Experimental observation of zero point effects in the susceptibility of CuCl2 × 2(NC5H5), an S = 12 linear Heisenberg antiferromagnet

Susceptibility measurements on a single crystal of Copper Dichloride Dipyridine (CDP) down to 0.1K reveal long range order below 1.15K with the spins aligned normal to the c-axis of the crystal. The observed transverse susceptibility (H parallel to c), extrapolated to zero temperature, is found to be 75.6% less than the classical molecular field value, in good agreement with Griffith's as well as Bonner and Fischer's prediction of 79.4%.     

Properties of the secondary order parameter in Rb2ZnCl4

The dielectric constants and the spontaneous polarization of Rb₂ZnCl₄-crystals have been investigated in order to characterize the nature of the transitions at 303, 195 and 74.5 K. The dielectric anomalies around 303 K hint at a critical exponent β = 0.36 ± 0.03 in the incommensurate phase. Close to 195 K the commensurate state can be induced by electric fields in agreement with an appropriate Clausius-Clapeyron relation. The polarization measurements further show that Rb₂ZnCl₄ has a (second) ferroelectric transformation at 74.5 K.     

Nd^3＋：La2Be2O5单晶的光谱特性

Ｎｄ（３＋）：Ｌａ２Ｂｅ２Ｏ５（Ｎｄ（３＋）：ＢＥＬ）是一种具有单斜结构的双轴晶体，发射线性偏振荧光谱，当Ｅ∥Ｘ时，荧光峰值波长为１．０７０μｍ，Ｅ∥ｙ（ｂ）时，＝１．０７９μｍ．本文测量了Ｎｄ（３＋）：ＢＥＬ单晶的吸收谱，偏振和非偏振荧光谱，低温荧光谱（７７Ｋ°）和荧光寿命，（τ＝１３５±５μｓ），并且分析了其光谱特性与Ｎｄ（３＋）：ＹＡＧ作了比较，认为Ｎｄ（３＋）：ＢＥＬ更适合激光二极管泵浦．     

BaTiO_3/La_(0.67)Sr_(0.33)MnO_(3-δ)复合薄膜的磁致电极化和磁介电特性研究

采用脉冲激光沉积法制备了BaTiO_3(BTO)与缺氧的铁磁绝缘态La_(0.67)Sr_(0.33)MnO_(3-δ)(LSMO)构成的磁电复合薄膜,研究了20—300K温度区间内磁场对电极化特性和介电特性的影响.研究发现,施加磁场使得电滞回线的剩余极化强度和矫顽场均增大,其变化率峰值分别为111.9%和89.6%,峰值温度分别为40K和60K.异质结具有显著的磁介电效应,在测量温度区间内,磁场使得介电常数增大,介电损耗减小.在0.8T场强下,介电常数的最大磁致变化率出现在60K,达到了300%,而介电损耗也在此温度实现了最大变化,减小为零场时的50.9%.该磁电复合薄膜的磁致电极化和磁介电特性的极值均出现在LSMO层的磁电阻峰值温度附近,这说明磁场对电滞回线和介电参数的调制应该源自电荷相关的耦合作用.其可能的机理是磁场使得锰氧化物中的Mn离子局域磁矩趋于有序排列,并通过自旋-轨道耦合以及界面效应间接影响了BTO的电极化特性.研究结果对于多铁器件的开发和应用具有重要意义.     

Exchange bias effect in multiferroic Eu0.75Y0.25MnO3

The exchange bias phenomenon has been investigated in multiferroic Eu_0.75Y_0.25MnO₃. The material shows a weak ferromagnetism with cone spin configuration induced by external magnetic field below 30 K. Consequently, the electric polarization coming from the cycloid spin order below 30 K can be suppressed by external magnetic fields. The magnetic hysteresis loops after cooling in a magnetic field exhibit characteristics of exchange bias below the spin glassy freezing temperature (T_g)∼16 K. The exchange bias field, coercivity field, and remanent magnetization increase with increasing cooling magnetic field. The exchange bias effect is ascribed to the frozen uncompensated spins at the antiferromagnetism/weak ferromagnetism interfaces in the spin-glass like phase.     

