Ca-doped YIG Research Articles

Investigating Phase and Electrical Properties of Calcium-Doped Yttrium Iron Garnet

Literature studies have shown that Ca0.5Y2.5Fe5O12-δ can be a promising cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC) 1. As it exhibits higher electrical and ionic conductivity compared to several other kinds of rare-earth garnets 2. However, given the high resistivity (> 1012 Ω cm at room temperature) and less-explored oxide ion conductivity of its parent phase yttrium iron garnet (Y3Fe5O12, YIG), the effect of calcium doping on the electronic and ionic properties has not been fully studied yet 3. In this work, polycrystalline samples of Ca-doped YIG, with general chemical formula CaxY3-xFe5O12-δ (x=0, 0.1, 0.3, 0.5 and 0.7), were prepared and phase characterized by powder X-ray diffractometer. The oxygen non-stoichiometry was determined by iodometric titration at room temperature and thermogravimetric analysis (TG) at elevated temperatures. Total electrical conductivity was measured by four-probe DC method, and ionic conductivity was calculated by using a modified Hebb-Wagner polarization method. As a result, the ionic transference number was calculated and discussed in relation to its potential applications. Reference: Zhong, W., Ling, Y., Rao, Y., Peng, R. & Lu, Y. Calcium doped Y3Fe5O12 as a new cathode material for intermediate temperature solid oxide fuel cells. J. Power Sources 213, 140–144 (2012).Kharton, V. V. et al. Ionic Transport in Gd3Fe5O12- and Y3Fe5O12-Based Garnets. J. Electrochem. Soc. 150, J33 (2003).Lehmann-Szweykowska, A., Wojciechowski, R. J., Gehring, G. A. & Tobijaszewski, I. Quasiparticles in Calcium Doped Yttrium-Iron Garnets. Acta Phys. Pol. A 91, 423–426 (1997).

A microscopic model of oxygen vacancies in Ca-doped YIG

Annealing experiments carried out on calcium doped yttrium iron garnet (Ca:YIG) in an oxygen-reducing atmosphere, show dramatic decreases in the magnetisation over the entire temperature range. Simultaneously, the electric resistivity and the electric activation energy significantly increase. Within the framework of the iron-oxygen cluster model, we analyse and compare with each other two situations, in which compensating holes can be found. The first is their occupying hybridised eigen-energy levels of the cluster. In this case, it can be shown at which ions the holes tend to remain for a given energy level. It turns out that the active holes remain localised most at the oxygen ions. The other situation means the hole's localisation in an attractive trap centre. The latter is created at the missing oxygen ion between the tetrahedral and octahedral iron-oxygen ionic clusters in the proximity of a doping Ca2+ ion. For a sufficiently shallow traps, their localised states can be considered within the framework of hydrogen-like model. The ground state energy of the vacancy can thus be estimated, considering the dielectric constant of the host material and the hole's effective mass as variational parameters. Communication between the tetrahedral and octahedral ironoxygen clusters with missing oxygen between them, is assumed to be maintained via the s − d hybridisation.

On the FMR linewidth in calcium-doped YIG films

FMR measurements at the K band were performed on Ca-doped YIG films. The size dependence of the FMR lineshape and linewidth was attributed to an inhomogeneous stress-induced anisotropy. Two distinct types of temperature dependence of the linewidth were observed, probably corresponding to Fe4+ and O− compensating centers.

Exchange stiffness of Ca-doped YIG

An effective medium theory for the zero-temperature exchange stiffness of uncompensated Ca-doped YIG is presented. The theory is based on the assumption that the effect of the Ca impurities is to produce strong, random ferromagnetic interactions between spins on the a and d sublattices. In the simplest version of the theory, a fraction, x, of the ad exchange integrals are large and positive, x being related to the Ca concentration. The stiffness is calculated as function of x for arbitrary perturbed ad exchange integral, Jxad. For Jxad≳(1/5)‖8Jaa+3Jdd‖, with Jaa and Jdd denoting the aa and dd exchange integrals, respectively, there is a critical concentration, Xc, such that when x≳Xc, the stiffness is complex. It is suggested that Xc delineates the region where there are significant departures from colinearity in the ground state of the Fe spins. Extension of the theory to a model where the Ca doping is assumed to generate Fe4+ ions on the tetrahedral sites is discussed. Possible experimental tests of the theory are mentioned.

