On the magnetic field acting upon moving domain walls in (Bi,Yb)3(Fe,Ga)5O12 garnet ferrite films

The dependence of the domain wall velocity V on the magnetic field strength H in (111)-oriented (Bi, Yb)₃(Fe,Ga)₅O₁₂ single crystal garnet ferrite films is studied by the method of pulsed remagnetization at the point of angular momentum compensation. It is established that the acting field strength H is decreased by an effective internal magnetic field existing in the films studied.     

Growth and scintillation properties of Yb doped aluminate, vanadate and silicate single crystals

For the purpose of quick screening for charge transfer (CT) transitions of Yb³⁺ in various hosts, (Lu_1−xYb_x)₃Al₅O₁₂ (Yb:LuAG) with x=0.05, 0.15, 0.30 and (Y_1−xYb_x)AlO₃ (Yb:YAP) with x=0.05, 0.10, 0.30 were grown by the micro-pulling-down method. (Y,Yb)VO₄ with strong wetting was grown by edge defined film-fed growth method and materials, which require moderate temperature gradient, such as Ca₈(La,Yb)₂(PO₄)₆O₂ and (Gd,Yb)₂SiO₅ were grown by Czochralski method. Strong dependence of the CT luminescence decay time and intensity on temperature was observed for Yb-doped LuAG and YAP. Super fast decay with 0.85 ns decay time was observed in Yb(30%) doped YAP at room temperature. Though the emission intensity is weak at room temperature, it exceeds several times that of PbWO₄. In addition, CT luminescence of Yb:YAP occurs at longer wavelength than in BaF₂, which enables the usage of glass-based photomultiplier for the detection. In addition, higher stopping power will be expected due to the higher density host compared with BaF₂.     

Synthesis of Brightly PEGylated Luminescent Magnetic Upconversion Nanophosphors for Deep Tissue and Dual MRI Imaging

A method is developed to fabricate monodispersed biocompatible Yb/Er or Yb/Tm doped β‐NaGdF₄ upconversion phosphors using polyelectrolytes to prevent irreversible particle aggregation during conversion of the precursor, Gd₂O(CO₃)₂?H₂O:Yb/Er or Yb/Tm, to β‐NaGdF₄:Yb/Er or Yb/Tm. The polyelectrolyte on the outer surface of nanophosphors also provided an amine tag for PEGylation. This method is also employed to fabricate PEGylated magnetic upconversion phosphors with Fe₃O₄ as the core and β‐NaGdF₄ as a shell. These magnetic upconversion nanophosphors have relatively high saturation magnetization (7.0 emu g^?1) and magnetic susceptibility (1.7 × 10^?2 emu g^?1 Oe^?1), providing them with large magnetophoretic mobilities. The magnetic properties for separation and controlled release in flow, their optical properties for cell labeling, deep tissue imaging, and their T₁‐ and T₂‐weighted magnetic resonance imaging (MRI) relaxivities are studied. The magnetic upconversion phosphors display both strong magnetophoresis, dual MRI imaging (r₁ = 2.9 mM^?1 s^?1, r₂ = 204 mM^?1 s^?1), and bright luminescence under 1 cm chicken breast tissue.     

Phase Relations in the Bi–Sr–Ca–Cu–M–O (M = Y, Dy, Ho, Er, Tm, Yb, Lu) Systems

The phase relations in the Bi–Sr–Ca–Cu–M–O (M = Y, Dy, Ho, Er, Tm, Yb, Lu) systems near Bi₂Sr₂CaCu₂O_{8+ x} (Bi-2212) were studied between 850 and 990°C. The results demonstrate that larger sized rare earths (Y, Dy, Ho, Er) substitute on the Ca site in Bi-2212, leading to the formation of CaO. Smaller sized rare earths (Tm, Yb, Lu) replace lesser amounts of Ca and form Sr₂MBiO₆phases. In the material of nominal composition Bi₂Sr₂Ca_0.85M_0.15Cu₂O_{8 + x} + 0.25Sr₂YBiO₆prepared via melt processing, a major fraction of the Y atoms substitute for Ca in Bi-2212, resulting in a low-T _cmultiphase superconductor. In the analogous Yb system, a Bi-2212–Sr₂YbBiO₆composite with T _c= 87 K is obtained.     

