Magnetic-field switching of crystal structure in an orbital-spin-coupled system: MnV2O4

Cubic MnV2O4 fabricated through a facile sol-gel process as an anode material for lithium-ion batteries: morphology and performance evolution

Metal vanadates have been popularly advocated as promising anode materials for lithium-ion batteries (LIBs) benefiting from their high theoretical specific capacity and abundant resources. Given that manganese and vanadium are reasonably economical elements and enjoy assorted redox reactions, they have extensive application prospects in energy storage systems. Here, we synthesized cubic MnV₂O₄ as an anode for LIBs by an efficient sol-gel process. As a result, the MnV₂O₄ electrode delivers distinguished electrochemical performance, including an appealing reversible specific capacity of nearly 1325 mA h g^-1 for 500 cycles at 200 mA g^-1, excellent cycling stability with a capacity of 399 mA h g^-1 up to 500 cycles at 2000 mA g^-1 and a favorable rate capability of 516/410 mA h g^-1 at 1000/2000 mA g^-1 (when the current density recuperates to 200 mA g^-1, the specific capacity still boosts as the number of cycles increases). What's more, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) under various scan rates and scanning electron microscopy (SEM) are executed to ascertain with a greater depth the electrochemical kinetic characteristics and morphology of the MnV₂O₄ electrode in different states. These results make known that MnV₂O₄ is a credible anode material for LIBs, and such a facile and economical synthetic route can be extended to the preparation of other metal vanadate materials.

Understanding the correlation between orbital degree of freedom, lattice-striction and magneto-dielectric coupling in ferrimagnetic Mn1.5Cr1.5O4

Dielectric anomaly observed in cubic Mn_1.5Cr_1.5O₄around ferrimagnetic ordering temperature (T_N) suggests a possible magneto-dielectric coupling in the system. This report confirms the presence of a weak but significant magneto-dielectric coupling in the system. Theab initiocalculations show a band gap of around 1.2 eV, with Fermi-level closer to the conduction band. The major features of conduction band nearest to the Fermi-level correspond tod_xzandd_3z²_-r²orbitals of Mn³⁺ion. Temperature-dependent neutron diffraction results show a rapid decay in structural parameters (lattice-striction and transition metal-oxygen bond length) aroundT_N.We confirmed that these changes in structural parameters atT_Nare not related to structural transition but the consequences of orbital-ordering of Mn³⁺. The rapid decay in transition metal-oxygen bond length under internal magnetism of the system shows that magnetism could certainly manipulate the electric dipole moment and hence the dielectric constant of the system. Magneto-striction acts as a link between magnetic and dielectric properties.

Sequence of superconducting states in field cooled FeCr2S4

In the present article we discuss theoretically the emergence of superconductivity in field cooled FeCr₂S₄. The chromium electrons form a triplett_2gstates and due to antiferromagnetic exchange with the iron spins have Zeeman splitting. Applied, during preparation, magnetic field along the moment of iron ions, successively compensates the Zeeman splittings. The chromium electrons with zero Zeeman energy form Cooper pairs induced by iron magnons. In that way, we predict theoretically the existence of sequence of superconducting states in field cooled FeCr₂S₄. Actually there are three different superconductors prepared applying, during preparation, different magnetic fields. In these compounds superconductivity coexist with the saturated magnetism of iron ions.

Orbital competition of Mn3+ and V3+ ions in Mn1+x V2-x O4

The structural and magnetic properties of Mn1+x V2-x O4 (0 < x ⩽ 1) have been investigated by the heat capacity, magnetization, x-ray diffraction and neutron diffraction measurements, and a phase diagram of temperature versus composition was built up. For x ⩽ 0.3, a cubic-to-tetragonal (c > a) phase transition was observed. For x > 0.3, the system maintained the tetragonal lattice. Although the collinear and noncollinear magnetic transitions of V3+ ions were obtained in all compositions, the canting angles between the V3+ ions decreased with Mn3+-doping, and the ordering of the Mn3+ ions was only observed as x > 0.4. In order to study the dynamics of the ground state, the first principles simulation was applied to analyze not only the orbital effects of Mn2+, Mn3+, and V3+ ions, but also the related exchange energies.

