Defect-induced magnetic fluctuations in YbB12

We present the results of nuclear spin–lattice relaxation rate (1/T₁) measurements in a typical Kondo insulator YbB₁₂ for ^10,11B and ¹⁷¹Yb nuclei. Above 20 K, 1/T₁ at the B sites shows thermally activated temperature dependence with a gap of about 100 K. However, it shows anomalous enhancement below 15 K, which is partially suppressed by magnetic field up to 16 T. No such anomaly was observed at the Yb sites. The ratio of 1/T₁ for ¹¹B and ¹⁰B nuclei indicates that the anomaly below 15 K is caused by dilute magnetic moments assisted by nuclear spin diffusion. The origin and the nature of the low temperature magnetic fluctuations are discussed.     

Study on ethyl groups with two different orientations in [N(C2H5)4]2CuBr4

The crystal structure and phase transition temperature of [N(C₂H₅)₄]₂CuBr₄ are studied using X-ray diffraction and differential scanning calorimetry (DSC); measurements revealed a tetragonal structure and the two phase transition temperatures T_C of 204 K and 255.5 K. The structural geometry near T_C is discussed in terms of the chemical shifts for ¹H magic angle spinning (MAS) nuclear magnetic resonance (NMR) and ¹³C cross-polarization (CP)/MAS NMR. The two inequivalent ethyl groups are distinguishable by the ¹³C NMR spectrum. The molecular motions are discussed in terms of the spin–lattice relaxation times T_1ρ in the rotating frame for ¹H MAS NMR and ¹³C CP/MAS NMR. The T_1ρ results reveal that the ethyl groups undergo tumbling motion, and furthermore that the ethyl groups are highly mobile.     

Dynamics of hydroxyl deuterons and bonded water molecules in NaDY(0.8) zeolite as studied by means of deuteron NMR spectroscopy and relaxation

Deuteron spin–lattice relaxation and spectra were measured for NaDY (0.8) zeolite containing some heavy water. Two subsystems of deuterons with different mobility were disclosed at low temperatures with their respective relaxation rates differing by two orders of magnitude. Spectra exhibit different shapes related directly to a specific motional model. Hydroxyl deuterons perform incoherent tunneling along the hydrogen bond, then on increasing temperature jumps to excited states and over the barrier appear. Hydrogen bonded water molecules perform 180° rotational jumps about the twofold symmetry axis. Spectral amplitudes are consistent with the water content of 13 D₂O molecules per unit cell. Above about 240 K translational mobility becomes significant and finally water molecules diffuse across the free space of cages. Diversity in temperature dependence of hydroxyl deuteron dynamics may indicate location of adsorbed molecules.     

7Li NMR analysis on perovskite structured Li0.15La0.28TaO3

The nanocrystalline perovskite material Li_0.15La_0.28TaO₃ has been synthesized by alkoxide-free Pechini type sol gel method. ⁷Li NMR measurements were carried out using a Bruker Avance 300 spectrometer at 116 MHz over the temperature range 150 to 400 K. Longitudinal spin-lattice relaxation times (T₁) measured by saturation recovery and longitudinal relaxation times in the rotating frame (T_1ρ) measured using the pulse sequence (π/2–spin lock τ acquisition) with lock radio-frequency field υ = 62.5 kHz and the T₂ relaxation time measured by Hahn echo are presented. The static Hahn-echo spectra show two different lithium sites in this perovskite oxide. Further, the relaxation measurements T₁ and T_1ρ show two different types of lithium cations with fast and slow dynamics.     

Proton diffusion in the room-temperature phase of [(NH4)1−xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation in the rotating frame

Proton diffusion in the room-temperature phase (phase II) of [(NH₄)_1?xRb_x]₃H(SO₄)₂ (0≤x≤1) has been studied by means of ¹H spin-lattice relaxation times in the rotating frame, T_1ρ. The ¹H T_1ρ values were measured at 200.13 MHz in the range of 380–490 K. The ammonium protons and the acidic protons have independent T_1ρ values in the higher temperature range of phase II, suggesting that the spin diffusion between the two species is ineffective. The translational diffusion of the acidic protons is the most dominant mechanism to relax both the ammonium protons and the acidic protons in phase II. The ¹H T_1ρ values in phase II are analyzed theoretically and the motional parameters are obtained. The results of NMR well explain the macroscopic proton conductivity.     

