Spin Glass

We investigate magnetic properties of a two-dimensional periodic structure with Ising spins and antiferromagnetic nearest neighbor interaction. The structure is topologically equivalent to the Archimedean (3, 12 2 ) lattice. The ground state energy is degenerate. In some ground states, the spin structure is translationally invariant, with the same configuration in each unit cell. Numerical results are reported on specific heat and static magnetic susceptibility against temperature. Both quantities show maxima at temperature T > 0. They reveal some sensitivity on the initial state in temperatures where the Edwards-Anderson order parameter is positive. For zero temperature and low frequency of the applied field, the magnetic losses are negligible. However, the magnetization curve displays some erratic behavior due to the metastable states.

We consider the random Erdős-Rényi network with enhanced clusterization and Ising spins s = ±1 at the network nodes. Mutually linked spins interact with energy J. Magnetic properties of the system as dependent on the clustering coefficient C are investigated with the Monte Carlo heat bath algorithm. For J > 0 the Curie temperature Tc increases from 3.9 to 5.5 when C increases from almost zero to 0.18. These results deviate only slightly from the mean field theory. For J < 0 the spin-glass phase appears below TSG; this temperature decreases with C, on the contrary to the mean field calculations. The results are interpreted in terms of social systems.

We present the experimental observation of the fluctuation-dissipation theorem (FDT) violation in an assembly of interacting magnetic nanoparticles in the low temperature superspin glass phase. The magnetic noise is measured with a two-dimension electron gas Hall probe and compared to the out of phase ac susceptibility of the same ferrofluid. For "intermediate" aging times of the order of 1 h, the ratio of the effective temperature T eff to the bath temperature T grows from 1 to 6.5 when T is lowered from Tg to 0.3 Tg, regardless of the noise frequency. These values are comparable to those measured in an atomic spin glass as well as those calculated for a Heisenberg spin glass.

We study Fermionic systems on a lattice with random interactions through their dynamics and the associated KMS states. These systems require a more complex approach compared with the standard spin systems on a lattice, on account of the difference in commutation rules for the local algebras for disjoint regions, between these two systems. It is for this reason that some of the known formulations and proofs in the case of the spin lattice systems with random interactions do not automatically go over to the case of disordered Fermion lattice systems. We extend to the disordered CAR algebra, some standard results concerning the spectral properties exhibited by temperature states for disordered quantum spin systems. We discuss the Arveson spectrum and its connection with the Connes and Borchers Γ-invariants for such W * -dynamical systems. In the case of KMS states exhibiting a natural property of invariance with respect to the spatial translations, some interesting properties, associated with standard spinglass-like behaviour, emerge naturally. It covers infinite-volume limits of finite-volume Gibbs states, that is the quenched disorder for Fermions living on a standard lattice Z d . In particular, we show that a temperature state of the systems under consideration can generate only a type III von Neumann algebra (with the type III 0 component excluded). Moreover, in the case of the pure thermodynamic phase, the associated von Neumann is of type III λ for some λ ∈ (0, 1], independent of the disorder. Such a result is in accordance with the principle of self-averaging which affirms that the physically relevant quantities do not depend on the disorder. The present approach can be viewed as a further step towards fully understanding the very complicated structure of the set of temperature states of quantum spin glasses, and its connection with the breakdown of the symmetry for the replicas.

We report smaH angle (SANS) and inelastic neutron scattering in zero and applied field for a-FeMn, NiMn and AuFe at composition where both ferromagnetic and frustration characters occur. We discuss the field evolution of the transverse correlations which arise below T,. A study of the field dependence of the spin wave anomalies in a-Fe-Mn is reported.

By neutron scattering, we have studied the spin correlations and spin fluctuations in frustrated Laves hydrides, where magnetic disorder sets in the topologically frustrated Mn lattice. Below the transition towards short range magnetic order, static spin clusters coexist with fluctuating and alsmost uncorrelated spins. The magnetic response shows a complexe lineshape, connected with the presence of the magnetic inhomogeneities. Its analysis shows the existence of two different processes, relaxation and local excitations, for the spin fluctuations below the transition.

We present the results of a neutron diffraction study of the antiferromagnet MnO embedded in a porous glass. The type of magnetic ordering and the structural distortion are similar to those of the bulk, but the ordered magnetic moment of 3.84͑4͒m B ͞ion is strongly reduced and the Néel temperature is enhanced. The magnetic transition is second order, in contrast to the first order transition of the bulk. The size of the magnetic region is smaller than the average size of the nanoparticles. The reasons for this behavior are discussed.

