We report a correlation between structural phase stability and magnetic properties of Co2FeO4 spinel oxide. We employed mechanical alloying and subsequent annealing to obtain the desired samples. The particle size of the samples changes from 25 nm to 45 nm. The structural phase separation of samples, except sample annealed at 9000C, into Co rich and Fe rich spinel phase has been examined from XRD spectrum, SEM picture, along with EDAX spectrum, and magnetic measurements. The present study indicated the ferrimagnetic character of Co2FeO4, irrespective of structural phase stability. The observation of mixed ferrimagnetic phases, associated with two Curie temperatures at TC1 and TC2 (>TC1), respectively, provides the additional support of the splitting of single cubic spinel phase in Co2FeO4 spinel oxide.
Mg batteries with oxide cathodes have the potential to significantly surpass existing Li-ion technologies in terms of sustainability, abundance, and energy density. However, Mg intercalation at the cathode is often severely hampered by the sluggish kinetics of Mg$^{2+}$ migration within oxides. Here we report a combined theoretical and experimental study addressing routes to identify cathode materials with an improved Mg-ion mobility. Using periodic density functional theory calculations, Mg$^{2+}$ migration in oxide spinels has been studied, revealing key features that influence the activatio…
Through direct nanoparticle nucleation and growth on nitrogen doped, reduced graphene oxide sheets and cation substitution of spinel Co3O4 nanoparticles, a manganese-cobalt spinel MnCo2O4/graphene hybrid was developed as a highly efficient electrocatalyst for oxygen reduction reaction (ORR) in alkaline conditions. Electrochemical and X-ray near edge structure (XANES) investigations revealed that the nucleation and growth method for forming inorganic-nanocarbon hybrid results in covalent coupling between spinel oxide nanoparticles and N-doped reduced graphene oxide (N-rmGO) sheets. Carbon K-edg…
The discovery of two-dimensional electron gases (2DEGs) at the heterointerface between two insulating perovskite-type oxides, such as LaAlO3 and SrTiO3, provides opportunities for a new generation of all-oxide electronic and photonic devices. However, significant improvement of the interfacial electron mobility beyond the current value of approximately 1,000 cm2V-1s-1 (at low temperatures), remains a key challenge for fundamental as well as applied research of complex oxides. Here, we present a new type of 2DEG created at the heterointerface between SrTiO3 and a spinel γ-Al2O3 epitaxial film w…
Sintered samples of a spinel oxide LiFeCr4O8, where Cr3+ and Fe3+ ions have localized moments, were found to show a large magnetic-field-induced volume increase approaching 500 ppm by applying a magnetic field of 9 T. This large volume increase appeared only at around 30 K. At 30 K, a spin-reorientation transition from ferrimagnetic to conical order occurs, giving rise to this large volume increase. The coexistence of ferrimagnetic and conical phases at this transition was found to be important, suggesting that such a large magnetic-field-induced volume change can be realized at various magnet…
We report high temperature synchrotron X-ray diffraction (SXRD), dc magnetization and current-voltage (I-V) characteristics for the samples of Co$_{2.75}$Fe$_{0.25}$O$_4$ ferrite. The material was prepared by chemical reaction of the Fe and Co nitrate solutions at pH = 11 and subsequent annealing at temperatures 200 0C, 500 0C and 900 0C. The measurements were performed by cycling the temperature from 300 K to high temperature (warming mode) and return back to 300 K (cooling mode). The SXRD patterns indicated a fine bi-phased cubic spinel structure in the highly Co rich spinel oxide. Magnetiza…
The normal spinel oxide MgCr2O4 is cubic at room temperature while the normal spinel CuCr2O4 is tetragonal as a consequence of the Jahn-Teller nature of Cu2+ on the tetrahedral sites. Despite different end-member structures, complete solid solutions of Mg_{1-x}Cu_xCr2O4 can be prepared that display a first-order structural transition with composition x = 0.43 at room temperature. Reverse Monte Carlo analysis of total neutron scattering on data acquired between 300 K and 15 K on samples with x = 0.10, 0.20, and 0.43 provides unbiased local and average structure descriptions of the samples, incl…
We have investigated the static and dynamic response of magnetic clusters in Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide, where a sequence of magnetic phase transitions, i.e., paramagnetic (PM) to ferromagnetic at T_{C} $\leq$ 270K and ferromagnetic to canted spin glass state at T_f$ $\leq$ 125K is observed.
