Anatomy of plastic events in magnetic amorphous solids

Effective forces in thermal amorphous solids with generic interactions

In thermal glasses at temperatures sufficiently lower than the glass transition, the constituent particles are trapped in their cages for a sufficiently long time such that their time-averaged positions can be determined before diffusion and structural relaxation takes place. The effective forces are those that hold these average positions in place. In numerical simulations the effective forces F_{ij} between any pair of particles can be measured as a time average of the bare forces f_{ij}(r_{ij}(t)). In general, even if the bare forces come from two-body interactions, thermal dynamics dress the effective forces to contain many-body interactions. Here, we develop the effective theory for systems with generic interactions, where the effective forces are derivable from an effective potential and in turn they give rise to an effective Hessian whose eigenvalues are all positive when the system is stable. In this Rapid Communication, we offer analytic expressions for the effective theory, and demonstrate the usefulness and the predictive power of the approach.

Mechanical avalanches promoted by magnetoelastic coupling in magnetic metallic glasses

The effect of magnetoelastic coupling on the mechanical behavior of magnetic metallic glasses is studied by superposing a magnetic field to the mechanical load during deformation experiments. The mechanical response is studied from the point of view of the global behavior, characterized by the Young's modulus, and from the perspective of the avalanches, estimated from the intermittency of the deformation signal. The influence of several intensities and orientations of magnetic field is analyzed in two amorphous alloys with different magnetostriction. For the highly magnetostrictive alloy, the decrease of the macroscopic Young's modulus under certain magnetic fields is accompanied by the promotion of mechanical avalanches, a fact that suggests that in this case the magnetoelastic coupling involves some extent of non-linear addition of the magnetic and mechanical energies.

Depinning and heterogeneous dynamics of colloidal crystal layers under shear flow

Using Brownian dynamics (BD) simulations and an analytical approach we investigate the shear-induced, nonequilibrium dynamics of dense colloidal suspensions confined to a narrow slit-pore. Focusing on situations where the colloids arrange in well-defined layers with solidlike in-plane structure, the confined films display complex, nonlinear behavior such as collective depinning and local transport via density excitations. These phenomena are reminiscent of colloidal monolayers driven over a periodic substrate potential. In order to deepen this connection, we present an effective model that maps the dynamics of the shear-driven colloidal layers to the motion of a single particle driven over an effective substrate potential. This model allows us to estimate the critical shear rate of the depinning transition based on the equilibrium configuration, revealing the impact of important parameters, such as the slit-pore width and the interaction strength. We then turn to heterogeneous systems where a layer of small colloids is sheared with respect to bottom layers of large particles. For these incommensurate systems we find that the particle transport is dominated by density excitations resembling the so-called "kink" solutions of the Frenkel-Kontorova (FK) model. In contrast to the FK model, however, the corresponding "antikinks" do not move.