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.     

Antiferromagnetic order and spin glass behavior in Dy2CuIn3

The magnetic properties of the intermetallic compound Dy₂CuIn₃ have been investigated. Ac and dc-susceptibility measurements indicate an onset of antiferromagnetic ordering at T_N=19.5 K and an additional frequency dependent transition at T_ds∼9 K. Neutron diffraction studies confirm the ordered transition at 19.5±1 K. The magnetic unit cell can be described by the propagation vector k=(0.25,0.25,0) with the magnetic moment μ=2.63(4)μ_B/Dy³⁺ parallel to the c-axis. Nevertheless, neutron diffraction reveals no additional magnetic phase transition around or below 9 K, which suggests that, at lower temperatures, a spin glass state may be formed in coexistence with the antiferromagnetic mode as a result of frustration and the antagonism between ferromagnetic and antiferromagnetic exchange interactions.     

Suppression of ferroelectricity and quantum fluctuations in Ru-doped TbMnO<Subscript>3</Subscript>

We investigate the effects of Ru-doping in polycrystalline TbMn_1−x Ru _x O₃ (x≤0.10) on the multiferroicity. It is observed that the Ru substitution gradually melts away the dielectric anomaly at the ferroelectric transition point and the ferroelectricity by suppressing the polarization, accompanied with a surprising low-temperature dielectric plateau. While it is reasonable to observe the significant suppression of ferroelectricity, owing to the fact that the Ru-doping disrupts the Mn spiral spin ordering and reduces the Mn–Mn spin angle, quantum fluctuations associated with the Ru substitution, responsible for the low-temperature dielectric plateau, seems to be significant.     

Eu2－xPbxRu2O7中的金属-绝缘体相变和自旋玻璃态行为

通过对Eu_2-xPb_xRu₂O₇(x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0和1.8)系列样品的结构、电阻和磁化率的观测，结果发现，随着Pb替代浓度x值的增加，样品的电阻率逐渐减小，系统在x=0.8附近发生了金属-绝缘体(M-I)相变；Ru⁴⁺的局域磁矩及其自旋玻璃冻结温度T_G也随之降低. 在该体系中，Pb²⁺对Eu³⁺的部分替代使样品中载流子浓度增加，Pb的6p能带与Ru 4d电子的T_2g能带混合，能带得以拓宽，Ru 4d电子的巡游性增强，导致该体系物性的系列变化. 关键词： 自旋几何受挫 2-xPb_xRu₂O₇体系')" href="#">Eu_2-xPb_xRu₂O₇体系 金属-绝缘体相变 自旋玻璃态     

High-pressure effects on the crystal and magnetic structure of the Nd0.78Ba0.22CoO3 cobaltite

The crystal and magnetic structure of the Nd_0.78Ba_0.22CoO₃ cobaltite is studied by neutron diffraction at high pressures up to 4.2 GPa in the temperature range 10–300 K. The pressure dependences of structural parameters are obtained. Ferromagnetic ordering of the Co sublattice is observed at normal pressure below T _C ～ 140 K, and ferrimagnetic ordering of the Co and Nd sublattices with an antiparallel direction of magnetic moments appears at T _F ～ 40 K. The magnetic moment of Co and the temperature T _C change slightly under pressure, which points to the stability of the initial intermediate-spin (S = 1) state of Co³⁺ ions. This behavior differs considerably from the characteristic behavior of cobaltites that are close in chemical composition and structure and exhibit ferromagnetic ordering of only the Co sublattice. In these cobaltites, the magnetic moment of Co is substantially suppressed and T _C decreases under pressure, which is related to the change in the state of Co³⁺ ions from the intermediate spin state to the nonmagnetic low-spin state (S = 0). The interplay between the appearance of the magnetic interaction of the R-Co sublattices and the stability of the spin state of Co³⁺ ions in the Nd_0.78Ba_0.22CoO₃ cobaltite is discussed.     

