Electronic structure and magnetic properties of substitutional transition-metal atoms in GaN nanotubes

The electronic structure and magnetic properties of the transition-metal （TM） atoms （Sc-Zn, Pt and Au） doped zigzag GaN single-walled nanotubes （NTs） are investigated using first-principles spin-polarized density functional calculations. Our results show that the bindings of all TM atoms are stable with the binding energy in the range of 6-16 eV. The Sc- and V-doped GaN NTs exhibit a nonmagnetic behavior. The GaN NTs doped with Ti, Mn, Ni, Cu and Pt are antiferromagnetic. On the contrary, the Cr-, Fe-, Co-, Zn- and Au-doped GaN NTs show the ferromagnetic characteristics. The Mn- and Co- doped GaN NTs induce the largest local moment of 4#B among these TM atoms. The local magnetic moment is dominated by the contribution from the substitutional TM atom and the N atoms bonded with it.

Optical Response of CeB6 Nanoparticles with Different Sizes and Shapes from Discrete-Dipole Approximation

The discrete dipole approximation is used to investigate the optical response of CeB6 nanoparticles with different sizes and different shapes. The extinction valley in the visible light range becomes narrower and the extinction peak at the near infrared region （NIR） is red-shifted with the increasing particle size. In addition, the extinction peak value of the spherical particle decreases more rapidly than that of cubic-shaped particle with an increase in the particle size, and the cubic-shaped particles exhibit better performance on blocking NIR radiation than spherical-shaped particles. The calculation results coincide well with the reported experimental results.

Synthesis,thermionic emission and magnetic properties of(Nd_xGd_(1-x))B_6

Polycrystalline rare-earth hexaborides （NdxGdl-x）B6 （x = 0, 0.2, 0.6, 0.8, 1） were prepared by the reactive spark plasma sintering （SPS） method using mixed powder of GdH2, NdH2 and B. The effects of Nd doping on the crystal structure, the grain orientation, the thermionie emission and the magnetic properties of the hexaboride were investigated by X-ray diffraction, electron backscattered diffraction and magnetic measurements. It is found that all the samples sintered by the SPS method exhibit high densities （〉95%） and high values of Vickers hardness （2319 kg/mm2）. The values are much higher than those obtained in the traditional method. With the increase of Nd content,the thermionic emission current density increases from 11 to 16.30 A/cm2 and the magnetic phase transition temperature increases from 5.85 to 7.95 K. Thus, the SPS technique is a suitable method to synthesize the dense rare-earth hexaborides with excellent properties.

Thermodynamics of warm axionic Abelian gauge inflation

Inflationary spectral index from the Langevin equation is calculated under the frame of warm inflationary scenario with inflaton interacting with U(1) gauge fields through the Chern–Simons coupling ∝ФF_(μv)F^(μv). Under the strong dissipative condition, the spectral index is calculated in terms of the ratio of Hubble parameter to temperature H/T. Then relation between H/T and other cosmic parameters is analytically expressed, based on which a spectral index related to the Chern–Simons coupling strength is further obtained. Numerical results show that cosmic temperature T closes to a constant during inflation and decreases after inflation without a reheating process. Meanwhile, the ratio H/T tends to a constant if the gauged coupling constant is less than a threshold. This phenomenon shows that cosmic temperature may be an important physical parameter with a special value and adiabatic approximation still holds. We obtain the estimate H/T ≤ 0.3377 during inflation from Planck data and other constraint conditions.

Built-in electric field effect on cyclotron mass of magnetopolarons in a wurtzite In_xGa_(1-x)N/GaN quantum well

The cyclotron mass of magnetopolarons in wurtzite InxGa1-xN/GaN quantum well is studied in the presence of an external magnetic field by using the Larsen perturbation method. The effects of the built-in electric field and different phonon modes including interface, confined and half-space phonon modes are considered in our calculation. The results for a zinc-blende quantum well are also given for comparison. It is found that the main contribution to the transition energy comes from half-space and interface phonon modes when the well width is very small while the confined modes play a more important role in a wider well due to the location of the electron wave function. As the well width increases, the cyclotron mass of magnetopolarons first increases to a maximum and then decreases either with or without the built-in electric field in the wurtzite structure and the built-in electric field slightly reduces the cyclotron mass. The variation of cyclotron mass in a zinc-blende structure is similar to that in a wurtzite structure. With the increase of external magnetic field, the cyclotron mass of polarons almost linearly increases. The cyclotron frequency of magnetopolarons is also discussed.

