Rare Earth Ferrites Research Articles

Electric-field-induced generation and reversal of ferromagnetic moment in ferrites

The prospect of controlling the magnetization (M) of a material solely with an external electric field (E) could enable the development of low-power spintronics. Although there has been some success towards this end, most approaches involve controlling interactions at the interface between two different materials rather than switching of a single bulk phase. Here we report the ability to exert complete control over the generation and reversal of the bulk spontaneous M of the single-component multiferroics RFeO3 (R = Dy0.70Tb0.30, Dy0.75Gd0.25) with an E alone. We achieve this by controlling the anisotropic character of rare-earth magnetism and exploiting the competition between different magnetoelectric phases. We also show that whether M is reversed or retained on the E-induced polarization reversal depends on the E modulation speed. This is ascribed to the different dynamical characteristics of ferroelectric and multiferroic domain walls governed by the reversal dynamics of rare-earth moments and iron spins, respectively. The ability to modify a material’s magnetization with an electric field could enable lower-power electronic devices. Such ‘magnetoelectric’ behaviour is usually only seen at the interface between magnetostrictive and electrostrictive materials, but has now been observed in the bulk of single-component rare-earth ferrites.

Change in the magnetostructural properties of rare earth doped cobalt ferrites relative to the magnetic anisotropy

The superparamagnetic properties of the doped cobalt ferrite nanocrystals have been demonstrated. The significance of the sol–gel autocombustion method in yielding the as obtained doped cobalt ferrite oxide powder in the nano-range has been very well complemented with structural, dimensional and morphological analytical techniques such as X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), particle size analysis and Scanning Electron Microscopy (SEM). The lattice strain and lattice parameters have been calculated by making use of the Williamson–Hall extrapolation. The valence states of the metal ions and single phase formation of the polycrystalline oxides have been confirmed with the help of X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The magnetic measurements M–H and M–T have been carried out demonstrating a change in the magnetic moment and a superparamagnetic–ferrimagnetic transition in the ferrite system. The influence of the distribution of the metal ions in the crystal lattice and the dimensions of the ferrite oxides on the resultant magnetic properties has been demonstrated. The contribution of the spin–orbit coupling generating from the Co2+ ions in the octahedral lattice towards higher magnetic anisotropy and hence the magnetic properties is investigated. The results provide an insight into the inter-relationship of the particle dimension, the spin–orbit coupling and the resulting superparamagnetic property.

Magnetoelectric effect in rare earth ferrites, LnFe2O4

Dielectric and magnetic properties of selected rare earth ferrites, LnFe2O4 (Ln=Y,Er,Yb), have been investigated. All these materials show ferroelectricity near the ferrimagnetic transition temperature around 250 K. More importantly, they exhibit a change in the dielectric behavior on the application of magnetic fields.

Effect of Gamma Irradiation on the Structural and Electrical Properties of Co0.5Zn0.5CeyFe2-yO4

The effect of γ - irradiation on structural and electrical properties of rare earth ferrite of the general formula Co0.5Zn0.5CeyFe2-yO4 ( 0.0≤y≤ 0.2) was discussed. X- ray diffraction pattern of irradiated and un- irradiated samples showed the formation of cubic spinel structure. The obtained data show that electrical conductivity ln σ as well as the dielectric constant e‘ are highly dependent on both Ce ion concentration and radiation dose. The number of ferrous ion at octahedral sites play a dominant role in the change of the crystal size due to irradiation effect. All samples exhibit a change in slope of the conductivity versus inverse temperature giving two regions with different conduction mechanisms.

The significant role of the rare earth ions on the elastic and thermodynamic parameters of LiCoDy- and ZnCoCe-ferrites

Two types of rare earth ferrites [Li 0.6Co 0.1Dy x Fe 2.3− x O 4; 0.0⩽ x⩽0.2] and [Zn 0.5Co 0.5Ce y Fe 2− y O 4; 0.0⩽ y⩽0.2] were prepared by standard ceramic technique with a view to investigate their elastic behavior and some essential thermodynamic parameters. The elastic properties were studied by measuring the ultrasonic velocities by adopting the pulse transmission technique. Longitudinal ( V L ) and shear ( V S ) velocities, Young's modulus ( E ), Debye temperature ( θ D ) and specific heat capacity ( C v ) have been evaluated for all the investigated samples. The rare earth content as well as its ionic radius plays a significant role in the evaluated parameters. According to the experimental results, the two investigated types of rare earth ferrite are considered as insulator magnetic solids. It was found that for each composition there exists a characteristic temperature, down to which the resonance frequency of the investigated samples drops smoothly, but above this temperature the resonance frequency stays constant. Accordingly, these samples seem to be of importance in industrial applications especially in the field of electronics.

