EPR powder spectra of Co(II) and Cr(III) in MgIn2S4 spinel

The EPR powder spectra of high spin Co(II) and Cr(III) diluted in partially inverse spinel matrix MgIn₂S₄ are reported. While the spectrum of Co(II) in octahedral sites is orthorhombic the Cr(III) one is axial. Evidence is given that the EPR spectra cannot be directly associated with the cation distribution of the matrix.     

Magnetodielectric coupling in MnCr2O4 spinel

We have investigated the magnetic and dielectric properties of polycrystalline samples of the spinel MnCr₂O₄. Below the ferrimagnetic ordering temperature at T_N∼43 K, both magnetization and dielectric measurements show signatures of the onset of a conical structure at T_s∼17 K and a lock-in temperature at T_f∼14 K. These values are similar to those previously reported for single-crystal samples, where the spiral structure is short-range ordered (SRO) at low temperatures. The application of magnetic field suppresses the dielectric anomaly at T_f indicating that the coherence length of the ordering increases. MnCr₂O₄ exhibits a symmetrical magnetodielectric response between T_f and T_s that scales with the square of the magnetization. This suggests that the magnetodielectric coupling originates from the P²M² term in the free energy expansion. The magnetodielectric response becomes asymmetric with respect to field below T_f.     

Immiscibility in the NiFe2O4-NiCr2O4 spinel binary

The solid solution behavior of the Ni(Fe_1−nCr_n)₂O₄ spinel binary is investigated in the temperature range 400-1200 °C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200 °C. Air-annealing for 1 year at 600 °C resulted in partial phase separation in a spinel binary having n=0.5. Spinel crystals grown from NiO, Fe₂O₃ and Cr₂O₃ reactants, mixed to give NiCrFeO₄, by Ostwald ripening in a molten salt solvent, exhibited single-phase stability down to about 750 °C (the estimated consolute solution temperature, T_cs). A solvus exists below T_cs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300 °C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.     

Lattice vibrations of MgAl2O4 spinel

Raman spectra have been measured on oriented single crystals of MgAl₂O₄ spinel. The fundamental frequencies are A_1g = 772, E_g = 410, and T_2g = 671, 492 and 311 cm^?1. A Kramers-Kronig analysis of the reflectance spectrum of spinel yields the i. r. -active vibrations T_1u = 670, 485, 428 and 305 cm^?1.     

Chemical synthesis of spinel nickel ferrite (NiFe2O4) nano-sheets

The present investigation is related to the deposition of single-phase nano-sheets spinel nickel ferrite (NiFe₂O₄) thin films onto glass substrates using a chemical method. Nano-sheets nickel ferrite films were deposited from an alkaline bath containing Ni²⁺ and Fe²⁺ ions. The films were characterized for their structural, surface morphological and electrical properties by means of X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and two-point probe electrical resistivity techniques. The X-ray diffraction pattern showed that NiFe₂O₄ nano-sheets are oriented along (3 1 1) plane. The FT-IR spectra of NiFe₂O₄ films showed strong absorption peaks around 600 and 400 cm⁻¹ which are typical for cubic spinel crystal structure. Microstructural study of NiFe₂O₄ film revealed nano-sheet like morphology with average sheet thickness of 30 nm. The room temperature electrical resistivity of the NiFe₂O₄ nano-sheets was 10⁷ Ω cm.     

Immiscibility in the Fe3O4-FeCr2O4 spinel binary

A recent thermodynamic model of mixing in spinel binaries, based on changes in cation disordering (x) between tetrahedral and octahedral sites [Am. Mineral. 68 (1983) 18, 69 (1984) 733], is investigated for applicability to the Fe₃O₄-FeCr₂O₄ system under conditions where incomplete mixing occurs. Poor agreement with measured consolute solution temperature and solvus [N. Jb. Miner. Abh. 111 (1969) 184] is attributed to neglect of: (1) ordering of magnetic moments of cations in the tetrahedral sublattice antiparallel to the moments of those in the octahedral sublattice and (2) pair-wise electron hopping between octahedral site Fe³⁺ and Fe²⁺ ions. Disordering free energies (ΔG_D), from which free energies of mixing are calculated, are modeled by