Quantum Degenerate Fermi Gases of Ytterbium Atoms

We performed evaporative cooling for dilute gases of ytterbium (Yb) isotopes in a crossed optical dipole trap and successfully cooled two fermionic and two bosonic species down to quantum degenerate regime, following the previous realization of Bose-Einstein condensation (BEC) in ¹⁷⁴Yb. The elastic collision rate of fermionic ¹⁷³Yb atoms with 6 spin components was found to be large enough to carry out efficient evaporation, which enables us to cool the atoms down to 0.6 T _F , where T _F is the Fermi temperature. In this regime, a plunge of evaporation efficiency was observed as an effect of the Fermi degeneracy. The other fermionic isotope ¹⁷¹Yb was cooled down to the temperature below T _F by sympathetic cooling with bosonic ¹⁷⁴Yb atoms. The sympathetic cooling technique has also been applied to ¹⁷⁴Yb-¹⁷⁶Yb Bose-Bose mixture. We have observed almost pure BEC of ¹⁷⁴Yb and the bimodal distribution of ¹⁷⁶Yb, showing the formation of BEC-BEC mixture. Moreover, we performed evaporative cooling of ¹⁷⁰Yb atoms and realized the BEC.     

Thermopower of Yb Heavy Fermion Compounds at High Pressure

The absolute thermopower S(T) of magnetic and non-magnetic Yb-based heavy fermions compounds was investigated at ambient pressure. High pressure experiments were performed for YbSiT _N =1.6 K) and YbCu ₂ Si ₂ . For the latter, which is not magnetically ordered at P=0, the chemical pressure effect was studied within the solid-solution Yb(Ni _x Cu _1–x ) ₂ Si ₂ (0x1). All S(T)-curves obtained at ambient pressure are characterised by a giant negative peak at a temperature T _{min, S} , and by an additional positive maximum at low temperature for the magnetic compounds. With increasing pressure, S(T _min,S ) and T _min,S were reduced, reflecting a reduction of the Kondo effect. At low temperature a magnetic transition is observed at P>8 GPa in the case of YbCu ₂ Si ₂ without a sign change in contrast to the alloys Yb(Ni _x Cu _1–x ) ₂ Si ₂.     

Relationship between crystal structure and magnetic properties of (RE)2V2O7; RE=Lu,Yb, Tm

Powder neutron diffraction data were collected to determine the atomic structures of (RE)₂V₂O₇ (RE=Lu, Yb, Tm). They all have the pyrochlore structure, crystallizing in the Fd3m space group. There has been an attempt to interpret the previously published magnetic behaviour of these compounds in terms of their structure.     

Paramagnetism and improved upconversion luminescence properties of NaYF4:Yb,Er/NaGdF4 nanocomposites synthesized by a boiling water seed-mediated route

In a route boiling water served as reaction medium, a stoichiometric amount of rare-earth compound and fluoride are put into this system to form α-NaYF₄:Yb, Er nuclei. Then prepared sample is heated at elevated temperature to improve the fluorescence intensity, and next a NaGdF₄ shell grows on the surface of NaYF₄ nuclei. NaYF₄:Yb,Er/NaGdF₄ core–shell structured upconversion nanoparticles (CSUCNPs) have been successfully synthesized by above route. The use of boiling water decreases the cubic-to-hexagonal phase transition temperature of NaYF₄:Yb,Er to 350°C and increases its upconversion (UC) luminescence intensity. A heterogeneous NaGdF₄ epitaxially growing on the surface of Ln³⁺-doped NaYF₄ not only improves UC luminescence, but also creates a paramagnetic shell, which can be used as contrast agents in magnetic resonance imaging (MRI). The solution of CSUCNPs shows bright green UC fluorescence under the excitation at 980 nm in a power density only about 50 mW·cm^–2. A broad spectrum with a dominant resonance at g of about 2 is observed by the electron paramagnetic resonance (EPR) spectrum of CSUCNPs. Above properties suggest that the obtained CSUCNPs could be potential candidates for dual-mode optical/magnetic bioapplications.     

