Field dependence of the NMR frequency in BaFe12O19

The field dependence of the NMR frequencies for the “a” and “c” sites of BaFe₁₂O₁₉ has been re-examined. In a polycrystalline grain-oriented sample at 4.2 K no indication for a deviation from the antiparallel orientation of the sublattice magnetization is found. For Fe⁵⁷, a gyromagnetic ratio of γ/2π = (138.0 ± 0.3) × 10⁴ Hz/T is found in fair agreement with the generally accepted value of (137.7 ± 1.0) × 10⁴ Hz/T.     

The influence of magnetic impurities on thermodynamic characteristics of antiferromagnets

The thermodynamic behaviour of an impurity spin interacting with both one and another sublattice of an antiferromagnet with the “easy axis” anisotropy type is considered in the framework of phenomenological theory. The direct interaction between impurity magnetic moments is assumed to be negligible. The ground (pure Neel) state of the host lattice is shown to become unstable in the presence of an impurity. At this condition a local magnetic distortion being “frozen” in the vicinity of an impurity takes place and gives rise to appearance of a local ferromagnetism vector perpendicular to the anisotropy axis. Such phenomenon may be described as a local phase transition within the host lattice.     

Mössbauer study of the magnetic high-Tc superconductor GdBa2Cu3O7−δ

The magnetic ordering of the Gd sublattice in superconducting GdBa₂Cu₃O_7-δ is studied by Mössbauer spectroscopy using the 86.5-keV gamma resonance of ¹⁵⁵Gd. Below the Néel temperature of T_N ≌ 2.4 K, the magnetic hyperfine field at the Gd nucleus reflects the increasing local sublattice magnetization extrapolating to a saturation value of B_eff(T=0 K) ≌ 31.5 T. The effective magnetic hyperfine field is found to be parallel to the main axis of the electric-field-gradient tensor, which is characterized by an asymmetry parameter of n = 0.40 ± 0.05. The observed isomer shift and the value of B_eff are typical for trivalent Gd compounds with negligible conduction-electron contributions.     

Induced moment antiferromagnet in an external magnetic field—II: Green's function theory

A two level two sublattice antiferromagnetic induced moment system is examined in the Green's function theory in the random phase approximation. Attentions are drawn to the effects of an externally applied magnetic field. It is shown that a “soft mode” behavior occurs at the transition field similar to that observed at the transition temperature in the absence of a field. The transition becomes first order in the RPA. The sublattice magnetizations and the transition field are significantly lowered compared to the molecular field theory results.     

Magnetic order observed in Er6Mn23H21 using the Mössbauer effect

The ferrimagnetic compound Er₆Mn₂₃ and its hydride Er₆Mn₂₃H₂₁ have been investigated using the ¹⁶⁶Er and ⁵⁷Fe (as dilute impurity) Mössbauer resonances. For the hydride a small ⁵⁷Fe magnetic hyperfine field is observed below 85 K indicating magnetic ordering. This observation is contrary to the previous assumption that the Mn sublattice must be non-magnetic. The ¹⁶⁶Er results demonstrate that the Er³⁺ magnetic moment is not “quenched” by crystal electric fields on hydrogenation.     

Dynamics of antiferromagnets exposed to ultrashort magnetic field pulses

An ultrashort magnetic field pulse generated by a laser pulse can excite spin oscillations in a “pure” antiferromagnet even when a weak ferromagnetic moment is absent. This mechanism features the spin oscillation amplitude independent of the sublattice structure and parameters of the ferromagnet and is governed only by the field pulse parameters.     

Hyperfine interactions and local magnetic ordering in Zr(Fe1-xCox)2 (0⩽x⩽0.2)

In the pseudobinary intermetallic compounds Zr(Fe_1-xCo_x)₂ (0?x?0.2) the hyperfine fields of all nuclei present are investigated by means of Mössbauer effect and NMR. While for the “nonmagnetic” site the Zr-hyperfine field depends on the configuration of the nearest Fe, Co neighbours, no such effect is observed for the hyperfine fields on the “magnetic” sites. A large pseudodipolar interaction is observed for the Fe and Co atoms, from which the coexistence of several directions of magnetization can be deduced. The easy direction seems to be determined by the respective Fe/Co configuration.     

Ordering of vacancies in LiAl

The intermetallic compound β-LiAl contains relatively high concentrations of constitutional vacancies on the Li sublattice for Li deficient compositions. Neutron diffraction measurements on a Li_0.486Al_0.514 single crystal show that these vacancies order on every tenth (840) plane at 97K. This vacancy ordering is responsible for the “100K” anomaly observed in several physical properties.     

