超声波在磁性液体中的传播特性

本文报道超声波在磁性液体中的传播特性,首先研究了磁化状态建立与消失的弛豫过程,弛豫时间约20min,为了保证平衡状态下进行测量,所有数据均为附加磁场60min后进行的测量,然后在f=4HMz下,测量了φ=0,π/2的声速与磁场的关系,声速与磁性液体中磁性颗粒浓度的关系,对有关理论进行了解释。 关键词：     

合金团簇(FeCr)n中的非共线磁序和自旋轨道耦合效应

利用密度泛函理论对合金团簇(FeCr)_n (n≤6)的几何结构、稳定性和磁性进行了系统的研究. 研究结果表明, 对n≤3的合金团簇, 其基态具有共线的反铁磁序; 而对于n≥4 的合金团簇, 其基态具有非共线磁序, 因此在n=4时体系发生了共线磁序向非共线磁序的“相变”. 此外, 虽然3d过渡金属原子中电子的自旋轨道耦合效应比较弱, 但计算结果表明对于某些小尺寸的合金团簇其轨道磁矩不能忽略. 对非共线磁性团簇的成键性质以及产生磁序“相变”的物理起源进行了详细讨论. 关键词： n合金团簇')" href="#">(FeCr)_n合金团簇 密度泛函理论 非共线磁序 自旋轨道耦合效应     

Anomalous magnetic relaxation in thin Pd<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript> films

The magnetic relaxation in Pd_0.99Fe_0.01 films, which have the thicknesses that are practically important for cryoelectronics (25 and 40 nm), is detected and experimentally studied. The relaxation is shown to be substantial only in thin films. The magnetization relaxation is found to be well described by the sum of two exponential functions with characteristic times that differ by an order of magnitude from each other. The characteristic relaxation time and the ratio of the contributions of two relaxations depend on temperature. The activation energies of the relaxation processes are determined. The activation volume is shown to correspond to a 20-nm ferromagnetic cluster. The results obtained agree with the model of two-component magnetization in thin PdFe films [6].     

Dipolar Relaxation of Multiple Quantum NMR Coherences as a Model of Decoherence of Many-Qubit Coherent Clusters

Dipolar relaxation of multiple quantum (MQ) nuclear magnetic resonance (NMR) coherence is investigated on the evolution period of the MQ NMR experiment in chains of ¹⁹F nuclei in a single crystal of calcium fluorapatite. The dependence of the relaxation time of the MQ coherence of the second order on the size of the coherent spin cluster formed on the preparation period is obtained. The dipolar relaxation of MQ NMR coherences is considered as a model for the investigation of decoherence of quantum states of many-qubit spin clusters.     

Micropore size analysis in oil-well cement by proton nuclear relaxation

The durability of cement depends mainly on cement microstructure parameters such as pore size distribution. In this study, we have observed pore size distribution in a Class G oil-well cement immersed for 1 year in brine at T = 293 K, P = 10(5) Pa and in a down-hole condition at T = 353 K, P = 7.10(6) Pa by proton nuclear magnetic spin-lattice relaxation (1H-NMR). After 4 months of immersion at T = 293 K, P = 10(5) Pa, the distribution of discrete relaxation rates reveals the fractal feature of the pore size distribution, whereas after 1 year at T = 353 K, P = 7.10(6) Pa, a dispersed distribution is still observed.     

Magnetic moments and adiabatic magnetization of free cobalt clusters

Magnetizations and magnetic moments of free cobalt clusters Co(N) (12 < N < 200) in a cryogenic (25 K < or = T < or = 100 K) molecular beam were determined from Stern-Gerlach deflections. All clusters preferentially deflect in the direction of the increasing field and the average magnetization resembles the Langevin function for all cluster sizes even at low temperatures. We demonstrate in the avoided crossing model that the average magnetization may result from adiabatic processes of rotating and vibrating clusters in the magnetic field and that spin relaxation is not involved. This resolves a long-standing problem in the interpretation of cluster beam deflection experiments with implications for nanomagnetic systems in general.     

