arXiv subjects

Thomas Nattermann

Publications and source records attributed to Thomas Nattermann.

40 records · Page 3Linked to original sources

arXiv1997

Theory of Hysteresis Loop in Ferromagnet

A theory of the hysteresis loop in ferromagnets controlled by the domain wall motion is presented. Domain walls are considered as plane or linear interfaces moving in a random medium under the action of the external ac magnetic field $H=H_0\sinωt$. We introduce important characteristics of the hysteresis loop, such as dynamic threshold fields, reversal field etc. together with well known characteristics as coercive field and hysteresis loop area (HLA) $\cal A$. We show that all these characteristics are regulated by two dimensionless combinations of the $H_0$ and $ω$and intrinsic characteristics of the ferromagnet. The moving domain wall can create magnetic bubbles playing the role of pre-existing nuclei of the reversed magnetization. We discuss a simple model of this process. For magnetization reversal determined by domain inflation we predict that HLA scales as ${\cal A}\propto ω^βH_0^α$ with $α=1/2$ and $β=1/2$. Numerical simulations confirmthis result.

cond-mat.mtrl-sci↗

arXiv1997

Hysteretic Depinning of Anisotropic CDW

We investigate the depinning transition in a dirty periodic medium considering a model of layered charge density waves as a prototype system. We find that depinning from strong disorder occurs via a two stage process, where, first, the pinned system experiences a continuous transition into a plastically sliding state and undergoes a second sharp hysteretic transition into a coherently moving 3D state at higher drives. In the weakly disordered system the depinning into a coherently sliding state remains continuous.

cond-mat.dis-nn↗

arXiv1996

Driven Interface Depinning in a Disordered Medium

The dynamics of a driven interface in a medium with random pinning forces is analyzed. The interface undergoes a depinning transition where the order parameter is the interface velocity $v$, which increases as $v \sim (F-F_c)^θ$ for driving forces $F$ close to its threshold value $F_c$. We consider a Langevin-type equation which is expected to be valid close to the depinning transition of an interface in a statistically isotropic medium. By a functional renormalization group scheme the critical exponents characterizing the depinning transition are obtained to the first order in $ε=4-D>0$, where $D$ is the interface dimension. The main results were published earlier [T. Nattermann et al., J. Phys. II France {\bf 2} (1992) 1483]. Here, we present details of the perturbative calculation and of the derivation of the functional flow equation for the random-force correlator. The fixed point function of the correlator has a cusp singularity which is related to a finite value of the threshold $F_c$, similar to the mean field theory. We also present extensive numerical simulations and compare them with our analytical results for the critical exponents. For $ε=1$ the numerical and analytical results deviate from each other by only a few percent. The deviations in lower dimensions $ε= 2,3$ are larger and suggest that the roughness exponent is somewhat larger than the value $ζ= ε/ 3$ of an interface in thermal equilibrium.

cond-mat↗

arXiv1994

Disorder-Induced Depinning Transition

The competition in the pinning of a directed polymer by a columnar pin and a background of random point impurities is investigated systematically using the renormalization group method. With the aid of the mapping to the noisy-Burgers' equation and the use of the mode-coupling method, the directed polymer is shown to be marginally localized to an arbitrary weak columnar pin in 1+1 dimensions. This weak localization effect is attributed to the existence of large scale, nearly degenerate optimal paths of the randomly pinned directed polymer. The critical behavior of the depinning transition above 1+1 dimensions is obtained via an $ε$-expansion.

cond-mat↗