EPR and magnetic susceptibility studies of xCr2O3-(1-x) 3B2O3-PbO glasses

By using X-ray diffraction analysis, EPR and magnetic measurements, the chromium-ion distribution in xCr₂O₃-(1–x) [3B₂O₃-PbO] glasses with 0x35 mol% Cr₂O₃ was studied. EPR investigation evidenced the presence of both Cr³⁺ and Cr⁵⁺ ions, the latter being in small proportion, in agreement with the atomic magnetic moment values. For concentrations x20 mol% Cr₂O₃ the isolated Cr³⁺ ions coexist with those coupled by super-exchange magnetic interactions, the isolated ones prevailing only for x<3 mol% Cr₂O₃. For x>20 mol % Cr₂O₃ microcrystalline precipitates were detected, giving rise to an antiferromagnetic transition with Néel temperature, N _N310 K and paramagnetic Curie temperature, _p=–480 K.     

Mictomagnetism in a new BaO.Fe2O3.B2O3 glass.

An homogeneous BaO ·Fe₂O₃ ·B₂O₃ glass containing 30% Fe₂O₃ is prepared by a splat-cooling technique. X-ray and electron diffraction reveal the product to be amorphous at room temperature. The crystallization, as shown by DTA studies, begins at 750 K, and up to 950 K, a temperature at which BaFe₁₂O₁₉ is shown to be present, the crystallizing products are mainly evolutive.The magnetic measurements show a probably mictomagnetic behaviour at low temperature with a maximum of magnetic susceptibility at 12 (±1) K. At higher temperature the susceptibility obeys a Curie-Weiss law with large negative Weiss constant and low Curie constant per Fe³⁺ ions. After crystallization the product is ferrimagnetic and could be used as a permanent magnet.Mössbauer study reveals that the glass mainly consists of Fe³⁺ ions in distorted sites and a hyperfine structure at low temperature; the magnetic ordering temperature is estimated to be about 44 (±1) K.     

Magnetic properties of the phases Fe2V4O13, FeVMoO7 and Fe4V2Mo3O20

The Curie constant,C, Weiss constant, , effective magnetic moment, _eff, and spectroscopic splitting factor,g, were determined for the Fe³⁺ ions in Fe₂V₄O₁₃, FeVMoO₇ and Fe₄V₂Mo₃O₂₀ at 76–300 K based on measurements of magnetic susceptibility of the phases. The Neel temperature,T _N, of interest was based on the temperature dependence of magnetization of the phases. It was shown that a local antiferromagnetic arrangement of the Fe³⁺ ions in Fe₂V₄O₁₃, FeVMoO₇ and Fe₄V₂Mo₃O₂₀ is already involved at a temperature much higher than the Neel temperature, resulting from the cation-anion-cation superexchange between the Fe³⁺ ions with ad ⁵ configuration.     

Magnetic ordering in Cu-Zn ferrite

The variation of magnetization with temperature of the Zn _x Cu_1–x Fe₂O₄ system has been obtained between 300 K and the Néel temperature at a constant magnetic field of 5.57×10⁵ A m^–1 for x=0 to 0.8. The observations indicate the existence of a Yafet-Kittel (Y-K) type of magnetic ordering in the mixed ferrites. A molecular field analysis of the Y-K spin-ordering using a three-sublattice model is shown to explain the experimental data satisfactorily. For the sake of verification, Néel temperatures of Cu-Zn ferrites were also determined from Mössbauer studies.     

Mössbauer and magnetic studies of dicalcium ferrite (Ca2Fe2O5)

In the antiferromagnetic compound Ca₂Fe₂O₅, the ferric ions occupy two non-equivalent sites. By means of Mössbauer studies in the temperature range 80 to 800 ° K, the variation of the magnetic hyperfine fields at both sites, as a function of temperature, is determined. It is found that all ferric ions order antiferromagnetically at a single Néel temperature, T _N=(725±2) ° K.From previous studies, the magnetic point group of this compound is known, and leads one to anticipate the existence of a ferromagnetic component in the ordered state. Magnetic measurements do, in fact, indicate the existence of such a component.The experimental values obtained for the quadrupole splittings at both ferric sites are compared with point charge calculations. This leads to a complete disagreement, both in the ordered region and in the paramagnetic region, showing the inadequacy of the point charge model for the present case.All the results are compared with the results of previous investigations on the same compound.Sponsored in part by the Air Force Materials Laboratory Research and Technology Division AFSC through the European Office of Aerospace Research, United States Air Force Contract F61052-67 C-0040.on leave from the Weizmann Institute.     