Magnetic properties of Fe_(3(1-x))Cr_(3x)C alloys

The magnetic properties of Fe3（1-x）Cr3xC alloys with x=0.05, 0.1, 0.15, and 0.2, which crystallize in the cementite Fe3C-type structure with space group Pnma, were investigated by means of magnetization measurements. These alloys show temperature-induced second-order magnetic phase transitions. The Curie temperature （Tc） of these alloys decreases with increasing x. The isothermal magnetic-entropy changes of these alloys were derived from the magnetic isotherms measured with increasing temperature and increasing field. The maximum values of the magnetic-entropy change are about 0.9 and 3.6 J·kg^-1·K^-1 at Tc =360 K for x = 0.05 in a magnetic field change from 0 to 1 T and 0 to 5 T, respectively.

Extended x-ray absorption fine structure study of MnFeP_(0.56)Si_(0.44) compound

The Mn Fe P0.56Si0.44 compound is investigated by x-ray diffraction, magnetic measurements, and x-ray absorption fine structure spectroscopy. It crystallizes in Fe2P-type structure with the lattice parameters a = b = 5.9823(0) and c = 3.4551(1) and undergoes a first-order phase transition at the Curie temperature of 255 K. The Fe K edge and Mn K edge x-ray absorption fine structure spectra show that Mn atoms mainly reside at 3g sites, while 3f sites are occupied by Fe atoms. The distances between the absorbing Fe atom and the first and second nearest neighbor Fe atoms in a 3f-layer shift from 2.65 and 4.01 in the ferromagnetic state to 2.61 and 3.96 in the paramagnetic phase. On the other hand, the distance between the 3g-layer and 3f-layer changes a little as 2.66 –2.73 below the Curie temperature and2.68 –2.75 above it.

Effects of electron–optical phonon interactions on the polaron energy in a wurtzite ZnO/Mg_xZn_(1-x)O quantum well

We investigated the properties of polarons in a wurtzite ZnO/MgxZn1-xO quantum well by adopting a modified Lee–Low–Pines variational method, giving the ground state energy, transition energy, and phonon contributions from various optical-phonon modes to the ground state energy as functions of the well width and Mg composition. In our calculations, we considered the effects of confined optical phonon modes, interface-optical phonon modes, and half-space phonon modes, as well as the anisotropy of the electron effective band mass, phonon frequency, and dielectric constant. Our numerical results indicate that the electron–optical phonon interactions importantly affect the polaronic energies in the ZnO/MgxZn1-xO quantum well. The electron–optical phonon interactions decrease the polaron energies. For quantum wells with narrower wells, the interface optical phonon and half-space phonon modes contribute more to the polaronic energies than the confined phonon modes. However, for wider quantum wells, the total contribution to the polaronic energy mainly comes from the confined modes. The contributions of the various phonon modes to the transition energy change differently with increasing well width. The contribution of the half-space phonons decreases slowly as the QW width increases, whereas the contributions of the confined and interface phonons reach a maximum at d ≈ 5.0 nm and then decrease slowly. However,the total contribution of phonon modes to the transition energy is negative and increases gradually with the QW width of d.As the composition x increases, the total contribution of phonons to the ground state energies increases slowly, but the total contributions of phonons to the transition energies decrease gradually. We analyze the physical reasons for these behaviors in detail.

Phase transitions and magnetocaloric effects in intermetallic compounds MnFeX(X=P,As,Si,Ge)

Since the discovery of giant magnetocaloric effect in MnFeP1-x As x compounds,much valuable work has been performed to develop and improve Fe2P-type transition-metal-based magnetic refrigerants.In this article,the recent progress of our studies on fundamental aspects of theoretical considerations and experimental techniques,effects of atomic substitution on the magnetism and magnetocalorics of Fe2P-type intermetallic compounds MnFeX（X=P,As,Ge,Si） is reviewed.Substituting Si（or Ge） for As leads to an As-free new magnetic material MnFeP1-xSi（Ge）x.These new materials show large magnetocaloric effects resembling MnFe（P,As） near room temperature.Some new physical phenomena,such as huge thermal hysteresis and ＇virgin＇ effect,were found in new materials.On the basis of Landau theory,a theoretical model was developed for studying the mechanism of phase transition in these materials.Our studies reveal that MnFe（P,Si） compound is a very promising material for room-temperature magnetic refrigeration and thermo-magnetic power generation.