High temperature magnetic ordering in La2RuO5

Magnetic susceptibility, heat capacity and electrical resistivity measurements have been carried out on a new ruthenate, La₂RuO₅ (monoclinic, space group P2₁/c) which reveal that this compound is a magnetic semiconductor with a high magnetic ordering temperature of 170 K. The entropy associated with the magnetic transition is 8.3 J/mol K close to that expected for the low spin (S=1) state of Ru⁴⁺ ions. The low temperatures specific heat coefficient γ is found to be nearly zero consistent with the semiconducting nature of the compound. The magnetic ordering temperature of La₂RuO₅ is comparable to the highest known Curie temperature of another ruthenate, namely, metallic SrRuO₃, and in both these compounds the nominal charge state of Ru is 4+.     

Re-entrant superconductivity in a magnetically ordered superconductor: La1-xGdxRu2

We have measured the critical temperature (T_c) and the upper critical magnetic field (H_c2) of La_1-xGdxRu₂. At low concentrations of the magnetic impurity (Gd), the suppression of T_c follows the expected Abrikosov-Gorkov (A-G) pair breaking curve. However, for larger concentrations, strong deviations below A-G are observed. Samples in this region (4. ? × ? 5. at. %) exhibit two T_c's. La_1-xGd_xRu₂ is known to order magnetically, probably as a spin glass, and the magnetic ordering temperature (T_M) has been measured in the normal state. This T_M curve intersects the T_c curve in the concentration range where the T_c curve is re-entrant and we therefore attribute the re-entrant T_c behavior to the magnetic ordering of the Gd³⁺ ions.     

Probing lattice defects in Sr2MgSi2O7:Eu2+,Dy3+

The Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ materials were prepared with a solid state reaction and their microscopic structure (at 295 K only) and luminescence were studied at selected temperatures between 150 and 295 K. Undisturbed Sr crystal planes were common in the TEM images of the undoped Sr₂MgSi₂O₇ material, whereas with Eu²⁺ doping more disturbed planes were observed even in the nanometer scale. With Dy³⁺ co-doping, a large number of small lattice domains created by the discontinuities in the crystal structure was observed. The domains with different orientations seem to be centered around point defects. The decay curves of Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ showed fast (ms scale) persistent luminescence. The intensity of persistent luminescence increased considerably between 200 and 250 K while remaining constant in the ranges of 150–200 and 250–295 K. The changes were used to study the depth of the traps. In general, Dy³⁺ co-doping was found to deepen the traps.     

Mössbauer Spectroscopy Study on the Effect of Al–Cr Co-Substitution in Yttrium and Yttrium–Gadolinium Iron Garnets

We have investigated the metal-insulator transition (MIT) of CuIr₂S₄ by a high resolution NMR measurement. The Cu-NMR spectrum below T _MI is broadened and split into four Cu signals with sizable electric quadrupole interactions. The NMR results are consistent with the charge ordering of Ir³⁺ and Ir⁴⁺ and the spin dimerization of Ir⁴⁺ spins, as revealed by a recent X-ray study.     

Thermodynamics of spin ice in staggered and direct (along the [111] axis) fields in the cluster approximation

We have analyzed the low-temperature thermodynamic properties of spin ice in the staggered and direct (acting along the [111] axis) fields for rare-earth oxides with the chalcolamprite structure and general formula Re₂ ³⁺Me₂ ⁴⁺O₇ ^2-. Calculations have been performed in the cluster approximation. The results have been compared with experimental temperature dependences of heat capacity and entropy for Dy₂Ti₂O₇ compound for different values of the external field in the [111] direction. The experimental data and calculated results have also been compared for the Pr₂Ru₂O₇ compound with the antiferromagnetic ordering of magnetic moments of ruthenium ions, which gives rise to the staggered field acting on the system of rare-earth ions. The calculated temperature dependences of heat capacity and entropy are in good agreement with experimental data.     