Optical Absorption and Faraday Rotation in Pb- and Ca-Substituted Y1G Films

Optical absorption losses a and Faraday rotation ? F in the wavelength region ?=0.5 to 2.5 ?m were investigated for YIG films doped with Pb, Ca and other ions. The contributions of Pb2+, Pb4+, Fe2+ and Fe4+ ions to the values of a and ? F were studied by reducing Ca-free and Ca-doped YIG films grown by LPE. The absorption enhancement due to Pb2+ is prominent near the optical absorption edge, and that due to Pb4+ is marked near 0.56 ?m. The strong absorption near ?=0.56 ?m is thought to originate in the combined presence of Fe3+ and Pb4+. The contribution of Fe4+ to the ? F spectra in the near-infrared is negative, similar to the case for Pb2+; but the contribution of Fe2+ is negligible in the wavelength range measured.

Hydrogen Annealing Effects on the Optical Absorption Loss and Lattice Constant of Ca-Doped YIG and BiGdAlGaIG Films

The effects of annealing in an H 2 atmosphere on the optical absorption losses and lattice constants of Ca- and Si-doped YIG and Ca-doped (BiGd) 3 (FeAlGa) 5 O 12 films grown by LPE, were studied. Hydrogen annealing was carried out at 150 to 500°C for 22 hours. The absorption losses and lattice constants in YIG films with poor impurities or with Si doping did not change with H 2 annealing at 150 to 450°C; however, the values in Ca-doped YIG and Bi-substituted garnet films changed notably. The changes in absorption loss are explained by the reduction of Fe ions. The minimum value of the absorption loss was about 250 dB/cm at ? = 0.8 ?m for both Ca-doped and Ca-free garnet films. These results show that H 2 annealing is effective for obtaining iron garnet films with a high figure of merit. The lattice constant increased with increasing annealing temperature, and only in Ca-doped YIG films did it decrease drastically, at 450°C.

Ｃａ添加ＹＩＧ及びＢｉＧｄＡｌＧａＩＧ膜の水素中熱処理による光損失吸収と格子定数の変化

Annealing effects in H2-atmosphere on the optical absorption loss and lattice constant are studied for Ca- and Si-doped YIG and Ca-doped (BiGd)3(FeAlGa)5O12 films grown by LPE technique. H2-annealing was done at 150∼500°C for 22 hrs. The absorption loss and lattice constant in YIG films with poor impurities or doped Si did not change by H2-annealing at 150 ∼450°C, however, those in Ca-doped YIG and Bi-substituted garnet films changed notably. The changes of the absorption loss are explained by the reduction of Fe ions. The minimum value of the absorption loss was about 250 dB/cm at λ = 0.8μm for both Ca-doped and Ca-free garnet films. These results show that H2-annealing is effective to get iron garnet films with a high figure of merit. The lattice constant increased with increase of annealing temperature, and only in Ca-doped YIG films it decreased drastically at 450°C.

Reducing treatment effects on the magneto-optical properties of Ca-doped YIG films

The change of the lattice constant, the absorption loss and Faraday rotation due to reduction of Ca:YIG films are investigated. It was found that the absorption loss of the reduced Ca:YIG films at 1.15 µm is 2∼3 dB/cm irrespective of the content of Ca,Pb,Pt ions and the vacancies, and the Faraday rotation increased in the near infrared region and decreased in the visible region. It turns out that the reduction of Pb4+→ Pb2contributes to the change of lattice constant, and that of Fe4+→ Fe3+to the change of absorption loss. The large absorption loss in as-grown Ca:YIG films is seems to be due to the electronic transitions of Fe4+absorption centers associated with or without oxygen vacancies.