     

Electronic structure of dimerized spinel ZnV2O4

Electronic structure calculations were performed for ZnV₂O₄, a material close to a metal-insulator transition. Structural optimization leads to the formation of V-V dimers along the off-plane chains. A strong spin-lattice coupling is expected close to the transition to itinerancy. No orbital ordering is observed in such a structure, and the experimentally found magnetic structure is naturally explained.     

Ground-state magnetism of chromium-substituted LiMn2O4 spinel

Comparative crystal structure and magnetic properties studies have been conducted on quaternary powder spinel samples LiMn_1.82Cr_0.18O₄ obtained by two different synthesis methods, glycine-nitrate (GN) and ultrasonic spray-pyrolysis (SP). Although both samples possess the same spinel structure of the cubic space group Fd3¯m, their low-temperature magnetic properties display significant differences. While the SP sample undergoes only spin-glass transition at the freezing temperature T_f=20 K, the GN sample possesses more complicated low-temperature magnetic behavior of the reentrant spin-glass type with the Néel temperature T_N=42 K and freezing temperature T_f=22 K. High-temperature magnetic susceptibility of both samples is of the Curie–Weiss type with the effective magnetic moments in agreement with the nominal compositions. This fact together with the results of the chemical analysis discards the existence of the diversity in chemical compositions as a possible cause for the observed differences in the low-temperature magnetism. On the other hand, the crystal structure analysis done by the Rietveld refinement of the X-ray powder diffraction data points to the strong influence of the cation distribution on the ground-state magnetism of these systems. An explanation of this influence is proposed within the framework of a collective Jahn–Teller effect.     

Structural information from EPR powder spectra of Mn(II)-doped Rb2MgCl4, Rb2MgCl3Br and Rb2MgCl2Br2

The structure of the compounds Rb₂MgCl₄, Rb₂MgCl₃Br and Rb₂MgCl₂Br₂ has been determined to be the tetragonal K₂NiF₄ structure. EPR spectra at X and Q-band frequencies of the polycrystalline samples of the Mn(II)-doped compounds, indicate that the Br^? ions prefer to order at special sites of the structure, as concluded from the fact that only octahedral groups [MgCl₆] and tetragonal groups [MgCl₄Br₂] are found.     

Spin-glass-like ordering in the spinel ZnFe2O4 ferrite

In the present study, spin-glass-like ordering has been observed in the spinel ZnFe₂O₄ ferrite. Field cooled (FC) and zero-field cooled (ZFC) DC magnetizations display divergence at low temperature, which indicates a frozen state with the freezing temperature of T_f=21 K. Frequency dependence of AC susceptibility measurement was performed on the sample. It shows a peak at around T_f, with the peak position shifting as a function of driving frequency, indicating a spin-glass-like transition of the sample. The sample shows a typical spin-glass behavior with a manifestation of non-equilibrium dynamics of the spin glass, such as aging, rejuvenation and memory effects. These experimental findings indicate that Zn-ferrite exhibits a spin-glass-like phase at low temperature and it is not canted antiferromagnetic.     

Effects of Mn substitution of Co and Fe in spinel CoFe2O4 thin films

Effects of Mn substitution for Co and Fe on the structural and magnetic properties of inverse-spinel CoFe₂O₄ have been investigated. Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ thin films were prepared by a sol–gel method. The observed increase of the lattice constant of Mn_xCo_1−xFe₂O₄ indicates that Mn²⁺ ions substitute the octahedral Co²⁺ sites. Conversion electron Mössbauer spectroscopy data indicate that a fraction of octahedral Co²⁺ ions exchange sites with tetrahedral Fe³⁺ ions through Mn doping. Vibrating-sample magnetometry data exhibit a large increase of saturation magnetization for both Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ films compared to that of the CoFe₂O₄ film. Such enhancement of magnetization can be explained in terms of a breaking of ferrimagnetic order induced by the Co²⁺ migration.     

