Effects of Gd substitution on the structural and magnetic properties of strontium hexaferrites

The effect of Gd substitution in M-type strontium hexaferrites has been examined in two series of samples, (_Sr1-xGdx)O·5.25$({\mathrm{Sr}}_{1-x}{\mathrm{Gd}}_x)\mathrm{O}·5.25$_Fe2O3${\mathrm{Fe}}_2{\mathrm{O}}_3$ and _{Sr1-xGdxFe12-xCoxO19}${\mathrm{Sr}}_{1-x}{\mathrm{Gd}}_x{\mathrm{Fe}}_{12-x}{\mathrm{Co}}_x{\mathrm{O}}_{19}$, both prepared by the ceramic method, where x=0–0.40$x=0–0.40$. The samples have been characterized by XRD, VSM and SEM-EDAX techniques. All substituted samples present primarily the hexaferrite structure. Sample (_{Sr0.95Gdx0.05})O·5.25$({\mathrm{Sr}}_{0.95}{\mathrm{Gd}}_{x0.05})\mathrm{O}·5.25$_Fe2O3${\mathrm{Fe}}_2{\mathrm{O}}_3$ is single phase. Formation of impurity phases is affected by stoichiometry and presence of Co. In Sr–Gd samples, coercivity showed a maximum value of 305 kA/m (3.8 kOe) for x=0.20$x=0.20$, while remanence and saturation magnetization did not decrease. Coercivity and magnetization in the Sr–Gd–Co series decreased steadily with substitution degree.     

On the propensity of magnetism in 3d transition-metal-MgCNi3 alloys

The effects of disorder and incipient magnetism in MgC(_Ni1-xTx)3$\mathrm{MgC}({\mathrm{Ni}}_{1-x}{\mathrm{T}}_x{)}_3$ (T≡Fe$\mathrm{T}\equiv \mathrm{Fe}$, Co or Cu) alloys are studied using coherent-potential approximation and Ginzburg–Landau coefficients. The first-principles, local-density-functional-based calculations for substitutionally disordered Fe and Co impurities in the Ni sub-lattice of MgCNi₃, in low concentrations, show that incipient magnetism resides in these materials. The overestimation of the calculated magnetic properties points to the limitations of the local-density approximation. However, using a phenomenological approach based on Ginzburg–Landau coefficients and the fixed-spin moment method, we show that MgC(_Ni1-xTx)3$\mathrm{MgC}({\mathrm{Ni}}_{1-x}{\mathrm{T}}_x{)}_3$ alloys remain paramagnetic. At expanded volumes, we also find the possibility of a ferromagnetic state for MgC(_{Ni0.95Fe0.05)3}$\mathrm{MgC}({\mathrm{Ni}}_{0.95}{\mathrm{Fe}}_{0.05}{)}_3$ and MgC(_{Ni0.90Co0.10)3}$\mathrm{MgC}({\mathrm{Ni}}_{0.90}{\mathrm{Co}}_{0.10}{)}_3$ alloys.     

Theoretical study of diluted Co/Cu alloys

This study employs the self-consistent Green's function method to study the magnetic properties of diluted _CoxCu1-x${\mathrm{Co}}_x{\mathrm{Cu}}_{1-x}$ alloys from a consideration of their spin dynamics characteristics. The numerical results show that in dilute cobalt concentrations (i.e. x?0.4$x?0.4$), the critical temperatures vary linearly with x for different itinerant carrier concentration conditions. Interestingly, the carrier concentration does not affect the degree of dependency of the temperature on the cobalt concentration when the carrier concentration is less than the atomic number concentration of the alloy.     