Orbital order and electron itinerancy in CoV2O4 and Mn0.5Co0.5V2O4 from first principles

In view of the recent experimental predictions of a weak structural transition in CoV₂O₄ we explore the possible orbital order states in its low temperature tetragonal phases from first principles density functional theory calculations. We observe that the tetragonal phase with I41/amd symmetry is associated with an orbital order involving complex orbitals with a reasonably large orbital moment at vanadium sites while in the phase with I41/a symmetry, the real orbitals with quenched orbital moment constitute the orbital order. Further, to study the competition between orbital order and electron itinerancy we considered Mn_0.5Co_0.5V₂O₄ as one of the parent compounds, CoV₂O₄, lies near itinerant limit while the other, MnV₂O₄, lies deep inside the orbitally ordered insulating regime. Orbital order and electron transport have been investigated using first principles density functional theory and Boltzmann transport theory in CoV₂O₄, MnV₂O₄ and Mn_0.5Co_0.5V₂O₄. Our results show that as we go from MnV₂O₄ to CoV₂O₄ there is enhancement in the electron's itinerancy while the nature of orbital order remains unchanged.

Pressure induced effects on the chemical and magnetic structure of spinel MnV2O4

The influence of external pressure (P  ⩽  5 GPa) on both the structural and magnetic ordering in MnV₂O₄ has been investigated using neutron diffraction technique. The volume and the V-V distance decrease with pressure while the c/a ratio increases, suggesting a lowering of the distortion with pressure. Under ambient conditions this compound exhibits a structural transition (T _S) from tetragonal to cubic at ~53 K and a magnetic transition (T _N ) at ~56 K. It is found that with an increase in pressure to 5 GPa, T _N increases (from 56 K to 80 K), dT _N /dP  >  0, while T _S decreases (from 53 K to 37 K). The non collinear magnetic structure in the tetragonal phase at 5 GPa and 10 K remains the same as at ambient pressure. However, the Mn and V sublattice, now exhibits distinct transition temperatures, [Formula: see text] ~ 80 K, and [Formula: see text] ~ 60 K. The transition to the cubic phase at T _S is accompanied by a collinear alignment of the Mn and V spins and a reduction in the Mn moment. The region in which the structure remains in the cubic phase with collinear magnetic structure increases with pressure from ~3 K at ambient pressure to ~43 K at 5 GPa pressure.

Spin-Orbital Correlated Dynamics in the Spinel-Type Vanadium Oxide MnV_{2}O_{4}

We investigate the magnetic dynamics in the spinel-type vanadium oxide MnV_{2}O_{4}. Inelastic neutron scattering around 10 meV and a Heisenberg model analysis have revealed that V^{3+} spin-wave modes exist at a lower-energy region than previously reported. The scattering around 20 meV cannot be reproduced with the spin-wave analysis. We propose that this scattering could originate from the spin-orbital coupled excitation. This scattering is most likely attributable to V^{3+} spin-wave modes, entangled with the orbital hybridization between t_{2g} orbitals.

Magnetocaloric Materials with Multiple Instabilities

The magnetocaloric effect is a well-known phenomenon where the temperature of a magnetic material varies upon application or removal of a magnetic field. This effect is anticipated to be applied to magnetic refrigeration technology, which is environmentally benign. For practical applications, it is essential to explore and expand the materials horizon of novel magnets that exhibit giant magnetocaloric effects to achieve sufficient cooling efficiency. In this article, several attempts to enhance the magnetocaloric effect are reviewed from the viewpoint of the competition or cooperation between the ferromagnetic interaction and other magnetic, electronic, and structural instabilities in strongly correlated materials. The results indicate that both the competition and cooperation between them promote the first-order nature of the magnetic transition, leading to giant magnetocaloric effects. Therefore, exploiting multiple instabilities is a promising strategy for exploring new magnetocaloric materials.