Magnetic and electrical properties of Al3+-substituted MgFe2O4

The structural, magnetic and electrical properties of the Al³⁺-substituted disordered spinel system Mg(Fe_2–x Al _x )O₄ have been investigated by X-ray diffraction, magnetization, a.c. susceptibility and electrical resistivity measurements. The cation distribution derived from the X-ray diffractometry data was found to agree very well with the cation distribution obtained through Mössbauer spectroscopy. The variation of saturation magnetization per formula unit as a function of aluminium context, x, has been satisfactorily explained on the basis of Neel's collinear spin model and the slight discrepancy between the observed and calculated n _B values can be explained in terms of a random canting model. The Néel temperatures calculated theoretically by applying molecular field theory agreed well with the experimentally determined values from thermal variation of susceptibility and electrical resistivity. An unusual metal-like thermo-electric behaviour was found for the compositions with x 0.3 which was attributed to the decrease in the Fe-Fe separation distance arising from aluminium substitution.     

Magnetization, magnetic susceptibility, and effective magnetic moment of the Fe3+ ions in Bi2Fe4O9

Bi₂Fe₄O₉ with an orthorhombic structure and lattice parameters a = 7.9595 Å, b = 8.4297 Å, c = 5.9912 Å, and V = 401.987 ų has been prepared by solid-state reactions method. Its molar magnetic susceptibility measured as a function of temperature in the range 5–950 K indicates that Bi₂Fe₄O₉ is an antiferromagnet with a Néel temperature of 258 K. In the range 280–750 K, its molar magnetic susceptibility exhibits Curie-Weiss behavior, which allowed us to determine the Weiss constant (Θ = ?1468 K) of this material and the effective magnetic moment of the Fe³⁺ ions $\left( {\mu _{eff}^{Fe^{3 + } } = 6.37\mu _B } \right)$ . Magnetization versus magnetic field data show no magnetic hysteresis, indicating that the Bi₂Fe₄O₉ sample studied exhibits no weak ferromagnetism.     

Effect of the addition of Fe2O3 and heat treatment duration on the magnetic susceptibility of V2O5-P2O5-B2O3 glass system

The effect of the addition of Fe₂O₃ and heat treatment duration on the magnetic susceptibility of vanadium borophosphate glass were studied. The magnetic susceptibility of glass samples was found to increase with increasing Fe₂O₃ content, which may be explained by the formation of the FeO₆ group and the change of Fe²⁺ to Fe³⁺ which has higher paramagnetic properties. No detectable changes in the magnetic susceptibility with heat treatment for the samples containing 0.0, 0.5 and 1.0 mol% Fe₂O₃ was observed. The magnetic susceptibility for the heat treated samples containing 2.5, 5.0 and 7.5 mol% Fe₂O₃ decreases sharply with increasing duration of heat treatment up to 6 h and then remains almost constant. The sharp decrease in magnetic susceptibility of 2.5 mol% Fe₂O₃ is attributed to the increase in the number of ferrous ions. The sharp decrease for samples containing 5.0 and 7.5 mol% Fe₂O₃ is attributed to the increase in the number of Fe³⁺ in tetrahedral co-ordination. The rate of crystallization owing to the heat treatment was calculated and was found to increase with increasing iron oxide content. The geometry of crystallization was found to be in three-, two-and one-dimension(s) for samples containing 2.5, 5.0 and 7.5 mol% Fe₂O₃, respectively.     

Magnetic susceptibility of heavy rare-earth molybdates

Results of the molar magnetic susceptibility (_m) measurements of heavy rare-earth (RE) molybdates of the type RE₂ (MoO₄)₃ with RE = Gd, Dy, Ho, Er, Tm and Yb are reported in the temperature range 3 to 300 K at a magnetic field of 3.2X10⁵ Am^–1. All molybdates show ferrimagnetic behaviour with ferrimagnetic Néel temperatures lying in the range 15 to 24 K. Evaluated magneton numbers agree fairly well with those for free tripositive rare-earth ions. Various magnetic parameters are evaluated using appropriate models.     

Magnetization Study in CuCr2O4 Spinel Oxide

We report magnetization and ac susceptibility as functions of the temperature and frequency for CuCr₂O₄ spinel oxide from 2 K to 300 K. Bulk CuCr₂O₄ crystallizes at room temperature in a tetragonal distorted spinel and above 865 K its structure is cubic spinel; distortion is caused by Jahn–Teller Cu²⁺ ions. The magnetization data of the polycrystalline sample indicates ferrimagnetic order below T _C =122 K. Magnetization isotherm resulted in an average magnetic moment of 0.08 μ_B/f.u. at 2 K values lowest to expected value. This discrepancy can be explained assuming a triangular configuration of spins Cr³⁺ and Cu²⁺. The ferromagnetic phase of the sample does not show glassy behavior. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferrimagnet.     

