Magnetic characteristics of R2(Fe, Co)14B systems (R = Y, Nd and Gd)

R₂(Fe, Co)₁₄B compounds (R = Y, Nd and Gd) were prepared in high purity. The magnetic behavior of R₂(Fe, Co)₁₄B compounds is reported over the temperature range 4 to 300 K. The effects of Fe substitution by Co on the saturation magnetization, Curie temperature and anisotropy are presented. The spin-reorientation temperature is lowered as Co replaces Fe. This also results in a reduced cone angle.

The R₂Fe_14−xCo_xB alloys crystallize in the tetragonal structure over the entire concentration range of 0 x 14. When Fe is substituted by Co, the Curie temperature increases significantly, the saturation magnetization increases to a maximum value around x = 2, and the anisotropy becomes planar for R = Y and Gd. The Nd₂(Fe, Co)₁₄B systems all exhibit uniaxial anisotropy at room temperature and Nd₂Co₁₄B is strongly uniaxial at 77 K. The Nd₂(Fe, Co)₁₄B systems are conical at 77 K.     

Magnetic and structural investigations on substituted Pr2Co17-xTx systems (T= Fe, Mn, Cr, Cu and Al)

Magnetic and structural characteristics of Pr₂Co₁₇-based ternaries, in which Co is partly replaced by other transition elements, namely Fe, Mn, Cr, Cu and Al, have been investigated. The objective of the work was to explore the possibility of enhancing the anisotropy of Pr₂Co₁₇ to the point that it would be useful for permanent magnet fabrication. X-ray diffraction indicates that all the systems studied occur in the rhombohedral Th₂Zn₁₇ structure. The cell constants increase as Fe, Mn, etc., are introduced in the system, which indicates that substitution occurs in the Co sublattice. The Curie temperatures, T_c, decrease monotonically with increasing x in the order Fe < Cu < Mn < Al < Cr. The saturation magnetization, M_s, increases with increasing x for the Fe system with x 8. For x #62; 8 in the Pr₂Co_17-xFe_x system and in all other ternary systems st died, M_s decreases with x, the magnitude of the effect being in the order Cr #62; Al #62; Mn #62; Cu. The rate of decrease is larger than that expected as a simple dilution except for the systems containing Cu. Significant modification of the anisotropy characteristics of Pr₂Co₁₇ is observed upon substitution. The planar anisotropy of Pr₂Co₁₇ becomes uniaxial with Fe (x 4) and Mn (x 2) substitutions. With Cr and Al substitutions, the plane-seeking tendency of Pr₂Co₁₇ is significantly weakened.     

Study on the superconducting composite material formation in the system Bi2Sr2CaCu2O8+x/Al-containing phases

Phase evolution in the Bi---Sr---Ca---Cu---Al---O system was studied. Two Al-containing phases BiSr_1.5Ca_0.5Al₂O_z and (Sr_1−xCa_x)₃Al₂O₆ (x = 0.4 − 0.45) were determined to be chemically compatible with Bi_2.18Sr₂CaCu₂O_8+x (Bi-2212) at temperatures of the samples processing. The phase equilibria in the title system were investigated above the solidus temperature. The BiSr_1.5Ca_0.5Al₂O_z was found to be in equilibrium only with the melt and the (Sr_1−xCa_x)₃Al₂O₆ phase. This latter aluminate equilibrated with Ca,Sr cuprates, CaO, the Cu-free phase, and the liquid. The melting and solidification in Bi-2212, doped with the aluminate, corresponded to the reversible reaction Bi-2212 + BiSr_1.5Ca_0.5Al₂O_z ↔ (Sr_1−xCa_x)₃Al₂O₆ + liquid. Two sets of superconducting composite materials with initial compositions Bi-2212 + nBiSr_1.5Ca_0.5Al₂O_z and Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ were prepared by solidification from the partial melt. The former material was composed mostly of large Bi-2212 lamellas separated by the BiSr_1.5Ca_0.5Al₂O_z phase, which destroyed superconducting links between Bi-2212 grains. The latter material consisted of a Bi-2212 polycrystalline matrix with high concentration of small (ca. 3 μm) grains of (Sr_1−xCa_x)₃Al₂O₆ imbedded in Bi-2212 lamellas. The Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ materials displayed a trend to enhance flux pinning at T = 60 K with the increase of aluminate phase content.     

Very strong interlayer exchange coupling in epitaxial Fe/Fe1−xSix/Fe trilayers (x=0.4–1.0)

Fe/Fe_1−xSi_x/Fe (x=0.4–1.0) wedge-type epitaxial trilayers with improved homogeneity are grown by co-evaporation from two electron-beam sources. The coupling strengths of the bilinear (J₁) and biquadratic (J₂) coupling terms are derived from Brillouin light scattering (BLS) spectra and longitudinal MOKE hysteresis loops. The total coupling strength J=J₁+J₂ increases dramatically with increasing x and reaches values in excess of 6 mJ/m².     