High-frequency dielectric properties of glassy As2Se3

The dielectric properties and the ac conductivity of glassy As₂Se₃ have been obtained for a wide frequency range at temperatures between 80 and 300 K. The results are described in terms of a non-Debye-type dielectric response due to a coupling of a polarization response to structural modes.     

顺磁性La2/3Sr1/3MnO3层对Bi0.8Ba0.2FeO3薄膜多铁性能的影响

界面效应在提升异质结构材料的多铁性能方面有着重要的作用. 本文采用脉冲激光沉积技术在SrTiO₃(STO)基片上制备了Bi_0.8Ba_0.2FeO₃(BBFO)/La_2/3Sr_1/3MnO₃(LSMO)异质结. X-射线衍射图谱表明异质结呈现单相外延生长, 利用高分辨透射电镜进一步证实了BBFO为四方相结构. X-射线光电子能谱证实异质结中只存在Fe³⁺ 离子, 没有产生价态的变化, 揭示了异质结铁电和铁磁性的增强与BBFO/LSMO的界面有关. 同时, 测试了磁电阻(MR)和磁介电(MD), 当磁场强度为0.8 T, 温度为70 K时, MR约为-42.2%, MD约为21.2%. 并且发现在180 K时出现磁相的转变. 实验结果揭示出异质界面效应在提升材料的多铁性和磁电耦合效应方面具有超常的优点, 是加快多铁材料实际应用的有效途径.     

Ferroelectric phase transitions in NH4HSeO4 crystals

Results are presented of measurements of spontaneous polarization, electric permittivity and electrical conductance of NH₄HSeO₄ crystals in three principal crystallographic directions from liquid nitrogen temperature to 320 K. NH₄HSeO₄ crystals exhibit ferroelectric properties in the a-axis direction between 106–250 K, whereas in the b-direction they are pyroelectrie over the entire temperature range. The calculated activation energy (1.42 eV) is indicative of semiconducting behavior.     

A polarized micro-Raman study of a 0.65PbMg1/3Nb2/3O3 - 0.35PbTiO3 single crystal

Polarized micro-Raman spectra of a 0.65PbMg_1/3Nb_2/3O₃-0.35PbTiO₃ (0.65PMN-0.35PT) single crystal poled in the [001] direction are obtained in a wide frequency range (50-2000 cm^-1) at different temperatures. The best fit to the Raman spectrum at 77 K is achieved using 17 Lorenzians to convolute into it, and this is proved to be a reasonable fit. According to the group theory and selection rules of overtone and combinational modes, apart from the seven Raman modes that are from first-order Raman scattering, the remaining ones are attributed to being from second-order Raman scattering. A comparison between the experimental results and theoretical predictions shows that they are in satisfactory agreement with each other. Our results indicate that at 77 K the sample belongs to the rhombohedral symmetry with the C_3v⁵ (R3m) space group (Z=1). In our study, on heating, the 0.65PMN-0.35PT single crystal undergoes a rhombohedral to tetragonal to cubic phase transition sequence. The two phase transitions occur at 340 and 440 K, which correspond to the disappearance of the soft mode near 106 cm^-1 recorded in VV polarization and the vanishing of the band around 780 cm^-1 in VH polarization, respectively.     

Temperature-dependent ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films

To investigate temperature-dependent ferroelectric and dielectric properties of ferroelectric films, Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were prepared on Pt-coated silicon substrates by pulsed laser deposition. The ferroelectric and dielectric behaviors have been studied in a wide temperature range from 80 K to room temperature. The saturated polarization (P_sat) decreases with decreasing temperature and decreasing electric field, whereas remnant polarization (P_r) shows a more complex temperature dependence. These results, which can be well explained based on a temperature-dependent charged defects-domain wall interaction model, might be helpful for further understanding the domain switching behavior. Based on these results, an alternative way to investigate temperature-dependent ferroelectric fatigue is proposed and experimentally carried out. The measured fatigue rate is found to be linearly dependent on temperature, consistent with the report on Pb(Zr,Ti)O₃ films. Temperature-dependent dielectric measurements of the films further confirm the above explanation.     