Novel superconducting characteristics and unusual normal-state properties in iron-based pnictide superconductors: 57FeNMR and 75AsNQR/NMR studies in REFeAsO1−y (RE = La,Pr, Nd) and Ba0.6K0.4Fe2As2

We discuss the novel superconducting characteristics and unusual normal-state properties of iron (Fe)-based pnictide superconductors REFeAsO_1?y (RE = La, Pr, Nd) and Ba_0.6K_0.4Fe₂As₂ (T_c = 38 K) by means of ⁵⁷FeNMR and ⁷⁵AsNQR/NMR. In the superconducting state of LaFeAsO_0.7 (T_c = 28 K), the spin component of the ⁵⁷Fe-Knight shift decreases to almost zero at low temperatures, which provide firm evidence of the superconducting state formed by spin-singlet Cooper pairing. The nuclear spin–lattice relaxation rates (1/T₁) in LaFeAsO_0.7 and Ba_0.6K_0.4Fe₂As₂ exhibit a T³-like dependence without a coherence peak just below T_c, indicating that an unconventional superconducting state is commonly realized in these Fe-based pnictide compounds. All these events below T_c are consistently argued in terms of an extended s_±-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, 1/T₁T decreases remarkably upon cooling for both the Fe and As sites of LaFeAsO_0.7. In contrast, it gradually increases upon cooling in Ba_0.6K_0.4Fe₂As₂. Despite the similarity between the superconducting properties of these compounds, a crucial difference was observed in their normal-state properties depending on whether electrons or holes are doped into the FeAs layers. These results may provide some hint to address a possible mechanism of Fe-based pnictide superconductors.     

Dielectric relaxation of CsHSeO4 above room temperature

Dielectric measurements of CsHSeO₄ show a distinct relaxation at low frequencies at several isotherms (T < 363 K). For example, the relaxation frequency is around 4 kHz at 323 K and increases to higher frequencies (~ 100 kHz) as the temperature increases. The relaxation has an activation energy of 0.8 eV, which is in close agreement with that associated with transport of charge carriers. We suggest that the observed dielectric relaxation could be produced by the H⁺ jump and SeO₄^? 2 reorientation that cause distortion and change the local lattice polarizability, inducing dipoles like HSeO₄^?.     

Spin state transition in Ca-doped Na0.7CoO2 with the nominal Co valence below 3.16

In order to clarify the nature of the transition around 300 K in Ca-doped Na_0.7CoO₂, the magnetism of Na_0.7Ca_yCoO₂ with y=0.035 and 0.07 was investigated in a positive muon spin rotation and relaxation (μ⁺SR) experiment. Transverse field μ⁺SR measurements showed that the spin state of the Co ions in Na_0.7Ca_0.07CoO₂ changes at around 300 K; i.e. the exponential relaxation rate vs. T curve exhibited a large maximum around 300 K with an accompanying small peak in the muonic Knight shift, whereas no changes were found in the asymmetry, similar to [Ca₂CoO₃]_0.62[CoO₂] at around 400 K.Although the spin-density-wave (SDW) state exists for Na_xCoO₂ with x≥0.75 at low temperatures, zero-field μ⁺SR spectra in the Ca-doped samples showed no muon spin precessions down to 1.8 K but only fast relaxations indicating disorder. This is probably because the Ca²⁺ ions in the Na planes alter the charge and/or spin distribution in the CoO₂ planes. As a result, the SDW order is hindered, as the nominal Co valence is decreased below 3.16.     