A bivariate multicanonical Monte Carlo simulation of the three-dimensional ±J Ising spin glass is described. The correlation time of the Monte Carlo dynamics is approximately proportional to the system size, which is a great improvement over previous spin-glass simulations. The order-parameter distribution function P (q) at T = 0.3 exhibits a feature of the droplet picture of the low-temperature phase, as opposed to the mean-field picture.

In this work we present low-field magnetic measurements on multilayers of Fe 1.5 nm and Cu 1.5 nm layers, repeated 10 times, grown on Si(111) and Fe 5 nm/CaF2 40 nm/Si(111) substrates. Magnetic characteristics of a spin-glass phase are exhibited by samples directly grown on Si(111), for which the glass temperatures (TG) depend strongly on the magnetic field. Above TG those samples exhibit a ferromagnetic behavior. In contrast, samples grown on Fe/CaF2 buffer structure also show the spin-glass characteristics, but are antiferromagnetic above TG. This work deals with the fundamental role played by the interface between Fe and Cu for these effects, which is clearly emphasized by the experimental results.

L&#39;objectif de cette these est d&#39;appliquer les methodes d&#39;approximation stochastique a l&#39;estimation de la densite et de la regression. Dans le premier chapitre, nous construisons un algorithme stochastique a pas simple qui definit toute une classe d&#39;estimateurs recursifs a noyau d&#39;une densite de probabilite. Nous etudions les differentes proprietes. De cet algorithme. Nous montrons en particulier que les deux estimateurs recursifs a noyau deja connus correspondent a deux elements bien particuliers de la classe d&#39;estimateurs definis par notre algorithme stochastique: l&#39;estimateur recursif introduit par Wolwerton et Wagner (1969) correspond a un choix de pas de l&#39;algorithme qui permet d&#39;obtenir un risque quadratique minimal, tandis que celui introduit par Duflo (1997) correspond a un choix de pas qui permet d&#39;obtenir une variance minimale. Dans le deuxieme chapitre, nous nous interessons a l&#39;estimateur propose par Revesz (1973, 1977) pour...

Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that SrCo2As2 is a candidate for frustrated itinerant magnetism. Using inelastic neutron scattering (INS), we find that antiferromagnetic (AF) spin fluctuations develop in the square Co layers of SrCo2As2 below T ≈ 100 K centered at the stripe-type AF propagation vector of ( 2 , 1 2 ), and that their development is concomitant with a suppression of the uniform magnetic susceptibility determined via magnetization measurements. We interpret this switch in spectral weight as signaling a temperature-induced crossover from an instability towards FM ordering to an instability towards stripe-type AF ordering on cooling, and show results from Monte-Carlo simulations for a J1-J2 Heisenberg model that illustrate how the crossover develops as a function of the frustration ratio -J1/(2J2). By putting our INS data on an absolute scale, we quantitatively compare them and our magnetization data to exact-diagonalization calculations for the J1-J2 model [N. Shannon et al., Eur. Phys. J. B 38, 599 (2004)], and show that the calculations predict a lower level of magnetic frustration than indicated by experiment. We trace this discrepancy to the large energy scale of the fluctuations (Javg 75 meV), which, in addition to the steep dispersion, is more characteristic of itinerant magnetism.

We report the a.c. susceptibility study of DyxTb2-xTi2O7 with x ∈ [0, 2]. In addition to the single-ion effect at Ts (single-ion effect peak temperature) corresponding to the Dy 3+ spins as that in spin ice Dy2Ti2O7 and a possible spin freezing peak at T f (T f < 3 K), a new peak associated with Tb 3+ is observed in χac(T ) at nonzero magnetic field with a characteristic temperature T * (T f < T * < Ts). T * increases linearly with x in a wide composition range (0 < x < 1.5 at 5 kOe). Both application of a magnetic field and increasing doping with Dy 3+ enhance T * . The T * peak is found to be thermally driven with an unusually large energy barrier as indicated from its frequency dependence. These effects are closely related to the crystal field levels, and the underlying mechanism remains to be understood.

We study the interplay between disorder and a quasi periodic coupling array in an external magnetic field in a spin-1 2 XXZ chain. A simple real space decimation argument is used to estimate the magnetization values where plateaux show up. The latter are in good agreement with exact diagonalization results on fairly long XX chains. Spontaneous susceptibility properties are also studied, finding a logarithmic behaviour similar to the homogeneously disordered case.