Microscopic and Macroscopic Magnetic Properties of the MgAl x Cr x Fe2 − 2x O4 Spinel Ferrite System

To determine the influence of the substitution of Al³⁺ and Cr³⁺ for Fe³⁺ in MgFe₂O₄ ferrites on the structural and magnetic properties, the MgAl _x Cr _x Fe_{2 – 2x} O₄ (x = 0.0–0.8) spinel systems were studied by using the X-ray diffraction analysis, magnetization in strong fields, magnetic susceptibility in weakly variable electric fields, and Mössbauer spectroscopy. Unlike previous investigations, it was discovered that a half of Al³⁺ occupies tetragonal positions. The system forms a noncollinear spin structure and a central paramagnetic doublet is superimposed over the magnetic sextet in the Mössbauer spectrum (0.5 > x > 0.2). The dependence of the magnetic susceptibility on temperature reveals the normal ferromagnetic properties of the material.     

Magnetic properties of RE2Mo2O7 pyrochlores

The magnetic properties of RE₂Mo₂O₇ (RE=Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, and Yb³⁺) and of solid solutions of the type (Nd_1?x A _x )₂Mo₂O₇ (A=Yb or Er; 0.05≤x≤0.4) have been studied using an ac mutual inductance bridge. All the compounds were found to exhibit magnetic ordering in the neighborhood of or below 77 K. The data have been analyzed using (i) the available susceptibility data on Y₂Mo₂O₇, (ii) a ferromagnetic coupling among the Mo⁴⁺ ions to obtain the contribution from the RE³⁺ ion, and (iii) the available susceptibility data on RE₂Ti₂O₇ to obtain the contribution from the Mo⁴⁺ ions. It was found that procedure (iii) gave the most satisfactory explanation of the magnetic ordering. The results indicated that (1) the Mo⁴⁺ ions become ordered magnetically, (2) the behavior of the RE³⁺ ions is almost the same as in the isostructural RE₂Ti₂O₇ and RE₂V₂O₇ compounds, and (3) the susceptibility values differ appreciably as we go from Nd³⁺ to Yb³⁺, possibly due to narrowing of the π* Mo-O conduction band.     

Magnetic Properties of Ca3Ru2O7 Grown by a Floating Zone Method

We have succeeded in growing single crystals of Ca₃Ru₂O₇ by a floating zone method. The temperature dependence of magnetic susceptibility was measured in the temperature range from 2 to 300 K for the field along ab-plane and c-axis. The magnetic susceptibility shows anisotropic behavior. For T>55 K, the susceptibilities of both directions increase like the Curie–Weiss behavior with decreasing temperature. In the temperature range between 55 and 48 K, the susceptibility along ab-plane decreases steeply, while the one along c-axis is almost constant, which implies that the magnetic moment is aligned antiferromagnetically in the ab-plane. The susceptibilities drastically decrease in both directions at 48 K and are almost independent of temperature below 30 K.     

Ferromagnetism and Morphology of Annealed Fe2O3/CoXOY/ZnO Thin Films

We have studied thermal solid‐state reactions in the Fe₂O₃/Co_XO_Y/ZnO thin film systems grown using the atomic layer deposition technique. The compound produced after annealing at 700 °C is found to be a complex mixture of three different spinel phases: ZnCo₂O₄, CoFe₂O₄, and ZnFe₂O₄. The magnetic properties of the compound strongly depend on the atomic ratio of Fe³⁺ and Co²⁺ atoms, which can be set by choosing the corresponding thicknesses of the Fe₂O₃ and Co_XO_Y films. In addition, we also find a formation of 100 nm voids at the interface between Fe–Co–Zn–O compound and remaining ZnO film after 1h annealing at 700 °C in argon atmosphere. The formation of these voids shows indirectly the preferential outward diffusion of Zn²⁺ ions from ZnO into the Fe₂CoO₄ phase layer what we prove via our magnetic measurements.     

Crystal,magnetic and electric behaviour of CoMnxFe2−xO4 cubic ferrites

Six powder samples of CoMn _x Fe_2–x O₄ (O x 1) were synthesized by using a ceramic sintering technique. X-ray powder diffraction patterns were obtained and confirmed the presence of single-phase spinel structure with no evidence of impurities. Lattice constants were determined. Differential thermal analysis measurements showed no variation in crystal phase with temperature. AC conductivity measurements at the temperature range (300–950 K) and for the frequency range (10²–10⁵ Hz) were also carried out and the Néel transition temperatures for all samples were determined. Mössbauer effect patterns revealed magnetic ordering for all compositions at room temperature. The obtained spectra were successfully analysed into two Zeeman sextets, could be attributed to the tetrahedral and octahedral sites. The different Mössbauer effect parameters were deduced and discussed. Neutron diffraction measurements were also performed where oxygen parameters and cation distributions were determined, and magnetic structure were studied. All obtained results from the different techniques support the Néel model of ferrimagnetism for such compounds.     

