Magnetic properties and magnetocaloric effects in(Ho_(1-x)Y_x)_5Pd_2 compounds

The crystal structure, magnetic and magnetocaloric properties of(Ho_(1-x) Y_(0.5))_5 Pd_2 compounds are investigated. All the compounds crystallize in a cubic Dy_5 Pd_2-type structure with the space group Fd3 m and undergo a second order transition from spin glass(SG) state to paramagnetic(PM) state. The spin glass transition temperatures T_g decrease from 26 K for x = 0 to 13 K for x = 0.5. In the PM region, the reciprocal susceptibilities for all the compounds obey the Curie–Weiss law. The paramagnetic Curie temperatures(θp) for Ho_5 Pd_2,(Ho_(0.75) Y_(0.25)_5 Pd_2, and(Ho_(0.5) Y_(0.5))_5 Pd_2 are determined to be 32 K, 30 K, and 22 K, respectively, and the corresponding effective magnetic moments(μeff) are10.8 μB/Ho, 10.3 μB/RE, and 7.5 μB/RE, respectively. Magnetocaloric effect(MCE) is anticipated according to the Maxwell relation, based on the isothermal magnetization curves. For a magnetic field change of 0–5 T, the maximum values of the isothermal magnetic entropy change-?SMof the(Ho_(1-x)Y_x)_5 Pd_2(x = 0, 0.25, and 0.5) compounds are determined to be 11.5 J·kg~(-1)·K~(-1), 11.1 J·kg~(-1)·K~(-1), and 8.9 K J·kg~(-1)·K~(-1), with corresponding refrigerant capacity values of 382.3 J·kg~(-1), 336.2 J·kg~(-1), and 242.5 J·kg~(-1), respectively.     

Magnetic properties and magnetocaloric effects in Er_(1-x)Gd_xCoAl intermetallic compounds

The magnetism and magnetocaloric effect in Er1-xGdxCoAl(x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) were investigated. The Er1-xGdxCoAl compounds were synthesized by arc melting. With the increasing Gd content, the N′eel temperature(T N)linearly increases from 14 K to 102 K, while the magnetic entropy change(-?S M) tends to decrease nonmonotonously.Under the field change from 0 T to 5 T, the-?S M of the compounds with x = 0.2–1 are stable around 10 J/kg·K, then a cooling platform between 20 K and 100 K can be formed by combining these compounds. For x = 0.6, 0.8, 1.0, the compounds undergo two successive magnetic transitions, one antiferromagnetism to ferromagnetism and the other ferromagnetism to paramagnetism, with increasing temperature. The two continuous magnetic transitions in this series are advantageous to broaden the temperature span of half-peak width(δT) in the-?S M–T curve and improve the refrigeration capacity.     

First-order transition from a Kondo insulator to a ferromagnetic metal in single crystalline FeSi(1-x)Ge(x)

The phase diagram of FeSi(1-x)Ge(x), obtained from magnetic, thermal, and transport measurements on single crystals, shows a discontinuous transition from Kondo insulator to ferromagnetic metal with x at a critical concentration, x(c) approximately 0.25. The gap of the insulating phase strongly decreases with x. The specific heat gamma coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with an insulator-metal transition induced by a reduction of the hybridization with x in conjunction with disorder on the Si/Ge ligand site.     

Modulation of magnetic properties and enhanced magnetoelectric effects in MnW_(1-x)Mo_xO_4 compounds

In this letter, we investigate the magnetic and ferroelectric properties of polycrystalline MnW1-xMoxO4 （x = 0, 0.05, 0.10, 0.20） compounds. The substitution of nonmagnetic Mo6＋ ions for W6＋ ions modifies the magnetic transition tem- peratures of MnW1-xMoxO4 by changing the Mn-O-Mn bond. As a result, distinct ferroelectric properties and enhanced magnetoelectric effects are observed in Mo6＋-doped MnWO4 compounds. The effects of substitution of Mo6＋ ions on magnetic properties and magnetoelectric coupling are discussed.     

Coexistence of positive and negative magnetic entropy changes in CeMn_2(Si_(1-x)Ge_x)_2 compounds

A series of CeMn2(Si1-xGex)2(x = 0.2, 0.4, 0.6, 0.8) compounds are prepared by the arc-melting method. All the samples primarily crystallize in the Th Cr2Si2-type structure. The temperature dependences of zero-field-cooled(ZFC) and FC magnetization measurements show a transition from antiferromagnetic(AFM) state to ferromagnetic(FM) state at room temperature with the increase of the Ge concentration. For x = 0.4, the sample exhibits two kinds of phase transitions with increasing temperature: from AFM to FM and from FM to paramagnetic(PM) at around TN~197 K and T C~300 K,respectively. The corresponding Arrott curves indicate that the AFM–FM transition is of first-order character and the FM–PM transition is of second-order character. Meanwhile, the coexistence of positive and negative magnetic entropy changes can be observed, which are corresponding to the AFM–FM and FM–PM transitions, respectively.     