A weakly coupled magnetic sublattice in NaMnFeF6, observed by Mössbauer spectroscopy and magnetic measurements

The thermal variations of the hyperfine fields of the two iron sites (1a, 2d) of NaMnFeF₆ (S.G.P321) indicate a weak magnetic coupling of the (2d) sublattice to the net magnetization, and suggest a magnetic phase transition near 32 K. The Mössbauer and magnetization data are consistent with a ferrimagnetic structure (T_N=44.6 ± 0.5 K) with low-temperature compensation point.     

Spontaneous spin-reorientation transition in (NdSmDy)(FeCo)B alloys

A magnetic transition accompanied by a sharp decrease in magnetization has been detected in an (NdSmDy)(FeCo)B alloy at temperature T = 110 K. It is found that the sample undergoes a spin-reorientation transition accompanied with a change in the type of magnetic anisotropy. The “easy axis”-type anisotropy corresponds to high temperatures T > 110 K. A magnetic structure of the type of “the cone of easy-magnetization axes” forms at low temperatures T < 110 K.     

Possibility of two-stage flux penetration in antiferromagnetic superconductors

An induced ferromagnetism in antiferromagnetic superconductors is possible, caused by a magnetic structure of vortex lines appearing in an external magnetic field strong enough to “flip over” the spins in the vortex core from their antiferromagnetic configuration. If the magnetic field is less than the “flip over” field the vortex line is entirely in the antiferromagnetic phase. Therefore the vortex interaction with the surface of a sample is altered when an applied magnetic field exceeds the “flip over” field. This mechanism makes the appearance of a new energy barrier that strongly influences the flux penetration possible. An estimation of the second critical entry field is made for DyMo₆S₈.     

Magnetic properties and spin order of Sr7Fe10O22

The magnetic properties of Sr₇Fe₁₀O₂₂ have been investigated by means of magnetic susceptibility and Mössbauer absorption measurements. This compound proved to be antiferromagnetic with a Néel temperature T_N = 425 K; the magnetic susceptibility is constant from the lowest measuring temperature (78 K) up to T_N.The Mössbauer measurements and the analogies with “brownmillerite” type compounds indicate that iron ions occupy one octahedral and two tetrahedral different sites. An antiferromagnetic spin configuration with moments lying in the ab plane appears to be consistent with the experimental results. A small spontaneous magnetic moment was observed at room temperature with features resembling those of strontium hexaferrite; a weak ferromagnetic behavior can not however be excluded taking into account the aforementioned susceptibility behaviour.     

One-loop Effective Potential in Higher-dimensional Yang-Mills Theory

We study the effective action in Euclidean Yang-Mills theory with a compact simple gauge group in one-loop approximation assuming a covariantly constant gauge field strength as a background. For groups of higher rank and spacetimes of higher dimensions such field configurations have many independent color components taking values in Cartan subalgebra and many “magnetic fields” in each color component. In our previous investigation it was shown that such background is stable in dimensions higher than four provided the amplitudes of “magnetic fields” do not differ much from each other. In the present paper we exactly calculate the relevant zeta-functions in the case of equal amplitudes of “magnetic fields”. For two “magnetic fields” with equal amplitudes the behavior of the effective action is studied in detail. It is shown that in dimensions d = 4,5,6,7 (8), the perturbative vacuum is metastable, i.e., it is stable in perturbation theory but the effective action is not bounded from below, whereas in dimensions d = 9,10,11 (8) the perturbative vacuum is absolutely stable. In dimensions d = 8 (8) the perturbative vacuum is stable for small values of the coupling constant but becomes unstable for large coupling constant leading to the formation of a non-perturbative stable vacuum with nonvanishing “magnetic fields”. The critical value of the coupling constant and the amplitudes of the vacuum “magnetic fields” are evaluated exactly. PACS numbers: 11.10Kk, 11.15Tk, 11.15.-q, 12.38Aw, 12.38Lg     

112型铁基化合物EuFeAs2的单晶生长与表征

铁基超导体中含有一类特殊的112型结构化合物,其层状结构中含有一层锯齿形的As链构型.本文报道了用CsCl助熔剂法生长新型铁基112型EuFeAs₂母体单晶的具体方法,以及对该单晶的结构和物性的详细表征.通过能量色散X射线能谱扫描对单晶样品进行的化学成分分析,以及单晶X射线衍射的结构解析,确定该单晶样品属于EuFeAs₂相,结构精修得到EuFeAs₂具有空间群为Imm2（No.44）的正交晶体结构,晶格常数分别是a=21.285（9）Å,b=3.9082（10）Å,c=3.9752（9）Å.通过低温电阻测量,发现在110 K附近和46 K附近存在两个异常电阻跳变.进一步分析表明,110 K附近存在两个邻近的相变,这两个相变与铁基母体材料中常见的结构相变和Fe²⁺的反铁磁相变相符合.结合磁化率测量分析,可知46 K附近的相变属于Eu²⁺的反铁磁相变.     

