Electrical and magnetic properties of RE-doped ZnO thin films (RE = Gd, Nd)

Due to the small magnetic moments observed for 3d transition metals in ZnO [M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, J. Lenzner, D. Spemann, A. Setzer, K.W. Nielsen, P. Esquinazi, M. Grundmann, Thin Solid Films 486 (2005) 117], there is still space for optimizing ZnO-based diluted magnetic semiconductors for spintronics applications. Motivated by the observation of magnetic moments as high as 4000μ_B/Gd atom in GaN:Gd [S. Dhar, O. Brandt, M. Ramsteiner, V.F. Sapega, K.H. Ploog, Phys. Rev. Lett. 96 (2005) 037205], we investigated ZnO films doped with 0.01, 0.1 or 1 at.% rare earth (RE) metals. The films, with thicknesses between 20 nm and 1 μm, have been grown by pulsed laser deposition on a-plane sapphire or fused silica substrates.The homogenous incorporation of the RE ions in ZnO was investigated by combined Rutherford backscattering and particle induced X-ray emission measurements. Hall measurements revealed an unexpected dependence of the electron concentration on film thickness, proving a non-uniform distribution of electrically active defects. Magnetotransport measurements at different temperatures were performed to study the magnetoresistance and the presence of the anomalous Hall effect. Large negative magnetoresistance was obtained at 5 K, while no anomalous Hall effect was observed. These results indicate that there are no exchange interactions between the RE ions.     

Growth and spectroscopic properties of RE, Mn:YAP (RE=Yb and Ce) photorefractive crystals

RE, Mn:YAP (RE=Yb and Ce) crystals with dimension of Φ 25×60 mm were successfully grown by the Czochralski method. The spectroscopic properties of RE, Mn:YAP (RE=Yb and Ce) crystals before and after γ-irradiation were investigated at room temperature. The results show that the content of Mn⁴⁺ ions was increased with the Yb³⁺ ions co-doping, but decreased by Ce³⁺ ions co-doping. Thermoluminescence (TL) spectra of the crystals indicate three steps of recombination, and the probable recombination processes were discussed.     

Transport property of a newly synthesized organic conductor, β″-(BEDT-TTF)2AuBr2

Transport phenomena of a newly synthesized organic conductor, β″-(BEDT-TTF)₂AuBr₂ were studied. All the crystals examined were found to be metallic down to very low temperatures. For samples which contain impurities, an increase in the resistivity with decreasing temperature was observed below 13 K.Galvanomagnetic effect was measured at liquid helium temperatures. Negative magnetoresistance at low magnetic field and positive magnetoresistance at high field were found to coexist in this temperature region. Kochler's rule applies to the positive component of the magnetoresistance.     

Physical properties and anisotropies of the RNiGe3 series (R = Y, Ce-Nd, Sm, Gd-Lu)

We have studied RNiGe₃ (R=Y, Ce-Nd, Sm, Gd-Lu) single crystals by measuring crystal structure and stoichiometry, magnetic susceptibility, magnetization, electrical resistivity, magnetoresistance, and specific heat. Clear anisotropies as well as antiferromagnetic ordering in the RNiGe₃ series (R=Ce-Nd, Sm, Gd-Tm) have been observed above 1.8 K from the magnetic susceptibility. A metamagnetic transition in this family (except for R=Sm) was detected at 2 K for applied magnetic fields below 70 kOe. The electrical resistivity of this series follows metallic behavior in the high temperature region. Below the antiferromagnetic ordering temperature a significant anisotropy is exhibited in the resistivity and magnetoresistance for different current directions. The anisotropic magnetic, transport, and thermal properties of RNiGe₃ compounds are discussed in terms of Ni site occupancy as well as a combination of the effect of formation of a magnetic superzone gap and the crystalline electric field.     

Crystal growth and optical properties of Cr3+, Er3+, RE3+: Gd3Ga5O12 (RE=Tm,Ho, Eu) for mid-IR laser applications

In this paper we report on the optical properties of triply Cr³⁺, Er³⁺, and RE³⁺ (RE=Tm, Ho, Eu) doped Gd₃Ga₅O₁₂ crystals that were grown by the Czochralski method. Optical absorption, near-infrared (NIR), and mid-infrared (mid-IR) fluorescence spectra were characterized for the fabricated crystals and corresponding luminescence decay measurements under 654 nm excitation were also carried out. Based on the analysis of energy transfer process between Er and RE (RE=Tm, Ho, Eu) ions, the energy transfer efficiency (ETE) values were evaluated, correspondingly. From the spectral data of all the studied crystals, it is observed that the co-doped Cr³⁺ ion highly increases the absorption pump power and the three kinds of co-doped RE³⁺ ions depopulate the Er:⁴I_13/2 energy level effectively. The spectral analysis shows that titled rare earth doped crystals are promising materials for ～3.0 μm mid-IR laser applications and among them Cr,Er,Eu:GGG is relatively more suitable due to its excellent optical properties compared with others.     