We investigate the magnetic properties of the isostructural spinel-spinel interface of NiMn2O4(NMO)-Fe3O4. Although the magnetic transition temperature of the NMO film is preserved, both bulk and interface sensitive measurements demonstrate that the interface exhibits strong interfacial magnetic coupling up to room temperature. While NMO thin films have a ferrimagnetic transition temperature of 60K, both NiFe2O4 and MnFe2O4 are ferrimagnetic at room temperature. Our experimental results suggest that these magnetic properties arise from a thin interdiffused region of (Fe,Mn,Ni)3O4 at the interf…
Magneto-Structural Coupling Enables Cryogenic Cation Redistribution in a Spinel Oxide
Hainan University（海南大学）；Arizona State University（亚利桑那州立大学）；Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；Spallation Neutron Source Science Center（散裂中子源科学中心）；University of Science and Technology Beijing（北京科技大学）；Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所北京凝聚态物理国家实验室）
The spinel cobalt oxide Co3O4 is a magnetic semiconductor containing cobalt ions in Co2+ and Co3+ oxidation states. We have studied the electronic, magnetic and bonding properties of Co3O4 using density functional theory (DFT) at the Generalized Gradient Approximation (GGA), GGA+U, and PBE0 hybrid functional levels. The GGA correctly predicts Co3O4 to be a semiconductor, but severely underestimates the band gap. The GGA+U band gap (1.96 eV) agrees well with the available experimental value (~ 1.6 eV), whereas the band gap obtained using the PBE0 hybrid functional (3.42 eV) is strongly overesti…
I review the microscopic spin-orbital Hamiltonian and ground state properties of spin one-half spinel oxides with threefold $t_{2g}$ orbital degeneracy. It is shown that for any orbital configuration a ground state of corresponding spin only Hamiltonian is infinitely degenerate in the classical limit. The extensive classical degeneracy is lifted by the quantum nature of the spins, an effect similar to order-out-of-disorder phenomenon by quantum fluctuations. This drives the system to a non-magnetic spin-singlet dimer manifold with a residual degeneracy due to relative orientation of dimers. Th…
Epitaxial Fe/magnesium gallium spinel oxide (MgGa2O4)/Fe(001) magnetic tunnel junctions (MTJs) were fabricated by magnetron sputtering. Tunnel magnetoresistance (TMR) ratio up to 121% at room temperature (196% at 4 K) was observed, suggesting a TMR enhancement by the coherent tunneling effect in the MgGa2O4 barrier. The MgGa2O4 layer had a spinel structure and it showed good lattice matching with the Fe layers owing to slight tetragonal lattice distortion of MgGa2O4. Barrier thickness dependence of the tunneling resistance and current-voltage characteristics revealed that the barrier height of…
Many positive electrode materials in lithium ion batteries include transition metals which are difficult to describe by electronic structure methods like density functional theory (DFT) due to the presence of multiple oxidation states. A prominent example is the lithium manganese oxide spinel Li$_x$Mn$_2$O$_4$ with $0\leq x\leq2$. While DFT, employing the local hybrid functional PBE0r, provides a reliable description, the need for extended computer simulations of large structural models remains a significant challenge. Here, we close this gap by constructing a DFT-based high-dimensional neural…
Magnesium oxide and sulfide spinels have recently attracted interest as cathode and electrolyte materials for energy-dense Mg batteries, but their observed electrochemical performance depends strongly on synthesis conditions. Using first principles calculations and percolation theory, we explore the extent to which spinel inversion influences Mg$^{2+}$ ionic mobility in MgMn$_2$O$_4$ as a prototypical cathode, and MgIn$_2$S$_4$ as a potential solid electrolyte. We find that spinel inversion and the resulting changes of the local cation ordering give rise to both increased and decreased Mg$^{2+…
Disentangling the contributions of individual cations to magnetic order in a spinel high entropy oxide
University of British Columbia（不列颠哥伦比亚大学）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；University of Saskatchewan（萨斯喀彻温大学）；Canadian Light Source（加拿大光源）