Jahn-Teller transition in Al doped LiMn2O4 spinel

Al-doped lithium manganese spinels, with starting composition Li_1.02Al_xMn_1.98−xO₄ (0.00<x≤0.06), are investigated to determine the influence of the Al³⁺ doping on the Jahn-Teller (J-T) cooperative transition temperature T_J-T. X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al³⁺ ions in the spinel phase. It is observed that Al³⁺ may distribute between the two cationic sublattices. The J-T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x>0.04 T_J-T it increases with increasing x, as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior.     

An orthorhombic chromium-center in the spinel MgAl2O4

In Cr-doped MgAl₂O₄ most of the Cr³⁺-ions occupy normal B-sites. In addition to this axial center we report here the presence of an orthorhombic Cr³⁺-center. The ESR-spectra of this new center can be described by the spin Hamiltonian:

$?=\mathit{D}\left[S_z^2?\frac{1}{3}\mathit{S}\left(S+1\right)\right]+g_x\beta H_x{S}_x+g_y\beta H_y{S}_y+g_z\beta H_z{S}_z+\mathit{E}\left(S_x^2?S_y^2\right)$

, with S=3/2. The principal axes of the center are along [111]=Z, $\left[111\right]=Z,\left[1\overline{1}0\right]=Y$ and $\left[\overline{1}\overline{1}2\right]=X$. The values of the parameters are: g_z=1·950±0·005, g_x=g_y=1·915±0·015, D=+0·903cm^?1±0·003 cm^?1 and E=+0·084 cm^?1±0·005 cm^?1. The intensity of the lines is about ten times lower than that of the axial Cr³⁺-center.     

Physical properties of the cubic spinel LiMn2O4

We have performed an ab initio study of structural, electronic, magnetic, vibrational and thermal properties of the cubic spinel LiMn₂O₄ by employing the density functional theory, the linear-response formalism, and the plane-wave pseudopotential method. An analysis of the electronic structure with the help of electronic density of states shows that the density of states at the Fermi level (N (E_F)) is found to be governed by the Mn 3d electrons with some contributions from the 2p states of O atoms. It is important to note that the contribution of Mn 3d states to N(_EF)$N\left(E_F\right)$ is as much as 85%. From our phonon calculations, we have obtained that the main contribution to phonon density of states (below 250 cm⁻¹) comes from the coupled motion of Mn and O atoms while phonon modes between 250 cm⁻¹ and 375 cm⁻¹ are characterized by the vibrations of all the three types of atoms. The contribution from Li increases rapidly at higher frequency (above 375 cm⁻¹) due to the light mass of this atom. Finally, the specific heat and the Debye temperature at 300 K are calculated to be 249.29 J/mol K and 820.80 K respectively.     

EPR and optical investigation of VO(II) in Zn(C3H3O4)2(H2O)2 single crystals: An interstitial site

EPR spectroscopic investigations on single crystals of diaquabis[malonato(1-)-κ²O,O′] zinc(II) doped with VO(II) ion have been carried out at X-band frequencies and at 300 K. The single crystal, rotated along the three mutually orthogonally axes, has yielded spin-Hamiltonian parameters g and A as: g_xx=1.980, g_yy=1.972, g_zz=1.937 and A_xx=8.4, A_yy=6.1, A_zz=18.1 mT, respectively. These spin-Hamiltonian parameters reflect a slight deviation from axial symmetry to rhombic, which is elucidated by the interstitial occupation of vanadyl ions. The isofrequency plots and powder EPR spectrum have been simulated. The percentage of metal-oxygen bond has been estimated. The optical absorption spectrum exhibits four bands at 257, 592, 720 and 764 nm suggesting a C_4v symmetry. The admixture coefficients and bonding parameters have also been calculated by collaborating EPR data with optical data.     