Phonon-assisted tunneling in the quantum regime of Mn12 acetate

Longitudinal or transverse magnetic fields applied on a crystal of Mn12 acetate allows one to observe independent tunnel transitions between m = -S+p and m = S-n-p ( n = 6-10, p = 0-2 in longitudinal field and n = p = 0 in transverse field). We observe a smooth transition (in longitudinal) from coherent ground-state to thermally activated tunneling. Furthermore, two ground-state relaxation regimes show a crossover between quantum spin relaxation far from equilibrium and near equilibrium, when the environment destroys multimolecule correlations. Finally, we stress that the complete Hamiltonian of Mn12 should contain odd spin operators of low order.     

Magnetic properties of NpM4Al8−x (M=Cr,Fe, Cu) intermetallics: Mössbauer effect and neutron diffraction studies

Mössbauer effect and neutron diffraction studies on the tetragonal NpM₄Al_8?x (M=Cr, Fe, Cu) of the 14/mmm space group are reported. In NpFe₄Al_8?x, both the Np and Fe sites order magnetically at close temperatures: the Np order ferromagnetically at 115(15) K (μ(Np)) ～ 0.6 μ_B and the Fe moments order antiferromagnetically at 130(10) K (μ(Fe) = 1.05(15) μ_B) with a ++?? sequence. In NpCr₄Al_8?x, the Np order magnetically around 50K. From the susceptibility data we conclude possible antiferromagnetic order of both Np and Cr sites. NpCu₄Al_8?x does not order magnetically down to at least 2 K. The magnetic hyperfine splitting observed below 45 K is explained by slow paramagnetic relaxation. The Np isomer shifts and also its magnetic behavior point to a trivalent ion (⁵I₄ Hund's rule ground state). The observed relaxation and magnetic phenomena are discussed in terms of crystalline electric field effects. In contrast to the isostructural rare-earth RM₄Al₈ compounds with practically independent order of R and M magnetic sublattices, we show that Np and M sublattices in NpM₄Al₈ are strongly coupled. This is caused by hybridization of both Np and M atoms with Al.     

Magnetic relaxation behavior in Tb-doped perovskite manganite

Tb-doped LaMnO₃ perovskite manganite has been synthesized by a conventional solid-state reaction method. The XRD patterns of the sample revealed that it has a single perovskite-type phase with orthorhombic symmetry at room temperature. The magnetic properties of the sample were investigated. The results of the static and dynamic magnetization measurements show that there is the magnetic relaxation behavior of the cluster (spin) glass in the Tb-doped LaMnO₃ sample. The outcomes of the ac nonlinear magnetization indicate that the magnetic relaxation behavior of the cluster (spin) glass was attributed to the coexistence and competition of a ferromagnetic double exchange between Mn³⁺ and Mn⁴⁺ and an antiferromagnetic superexchange coupling among Tb³⁺ and Tb³⁺ as well as Mn³⁺ and Mn³⁺.     

Nin(n≤8)团簇的结构

用高斯程序的B3LYP交换相关函数和全电子基组DGDZVP、TZVP优化Ni_n、Ni_n⁺和Ni_n^-(n=2~8),得到这些体系的基态几何构型、离化能、电子亲和能及键能.结果表明:Ni₂,Ni₃,Ni₄,Ni₅和Ni₆团簇的基态分别是5,7,9,11和13重态.表明:Ni₂,Ni₃,Ni₄,Ni₅和Ni₆团簇中分别有4,6,8,10和12个平行自旋的电子,在每一个Ni_n团簇中非耦合的电子最多,这些电子占据不同的轨道,导致每一个Ni_n团簇的能量最小,说明在这些Ni_n团簇中电子平行的效应大于电子耦合的效应,这显然与d电子的离域效应有关,称为自旋极化效应,符合宏特规则.Ni_n、Ni_n⁺和Ni_n^-(n=2~8)体系的磁矩分别是Ni_n具有8μ_B(n=6~8),Ni_n⁺具有9μ_B(n=6~8),Ni_n具有7μ_B(n=4~8).     