Single-crystalline thin film of an iridium dioxide polymorph Ir2O4 has been fabricated by the pulsed laser deposition of LixIr2O4 precursor and the subsequent Li-deintercalation using soft chemistry. Ir2O4 crystallizes in a spinel (AB2O4) without A cations in the tetrahedral site, which is isostructural to lambda-MnO2. Ir ions form a pyrochlore sublattice, which is known to give rise to a strong geometrical frustration. This Ir spinel was found to be a narrow gap insulator, in remarkable contrast to the metallic ground state of rutile-type IrO2. We argue that an interplay of strong spin-orbit …
Dynamical properties of the lattice structure was studied by optical spectroscopy in ACr2O4 chromium spinel oxide magnetic semiconductors over a broad temperature region of T=10-335K. The systematic change of the A-site ions (A=Mn, Fe, Co, Ni and Cu) showed that the occupancy of 3d orbitals on the A-site, has strong impact on the lattice dynamics. For compounds with orbital degeneracy (FeCr2O4, NiCr2O4 and CuCr2O4), clear splitting of infrared-active phonon modes and/or activation of silent vibrational modes have been observed upon the Jahn-Teller transition and at the onset of the subsequent …
We experimentally investigated the magnetic properties of NiCo$_2$O$_4$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magneto-torque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature ($T_{\rm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although $T_{\mathrm{SR}}$ is dep…
A unique type of frustrated lattice is found in two A-site ordered spinel oxides, LiGaCr4O8 and LiInCr4O8. Because of the large size mismatch between Li+ and Ga3+/In3+ ions at the A site, the pyrochlore lattice, made up of Cr3+ ions carrying spin 3/2, becomes an alternating array of small and large tetrahedra, i.e., a "breathing" pyrochlore lattice. We introduce a parameter, the breathing factor Bf, which quantifies the degree of frustration in the pyrochlore lattice: Bf is defined as J'/J, where J' and J are nearest-neighbor magnetic interactions in the large and small tetrahedra, respectivel…
Spinel oxides are an ideal setting to explore the interplay between configurational entropy, site selectivity, and magnetism in high entropy oxides. In this work we characterize the magnetic properties of the spinel (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and study the evolution of its magnetism as a function of non-magnetic gallium substitution. Across the range of compositions studied here, from 0% to 40% Ga, magnetic susceptibility and powder neutron diffraction measurements show that ferrimagnetic order is robust in the spinel HEO. However, we also find that the ferrimagnetic order is highly tunable, wi…
Magnetic insulators are important materials for a range of next generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators which can be synthesized with surface qualities that would allow these smooth interfaces and precisely tuned interfacial magnetic exchange coupling which might be applicable at room temperature. In this work, we demonstrate an example of …
High-temperature Cu-Mn-Fe spinel-oxide nanoparticle solar selective absorber coatings are investigated experimentally and theoretically. A reliable, general approach to evaluate absorption coefficient spectra from the optical measurements of the nanoparticle-pigmented coatings is developed based on solving the inverse problem using four-flux-radiative method. The derived absorption properties of NP materials can be directly applied to predict the solar absorptance, optimize the nanoparticle-pigmented coatings, and analyze the thermal degradation, which agree well with the experimental results.…
LiTi2O4 is a unique material in that it is the only known oxide spinel superconductor. Although bulk studies have demonstrated that superconductivity can be generally described by the Bardeen-Cooper-Schreiffer theory, the microscopic mechanisms of superconductivity are not yet resolved fully. The sensitivity of the superconducting properties to various defects of the spinel crystal structure provides insight into such mechanisms. Epitaxial films of LiTi2O4 on single crystalline substrates of MgAl2O4, MgO, and SrTiO3 provide model systems to systematically explore the effects of lattice strain …
We have studied the structural, electronic and magnetic properties of spinel $\rm Co_3O_4$(111) surfaces and their interfaces with ZnO (0001) using density functional theory (DFT) within the Generalized Gradient Approximation with on-site Coulomb repulsion term (GGA+U). Two possible forms of spinel surface, containing $\rm Co^{2+} $ and $\rm Co^{3+} $ ions and terminated with either cobalt or oxygen ions were considered, as well as their interface with zinc oxide. Our calculations demonstrate that $\rm Co^{3+} $ ions attain non-zero magnetic moments at the surface and interface, in contrast to…
