Magnetic frustration in the antiferromagnetic Kondo lattice YbPtAl: Anomalous magnetic behavior at high pressures

We have investigated the magnetic and electronic properties of the antiferromagnetic Kondo lattice YbPtAl using the ¹⁷⁰Yb Mössbauer effect at ambient pressure (1.8<T<10 K), electrical resistance (1.8<T<300 K) and X-ray diffraction (T=300 K) techniques at high pressures up to 26 GPa. We find a complex magnetic state in YbPtAl at ambient pressure and an unusual volume-induced change of T_N. It is suggested, that the anomalous volume dependence of T_N is due to the interplay between frustrated anisotropic exchange interactions and magnetocrystalline anisotropy. The magnetic frustration originates from the topology of the crystal lattice.     

Magnetoabsorption and magnetization of Zn1−xMnxTe mixed crystals

Excitonic magnetoabsorption (on samples of thickness less than 1 μm) of Zn_1?xMn_xTe (x < 0.1) was measured for magnetic fields up to B = 5T at T = 2.2 K. A strong (about 80 meV) exchange splitting of the exciton was observed. In the absence of magnetic field the variation of energy gap with composition of the crystals, as well as the coefficients dEg/dT were studied at 2.2 < T < 275 K. The magnetization measurements were performed at temperatures of T = 2.2 K and 4.2 K for B < 15T on the same samples that were optically tested. As a result, the s-d exchange constants were evaluated: N_oα = 0.19 eV, N_oβ = ?1.1 eV.     

Heat capacity of CeIrSi3 under pressure

We measured the heat capacity of CeIrSi₃ (100 mK<T<6 K) under high pressure up to P=1.38 GPa. The measurements have been used a quasiadiabatic method utilizing a CuBe piston-cylinder pressure cell in a dilution refrigerator. At 0 GPa, a sharp anomaly which indicates the antiferromagnetically transition is observed at T_N=5 K. T_N decreases monotonically with increasing pressure up to P=1.38 GPa. The magnetic entropy is released below T_N only 19% of R ln 2 at 0 GPa. And the magnetic entropy decreases with increasing pressure up to 1.38 GPa, 64% compared to that at 0 GPa.     

Transition into a magnetic state without magnetic scattering in a rare earth system: CeAl3

Susceptibility measurements on CeAl₃ down to 0.5 K show both the absence of magnetic ordering and a reduced effective moment such as found in dilute Kondo alloys. The resistance decreases a factor of 30 below the maximum at T = 37 K and the magneto resistance which is negative above 0.5 K apparently becomes positive for T < 0.5 K.     

Influence of irradiation induced atomic disorder on Hc2 of Nb3Sn

A significant monotonous decrease of H_c2(0) almost proportional to T_c has been observed after heavy ion irradiation of the A15 superconductor Nb₃Sn at low temperature < 30 K and isochronal annealing up to 300 K has been measured. The effects are discussed in connection with the Goodman—Gorkov-relation.     

Natural variability in the surface roughness of combinatorial libraries of materials

Using a conventional method for the fabrication of solid-state combinatorial libraries, we have produced the binary libraries Fe_1−xCr_x and Mo_1−xSn_x (0 < x < 1) and found that their surface roughness is highly dependent on chemical composition. Roughness enhancements of up to 20-fold have been observed on the nanometer scale and were manifested in the boundaries between structurally dissimilar phases. It is anticipated that this is a common occurrence in thin-film libraries of metals and an obstacle in separating the influence of roughness and composition on the performance of library materials in biomaterial studies.     

Magnetooptical investigation of Hg1−xCdxSe mixed crystals

Interband magneto-absorption was used for the investigation of the Hg_1?xCd_xSe alloys in the composition range x < 0.13. Magnetotransmission was measured at 4.2. K up to 60 kG in the spectral region 120 ? ?ω ? 310 meV, on unoriented samples, in the Faraday and Voigt geometries. The interpretation of the results was based on the multi-band model of Pidgeon and Brown. The interaction gap, the Kane matrix element, and the Luttinger parameters were determined for each alloy composition from fitting the theory with experiments.     

Nonlinear conductivity of quasi-one-dimensional layered compound TiS3

The crystal structure and transport properties of TiS₃ whiskers in the plane of layers (ab) have been studied. Maxima of the logarithmic derivative of resistance, dln R/d(1/T), are observed at 17, 60 and 120 K both along and across the chains. Strong nonlinearity of the current–voltage characteristics has been revealed in both directions. Nonlinear conductivity along the chains is observed up to T=60 K, while in the transverse direction it is observed up to T=130 K. The results indicate possible phase transitions of electrons to collective states, probably, charge density waves.     