High pressure Raman spectroscopic study of spinel MgCr2O4

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of MgCr₂O₄ spinel up to pressures of 76.4 GPa. Results indicate that MgCr₂O₄ spinel undergoes a phase transformation to the CaFe₂O₄ (or CaTi₂O₄) structure at 14.2 GPa, and this transition is complete at 30.1 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish transition mechanism. No evidence was observed to support the pressure-induced dissociation of MgCr₂O₄ at 5.7-18.8 GPa, predicted by the theoretical simulation. This high pressure MgCr₂O₄ polymorphism remains stable upon release of pressure, but at ambient conditions, it transforms to the spinel phase.     

Al掺杂的尖晶石型LiMn2O4的结构和电子性质

采用基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)和GGA+U方法下对尖晶石型LiMn₂O₄及其Al掺杂 的尖晶石型LiAl_0.125Mn_1.875O₄晶体的结构和电子性质进行了计算. 结果表明: 采用GGA方法得到尖晶石型LiMn₂O₄是立方晶系结构, 其中的Mn离子为+3.5价, 无法解释它的Jahn-Teller 畸变. 给出的LiMn₂O₄能带结构特征也与实验结果不符. 而采用GGA+U方法得到在低温下的LiMn₂O₄和其掺杂 体系LiAl_0.125Mn_1.875O₄的晶体都是正交结构, 与实验一致. 也能明确地确定Mn的两种价态Mn³⁺/Mn⁴⁺的分布并且能够说明Mn³⁺O₆的z方向有明显的Jahn-Teller 畸变, 而Mn⁴⁺O₆则没有畸变. LiMn₂O₄的能带结构与实验比较也能够符合. 采用GGA+U方法对Al掺杂体系的LiAl_0.125Mn_1.875O₄的研究表明, 用Al替换一个Mn不会明显地改变晶体的电子性质, 但可以有效地消除Al³⁺O₆ 八面体的Jahn-Teller畸变, 从而改善正极材料LiMn₂O₄的性能, 这与电化学实验的观察结果相一致.     

Mössbauer hyperfine spectra of dilute Fe3+ in synthetic MgAl2O4 spinel

Mössbauer effect spectra of ⁵⁷Fe diffused into powdered synthetic spinel, MgAl₂O₄, exhibit paramagnetic hyperfme structure. The iron was determined to be equally distributed between the tetrahedral and octahedral sites, with the tetrahedrally coordinated iron having a longer spin-lattice relaxation time. Analysis of the spectra taken in small applied fields between 30 and 1000 Oersted at 4.2 and 1.5 K yielded the electronic zero field splitting parameters and showed both sites to have significant departures from axial symmetry.     

Unusual magnetic hysteresis behavior of oxide spinel MnCo2O4

Magnetic hysteresis behavior of the oxide spinel MnCo₂O₄ has been studied at different temperatures below its T_c≈184 K. Normal hysteresis behavior is observed down to 130 K whereas below this temperature the initial magnetization curve, at higher magnetic fields, lies outside the main loop. No related anomaly is observed in the temperature variation of magnetization or coercivity. However, the anisotropy field overcomes the coercivity below 130 K. The unusual magnetic hysteresis behavior of MnCo₂O₄, at low temperatures, may be associated with irreversible domain wall movements due to the rearrangement of the valence electrons.     

The micromagnetic study of the vanadate spinel MnV2O4

MnV₂O₄ exhibits a paramagnetic to ferrimagnetic transition at 57 K and shows significant magnetic hysteresis below 55 K. By performing detailed powder X-ray diffraction at the same temperature during cooling and warming sequences, it is found that the magnetic hysteresis observed here is owing to strains induced by the structural phase separation. The intensity of the electron spin resonance spectra shows unusual temperature dependence, which might be related to the phase separation induced by the structural transition. By performing a mean field analysis, we obtained the exchange energies among the different magnetic moments and qualitatively understood the micromagnetic properties.