High-performance thermoelectric oxides could offer a great energy solution for integrated and embedded applications in sensing and electronics industries. Oxides, however, often suffer from low Seebeck coefficient when compared with other classes of thermoelectric materials. In search of high-performance thermoelectric oxides, we present a comprehensive density functional investigation, based on GGA$+U$ formalism, surveying the 3d and 4d transition-metal-containing ferrites of the spinel structure. Consequently, we predict MnFe$_2$O$_4$ and RhFe$_2$O$_4$ have Seebeck coefficients of $\sim \pm …
Ni incorporation has been studied in a comprehensive range of Zn/Co-based magnetic oxides to elucidate it valence state and lattice incorporation. The resulting structural and magnetic properties are studied in detail. To the one end Ni in incorporated by in-diffusion as well as reactive magnetron co-sputtering in wurtzite ZnO where only the Ni-diffused ZnO exhibits significant conductivity. This is complemented by Ni and Co codoping of ZnO leading. To the other end, the ZnCo$_2$O$_4$ spinel is co-doped with varying amounts of Ni. In the wurtzite oxides Ni is exclusively found on tetrahedral l…
We report the magnetic properties of mechanically milled Co$_{0.2}$Zn$_{0.8}$Fe$_2$O$_4$ spinel oxide. After 24 hours milling of the bulk sample, the XRD spectra show nanostructure with average particle size $\approx$ 20 nm. The as milled sample shows an enhancement in magnetization and ordering temperature compared to the bulk sample. If the as milled sample is annealed at different temperatures for the same duration, recrystallization process occurs and approaches to the bulk structure on increasing the annealing temperatures. The magnetization of the annealed samples first increases and the…
Ultra-Soft Ferrimagnetism in a High-Entropy Spinel Oxide Driven by Site-Selective Cation Disorder
Thapar Institute of Engineering and Technology（塔帕尔工程技术学院）；UGC-DAE Consortium for Scientific Research（UGC-DAE科学研究联盟）；Univ. Bordeaux, CNRS, Bordeaux INP, ICMCB, UMR 5026（波尔多大学、法国国家科学研究中心、波尔多国立理工学院、材料与化学研究组合中心）；Laboratory Crismat, UMR6508 CNRS, Normandie University, ENSICAEN, UNICAEN（Crismat实验室、法国国家科学研究中心联合研究实验室6508、诺曼底大学、恩西辛高等学院、卡昂大学）
High-temperature concentrating solar power (CSP) system is capable of harvesting and storing solar energy as heat towards cost-effective dispatchable solar electricity. Solar selective coating is a critical component to boost its efficiency by maximizing solar absorptance and minimizing thermal emittance losses. However, maintaining a high solar-thermal conversion efficiency >90% for long-term operation at >750 degrees C remains a significant challenge. Herein, we report spray-coated spinel Cu-Mn-Cr oxide nanoparticle-pigmented solar selective coatings on Inconel tube sections maintaining >94%…
We report on the electronic and optical properties of two theoretically predicted stable spinel compounds of the form ZnB2O4, where B = Ni or Cu; neither compound has been previously synthesized, so we compare them to the previously studied p-type ZnCo2O4 spinel. These new materials exhibit spin polarization that is ideal for spintronics applications, and broad conductivity maxima near the valence band edge that facilitate p-type dopability. We show that 3d electrons on the octahedrally coordinated Zn atom fall deep within the valence band and do not contribute significantly to the electronic …
Co$_3$O$_4$, ZnFe$_2$O$_4$, CoFe$_2$O$_4$, ZnCo$_2$O$_4$, and Fe$_3$O$_4$ thin films were fabricated by pulsed laser deposition at high and low temperatures resulting in crystalline single-phase normal, inverse, as well as disordered spinel oxide thin films with smooth surface morphology. The dielectric function, determined by spectroscopic ellipsometry in a wide spectral range from 0.5 eV to 8.5 eV, is compared with the magneto-optical response of the dielectric tensor, investigated by magneto-optical Kerr effect (MOKE) spectroscopy in the spectral range from 1.7 eV to 5.5 eV with an applied …