Semiconducting properties of a low resistive Ca-doped YIG film

A low resistivity p-semiconductor Ca:YIG film containing Fe4+ ions has been investigated. In particular, the temperature dependence, the magnetogradient conductivity, and the bolometer responsivity of this material have been analyzed in detail. Main results can be summarized as: (i) a large increase of the conductivity causing a negative magnetoresistance of 26% for gradient field of 1500 G/cm; (ii) high responsivity values exhibited by this material when it is used in bolometer applications where resistance temperature coefficient of about −0.04/°C has been measured.

Stability and site occupancy of Fe<sup>4+</sup>in Ca-doped YIG films

The nature and site occupancy of Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> centers in Ca-doped YIG films is investigated by studying the symmetry properties of the near-infrared absorption. We apply the recently proposed AVIA (angular variation of induced anisotropy) method: we measure, as the single- -crystal film is rotated relative to a magnetic field, the angular variation of the magnetic linear dichroism, which is the anisotropy property utilized in this case. The AVIA pattern thus obtained is compared with the theoretical angular variations appropriate to the symmetry of the garnet sites. The result we find is a uniform distribution of Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> over the a (octahedral) and d (tetrahedral) sublattices. This uniform distribution does not change, in spite of varying Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> concentration, after successive annealing treatments in oxidizing or reducing atmospheres. The structural changes through the annealing steps are monitored by X-ray diffraction and the lattice is found to expand, under reducing treatments, only as long as there are Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> centers. After all Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> ions have been reduced (namely at zero near-infrared absorption), rapid diffusion of oxygen anions is hindered, in spite of the large concentration of oxygen vacancies. All observed effects are explained by a single mechanism: electron deficiency (or holes), in Ca-doped YIG, results in a widespread distribution of Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</inf> and, effectively, in the presence of O- oxygens.

Effect of temperature on magnetostatic mode spectra of reduced Ca-doped YIG thin films

Magnetostatic modes have been observed on reduced Ca-doped YIG thin films of thickness 16 μm, in which the Fe4+ion, a fast relaxer, was converted into the Fe3+ion by introducing oxygen vacancies. The spectra can be well explained by the Damon-Eshbach theory. A magnetic field gap found between the surface exitations can be explained by an easy-plane uniaxial anisotropy energy developed during the reduction process. This field gap has also been studied as a function of temperature.

Magnetostatic mode spectra of uniaxial ferromagnets

The Damon-Eshbach dispersion relation for magnetostatic modes in a rectangular platelet is expressed in terms of the permeability tensor of the magnetic medium and the consequences of a uniaxial easy-plane anisotropy are investigated. The anisotropy modifies the critical angle between bulk and surface modes which results in the existence of bulk excitations within the series of DE-surface modes. Two systems are analyzed. In the case of a ferromagnetic layer compound, the anisotropy is large compared with the demagnetizing field and the series of surface modes is replaced by a second series of volume modes. Recent measurements are reported on Ca-doped YIG. Several hundred modes of forward and backward volume modes are in good agreement with the calculated dispersion function using the parameters of pure YIG and Hu = −250 Oe. It is suggested that a measured anisotropy profile along the film normal suppresses a few bulk modes within the series of surface modes and accounts for a gap observed on the low field side of the uniform resonance.

Enhancement of uniaxial anisotropy constant by introducing oxygen vacancies in Ca-doped YIG

Ca-doped YIG films having composition Y3−x3+ Cax2+ Fe3+5−x Fex4+ O1 2 have been grown with a value of x = 0.25 and thickness 1.15 μm. Upon annealing in saturated FeCl2 solution, oxygen vacancies are introduced, which compensate for Ca2+ ions and reduce Fe4+ ions to Fe3+ ions. The uniaxial anisotropy constant changed from −5.89×103 erg/cm3 to −17.0×103 erg/cm3, which is consistent with a change in the crystal field effects reduction as proposed by Stacy and Rooymans. The exchange stiffness coefficient changed from 3.19×10−7 erg/cm to 3.66×10−7 erg/cm. The exchange stiffness parameter and the anisotropy energy have been studied as a function of temperature and oxygen vacancy concentration.