Emergent phenomena in perovskite-type manganites

Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. One such example is the keen competition between phases with different spin/charge/orbital orders. Keen competition between antiferromagnetic metal and orbital-ordered insulator is found in the slightly electron-doped regime near Mn⁴⁺ state which is stabilized by the high oxygen-pressure condition. Another one is the emergence of ferroelectricity either induced by the magnetic ordering or independently of the magnetic ordering. As the respective examples, perovskite-type YMnO₃ and Sr_1−xBa_xMnO₃ are discussed. In the YMnO₃, the ferroelectric lattice distortion associated with the E-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr_0.5Ba_0.5MnO₃, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.     

Observation of long-range magnetic ordering in the proposed spin-singlet state of Hg2Ru2O7

The magnetic ground-state of the pyrochlore material Hg₂Ru₂O₇, which undergoes a metal to Mott insulator transition (MIT) at T=107 K, was investigated using muon spin relaxation and neutron powder diffraction (both non-polarized and polarized) measurements. Unlike the expected behavior associated with a spin-singlet ground-state, these surprisingly revealed the onset of long-range magnetic ordering below the MIT.     

The giant dielectric tunability effect in bulk La2NiMnO6 around room temperature

The effect of applied dc bias electric field on dielectric permittivity in bulk La₂NiMnO₆ is investigated in this paper. It is found that a small bias field of 40 V/cm can greatly reduce the dielectric permittivity around the room temperature, compared to the much larger electric field that is required for conventional ferroelectric materials. The observed giant dielectric tunability is retained over a broad range of around room temperature and is most likely related to the charge ordering of Ni²⁺ and Mn⁴⁺ ions, further confirming the existence of electronic ferroelectricity in La₂NiMnO₆.     

Photoexcited Eu2+ spin dynamics in EuFe2As2

Employing temperature dependent time-resolved optical femtosecond spectroscopy, we investigated the quasiparticle and Eu²⁺ spin relaxation dynamics in EuFe₂As₂ (EFA). As previously reported in other undoped iron-based pnictides, we observe the quasiparticle relaxation bottleneck due to the charge gap opening in the spin density wave (SDW) state below T _SDW = 189 K. Below the Eu²⁺ antiferromagnetic (AFM) spin ordering temperature, T _AFM = 19 K, we observe another slower relaxation component, which we attribute to the Eu²⁺ AFM order dynamics. The slow dynamics of this component suggests a weak coupling between the Eu²⁺ spins and the carriers in the Fe-d derived bands.     

Magnetic ordering in the T* phase La1.2Tb0.8CuO4

We report a muon spin relaxation study of the magnetic properties of the La_1.2Tb_0.8CuO₄ phase with the T^* structure. Random magnetic order is revealed between 280 and 170 K by the zero field data. A spontaneous muon precession then appears below 170 K, arising from antiferromagnetic long range order of the Cu²⁺ spins. Evidence exists below 20 K for ordering of the Tb³⁺ ions. We find that the T^* phase adopts the same magnetic structure as the (T/O) phase La₂CuO₄.     

Magnetic properties and charge ordering in Pr0.75Na0.25MnO3 manganite

The structural and magnetic properties of Pr_0.75Na_0.25MnO₃ have been investigated experimentally. At room temperature, the compound shows paramagnetic characteristic. Along with decreasing temperature, a peak appears in the magnetization versus temperature curve around 220 K. To clarify whether this peak is associated with the ordering arrangement of Mn³⁺ and Mn⁴⁺ ions, electron diffraction experiments were carried out below and above 220 K respectively. Only basic Brag diffraction spots can be observed at high temperatures, however, superlattice diffraction appears below 220 K. This provides direct evidence for the existence of charge ordering in Pr_0.75Na_0.25MnO₃. We find the Mn³⁺ and Mn⁴⁺ cations form zigzag chains in a-c plane by analyzing the diffraction patterns. Combining with the magnetization measurements and the results of electron spin resonance, we confirm the antiferromagnetic phase and ferromagnetic component coexist in Pr_0.75Na_0.25MnO₃ below 120 K.     