Spin reorientation and magnetocrystalline anisotropy of (Nd1−xDyx) Fe14B

Spin reorientation and magnetocrytalline anisotropy of (Nd_1−xDy_x)₂Fe₁₄B (x=0.25, 0.5, 0.75) have been studied from mangetization curves of magnetically aligned powders. In (Nd_1−xDy_x)₂Fe₁₄B, the spin reorientation temperature (T_SR) decreases linearly on increasing Dy-substitution from 135 to 56 K with the ratio of ΔT_SR=−1.11 K/Dy at% in the composition range of 0⩽x⩽0.75. The spin reorientation angle at 4.2 K decreases on Dy-substitution from 30.4° at x=0 to 14.7° at x=0.75. From the investigation of the magnetocrystalline anisotropy at 4.2 K, the disappearance of the spin reorientation for compositions x≳0.85 is expected.     

Giant magnetic refrigerant capacity in Ho3Al2 compound

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Ho₃Al₂ compound are investigated by magnetization and heat capacity measurements. Two successive magnetic transitions, a spin-reorientation (SR) transition at T_SR=31 K followed by a ferromagnetic (FM) to paramagnetic (PM) transition at T_C=40 K, are observed. Both magnetic transitions contribute to the MCE and result in a large magnetic entropy change (ΔS_M) in a wide temperature range. The maximum values of ?ΔS_M and adiabatic temperature change (ΔT_ad) reach 18.7 J/kg K and 4.8 K for the field changes of 0–5 T, respectively. In particular, a giant value of refrigerant capacity (RC) is estimated to be 704 J/kg for a field change of 5 T, which is much higher than those of many potential refrigerant materials with similar transition temperatures.     

Magnetic properties and magnetocaloric effects in Gd1−xHoxNiIn intermetallic compounds

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Gd_1?xHo_xNiIn (x=0?1) compounds have been evaluated by magnetization and heat capacity measurements. The Curie temperature T_C can be tuned from near 100 K to 20 K by substituting Ho for Gd atoms. In addition, all the compounds with Ho atoms undergo two successive magnetic transitions with the decrease of temperature: a paramagnetic (PM) to ferromagnetic (FM) transition around T_C and a spin-reorientation (SR) transition around 7?9 K. It is found that both transitions contribute to the magnetic entropy change (ΔS_M). For a field change of 5 T, the maximum values of ?ΔS_M for Gd_0.4Ho_0.6NiIn are 6 J/kg K at T_t=9 K and 10 J/kg K at T_C=52 K, respectively. These two ?ΔS_M peaks overlap partly and result in a wide working temperature range of MCE, and thus leading to the largest RC value of 443 J/kg in the Gd_1?xHo_xNiIn system.     

Electrical conductivity and NMR studies in paraelastic phase of Tl2SeO4

We have carried out the electrical conductivity and NMR measurements and investigated the characteristic features of the electrical conductivity in a paraelastic phase of Tl₂SeO₄. It was found from electrical conductivity measurement that the activation energy in the paraelastic phase (above T_c(= 661 K)) is 0.98 eV. We also found that ²⁰⁵Tl-NMR line width drastically decreases above T_c and becomes approximately 0.5 Gauss. This result indicates that in the paraelastic phase the mobile Tl ions exist. Moreover this result is consistent with the existence of the anomalously large hopping motion of Tl ions observed in X-ray diffraction measurement. Furthermore the activation energy estimated from the motional narrowing of ²⁰⁵Tl-NMR absorption line is 0.92 eV and is in agreement with that obtained from the conductivity measurement. From these results, it is deduced that the mobile Tl ions play an important role in the appearance of electrical conductivity in the paraelastic phase.     

The effect of chemical pressure in rutheno-cuprates

We have studied the effect of negative chemical pressure in the RuGd_1.5(Ce_0.5?xPr_x)Sr₂Cu₂O_10?δ with Pr content of 0.0 ? x ? 0.2. This is also investigated using the bond length results obtained from the Rietveld refinement analysis. The c parameter and cell volume increase with x for 0.0 ? x ? 0.15. The width of the resistivity transition also increases with Pr concentration, indicating higher inhomogeneity and oxygen deficiency. The difference in the ionic valences of Pr^3+,4+ and Ce⁴⁺ causing different hole doping, the difference in the ionic radii, and oxygen stoichiometry affect the superconducting transition. The magnetoresistance shows a cusp around 135 K which lies between the antiferromagnetic and ferromagnetic transition temperatures, which is probably due to the presence of a spin glass region. There exist two magnetic transition temperatures for 0.0 ? x ? 0.2 which respectively change from T_M = 155 K to 144 K and from T_irr = 115 K to 70 K. The magnetization versus applied magnetic field isotherms at 77 K and 300 K show that the remanent magnetization and coercivity are lower for samples with higher Pr content.     