We discuss the effect of weak bond disorder in two-leg spin ladders on the dispersion relation of the elementary triplet excitations with a particular focus on the appearance of bound states in the spin gap. Both the cases of modified exchange couplings on the rungs and the legs of the ladder are analyzed. Based on a projection on the single-triplet subspace, the single-impurity and small cluster problems are treated analytically in the strong-coupling limit. Numerically, we study the problem of a single impurity in a spin ladder by exact diagonalization to obtain the low-lying excitations. At finite concentrations and to leading order in the interrung coupling, we compare the spectra obtained from numerical diagonalization of large systems within the single-triplet subspace with the results of diagrammatic techniques, namely, low-concentration and coherentpotential approximations. The contribution of small impurity clusters to the density of states is also discussed.

Local magnetic structure of the spin-glass material Mn 0.5 Fe 0.5 TiO 3 is investigated by magnetic pair distribution function (mPDF) obtained from powder neutron diffraction data. The magnetic correlation peak in the mPDF decays with increasing the distance r, indicating the short-range magnetic ordering in the spin glass state. The mPDF can be almost reproduced by the superposition of the mPDFs calculated using the antiferromagnetic structures of MnTiO 3 and FeTiO 3 multiplied a Gaussian-type decay function. These results are consistent with the coexistence and the competition of the two different antiferromagnetic structures in the spin glass state, suggested by the preceding study.

This paper models the coupling between the human Noosphere and the global Technosphere. Using phase transition mathematics, it frames societal crises—climate disruption, AI acceleration, political instability—as critical bifurcation points. The SCPN framework is applied to predict large-scale transitions in the hybrid noosphere-technosphere system, offering both diagnostic tools and strategies for resilience.

We report here SQUID (magnetization) measurements, along with supporting specific heat, Raman, SEM (scanning electron microscope), EDX (energy dispersive X-ray) and XRD (X-ray diffraction) measurements, on Cudoped and undoped double perovskite Sr 2+ 2 Y 3+ Ru 5+ O 2- 6 (abbreviated as SrY2116) system grown as single crystal using high-temperature solution growth technique. These measurements show the undoped system to be a nonmetallic (insulating) spin glass (SG) and the ∼ 5-30% Cu-doped (i.e. Cu-concentration/(Cu + Ru-concentration) ∼ 5-30%) system to be a spin glass superconductor (SGSC) with T c (critical temperature) ∼ 28-31 K and superconducting volume fraction, f sc ∼ 2.2-9%. To mention, similar measurements done on undoped and Cu-doped BaY2116 and BaPr2116 systems show for them the same (SG, SGSC) behaviors. However they show a decrease in T c and f sc when diamagnetic Y 3+ ions are replaced by Pr 3+ spins, presumably due to enhanced internal pair breaking, and also decreased Cu-

The time decay of the thermoremanent magnetization(TRM) of the Fe 80-x Ni x Cr 20 (14 ≤ x ≤ 30) alloys has been measured for four different magnetic phases within the fcc γ-phase using a SQUID magnetometer. In the spin-glass phase(SG)(X=19) very distinct ageing effects are observed where M(t) can be described as M (t) = M 0 (t/t w ) -γ exp[-(t/τ ) 1-n ] for the entire time domain. In the reentrant spin-glass(RSG)(X= 23 and 26), M(t) can be better represented by the stretched exponential with an addition of a constant term which can be well explained by the Gabay-Toulouse(GT) model. We have also measured the linear and non-linear ac susceptibilities for the sample X=23 and confirmed the presence of the ferromagnetic(FM) ordering down to the lowest temperature. In the RSG(X=23), the TRM shows a minimum near T c and a local maximum just above T c . In the FM phase (X=30) the popular prediction of the power law decay of the TRM is observed. The latter is indistinguishable from the stretched exponential in the antiferromagnetic(AF) phase (X=14).

Random fields disorder Ising ferromagnets by aligning single spins in the direction of the random field in three space dimensions, or by flipping large ferromagnetic domains at dimensions two and below. While the former requires random fields of typical magnitude similar to the interaction strength, the latter Imry-Ma mechanism only requires infinitesimal random fields. Recently, it has been shown that for dilute anisotropic dipolar systems a third mechanism exists, where the ferromagnetic phase is disordered by finite-size glassy domains at a random field of finite magnitude that is considerably smaller than the typical interaction strength. Using large-scale Monte Carlo simulations and zero-temperature numerical approaches, we show that this mechanism applies to disordered ferromagnets with competing short-range ferromagnetic and antiferromagnetic interactions, suggesting its generality in ferromagnetic systems with competing interactions and an underlying spin-glass phase. A finite-size-scaling analysis of the magnetization distribution suggests that the transition might be first order.