Mn3TeO6 – a new multiferroic material with two magnetic substructures

From magnetic susceptibility, dielectric permittivity, electric polarization and specific heat measurements we discover spin‐induced ferroelectricity and magnetoelectric coupling in Mn₃TeO₆ and observe two successive magnetic transitions at low temperatures. A non‐ferroelectric intermediate magnetic state occurs below 23 K and a multiferroic ground state emerges below 21 K. Moreover, Mn₃TeO₆ is a candidate for a multiferroic material where two types of incommensurate spin structures, cycloidal and helical, coexist. Theoretically, both spin substructures may contribute to the macro electric polarization via different mechanisms. This could open new ways of manipulating the ferroelectric polarization in a multiferroic material. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Antiferromagnetic resonance and high magnetic field properties of NaNiO2

Magnetisation measurements up to 23 T and submillimeter wave ESR in the frequency region 48-380 GHz have been performed on NaNiO₂ powders at low temperature. Also typical spectra above the Néel temperature are shown. At 4 K the magnetisation shows a spin-flop transition at 1.8 T and saturation at 10 T. /Ni confirms the low spin state of the Ni³⁺ ions. The susceptibility exhibits a maximum at K with and K. NaNiO₂ is an A-type antiferromagnet: we derive K and K for the interactions between Ni ions within and between adjacent layers, respectively. The AFMR spectra yield an energy gap of 52.5 GHz, in agreement with the spin-flop value derived from the magnetisation. The anisotropy of the g factor observed at 100 K with can be attributed to the Jahn-Teller effect for Ni³⁺ ions in the low spin state, which stabilises the occupation. We finally comment on the isomorphic controversial Li_1-xNi_1+xO₂ compound. Received 9 March 2000 and Received in final form 13 July 2000.     

Effect of 100 MeV swift Si8+ ions on structural and thermoluminescence properties of Y2O3:Dy3+nanophosphor

Nanoparticles of Y₂O₃:Dy³⁺ were prepared by the solution combustion method. The X-ray diffraction pattern of the 900°C annealed sample shows a cubic structure and the average crystallite size was found to be 31.49?nm. The field emission scanning electron microscopy image of the 900°C annealed sample shows well-separated spherical shape particles and the average particle size is found to be in a range 40?nm. Pellets of Y₂O₃:Dy³⁺ were irradiated with 100?MeV swift Si⁸⁺ ions for the fluence range of 3?×?10^11_3?×?10¹³ ions cm^?2. Pristine Y₂O₃:Dy³⁺ shows seven Raman modes with peaks at 129, 160, 330, 376, 434, 467 and 590?cm^?1. The intensity of these modes decreases with an increase in ion fluence. A well-resolved thermoluminescence glow with peaks at ～414?K (T_m1) and ～614?K (T_m2) were observed in Si⁸⁺ ion-irradiated samples. It is found that glow peak intensity at 414?K increases with an increase in the dopant concentration up to 0.6?mol% and then decreases with an increase in dopant concentration. The high-temperature glow peak (614?K) intensity linearly increases with an increase in ion fluence. The broad TL glow curves were deconvoluted using the glow curve deconvoluted method and kinetic parameters were calculated using the general order kinetic equation.     

The giant dielectric tunability effect in bulk Y<Subscript>2</Subscript>NiMnO<Subscript>6</Subscript> around room temperature

The effect of applied dc bias electric field on dielectric permittivity in bulk Y₂NiMnO₆ is investigated in this paper. It is found that a small bias field of 40 V/cm can greatly reduce the dielectric permittivity around the room temperature, compared to the much larger electric field that is required for conventional ferroelectric materials. The observed giant dielectric tunability is retained over a broad range of around room temperature and is most likely related to the charge ordering of Ni²⁺ and Mn⁴⁺ ions. This may further confirm the existence of electronic ferroelectricity in Y₂NiMnO₆.