Observation of a time-dependent spatial correlation length in a metallic spin glass

Waiting time dependence of aging

Aging phenomena have been observed in many non-equilibrium systems such as polymers and glasses, where physical properties depend on the waiting time between the starting time of observation and the time when the temperature is changed. The aging is classified into two types on the basis of the waiting time dependence of an instantaneous relaxation time: When the relaxation time is always an increasing function of the waiting time, the aging is called Type I and when it depends on the protocol of the temperature change, the aging is called Type II. Aging of a random walk in three dimensions is investigated when the free energy landscape controlling the jump rate responds to temperature change with a delay. It is shown that the intermediate scattering function of the random walk model exhibits Type II aging. It is also shown that the relaxation time of the free energy landscape can be deduced from the waiting time dependence of the instantaneous relaxation time.

Anomalous dielectric response in the triple perovskite ruthenate Ba3BiRu2O9

We have investigated the magnetic, dielectric and thermal properties of theSRu= 1 magnetic dimer Ba₃BiRu₂O₉, which is known to exhibit a spin-gap opening in conjunction with a first-order magneto-elastic phase transition at ∼175 K. Above the spin-gap temperature, the temperature dependence dielectric constant shows a peak like feature with pronounced frequency dependence. The critical slowing down behavior of this frequency dispersion suggests that a ferroelectric relaxor like electrical glassy state exists above the spin-gap opening temperature regime. The extermination of frequency dispersion-right at the magneto-elastic phase transition, is suggestive of a strong coupling between the lattice and charge-spin degrees of freedom in this triple perovskite system.

Memory effect and phase transition in a hierarchical trap model for spin glasses

We introduce an efficient dynamical tree method that enables us to explicitly demonstrate the thermoremanent magnetization memory effect in a hierarchical energy landscape. Our simulation nicely reproduces the nontrivial waiting-time and waiting-temperature dependences in this nonequilibrium phenomenon. We further investigate the condensation effect, in which a small set of microstates dominates the thermodynamic behavior in the multilayer trap model. Importantly, a structural phase transition of the multilayer tree model is shown to coincide with the onset of the condensation phenomenon. Our results underscore the importance of hierarchical structure and demonstrate the intimate relation between the glassy behavior and structure of barrier trees.

Synthesis of a d2 kagome lattice antiferromagnet, (CH3NH3)2NaV3F12

A new d²-vanadium-based KLAF, (CH₃NH₃)₂NaV₃F₁₂, was synthesized hydrothermally and is a candidate to study S = 1 KLAF physics.

Magnetic ordered states induced by interparticle magnetostatic interaction in α-Fe/Au mixed nanoparticle assembly

The magnetic behavior of α-Fe/Au nanoparticle (NP) assemblies is studied over a very wide range of dipolar interactions among α-Fe NPs, by changing the volume density of the α-Fe NP. The assembly whose α-Fe NP density is lower than 0.1% exhibits typical superparamagnetic behavior. When Fe NP density exceeds 8.6% the magnetic dynamics changes to that resembling superspin glass. Moreover, NP assembly with highest Fe concentration (43%), whose dipolar interaction is enormously strong compared with previous studies, exhibits a two-stage magnetic transition, i.e., ferromagnetic and spin glass-like transitions at 385 K and around 150 K, respectively. Therefore, we first observed the reentrant spin glass-like magnetism at the limit of strong interaction in a close-packed NP assembly. Based on these observations, the magnetic phase diagram of the interacting α-Fe NP assembly is determined over a very wide range of interaction.

Thermal shifts and intermittent linear response of aging systems

At time t after an initial quench, an aging system responds to a perturbation turned on at time tw<t in a way mainly depending on the number of intermittent energy fluctuations, so-called quakes, which fall within the observation interval (tw,t] [P. Sibani, G. F. Rodriguez, and G. G. Kenning, Phys. Rev. B 74, 224407 (2006); P. Sibani, Eur. J. Phys. B 58, 483 (2007)]. The temporal distribution of the quakes implies a functional dependence of the average response on the ratio t/tw . Further insight is obtained imposing small temperature steps, so-called T shifts. The average response as a function of t/tw,eff , where tw,eff is the effective age, is similar to the response of a system aged isothermally at the final temperature. Using an Ising model with plaquette interactions, the applicability of analytic formulas for the average isothermal magnetization is confirmed. The T -shifted aging behavior of the model is approximately described using effective ages. Large positive shifts nearly reset the effective age. Negative T shifts offer a more detailed probe of the dynamics. Assuming the marginal stability of the "current" attractor against thermal noise fluctuations, the scaling form tw,eff=tw x and the dependence of the exponent x on the aging temperatures before and after the shift are theoretically available. The predicted form of x has no adjustable parameters. Both the algebraic scaling of the effective age and the form of the exponent reasonably agree with the data. The present simulations thus confirm the crucial role of marginal stability in glassy relaxation.

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.

Aging and self-organized criticality in driven dissipative systems

We study the noisy dynamics of a close relative to the original sandpile model. Depending on the type of noise and the time scale of observation, we find stationary fluctuations (similar to self-organized criticality) or an aging dynamics with punctuated equilibria, a decreasing rate of events and reset properties qualitatively similar to those of glassy systems, evolution models, and vibrated granular media.

Phenomenological glass model for vibratory granular compaction

A model for weakly excited granular media is derived by combining the free volume argument of Nowak et al. [Phys. Rev. E 57, 1971 (1998)] and the phenomenological model for supercooled liquids of Adam and Gibbs [J. Chem. Phys. 43, 139 (1965)]. This is made possible by relating the granular excitation parameter Gamma, defined as the peak acceleration of the driving pulse scaled by gravity, to a temperaturelike parameter eta(Gamma). The resulting master equation is formally identical to that of Bouchaud's trap model for glasses [J. Phys. I 2, 1705 (1992)]. Analytic and simulation results are shown to compare favorably with a range of known experimental behavior. This includes the logarithmic densification and power spectrum of fluctuations under constant eta, the annealing curve when eta is varied cyclically in time, and memory effects observed for a discontinuous shift in eta. Finally, we discuss the physical interpretation of the model parameters and suggest further experiments for this class of systems.