Electrical conduction of a quasi-one dimensional Nb3S4 single crystal

With respect to single crystals of Nb₃S₄ the electrical resistivity from 2.8 K to 300 K and the magnetoresistance at 4.2 K were measured. The resistivity is represented as a sum of a temperature independent and an intrinsic temperature dependent component. The temperature dependence of the intrinsic resistivity subjects to T₃ form between 7 and 50 K above which it becomes weaker than T₃ approaching a T linear from. This behaviour is discussed in terms of the electron-electron Umklapp scattering. The ratio of the resistivities perpendicular and parallel to the c-axis takes about 15 between room temperature and 50 K. The transverse magnetoresistance is proportional to the magnetic field. The longitudinal magnetoresistance is too small to be measured.     

Transport properties and magnetoresistance behavior of Nd0.7Pb0.3MnO3 single crystals

Single crystals of Nd_0.7Pb_0.3MnO₃ with large crystal size were obtained by the flux growth. The temperature dependence of the resistivity and magnetoresistance was measured. It has been discovered that a phase transition occurs from a ferromagnetic-metal (FM) to a ferromagnetic-insulator (FI) phase at 120 K, below the Curie temperature. At the Curie temperature, the samples exhibit a larger value of magnetoresistance (MR) than polycrystalline samples. The second peak of the magnetoresistance has been observed on MR(T) curve below the Curie temperature. The transition properties and magnetoresistance behavior were discussed.     

High-field magnetoresistance and Hall effect in Bi2Sr2CuO x single crystals

We investigated the in-plane magnetoresistance and the Hall effect of high-quality Bi₂Sr₂CuO_x single crystals with T _c (midpoint) = 3.7–9.6 K in dc magnetic fields up to 23 T. For T < 10 K, the crystals show the classical positive magnetoresistance. Starting at T ≈ 14 K, an anomalous negative magnetoresistance appears at low magnetic fields; for T ≥ 40 K, the magnetoresistance is negative in the whole studied range of magnetic fields. Temperature and magnetic field dependences of the negative-magnetoresistance single crystals are qualitatively consistent with the electron interaction theory developed for simple semiconductors and disordered metals. As is observed in other cuprate superconductors, the Hall resistivity is negative in the mixed state and changes its sign with increasing field. The linear T-dependence of cotθ_H for the Hall angle in the normal state closely resembles that of the normal-state resistivity as expected for a Fermi liquid picture.     

Resistivity and magnetoresistance of Fe80B20 and Fe78B13Si9 amorphous glasses

Summary The electrical resistivity of Fe₈₀B₂₀ and Fe₇₈B₁₃Si₉ amorphous glasses as a function of temperature from 293 K down to 15 K was measured, and it was found to fit quite well with the model given by Cote and Meisel. Comparison between our resistivity measurements of Fe₈₀B₂₀ and others was made, where some differences were found. These resistivity differences are evidence for a variety of amorphous atomic arrangements of the samples. The longitudinal magnetoresistance of Fe₈₀B₂₀ and Fe₇₈B₁₃Si₉ at 293K and 77K was measured in a low magnetic field. The observed magnetoresistance shows a typical field dependence known for ferromagnetic materials.     

Electrical resistivity and magnetoresistance of CeB6

Electrical resistivity and magnetoresistance of CeB₆ single crystal have been measured in the temperature range from 1.3 to 300 K under the magnetic field up to 85 kOe. Three characteristic phases are distinguished consistently with other measurements. The Kondo like behaviour in the resistivity observed in the high temperature phase is fitted by the conventional form for the dilute Kondo state with the Kondo temperature T_K = 5 ～ 10K and the unitarity limit resistivity $\text{?}{}_{\mathrm{u}}\text{? 110 μΩ}$ cm/Ce-atom. The negative magnetoresistance in the middle phase is stronger with increasing magnetic field and with decreasing temperature suggesting rapid destruction of the Kondo state. The magnetoresistance in the low temperature phase exhibits some anomalies suggesting sub-phases corresponding to several kinds of spin ordering.     