Near-neighbour impurity effect on the spin-state transitions in LaCoO3 at low temperature (12<T<300 K)

The low-temperature magnetic susceptibility behaviour of LaCoO₃ prepared under different conditions as well as substituted samples such as LaCo_0.95M_0.05O₃ (M=Al, Ga, Cr, Fe, Mn, Ni) and La_0.98Sr_0.02CoO₃ have been investigated in the temperature range 12–300 K. Earlier interpretations of the magnetic susceptibility have been reexamined. In the case of LaCoO₃ samples containing Al, Ga, Cr and Fe impurities spin-state transitions involving a temperature independent activation energy (∼0.01 eV) are observed atT<200 K. Analysis of the data indicates that either the excited state has an intermediate-spin (t _2g ⁵ e _g ¹ ) configuration or only half the Co ions are involved in the activated transition to the high-spin (t _2g ⁴ e _g ² ) configuration. Al³⁺, and Cr³⁺ increases the activation energy considerably. Substitution of L ≠ 0 ions such as Mn³⁺ and Ni³⁺ or Co⁴⁺ (low-spin) seems to introduce ferromagnetic interactions and stabilizes the paramagnetic state. LaCoO₃, when Co is substituted by Mn (5%) or La is substituted by Sr (2%) show giant magnetic moments. When Co is substituted by Ni (5%) a ferromagnetic ground state is observed. Communication No. 226 from the Solid State and Structural Chemistry Unit.     

EPR and magnetic susceptibility studies on V2 O5-P2O5-PbO glasses

EPR and magnetic susceptibility investigations on the glass system with 04+ ion content are explained using a simulation program on the assumption of the superposition of two signals, one with hyperfine structure (hfs) typical for isolated ions and another one consisting of a broad line without hfs characteristic for clustered ions. The hfs is not shown for x<3mol%V₂O₅. The EPR data show the presence of V₄₊ ions in a square-pyramidal co-ordination (C_4V) for 3x20mol%V₂O₅. The progressive disappearance of hfs for high V²O⁵ content (>20 mol %) suggests the increase of the associated ion number coupled by superexchange interactions. This result is consistent with magnetic susceptibility data for >3 mol % where the temperature dependence of magnetic susceptibility is described by the Curie–Weiss type law with a negative paramagnetic Curie temperature. The magnetic susceptibility results allowed the estimation of the V⁴⁺/V⁴⁺ + V⁵⁺ ratio in the sample studied.     

Fast Li+ ion conduction in Li2O–(Al2O3 Ga22O3)–TiO2–P2O5 glass–ceramics

Fast lithium ionic conducting glass-ceramics have been obtained by heat-treatment of glasses in the systems Li₂O–M₂O₃–TiO₂–P₂O₅ (M = Al and Ga). The glass–ceramics were mainly composed of LiTi₂(PO₄)₃ in which Ti⁴⁺ ions were partially replaced by M³⁺ ions. Considerable enhancement of the conductivity with the substitution of M³⁺ ions for Ti⁴⁺ ions was observed. The maximum conductivity obtained at room temperature was 1.3 × 10^–3 S cm^–1 for the aluminium system and 9 × 10^–4 S cm^–1 for the gallium system.     

Anisotropic gap distortion due to superflow and the depairing critical current in superfluid 3He-B

The specific heats of UAs and the isostructural nonmagnetic homolog ThAs have been measured in the temperature range 5–300 K. While the latter compound displays a regular smooth curve C _p (T), UAs shows two sharp anomalies. The first anomaly, around 64 K, may be ascribed to the magnetic transition from type IA to type I antiferromagnetic structure; the second anomaly, at 122.8 K, corresponds to the Néel temperature. An analysis of the experimental curve C _p (T) for UAs has been carried out by several different methods to get the magnetic contribution to the specific heat with the best possible accuracy. The resulting magnetic entropy depends on the method and its maximum value at 250 K is 0.8 R ln 4, assuming a high-temperature value of the electronic heat capacity coefficient – 33 mJ/K² mole. No anomaly at 41 K was observed whatever thermal treatment was used to prepare the UAs samples.     

Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Nanocrystallites of tricobalt tetraoxide (Co₃O₄) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC₂O₄·2H₂O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O²⁻) → e_g(Co³⁺), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t_2g(Co³⁺) → t₂(Co²⁺), 1.77 eV, (b) t₂(Co²⁺) → e_g(Co³⁺), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co²⁺ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance (C)–voltage (V) measurements is found to be 1.05 × 10¹⁶ m⁻³. The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature (T_B) is centered at 35 K. The observed Néel temperature (T_N  25 K) is significantly lower than the bulk Co₃O₄ value (T_N = 40 K) possibly due to the associate finite size effects.