Influence of the Magnetic Moment on the Atomic Distance in Amorphous Ce_xRu_(100-x)

We perform the extended x-ray absorption tlne structure （EXAFS） measurement to investigate the local structure of amorphous alloys CexRu100-x （x = 9, 43 and 80）. The interatomic distances of the nearest Ru-Ce and Ru-Ru pairs derived from EXAFS are fully independent of Ce concentration. On the other hand the distance between neighboring Ce atoms increases sharply with the Ce content, which is exactly proportional to the Ce effective magnetic moment. We discuss the relation between the atomic distance and the effective magnetic moment from the point of view of the magneto-volume effect.

Mechanism for transmittance light tunable property of nanocrystalline Eu-doped SmB_(6):Experimental and first-principles study

Binary nanocrystalline rare-earth hexaboride is regarded as one of the promising optical absorption materials that possess the high transparency for visible light and the strong absorption for NIR due to the effects of localized surface plasmon resonance.In present work,we experimentally investigated the structural and optical absorption of ternary nanocrystalline Eu-doped SmB6 powder.As a result,it is found that the reaction temperature as an important factor not only refines the grain size,but also improves the powder dispersion of synthesized samples.The optical absorption results reveal that the transmittance wavelength of nanocrystalline SmB6 increases from 696 nm to exceeding 1000 nm with the Eu doping content increasing to x=0.8.Theo retically,in order to study the transmittance wavele ngth tunable mechanisms of nanocrystalline Eu-doped SmB6,the electronic structure,density of states and energy loss function were calculated by first-principle calculation.It shows that the En2+doping into SmB6 effectively reduces the plasma frequency excitation energy from 1.48 to 1.25 eV and leads to the transmittance wavelength redshift toward a higher wavelength.Based on above meaningful studies,present work is helpful for extension of optical applications of nanocrystalline SmB6 powder.

Influence of rare earth element terbium doping on microstructure and magnetostrictive properties of Fe_(81)Al_(19) alloy

Fe_(81)Al_(19)Tb_(x)(x=0,0.05,0.1,0.2,0.3,0.4)alloys were prepared by a non-consumable vacuum arc melting technique under an inert argon gas atmosphere.The microstructures of the alloys were studied by X-ray diffraction(XRD)and scanning electron microscopy combined with an energy dispersive spectroscopy(SEM/EDS).The magnetic properties and magnetostriction coefficients of the alloys were measured by a vibrating sample magnetometer(VSM)and adifferential resistive strain sensor,respectively.The results show that the Fe_(81)Al_(19)alloy consists of a single A2 phase with bcc structure,whereas the Tb doped Fe_(81)Al_(19)alloys are composed of the A2 phase and a small amount of rare earth-rich phase.The doping of the rare earth element Tb makes the Fe_(81)Al_(19)alloy preferentially oriented along with the<100>crystal direction.With the increase of Tb content,the magnetostriction coefficient of the alloy first increases and then decreases.When x=0.1,the magnetostriction coefficient reaches the maximum,which is 146×10^(−6).Compared with the as-cast Fe_(81)Al_(19)alloy(27×10^(−6)),it increases by 441%.The enhanced magnetostrictive properties are mainly attributed to the preferred orientation along<100>of A2 phase of the Tb doped Fe_(81)Al_(19)alloys and lattice distortion caused by the small amount of rare earth atoms entering the Fe-Al alloy lattice.