Magnetic-field-induced localization of quasiparticles in underdoped La(2-x)SrxCuO4 single crystals

Magnetic-field-induced ordering of electrons around vortices is a striking phenomenon recently found in high-T(c) cuprates. To identify its consequence in the quasiparticle dynamics, the magnetic-field (H) dependence of the low-temperature thermal conductivity kappa of La(2-x)SrxCuO4 crystals is studied for a wide doping range. It is found that the behavior of kappa(H) in the subkelvin region changes drastically across optimum doping, and the data for underdoped samples are indicative of unusual magnetic-field-induced localization of quasiparticles; this localization phenomenon is probably responsible for the unusual "insulating normal state" under high magnetic fields.     

CdSxTe1-x多晶薄膜的制备与性质研究

采用真空共蒸发方法制备了CdSxTe1 -x 多晶薄膜 ,并用原子力显微镜、x射线衍射和光学透过率谱等研究了CdSxTe1 -x多晶薄膜的结构和性质 .结果表明 :薄膜均匀、致密、无微孔 ,当x≥ 0 5时为n型半导体 ,x <0 5时为p型半导体 .CdSxTe1 -x多晶薄膜的光学能隙随x变化 .结合薄膜的晶格常数和光学能隙得到了薄膜发生相变的组分 ,当x<0 2 5时CdSxTe1 -x 多晶薄膜为立方相 ,当x >0 2 5时为六方结构 .退火后结构没有改变 ,能隙减小 .提出了用CdSxTe1 -x多晶薄膜作为缓冲层的新型结构太阳电池 .     

Electronic structure and optical properties of Sn_(2x)Ga_(2(1-x))O_3 compounds

Band structure, density of states, electron density difference, and optical properties of intrinsic β-Ga2O3 and Sn2xGa2(1-x)O3 (x= 3.125%-6.25%) compounds are studied using first-principle calculations based on the density functional theory. The anisotropic optical properties are investigated by means of the complex dielectric function, which are explained by the selection rule of band-to-band transitions. All the calculation results indicate that the conductivity of Sn2xGa2(1-x)O3 is super to β-Ga2O3, and ...     

The Peierls instability and superconductivity in quasi-2-d La2-x(Ba,Sr)xCuO4 compounds

The Peierls instability in the quasi-2-d La_2-x(Ba,Sr)_xCuO₄ is reexamined by taking the weak interlayer coupling into account. A two-step Peierls transition theory is developed, the general misunderstandings on the Peierls instability are therefore clarified, and a number of experimental anomalies observed in La₂CuO₄ are well explained. By suggesting an additional change of space group symmetries at the lower transition point, the calculated band structures are also intepreted. The possibility of CDW coexistence with the high-T_c superconductivity, the possible evidence for the 3-d superconductivity of the high-T_c materials are discussed with this theory.     

(Nd_(1-x)Er_x)_2Co_(15.5)V_(1.5)的结构转变与磁性

利用电弧熔炼制备了 (Nd1 xErx) 2 Co1 5 5V1 5(x=0— 1 0 )化合物样品 .通过x射线衍射分析和磁性测量研究了Er替代Nd2 Co1 5 5V1 5中的Nd时对化合物结构和磁性的影响 .研究结果表明 ,低Er含量 (x <0 4 ) ,化合物为Th2 Zn1 7型结构 ;高Er含量时 (x >0 5 ) ,化合物转变为Th2 Ni1 7结构 ;Er含量为x =0 4和 0 5时 ,两种结构共存 .两种结构的晶胞参数a ,c和晶胞体积V随着Er含量的增加都呈现递减的趋势 .随着Er含量的增加 ,(Nd1 xErx) 2 Co1 5 5V1 5化合物的居里温度和饱和磁化强度都单调下降 .(Nd1 xErx) 2 Co1 5 5V1 5化合物的室温各向异性由低Er含量时的易锥型转变为高Er含量时的易轴型 .x =0— 0 5的化合物在温度升高时发生自旋重取向转变 ,自旋重取向温度Tsr随Er含量的增加而减小     

Current percolation and anisotropy in polycrystalline MgB(2)

The influence of anisotropy on the transport current in MgB(2) polycrystalline bulk samples and wires is discussed. A model for the critical current density is proposed, which is based on anisotropic London theory, grain boundary pinning, and percolation theory. The calculated currents agree convincingly with experimental data, and the fit parameters, especially the anisotropy, obtained from percolation theory agree with experiment or theoretical predictions.     

