A 57Fe and 119Sn Mössbauer study of BaFe4Sn2O11

The Mössbauer spectrum of BaFe₄Sn₂O₁₁ has been recorded for both ⁵⁷Fe and ¹¹⁹Sn isotopes at a variety of temperatures. In the paramagnetic state the ⁵⁷Fe spectra are interpreted in terms of three iron environments. Magnetic ordering begins at 77 K and is virtually complete by 4.2 K to give an average magnetic hyperfine field of 504 kG. The ¹¹⁹Sn spectra also reflect the magnetic ordering and a magnetic hyperfine field of 45 kG is transferred to the tin nuclei.     

Ni54Fe19Ga27单晶的形状记忆和超弹性

本文研究了单晶Ni₅₄Fe₁₉Ga₂₇不同方向的形状记忆效应、超弹性和磁性. 研究发现,单晶样品具有良好的双向形状记忆效应.不同晶体学方向的相变应变随着热循环次数的变化而改变. 在外应力作用下,通过应力诱发马氏体相变,样品在[001],[110],[111]方向分别产生了3.3%, 2% 和3%的可回复应变平台.磁性测量结果表明马氏体的磁晶各向异性能约为4.8× 10⁵ erg/cm³,远远小于变体孪生所需机械应力能,因此磁场的作用是使磁矩发生转动而不是使孪晶界移动, 成功揭示了不能在NiFeGa中获得大磁感生应变的物理根源.     

Interactions hyperfines magnetiques et mecanisme de compensation de la charge pour les ions d'impurete 119Sn4+ dans Cr2O3

Mössbauer spectra of Cr₂O₃ sesquioxide doped with ¹¹⁹Sn⁴⁺ ions lead to the hypothesis of three non-equivalent magnetic sites for Sn⁴⁺. The identification of these sites has allowed to estimate the relative importance of exchange interactions between Cr³⁺ ions along [111] axis and in (111) plane.     

Zeeman effect on the 4E level of Mn2+ in ZnS

The fine structure lines of the ⁴E level of Mn²⁺ in ZnS have been studied under a high magnetic field. Experiments were performed with the magnetic field parallel to the [111]_W crystallographic axis. The computed positions and intensities for polarized and non-polarized light are in agreement with the experiments.     

Hyperfine spectroscopic study of the metal-insulator transition in V2O3

The metal-insulator (M-I) transition in vanadium sesquioxide V₂O₃ has been investigated by time differential perturbed angular correlation measurements of the electric fieldgradient (EFG) and the magnetic hyperfine field at dilute¹¹¹Cd impurities. The EFG undergoes a first-order change at the M-I transition at T_t=160 K, but does not reflect the high temperature resistivity anomaly. The increase of the EFG with temperature in the metallic phase can be attributed to thermal variations of the oxygen sublattice. The temperature dependence of the magnetic hyperfine field in the insulating phase follows a Brioullin function with a saturation value of H_hf(O)=15 KOe and an extrapolated Neel temperature, which, depending on the impurity concentration, varies between 188 and 230 K.     

Isolated A+ states in boron doped silicon under high compressive stress

Long-wavelength photoconductivity spectra of isolated A⁺ states in B-doped Si under uniaxial [111] or [100] compressive stress have been studied at 1.5 K. Besides, far-infrared photoconductivity and adsorption spectra of acceptor states in the crystal have been measured to obtain the experimental ionization energy of the acceptor. The binding energy of the second hole in the A⁺ center is rather close to, and a tittle less than, the theoretical value of 0.055 of the effective Rydberg obtained with the effective mass approximation.     

Magnetic anisotropy of Sm2Fe14B single crystal

Magnetization measurements have been made on a Sm₂Fe₁₄B single crystal in magnetic fields up to 140 kOe. The easy direction of the magnetization lies along [100] in the tetragonal structure P4₂/mnm. Magnetic anisotropy energies at 290K along [110] and [001] have been estimated to be 5.8×10⁶ and 1.1×10⁸ erg/cm³, respectively, both becoming much larger at lower temperature. No evidence of the spin canting of Nd₂Fe₁₄B type is observed even at 4.2K.     

