁺ and Co²⁺? Fe²⁺ ion pairs are missing. This fact explains the uniaxial antiferromagnetic ordering in FeCO₃ and the presence of a low-lying oscillation branch for Fe²⁺ impurity ions in antiferromagnetic CoCO₃. The EPR spectra of exchange-coupled Co²⁺?Co²⁺, Fe²⁺?Fe²⁺, and Co²⁺?Fe²⁺ pairs occupying nonequivalent positions have been calculated and their parameters have been numerically estimated.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Effect of hydrogenation vs. re-heating on intrinsic magnetization of Co doped In2O3

Influence of Co doping for In in In₂O₃ matrix has been investigated to study the effect on magnetic vs. electronic properties. Rietveld refinement of X-ray diffraction patterns confirmed formation of single phase cubic bixbyite structure without any parasitic phase. Photoelectron spectroscopy and refinement results further revealed that dopant Co²⁺ ions are well incorporated at the In³⁺ sites in In₂O₃ lattice and also ruled out formation of cluster in the doped samples. Magnetization measurements infer that pure In₂O₃ is diamagnetic and turns to weak ferromagnetic upon Co doping. Hydrogenation further induces a huge ferromagnetism at 300 K that vanishes upon re-heating. Experimental findings confirm the induced ferromagnetism to be intrinsic, and the magnetic moments to be associated with the point defects (oxygen vacancies V_o) or bound magnetic polarons around the dopant ions.     

Identification of local magnetic contributions in a Co2FeBO5 single crystal by XMCD spectroscopy

The temperature dependences of the X-ray absorption spectra (XAS) and of the spectra of X-ray magnetic circular dichroism (XMCD) are measured near the L _3,2 absorption edges of Co and Fe in ludwigite Co₂FeBO₅ single crystals. The antiparallel orientation of the magnetic moments of cobalt and iron is demonstrated. The coercive fields related to cobalt and iron ions are determined. The orbital (m _l ) and (m _s ) spin contributions to the total magnetic moments of cobalt and iron ions are identified. The ratios and relative directions of m _l and m _s are found.     

Doping induced spin state transition in Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> as studied by the GGA + DMFT calculations

The magnetic properties of Li _x CoO₂ for x = 0.94, 0.75, 0.66, and 0.51 are investigated within the method combining the generalized gradient approximation with dynamical mean field theory (GGA + DMFT). A delicate interplay between Hund’s exchange energy and t _2g ?e _g crystal field splitting is found to be responsible for the high-spin to low-spin state transition for Co⁴⁺ ions. The GGA + DMFT calculations show that the Co⁴⁺ ions at a small doping level adopt the high-spin state, while delithiation leads to an increase in the crystal field splitting and low-spin state becomes preferable. The Co³⁺ ions are found to stay in the low-spin configuration for any x values.     

Investigations on magnetically-dilute CoxZn1−x Rh2O4 spinel solid solution

Magnetic measurements have been performed on polycrystalline solid solutions Co_xZn_1?xRh₂O₄ with a spinel structure. The samples with 0.50? x ? 1.00 are antiferromagnets. The samples with x?0.40 do not show a magnetic order, and their magnetic behavior can be explained taking into account the presence of finite clusters of Co²⁺ ions and paramagnetic isolated ions.     

Crystal field effect on the magnetic moment's direction of rare earth ions with low point symmetry in r4co3 (r = Tb, Ho, Dy, Er, Tm) compounds

Neutron diffraction measurements, made on powder samples, show that Ho₄Co₃ and Er₄Co₃ intermetallic compounds are ferrimagnetic at 4.2 K. The magnetic moments of the 2 holmium sites are 8.7 and 2.1 μ_B and those of the erbium sites are equal to 8.7 and 8.1μ_B. The cobal⁺ magnetic moment is 0.2μ_B for both compounds. The easy magnetization direction lies on the hexagonal plane for Ho₄Co₃ while for Er₄Co₃ there are 2 anisotropy directions. Exchange interactions between rare-earth ions of both sites are very weak compared with the total crystal field splitting of the ground state multiplet J. The crystal field parameters are calculated and the magnitude and direction of the rare-earth magnetic moments in each site is determined.     

Temperature dependence of the spin state of a Co3+ Ion in RCoO3 (R = La,Gd) cobaltites

Changes in the spin state of Co³⁺ ions in LaCoO₃ and GdCoO₃ compounds are studied through the use of the temperature dependence of the magnetic susceptibility and the modified crystal field theory. It is shown that the spin subsystem of Co³⁺ ions in LaCoO₃ and GdCoO₃ undergoes the spin-crossover type transition between the high-spin (S = 2) and low-spin (S = 0) states without any contribution of the intermediate-spin state (S = 1).     