Coexistence of antiferromagnetic and spin cluster glass order in the magnetoelectric relaxor multiferroic PbFe 0.5 Nb 0.5 O3

The coexistence of cluster glass with long-range antiferromagnetic order in the relaxor ferroelectric PbFe 0.5 Nb 0.5 O3 is elucidated. While the transition at T(N) = 153 K on the infinite antiferromagnetic cluster induces 3m symmetry with large EH2 magnetoelectric response, the disconnected subspace of isolated Fe3+ ions and finite clusters accommodates the cluster glass below T(g) = 10.6 K with field-induced m' symmetry and EH-type magnetoelectric response. Critical slowing-down, memory and rejuvenation after aging, occurrence of a de Almeida-Thouless phase line, and stretched exponential relaxation of remanence corroborate the glass nature.     

Observation of magnetic level repulsion in Fe6:Li molecular antiferromagnetic rings

Heat capacity (C), magnetic torque, and proton NMR relaxation rate (1/T(1)) measurements were performed on Fe6:Li single crystals in order to study the crossings between S = 0 and S = 1 and between S = 1 and S = 2 magnetic states of the molecular rings, at magnetic fields B(c1) = 11.7 T and B(c2) = 22.4 T, respectively. C vs B data at 0.78 K show that the energy gap between two states remains finite at B(c)'s (Delta(1)/k(B) = 0.86 K and Delta(2)/k(B) = 2.36 K) thus proving that levels repel each other. The large Delta(1) value may also explain the anomalously large width of the peak in 1/T(1) vs B, around B(c1). This anticrossing, unexpected in a centrosymmetric system, requires a revision of the Hamiltonian.     

Spin-glass state and ferromagnetic order in Cu(II)－Fe(III) cyanides

The static and dynamic magnetic properties of a Prussian blue analogue, molecular magnet Cu^Ⅱ_{3}[Fe^Ⅲ(CN)_6]_2·3NH_3·6H_2O were investigated in detail. The H dependence of the linear AC susceptibility, the irreversibility in the field-cooled (FC)/zero-field-cooled (ZFC) magnetization (M_{FC}/M_{ZFC}) and the relaxation of M_{ZFC} suggest that the magnetic system can be visualized as containing a ferromagnetic cluster of spin below T_C, mixed with small spin-glass clusters formed below temperature T_g less than T_C. The observed magnetic properties are explained with a ferromagnetic-spin-glass phase model. The magnetic ordering of the sample occurs below 19.8K.     

Dynamic scaling approach to glass formation

Experimental data for the temperature dependence of relaxation times are used to argue that the dynamic scaling form, with relaxation time diverging at the critical temperature T(c) as (T-T(c))(-nuz), is superior to the classical Vogel form. This observation leads us to propose that glass formation can be described by a simple mean-field limit of a phase transition. The order parameter is the fraction of all space that has sufficient free volume to allow substantial motion, and grows logarithmically above T(c). Diffusion of this free volume creates random walk clusters that have cooperatively rearranged. We show that the distribution of cooperatively moving clusters must have a Fisher exponent tau=2. Dynamic scaling predicts a power law for the relaxation modulus G(t) approximately t(-2/z), where z is the dynamic critical exponent relating the relaxation time of a cluster to its size. Andrade creep, universally observed for all glass-forming materials, suggests z=6. Experimental data on the temperature dependence of viscosity and relaxation time of glass-forming liquids suggest that the exponent nu describing the correlation length divergence in this simple scaling picture is not always universal. Polymers appear to universally have nuz=9 (making nu=3 / 2). However, other glass-formers have unphysically large values of nuz, suggesting that the availability of free volume is a necessary, but not sufficient, condition for motion in these liquids. Such considerations lead us to assert that nuz=9 is in fact universal for all glass- forming liquids, but an energetic barrier to motion must also be overcome for strong glasses.     