Electrical behavior of some silver-doped lanthanum-based CMR materials

With a view to understand the structural, magnetic and electrical properties of La_1−xAg_xMnO₃ (x=0.05-0.3), a series of samples were prepared by polyvinyl alcohol (PVA) gel route. It has been found that both the metal-insulator and ferro- to paramagnetic transition temperatures after increasing up to the composition x=0.20, are found to remain constant thereafter. The electrical resistivity vs. temperature plot of the sample x=0.10 is found to exhibit an insulating behavior below 36 K, while the sample, x=0.20 exhibits two peaks, and the observed behavior is explained on the basis of the phase separation model. The low-temperature (T<T_P), electrical resistivity data were analyzed by a theoretical model, ρ=ρ₀+ρ₂T²+ρ_4.5T^4.5, indicating the importance of grain/domain boundary effects, electron-electron and two-magnon scattering processes. The low-temperature resistivity data (T<50 K) were fitted to an equation, which is based on the combined effect of weak localization, electron-electron and electron-phonon scattering.     

Structural,magnetic and transport properties of half-metallic ferrimagnet Mn2VGa

Polycrystalline Mn₂VGa samples were synthesized using an arc furnace. X-ray diffraction (XRD) pattern was analyzed using General Structural Analysis System (GSAS) package and the refined lattice parameter was found to be 5.905 Å. We found magnetic ordering in the system below 783 K and the spontaneous magnetization was observed to be following the Bloch T^3/2 law below 80 K. The magnetic moment per formula unit at 5 K was observed to be 1.88 μ_B. The temperature variation of the electrical resistance was found to follow the relation R_n=R_0n+aT^α (α=1.616) and (R_n—normalized electrical resistance) in the temperature range of 25–300 K and we observed almost a temperature independent variation of the electrical resistance below 25 K indicating the absence of spin-flip scattering.     

Observation of the strongly anomalous temperature and composition dependence of (δp/δT)s in liquid Li/Pb alloys

(δp/δT)_s which is proportional to the specific heat has been measured between the liquidus temperature and 1250 K for the whole concentration range of liquid Li/Pb alloys. For all concentrations it shows a positive deviation from the values expected for an ideal solution and it exhibits a narrow peak close to the composition Li₄Pb. The variation of T(δp/δT)_s with T is linear only for pure Pb. At 0.2 < x_Li < 0.6 a fall-off after melting is followed by a broad maximum at about 1000 K.     

Metastable states in phase separated electron doped Sm0.15Ca0.85MnO3

In order to study the mechanism behind the phase separation scenario in the Sm_0.15Ca_0.85MnO₃ compound, magnetization and resistivity measurements have been carried out in pulsed magnetic fields up to 50 T at temperatures 4.2 K<T<200 K. It is found that external magnetic field causes a collapse of a C-type AFM (P2₁/m) phase resulting in field-induced insulator-metal transition, which is irreversible below T₁=75 K. In zero field the content of a G-type phase in the mixed C-G state can vary from 10 to 17% at T=10 K. A set of metastable states with different volume ratios of G-type to C-type phases is observed below T₁ depending on the history of the sample. The obtained results indicate that the phase separation plays a dominant role for the electric and the magnetic properties of this material.     

Complex competition between ferromagnetism and antiferromagnetism in the CMR manganites Pr1−x Ca x MnO3

The systematic investigation of the magnetic susceptibility of the CMR manganites Pr_1?x Ca _x MnO₃ versus temperature has been performed for 0.25 ≤ x ≤ 0.50. Due to the similar size of calcium and praseodymium, these results show the important role of the mixed valence of manganese upon the complex magnetic behaviour of these compounds. They demonstrate that the appearance of antiferromagnetism coincides with charge ordering, T _N = T _CO varying with x, from 250K for x = 0.50 to 225K for x = 0.35. A strong competition between ferromagnetism and antiferromagnetism is observed for 0.35 ≤ x < temperature (T > 170 K) and going through canted antiferromagnetic or weak ferromagnetic states for intermediate temperatures (70 K < T < 170 K). For 0.25 ≤ x ≤ 0.30, a strong ferromagnetic state is observed for 95 K ≤ T ≤ 150 K, with a transition to a spin glass like state below 95–110 K.     