Ferromagnetic Ising systems with competing interactions are considered in the presence of a random field. We find that in three space dimensions the ferromagnetic phase is disordered by a random field which is considerably smaller than the typical interaction strength between the spins. This is the result of a novel disordering mechanism triggered by an underlying spin-glass phase. Calculations for the specific case of the long-range dipolar LiHoxY1-xF4 compound suggest that the above mechanism is responsible for the peculiar dependence of the critical temperature on the strength of the random field and the broadening of the susceptibility peaks as temperature is decreased, as found in recent experiments by Silevitch et al. [Nature (London) 448, 567 (2007)]. Our results thus emphasize the need to go beyond the standard Imry-Ma argument when studying general random-field systems.

As a result of the interplay between the intrinsic off-diagonal terms of the dipolar interaction and an applied transverse field Ht, the diluted LiHoxY1-xF4 system at x> 0.5 is equivalent to a ferromagnet in a longitudinal random field (RF). At low Ht the quantum fluctuations between the Ising like doublet states are negligible, while the effective induced RF is appreciable. This results in a practically exact equivalence to the classical RF Ising model. By tuning Ht, the applied longitudinal field, and the dilution, the Ising model can be realized in the presence of an effective RF, transverse field, and constant longitudinal field, all independently controlled. The experimental consequences for D = 1, 2, 3 dimensions are discussed.

Both orientational glasses and dipolar glasses possess an intrinsic random field, coming from the volume difference between impurity and host ions. We show this suppresses the glass transition, causing instead a crossover to the low T phase. Moreover the random field is correlated with the inter-impurity interactions, and has a broad distribution. This leads to a peculiar variant of the Imry-Ma mechanism, with 'domains' of impurities oriented by a few frozen pairs. These domains are small: predictions of domain size are given for specific systems, and their possible experimental verification is outlined. In magnetic glasses in zero field the glass transition survives, because the random fields are disallowed by time-reversal symmetry; applying a magnetic field then generates random fields, and suppresses the spin glass transition.

Nd still orders cooperatively at T N = 0.95 K. The evolution of the magnetic ordering of Nd with the Fe content is studied by low-temperature specific-heat, ac susceptibility and neutron scattering measurements. In the Fe-rich side of the phase diagram, Nd order is inhibited by Fe-Nd internal field at very low Ga content, while in the Fe-poor side, Nd order is more robust, reaching the Fe magnetic percolation concentration (x c = 0.33). We have used a mean field model as well as MonteCarlo simulations to clarify the interpretation of the results. The introduction of Ga ions induces discompensation of the ordered ferromagnetic Fe sublattice which strongly inhibits Nd order. However, this is only effective at every temperature if the Fe sublattice is long range ordered, while below Fe percolation, Nd is able to orders at sufficiently low T.

Using the method of selective excitation of the exciton luminescence in Cd& "Mn, Te epilayers we have measured energies of localized magnetic polarons (LMP's) for alloys with manganese mole fractions x 0.34. The suppression of the LMP energy has been studied in external magnetic fields and with tem- perature increase. Polaron formation times and exciton lifetimes have been measured by time-resolved photoluminescence. We have found that in alloys with x &0.17 the polaron formation process is inter- rupted by exciton recombination and, as a result, the LMP does not reach its equilibrium energy. This dynamical effect on the polaron energy together with the strong sensitivity of the LMP formation to the conditions of primary exciton localization causes the absence of the LMP formation in layers with x &0. 1. Antiferromagnetic clustering of Mn ions, which leads to the spin-glass phase formation at low temperatures, affects the polaron energy and results in the increasing stability of LMP s against suppres- sion by temperature increase and magnetic fields. In Cd, "Mn"Te with x &0.20 a considerable part of the polaron energy is controlled by the input of clusters of antiferromagnetically coupled Mn spins locat- ed in the nonuniform molecular field of localized excitons. The comparison of the exciton Zeeman split- ting and the LMP magnetic-field suppression provides insight into the internal structure of LMP s.

Thermodynamics studies of a prototypical quasi-two-dimensional frustrated magnet Ba2Sn2ZnCr7pGa10-7pO22 where the magnetic Cr 3+ ions are arranged in a triangular network of bipyramids show that the magnetic zero-point entropy for p = 0.98 is 55(1)% of the entropy expected when the Cr 3+ moments are fully disordered. Furthermore, when combined with a previous neutron scattering study and the perimeter scaling entropy of a spin jam, the analysis reveals that with decreasing p, i.e., doping of the nonmagnetic Ga 3+ ions, the variation in the magnetic zero-point entropy can be well explained by the combined effects of the zero-point entropy of the spin jam state and that of weakly coupled orphan spins, shedding light on the coexistence of the two types of spin states in quantum magnetism.

