Influence of magnetic ordering on the structural properties of dilute copper ferrites

The x-ray structural properties of samples in the CuGa_xAl_xFe_2?2x O₄ (x = 0?0.7) and CuGa_xAl_2x Fe_2?3x O₄ (x = 0?0.5) systems are studied. It is found that magnetic ordering in dilute copper ferrites affects their structural properties. It is concluded that the frustration of magnetic coupling leads to suppression of the cooperative Jahn-Teller effect in dilute copper ferrites with a frustrated magnetic structure.     

Enhancement of local superconductivity in ferromagnetic FeCrB metallic glass by Ar+ ion irradiation

We have reinforced local superconductivity in ferromagnetic Fe(67)Cr(18)B(15) metallic glasses by ion irradiation. Superconductivity in this medium appears due to the presence of large-scale layered clusters of metallic Fe-Cr phase, 150-230 ? in size, with a ferromagnetic (or superparamagnetic) Fe-rich core and nonmagnetic Cr-rich superconducting shell. Here we show that due to the intensification of concentration phase separation in the Fe-Cr clusters under ion (Ar(+)) irradiation, the volume of the superconducting phase increases from the initial 0.4-0.5% up to 7-8%. After irradiation, the resistivity jump Δρ/ρ in the temperature range T=3.1-3.6 K increases ～14 times, reaching 19%, as compared to 1.36% for the initial sample. In the interval of T=3.1-3.6 K, the rate of resistance change reaches 79% K(-1) for the irradiated sample instead of 3.6% K(-1) for the initial sample. In the same temperature interval, the rate of magnetoresistance change increases from 3% K(-1) for the initial sample up to 70% K(-1) after irradiation.     

High precision delta(17)O isotope measurements of oxygen from silicates and other oxides: method and applications

The use of infrared laser-assisted fluorination to release oxygen from milligram quantities of silicates or other oxide mineral grains is a well-established technique. However, relatively few studies have reported the optimisation of this procedure for oxygen-17 isotope measurements. We describe here details of an analytical system using infrared (10 μm) laser-assisted fluorination, in conjunction with a dual inlet mass spectrometer of high resolving power ( approximately 250) to provide (17)O and (18)O oxygen isotope measurements from 0.5-2 mg of silicates or other oxide mineral grains. Respective precisions (1) of typically 0.08 and 0.04 per thousand are obtained for the complete analytical procedure. Departures from the mass-dependent oxygen isotope fractionation line are quantified by Delta(17)O; our precision (1) of such measurements on individual samples is shown to be +/-0.024 per thousand. In turn, this permits the offset between parallel, mass-dependent fractionation lines to be characterised to substantially greater precision than has been possible hitherto. Application of this system to investigate the (17)O versus (18)O relationship for numerous terrestrial whole-rock and mineral samples, of diverse geological origins and age, indicates that the complete data set may be described by a single, mass-dependent fractionation line of slope 0.5244+/- 0.00038 (standard error). Copyright 1999 John Wiley & Sons, Ltd.     

Mössbauer study of copper ferrite diluted by Ga<Superscript>3+</Superscript> and Al<Superscript>3+</Superscript> ions

Mössbauer spectra of the CuGa _x Al_2x Fe_{2 ? 3x} O₄ system (x = 0.1, 0.2, 0.3, 0.4, 0.5) have been studied at a temperature of 295 K. The results obtained are compared with Mössbauer data for the CuGa _x Al _x Fe_2?2x O₄ system. It is established that the hyperfine magnetic fields H _B (for octahedral sites) and H _A (for tetrahedral sites) for ferrites in both systems with ferrimagnetic ordering decrease linearly depending on the total number of nonmagnetic ions in octahedral and tetrahedral sites of the two systems.     