X-ray magnetic circular dichroism at rare-earth L23 absorption edges in various compounds and alloys

Theoretical interpretations of X-ray magnetic circular dichroism (XMCD) at rare-earth (called R hereafter) L₂₃ absorption edges are reviewed using differing models, depending on the material under investigation. In the first chapter, we present an overview of recent developments for XMCD in XAS with a few general remarks, especially at R atom absorption edges. In Section 2, we first describe basic mechanism of XMCD at L₂₃ edges of R systems, and then we essentially discuss two examples of XMCD spectra in: (i) R₂Fe₁₄B metallic compounds, with the help of a cluster model, and (ii) RFe₂ Laves-phase compounds, using a tight-binding approximation for R 5d and Fe 3d conducting states. A good agreement between theory and experiment for R₂Fe₁₄B suggests that a cluster model provides a valuable method to quantitatively calculate XMCD spectra of R systems, even with quite complicated atomic arrangements. For RFe₂ systems the XMCD spectral shape, especially for the L₂ edge of heavy R elements, is more complicated than those of R₂Fe₁₄B systems, and this is explained by the competition of some different XMCD mechanisms. In Section 3, we focus on special series of Ce systems, related to XAS and XMCD studies at the Ce L₂₃ edges. Two clearly differing cases are interpreted: (i) A well localized 4f¹ system, i.e. CeRu₂Ge₂; (ii) A less localized 4f¹ system, i.e. CeFe₂, with a 3d partner. Then, from a more experimental point of view, we investigate the influence of substitution on the low temperature properties of CePd₃ compounds, i.e. Ce(_Pd1-xMnx)3$\mathrm{Ce}({\mathrm{Pd}}_{1-x}{\mathrm{Mn}}_x{)}_3$ alloys where x   is about 0.03, giving rise to (_CePd3)8Mn$({\mathrm{CePd}}_3{)}_8\mathrm{Mn}$ ordered structure. We give another example: Ce(_Pd1-xNix)3$\mathrm{Ce}({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x{)}_3$ alloys with x taken up to about 0.25. Also the Ce L₂₃ XMCD signal measured in pure CePd₃ demonstrates that in the Ce based dense Kondo materials, only the 4f¹ channel yields a magnetic response.     

First-principles materials design of CuInSe2-based high-efficiency photovoltaic solar cells

Based on ab initio   electronic structure calculations by self-interaction-corrected local-density-approximation (SIC-LDA) with the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA), we propose a materials design for high efficiency photovoltaic solar cells (PVSCs). It is shown that (i) the concentration dependence of the mixing energy of CuIn_1−xGa_xSe₂${\mathrm{CuIn}}_{1-x}{\mathrm{Ga}}_x{\mathrm{Se}}_2$ shows upward convexity, thus this system favors phase separation. Due to the type II band alignment between CuInSe₂${\mathrm{CuInSe}}_2$ and CuGaSe₂${\mathrm{CuGaSe}}_2$, efficient electron–hole separation is realized in decomposed phase of this system. (ii) CuIn_1−xZn_0.5xSn_0.5xSe₂${\mathrm{CuIn}}_{1-x}{\mathrm{Zn}}_{0.5x}{\mathrm{Sn}}_{0.5x}{\mathrm{Se}}_2$ has a direct band gap and no impurity state appears in the gap. Therefore, cost reduction is possible by using Zn and Sn instead of In. (iii) n-type CuAl_1−xSn_xS₂${\mathrm{CuAl}}_{1-x}{\mathrm{Sn}}_x{\mathrm{S}}_2$ and p-type Cu_1−xV_CuxAlS₂${\mathrm{Cu}}_{1-x}{\mathrm{V}}_{\mathrm{Cu}x}{\mathrm{AlS}}_2$ have negative activation energy for doped impurities and are expected to be low-resistive transparent conducting sulfides, which should be useful for CuInSe₂${\mathrm{CuInSe}}_2$-based PVSCs.     

Improvement of initial permeability for Z-type ferrite by Ti and Zn substitution

We have found that the initial permeability μ^′${\mu }^{\prime }$ of _Co2Z${\mathrm{Co}}_2\mathrm{Z}$ ferrite is improved by the substitution of Ti⁴⁺${\mathrm{Ti}}^{4+}$ and Zn²⁺${\mathrm{Zn}}^{2+}$ ions for Fe³⁺${\mathrm{Fe}}^{3+}$ ions. The substituted sample of _{Ba3Co2TixZnxFe24-2xO41}${\mathrm{Ba}}_3{\mathrm{Co}}_2{\mathrm{Ti}}_x{\mathrm{Zn}}_x{\mathrm{Fe}}_{24-2x}{\mathrm{O}}_{41}$ with x=0.85$x=0.85$ has a maximum μ^′${\mu }^{\prime }$ of 24, which is twice as large as that of the non-substituted sample with x=0$x=0$. The particle size and shape are changed by the substitution. This is influential in the densification and the preferential orientation of a toroidal-shape sample, which results in the improvement of μ^′${\mu }^{\prime }$.     