Transport Properties of Heavy Fermion Compounds: YbIn1−x Cu4+x and YbIn1−y Ag y Cu4

The intermetallic compounds of Yb with In and Cu (YbIn_1–x Cu_4+x ) and Yb with In, Ag, and Cu (YbIn_1–y Ag _y Cu₄) exhibit interesting magnetic and transport properties. Of the compounds of Yb with In and Cu the compound with x=0, namely YbInCu₄, has attracted particular attention, because—while being a Curie–Weiss paramagnet—it undergoes a first-order isostructural phase transition at T _v =approx. 40 to 80 K and atmospheric pressure. Below T _v the ytterbium in this compound is in a mixed-valence state and the compound as a whole is sometimes called a light heavy-fermion system. Above T _v , the compound is known as a Curie–Weiss paramagnet of localized magnetic moments and, below T _v , a Pauli paramagnet in a nonmagnetic Fermi-liquid state. In the present paper the results of measurements of the thermal conductivity of polycrystalline samples, YbIn_1–x Cu_4+x with x=0,0.015, 0.095, and 0.17 and YbIn_1–y Ag _y Cu₄ with y=0, 0.3, 0.7, and 1.0, are reported. The thermal conductivity is separated into the phonon thermal conductivity ( _ph ) (i.e., related to the heat carried by phonons) and into the electronic thermal conductivity ( _e ) (related to the heat carried by electrons). The electrical resistivity of the compounds was measured to determine the temperature dependence of the Lorenz number. The results show that in the temperature interval 4.2 to 300 K the latter quantity behavior follows the theoretical predictions for heavy fermion materials.     

The magnetic properties of ytterbium trihydride

Ytterbium trihydrides (2.02 < H/Yb < 2.96) were synthesized at high hydrogen gas pressures by thermally decomposing.aluminium hydride under high pressure. The magnetic susceptibility of ytterbium trihydrides was measured by the Faraday method at temperatures 78 to 300 K. The gram magnetic susceptibility of YbH_2.96 in the temperature range 78 to 295 K obeys the Curie-Weiss law withP _eff = 4.37 µ_B andT _gq = –62.7 K. The electrical resistance of ytterbium dihydrides were measured by the d.c. method at temperatures of 78 to 273 K. YbH showed a metallic resistivity in the temperature range 78 to 273 K, but YbH_1.2 and YbH_1.5 showed semiconducting characteristics at temperatures of 241 to 273 K and at 163 to 273 K, respectively. The band gap energy of YbH_1.2 which appeared to show semiconducting behaviour was 0.241 eV, and of YbH_1.5 was 0.294 eV.     

Synthesis of New Pb-Based 1222 Cuprates Containing Phosphorus, (Pb0.75P0.25)Sr2(RE2–x–y Ce x Sr y )Cu2O z (RE = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y)

New Pb-based 1222 cuprates containing phosphorus have been synthesized in the (Pb_0.75P_0.25)Sr₂- (RE_2–x–y Ce _x Sr _y )Cu₂O _z (RE = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y) systems. The almost-single phase samples are obtained for 0.5≤x≤1.0 and y=0.1 in the cases of RE = Nd, Dy, Ho, Er, Tm, Yb and Y, while for 0.3≤x≤1.0 and y=0.1 in the cases of RE = Sm, Eu and Gd. The samples crystallize in a tetragonal lattice, the lattice parameters a and c are decreasing with the decrease of the ionic radius of the RE element. Even after annealing under 143 atm O₂ atmosphere at 400 °C, almost all the samples with the common values x=0.5 and y=0.1 are semiconductors with a transport process followed by three-dimensional variable range hopping. However, the samples of RE = Sm, Eu and Gd, which are of almost single 1222 phase, even for x=0.3 and y=0.1, show superconductivity with the onsets at about 25 K, 20 K and 22 K, respectively. Through this study, we find very important procedure for discovery of new superconducting 1222 compound.     