Structural- and chemical-phase-separation on single crystalline sapphire (0001) surfaces

Atomic force microscopy (AFM) was used to characterize morphology and chemistry on step-controlled single crystalline sapphire (0001) surfaces. Hydrophilicity on the surface can be evaluated using frictional force imaging. A sapphire surface that is covered with bunched steps accompanied with crossing steps clearly exhibits the phase-separation into two domains with different hydrophilicity, where an almost hydrophobic domain forms inside a hydrophilic area. By analyzing the histogram of the step height, we concluded that the difference in the hydrophilicity is associated with the surface reconstruction. We found that the phase-separation characterized by the hydrophilicity and charge potential self-organizes on the sapphire surface with specific step arrangements. We demonstrated selective adsorption of biomolecules on step-controlled surfaces as an example of the application.     

Magnetic properties of bulk polycrystalline Pr1-x Ndx（x=0,0.8）FeB permanent magnets

For spin reorientation （SRT）, the applications of sintering NdFeB permanent magnets are limited at low temperature. The sintering PrFeB permanent magnet （PM） presents no SRT and shows excellent magnetic properties at low temperature. The magnetic properties of bulk polycrystalline sintering Prl-xNdxFeB （x = 0 and 0.8 correspond to P42H and N50M respectively） are studied in this paper. The results show that magnetic properties and stability of N50M are better than those of 42H at room temperature. With the decrease of temperature, the parameters ofBr, Hcb, and Hci of P42H present a nearly linear increasing trend; Br and Hcb of N50M first increase and then decline, Hci presents an increasing trend. At 77 K, Br, Hci, and Jr of P42H are increased by 18.7%, 308%, and 17.1% respectively over than those at 300 K; at 120 K, Br, Hci, and Jr of N50M are increased by about 16.19%, 245%, and 12.6% respectively over than those at 300 K. The magnetic properties of P42H are better than those of N50M at low temperature. The sintering PrFeB is the preferred PM in various low-temperature devices.     

From stable divalent to valence-fluctuating behaviour in Eu(Rh(1-x)Ir(x))2Si2 single crystals

We have succeeded in growing high-quality single crystals of the valence-fluctuating system EuIr(2)Si(2), the divalent Eu system EuRh(2)Si(2) and the substitutional alloy Eu(Rh(1-x)Ir(x))(2)Si(2) across the range 0 < x < 1, which we characterized by means of x-ray diffraction, energy-dispersive x-ray spectroscopy, specific heat, magnetization and resistivity measurements. On increasing x, the divalent Eu ground state subsists up to x = 0.25 with a slight increase in Néel temperature, while for 0.3≤x < 0.7 a sharp hysteretic change in susceptibility and resistivity marks the first-order valence transition. For x?0.7 the broad feature observed in the physical properties is characteristic of the continuous valence evolution beyond the critical end point of the valence transition line, and the resistivity is reminiscent of Kondo-like behaviour while the Sommerfeld coefficient indicates a mass renormalization of at least a factor of 8. The resulting phase diagram is similar to those reported for polycrystalline Eu(Pd(1-x)Au(x))(2)Si(2) and EuNi(2)(Si(1-x)Ge(x))(2), confirming its generic character for Eu systems, and markedly different to those of homologue Ce and Yb systems, which present a continuous suppression of the antiferromagnetism accompanied by a very smooth evolution of the valence. We discuss these differences and suggest them to be related to the large polarization energy of the Eu half-filled 4f shell. We further argue that the changes in the rare earth valence between RRh(2)Si(2) and RIr(2)Si(2) (R = Ce, Eu, Yb) are governed by a purely electronic effect and not by a volume effect.     

层状铁电体Ba_(1-x)Bi_(2+x)Ti_xNb_(2-x)O_9的结构相变研究

本文应用拉曼光谱方法研究了Ba1-xBi2+xTixNb2-xO9的结构相变。与x≠0的情况相比,x=0的情况BaBi2Bn2O9表现出了明确不同的拉曼光谱。前者有与Aurivillius层状铁电体家族的铁电相相似的拉曼光谱。因此认为BaBi2Nb2O9发生了铁电—非铁电相变。此外,用内模方法对层次铁电体Ba1-xBi2+xTixNb2-xO9的拉曼振动模式进行了初步的指认     

(Nd1－xErx)2Co15.5V1.5的结构转变与磁性

利用电弧熔炼制备了(Nd1-xErx)2Co15.5V1.5（x=0—1.0）化合物样品.通过x射线衍射分析和磁性测量研究了Er替代Nd2Co15.5V1.5中的Nd时对化合物结构和磁性的影响.研究结果表明，低Er含量（x<0.4），化合物为Th2Zn17型结构；高Er含量时(x>0.)，化合物转变为Th2Ni17结构；Er含量为x＝0.4和0.5时，两种结构共存.两种结构的晶胞参数a,c和晶胞体积V随着Er含量的增加都呈现递减的趋势.随着Er含量的增加，(Nd1-xErx)2Co15.5V1.5化合物的 关键词： (Nd1-xErx)2Co15.5V1.5 结构转变 磁性