Direct observation of chiral susceptibility in the canonical spin glass AuFe

The extraordinary Hall resistivity rho(xy) and the magnetization M of a canonical spin glass AuFe (8 at.% Fe) were measured simultaneously as functions of temperature with the best care to the thermal and the magnetic field hysteresis. The data of rho(xy) show an anomaly at the spin glass transition temperature T(g) and have different zero field cooling (ZFC) and field cooling (FC) measurements below T(g). Moreover, the value of rho(xy)/M, which represents the chiral susceptibility of the system in the present case, also shows the difference between ZFC and FC measurements. The results are consistent with the predictions of the chirality scenario of canonical spin glasses by Kawamura.     

Dielectric investigation of the temperature dependence of the nonexponentiality of the dynamics of polymer melts

Using broad band dielectric spectroscopy (10(-5)-10(9) Hz), combining time domain and frequency domain techniques, we study the temperature dependence of the non-Debye character of the alpha relaxation of polymer melts in the glass transition temperature T(g) range. The alpha relaxation process is described in terms of the Kohlrausch-Williams-Watts relaxation function which has a single parameter beta to characterize the nonexponentiality of the relaxation. At high temperatures, beta remains nearly insensitive to temperature changes, whereas in the vicinity of T(g) a nearly linear increasing of beta with temperature is found. The temperature range where the change of the beta(T) behavior occurs is located for all the polymers investigated around 1.2T(g). Moreover, our results indicate a common value of beta approximately equal to 1/3 at the temperature where the relaxation time diverges. The beta(T) behavior near T(g) is discussed in terms of a "rugged landscape" phase space which allows us to rationalize both the beta(T) behavior observed as well as the similarities of our findings near T(g) with the results reported in simulations on Ising spin glasses and other model systems.     

自旋玻璃LaFeyNi5-y的磁性研究

本文报道了金属间化合物LaFe_yNi_5-y(0.5≤y≤1.1)的自旋玻璃特性。测定了样品的自旋玻璃冻结温度T_f。y≤0.9时,在一定温度下,样品中发生顺磁-自旋玻璃转变,T_f近似为y的线性函数,y≥1.0时,材料将发生顺磁-铁磁-自旋玻璃转变。 关键词：     

Extended scaling in the magnetic critical phenomenology of the σ-phase Fe0.53-Cr0.47 and Fe0.52-V0.48 alloys

The magnetization of the sigma-phase Fe(0.53)Cr(0.47) and Fe(0.52)V(0.48) alloys was studied as a function of temperature and field. The experiments show that both materials behave magnetically as re-entrant spin glass systems. Field versus temperature diagrams were obtained where the locations of the paramagnetic phase, the intermediate ferromagnetic-like phase and the spin glass fundamental state were displayed. These diagrams are in qualitative agreement with the predictions of the mean field theory for the interplay between the ferromagnetic and spin glass orderings. The critical phenomenology near the para-ferromagnetic transition could be investigated. It was found that the paramagnetic susceptibility is quite well described by the extended scaling scheme, where the reduced temperature is written as τ = (T - T(c))/T. The value obtained for the susceptibility critical exponent γ is intermediate between the prediction of the 3D Heisenberg universality class and the large values observed in spin glasses, as previously found in other re-entrant systems. The data do not confirm the validity of the extended scaling in the ferromagnetic-like phase. Using either the conventional or extended scaling protocols, the exponents β and δ were found to have values close to those reported for spin glass transitions. Despite the relevance of disorder and the anomalous values determined for β, γ and δ, the Widom scaling relation holds as an equality.     

Reentrant spin glass behavior in Cr-doped perovskite manganite

The magnetic and transport properties of the Cr-doped manganites La(0.46)Sr(0.54)Mn(1-y)Cr(y)O3 ( 0 < or = y < or = 0.08) with the A-type antiferromagnetic structure were investigated. Upon cooling, we find multiple magnetic phase transitions, i.e., paramagnetic, ferromagnetic (FM), antiferromagnetic (AFM), and spin glass in the y = 0.02 sample. The low temperature spin glass state is not a conventional spin glass with randomly oriented magnetic moments but has a reentrant character. The reentrant spin glass behavior accompanied by the anomalous multiple magnetic transitions might be due to the competing interactions between the FM phase and the A-type AFM matrix induced by the random Cr impurity.     