Missing superconductivity in BaAlSi with the AlB2 type structure

The solid solutions BaAl_1−xSi_1+x (0 ⩽ x ⩽ 0.5) were prepared. The compound with the stoichiometric composition (x = 0) did not show superconductivity as reported by other investigators, but the solid solutions with x > 0 became superconductors with a transition temperature T_c = 2.8 K. The comparison of the lattice parameters with those of the other isotypic ternary superconductors MAlSi (M = Ca, Sr) suggested that the superconductivity could be related to the lattice parameter within the (AlSi) plane rather than the interlayer spacing. The band structures near the Fermi level of MAlSi (M = Ca, Sr, Ba) were measured using soft X-ray photoelectron spectroscopy, which were in good agreement with the calculated ones, confirming that the contribution of the d orbitals of the alkaline-earth metals were predominant in the conduction bands.     

Hydrothermal synthesis and magnetic studies of transition metal nocerites M3(BO3)F3 (M=Fe,Co, Ni)

Polycrystalline samples of M₃(BO₃)F₃ (M=Fe, Co, Ni), isostructural with nocerite Mg₃(BO₃)(OH,F)₃, have been prepared in supercritical hydrothermal conditions. These compounds represent with boracites, M₃B₇O₁₃F (M=Mg, Cr, Mn, Fe, Co, Zn), the only transition metal fluoride borates known to date. Co₃(BO₃)F₃ and Ni₃(BO₃)F₃ are antiferromagnetic with T_N=17(2) and 40(2) K, respectively. Spin-flop transitions at B_C1=4.0 T and B_C2=7.5 T occur at 1.6 K in Co₃(BO₃)F₃, while a parasitic ferromagnetism (0.02 μ_B/Ni²⁺ at 1.6 K) appears below T_N in Ni₃(BO₃)F₃. The magnetic structures consist of three spin sub-lattices of double rutile-type ferromagnetic chains.     

Proton diffusion in the superprotonic phase of [(NH4)1 − xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation

Proton diffusion in [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 < x < 1) has been studied by means of ¹H spin-lattice relaxation times, T₁. The relaxation times were measured at 200.13 MHz in the range of 296–490 K and at 19.65 MHz in the range of 300–470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 ≤ x ≤ 0.54). Rb₃H(SO₄)₂ does not obey this trend. The results of NMR well explain the macroscopic proton conductivity.     

Crystal-field effect on the hyperfine splittings,Mössbauer spectra and specific heat of Er3+ in pyrogermanate and diglycollate hosts