Orbital reorientation in MnV2O4 observed by V NMR

The simultaneous occurrence of the structural and magnetic phase transitions observed in MnV₂O₄ is one clear example of strong interplay among the spin, orbital and lattice degrees of freedom. The structure of MnV₂O₄ is switched by the magnetic field and the linear magnetostriction is very high. The orbital order mediates the interaction between the spin and the lattice generating these phenomena. In this work, we present experimental evidence of an orbital order in MnV₂O₄ and its reorientation under a rotating magnetic field as obtained by nuclear magnetic resonance(NMR). The shift in the resonance frequency of the V NMR spectrum is symmetrical with respect to 45° as an external magnetic field of 7 T rotates from the c-axis to the b-axis, indicating that the initial easy axis flips to the orthogonal direction most parallel to the field direction. The spectrum of V³⁺ ions splits into four peaks with a maximum shift of 40 MHz. Our analysis revealed that this is the combined effect of the anisotropic hyperfine field due to an ordered orbital and the dipolar hyperfine field. Reorientation of the orbital order in response to an external magnetic field accompanies the macroscopically observed magnetostriction in MnV₂O₄.

Application of strain to orbital-spin-coupled system MnV2O4 at cryogenic temperatures within a transmission electron microscope

The impact of mechanical stress on the morphology of crystallographic and magnetic domains in shape-controlled specimens of an orbital-spin-coupled system, MnV2O4, was examined by cryogenic Lorentz microscopy. Because of the difference in thermal expansion coefficients of MnV2O4 and the supporting Mo mesh, compression on the order of 0.01% was applied to the thin-foil specimens near the structural/magnetic phase transformation temperatures. The extent of compression was comparable to the lattice striction associated with the cubic-to-tetragonal phase transformation in MnV2O4 The applied strain thus clearly influenced the morphology of crystallographic domains (i.e. twinning configuration in the tetragonal phase) produced during cooling. The magnetic domain structure was entirely dependent on the configuration of twinning in the tetragonal phase. The observations in this study provided useful information for understanding the relationship between the crystallographic domains and the magnetic domains in MnV2O4.

Magnetic, structural, and thermal properties of CoV₂O₄

In this paper, we investigate the electric, magnetic, structural, and thermal properties of spinel CoV(2)O(4). The temperature dependence of magnetization shows that, in addition to the paramagnetic-to-ferrimagnetic transition at T(C) = 142 K, two magnetic anomalies exist at 100 K, T(1) = 59 K. Consistent with the anomalies, the thermal conductivity presents two valleys at 100 K and T(1). At the temperature T(1), the heat capacity shows one peak, which cannot be attributed to the structural transition as revealed by the x-ray diffraction patterns for CoV(2)O(4). Below the transition temperature T(1), the ac susceptibility displays the characteristics of a glass. The series of phenomena at T(1) and the orbital state on V(3+) sites are discussed.

Co[V2]o4: a spinel approaching the itinerant electron limit

Studies of the structure, magnetization, and resistivity under pressure on stoichiometric normal spinel Co[V(2)]O(4) single crystals show (i) absence of a structural distortion, (ii) abnormal magnetic critical exponents, and (iii) metallic conductivity induced by pressures at low temperatures. All these results prove that Co[V(2)]O(4) sits on the edge of the itinerant-electron limit. Compared with similar measurements on Fe[V(2)]O(4) and other A[V(2)]O(4) studies, it is shown that a critical V-V separation for a localized-itinerant electronic phase transition exists.