Attenuation of second sound in superfluid 3He-A1

The attenuation of second sound (spin-entropy) wave in the superfluid A1 phase has been measured in magnetic fields up to 11 T and to sufficiently high frequency to observe the bulk attenuation proportional to the square of frequency. The measured attenuation coefficient is compared with the existing theories of hydrodynamics and dissipative coefficients. The resulting "excess" attenuation is discussed in terms of the temperature dependent spin diffusion coefficient in the superfluid.     

Antiferromagnetic Ising spin glass competing with BCS pairing interaction in a transverse field

The competition among spin glass (SG), antiferromagnetism (AF) and local pairing superconductivity (PAIR) is studied in a two-sublattice fermionic Ising spin glass model with a local BCS pairing interaction in the presence of an applied magnetic transverse field Γ. In the present approach, spins in different sublattices interact with a Gaussian random coupling with an antiferromagnetic mean J₀ and standard deviation J. The problem is formulated in the path integral formalism in which spin operators are represented by bilinear combinations of Grassmann variables. The saddle-point Grand Canonical potential is obtained within the static approximation and the replica symmetric ansatz. The results are analysed in phase diagrams in which the AF and the SG phases can occur for small g (g is the strength of the local superconductor coupling written in units of J), while the PAIR phase appears as unique solution for large g. However, there is a complex line transition separating the PAIR phase from the others. It is second order at high temperature that ends in a tricritical point. The quantum fluctuations affect deeply the transition lines and the tricritical point due to the presence of Γ.     

Exchange-mediated spin-lattice relaxation of Fe3+ ions in borate glasses

Spin-lattice relaxation times (T1) of two borate glasses doped with different concentrations of Fe2O3 were measured using the Electron Spin-Echo (ESE) technique at X-band (9.630 GHz) in the temperature range 2-6K. In comparison with a previous investigation of Fe3+-doped silicate glasses, the relaxation rates were comparable and differed by no more than a factor of two. The data presented here extend those previously reported for borate glasses in the 10-250K range but measured using the amplitude-modulation technique. The T1 values were found to depend on temperature (T) as T(n) with n approximately 1 for the 1% and 0.1% Fe2O3-doped glass samples. These results are consistent with spin-lattice relaxation as effected by exchange interaction of a Fe3+ spin exchange-coupled to another Fe3+ spin in an amorphous material.     

Magnetic and electrical properties of the ZnGeAs<Subscript>2</Subscript>: Mn chalcopyrite

Doping of the ZnGeAs₂ semiconductor with manganese has produced compositions with spontaneous magnetization and high Curie temperatures of up to 367 K for the composition 3.5 wt% Mn. Their magnetic properties are characteristic of spin glasses at temperatures T < T _S and magnetic fields H < 11 kOe. In stronger fields, the spin glass state transforms into a phase with a spontaneous magnetization 4–5 times weaker than that to be expected under ferromagnetic ordering of all Mn ions. This is obviously a singly-connected ferromagnetic phase containing regions with frustrated bonds. The frustrated regions and the spin glass phase have inclusions of noninteracting ferromagnetic clusters, because these regions and the spin glass phase at low temperatures exhibit a strong increase in the magnetization M, with the dependence M(T) being described by the Langevin function. Measurements of the electrical resistivity ρ and the Hall effect have revealed that, for T < 30 K, the resistivity ρ of compositions with 1.5 and 3.5 wt % Mn is higher that at 30 K, which makes superexchange dominant and gives rise to the onset of the spin glass state. The nonuniform distribution of Mn ions in the spin glass phase accounts for the existence of isolated ferromagnetic clusters, their ferromagnetism being generated by carrier-mediated exchange. As the temperature increases still more, the increase in the mobility occurs faster than the decrease in the concentration, thus promoting an enhancement of the carrier-mediated exchange and growth of the ferromagnetic clusters in size, which at T = T _S come in contact. This signifies a transition from a multiply-to a singly-connected ferromagnetic phase, which contains microregions with frustrated bonds.     