High field magnetization of the Laves phase compounds M(Co1−xAlx)2 (M = Y, Lu)

Magnetization measurements have been carried out on the Laves phase compounds M(Co_1−xAl_x)₂(M = Y, Lu) up to 42 T. Sharp metamagnetic transitions with small hysteresis are found in Y(Co_1−xAl_x)₂, while broad transitions with large hysteresis are found in Lu(Co_1−xAl_x)₂. The results suggest that the former compounds are magnetically homogeneous but the latter inhomogenous.     

Effect of Co substitution on magnetic properties of Sr14Cu24O41

A series samples of Sr₁₄(Cu_1−xCo_x)₂₄O₄₁ (x=0, 0.02, 0.06, 0.14, 0.18) were prepared by standard solid-state reaction. X-ray diffraction measurements show that all the samples are single phase and their lattice parameter hardly changes by Co dopant. Electron diffraction experiments and X-ray photo-emission spectroscopy measurements reveal that Co ions substituted Cu ions in the chain. The measurements of magnetic susceptibility from 10 to 300 K in an applied magnetic field of 1.0 T show that Co dopant induces increase in susceptibility. The spin gaps are observed in all the samples, and decrease with increase in Co doping concentration. Fitting of the date indicates that strong antiferromagnetic interaction is induced and antiferromagnetic dimeried state may be formed due to Co³⁺ ions doping in these compounds.     

Guest-guest interactions in cointercalation compounds of FexCo1/4TiS2 and (FeyCo1−y)1/3TiS2

Lattice spacings, as magnetic susceptibilities, ESR, specific heats, and electrical properties have been measured on the cointercalation compounds of 1T-CdI₂ type host TiS₂, Fe_xCo TiS₂ (0 x ) and (Fe_yCo_1−y) TiS₂(0 y 1).

While the interlayer spacings are expanded rather smoothly with increasing the cointercalated Fe metals, the magnetic, thermal, and transport properties depend sensitively on a small amount of the added Fe metals, showing the presence of strong magnetic interactions, or guest-guest interactions between the cointercalated Fe and Co 3d metals in these systems.     

Raman phonon anomalies in Sr(Fe1-xCox)2As2

Phonon anomalies have been reported in iron-pnictide superconductors indicating a diverse interplay between different orders in the materials.Here,we report Raman scattering measurements on Sr(Fe_1-xCo_x)₂As₂(x=0 and x=0.04)single crystals in the B;symmetry with respect to a 1 Fe unit cell.Upon cooling,we observe a larger split(13 cm;)of Eg Raman phonon modes pertaining to in-plane Fe and As displacements as the crystals undergo the tetragonal-toorthorhombic structural phase transition,although a considerable split(9 cm;)has been reported in BaFe_1-xCo_x)₂As₂.Furthermore,the splitting of E;phonon modes is strongly reduced upon doping.We perform an order-parameter analysis revealing a similar doping dependence of E;phonon splitting as reported in other compounds of the 122 family,indicating these phonon anomalies widely exist in 122 iron-based superconductors and might share the same mechanisms.     

The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.     

Identification of first- and second-order magnetic phase transitions in ferromagnetic perovskites

A criterion used for the determination of first- and second-order magnetic phase transitions from purely magnetic methods is applied to manganese perovskites of formula La_2/3(Ca_1−xSr_x)_1/3MnO₃. A crossover from first- to second-order character at a tolerance factor t=0.92 is found, which also brings about several variations in other physical properties. At t=0.92 a change from orthorhombic to rhombohedral symmetry also takes place. The impossibility of establishing static cooperative Jahn–Teller distortions in the rhombohedral symmetry is suggested as being responsible for the observed behaviour.     

Magnetic properties of Sm–Co–B-based nanocomposite magnets

The magnetic properties of nanocomposite melt-spun magnets with composition Sm_16−xCo_68+xB₁₆ (x=0–10, 2 at% interval) and Sm₈Co_92−yB_y (y=10–18, 2 at% interval) have been studied systematically. Several ribbons were fabricated with a wheel speed of 50 m/s, followed by annealing in the temperature range of 700–800°C for 2.5–40 min. XRD results and magnetization versus temperature curves showed that almost all of the samples were composed of the tetragonal Sm₂Co₁₄B and rhombohedral SmCo₁₂B₆ phases which are not magnetically hard at room temperature. However, a relatively high coercivity in the range of 3.5–5.5 kOe has been obtained in these samples. The highest coercivity of 5.5 kOe and a very promising β value of −0.28%/°C were obtained in Sm₈Co₇₄B₁₈ ribbons annealed at 750°C for 5 min. The high coercivities are attributed to the small grain size of the 2 : 14 : 1 phase, in which the large surface areas enhance its effective anisotropy, and make it uniaxial type.     