Phonon modes in KH2PO3-type crystals

New infrared reflectivity spectroscopic measurements for polarization of the electric field perpendicular to the ferroelectric axis of KDP in its ferroelectric phase at 7K, are presented and discussed. The optical mode parameters of modes belonging to B₁ and B₂ symmetry are deduced from the experimental infrared reflectivity, together with the fractional number of dipoles pointing in the + c direction.     

Dielectric properties for the ordered-perovskite cuprate Sr2Cu(Re0.69Ca0.31)O6

Dielectric properties are reported on polycrystalline cubic ordered-perovskite cuprate Sr₂Cu(Re_0.69Ca_0.31)O₆ in the frequency range 10 Hz-100 kHz at temperature from 300 to 500 K. Both the dielectric permittivity and dielectric loss factor are found to be frequency and temperature dependent. The enhanced value of the low frequency dielectric permittivity is associated to ionic polarization and interfacial phenomena. The material is found to possess significantly high dielectric permittivity. The calculated ac conductivity suggests semiconducting behaviour for the Sr₂Cu(Re_0.69Ca_0.31)O₆.     

Temperature evolution of the local structure in relaxor ferroelectric Ba(Ti0.7Zr0.3)O3 studied using neutron total scattering analysis

We performed neutron total scattering measurements on relaxor ferroelectric Ba(Ti_0.7Zr_0.3)O₃ from T=350 to 50 K. Using reverse Monte Carlo modeling on the total scattering data in k-space as well as in real space, we show that the off-centering of Ti ion is the source of local polarization at all temperatures. In addition, we estimated that Ti displacement is toward the 〈111〉 direction with a magnitude of about ∼0.11 Å. We also present an evolution of Ti-O bond length distribution across the maximum permittivity temperature, which suggests a freezing of local polarization with decreasing temperature.     

Cs3BiCl6 : Un compose ferroelastique presentant une polarisation remanente a temperature ambiante

X-ray diffraction, calorimetric, dielectric and thermocurrent measurements have been carried out on Cs₃BiCl₆ ceramic samples. Three phase transitions have been detected at T₁ = 395 K, T₂ = 684 K and T₃ = 727 K. The T₃ transition is of ferroelastic-paraelastic type. The low temperature phase is characterized by remanent polarization. The depolarization current results probably from charge traps in potential minima which disappear at rising temperature.     

Nd含量对Bi_(6-x)Nd_xFe_(1.4)Ni_(0.6)Ti_3O_(18)多晶材料多铁性的影响

采用柠檬酸-硝酸盐法制备了Bi_(6-x)Nd_xFe_(1.4)Ni_(0.6)Ti_3O_(18)(BNFNT-x,x=0.00,0.10,0.20,0.25和0.30)前驱液,再经过干燥、烧结过程制备了单相多晶材料.研究发现,少量Nd掺杂有助于提高样品的铁电性能,BNFNT-0.25样品的铁电性能(2Pr)最大,约达到19.7μC/cm~2.室温下BNFNT-0.20样品磁性能(2Ms)最大约达到4.132 emu/g(1 emu/g=10–3 A·m~2/g).变温介电损耗结果表明Nd掺杂降低了Fe~(3+)和Fe~(2+)间的电子转移或跃迁的激活能.X射线光电子能谱结果表明小量Nd掺杂有助于增强Bi离子稳定性,对改善样品的铁电性能有积极意义.     