Hyperfine splittings and the Mössbauer spectra (MS) of the two isomers ¹⁶⁶Er and ¹⁶⁷Er in the pyrogermanate (or ErPG) and diglycollate (or ErDG) hosts were calculated using the previously determined values of the crystal field (CF) parameters and the CF levels. MS for the 166 isomer in both the hosts show only 5 lines, whereas 27 lines are expected for the 167 isomer. In case of ErDG, the quadrupole interaction P increased considerably between 300 and 4 K, i.e., from 1.5 to 21.3 MHz in case of ¹⁶⁶Er and from −1.0 to −14.0 MHz in ¹⁶⁷Er. However for ErPG, the corresponding changes in P for both the isomers are insignificant ∼1 MHz. The hyperfine magnetic fields H_hf in ErDG and ErPG were found to be nearly the same being equal to 4.4±0.01 MG because the g_∥-values are close, i.e., 10.31 and 10.58, respectively. Specific heat C_p of ErPG was measured between 30 and 0.65 K and a sharp λ peak was detected with a transition temperature T_N=0.95±0.01 K. The calculated hyperfine and Schottky specific heat components, C_hf, and C_sch, respectively showed peaks at 32 mK and 110 K. From the experimental results of C_p, the lattice specific heat C_L/R in ErPG was found to be 3.34×10⁻⁵ T³ and the thermal characteristics of the magnetic specific heat C_M was determined. The internal energy U_M(T) was obtained from C_M by numerical integration and its change ΔE/R=[U_M(∞)/R–U_M(0)/R] was found to be 1.02 K and the corresponding value of the Weiss constant θ in the ordered state was 2.01 K. The value of the exchange interaction constant |J_ex/k| and the dipole–dipole interaction constant for nearest neighbours were found to be ∼0.5 and ∼1.03 K, respectively. The molecular field (H_mol) for ErPG was equal to 4.1 kG. From future epr and MS measurements, interesting results are expected regarding the magnetic ordering in ErPG and the phase change in ErDG.     

Electronic excitations and luminescence of SrAlF5 crystals doped with Ce3+ ions

This paper reports the results of a time-resolved photoluminescence and energy transfer processes study in Ce³⁺ doped SrAlF₅ single crystals. Several Ce³⁺ centers emitting near 4 eV due to 5d-4f transitions of Ce³⁺ ions substituting for Sr²⁺ in non-equivalent lattice sites were identified. The lifetime of these transitions is in the range of 25–35 ns under intra-center excitation in the energy region of 4–7 eV at T = 10 K. An effective energy transfer from lattice defects to dopant ions was revealed in the – 7–11 eV energy range. Both direct and indirect excitation channels are efficient at room temperature. Excitons bound to dopants are revealed at T = 10 K under excitation in the fundamental absorption region above 11 eV, as well as radiative decay of self-trapped excitons resulting in luminescence near 3 eV.     

Effect of structural disorder on magnetic frustrations in the Fe80Nb6B14 amorphous alloy

The paper presents measurements of magnetic frustrations for the Fe₈₀Nb₆B₁₄ amorphous alloy preliminary annealed for 1 h at temperatures ranging from 300 to 770 K in comparison with other parameters characterizing structural relaxation. It was shown that annealing out of free volume and internal stresses cause a reduction in number of frustrated states observed in the range 400>T>50 K. In the range 2<T<50 K additional frustrations due to iron clusters or nanograins were detected.     

Magnetic properties of GdCo12B6 compound under high pressures

The effects of hydrostatic pressure up to 10 kbar on Curie temperature T_C, compensation temperature T_COMP and spontaneous magnetization M_S of ferrimagnetic GdCo₁₂B₆ compound have been studied. Two antiferromagnetically coupled sublattices that are carrying magnetization of typically 0.42 μ_B/Co atom and 7 μ_B/Gd cancel out at compensation temperature at about 50 K and magnetic ordering temperature T_C=163±2 K. The volume dependence of intrinsic magnetic properties of the GdCo₁₂B₆ compound has been determined by studying it under hydrostatic pressure. The observed increase of M_S with pressure (dM_S/dp=+0.005 μ_B kbar^?1 at 5 K) is attributed predominantly to the pressure induced decrease of Co magnetic moments. The crucial role of Co in this behavior is confirmed by the change of sign of the pressure slope at temperatures above T_COMP and by the fact that the estimated decrease of m_Co is also quite comparable with pressure induced decrease of M_S in YCo₁₂B₆ (dM_S/dp=?0.007 μ_B kbar^?1). The decrease of m_Co is also responsible for the increase of T_COMP with pressure (dT_COMP/dp=+0.06 K kbar^?1). The decrease of T_C with pressure (dT_C/dp=?0.55 K kbar^?1) is comparable to the decrease observed on RCo₁₂B₆ compounds with non-magnetic R and can be attributed to the volume dependence of Co–Co exchange interactions. The remarkable role of the hybridization as a consequence of small distances between Co and B atoms could be a background of this rather unexpected volume stability of magnetic properties.