Effect of mechanical pressure and sintering temperature on the electrical properties of the rare earth ferrite Li0.6Co0.1DyxFe2.3-xO4; 0.0 x ≤ 0.2

Polycrystalline samples of mixed ferrite Li0.6Co0.1Dy x Fe2.3−x O4; 0.0 ≤ x ≤ 0.2 were prepared by the standard ceramic technique. X-ray diffraction analysis was carried out to assure the formation of the samples. The resistance was measured at different temperatures (300–800 K) as a function of frequency (200 kHz–5 MHz) for the investigated samples prepared at different sintering temperatures (1100°C ≤ T s ≤ 1250°C). It is found that this type of rare earth ferrite gives a distinguishable behavior at T s = 1200°C where the resistance of the samples at room temperature reduced to ≈90% of their values of the others sintering temperatures. The data showed also that the resistance of these samples sintered at 1100°C is changed at 100 kHz from 12 MΩ without applying pressure to 19 kΩ with 0.8 MPa. This change occurs in a reversible manner (i.e., after removing the mechanical pressure effect the resistance of the samples well return to their initial values again immediately). Thus this type of rare earth ferrite could be more applicable in several advanced electronic devices can be used as a sensor for the mechanical pressure.

γ irradiation effect on the polarization and resistance of Li–Co–Yb-ferrite

The effect of γ irradiation on some physical properties of rare earth ferrite of the general formula Li 0.5+ z Co z Yb x Fe 2.5−2 z− x O 4, ( z=0.1, x=0.00, 0.025, 0.050, … , 0.200) is discussed. The temperature dependence of the polarization and resistance is studied in the range (300 K≤ T≤700 K) at different frequencies (10 kHz≤ f≤1 MHz). The relaxation time and the activation energy have been calculated before and after irradiation with γ rays doses of 1 and 3 Mrad. A comparison was made between the ac resistance before and after irradiation for the samples with (0.0≤ x≤0.2). The results after irradiation with 1 Mrad γ rays showed that the resistance at the critical concentration decreases from 800 to 25 kΩ at room temperature. Furthermore, with increasing temperature the resistance ranged from R≈130 kΩ at T≈310 K to R≈0.13 kΩ at T≈640 K. Thus, it is possible to improve the conductivity of this type of rare earth ferrite materials to be used in technological applications at room as well as at high temperature.

Effect of iron oxide on the phase composition of zirconia refractories

It has been established that in the compositions ZrO2∶Y2O3∶Fe2O3 and ZrO2∶Nd2O3∶Fe2O3 the processes of formation of rare earth ferrites and ZrO2 cubic solid solutions occur simultaneously. At temperatures of up to 1300°C the ferrite-formation process is fastest; with a further increase in the temperature the synthesis of ZrO2 cubic solid solutions is significantly intensified. In the mixture of CaO, ZrO2, and Fe2O3, the calcium oxide reacts only with ZrO2. The greatest stability in relation to the action of Fe2O3 is clearly found in the case of the zirconia-calcium oxide solid solutions. In the mixtures of cubic solid solutions of Nd2Zr2O7-ZrO2 or ZrO2-Y2O3 and Fe2O3, an interaction is only possible in the case where the synthesis of ferrites is not accompanied by the restructuring of the crystal lattice of ZrO2, i. e., by destabilization of the material. The presence of Fe2O3 prevents the decomposition of zirconia-calcium oxide and zirconia-neodymium oxide solid solutions under conditions of prolonged heating in air at 1300°C. The zirconia-yttria cubic solid solution is the most stable and does not decompose, regardless of the concentration of Fe2O3, after a dwell of 250 h at 1300°C.