Magnetic, magnetocaloric, and magnetotransport properties of RCo1.8Mn0.2 (R=Er, Ho, Dy, and Tb) compounds

The magnetic, magnetocaloric, and magnetotransport properties of RCo_1.8Mn_0.2 (R=Er, Ho, Dy, and Tb) were studied by room temperature X-ray diffraction, magnetization, and resistivity measurements at a temperature interval of 5-400 K and magnetic fields up to 5 T. The Curie temperature of RCo₂ was found to increase significantly when 10% Mn was substituted for Co. The effective paramagnetic moments were found to be in reasonable agreement with their theoretical values. A large magnetoresistance (MR) of Δρ/ρ_o≈−13.5% for R=Ho at T≈153 K for ΔH=5 T has been observed. The maximum relative cooling capacities vary from 467 J/kg at low temperature for R=Er to 202 J/kg at the near room temperature for R=Tb.     

Low-temperature magnetic ordering in the perovskites Pr1−xAxCoO3 (A=Ca,Sr)

The magnetic and electrical properties of polycrystalline Pr_1?xA_xCoO₃ cobaltites with A=Ca, Sr and 0≤x≤0.5 were studied in the temperature range 4 K≤T≤1000 K and field up to 7 T. The X-ray analyses show the presence of only one phase having monoclinic or orthorhombic symmetry. The magnetic measurements indicate that the Ca-doped samples have at low temperatures, similar properties to the frustrated magnetic materials. PrCoO₃ is a paramagnetic insulator in the range from 4 to 1000 K. The Sr-doped cobaltites exhibit two phase transitions: a paramagnetic–ferromagnetic (or magnetic phase separated state) phase transition at about 240 K and a second one at about 100 K. The magnetic measurements suggest the presence of magnetic clusters and a change in the nature of magnetic coupling between Co ions at low temperatures. A semiconducting type behavior and high negative magnetoresistance was found for the Ca-doped samples, while the Sr-doped ones were metallic and with negligible magnetoresistance. The results are analyzed in the frame of a phase separation scenario in the presence of the spin-state transitions of Co ions.     

X-ray investigations and possible mechanism for the ionic conductivity in the Na5RESi4O12 (RE=Y,Sc) class of compounds

X-ray investigations on single crystals of the series of rare-earth compounds Na₅RESi₄O₁₂ with RE as Sc or Y were carried out at room temperature and 300°C. They show a dependence of the inner symmetry of the main conductive channels on the type of the RE ion. A simple analysis of the conductivity mechanism is presented.     

Influence of high-energy electron irradiation on the transport properties of La1-xCaxMnO3 films ($$(x \approx 1/3)$$ )

The effect of crystal lattice disorder on the conductivity and colossal magnetoresistance in La_1-xCa_xMnO₃ ( ) films has been examined. The lattice defects are introduced by irradiating the film with high-energy ( MeV) electrons with a maximal fluence of about cm^-2. This comparatively low dose of irradiation produces rather small radiation damage in the films. The number of displacements per atom (dpa) in the irradiated sample is about 10^-5. Nevertheless, this results in an appreciable increase in the film resistivity. The percentage of the resistivity increase in the ferromagnetic metallic state (below the Curie temperature ) was much greater than that observed in the insulating state (above ). At the same time irradiation has much less effect on or on the magnitude of the colossal magnetoresistance. A possible explanation of such behavior is proposed. Received 21 July 1999 and Received in final form 27 December 1999     

Effect of strontium deficiency on the transport and magnetic properties of Pr0.7Sr0.3MnO3

Ceramic samples of Pr_0.7Sr_0.3−x□_xMnO₃ with x⩽0.2 have been investigated by various techniques including ⁵⁵Mn nuclear magnetic resonance, SQUID magnetometry, resistivity and magnetoresistance measurements. On increasing x, the samples remain ferromagnetic at a low temperature with a decreasing Curie temperature, but a metal–insulator transition is observed. Besides, a positive magnetoresistance (MR) at very low temperatures with respect to a negative colossal MR at high temperatures is observed in the Pr_0.7Sr_0.2□_0.1MnO₃ sample.     

