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Dynamics of a Ferromagnetic Domain Wall and the Barkhausen Effect

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Dynamics of a Ferromagnetic Domain Wall and the Barkhausen Effect VOLUME 79, NUMBER 23 PHYSICAL REVIEW LETTERS 8DECEMBER 1997 Dynamics of a Ferromagnetic Domain Wall and the Barkhausen Effect Pierre Cizeau,1 Stefano Zapperi,1 Gianfranco Durin,2 and H. Eugene Stanley1 1Center for Polymer Studies and Department of Physics, Boston University, Boston, Massachusetts 02215 2Istituto Elettrotecnico Nazionale Galileo Ferraris and GNSM-INFM, Corso M. d'Azeglio 42, I-10125 Torino, Italy (Received 2 July 1997) We derive an equation of motion for the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium, and we study the associated depinning transition. The long-range dipolar interactions set the upper critical dimension to be dc ­ 3, so we suggest that mean-field exponents describe the Barkhausen effect for three-dimensional soft ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a function of the field driving rate and the intensity of the demagnetizing field, and find results in quantitative agreement with experiments on crystalline and amorphous soft ferromagnetic alloys. [S0031-9007(97)04766-2] PACS numbers: 75.60.Ej, 68.35.Ct, 75.60.Ch The magnetization of a ferromagnet displays discrete Here, we present an accurate treatment of magnetic in- jumps as the external magnetic field is increased. This teractions in the context of the depinning transition, which phenomenon, known as the Barkhausen effect, was first allows us to explain the experiments and to give a micro- observed in 1919 by recording the tickling noise produced scopic justification for the model of Ref. [13]. We study by the sudden reversal of the Weiss domains [1]. The a single domain wall separating two regions with magne- Barkhausen effect has been widely used as a nondestruc- tization of constant magnitude M and opposite directions. tive method to test magnetic materials, and the statistical We assume that the wall surface does not form overhangs, properties of the noise have been analyzed in detail [2,3]. and describe the position of the wall by its height hsr$, td. In particular, it has been observed that the distributions of The motion of the wall is overdamped because of eddy sizes and durations of Barkhausen jumps decay as power currents so, neglecting thermal fluctuations, the evolution laws at low applied field rates [4–6]. In addition to its of hsr$, td is governed by practical and technological applications, the Barkhausen $, $, ­hsr td ­ dEsshhsr tdjdd effect has recently attracted a growing interest as an ex- 2 $ , (1) ample of a complex dynamical system displaying critical ­t dhsr, td behavior [7–11]. where Esshhsr$, tdjdd is the energy of the system [14,15]. In soft ferromagnetic materials the magnetization We will show that by incorporating the effects of ferro- process is composed of two distinct mechanisms [12]. magnetic, magnetocrystalline, magnetostatic and dipolar (i) When the field is increased from the saturated region, interactions, and disorder, the equation of motion is domains nucleate in the sample, typically starting from the $ boundaries. (ii) In the central part of the hysteresis loop, ­hsr, td 2 2 0 0 ­ n0= hsr$, td 1 H 2 Hd 1 d r Ksr$ 2 r$ d around the coercive field, the magnetization process is ­t Z due mainly to domain wall motion. The disorder present 3 fhsr$ 0, td 2 hsr$, tdg 1hsr$,hd. (2) in the material (due to nonmagnetic impurities, lattice dislocations, residual stresses, etc.) is responsible for Here n0 is the surface tension, H is the external magnetic the jerky motion of the domain walls, giving rise to the field, the demagnetizing field Hd and the nonlocal term jumps observed in the magnetization. The moving walls ± are due to dipolar interactions, and we model the disorder are usually parallel to the magnetization (180 walls), and with a random force, with Gaussian distribution and short span the sample from end to end [12]. The statistical range correlations, properties of the Barkhausen effect are normally studied in the central part of the hysteresis loop and can therefore khsr$, hdhsr$ 0, h0dl ­ d2sr$ 2 r$ 0dDsh 2 h0d , (3) be understood by studying domain wall motion. It has recently been proposed to relate the scaling where Dsxd decays very rapidly for large values of the properties of the Barkhausen noise to the critical behavior argument. expected at the depinning transition of an elastic interface The dipolar interactions are treated considering effective [9,11]. The numerical values of the scaling exponents, magnetic charges induced by the discontinuities of the