(Sr_(1-1.5x)Y_x)TiO_3模拟固化~(90)Y的稳定性研究

为了研究钙钛矿结构的SrTiO_3固化Y~(3+)(~(90)Y的模拟物)的化学稳定性,以Sr(NO_3)_2,TiO_2及Y_2O_3粉体作为原料,按照化学计量比Sr_(1-1.5x)Y_xTiO_3(0≤x≤0.12)设计配方,采用高温固相法制备一系列固化体.利用X射线衍射、扫描电镜和Raman对制备固化体的物相、结构和微观形貌进行分析表征,并对其抗浸出性能进行了研究.结果表明:当x0.08时,固化体为单一物相,当x≥0.08时,固化体中出现部分烧绿石相;固化体中的Sr~(2+),Y~(3+)的浸出浓度随浸泡时间延长而增大,在浸泡42d时,Sr~(2+)的最大浸出浓度为0.004μg·mL~(-1),Y~(3+)的最大浸出浓度为0.02μg·mL~(-1).     

Effects of cation- or oxide ion-defect on conductivities of apatite-type La–Ge–O system ceramics

A series of apatite-type La–Ge–O ceramics were prepared and their cation-defect at the 4f+6h sites and oxide ion-defect at 2a site were investigated. In La_xGe₆O_12+1.5x ceramics of x=6–12, the higher conductivities were obtained in the region of apatite composition, La_x(GeO₄)₆O_1.5x−12 (x=8–9.33), and the highest conductivity was achieved for La₉(GeO₄)₆O_1.5 (x=9), where the number of cation (La³⁺) occupying the 4f+6h sites is 9 and the number of oxide ion occupying the 2a site is 1.5. The ceramics with cation- and oxide ion-defects were La_9−0.66xSr_x(GeO₄)₆O_1.5 (x=0–1), La_9−1.33xZr_x(GeO₄)₆O_1.5 (x=0–1), La_9−xSr_x(GeO₄)₆O_1.5−0.5x (x=0–3), La_9−xZr_x(GeO₄)₆O_1.5+0.5x (x=0–1), La_x(GeO₄)_3x−21(AsO₄)_27−3xO_1.5 (x=0–3), La_x(GeO₄)_33−3x(AlO₄)_3x−27O_1.5 (x=0–3), La₉(GeO₄)_6−x (AlO₄)_xO_1.5−0.5x (x=0–3), La₉(GeO₄)_6−x(AsO₄)_xO_1.5+0.5x (x=0–1), La_9.33−xSr_x(GeO₄)₆O_2−0.5x (x=0–1.2) and La_x(GeO₄)_4.5(AlO₄)_1.5O_1.5x−12.75 (x=8.8–9.83), which were prepared by the partial substitution of La³⁺and GeO₄⁴⁻of the basic apatite La₉(GeO₄)₆O_1.5 with Sr²⁺ or Zr⁴⁺ and AlO₄⁵⁻ or AsO₄³⁻. Such substitutions lowered the conductivity of La₉(GeO₄)₆O_1.5. These results were discussed by the electrostatic interaction between Sr²⁺, Zr⁴⁺, AlO₄⁵⁻ or AsO₄³⁻ and oxide ion as a conductive species.     

Structural and lithium intercalation studies of Mn(0.5−x)CaxTi2(PO4)3 phases (0≤x≤0.50)

Materials from the Mn_(0.5−x)Ca_xTi₂(PO₄)₃ (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, ³¹P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R space group. Mn²⁺/Ca²⁺ ions occupy only the M1 sites in the Ti₂(PO₄)₃ framework. The divalent cations are ordered in one of two M1 sites, except for the Mn_0.50Ti₂(PO₄)₃ phase, where a small departure from the ideal order is observed by XRD and ³¹P MAS NMR. The electrochemical behaviour of Mn_0.50Ti₂(PO₄)₃ and Mn_(0.5−x)Ca_xTi₂(PO₄)₃ phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti₂(PO₄)₃ framework. In the 0≤y≤2 range, the OCV curves of Li//Li_yMn_0.50Ti₂(PO₄)₃ cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li_(1+y)Ti₂(PO₄)₃ and Li//Li_yMn_0.50Ti₂(PO₄)₃ shows that the intercalation occurs first in the unoccupied M1 site of Mn_0.50Ti₂(PO₄)₃ at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn_0.50Ti₂(PO₄)₃ on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn_0.50Ti₂(PO₄)₃, the stability of Mn²⁺ either in an octahedral or tetrahedral environment facilitates cationic migration.     