Gd3Ga5-xAlxO12单晶低温磁性的实验研究

采用提拉法制备了Gd₃Ga_5-xAl_xO₁₂(以下简称GGAG)单晶,经X射线粉末照相证实为石榴石型单相结构。在T=1.5—77K温区里,场强从45—60000Oe范围内,沿[111]晶轴方向测量了磁化强度和磁化率,并与本文所用钇镓石榴石Gd₃Ga₅O₁₂(以下简称GGG)和文献[1,2]报道的GGG单晶数据作了比较。实验结果表明:(1)在场强H<500Oe时,磁化率X与温度T的关系遵守居里-外斯定律,其有序温度T_N远低于1.5K;(2)H>10000Oe时,该材料的磁化强度(或磁化率)比GGG高3—4％;(3)在H～30000Oe时,特定温度T_min对应的临界场H_c比GGG约高4000Oe。实验研究表明,采用离子代换法寻找新的性能良好的磁致冷剂的途径是可行的。 关键词：     

Cooperative Jahn-Teller ordering and magnetic structure - pyridine-N-oxide complexes of Cu2+: Cu(ONC5H5)6X2 [X = ClO−4, BF−4]1

Pyridine-N-oxide complexes CuL₆X₂ [L: C₅H₅NO ; X: BF^?₄(I), ClO^?₄(II)] with trigonal unit cells at 298 K and CuL₆²⁺ octahedra in approximately cubic primitive arrangement exhibit dynamic to static Jahn-Teller phase transitions to pseudopetragonal modifications at ≈ 90 K (I) and below 70 K (II). It found by single crystal and powder EPR-spectroscopy, that the low-temperature β-phases are characterised by a cooperative Jahn-Teller order of elongated CuO₆-octahedra, which is ferrodistortive (I) and antiferrodistortive (II) respectively. These orbital order patterns are in agreement with the symmetries of the magnetic structures found for the two complexes.     

Temperature studies of Fe−Mn nodules

Sea nodules were extensively studied over a wide range of temperature ranging from 77K to 1175K using Mossbauer effect and ESR. The Mossbauer studies of the nodules at 77K and room temperature show a quadrupole doublet whereas at higher temperatures magnetic spectra were obtained starting at around 875K which ultimately gives a hyperfine field of around 390 KOe at 1175 K. The Mossbauer spectra recorded at 30K did not show any significant change in the room temperature spectra. The analysis of the spectra upto 775K showed two positions of Fe^R+, vie., octahedral and tetrahedral which were varified by ESR of the diluted samples.     

Spin reorientation and magnetization anomaly in Er2Fe14B and Tm2Fe14B

Static magnetic measurements have been carried out on single crystals of Er₂Fe₁₄B and Tm₂Fe₁₄B in a temperature range between 77 and 590 K. Spin reorientation phenomena have been found in both compounds slightly above room temperature. In Er₂Fe₁₄B, the easy direction of magnetization changes from [100] to [001] at 316 K as temperature increases, and Tm₂Fe₁₄B from [100] to [001] at 310 K. Anomalously large anisotropy in the saturation magnetization has been detected around the spin reorientation temperature.     

Proton NMR study of α-MnH0.06

Proton nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates for the solid solution α-MnH_0.06 have been measured over the temperature range 11-297 K and the resonance frequency range 20-90 MHz. A considerable shift and broadening of the proton NMR line and a sharp peak of the spin-lattice relaxation rate are observed near 130 K. These effects are attributed to the onset of antiferromagnetic ordering below the Néel temperature T_N≈130 K. The proton NMR line does not disappear in the antiferromagnetic phase; this suggests a small magnitude of the local magnetic fields at H-sites in α-MnH_0.06. The spin-lattice relaxation rate in the paramagnetic phase is dominated by the effects of spin fluctuations.     