T2 relaxation induced by clusters of superparamagnetic nanoparticles: Monte Carlo simulations

Clustering strongly affects the transverse (T2) relaxation induced by superparamagnetic nanoparticles in magnetic resonance experiments. In this study, we used Monte Carlo simulations to investigate systematically the relationship between T2 values and the geometric parameters of nanoparticle clusters. We computed relaxation as a function of particle size, number of particles per cluster, interparticle distance, and cluster shape (compact vs. linear). We found that compact clusters induced relaxation equivalent to similarly sized single particles. For small particles, the shape and density of clusters had a significant effect on T2. In contrast, for larger particles, T2 relaxation was relatively independent of cluster geometry until interparticle distances within a cluster exceeded ten times the particle diameter. Results from our simulations suggest principles for the design of nanoparticle aggregation-based sensors for MRI.     

On the magnetic properties of Pr[Mn(CN)<Subscript>6</Subscript>]. 4H<Subscript>2</Subscript>O

A Prussian Blue type compound of the nominal composition Pr[Mn(CN)₆].4H₂O has been prepared. It is shown that the compound exhibits ferrimagnetism with a Curie temperature T_C=38.9 K. Observed magnetic relaxation displays a logarithmic behavior.     

Fast nuclear spin relaxation in hyperpolarized solid 129Xe

We report extensive new measurements of the longitudinal relaxation time T1 of 129Xe nuclear spins in solid xenon. For temperatures T<120 K and magnetic fields B>0.05 T, we found T1 on the order of hours, in good agreement with previous measurements and with the predicted phonon-scattering limit for the spin-rotation interaction. For T>120 K, our new data show that T1 can be much shorter than the phonon scattering limit. For B = 0.06 T, a field often used to accumulate hyperpolarized xenon, T1 is approximately 6 s near the Xe melting point T(m) = 161.4 K. From T = 50 K to T(m), the new data are in excellent agreement with the theoretical prediction that the relaxation is due to (i) modulation of the spin-rotation interaction by phonons, and (ii) modulation of the dipole-dipole interaction by vacancy diffusion.     

Fe-B团簇的结构与磁性的第一性原理研究

采用局域密度近似下的密度泛函理论和原子轨道的线性组合方法,通过离散变分法自洽求解Kohn-Sham方程,详细地研究了Fe_nB(n≤6)团簇的结构和磁性,所得主要结论如下:第一,B原子更倾向于在团簇的表面而不是在团簇的内部,通常计算非晶态的四面体结构和三棱柱结构,对于孤立团簇而言是不稳定的,这说明环境对团簇的结构稳定性有重要影响;第二,当Fe_n+1团簇中的一个Fe原子被B原子取代形成Fe_n团簇时,其结合能增大而Fe原子的磁矩减小;第三,Fe<     

线极化Bell-Bloom测磁系统中抽运光对磁场灵敏度的影响

利用适用于线极化Bell-Bloom测磁系统的布洛赫方程和含有自旋弛豫的速率方程,以铯原子为研究对象,分析了抽运光对磁场灵敏度的影响,并在实验上分别采用与铯原子D1线和D2线共振的线偏光作为抽运光和探测光,用充有缓冲气体的气室进行了实验.实验结果与理论分析一致,均表明只有在一定的光强范围内,增大抽运光光强可以提高磁场灵敏度.且利用这一方法分析了原子的自旋弛豫对磁场灵敏度的影响.这项研究对于深入认识线极化的Bell-Bloom测磁系统,以及如何通过优化系统实现磁场灵敏度的提高具有重要的意义.     

Measurement of the relaxation rate of the magnetization in Mn12O12-acetate using proton NMR echo

We present a novel method to measure the relaxation rate W of the magnetization of Mn 12O (12)-acetate (Mn12) magnetic molecular cluster in its S = 10 ground state at low T. It is based on the observation of an exponential growth in time of the proton NMR signal during the thermal equilibration of the magnetization of the molecules. We can explain the novel effect with a simple model which relates the intensity of the proton echo signal to the microscopic reversal of the magnetization of each individual Mn12 molecule during the equilibration process. The method should find wide application in the study of magnetic molecular clusters in off-equilibrium conditions.