Magnetocaloric effects in Fe_4MnSi_3B_x interstitial compounds

The magnetic properties and magnetocaloric effect in Fe4MnSi3B~ compounds with x=0, 0.05, 0.10, 0.15, 0.20, 0.25 have been investigated. X-ray diffraction study shows that all these compounds investigated crystallize in the MnsSi3-type structure with space group P63/mcm. Boron insertion in the host ternary silicide Fe4MnSi3 does not change the crystal symmetry, only leads to an increase of the lattice parameters, indicating the B atoms entered the interstitial sites. With increasing B content, the Curie temperature shifts to higher temperatures. The maximal magnetic-entropy changes of the Fe4MnSi3Bx compounds with x=0, 0.10 and 0.20 are about 1.8 J/（kg.K）, 1.8 J/（kg-K） and 1.6 J/（kg.K）, respectively, for a field change from 0 to 1.5 T.

Magnetocaloric effects in Mn_(1.35)Fe_(0.65)P_(1-x)Si_x compounds

The structural and magnetocaloric properties of Mn1.35Fe0.65P1-xSix compounds are investigated.The Sisubstituted compounds,Mn1.35Fe0.65P1-xSix with x = 0.52,0.54,0.55,0.56,and 0.57,are prepared by high-energy ball milling and the solid-state reaction.The X-ray diffraction shows that the compounds crystallize into the Fe 2 P-type hexagonal structure with space group P■2m.The magnetic measurements show that the Curie temperature of the compound increases from 253 K for x = 0.52 to 296 K for x = 0.56.The isothermal magnetic-entropy change of the Mn1.35Fe0.65P1-xSix compound decreases with the Si content increasing.The maximal value of the magnetic-entropy change is about 7.0 J/kg·K in the Mn1.35Fe0.65P0.48Si0.52 compound with a field change of 1.5 T.The compound quenched in water possesses a larger magnetic entropy change and a smaller thermal hysteresis than the non-quenched samples.The thermal hysteresis of the compound is less than 3.5 K.The maximum adiabatic temperature change is about 1.4 K in the Mn1.35Fe0.65P0.45Si0.55 compound with a field change of 1.48 T.

Enhanced orange emission by doping CeB_6 in CaAlSiN_3:Ce^(3+) phosphor for application in white LEDs

A series of CeB6-doped and CeO2-doped Ca1-xSiAlN3:xCe3+(denoted as CASN:Ce3+@CeB6 and CASN:Ce3+@CeO2, respectively) were synthesized by alloy-nitridation method under high-purity nitrogen atmosphere. The morphologies, crystal phases, and luminescence properties were investigated in detail.With an increase in the concentration of CeB6, the unit cell volume of CASN:Ce3+@CeB6slightly increases due to the substitution between ions, which leads to a change of micro structure around Ce3+. CASN:Ce3+@CeB6 efficiently emits yellow-orange light with a maximum emission intensity at around 550 nm for the content x of 0.01(being in comparable situation, CASN:Ce3+@CeO2is x = 0.04) when excited at460 nm. Compared with CASN:Ce3+@CeO2, the red emission component of Ce3+ in CASN:Ce3+@CeB6 is much stronger. This is ascribed to energy transfer of intra-Ce3+(within one Ce3+ ion) and inter-Ce3+(between Ce3+ and Ce3+ ions). In addition, the replacements of N3-(0.132 nm for CN = 4) and O2-(0.124 nm for CN = 4) by B2-(0.140 nm for CN = 4), which can lead to a marked expansion of the host lattice and a decrease of the oxidation of samples, are also responsible for the increase of red emission component. Furthermore, CASN:Ce3+@CeB6phosphor has an excellent thermal stability because of the partial substitution of Ce-O(Ce-N) bonds by more covalent Ce-B. As a result, the outstanding luminescent properties of CASN:Ce3+@CeB6 phosphor make it practical to use in the single phosphor-coated high-color-rendering power white LED.

Enhanced magnetostrictive properties of lightly Pr-doped Fe83Ga17 alloys

Fe83Ga17Prx alloys,where x=0,0.2,0.4,0.6 and 1.0,were prepared by a non-consumable vacuum arc melting technique under an inert argon gas atmosphere.The crystal structure and surface morphologies of the alloys were studied by X-ray diffraction(XRD) and scanning electron microscopy(SEM),respectively.Local compositional variations were measured by energy dispersive X-ray spectroscopy(EDXS).The magnetostriction coefficients of the alloys were measured by means of a differential resistive strain sensor.The magnetism of the sample was measured by a vibrating sample magnetometer(VSM).The results show that the parent Fe83Ga17 alloy consists of a single A2 phase of bcc symmetry,whereas the Pr doped Fe83Ga17 alloys are composed of the A2 phase of bcc structure and a small volume fraction of PrGa2 as a secondary phase.Most importantly,with increasing x,the magnetostriction coefficient of the Fe83Ga17Prx alloys first increases reaching a maximum saturation magnetostriction coefficient of192 ppm for x=0.6 at a magnetic field strength of 486 kA/m,then decreased.This maximum represents a 400% increase over the parent alloy with a mere 0.6 at% Pr doping.