A pulsed EPR study of the catalytic oxidation of CO over Cu/SnO2

The role of the Cu(II) in the catalytic oxidation of CO over Cu/SnO₂ with low Cu(II) content was studied by continuous wave EPR, electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectroscopes. Three methods were employed for introducing the copper: (i) by coprecipitation, (ii) impregnation onto SnO₂ gel and (iii) impregnation onto calcined SnO₂. Two types of Cu(II) species were identified in these calcined Cu/SnO₂ materials. Those belonging to the first type, termed B and C, exhibit highly resolved EPR spectra with well defined EPR parameters and are located within the bulk of the oxide. The other group comprises a distribution of surface Cu(II) species with unresolved EPR features and are referred to as S. While the latter were readily reduced by CO the former required long exposures at high temperatures (> 673 K). The specific interactions of the different Cu(II) species with CO were investigated through the determination of the¹³C hyperfine coupling of enriched¹³CO. The ESEEM spectra of calcined samples, generated either by coprecipitation or impregnation, show after the adsorption of CO signals at the Larmor frequencies of^{117, 119}Sn and¹³C and at twice these Larmor frequencies. Although these signals indicate that^{117, 119}Sn and¹³C are in the close vicinity of Cu(II), they cannot provide the hyperfine couplings of these nuclei. This problem was overcome by the application of the HYSCORE experiment. The 2D HYSCORE spectra show well resolved cross peaks which provide the hyperfine interaction of these nuclei. Simulations of the HYSCORE spectra yield for^{117, 119}Sn an isotropic hyperfine constant,a _iso, of ±4.0 MHz and an anisotropic component,T _?, of ±2.0 MHz. Pulsed ENDOR spectra also showed^{117, 119}Sn signals which agree with the above values. The¹³C cross peaks yielda _iso=±1.0 MHz andT _?=±2.0 MHz. Similar C cross peaks were observed in spectra of calcined Cu/SnO₂ after the adsorption of CO₂. Based on the same hyperfine couplings in the samples exposed to¹³CO and¹³CO₂ the signals were assigned to surface carbonate species generated by part of the Cu(II) S type species rather then by species B and the role of the Cu(II) in the oxidation process is discussed.     

Magnetic anisotropy of the antiferroquadrupole phase in Ce0.50La0.50B6

Magnetic phase diagrams for antiferroquadrupole (AFQ) phase II and antiferromagnetic (AFM) phase III in Ce0.50La0.50B6 with a Gamma(8) ground state have been investigated by ultrasonic measurements. The hybrid magnet (Gama) in the National Institute for Materials Science was employed for high-field measurements up to 30 T and a 3He-4He dilution refrigerator was used for low-temperature experiments down to 20 mK. The phase boundary from paramagnetic phase I to AFQ phase II under [001] magnetic fields closes at H(I-II) approximately 29 T, while the boundary is still open under fields along the [110] and [111] directions even up to 30 T. This anisotropic character of phase II in fields is consistent with the theoretical calculation based on the O(xy)-type AFQ ordering. We also found that AFM phase III reduces considerably in fields turning from the [001] to [110] and [111] directions.     

Ferromagnetic resonance of magnetic field oriented Fe3O4 nanoparticles in frozen ferrofluids

Measurements of the orientational dependence of the ferromagnetic resonance (FMR) spectra are made on Fe₃O₄ nanoparticles in ferrofluids solidified in dc magnetic fields. The in field solidification locks the direction of magnetization parallel to the direction of the cooling field enabling measurements as a function of orientation with respect to the direction of magnetization in the frozen state. The g value of the FMR spectra at 77 K is 2.16 and the anisotropy constant is −1.23 J/m³. A marked reduction of the difference between the field position in the parallel and perpendicular orientation onsets on warming to 140 K well below the melting temperature of the fluid carrier and is attributed to the onset of fluctuations in the direction of the magnetization in the solid phase. The phase transition of the magnetic symmetry observed in bulk Fe₃O₄ occurs at much lower temperature in the nanoparticles.     