We show that the problem of a directed polymer on a tree with disorder can be reduced to the study of nonlinear equations of reaction-diffusion type. These equations admit traveling wave solutions that move at all possible speeds above a certain minimal speed. The speed of the wavefront is the free energy of the polymer problem and the minimal speed corresponds to a phase transition to a glassy phase similar to the spin-glass phase. Several properties of the polymer problem can be extracted from the correspondence with the traveling wave: probability distribution of the free energy, overlaps, etc.

A position-space renormalization-group study is done for the Ashkin-Tellerized XY model, as an exact solution on the d = 2 hierarchical lattice and an approximate solution on the square lattice. A multiplicity of algebraic order is found in the phase diagram, in the form of renormalization-group fixed lines composed of a continuous sequence of interaction potential surfaces. In the Ashkin-Tellerized XY model, each site has two continuously varying spins, each spin being an XY spin, that is, having orientation continuously varying in 2π radians. Nearestneighbor sites are coupled by two-spin and four-spin interactions. The phase diagram has algebraically ordered phases that are ferromagnetic and antiferromagnetic in each of the spins, and algebraically ordered phases that are ferromagnetic and antiferromagnetic in the combined spin variable. These phases are subtended by fixed lines of potential surfaces that are multiplicatively different Berezinskii-Kosterlitz-Thouless fixed potentials. The evolution of continuously varying criticality is traced within each of the four phases. The renormalization-group flows, the fixed lines, and the interaction surfaces are in terms of the doubly composite Fourier coefficients of the exponentiated energy of the four nearest-neighbor spins. The disordered phase is maintained along two semi-infinite a priori quasi-disorder lines.

The density matrix renormalization group (DMRG) has been extended in order to analyse the quantum spin glass transition (QSGT) for a random Ising model in a transverse field --$\Gamma$-- on a random graph with fixed connectivity $K=3$. The system is solved ...

Phases and phase transitions in pure and doped spin-Peierls system CuGeO3 are considered on the basis of a Landau-theory. In particular we discuss the critical behaviour, the soliton width and the low temperature specific heat of the incommensurate phase. We show, that dilution leads always to the destruction of long range order in this phase, which is replaced by an algebraic decay of correlations if the disorder is weak.

Entanglement, its generation, manipulation, measurement and fundamental understanding is at the very heart of quantum mechanics. We here report on the creation and characterization of entangled states of up to 8 trapped ions, the investigation of long-lived two-ion Bell-states and on experiments towards entangling ions and photons.

Ferrimagnetism in orthorhombic Ni 4 Nb 2 O 9 below its Néel temperature, T FN ∼ 76 K is reported to result from two inequivalent Ni 2+ ions having different magnetic moments. However, a clear understanding of the temperature variation of its magnetization [M(T )] for T > T FN and T < T FN in terms of a single set of exchange parameters is still lacking. In this work, experimental results obtained from a detailed analysis of the temperature and magnetic field dependence of magnetization [M(T, H )], ac-magnetic susceptibility [χ ac ( f , T, H )], and heat-capacity [C P (T, H )] measurements are combined with theoretical analysis to provide new insights into the nature of ferrimagnetism in Ni 4 Nb 2 O 9 . X-ray diffraction/Rietveld analysis of the prepared sample yielded the structural parameters of the orthorhombic crystal in agreement with previous studies, whereas x-ray photoelectron spectroscopy confirmed the Ni 2+ and Nb 5+ electronic states in Ni 4 Nb 2 O 9 . Analysis of χ ac (T ) shows the paramagnetic-to-ferrimagnetic transition occurs at 76.5 K (T FN ), which increases with applied field H as T FN ∝ H 0.35 due to the coupling of the ferromagnetic component with H . For T > T FN , the χ dc versus T data are fitted to the Néel's expression for ferrimagnets, yielding the g-factors for the two Ni 2+ ions as g A = 2.47 and g B = 2.10. Also, the antiferromagnetic molecular field constants between the A and B sublattices were evaluated as N AA = 26.31, N BB = 8.59, and N AB = 43.06, which, in turn, yield the antiferromagnetic exchange parameters: J AA /k B = 4.27 K, J BB /k B = 1.40 K, and J AB /k B = 6.98 K. For T < T FN , the M versus T data clearly show the magnetic compensation point at T COM ∼ 33 K. The mathematical model presented here using the magnitudes of N AA , N BB , and N AB correctly predicts the position of T COM as well the temperature variation of M both above and below T COM . The data of C P (T ) versus T shows a λ-type anomaly across T FN . After subtracting the lattice contribution, the C P (T ) data are fitted to C P = A(T -T N ) (-α) yielding the critical exponent α = 0.14(0.12) for T < T FN (T > T FN ), which is a characteristic of second-order phase transition. Magnetic entropy changes determined from the M-H isotherms shows that the applied field H enhances the magnetic ordering for T > T FN and T < T COM , but for T COM < T < T FN , the spin disorder increases with the increase in H . The temperature variation of the measured coercivity H C (T ) and remanence M R (T ) from 1.9 K to T FN initially show a decreasing trend, becoming zero at T COM , then followed by an increase and eventually becoming zero again at T FN .