Field induced sign reversal of magnetocaloric effect in Gd2In

We report the magnetocaloric effect in the metamagnetic compound Gd₂In obtained from magnetization measurement. Gd₂In was previously reported to have two magnetic transitions: (i) a paramagnetic to ferromagnetic transition below 190 K and (ii) a ferromagnetic to an antiferromagnetic state below 105 K. The low temperature antiferromagnetic state is unstable under an applied magnetic field and undergoes metamagnetic transition to a ferromagnetic like state. We observe conventional positive magnetocaloric effect (the magnetic entropy change, ΔS_M<0) around 190 K at all applied fields. The magnetocaloric effect is found to be inverse (negative) at low fields around 105 K (ΔS_M>0), however it turns positive at higher fields (ΔS_M<0). The observed anomaly is found to be related to the field induced transition which drives the system from an antiferromagnetic to a ferromagnetic state.     

Pressure and the quadratic temperature term in the electrical resistance of NbTi

We have measured the electrical resistance of an Nb_0.53Ti_0.47 alloy sample as a function of temperature T (4-300 K) and pressure (<20 GPa). At low temperatures, above the superconducting transition, we observe a T² term whose coefficient decreases with pressure. It is linearly dependent on the residual resistance, that also decreases with pressure, in strong agreement with a Koshino-Taylor origin, i.e. inelastic carrier scattering against impurities.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Magnetic and crystallographic characteristics of Th6–xYxMn23 alloys

Crystallographic studies of Th_6–xY_xMn₂₃ ternaries show that Y₆Mn₂₃ and Th₆Mn₂₃ are miscible in all proportions with lattice parameters that are essentially independent of composition. Magnetic properties of the ternaries have been examined over the temperature range 4 to 500 K. The Curie temperatures and the saturation magnetizations decrease monotonically with decreasing Y content; magnetic ordering is observed for all compositions within the range 1.50 < x ? 6. Since the Mn-Mn spacing is nearly invariant with composition, it follows that electron concentration variation is responsible for the compositional variation of magnetic properties.     

Transition De Phase Metamagnetique Du Bromure Ferreux

In a magnetic field parallel to the magnetization axis of an antiferromagnetic Fe Br₂ single crystal, a caracteristic metamagnetic behaviour is observed. The transition from an antiferromagnetic phase to a paramagnetic phase is studied by help of magnetization measurements in a steady field (H < 60 kOe). The measurement precision has allowed a detailed study of the magnetization isotherms, caracteristic of a first order magnetization phase transition (T < T_c = 4, 7 K) and of a second order phase transition (T_c < T < T_N = 14, 2 K).We have observed an original phase diagram. In a certain temperature and field range, the ordered phase is stable on the high temperature side of the transition point. Some theoretical studies in an Ising model, or in the hypothesis of a strong magnetoelastic coupling forecast the existence of such a magnetic phase diagram.At present, we proceed to a theoretical study, in a molecular field approximation, of the magnetic phase diagram of compounds similar to Fe Br₂ where we take into account the relative values of parameters J₁, J₂ and D associated with ferromagnetic and antiferromagnetic interactions and crystalline anisotropy.     

Superconductivity in the Zr-D system under pressure

The superconducting transition temperature T _c of the ZrD_0.48 alloy is measured in the pressure range up to 41.5 GPa. The measurements are carried out in a high-pressure chamber with diamond anvils by the inductometric method. It is found that T _c(P) increases to 3.1 K at a pressure below 30 GPa, exhibits a sharp increase up to 8 K near 30 GPa, and then smoothly decreases to ～6.5 K at 41.5 GPa. A similar dependence T _c(P) is obtained for pure Zr. The similarity of the T _c(P) curves suggests that the dependence T _c(P) observed for ZrD_0.48 is due to the presence of ω-phase in this alloy at pressures P<30 GPa and the ω-β transition at P≈30 GPa, which leads to the establishment of new ratios between the phases in the Zr-D system. In the pressure range studied, no indications are observed for new superconducting phases similar to the phases of intermediate composition in the Ti-H(D) system, which are formed by the hydrogen transfer from tetrahedral to octahedral interstitials.     

FMR linewidth and eddy-current losses in Ag-doped CdCr2Se4 single crystals

We report FMR linewidth data on Ag-doped CdCr₂Se₄ single crystals, with 0.1, 0.75 and 5.4% of Ag doping, in the range 70 K < T < 170 K. The contribution of eddy-current losses to the linewidth is analyzed for samples of spherical and slab shapes. Together with two-magnon scattering due to surface and volume pits, these losses explain the observed linewidths. The microwave conductivities obtained are consistent with the model for conduction and the results for n-type CdCr₂Se₄ and EuO.