Etude par spectroscopie Mössbauer d'une serie de ferrites mixtes de terre rare et d'alcalinoterreux: SrLn 2Fe 2O 7

The room-temperature magnetic structure of some mixed alkaline-earth and rare-earth ferrites, with the formula Sr(Ba)Ln 2Fe 2O 7, consists of a pattern perpendicular to the c tetragonal axis, either along the [110] or the [1 1 0] direction. One of the G x − ± A y − magnetic modes is obtaine for SrTb 2Fe 2O 7 and one of the A x − ± G y − modes is obtained for SrNd 2Fe 2O 7 and for SrPr 2Fe 2O 7. This ambiguity is removed by the use of powder and single crystal Mössbauer spectroscopy, which also allows us to determine the magnetic structures of all other members of the series; at room temperature the magnetic moments are aligned along the [110] diagonal for all compounds. In lowering the temperature to 4.2 K, different magnetic moment reorientations have been observed. In SrTb 2Fe 2O 7 and SrNd 2Fe 2O 7 the magnetic moments go from the basal plane to along the c axis, whereas in BaLa 2Fe 2O 7 and SrGd 2Fe 2O 7 they rotate in the basal plane itself, the rotation angle being π/4 and π/2, respectively.

Magnetic properties of a new series of mixed alcaline earth and rare earth ferrites with formula SrLn3Fe2O7

Abstract Magnetic structures and spin reorientations in the SrLn2Fe2O7 compounds have been studied by neutron diffraction and Mossbauer spectroscopy. The results as a whole show that the rare earth's order is induced by the exchange interactions between the two sublattices; their anisotropy allow the observed spin reorientations to be explained.

Magnetic and Crystallographic Investigations of Some Rare Earth Ferrite Compounds

Lattice parameters and magnetic moments are given for a number of ternary compounds isomorphous with Sr2EuFeO5. These compounds are paramagnetic above 80°K. The magnetic susceptibility data can be interpreted by a simple paramagnetism.

Magnetic properties of some rare earth ferrites

The tetragonal SrEu 2Fe 2O 7 and BaEu 2Fe 2O 7 have been investigated using Mössbauer spectroscopy and magnetic susceptibility techniques. The measurements of magnetic susceptibility have been carried out in the temperature range 100 to 700°K. The data have been fitted to Curie-Weiss law χ = C T + θ + A using a non-linear least square curve fitting procedure. These ferrite compounds are paramagnetic above 534°K and 537°K respectively, with antiferromagnetic ordering below these temperatures.

The infra-red Faraday effect andgvalues in rare-earth garnets

Measurement of the Faraday effect in single crystals of rare-earth garnet ferrites at wavelengths around 5 μm allows the determination of the g values of ions in the system. Experimental results are given in the temperature range 77 °K to 300 °K for the materials R3Fe5O12, where R = Y, Gd, Y0 5Gd0 5, Tm or Dy. The g values of gadolinium and ferric ions are observed to be constant over the whole temperature range and are in reasonably good agreement with previous measurements by alternative methods. In thulium ions the g value is observed to be 1.36 ± 0 03 at 77 °K and 1 64 ± 0 06 at 300 °K with a roughly linear variation at intermediate temperatures. In dysprosium ions the respective values are 0.77 ± 0 02 and 1 06 ± 0 04. These values are in poor agreement with measurements at microwave resonance. There is evidence that Kittel fast-relaxation effects explain the differnce. Such effects are not present at optical frequencies.

Measurement of Ferromagnetic Relaxation by a Modulation Technique

A modulation method is introduced as a novel means of studying directly the relaxation process in the case of ferrimagnetic rare earth garnets and ferrites. The method directly measures the relaxation of the spin system of the material which is indentified with the transverse relaxation time by means of the standard equations of motion. A low-power fixed frequency X-band microwave field is amplitude modulated at an rf rate up to 50 Mc while maintaining the sample in ferromagnetic resonance. The resulting precession of the magnetic moment will then contain an rf component which is a function of both the transverse relaxation time and the frequency of the rf modulation. The precession is observed by a single turn pickup loop in which the induced microwave voltage is proportional only to the rotating component of magnetization. The rf signal resulting from demodulation of this voltage then characterizes the response of the precession to the modulation frequency of the driving microwave field. The falloff in this response with increasing modulation frequency is shown to be related to the transverse relaxation time by means of the equations of motion. The predicted form of this falloff is a function proportional to the factor (1+β2τ2)−12 from which the relaxation time τ is obtained. Certain conclusions may be drawn from the excellent agreement of the experimental data with theory.