Study on the spin-states of cobalt-based double-layer perovskite Sr<Subscript>2</Subscript>Y<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>Co<Subscript>2</Subscript>O<Subscript>7</Subscript>

The spin-states of cobalt based perovskite compounds depend sensitively on the valence state and local crystal environment of Co ions and the rich physical properties arise from strong coupling among charge, spin, and orbital degrees of freedom. While extensive studies have been carried out in the past, most of them concentrated on the isotropic compound LaCoO₃. In this paper, using the unrestricted Hartree-Fock approximation and the real-space recursion method, we have investigated the competition of various magnetically ordered spin-states of anisotropic double-layered perovskite Sr₂Y_0.5Ca_0.5Co₂O₇. The energy comparison among these states shows that the nearest-neighbor high-spin-intermediate-spin ferromagnetically ordered state is the relevant magnetic ground state of the compound. The magnetic structure and sizes of magnetic moments are consistent with the recent experimental observation.     

Effects of calcium and zinc on the adsorption of cobalt on γ-Fe2O3

The effects of calcium and zinc on the room-temperature coercivity of γ-Fe₂O₃ particles having cobalt ions adsorbed in 3M NaOH solution at 373K have been studied. When the Ca²⁺ ions are adsorbed on the γ-Fe₂O₃ prior to Co²⁺ ions adsorption, the coercivity of Co-modified γ-Fe₂O₃ significantly increases compared with that of γ-Fe₂O₃ modified only with Co²⁺ ions. In the case of Zn²⁺ ions, the coercivity of Co-modified γ-Fe₂O₃ is the same as that of γ-Fe₂O₃ modified only with Co²⁺ ions. The emission Mössbauer spectrum of⁵⁷Co²⁺ adsorbed on the surfaces of γ-Fe₂O₃ after pretreating with Ca²⁺ ions consists of a typical sextet of Fe³⁺ ions with hyperfine magnetic field, whereas those of γ-Fe₂O₃ modified only with Co²⁺ ions and with both Zn²⁺ and Co²⁺ ions show nonmagnetic components in addition to magnetic one. The effects of Ca²⁺ and Zn²⁺ ions on the adsorption of cobalt on the surface of γ-Fe₂O₃ are discussed from the viewpoint of site preference energy of cations in ferrite and distribution ratio of each cation.     

Temperature dependent Mössbauer and neutron diffraction studies of Cu x Fe1−x Cr2S4 compounds

The cation distribution and magnetic structure of Cu _x Fe_1?x Cr₂S₄ (x?=?0.1, 0.2, 0.3, 0.4, and 0.5) has been studied by X-ray and neutron diffraction, vibrating sample magnetometer (VSM), and Mössbauer spectroscopy. The charge state of Fe is found to be ferrous (Fe²⁺) for the x?=?0.1 sample; ferric (Fe³⁺) for the x?=?0.5 sample; mixed state (Fe²⁺, Fe³⁺) for the x?=?0.2, 0.3, and 0.4 samples. The Mössbauer spectra of the x?=?0.1 sample show asymmetric line broadening, which is considered to be due to the Jahn–Teller effect of Cu²⁺ ions, and a symmetrical six-line pattern is shown for the x?=?0.5 sample. The valence state of the Cu ions for the x?=?0.1 and 0.5 samples is found to be divalent and monovalent, respectively. The magnetic structure of the samples was determined to be a ferrimagnetic structure with antiparallel alignment of the Fe and Cr ion magnetic moments.     

Crystal and magnetic structure of KFeO2

The crystal and magnetic structures of KFeO₂ have been determined by neutron and X-ray powder-diffraction and Mössbauer-effect techniques. The crystal structure at 4.2 K and 300 K is orthorhombic and the magnetic space group is Pbca'. The Fe³⁺-ions in this structure are tetrahedrally coordinated by oxygen ions, and each Fe³⁺-ion has a magnetic moment which is antiferromagnetically coupled to the moments of four Fe³⁺-neighbours. The direction of the moments is parallel to the a-axis. A crystal phase transition has been observed near the Néel temperature?960 K.     

Theory of magnetic excitations in KCoF3 at finite temperatures

Magnetic excitations in KCoF₃ at finite temperatures are studied by taking into account the residual orbital moment of the Co²⁺ ions. Equations of motion of Green functions related to all excitation operators between the molecular field states of the ⁴T₁ symmetry of the Co²⁺ ions are solved using the RPA approximation, and temperature-dependent dispersion curves are calculated.     

Effect of the diamagnetic dilution on the magnetic ordering and electrical conductivity in the Co3O2BO3: Ga ludwigite

Single crystals of cobalt ludwigite Co₃O₂BO₃ with diamagnetic substitution of Ga³⁺ ions for a part of the cobalt ions have been grown by the flux method. A detailed investigation of the crystal structure and magnetic properties of the compound has been carried out. A preferred character of the occupation of nonequivalent crystallographic positions by gallium has been revealed. It has been found that the effective magnetic moment and the magnetic ordering temperature are decreased compared to those in the original crystal of the Co₃O₂BO₃ ludwigite. It has been noted that the pronounced magnetic anisotropy observed in the crystallographic ab plane of the original material of the Co₃O₂BO₃ composition disappears in the presence of gallium.