Graphene oxide continues to amaze scientific community for multiple potentials in a broad span of applications such as catalysts, adsorbents, oxidants, etc., determined by constant unveiling of its fantastic natures. Of them, magnetism is not ultimately identified and directly observed by naked eyes. Herein, we report graphene oxide directionally migrated and deposited together simply under external magnetic field from common Nd-Fe-B magnet, whereas the ferromagnetism of graphene oxide did not exhibit that strong as iron. Therefore, we illustrated this interesting pathway to keep close to such…
This review article aims at illustrating the recent progresses in the electrical control of magnetism in oxides with profound physics and enormous potential applications. In the first part, we provide a comprehensive summary of the electrical control of magnetism in the classic multiferroic heterostructures and clarify their various mechanisms lying behind. The second part focuses on the novel route of electric double layer gating for driving a significantly electronic phase transition in magnetic oxides by a small voltage. The electric field applied on the ordinary dielectric oxide in the thi…
Over the past few years, intensive studies of ultrathin epitaxial films of perovskite oxides have often revealed exciting properties like giant magnetoresistive tunnelling and electric field effects. Spinel oxides appear as even more versatile due to their more complex structure and the resulting many degrees of freedom. Here we show that the epitaxial growth of nanometric NiFe2O4 films onto perovskite substrates allows the stabilization of novel ferrite phases with properties dramatically differing from bulk ones. Indeed, NiFe2O4 films few nanometres thick have a saturation magnetization at l…
Bulk NiFe2O4 is an insulating ferrimagnet. Here, we report on the epitaxial growth of spinel NiFe2O4 ultrathin films onto SrTiO3 single-crystals. We will show that - under appropriate growth conditions - epitaxial stabilization leads to the formation of a spinel phase with magnetic and electrical properties that radically differ from those of the bulk material : an enhanced magnetic moment (Ms) - about 250% larger - and a metallic character. A systematic study of the thickness dependence of Ms allows to conclude that its enhanced value is due to an anomalous distribution of the Fe and Ni catio…
Lithium ion batteries often contain transition metal oxides like Li$_{x}$Mn$_2$O$_4$ ($0\leq x\leq2$). Depending on the Li content different ratios of Mn$^\text{III}$ to Mn$^\text{IV}$ ions are present. In combination with electron hopping the Jahn-Teller distortions of the Mn$^\text{III}$O$_6$ octahedra can give rise to complex phenomena like structural transitions and conductance. While for small model systems oxidation and spin states can be determined using density functional theory (DFT), the investigation of dynamical phenomena by DFT is too demanding. Previously, we have shown that a hi…
We present the magnetic properties of complete solid solutions of ZnCr$_2$O$_4$ and CoCr$_2$O$_4$: two well-studied oxide spinels with very different magnetic ground states. ZnCr$_2$O$_4$, with non-magnetic $d^{10}$ cations occupying the A site and magnetic $d^3$ cations on the B site, is a highly frustrated antiferromagnet. CoCr$_2$O$_4$, with magnetic $d^7$ cations (three unpaired electrons) on the A site as well, exhibits both Néel ferrimagnetism as well as commensurate and incommensurate non-collinear magnetic order. More recently, CoCr$_2$O$_4$ has been studied extensively for its polar b…
The discovery of conducting two-dimensional electron gas (2DEG) and magnetism at the interface between insulating nonmagnetic oxides, as exemplified by the polar LaAlO3 and nonpolar SrTiO3 has raised prospects for attaining interfacial functionalities absent in the component materials. Yet, the microscopic origin of such emergent phenomena remains unclear, posing obstacles to design of improved functionalities. Using first principles defect calculations, we reveal a unifying polarity-induced defect mechanism for both conductivity and magnetism at polar-nonpolar interfaces of nonmagnetic insula…
We have grown epitaxial films a few atomic layers thick of iron oxides on ruthenium. We characterize the growth by low energy electron microscopy. Using selected area diffraction and intensity vs. voltage spectroscopy, we detect two distinct phases which are assigned to wustite and magnetite. Spin polarized low energy electron microscopy shows magnetic domain patterns in the magnetite phase at room temperature.