The energy crisis and climate change have promoted a growing interest in non-fossil sources, such as nuclear, with uranium carbides being seen as potential fuel candidates for Generation IV nuclear reactors. However, the need of accurate thermophysical data for the fuel and its compatibility with core materials during the extreme fission conditions is still an issue. Here a study of the ternary uranium-iron-carbon system performed at 1100 °C using powder x-ray diffraction and Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry is presented. The U-Fe-C isothermal section is characterized by two ternary compounds, thirteen 3-phase regions and five 2-phase regions. UFeC 2 and * U 11 Fe 12 C 18 were confirmed to be present at 1100 °C and crystallize in structures related to the binary uranium carbides. UFeC 2 crystallizes in an original structure type, a distorted variant of the UCoC 2 structure, with space group P4/n and a = 3.503(5) A ˚and c = 7.405(5) A ˚lattice parameters. * U 11 Fe 12 C 18, has a crystal structure related to the Th 11 Ru 12 C 18 structure-type (space group I 4 3m) with the lattice parameter a & 10 A ˚. Furthermore, an island of a a-UC 2 -based phase with 32U:4Fe:64C composition was found in the 1100 °C isothermal section, indicating the inclusion of Fe in the a-UC 2 binary compound.

Nanocrystalline ribbons of inverse Heusler alloy Mn2Ni1.6Sn0.4 have been synthesised by melt spinning of the arc melted bulk precursor. The single phase ribbons crystallize into a cubic structure and exhibit very fine crystallite size of < 2 nm. Temperature dependent magnetization (M-T) measurements reveal that austenite (A)-martensite (M) phase transition begins at TCA C 248 K and finishes at TM C 238 K during cooling cycle and these values increase to TCA W 267 K and TM W 259 K while warming. In cooling cycle, the A-phase shows ferromagnetic (FM) ordering with a Curie temperature T 267 K, while both the FMantiferromagnetic (AFM) and M-transitions occur at T 242 K. The M-phase undergoes FM transition at T 145 K. These transitions are also confirmed by temperature dependent resistivity (-T) measurements. The observed hysteretic behaviour of M-T and -T in the temperature regime spanned by the A-M transition is a manifestation of the first order phase transition. M-T and -T data also provide unambiguous evidence in favour of spin glass at . The scaling of the glass freezing temperature (Tf) with frequency, extracted from the frequency dependent AC susceptibility measurements, confirms the existence of canonical spin glass at T T 145 K. The occurrence of canonical spin glass has been explained in terms of the nanostructuring modified interactions between the FM correlations in the martensitic phase and the coexisting AFM.

Magnetic properties of a sigma-phase Fe 60 V 40 intermetallic compound were studied by means of ac and dc magnetic susceptibility and magnetocaloric effect measurements. The compound is a soft magnet yet it was found to behave like a re-entrant spin-glass system. The magnetic ordering temperature was found to be T C 170 K, while the spin-freezing temperature was 164 K. Its relative shift per decade of ac frequency was 0.002, a value smaller than that typical of canonical spin-glasses. Magnetic entropy change, S, in the vicinity of T C was determined for magnetic field, H, ranging between 5 and 50 kOe. Analysis of S in terms of the power law yielded the critical exponent, n, vs. temperature with the minimum value of 0.75 at T C , while from the analysis of a relative shift of the maximum value of S with the field a critical exponent =1.7 was obtained. Based on scaling laws relationships values of other two exponents viz. =0.6 and =1 were determined.