Raman scattering study of anomalous spin, charge, and lattice dynamics in the charge-ordered phase of Bi1-xCaxMnO3 (x > 0.5)

We report an inelastic light scattering study of the effects of charge ordering on the spin, charge, and lattice dynamics of Bi1-xCaxMnO3 (x>0.5). We find that charge ordering results in anomalous phonon behavior, such as the appearance of "activated" modes. More significantly, however, the transition to the charge-ordered phase results in the appearance of a quasielastic scattering response with the symmetry of the spin-chirality operator ( T(1g)); this scattering response is thus indicative of magnetic or chiral spin fluctuations in the antiferromagnetic charge-ordered phase.     

Electron spin decoherence in isotope-enriched silicon

Silicon is promising for spin-based quantum computation because nuclear spins, a source of magnetic noise, may be eliminated through isotopic enrichment. Long spin decoherence times T2 have been measured in isotope-enriched silicon but come far short of the T2=2T1 limit. The effect of nuclear spins on T2 is well established. However, the effect of background electron spins from ever present residual phosphorus impurities in silicon can also produce significant decoherence. We study spin decoherence decay as a function of donor concentration, 29Si concentration, and temperature using cluster expansion techniques specifically adapted to the problem of a sparse dipolarly coupled electron spin bath. Our results agree with the existing experimental spin echo data in Si:P and establish the importance of background dopants as the ultimate decoherence mechanism in isotope-enriched silicon.     

磁性物质中冷无序能的作用

在研究物质的磁性时,考虑了电子之间的正交换能(A1>0,导致电子自旋平行排列)和负交换能(A2<0,导致电子自旋反平行排列)两项各自的物理作用,不是简单地只以它们的代数和为判据.提出冷无序能的概念:当A1>|A2|(A=A1-|A2|>0)时,A1为有序能,A2为冷无序能;当A1<|A2|(A<0)时,A2为有序能,A1为冷无序能.物质的磁性决定于热运动能、有序能以及冷无序能之间的竞争.考虑了冷无序能导致“冷无序”的物理功能,将冷无序能变换为等效温度,在统计物理的框架内处理了铁磁性、反铁磁性转变和自旋玻璃冻结问题.A2=0的体系具有Weiss铁磁性,|A2|A1=1的体系表现自旋玻璃磁性,1>|A2|A1>0的体系同时具有铁磁性和自旋玻璃磁性,1>A1|A2|>0的体系同时具有反铁磁性和自旋玻璃磁性. 关键词： 交换能 冷无序能 铁磁性 反铁磁性 自旋玻璃     

Thermal studies of the spin liquid state and analog to the He melting curve in a geometrically frustrated magnet

Gadolinium gallium garnet, Gd₃Ga₅O₁₂ (GGG) has an extraordinary low-temperature phase diagram. Although the Curie–Weiss temperature of GGG is −2 K, GGG shows no long-range order down to T0.4 K. At low temperatures GGG has a spin glass phase at low fields (0.1 T), a field-induced long-range ordered antiferromagnetic state at fields of between 0.7 and 1.3 T, and, at intermediate fields, an apparent spin-liquid state without long-range order. We have characterized the intermediate field (IF) state through heat capacity, thermal conductivity, and magnetocaloric measurements. Our results show a sharp high-field phase boundary of the thermal irreversibility of the spin glass phase of GGG implying that the intermediate field phase is distinct from the spin glass. The lower field boundary of the AFM phase is shown to have distinct minimum at T0.2 K, in analogy to the minimum in the melting curve of ⁴He. The existence of such a minimum is confirmed by measurements of the latent heat of the transition below that temperature.     