Effect of La doping on magnetotransport and magnetic properties of double perovskite Sr2FeMoO6 system

We have investigated the magnetotransport and magnetic properties on polycrystalline samples of Sr_2−xLa_xFeMoO₆ (x=0$x=0$, 0.2, 0.4, 0.6, 0.8 and 1.0). The magnitude of intergrain tunneling magnetoresistance with low magnetic field of 0.88 T for x=0.2$x=0.2$ and 0.4$0.4$ samples are as large as 5% and 7% at room temperature and 13% and 10% at 10 K, respectively. The increase of coercivity (_Hc$H_{\mathrm{c}}$), ratio of remanent magnetization with respect to saturation magnetization (_Mr/_Ms$M_{\mathrm{r}}/M_{\mathrm{s}}$), high saturation fields, and reduction of the saturation magnetization indicate that random disorder of spin orientation is mainly responsible for enhancement of the low-field magnetoresistance for samples with x?0.4$x?0.4$. Whereas rapid drop of _Hc$H_{\mathrm{c}}$, _Mr/_Ms$M_{\mathrm{r}}/M_{\mathrm{s}}$, _Mr$M_{\mathrm{r}}$, and saturation fields for samples with x>0.4$x>0.4$ signifies the growth of antiphase boundary, which gives rise to lower values of low-field MR.     

Magnetic,dielectric and magnetoelectric properties of new family of orthorhombic multiferroic Eu1−xYxMNO3 manganites

Multiferroic ground states with a spatially modulated antiferromagnetic structure and electric polarization have been revealed in Eu_1−xY_xMnO₃ (0.2?x?0.5$0.2?x?0.5$) single crystals. While the slightly substituted (x?0.1$x?0.1$) compounds exhibited a transition from the incommensurate (IC) to the canted antiferromagnetic (CAF) state at _TCA<_TN$T_{\mathrm{CA}}<T_{\mathrm{N}}$, the transitions from IC to commensurate ferroelectric (C/FE) phase were observed at _Tlock<_TN$T_{\mathrm{lock}}<T_{\mathrm{N}}$ for x>0.2$x>0.2$. Various phase transitions were observed in the magnetic fields up to 250 kOe along a, b, c axes by magnetization, magnetostriction and electric polarization measurements which show an existence of a spontaneous electric polarization below T_lock.     

Structural,magnetic, and transport properties in La(2+4x)/3Sr(1−4x)/3Mn1−xCuxO3 (0⩽x⩽0.20) system

A series of the double-doping samples La_(2+4x)/3Sr_(1−4x)/3Mn_1−xCu_xO₃(0?x?0.2)$(0?x?0.2)$ with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been prepared. The structural, magnetic, transport properties and magnetoresistance of the series samples have been investigated. It is found that no apparent crystal structure change is introduced by Cu doping up to x=0.20$x=0.20$. But the Curie temperature _TC$T_{\mathrm{C}}$ and magnetization M   are strongly affected by Cu substitution. A remarkable magnetotransport behavior, characterized by double bumps, is observed, and an obvious low-temperature upturn is found in the range of 0.07?x?0.12$0.07?x?0.12$. As a result, the temperature range of colossal magnetoresistance (CMR) is greatly broadened. Moreover, it is found that the room temperature magnetoresistance (MR) of double-doping samples is obviously larger that the undoped La_2/3Sr_1/3Mn_1−xCu_xO₃ at 300 K, which can give a guide for the adequate selection of the room temperature CMR materials.     