A -phase Fe 65.9 V 34.1 alloy was investigated with the AC magnetic susceptibility as a function of temperature, frequency and external magnetic field. An evidence was found that its magnetism shows features characteristic of a reentrant behavior viz. two transitions: first at T C 312K from the paramagnetic state into the collinear ferromagnetic one, and second at T f 302K to a mixed state (sometimes termed as a ferromagnetic re-entrant spin glass) which, finally, at a lower temperature (T RSG 60K) transforms to a state where replica symmetry is broken. The frequency dependence of T f is lower than that of canonical spin glasses, a feature, that in the light of a high concentration of magnetic carriers, can be understood in terms of a weak coupling between magnetic clusters.

A multi-time probing of density fluctuations is introduced to investigate hidden time scales of heterogeneous dynamics in glass-forming liquids. Molecular dynamics simulations for simple glassforming liquids are performed, and a three-time correlation function is numerically calculated for general time intervals. It is demonstrated that the three-time correlation function is sensitive to the heterogeneous dynamics and that it reveals couplings of correlated motions over a wide range of time scales. Furthermore, the time scale of the heterogeneous dynamics τ hetero is determined by the change in the second time interval in the three-time correlation function. The present results show that the time scale of the heterogeneous dynamics τ hetero becomes larger than the α-relaxation time at low temperatures and large wavelengths. We also find a dynamical scaling relation between the time scale τ hetero and the length scale ξ of dynamical heterogeneity as τ hetero ∼ ξ z with z = 3.

We report the magnetic properties of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O, a high-entropy oxide with a rocksalt structure, and the influence of substitutions on these properties. From the magnetic susceptibility and neutron diffraction measurements, we found that this compound exhibits long-range magnetic order below 120 K despite the substantial structural disorder. Other rocksalt-type high-entropy oxides with various chemical substitutions were found to host either an antiferromagnetic order or spin-glass state depending on the amount of magnetic ions. The presence of magnetic order for such a disordered material potentially provides a route to explore exotic magnetic properties and functions.

We study the exact ground state of the two-dimensional random-field Ising model as a function of both the external applied field B and the standard deviation of the Gaussian random-field distribution. The equilibrium evolution of the magnetization consists in a sequence of discrete jumps. These are very similar to the avalanche behavior found in the out-of-equilibrium version of the same model with local relaxation dynamics. We compare the statistical distributions of magnetization jumps and find that both exhibit power-law behavior for the same value of . The corresponding exponents are compared.

We present a simple model that allows hysteresis loops with exchange bias to be reproduced. The model is a modification of the Tϭ0 random-field Ising model driven by an external field and with synchronous local relaxation dynamics. The main novelty of the model is that a certain fraction f of the exchange constants between neighboring spins is enhanced to a very large value J E . The model allows the dependence of the exchange bias and other properties of the hysteresis loops to be analyzed as a function of the parameters of the model: the fraction f of enhanced bonds, the amount of the enhancement J E , and the amount of disorder, which is controlled by the width of the Gaussian distribution of the random fields.

We study low-temperature properties of Pr 1Ϫx La x Pb 3 with the ground state of a non-Kramers ⌫ 3 for 0рx р0.8. From the concentration dependence of the specific heat, quadrupolar ordering is expected to occur only up to xϳ0.02. For a wide range of La concentrations where the ordering is absent, the specific heat shows a T linear variation, which is in an excellent agreement with the result obtained by the model for amorphous materials with a random configuration of a two-level system. Impurity effects on quadrupolar moments will be discussed.

Muon spin rotation experiments are carried out on clinoatacamite, Cu 2 ClOH 3 , which is a new geometrically frustrated system featuring a three-dimensional network of corner-sharing tetrahedral 3d Cu 2 spins. A long-range antiferromagnetic order occurs below 18.1 K with a surprisingly small entropy release of about 0:05R ln2=Cu. Below 6.5 K, the static long-range order transforms abruptly into a metastable state with nearly complete depolarization of muon spins which suggests strong fluctuation. The system then enters a state in which partial long-range order and spin fluctuation coexist down to the lowest experimentally attainable temperature of 20 mK. This work presents a novel system for studying geometric frustration.