永磁性大块金属玻璃Nd60Al10Fe20Co10低温磁性的研究

研究了Nd6 0 Al1 0 Fe2 0 Co1 0 大块金属玻璃磁性随温度的变化关系 ,结果表明Nd6 0 Al1 0 Fe2 0 Co1 0 在室温下表现为永磁性 ,随着温度的降低 ,矫顽力和磁滞回线形状都有很大的变化 .交流磁化率在 18K左右出现尖峰而且峰值温度随频率变化 ,表明该大块非晶体系中存在自旋玻璃态     

自旋玻璃的重入行为及CoxZn1-x(FeyCr1-y)2O4尖晶石系统的磁相图

本文研究了Co_xZn_1-x(Fe_yCr_1-x2O₄尖晶石系统的磁性,测量了不同成份样品的低频弱场交流磁化率与低场直流磁化强度的温度关系。根据实验结果,给出了该系统可能的磁相图。发现在该系统中相当宽的成份范围内,都存在自旋玻璃的重入现象。同时发现,自旋玻璃的重入温度随磁性离子浓度增加而增加。这些行为是磁性离子浓度含量较高和多种磁性离子共存系统的共同特性。还讨论了自旋玻璃重入行为 关键词：     

On the bethe approximation for the random bond Ising model

A random bond Ising model is considered in terms of the pair approximation, which is equivalent to the Bethe approximation, of the cluster variation method. On taking the configurational average over the random distribution of bonds ±J, we take into account the nearest neighbor correlations between effective fields and bonds. We investigate their effects to the phase transition temperature from the paramagnetic phase to the ferro- (or antiferro-) magnetic phase and to the spin glass phase for the Ising model on the square lattice. It turns out that the correlation effects act favorably to the spin glass phase and bend upward the line of transition temperature from the paramagnetic phase to the spin glass phase as the concentration being apart from 0.5. In the appendix, we derive the expression of free energy in the weak interaction limit.     

Symmetrical temperature-chaos effect with positive and negative temperature shifts in a spin glass

Aging in a Heisenberg-like spin glass Ag(11 at. % Mn) is investigated by measurements of the zero-field-cooled magnetic relaxation at a constant temperature after small temperature shifts absolute value of Delta T/T(g) < 0.012. A crossover from fully accumulative to nonaccumulative aging is observed, and by converting time scales to length scales using the logarithmic growth law of the droplet model, we find quantitative evidence that positive and negative temperature shifts cause an equivalent restart of aging (rejuvenation) in terms of dynamical length scales. This result supports the existence of a unique overlap length between a pair of equilibrium states in the spin-glass system.     

Optical decoherence in eR3+-doped silicate fiber: evidence for coupled spin-elastic tunneling systems

We measured the optical decoherence times T2, or, equivalently, the homogeneous line width, in an Er-doped optical fiber at low temperature as a function of external magnetic field and temperature using two-pulse photon echoes. The decoherence times were up to 230 ns at fields above 3 T. The magnitude of the line narrowing induced by a magnetic field of 3 T is 2.5 MHz, which is anomalously large compared to that typical for oxide crystals with similar Er3+ concentration. This is interpreted as evidence for dephasing by coupled spin-elastic tunneling modes where the normal glass tunneling modes acquire a magnetic character by coupling to the Er3+ spin.     

Conductivity of Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites in a magnetic field within the temperature range 78–300 K

The electrical conductivity of the family of Ce _x Sr_1?x MnO₃ (x = 0.50, 0.67) alloys is studied in magnetic fields of up to 0.6 T in the temperature range 78–300 K. The semiconductor-metal phase transition is observed in unannealed samples with x = 0.5 and in both annealed and unannealed samples with x = 0.67. All samples exhibit giant negative magnetoresistance. The temperature dependence of the giant negative magnetoresistance effect, the dependence of the electrical resistivity on the magnetic field at 78 K, and the time dependence of the magnetoresistance at 78 K are measured for the first time. Some samples reveal the properties of spin glass and strong ferromagnets. The reproducibility of the data obtained for these samples depends on the prehistory of the samples, specifically on the conditions of annealing and exposure to a magnetic field.     

Imaging of laser-polarized solid xenon

The enhanced spin polarization produced by optical pumping of gaseous rubidium/xenon samples has made possible a number of recent experiments in nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI). Here we report MRI of laser-polarized xenon in the solid phase at low temperature. Due to the high xenon density in the solid phase and the enhanced spin polarization, it is possible to achieve high intensity and spatial resolution of the image. Signals were observed from xenon films solidified onto the glass container walls and not from an enclosed chili pepper.