Thermal Expansion and Electroabsorption in the TlInS2–TlYbS2 System

The thermal expansion and electroabsorption of TlIn_{1 – x} Yb _x S₂ solid solutions were measured as a function of temperature. The introduction of Yb into TlInS₂ was found to fully suppress the phase transition. It is shown that there is a perfect correlation between the band gap and thermal expansion of the TlIn_{1 – x} Yb _x S₂ solid solutions: both decrease with increasing Yb content. This finding is interpreted in terms of the effect of Yb substitution on bond strength.     

Electrical properties of In/Yb/n-Hg1 − x Cd x Te contacts

The specific contact resistance, _c, and the modified sheet R _sk, of In/Hg_{1 – x} Cd _x Te contacts incorporating a Yb diffusion barrier were measured as a function of the layer thickness and composition (x = 0.32–0.65). Significant increases in _c, were evident only for x 0.56 and at Yb thicknesses between 2.5 and 6.0 nm, depending on the x-value. Analytical examination of the interfaces by Rutherford backscattering spectrometry (RBS) also showed a progressive reduction in the extent of inward diffusion of In with increasing thickness of the Yb interlayer.     

Structural design and synthesis of new MOO3-x interlayer bi-functional nanomaterials for enhanced up-conversion luminescence properties

A novel imaging materials based on bi-functional Fe₃O₄@M_OO_3-x@YF₃:Yb/Er nanoparticles (NPs) with strong up-conversion luminescence and magnetic properties was designed and synthesized by inlaying M_OO_3-x with localized surface plasmon resonance (LSPR) and ferromagnetic property in the bi-functional Fe₃O₄@YF₃:Yb/Er NPs. The morphology, structure and properties of Fe₃O₄@M_OO_3-x@YF₃:Yb/Er NPs are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), photoluminescence (PL) spectra, UV–Vis-NIR spectrophotometer and superconducting quantum interference devices (SQUID). It was found that the experimental conditions (reaction temperature, reaction time, the mass ratio of core:shell and oxygen pressure in heat treatment) have a certain effect on the oxygen defect concentration of the M_OO_3-x interlayer. XRD also showed the crystal structure of Fe₃O₄, Fe₃O₄@M_OO_3-x and Fe₃O₄@M_OO_3-x@YF₃:Yb/Er NPs. The SEM and TEM results showed that the average grain size of the prepared NPs were 200 nm. The SQUID and PL results showed that the Fe₃O₄@M_OO_3-x@YF₃:Yb/Er NPs have stronger magnetism (14.3 emu/g) and excellent up-conversion luminescence performance (the emission peak at 525 nm is nearly 20 times higher than the corresponding emission intensity of the Fe₃O₄@YF₃:Yb/Er). The Fe₃O₄@M_OO_3-x@YF₃:Yb/Er NPs can be easily used to magnetic resonance and fluorescence dual-mode image-guided visual delivery drug and also increase the accurate diagnosis and treatment effect of malignant tumor.     

Superconductivity and Magnetism in R 2CaBa2Cu5O z (R=La, Pr, Nd and Eu)

A series of compounds R ₂CaBa₂Cu₅O _z (R=La, Pr, Nd and Eu) have been synthesized by conventional solid state reaction and characterized for their structural, superconducting and magnetic properties. All compounds crystallize with the tetragonal LaBa₂Cu₃O _z type structure, space group P4/mmm. Among the four compounds studied here, R=La, Nd and Eu are superconductors with superconducting transition temperatures (T _c ^R=0) of 72, 40 and 55 K, respectively. On the other hand, neither superconductivity nor magnetic ordering is seen for R=Pr down to 3 K. The effect of the magnetic field on the susceptibility and (magneto) resistance for the superconducting samples has been investigated. The superconductivity of compounds with magnetic rare-earth ions Nd and Eu exhibit a profound influence of the magnetic field, whereas the application of the magnetic field has a limited effect on the La compound. The Pr compound is paramagnetic and does not exhibit magnetic ordering either.     