High field magnetization in Pr(Ni1-xCox)5 single crystals

Magnetization measurements in fields up to 38 T performed at low temperature on single crystals of the hexagonal Pr(Ni_1-xCo_x)₅ compounds for x = 0, 0.05, 0.10 and 0.20 are presented. In PrNi₅ we observe highly original behaviour predicted by the knowledge of the Crystalline Electric Field parameters and arising from the existence of a non-magnetic singlet ground state; namely transitions associated with the field induced “anticrossing” and “crossing” of the two lowest states along the [100] and [120] directions, respectively. The measurements performed on the other compounds have allowed us to study the dependence of this behaviour on Co substitutions.     

Itinerant metamagnetic transition in Y(Co1−xFex)2

High field magnetization measurements have been performed to examine the existence of itinerant metamagnetism in exchange-enhanced systems related to YCo₂ together with Fe_1−x Co_xSi. In the Y(Co_−xFe_x)₂ system, the meta magnetism inherent in YCo₂ has been observed in 0.04 x0.07. The transition is not as sharp as in the Y(Co_1−xAl_x)₂ system. Other exchange-enhanced paramagnets Y(Co_−xCu_x)₂ and Y_1−xLa_xCo_x2 and weakly itinerant ferromagnet Fe_1−xCo_xSi exhibit no metamagnetic transition up to 430 kOe.     

Dielectric and ferroelectric properties of the mixed crystals system (CH3NH3)5Bi2(1 − x)Sb2xCl11

Dielectric and pyroelectric properties of the mixed crystals system, (CH₃NH₃)₅Bi_{2(1 − x)}Sb_2xCl₁₁ (0 < x < 0.25) were systematically investigated. Temperature dependencies of ′_c in the vicinity of ferro-paraelectric phase transition were measured for the mixed crystals with x = 0.05, 0.07, 0.11, 0.13 and 0.25 in the frequency region 1 kHz–1 MHz. The substitution of bismuth atoms by antimony drastically reduces the magnitude of ′_c and shifts the ferro-paraelectric phase transition towards higher temperatures. The dielectric dispersion of the complex electric permittivity, _c^*, in x = 0.05 crystals was studied in the frequency range from 30 to 1000 MHz. Around 321 K phase transition, two dielectric relaxators are postulated; a low-frequency one in the megahertz region showing a critical slowing down and a high-frequency one in the gigahertz region.     

A contribution to the invar problem by hyperfine field distribution analyses

The Mössbauer spectra of Invar type Fe₆₅(Ni_1-xMn_x)₃₅ alloys (0 x 0.3) were analyzed to yield hyperfine field distribution (P(H)) curve. The P(H) curves of the alloy with x = 0, that is the “classical” Invar alloy, at 4.2 K is character ized by a relatively sharp main peak at 350 kOe and a weak low field component around 50 kOe. With increasing x, the low field component grows and the high field main peak spreads toward a low field. By raising temperature, the main peak exhibits a remarkable broadening as well as a decrease in the average internal field and a growth of the low field component has not been detected. On the basis of these observations, models of the Invar alloy so far proposed are discussed.     

Effect of hydrogen absorption on the magnetic properties of some Zr-3d laves phase binary and pseudobinary intermetallics

Magnetization measurements have been performed on ZrCr₂, ZrMn₂, Zr(Cr_xFe_1−x)₂(x = 0.2, 0.5, 0.75) and Zr(Cr_x Co_1−x)₂ (x = 0.75, 0.4) before and after hydrogenation. In all cases both the magnetic moments and the magnetic ordering temperatures (whenever present) appeared to be significantly larger in the hydride phases. The increase of the magnetic ordering temperatures is in a qualitative agreement (but not a quantitative one) with the negative pressure effects on T_c in ZrFe₂.     

W doping-dependent structural and ferroelectric properties of SrBi2Nb2O9 ferroelectric ceramics

The structural and ferroelectric characteristics of SrBi₂(Nb_1−xW_x)₂O₉ (x=0–0.12) ferroelectric ceramics were investigated. SrBi₂(Nb_1−xW_x)₂O₉ ceramics consisted of a single-phase layered perovskite structure when x was less than 0.06. Uniform microstructure and grain size reduction were observed after the introduction of W. The maximum remanent polarization of 16 μC/cm² appeared at x=0.03, and the coercive field decreased with increasing concentration of W. The ferroelectric behavior of SrBi₂(Nb_1−xW_x)₂O₉ ceramics is interpreted based on the Raman measurement.