Uniform and nonuniform thermal switching of magnetic particles

Abstract

The pulse-noise approach to systems of classical spins weakly interacting with the bath has been applied to study thermally-activated escape of magnetic nanoparticles over the uniform and nonuniform energy barriers at intermediate and low damping. The validity of approximating a singledomain particle by a single spin is investigated. Barriers for a non-uniform escape of elongated particles for the uniaxial model with transverse and longitudinal field have been worked out. Pulsenoise computations have been done for finite magnetic chains. The linear stability of the uniform barrier state has been investigated. The crossover between uniform and nonuniform barrier states has been studied with the help of the variational approach.

Thermal collapse of a skyrmion

“…where T is the temperature. To speed up the numerical integration of the LLL equation, one can replace the continuous white noise by the pulse noise with the period ∆t 43,44 . Noiseless evolution during the interval ∆t between the pulses can be computed by an efficient highorder ODE solver such as fourth-order Runge-Kutta method with the integration step δt ∆t for weak damping.…”

“…The collected statistics of collapse times was used to extract the collapse rate Γ with the help of the new algorithm that does not require that all skyrmions collapse (see the Appendix of Ref. 44 ). Finally, the obtained data for Γ were fit to the Arrhenius law, Γ = Γ 0 exp(−U/T ), to extract the exponent U and the prefactor Γ 0 .…”

The optical tweezer of skyrmions

“…The analytical estimation of the barrier is valid only in the critical region. Away from the critical region, the rate of skyrmion collapse can be estimated numerically as 56 x as a function of the frequency f e plotted for electric fields: E 0 ¼ E l0 þ E l1 sinð2πf e tÞ, with E l0 = 1.2 MV cm −1 and E l1 = 0.07E l0 . c Dependence of the critical velocity on the E l1 /E l0 at the frequency f e = 0.22 GHz.…”

Single-particle versus collective effects in assemblies of nanomagnets: Screening

“…The leading terms have coefficients K 2 and K 4 that are functions of the atomistic parameters (J, K c , K s , z, etc) and of the size and shape of the NM [13][14][15]. Note that both the core and surface may contribute to K 2 and K 4 .…”

“…In fact, even in this case the quartic term appears and is a pure surface contribution. Regarding the coefficient of the quartic contribution (k ≡ K/J), for a sphere we have k 4 = κk 2 s /zJ where κ is a surface integral [13] and for a cube k 4 = 1 − 0.7/N 1/3 4 k 2 s /zJ [15]. The most relevant parameter of this effective model is the ratio which roughly represents the relative contribution of the surface disorder and the ensuing spin noncolinearities.…”