Magnetic properties of RE x Mo6Se8 compounds between 0.7 and 295 K

Magnetization measurements are reported for seven members of the series of compounds RE _x Mo ₆ Se ₈ (x1; RE=La, Ce, Pr, Eu, Gd, Yb, and Lu). These measurements establish that Ce, Pr, and Gd ions are trivalent in these compounds, but that Eu and Yb are primarily divalent, in agreement with crystallographic data. Large deviations of the susceptibilities for CeMo ₆ Se ₈ , PrMo ₆ Se ₈ , and YbMo ₆ Se ₈ from Curie-Weiss behavior above 10 K are attributed predominantly to crystal field effects, and quantitative agreement between crystal field theory and the experimental data was obtained for CeMo ₆ Se ₈ above 25 K. Further, we find for Gd _1.2 Mo ₆ Se ₈ an unusually small spin exchange interaction between the Gd ions and conduction electrons, consistent with previous estimates of this quantity based on superconductivity data. For the Gd, Eu, and Ce compounds, the low-temperature data indicate very small Curie-Weiss temperatures of –1.7, –0.6, and +0.4 K, respectively. No evidence for magnetic ordering of the RE ions in any of the above compounds was observed, with the exception of CeMo ₆ Se ₈ , for which the magnetization data at 0.7–1.3 K suggest the onset of ferromagnetic order.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR Contract No. AFOSR/F-44620-72-C-0017.     

Growth and optical properties of Er,Yb:YAl3(BO3)4 crystal

Single crystal of erbium, ytterbium-codoped yttrium aluminum tetraborate Er,Yb:YAl₃(BO₃)₄(Er,Yb:YAB) has been grown by the flux method. The absorption spectrum in the visible and NIR regions of Er,Yb:YAl₃(BO₃)₄ crystal are measured at room temperature and fluorescence spectrum of Er,Yb:YAl₃(BO₃)₄ crystal are also measured at room temperature, excited by 976 nm laser. Not only the strong NIR emission peaks located at 1548 nm was observed, but also the visible up-conversion luminescence has been found. The specific heat of the Er/Yb:YAB crystal at room temperature is 0.81 J/g °C.     

Preparation of [001]- and [107]-Oriented Epitaxial YbBa2Cu4O8 Films Prepared by Coating-Pyrolysis Process

YbBa ₂ Cu ₄ O ₈ (Yb124) films were prepared on SrTiO ₃ (001) and SrTiO ₃ (110) substrates by coating-pyrolysis process at ambient pressure, and the alignments of these films were investigated by XRD -2 scanning and XRD pole-figure analysis. Films prepared on SrTiO ₃ (001) consisted of almost single-phase [001]-oriented epitaxial Yb124. On the other hand, films on SrTiO ₃ (110) consisted of [107]-oriented epitaxial Yb124 and other impurity phases, one of which was considered to be so-called other perovskite phase.     

XRD EPR studies of ceramics Pr0.5Re0.5Ba2Cu3O7−δ in the orthorhombic and tetragonal phase

EPR, XRD, and magnetic studies are presented for Pr_0.5Re_0.5Ba₂Cu₃O_7?δ compounds (Re= La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, Yb, and Lu) in the orthorhombic and tetragonal (large oxygen deficiency) phase. For the samples with Re=Dy, Ho, Y, Er, Tm, Yb, and Lu in the orthorhombic phase, the transition to the superconducting state has been observed in the temperatures rangeT _c between 18 and 40 K. For the samples with Nd and Yb in the tetragonal phase, EPR spectra coming from trivalent rare earth ions have been recorded. In the nonoxygenated sample Pr_0.5La_0.5Ba₂Cu₃O_7?δ the EPR spectrum arising from the non-Kramers trivalent praseodymium ion has been observed. A broad EPR line appearing in all our samples was attributed to superexchange interaction between copper ions over oxygen (O ₂ ^? ) bridges. Interestingly, for the Pr_0.5Re_0.5Ba_0.5Cu₃O_7?δ (Re = Er and Lu) compounds in the tetragonal phase at liquid-nitrogen temperature, a nonresonant microwave absorption in low magnetic fields has been detected.