The tetragonal-like distortion of Fe-Ga alloys interstitial doped with Cu

The as-cast Cu-doped Fe_(83)Ga_(17)alloys were prepared by arc-melting.The microstructures and magnetostrictions of(Fe_(83)Ga_(17))_(100-x)Cu_(x)(x=0,3,6,9,12,15)alloys were investigated by X-ray diffraction,scanning electron microscopy,energy dispersive spectroscopy and strain gauge magnetostriction measurement.Vibrating sample magnetometer was employed to investigate the magnetic properties of(Fe_(83)Ga_(17))_(100-x)Cu_(x)(x=0,3,6,9,12,15).First-principal calculation was employed to make lattice structure and energy simulation.The results indicate that Cu dopants dissolve into A2 lattice by interstitial doping,which induces tetragonal-like distortion of A2 lattice.And the magnetostriction for Cu-doped Fe_(83)Ga_(17)alloys is lower than that of Fe_(83)Ga_(17)alloy.The reduction of magnetostriction is attribute to the increase of Young’s modulus caused by Cu dopants dissolve into A2 lattice by interstitial doping and the decrease of magnetoelastic coupling coefficient for Cu-doped Fe_(83)Ga_(17)alloys caused by Cu dopants dissolve into A2 lattice.

Magnetocaloric Effect in MnCo_(1-x)Al_xGe Compounds

The effects of substitution of AI for Co on magnetic and magnetocaloric properties of MnCo1-xAlxGe （x=0.00, 0.03, 0.05, 0.08, 0.10, 0.13, 0.15, and 0.20） compounds have been investigated by X-ray diffraction （XRD） and magnetization measurements. XRD exhibits that MnCo1-xAlxGe compounds crystallize in the orthorhombic TiNiSi-type structure for x〈0.03 and in the hexagonal Ni2In-type crystal structure for x〉0.03. Magnetic measurements show that the Curie temperature can be tuned between 286 and 347 K by changing the Co/Al ratio. The maximum magnetic entropy change determined from the isothermal magnetization measurement by Maxwell relation reaches 1.52 J/（kgK） for x=0.08 in a field change from 0 to 1.5 T around 310 K.

Fe_(2–x)Mn_xP_(0.4)Si_(0.6)合金的显微组织、磁性和磁热效应（英文）

本文系统研究了富Si,Mn的FeMn(P,Si)合金的显微组织、磁性和磁热效应.利用高能球磨和固态烧结法制备了Fe_(2-x)Mn_xP_(0.4)Si_(0.6)(x=1.25,1.30,1.35,1.40,1.45和1.50)系列合金.实验结果表明,合金主要形成了Fe_2P型六角结构相,伴有少量(Mn,Fe)_3Si和(Mn,Fe)_5Si_3相的析出.合金的Curie温度(T_c)随着Mn含量的增加而逐渐降低,由x=1.25时的321 K降低到x=1.5时的266 K.随着Mn含量的增加,合金逐渐由一级磁相变过程向二级磁相变过程转变.Fe_(0.75)Mn_(1.25)P_(0.4)Si_(0.6)合金具有最大等温磁墒变(-△S_(max),在0-1.5 T磁场变化下的-△S_(max)为7.2J(kg K)^(-1).Fe_(0.7)Mn_(1.3)P_(0.4)Si_(0.6)和Fe_(0.65)Mn_(1.35)P_(0.4)Si_(0.6)合金在0-1.48 T磁场变化下的最大绝热温变(△T_(ad))为1.8 K.所有合金的热滞部低于4 K.Fe_(2-x)Mn_xP_(0.4)Si_(0.6)系列合金可作为室温磁制冷的理想候选材料.