however, do not agree with most experimental data [4–6]. magnetization across the boundaries of the sample and the Interestingly, a quantitative description of the phenomenon domain wall [16]. The corresponding magnetic surface can be obtained by a simple phenomenological model, charge is given by where the wall is described as a single point moving in $ $ a correlated random pinning field [13]. s ­ sM1 2 M2d ? nà , (4) 0031-9007y97y79(23)y4669(4)$10.00 © 1997 The American Physical Society 4669 VOLUME 79, NUMBER 23 PHYSICAL REVIEW LETTERS 8DECEMBER 1997 $ $ where nà is normal to the surface and M1 and M2 are the For k ­ 0 (no demagnetizing field), Eq. (2) displays a magnetization vectors on each side of the surface. The depinning transition; i.e., there exists a critical field Hc surface charge induced at the boundary of the sample gives such that for H , Hc the interface is pinned while for rise to a demagnetizing field which opposes the external H . Hc, it moves with nonzero average velocity. At field [12]. The simplest approximation is to consider H ­ Hc, the system exhibits scaling properties: The in- this field to be constant throughout the sample, and to terface moves by avalanches whose sizes s and durations be proportional to the total magnetization [9,13]. This T distributions follow power laws, 2 yields the term H ­ 2k d rhsr$,td in Eq. (2), where 2t 2a d Pssd,s , PsTd,T . (8) the demagnetizing factor k takesR into account the geometry of the domain structure, the shape of the system, and When k . 0, the demagnetizing field provides an addi- its size. tional restoring force that keeps the interface at the de- Magnetic surface charges will also appear on the domain pinning transition [9,11] if the external field is increased wall when its surface is not parallel to the magnetization. adiabatically. In the limit of infinite anisotropy [17] and small bending Using the functional renormalization group scheme in- of the surface, we can express the surface charge as troduced in Ref. [23], we find that—due to the long- range kernel Ksr$ 2 r$ 0d in Eq. (6)—the critical behavior ­hsr$, td ssr$d ­ 2M cos u . 2M , (5) of Eq. (2) differs from that of elastic interfaces: The up- ­x per critical dimension becomes dc ­ 3 [18,24,25], instead where u is the local angle between the vector normal to of dc ­ 5 [21,22]. Hence we predict that, for d ­ 3, the the surface and the magnetization (see Fig. 1). The inter- motion of the domain wall will be described by mean-field 2 2 0 action energy of these charges Ed ­ s1y2d d rd r 3 theory (apart from logarithmic corrections), which yields ssr$dssr$0dyjr$ 2 r$ 0j gives rise to the nonlocalR kernel [18,22] t ­ 3y2, a ­ 2, and that the surface is flat (the present in Eq. (2) [14,15,18]: roughness exponent z is zero). These values differ signifi- cantly from the results of elastic interfaces which in d ­ 3 2 0 2 0 2M 3sx 2 x d are t 1.3, a 1.5, and z 0.7 [22,26]. Ksr$ 2 r$ d ­ 1 1 . (6) . jr$ 2 r$ 0j3 µ jr$ 2 r$0j2 ¶ Next we make contact between our approach and the conventional approach that reduces the domain wall to a The interaction (6) is long range and anisotropic, as can single point moving in a random pinning field [3,13,27]. also be seen by considering the Fourier transform To this end, we introduce an infinite range version sd ! `d 2M2 p2 of Eq. (2), which should have the same critical behavior Ksp, qd ­ , (7) p p2 1 q2 as Eq. (2) but has the advantage of being much simpler to p analyze. where p and q are the two components of the Fourier To treat the infinite-range model, we discretize the vector. Moreover, an estimate of the order of magnitude of interface and consider that all N elements are at the same $ $ 0 Ksr 2 r d shows that it dominates over the surface tension distance from each other. Equation (2) then becomes [28] for all length scales of interest [19]. Apart from the nonlocal kernel, Eq. (2) is the equation ­histd ­ Hstd 2xhÅ1JfhÅ2hstdg 1hshd, (9) proposed in Ref. [9] which, in turn, reduces when k ­ 0 ­t i i to a driven elastic interface in the presence of quenched Å N 2 disorder [20–22]. where h ; i­1 hiyN, x ; Nk, J ;sn012M d,and the externalPfield Hstd increases at a finite constant rate. Summing Eq. (9) over all sites i, we obtain an equation for the total magnetization m ; NhÅ, dm N ­ ctÄ 2xm1 hishd, (10) dt iX­1 where the time dependence of the field has been made explicit. We can approximate hi by an effective random pinning field Wsmd, dependingPonly on the magnetization. When the interface moves between two configurations, the change in W is 0 0 Wsm d 2 Wsmd ­ Dhi , (11) FIG. 1. The domain wall separating two regions of opposite Xi magnetization.
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