Effect of hydrostatic pressure on the magnetocrystalline anisotropy constant K1 of iron and nickel

Previous values of the pressure dependence of the magnetocrystalline anisotropy constant K₁ of iron and nickel were revised. These values of K₁^?1 ($\text{dK}{}_1\text{dp}$) depend on the magnetic field for iron, and do not for nickel. The value in iron extrapolated to infinitely strong magnetic field is ?7.8×10^?6 bar^?1 at room temperature and ?7.3×10^?6 bar^?1 at 77K, and in nickel at 15 KOe is ?7.5×10^?6 bar^?1 at room temperature and ?2.8×10^?6 bar^?1 at 77K.     

Exchange interaction and spin dynamics in pentanuclear clusters, Cu3Ln2(ClCH2COO)12(H2O)8 (Ln = Nd3+, Sm3+, Pr3+)

New compounds, [Cu₃Ln₂(ClCH₂COO)₁₂(H₂O)₈]·2H₂O with Ln = Nd³⁺ (I), Sm³⁺ (II), Pr³⁺ (III), built up of pentanuclear clusters were synthesized and studied by means of X-ray analysis and electron paramagnetic resonance (EPR). X-ray data show that all compounds are isostructural and the pentanuclear clusteres may be considered as a linear system with alternating Cu(II) and Ln(III) ions: Cu(2)-L¹-Ln-L²-Cu(1)-L²-Ln-L²-Cu(2) with L¹ and L² being bridging fragments and Cu(1) and Cu(2) being structurally nonequivalent copper complexes. EPR studies demonstrate that in the temperature range of 100–293 K the signals due to only one type of the copper complexes are observed in the spectra of I–III. AtT<100 K the spectral temperature dependence is nontrivial. AtT<30 K new signals are detected in the spectra of I and II. The temperature dependence of the EPR spectra is interpreted under the assumption that the parameter of the exchange interaction Cu(2)-Ln considerably exceeds the parameter of the interaction Cu(1)-Ln. EPR spectra are calculated for the fragments of five paramagnetic centers in the frames of the model taking into account the nonequivalence of two copper complexes, short longitudinal and transverse paramagnetic relaxation times of the rare-earth ions at room temperature and the change of the relaxation rates when the temperature decreases. The results of the calculations show that it is possible to obtain information about the interactions in the system on the basis of the analysis of the temperature dependence of the EPR spectra of the central copper complex. The parameter of the isotropic part of the exchange interaction between copper and neodymium ions (for the interaction Cu(2)-Nd) is estimated as about 15 cm⁻¹. A considerable rearrangement of the spin states when the temperature changes is found for all complexes.     

Evidence for low-temperature internal dynamics in Cu12As4S13 according to copper NQR and nuclear relaxation

^63,65Cu nuclear quadrupole resonance (NQR) was applied to study the natural mineral Cu₁₂As₄S₁₃ (tennantite) in the temperature range 4.2–210 K. The obtained results point to the presence of field fluctuations caused by internal motions in tennantite. Consistently with the crystal structure, the experimental data can be described by an occurrence of a magnetic phase transition, which takes place near 65 K. The low-temperature phase is characterized by Cu(II) electron magnetic moments freezing in the form of a spin-glass-like constitution.     

K3C60在200K附近的取向相变机理

研究了K₃C₆₀单晶薄膜在200K附近的导带结构.样品温度为190K时,同步辐射角分辨光电子谱能够观察到[111]方向有规律的能带色散.而在220K附近色散不存在.这一实验结果与K₃C₆₀在200K存在取向相变相符合.用反铁磁Ising模型对实验结果进行了分析.结果表明,K₃C₆₀在200K的相变是由低温下的一维无序取向结构转变为200K以上的双取向结构畴与无序分子(约占40 关键词： 3C₆₀')" href="#">K₃C₆₀ 取向相变机理     

Non-colinear magnetic structure of U3P4 and U3As4

Neutron diffraction experiments have been performed on single crystal samples of U₃P₄ and U₃As₄. The magnetic ordering is found to be a non-collinear three axial structure in which magnetic moments of U⁴⁺ ions are tilted from the [111] axis by an angle of about twenty degrees within (110) planes.