Structure and magnetic properties of the rhombohedral-structure compound Co 2 ͑OH͒ 3 Br, a member of the geometrically frustrated series of the compounds M 2 ͑OH͒ 3 X where the magnetic ions form a deformed pyrochlore lattice, were studied using dc and ac magnetic susceptibilities, heat-capacity, neutron powder-diffraction, and muon-spin-rotation/-relaxation ͑SR͒ measurements. The structure of Co 2 ͑OH͒ 3 Br is featured by alternatively stacked layers of perfect kagome-lattice planes and triangular-lattice planes with a 10% distortion along the stacking direction ͑the c axis͒. Despite a very small difference in the distortion ͓0.42% larger in Co 2 ͑OH͒ 3 Br͔, Co 2 ͑OH͒ 3 Br was found to show contrasting antiferromagnetism that is strikingly different from the previously reported ferromagnetic Co 2 ͑OH͒ 3 Cl. Successive antiferromagnetic transition was observed at T N1 = 6.2 K and T N2 = 4.8 K, respectively. The antiferromagnetic ground state is metastable and an intermediate magnetic phase was induced by applying a relatively low magnetic field of H ϳ 5 kOe. When the field was further increased above H ϳ 20 kOe spin reorientation occurred to form a configuration similar to ferromagnetic Co 2 ͑OH͒ 3 Cl. The successive antiferromagnetic transitions in zero field were found to occur with propagation vector of k 1 = ͑0 -1/ 2 1/ 2͒ at T N1 and an additional k 2 = ͑0 0 3/ 2͒ at T N2 . Refinement of the neutron powder-diffraction patterns revealed an unconventional multi-k and noncoplanar spin structure for the antiferromagnetic phases. Multiple measurements, in particular, the SR study, consistently demonstrated magnetic coupling at high temperatures, and persistent fluctuations well below the T N . This work presents a unique system to investigate the orbital effect and the critical role of lattice distortion in geometric frustration, and provides a single material system to study multiple phase transitions and competing exchange interactions.

Unconventional magnetic properties are found for a quantum spin system of mineral clinoatacamite, Cu 2 Cl͑OH͒ 3 . Clinoatacamite is found to undergo an antiferromagnetic transition at 18.1 K with a small entropy fall ͑0.05R ln 2͒. At temperatures below 6.4 K, it transits into a disordered spin-glass-like state but with large specific heat anomalies. The entire entropy fall for the spin system is about 0.31R ln 2, which is smaller than those for dipolar spin ice ͑0.67R ln 2͒ and for ice I h ͑0.71R ln 2͒. Geometric frustration for the quantum Cu 2+ spins is suggested based on its crystal structure, magnetization, and specific heat studies.

A systematic study of magnetic susceptibility measurements is carried out on distorted tetrahedral lattice hydroxyhalides Fe 2 (OH) 3 Cl, Mn 2 (OH) 3 Cl, Mn 2 (OH) 3 Br, and Co 2 (OH) 3 Br, following our recent finding of coexisting antiferromagnetic order and disorder in Cu 2 (OH) 3 Cl (clinoatacamite). These transition metal hydroxyhalides are found to undergo antiferromagnetic transitions, at T N = 14 K for Fe 2 (OH) 3 Cl, at two successive transitions of T N1 = 3.4 K and T N2 = 2.7 K for Mn 2 (OH) 3 Cl, at T N1 = 3.3 K and T N2 = 2.4 K for Mn 2 (OH) 3 Br, and at T N = 5 K for Co 2 (OH) 3 Br, respectively. All hydroxychlorides show coexisting glassiness below the magnetic transitions; meanwhile, the glassiness is not found in the hydroxybromides. The distorted tetrahedral lattice for the magnetic ions consists of stacked layers of triangular lattice planes and Kagome lattice planes, with the magnetic ions on the Kagome lattice planes being bonded by the halogen ions. The contrasting behaviours in Mn 2 (OH) 3 Cl and Mn 2 (OH) 3 Br suggest that the glassiness arises from competing magnetic interactions in the tetrahedron, specifically on the Kagome lattice planes.

We investigate theoretically the low-temperature physics of a two-component ultracold mixture of bosons and fermions in disordered optical lattices. We focus on the strongly correlated regime. We show that, under specific conditions, composite fermions, made of one fermion plus one bosonic hole, form. The composite picture is used to derive an effective Hamiltonian whose parameters can be controlled via the boson-boson and the bosonfermion interactions, the tunnelling terms and the inhomogeneities. We finally investigate the quantum phase diagram of the composite fermions and show that it corresponds to the formation of Fermi glasses, spin glasses and quantum percolation regimes.

Starting from microscopic and symmetry considerations, we derive the Hamiltonian describing the exchange interaction between the localized Mn spins and the valence band holes in Ga1-xM nxAs. We find that due to the strong spin-orbit coupling in the valence band, this exchange interaction has a rather complex structure and generates a highly anisotropic effective interaction between the Mn spins. The corresponding ground state has a finite ferromagnetic magnetization but is intrinsically spin-disordered even at zero temperature.