A theoretical perspective is provided on the glass transition in molecular liquids at thermal equilibrium, on the spatially heterogeneous and aging dynamics of disordered materials, and on the rheology of soft glassy materials. We start with a broad introduction to the field and emphasize its connections with other subjects and its relevance. The important role played by computer simulations in studying and understanding the dynamics of systems close to the glass transition at the molecular level is given. The recent progress on the subject of the spatially heterogeneous dynamics that characterizes structural relaxation in materials with slow dynamics is reviewed. The main theoretical approaches are presented describing the glass transition in supercooled liquids, focusing on theories that have a microscopic, statistical mechanics basis. We describe both successes and failures and critically assess the current status of each of these approaches. The physics of aging dynamics in disordered materials and the rheology of soft glassy materials are then discussed, and recent theoretical progress is described. For each section, an extensive overview is given of the most recent advances, but we also describe in some detail the important open problems that will occupy a central place in this field in the coming years.

Theoretical perspective on the glass transition and amorphous materials

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

Facing the ever-growing demand for data storage will most probably require a new paradigm. Nanoscale magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin ﬁlms in nature. We designed cobalt-based multilayered thin ﬁlms in which the cobalt layer is sandwiched between two heavy metals and so provides additive interfacial Dzyaloshinskii-Moriya interactions (DMIs), which reach a value close to 2 mJ m(-2) in the case of the Ir|Co|Pt asymmetric multilayers. Using a magnetization-sensitive scanning X-ray transmission microscopy technique, we imaged small magnetic domains at very low fields in these multilayers. The study of their behaviour in a perpendicular magnetic ﬁeld allows us to conclude that they are actually magnetic skyrmions stabilized by the large DMI. This discovery of stable sub-100 nm individual skyrmions at room temperature in a technologically relevant material opens the way for device applications in the near future.

https://hal.archives-ouvertes.fr/hal-01344269/document

Additive interfacial chiral interaction in multilayers for stabilization of small individual skyrmions at room temperature.

Within the past 20 years or so, there has occurred an explosion of interest in the magnetic behavior of pyrochlore oxides of the type $A_{2}^{3+}$$B_{2}^{4+}$O$_{7}$ where $A$ is a rare-earth ion and $B$ is usually a transition metal. Both the $A$ and $B$ sites form a network of corner-sharing tetrahedra which is the quintessential framework for a geometrically frustrated magnet. In these systems the natural tendency to form long range ordered ground states in accord with the Third Law is frustrated, resulting in some novel short range ordered alternatives such as spin glasses, spin ices and spin liquids and much new physics. This article attempts to review the myriad of properties found in pyrochlore oxides, mainly from a materials perspective, but with an appropriate theoretical context.

/pdf/magnetic-pyrochlore-oxides-3nd6sywsbi.pdf

Magnetic Pyrochlore Oxides

Magnetocaloric effect: From materials research to refrigeration devices

Many dense magnetic nanoparticle systems exhibit slow dynamics which is qualitatively indistinguishable from that observed in atomic spin glasses and its origin is attributed to dipole interactions among particle moments (or superspins). However, even in dilute nanoparticle systems where the dipole interactions are vanishingly small, slow dynamics is observed and is attributed solely to a broad distribution of relaxation times which in turn comes from that of the anisotropy energy barriers. To clarify characteristic differences between the two types of slow dynamics, we study a simple model of a noninteracting nanoparticle system (a superparamagnet) analytically as well as ferritin (a superparamagnet) and a dense ${\mathrm{Fe}}_{3}\mathrm{N}$ nanoparticle system (a superspin glass) experimentally. It is found that superparamagnets in fact show aging (a waiting time dependence) of the thermoremanent magnetization as well as various memory effects. We also find some dynamical phenomena peculiar only to superspin glasses such as the flatness of the field-cooled magnetization below the critical temperature and memory effects in the zero-field-cooled magnetization. These dynamical phenomena are qualitatively reproduced by the random energy model, and are well interpreted by the so-called droplet theory in the field of spin-glass study.

Aging and memory effects in superparamagnets and superspin glasses

Magnetic properties of nanoparticle systems and spin glasses have been investigated theoretically, and experimentally by squid magnetometry.Two model three-dimensional spin glasses have been studie ...

/pdf/superparamagnetism-and-spin-glass-dynamics-of-interacting-4asrz264pw.pdf

Superparamagnetism and Spin Glass Dynamics of Interacting Magnetic Nanoparticle Systems

The physical properties of magnetic nanoparticles have been investigated with focus on the influence of dipolar interparticle interaction. For weakly coupled nanoparticles, thermodynamic perturbation theory is employed to derive analytical expressions for the linear equilibrium susceptibility, the zero-field specific heat and averages of the local dipolar fields. By introducing the averages of the dipolar fields in an expression for the relaxation rate of a single particle, a nontrivial dependence of the superparamagnetic blocking on the damping coefficient is evidenced. This damping dependence is interpreted in terms of the nonaxially symmetric potential created by the transverse component of the dipolar field. 
Strongly interacting nanoparticle systems are investigated experimentally in terms of spin glass behavior. Disorder and frustration arise in samples consisting of frozen ferrofluids from the randomness in particle position and anisotropy axis orientation. A strongly interacting FeC system is shown to exhibit critical dynamics characteristic of a spin glass phase transition. Aging, memory and rejuvenation phenomena similar to those of conventional spin glasses are observed, albeit with much weaker rejuvenation effects than in both a Ag(11 at% Mn) Heisenberg and an Fe_{0.5}Mn_{0.5}TiO_3 Ising spin glass. Differences in the nonequilibrium dynamics of the strongly interacting nanoparticle system and the two spin glass samples are discussed in terms of anisotropy and different timescales, due to the much longer microscopic flip time of a magnetic moment than of an atomic spin.

Nonequilibrium dynamics in a Ag(Mn) spin glass are investigated by measurements of the temperature dependence of the remanent magnetization. Using specific cooling protocols before recording the thermo- or isothermal remanent magnetizations on reheating, it is found that the measured curves effectively disclose nonequilibrium spin glass characteristics such as aging and memory phenomena as well as an extended validity of the superposition principle for the relaxation. The usefulness of this ``simple'' dc method is discussed, as well as its applicability to other disordered magnetic systems.

Memory and superposition in a spin glass

Effects of dipole-dipole interactions on the magnetic relaxation have been investigated for three Fe-C nanoparticle samples with volume concentrations of 0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective behaviour due to dipolar interactions, only the 17 vol% sample displays critical behaviour close to its transition temperature. The behaviour of the 5 vol% sample can be attributed to a mixture of collective and single-particle dynamics.

https://iopscience.iop.org/article/10.1088/0953-8984/14/19/314/pdf

Petra E. Jönsson

Papers

Aging and memory effects in superparamagnets and superspin glasses

Superparamagnetism and Spin Glass Dynamics of Interacting Magnetic Nanoparticle Systems

Superparamagnetism and Spin Glass Dynamics of Interacting Magnetic Nanoparticle Systems

Memory and superposition in a spin glass

Critical dynamics of an interacting magnetic nanoparticle system