Role of Dirac cones in magnetotransport properties of REFeAsO (RE = rare earth) oxypnictides

In this work we study the effect of the rare earth element in iron oxypnictides of composition REFeAsO (RE = rare earth). On one hand we carry out density functional theory calculations of the band structure, which evidence the multiband character of these compounds and the presence of Dirac cones along the Y-Γ and Z-R directions of the reciprocal space. On the other hand, we explore transport behavior by means of resistivity, Hall resistance and magnetoresistance measurements, which confirm the dominant role of Dirac cones. By combining our theoretical and experimental approaches, we extract information on effective masses, scattering rates and Fermi velocities for different rare earth elements.     

Low temperature magnetization measurements on heavy rare earth molybdenum sulfides, (RE)1.2Mo6S8

Magnetization measurements on the heavy rare earth molybdenum sulfides, (RE)_1.2Mo₆S₈ (RE = Gd, Tb, Dy and Ho) were performed to further clarify the natures of their magnetic orders found in our recent resistivity and ac susceptibility measurements below 1 K. The variations of their magnetic and superconducting transition temperatures along the series of the rare earth ions in the Periodic Table are interpreted by means of available theories.     

Influence of Se-deficiencies on the properties of In-doped CdCr2Se4 single crystals

Lattice constant, Curie temperature, and electrical conductivity of CdCr₂Se₄:In single crystals have been measured after heat treatments of the crystals in Se atmosphere and under streaming hydrogen. By these treatments, the concentration of the Se vacancies and of the charge carrier concentration is altered drastically. The lattice constant as well as the magnetic ordering temperature have been found not to be affected by these heat treatments.Since the Se vacancies act as doubly changed donors, the electrical conductivity is strongly dependent on the concentration of the Se vacancies. A resistivity anomaly and large magnetoresistance are observed only in crystals with considerable Se deficiency. From these results it is concluded that the magnetoresistance is caused by hopping conduction between donor sites partly emptied by compensating A-site vacancy acceptors. Large magnetoresistance is found in samples with considerable Se deficit because only in this case the conduction at lower temperatures is dominated by the impurity band.     

Readdressing the issue of low-temperature resistivity minimum in La0.7Ca0.3MnO3 thin films

We have investigated the origin of low-temperature resistivity minima observed in epitaxial thin films of La_0.7Ca_0.3MnO₃ (thicknesses—300 Å and 3000 Å) using electrical and magneto-transport property measurements. We observe considerably smaller hysteresis in the magnetoresistance measurements for the thicker film than the thinner film. 300 Å film shows meta-stability in the resistivity measurements at low temperature and for this film the sample current ‘I’ shows large effect on the resistivity and its minima temperature. These observations suggest that the strain induces electronic intra grain inhomogeneity in these samples and these inhomogeneities consist of regions of different resistive phases. It appears that the high resistive phase prevents the transport of charge carriers between two low resistive regions thus giving rise to the resistivity minimum in these samples.     

Magnetic properties and structural chemistry of ternary silicides (RE,Th, U)Ru2Si2 (RE = RARE EARTH)

Ternary silicides (RE, Th, U)Ru₂Si₂ have been synthesized from the elements. All the compounds (RE = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were found to be isotypic and to crystallize with the structure type of ThCr₂Si₂ (ordered derivative of the BaAl₄-type). The magnetic behavior of these alloys was studied in the temperature range 1.5 K < T < 1100 K. Magnetic susceptibilities at temperatures T > 300 K closely follow a typical Van Vleck paramagnetism of free RE³⁺-ions. In the case of CeRu₂Si₂ susceptibilities are well described for 20 K < T < 1100 K by a Van Vleck paramagnetism of widely spaced multiplets; the observed effective paramagnetic moment μ_eff = 2.12 BM indicates a high percentage (85%) of Ce³⁺. SmRu₂Si₂ yields an effective moment μ_eff = 0.54 BM, which compares reasonably well with the Hund's rule J = 5/2 ground level for free Sm⁺ and a low-lying excited level with J = 7/2. For temperatures T > 15 K the magnetic susceptibility as a function of temperature follows the “Van Vleck behavior” for free Sm³⁺. At low temperatures ferromagnetic ordering was encountered for (Pr, Nd, Ho, Er, Tm)Ru₂Si₂, whereas antiferromagnetic ordering was observed for (Sm, Gd, Tb, Dy)Ru₂Si₂. The ordering temperatures are generally below 55 K. No superconductivity was found for temperatures as low as 1.8 K.