Competition of magneto-dipole, anisotropy and exchange interactions in composite multiferroics

We study the competition of magneto-dipole, anisotropy and exchange interactions in composite three dimensional multiferroics. Using Monte Carlo simulations we show that magneto-dipole interaction does not suppress the ferromagnetic state caused by the interaction of the ferroelectric matrix and magnetic subsystem. However, the presence of magneto-dipole interaction influences the order-disorder transition: depending on the strength of magneto-dipole interaction the transition from the ferromagnetic to the superparamagnetic state is accompanied either by creation of vortices or domains of opposite magnetization. We show that the temperature hysteresis loop occurs due to non-monotonic behavior of exchange interaction versus temperature. The origin of this hysteresis is related to the presence of stable magnetic domains which are robust against thermal fluctuations.

Single electron tunneling with slow insulators

Usual paradigm in the theory of electron transport is related to the fact that the dielectric permittivity of the insulator is assumed to be constant, no time dispersion. We take into account the slow polarization dynamics of the dielectric layers in the tunnel barriers in the fluctuating electric fields induced by single-electron tunneling events and study transport in the single electron transistor (SET). Here slow dielectric implies slow compared to the characteristic time scales of the SET charging-discharging effects. We show that for strong enough polarizability, such that the induced charge on the island is comparable with the elementary charge, the transport properties of the SET substantially deviate from the known results of transport theory of SET. In particular, the coulomb blockade is more pronounced at finite temperature, the conductance peaks change their shape and the current-voltage characteristics show the memory-effect (hysteresis). However, in contrast to SETs with ferroelectric tunnel junctions, here the periodicity of the conductance in the gate voltage is not broken, instead the period strongly depends on the polarizability of the gate-dielectric. We uncover the fine structure of the hysteresis-effect where the large hysteresis loop may include a number of smaller loops. Also we predict the memory effect in the current-voltage characteristics $I(V)$, with $I(V) eq -I(-V)$.