Structural characterization and magnetization of Mg0.7Zn0.3SmxFe2−xO4 ferrites

Mg_0.7Zn_0.3Sm_xFe_2−xO₄ ferrites were prepared by the solid-state reaction method and were characterized by X-ray diffraction and magnetization measurements. A single spinel phase was obtained in the range 0.00?x?0.03$0.00?x?0.03$. The lattice parameter was found to increase at x=0.01$x=0.01$ and then decreases up to x=0.03$x=0.03$, which may indicate a distortion in the spinel lattice. The saturation magnetization was found to decrease with the increase in x up to 0.04, due to the replacement of the Fe³⁺ ions by the Sm³⁺ ions.     

Permeability-frequency spectra of (Fe1−xcox)73.5Cu1Nb3Si13.5B9 nanocrystalline alloys under different magnetic fields

Under various amplitude of AC magnetic fields domain wall motion is the main mechanism in the magnetization process. This includes domain wall bulging and domain wall displacing. In this paper complex permeability-frequency spectra of (Fe_1−xCo_x)_73.5Cu₁Nb₃Si_13.5B₉ (x=0,0.5$x=0,0.5$) nanocrystalline alloys were measured as a function of the AC magnetic field, ranging from 0.001 to 0.04 Oe. Obvious changes have been found in complex permeability spectra for alloy x=0$x=0$ with the change of the amplitude of AC magnetic field, but variation of AC magnetic field has little effect on complex permeability spectra for alloy x=0.5$x=0.5$. This is attributed to the increased pinning field after substitution of Fe with Co in Fe_73.5Cu₁Nb₃Si_13.5B₉ nanaocrystalline alloy.     

Magnetic entropy-change in La0.67−xBixCa0.33MnO3 compound

Bi doped lanthanum manganites with the chemical composition of La_0.67−xBi_xCa_0.33MnO₃ (x=0$x=0$, 0.05, 0.1, 0.2) were prepared by the standard solid-state process. The Curie temperatures were measured to be 267 K for x=0$x=0$, 248 K for x=0.05$x=0.05$, 244 K for x=0.1$x=0.1$ and 229 K for x=0.2$x=0.2$ samples. It was found that the maximum value of the magnetic entropy change ∣ΔS_m∣ has reached the highest value of 6.08 J/kg K at 3 T for the composition with x=0.05$x=0.05$. Nearly the same maximum entropy change was observed for the x=0$x=0$ sample. A large decrease in the magnitude of the entropy change was observed for the x=0.2$x=0.2$ sample.     

Effect of external magnetic field on superconducting and spin density wave gaps of high-Tc superconductors

A theoretical model is addressed here to study the interplay of the superconductivity (SC) and the spin density wave (SDW) long range orders in underdoped region in the vicinity of on-set of superconductivity in presence of an external magnetic field. The order parameters are calculated by using Zubarev’s technique of Green’s functions and determined numerically self-consistently. The gap parameters are found to be strongly coupled to each other through their coupling constants. The interplay displays BCS type two gaps in the quasi-particle density of states (DOS) which resemble the tunneling conductance of STM experiments. The gap edges in the DOS appear at ±(z+z₁)$\pm (z+z_1)$ and ±(z-z₁)$\pm (z-z_1)$. The applied magnetic field further induces Zeeman splitting which is explained on the basis of spin-filter effect of tunneling experiment.     

The influence of 3d-impurities on magnetic and transport properties of CoSiB metallic glasses

Temperature dependencies of magnetic susceptibility and electric resistivity of Co-based metallic glasses (MGs) of the general composition _Co(72-x)Mex(Si,B₎₂₈${\mathrm{Co}}_{(72-x)}{\mathrm{Me}}_x(\mathrm{Si},\mathrm{B}{)}_{28}$(Me=Fe,Cr,Si:B=18:10)$(\mathrm{Me}=\mathrm{Fe},\mathrm{Cr},\mathrm{Si}:\mathrm{B}=18:10)$ have been studied up to 950 K. The studied MGs were found to be ferromagnets at the room temperature and their Curie point _TC$T_{\mathrm{C}}$ ranges within 260–560 K depending on the dopant contents. At the temperatures higher than _TC$T_{\mathrm{C}}$, a wide paramagnetic region exists. The regularities of magnetic moment variation upon Cr doping evidence a formation of antiferromagnetic clusters, which determine the anomalous behavior of resistivity.