Temperature dependence of the spontaneous magnetization of Cu x Ni1-x Fe0.6Cr1.4O4 chromite ferrites with a frustrated magnetic structure

The behavior of the spontaneous magnetization σ _s (T) of samples in the Cu _x Ni_1-x Fe_0.6Cr_1.4O₄ system (x = 0, 0.1, 0.2, 0.3, 0.4) with variations in temperature is studied. The conclusion is drawn that the degree of frustration of magnetic interactions in the A sublattice increases with increasing x, with the result that the dependence σ _s (T) transforms from an N-type curve to an anomalous dependence of a new type.     

Anomalous magnetic properties of ferrites of the CuFe2−x CrxO4 system with high Cr3+ content

The paper reports on a comprehensive study of magnetic properties of CuFe_2−x Cr_xO₄ ferrites (x=0.0, 0.2, 0.3, 1.0, 1.4, 1.6, and 2.0). The curves of magnetization versus temperature, σ _s(T), of the ferrites with x=1.0, 1.4, and 1.6 have anomalous shapes: the magnetization begins to fall off at lower temperatures than the Curie point TC. The experimental results and analysis of exchange interactions suggest that in ferrites with high Cr³⁺ content, magnetic transitions to either a frustrated magnetic structure or a clustered spin glass can take place. Zh. éksp. Teor. Fiz. 111, 1732–1737 (May 1997)     

Origin of grey spots on the surface of metallic glasses under Ar ion irradiation: The surface and volume properties

The nature of grey spots on the surface of amorphous Fe₆₇Cr₁₈B₁₅ metglass ribbons irradiated by 30 keV Ar⁺ ions is investigated. Changes in the surface and volume properties of samples are analyzed bearing in mind the presence of competitive processes of ordering and disordering in the structure under irradiation at initial stage of crystallization (T = 500 °C). Changes in the volume properties of the samples become apparent in their structure, electrical and magnetic properties and are caused by radiation-enhanced diffusion at high gradients in the concentration of defects generated by Ar⁺ irradiation. It is established that grey spots are shown by decrease in the reflection coefficient in a visible range. They emerge as a result of reorganization in the electronic structure of irradiated ribbons due to damage in the short-range order and formation of non-uniformly scaled atomic structure during transition from the medium-range to the long-range atomic order. Experimental data are in accord with the calculations in the framework of the free electrons model.     

Effect of Ion (Ar+) Irradiation on Cluster Magnetism and Magnetic Interactions in Fe67Cr18B15 Amorphous Alloy

Physics of the Solid State - The comparison of the resonance magnetic absorption spectra with the data on structure and magnetic properties of Fe67Cr18B15 amorphous samples has shown that a...     

Susceptibility of the paraprocess in spinel ferrites with a frustrated magnetic structure

The temperature dependence of the susceptibility of the paraprocess χ_para (T) is investigated for samples in the CuGa_xAl_xFe_2?2xO₄ (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7), CuGa_xAl_2xFe_2?3xO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5), and Ga_xFe_{1? x}NiCrO₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8) systems. It is found that long-range magnetic order arises in spinel ferrites at the temperature T_tran of the transition from a cluster spin-glass state to a frustrated magnetic structure with a maximum in the temperature dependence of the susceptibility of the paraprocess.     

Magnetic structure of the NiCr<Subscript>2</Subscript>O<Subscript>4</Subscript> nickel chromite

The magnetic properties of the NiCr₂O₄ chromite are investigated and compared with similar properties of the NiFe_1.1Cr_0.9O₄ nickel ferrite-chromite material. It is found for the first time that the tetrahedral (A) sublattice of the NiCr₂O₄ chromite is responsible for the total magnetic moment. Moreover, it is revealed that the NiCr₂O₄ chromite exhibits a giant magnetostriction of the paraprocess (λ _para ～ 200×10^?6) and an anomalously large volume magnetostriction (ω ～ 500×10^?6) at a temperature of 4.2 K in magnetic fields up to ～50 kOe. The inference is made that the observed paraprocess is caused by an increase in the degree of noncollinearity of the magnetic moments induced in the octahedral (B) sublattice of the NiCr₂O₄ chromite in an external magnetic field.