Reentrant canonical spin-glass dynamics and tunable field-induced transitions in (GeMn)Co2O4Kagomé lattice

We report on the reentrant canonical semi spin-glass characteristics and controllable field-induced transitions in distorted Kagomé symmetry of (GeMn)Co2O4. ThisB-site spinel exhibits complicated, yet interesting magnetic behaviour in which the longitudinal ferrimagnetic (FiM) order sets in below the Néel temperatureTFN∼ 77 K due to uneven moments of divalent Co (↑ 5.33μB) and tetravalent Mn (↓ 3.87μB) which coexists with transverse spin-glass state below 72.85 K. Such complicated magnetic behaviour is suggested to result from the competing anisotropic superexchange interactions (JAB/kB∼ 4.3 K,JAA/kB∼ -6.2 K andJBB/kB∼ -3.3 K) between the cations, which is extracted following the Néel's expression for the two-sublattice model of FiM. Dynamical susceptibility (χac(f, T)) and relaxation of thermoremanent magnetization,MTRM(t) data have been analysed by means of the empirical scaling-laws such as Vogel-Fulcher law and Power law of critical slowing down. Both of which reveal the reentrant spin-glass like character which evolves through a number of intermediate metastable states. The magnitude of Mydosh parameter (Ω ∼ 0.002), critical exponentzυ= (6.7 ± 0.07), spin relaxation timeτ0= (2.33 ± 0.1) × 10-18s, activation energyEa/kB= (69.8 ± 0.95) K and interparticle interaction strength (T0= 71.6 K) provide the experimental evidences for canonical spin-glass state below the spin freezing temperatureTF= 72.85 K. The field dependence ofTFobtained fromχac(T) follows the irreversibility in terms of de Almeida-Thouless mean-field instability in which the magnitude of crossover scaling exponent Φ turns out to be ∼2.9 for the (Ge0.8Mn0.2)Co2O4. Isothermal magnetization plots reveal two field-induced transitions across 9.52 kOe (HSF1) and 45.6 kOe (HSF2) associated with the FiM domains and spin-flip transition, respectively. Analysis of the inverse paramagnetic susceptibilityχp-1χp=χ-χ0after subtracting the temperature independent diamagnetic termχ0(=-3 × 10-3emu mol-1Oe-1) results in the effective magnetic momentμeff= 7.654μB/f.u. This agrees well with the theoretically obtainedμeff= 7.58μB/f.u. resulting the cation distributionMn0.24+↓A[Co22+↑]BO4in support of the Hund's ground state spin configurationS=3/2andS= 1/2of Mn4+and Co2+, respectively. TheH-Tphase diagram has been established by analysing all the parameters (TF(H),TFN(H),HSF1(T) andHSF2(T)) extracted from various magnetization measurements. This diagram enables clear differentiation among the different phases of the (GeMn)Co2O4and also illustrates the demarcation between short-range and long-range ordered regions.

The Renewed Interest on Brunogeierite, GeFe2O4, a Rare Mineral of Germanium: A Review

GeFe₂O₄, also known as brunogeierite, is a rare mineral of germanium. It has a normal spinel structure and, as with many other spinels, amazing functional properties thanks to its peculiar structural features. In the past, its spectroscopic, optical, magnetic and electronic properties were determined; then, for many years, this compound was left behind. Only recently, a renewed interest in this oxide has arisen, particularly for its application in the electrochemical field. In this review paper, the crystal structure of GeFe₂O₄ will be described, as well as the synthesis methods required to obtain single crystals or polycrystalline powders. Its spectroscopic, magnetic, optical and electrical properties will be reported in detail. Then, successful applications known so far will be described: its use as anode in Lithium Ion and Sodium Ion Batteries and as electrocatalyst for urea oxidation reaction.

Tailoring the electronic structure and magnetic properties of pyrochlore Co2Ti1-xGexO4: a GGA +Uab initiostudy

We report the electronic structure and magnetic properties of Co₂Ti_1-xGe_xO₄(0 ⩽x⩽ 1) spinel by means of the first-principle methods of density functional theory involving generalized gradient approximation along with the on-site Coulomb interaction (U_eff) in the exchange-correlation energy functional. Special emphasis has been given to explore the site occupancy of Ge atoms in the spinel lattice by introducing the cationic disorder parameter (y) which is done in such a way that one can tailor the pyrochlore geometry and determine the electronic/magnetic structure quantitatively. For all the compositions (x), the system exhibits weak tetragonal distortion (c/a≠ 1) due to the non-degeneratedz2anddx2-y2states (e_gorbitals) of the B-site Co. We observe large exchange splitting (Δ_EX∼ 9 eV) between the up and down spin bands oft_2gande_gstates, respectively, of tetrahedral and octahedral Co²⁺(⁴A_2(g)(F)) and moderate crystal-field splitting (Δ_CF∼ 4 eV) and the Jahn-Teller distortion (Δ_JT∼ 0.9 eV). These features indicate the strong intra-atomic interaction which is also responsible for the alteration of energy band-gap (1.7 eV ⩽E_g⩽ 3.3 eV). The exchange interaction (J_BB∼ -4.8 meV, for (x,y) = (0.25, 0)) between the Co²⁺dominates the overall antiferromagnetic behaviour of the system for all 'x' as compared toJ_AA(∼-2.2 meV, for (x,y) = (0.25, 0)) andJ_AB(∼-1.8 meV, for (x,y) = (0.25, 0)). For all the compositions without any disorderness in the system, the net ferrimagnetic moment (Δμ) remains constant, however, increases progressively with increasingxdue to the imbalance of Co spins between the A- and B-sites.

Phase Dynamics on Conversion-Reaction-Based Tin-Doped Ferrite Anode for Next-Generation Lithium Batteries

The conventional view of conversion reaction is based on the reversibility, returning to an initial material structure through reverse reaction at each cycle in cycle life, which impedes the complete understanding on a working mechanism upon a progression of cycles in conversion-reaction-based battery electrodes. Herein, a series of tin-doped ferrites (Fe_{3- x}Sn _xO₄, x = 0-0.36) are prepared and applied to a lithium-ion battery anode. By achieving the ideal reoxidation into SnO₂, the Fe_2.76Sn_0.24O₄ composite anchored on reduced graphene oxide shows a high reversible capacity of 1428 mAh g^-1 at 200 mA g^-1 after 100 cycles, which is the best performance of Sn-based anode materials so far. Significantly, a newly formed γ-FeOOH phase after 100 cycles is identified from topological features through synchrotron X-ray absorption spectroscopy with electronic and atomic structural information, suggesting the phase transformation from magnetite to lepidocrocite upon cycling. Contrary to the conventional view, our work suggests a variable working mechanism in an iron-based composite with the dynamic phases from iron oxide to iron oxyhydroxide in the battery cycle life, based on the reactivity of metal nanoparticles formed during reaction toward the solid electrolyte interface layer.