Oxidizing effects of high temperature annealing in reducing atmosphere in Ca-doped YIG films

The existence of an oxidation mechanism is reported in Ca-doped YIG films, when high temperature annealing is carried out in N 2 atmosphere. The annealing were performed at successively increasing temperatures, and optical absorption, lattice parameter and thermoelectric power were measured at each step. Optical absorption was observed going down to a minimum ( T = 450°C in one cycle of annealing, T = 400°C in a different cycle) and then rise again. The minimum in absorption corresponds to a maximum of the lattice parameter, while the thermoelectric power is always p-type. Analysis of the data leads to the conclusion that we are observing a reoxidation process, triggered both by temperature and oxygen vacancy concentration. This process is subject to exhaustion. Previous annealing experiments are analyzed in the light of these results.

Eddy current damping in Ca-doped YIG films

A series of Ca-doped YIG films in which the atomic ratio of Ca to Y in the melt varied from 0 to 1.18 were grown. Resistivities of the virgin state of these films were measured. The dependence of the linewidth when the field is applied at 30° from the film normal on the Ca/Y atomic ratio was found to be linear and the dependence of the field is applied normal to the film surface. These are clear indications of the eddy current damping mechanism. The linewidth contribution due to this mechanism is given by ΔHe.c.= (2π/c2γ) σAω2 where σ is the conductivity, A is the disc area, ω is the radial frequency and the magnetic field is applied normal to the film plane.

Investigation of increasing Fe4+concentrations in Ca-doped YIG films

A magneto-optical investigation has been carried out, at room temperature, on a series of calcium-doped YIG films, epitaxially grown on a

Photomagnetic effect and electron diffusion after-effect in Si- and Ca-doped YIG

physica status solidi (a)Volume 28, Issue 1 p. K41-K44 Short Note Photomagnetic effect and electron diffusion after-effect in Si- and Ca-doped YIG M. Wurlitzer, M. Wurlitzer Sektion Physik der Karl-Marx-Universität LeipzigSearch for more papers by this authorW. Schaller, W. Schaller Sektion Physik der Karl-Marx-Universität LeipzigSearch for more papers by this author M. Wurlitzer, M. Wurlitzer Sektion Physik der Karl-Marx-Universität LeipzigSearch for more papers by this authorW. Schaller, W. Schaller Sektion Physik der Karl-Marx-Universität LeipzigSearch for more papers by this author First published: 16 March 1975 https://doi.org/10.1002/pssa.2210280156Citations: 10AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume28, Issue116 March 1975Pages K41-K44 RelatedInformation

Photoinduced Changes in the Crystal Anisotropy of Si-Doped YIG

It is found that the cubic magnetocrystalline anisotropy of Si-doped YIG can be changed by exposing a single crystal to light at 77 or 4.2 K. The experiment is done using a (110) plate sample in a rotating sample magnetometer (RSM). Doping levels of 0.01 to 0.017 Si per formula unit Y3Fe5O12 have been studied. The experimental problem is to distinguish changes in the crystal anisotropy from changes in the induced anisotropy, since both are influenced by light. This separation is possible by analysis of the RSM outputs at different frequencies and phase angles. We observe changes in the quantity (−32K1−14K2); the changes are a few percent in magnitude, occur within a few seconds or tens of seconds after the light is turned on, and persist for long times (many minutes) after the light is turned off. For low levels of Si doping, irradiation at 77 K produces large changes in the part of the cubic anisotropy attributable to the Fe2+ present. The measured change in anisotropy for all samples is an increase in magnitude at 77 K, but a decrease at 4.2 K. The effect is not observed in Ca-doped YIG, confirming previous conclusions that ferrous ions are principally responsible for photoinduced anisotropy effects.