Isolated structures in two-dimensional optical superlattice

Overlaying commensurate optical lattices with various configurations called superlattices can lead to exotic lattice topologies and, in turn, a discovery of novel physics. In this study, by overlapping the maxima of lattices, a new isolated structure is created, while the interference of minima can generate various “sublattice” patterns. Three different kinds of primitive lattices are used to demonstrate isolated square, triangular, and hexagonal “sublattice” structures in a two-dimensional optical superlattice, the patterns of which can be manipulated dynamically by tuning the polarization, frequency, and intensity of laser beams. In addition, we propose the method of altering the relative phase to adjust the tunneling amplitudes in “sublattices”. Our configurations provide unique opportunities to study particle entanglement in “lattices” formed by intersecting wells and to implement special quantum logic gates in exotic lattice geometries.     

金属间化合物PrMn6Sn6的结构、磁性与119Sn穆斯堡尔谱研究

采用电弧熔炼法制备了金属间化合物PrMn₆Sn₆.X射线衍射表明该化合物具有HoFe₆Sn₆型(空间群为Immm)晶体结构.磁测量表明该化合物为铁磁性，居里温度为325　K.在15—360　K范围内测量了¹¹⁹Sn穆斯堡尔谱，得到了8个Sn原子晶位的转移超精细场随温度的变化，并且讨论了Mn亚晶格与Pr亚晶格的磁有序方向. 关键词： 6Sn₆')" href="#">PrMn₆Sn₆ 穆斯堡尔谱 磁结构     

Co (III) ions high-spin configuration in nonstoichiometric Co3O4 films

Optical spectra in UV, visible and near IR wave-length range have been measured and the SCF X_∝SW method calculation of Co₃O₄ electronic structure has been carried out. The nature of absorption bands observed has been revealed. As shown, the principal peculiarity of the nonstoichiometric films electronic structure compared with “bulk” samples is a stabilization of Co(III) ions high-spin configuration in Co₃O₄ spinel octahedral sublattice.     

Magnetic order in RCr\mathsf{_{2}}Si\mathsf{_{2}} intermetallics

The magnetic structure and ordering temperatures of three intermetallic compounds which crystallize in the tetragonal ThCr₂Si₂ structure, TbCr₂Si₂, HoCr₂Si₂ and ErCr₂Si₂, have been determined by neutron diffraction, differential scanning calorimetry and magnetization measurements. The Cr-sublattice orders anti-ferromagnetically with Néel temperatures of 758 K for TbCr₂Si₂, 718 K for HoCr₂Si₂ and 692 K for ErCr₂Si₂. Chromium atoms located at 4d crystallographic sites are aligned anti-parallel along the c-axis, with G_ZCr magnetic modes. In contrast with metallic bcc Cr, the refined room temperature value of the ordered Cr moment is anomalously large for all three compounds. No long range magnetic order of the R sublattice in TbCr₂Si₂ and HoCr₂Si₂ is observed, whilst the Er sublattice in ErCr₂Si₂ orders independently of the Cr sublattice below 2.4 K with moments ferromagnetically aligned in the basal plane.Received: 4 November 2003, Published online: 30 January 2004PACS: 75.25. + z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) - 75.30.Cr Saturation moments and magnetic susceptibilities - 75.50.Ee Antiferromagnetics     

Incoherent Kondo ground state in CeCu1.54Si1.46

Measurements of the magnetic susceptibility, magnetization, specific heat and electrical resistivity on a new Kondo lattice compound CeCu_1.54Si_1.46 have revealed an antiferromagnetic phase transition at 6.9 K. The analysis of the specific heat demonstrates that this compound is a moderately heavy electron system with strong spin fluctuations. Based on the resistivity result, we maintain that the coherence between Kondo states at Ce sites is hindered by the disorder in the Cu and Si sublattice in this non-stoichiometric compound.     

Size and thickness effect on magnetic structures of maghemite hollow magnetic nanoparticles

The effect of surface anisotropy on the magnetic ground state of hollow maghemite nanoparticles is investigated using atomistic Monte Carlo simulation. The computer modeling is carried on hollow nanostructures as a function of size and shell thickness. It is found that the large contribution of the surface anisotropy imposes a “throttled” spin structure where the moments located at the outer surface tend to orient normal to the surface while those located at the inner surface appear to be more aligned. For increasing values of surface anisotropy in the frame of a radial model, the magnetic moments become radially oriented either inward or outward giving rise to a “hedgehog” configuration with nearly zero net magnetization. We also show the effect of the size of hollow nanoparticle on the spin behavior where the spin non-collinearity increases (for fixed value of surface anisotropy) as the diameter of the hollow nanoparticle increases due to the significant increase in surface-to-volume ratio, the thickness being constant. Moreover, the thickness of the hollow nanoparticle shell influences the spin configuration and thus the relation between surface anisotropy and the size or the thickness of the hollow nanoparticle is established.