The crystal and magnetic structures of stoichiometric ZnCr2Se4 have been investigated using synchrotron X-ray and neutron powder diffraction, muon spin relaxation (muSR) and inelastic neutron scattering. Synchrotron X-ray diffraction shows a spin-lattice distortion from the cubic spinel to a tetragonal I41/amd lattice below TN = 21 K, where powder neutron diffraction confirms the formation of a helical magnetic structure with magnetic moment of 3.04(3) μB at 1.5 K; close to that expected for high-spin Cr3+. MuSR measurements show prominent local spin correlations that are established at temper…
The occurence of spin-polarization at ZrO$_{2}$, Al$_{2}$O$_{3}$ and MgO surfaces is proved by means of \textit{ab-initio} calculations within the density functional theory. Large spin moments, as high as 1.56 $μ_B$, develop at O-ended polar terminations, transforming the non-magnetic insulator into a half-metal. The magnetic moments mainly reside in the surface oxygen atoms and their origin is related to the existence of $2p$ holes of well-defined spin polarization at the valence band of the ionic oxide. The direct relation between magnetization and local loss of donor charge makes possible t…
From Chemical Complexity to Tunable Magnetic Ordering in Highly Disordered High-Entropy Spinel Oxides
Thapar Institute of Engineering and Technology（塔帕尔工程技术学院）；UGC-DAE Consortium for Scientific Research（UGC-DAE科学研究联盟）；Freie Universität Berlin（柏林自由大学）；Helmholtz Zentrum Berlin für Materialien und Energie（柏林亥姆霍兹材料与能源中心）；Normandie University（诺曼底大学）
We report neutron scattering measurements of the spinel oxide LiGaCr$_{4}$O$_{8}$, in which magnetic ions Cr$^{3+}$ form a breathing pyrochlore lattice. Our experiments reveal the coexistence of a nearly dispersionless resonance mode and dispersive spin wave excitations in the magnetically ordered state, which can be quantitatively described by a quantum spin model of hexagonal loops and linear spin wave theory with the same set of exchange parameters, respectively. Comparison to other Cr spinel oxides reveals a linear relationship between the resonance energy and lattice constant across all t…
We have employed first-principles electronic structure calculations to examine the hypothetical (but plausible) oxide spinel, LiCr2O4 with the d^{2.5} electronic configuration. The cell (cubic) and internal (oxygen position) structural parameters have been obtained for this compound through structural relaxation in the first-principles framework. Within the one-electron band picture, we find that LiCr2O4 is magnetic, and a candidate half-metal. The electronic structure is substantially different from the closely related and well known rutile half-metal CrO2. In particular, we find a smaller co…
We perform a theoretical study of the magnetism induced in transition metal dioxides ZrO2 and TiO2 by substitution of the cation by a vacancy or an impurity from the groups 1A or 2A of the periodic table, where the impurity is either K or Ca. In the present study both supercell and embedded cluster methods are used. It is demonstrated that the vacancy and the K-impurity leads to a robust induced magnetic moment on the surrounding O-atoms for both the cubic ZrO2 and rutile TiO2 host crystals. On the other hand it is shown that Ca-impurity leads to a non magnetic state. The native O-vacancy does…
A structural phase transition from cubic $Fd\bar{3}m$ to tetragonal $I$4$_1$/$amd$ symmetry with $c/a >$ 1 is observed at $T_{\rm{S}}$ = 16 K in spinel GeCo$_2$O$_4$ below the Néel temperature $T_N$ = 21 K. Structural and magnetic ordering appear to be decoupled with the structural distortion occurring at 16 K while magnetic order occurs at 21 K as determined by magnetic susceptibility and heat capacity measurements. An elongation of CoO$_6$ octahedra is observed in the tetragonal phase of GeCo$_2$O$_4$. We present the complete crystallographic description of GeCo$_2$O$_4$ in the tetragonal $I…
We report on x-ray diffraction, magnetic susceptibility, electron- spin resonance and heat- capacity studies of Co[Al_1-xCo_x]_2O_4 for Co concentrations 0<x<1. In this spinel system only the A-site Co^2+ cation is magnetic, while the non-magnetic B-site Al^3+ is substituted by the low-spin non-magnetic Co^3+, and it is possible to investigate the complete phase diagram from Co^2+Al^3+_2O_4 to Co^2+Co^3+_2O_4. All samples reveal large negative Curie-Weiss temperatures Theta_CW of the order of -110 K independent of concentration, which is attributed to a high multiplicity of the superexchange i…