Observation of the large magnetocaloric effect in an orbital-spin-coupled system MnV(2)O(4)

The magnetocaloric effect (MCE) in an orbital-spin-coupled spinel vanadate MnV(2)O(4) is investigated by magnetization measurement. MnV(2)O(4) has ferrimagnetic ordering occurring at T(C) = 57 K. The maximum magnetic entropy change reaches 14.8 and 24.0 J kg(-1) K(-1) for field changes of 0-2 and 0-4 T, respectively. The maximum adiabatic temperature is about 2.9 K for a magnetic field change of 2 T. Except for the spin entropy change, the observed giant MCE is suggested to be related to the orbital entropy change due to the change of the orbital state of V(3+) induced by an applied magnetic field around T(C).

Proposed orbital ordering in MnV2O4 from first-principles calculations

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation +U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favors a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.

Towards the theory of ferrimagnetism: II

The present paper is a sequel to the paper by Karchev (2008 J. Phys.: Condens. Matter 20 325219). A two-sublattice ferrimagnet, with spin- s(1) operators S(1i) at the sublattice A site and spin-s(2) operators S(2i) at the sublattice B site, is considered. Renormalized spin-wave theory, which accounts for the magnon-magnon interaction, and its extension are developed to describe the two ferrimagnetic phases (0,T(*)) and (T(*),T(N)) in the system, and to calculate the magnetization as a function of temperature. The influence of the parameters in the theory on the characteristic temperatures T(N) and T(*) is studied. It is shown that, increasing the inter-sublattice exchange interaction, the ratio T(N)/T(*)>1 decreases approaching one, and above some critical value of the exchange constant there is only one phase T(N) = T(*), and the magnetization-temperature curve has the typical Curie-Weiss profile. When the intra-exchange constant of the sublattice with stronger intra-exchange interaction increases the Néel temperature increases while T(*) remains unchanged. Finally, when the magnetic order of the sublattice with smaller magnetic order decreases, T(*) decreases. The theoretical predictions are utilized to interpret the experimentally measured magnetization-temperature curves.

Derivation of the heat capacity anomaly at a first-order transition by using a semi-adiabatic relaxation technique

This paper deals with the problem of determining the heat capacity anomaly associated with a first-order transition when using relaxation calorimetry. A method of data recording and analysis is proposed, which is shown to be well suited to investigate such a feature, including its hysteretical character. This technique is applied to spinel vanadates, allowing us to shed light on a recent controversy about the double-transition which takes place in these oxides.

Ferrimagnetism of MnV(2)O(4) spinel

The spinel MnV(2)O(4) is a two-sublattice ferrimagnet, with site A occupied by the Mn(2+) ion and site B by the V(3+) ion. The magnon of the system, the transversal fluctuation of the total magnetization, is a complicated mixture of the sublattice A and B transversal magnetic fluctuations. As a result, the magnons' fluctuations suppress in a different way the manganese and vanadium sublattice magnetization and one obtains two phases. At low temperature (0,T(*)) the sublattice Mn magnetization and sublattice V magnetization contribute to the magnetization of the system, while at a high temperature (T(*),T(N)), the vanadium sublattice magnetization is suppressed by magnon fluctuations, and only the manganese ions have non-zero spontaneous magnetization. A modified spin-wave theory is developed to describe the two phases and to calculate the magnetization as a function of temperature. The anomalous M(T) curve reproduces the experimentally obtained zero-field-cooled (ZFC) magnetization.

Magnetic and orbital ordering in the spinel MnV2O4

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.

Orbital ordering and magnetic field effect in MnV2O4

We studied the structural properties of an orbital-spin-coupled spinel oxide, MnV2O4, mainly by single-crystal x-ray diffraction measurement. It was found that a structural phase transition from cubic to tetragonal and ferrimagnetic ordering occur at the same temperature (Ts,TN=57 K). The structural phase transition was induced also by magnetic field above Ts. In addition, magnetic-field-induced alignment of tetragonal domains results in large magnetostriction below Ts. We also found that the structural phase transition is caused by the antiferro-type ordering of the V t2g orbitals.