碱金属Li,Na,K,Rb,Cs,Fr的ns2S1/2,np2P1/2,3/2, nd2D3/2,5/2 和nf2F5/2,7/2 里德伯能级理论研究

本文在多通道量子数亏损理论(MQDT)框架下,利用相对论多通道理论(RMCT),分别在冻结实近似、 考虑Δl=-1的偶极极化效应、Δl=+1的偶极极化效应、Δl=± 1的偶极极化效应、伸缩模效应以及同时考虑偶极极化效应和伸缩模效应等不同层次近似下,系统地计算了碱金属Li, Na, K, Rb, Cs和Fr七个里德伯系列的能级,即ns²S_1/2, np²P_1/2, np²P_3/2, nd²D_3/2, nd²D_5/2, nf²F_5/2 和nf²F_7/2.计算结果表明,电子关联效应对碱金属原子的里德伯能级的影响很大.总的来说,偶极极化效应比伸缩模效应重要,而在偶极极化效应中, Δl = + 1的偶极极化效应比Δl = - 1的偶极极化效应重要.但对于Na的ns²S_1/2,(nd²D_3/2,nd²D_5/2)里德伯系列的能级,和Li的(np²P_1/2,np²P_3/2)里德伯系列的能级,是伸缩模效应比较重要.     

外延PbZr0.4Ti0.6O3薄膜厚度对其铁电性能的影响

从Landau-Devonshire唯象理论出发，考虑到晶格失配导致的NFDA3位错应力场与极化场的耦合，研究了在SrTiO₃衬底上外延生长的PbZr_0.4Ti_0.6O₃薄膜厚度对其自发极化强度、电滞回线的影响. 结果表明，产生刃位错的PbZr_0.4Ti_0.6O₃薄膜临界厚度为～1.27nm，当薄膜厚度大于临界厚度时，在所形成的位错附近，极化强度出现急剧变化，形成自发极化强度明显减弱的“死层”；随着薄膜厚度的减小，位错间距增大，“死层”厚度与薄膜总厚度之比增加. 由薄膜电滞回线的变化情况可知，其剩余极化强度随着薄膜厚度的减小而逐渐减小. 关键词： 铁电薄膜 自发极化强度 电滞回线 位错     

Asymmetry and polarization of the recoil nucleus in the reaction μ−+12C→12B+vμ

Expressions for the resultant polarizations of ¹²B nuclei obtained in the experimental arrangement of Louvain-Saclay collaboration are derived. How these polarizations are related to the average polarization, the longitudinal polarization and the nuclear recoil asymmetry is investigated.     

铁电磁体Pb(Fe1/2Nb1/2)O3相变特征

借助与示差扫描量热法、磁化率测量、电子自旋共振、铁电与介电性质测量及电子衍射系统地研究了Pb(Fe_1/2Nb_1/2)O₃(PFN)的电、磁性质和相变特征．结果表明发生在380K附近的顺电－铁电转变和发生在145K附近的顺磁 反铁磁转变分别为一级相变和二级相变或弱一级相变．在室温下,PFN的剩余极化与矫顽场分别为11.5μC/cm²和3.04kV/cm.介电测量表明PFN的顺电-铁电相变为弥散型相变．其弥散指数为1.62.电子衍射表明Fe³⁺与Nb⁵⁺离子在B位置上是无序分布的,正是这种与无序分布相关联的成分涨落导致铁电相变的弥散性. 关键词：     

Magnetic properties of GdxY1−xNi5 alloys

Magnetic measurements have been performed on single crystals of Yni₅ and GdNi₅ and on polycrystalline samples of Gd_xY_1?xNi₅. YNi₅ is a Pauli paramagnet, the susceptibility of which is enhanced by exchange and slightly temperature dependent (χ = 23 × 10^?4 emu/mole at 4.2 K). In GdNi₅, where the anisotropy is very weak, a polarization of the d band opposite to the Gd magnetization is observed. At 4.2 K, this polarization is 0.16 ± 0.02μ_B/Ni. The decrease of this polarisation with decreasing magnetic interactions has been studied in Gd_xY_1?xNi₅-type alloys. It is concluded that the polarization is more homogeneous than that of cobalt observed in Gd_xY_1?xCo₂.