Ben Van de Wiele

Femke Olyslager

Landau L. D.

Luc Dupré

English

Van de Wiele, Ben

Dupré, Luc

Olyslager, Femke

Ghent University Academic Bibliography

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Memory properties in a Landau-Lifshitz hysteresis model for thin ferromagnetic sheets

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JOURNAL OF APPLIED PHYSICS 99, 08G101 2006Memory properties in a Landau-Lifshitz hysteresis model for thinferromagnetic sheetsBen Van de WieleDepartment of Information Technology, Ghent University, Sint-Pietersnieuwstraat 41, Ghent B-9000,BelgiumLuc DupréaDepartment of Electrical Energy, Systems and Automation, Ghent University, Sint-Pietersnieuwstraat 41,Ghent B-9000, BelgiumFemke OlyslagerDepartment of Information Technology, Ghent University, Sint-Pietersnieuwstraat 41, Ghent B-9000,BelgiumPresented on 1 November 2005; published online 20 April 2006The paper deals with a two-dimensional numerical model for the evaluation of the electromagnetichysteretic behavior of thin magnetic sheets when applying a unidirectional magnetic field. The timevariation of the magnetization vector m in each space point obeys the Landau-Lifshitz equation. Theeffective field is the result of several contributions: the applied field, the magnetostatic field, theanisotropy field, and the exchange field. Microstructural features, such as grain size andcrystallographic texture, are introduced in the micromagnetic model by dividing the geometry insubregions, each with its own magnetic preferable directions. In the article, numerical experimentsare presented aiming at low-frequency applications. The presented micromagnetic model is used tostudy magnetic memory material properties. © 2006 American Institute of Physics.DOI: 10.1063/1.2165585I. INTRODUCTIONFerromagnetic polycrystalline material can be magne-tized by an externally imposed time varying magnetic field.However, between the magnetization of the material and theapplied magnetic field, there exists no unique relation: nosingle-valued magnetization curve is described, but instead amagnetization loop or a hysteresis loop. The shape of thesemagnetization loops is determined mainly by the imposedmagnetic field on the one hand and the composition andmicrostructural material properties on the other. In particular,these properties concern the density of dislocations, the size,and size partitioning of the present crystal grains, crystallo-graphic texture, etc. Indeed, during the magnetization pro-cess, the mobility of the magnetic domain walls is affectedby these microstructural quantities.The presented two-dimensional micromagnetic model isbased on the Landau-Lifshitz formalism,1 which describesthe electromagnetic action at the level of magnetic dipoles,resulting in the damped precession movement of the localmagnetization vector with constant amplitude. We recall that,by means of the local effective field, the formalism allows usto account for both the magnetic short-distance effect e.g.,exchange field and anisotropy field and the magnetic long-distance effects magnetostatic field, imposed magneticfield, after defining the abovementioned microstructuralproperties in each point of the material.Starting from a predefined microstructural state, the mi-cromagnetic model must permit us to describe the macro-aElectronic mail: luc.dupre@ugent.be0021-8979/2006/998/08G101/3/$23.00 99, 08G1scopic magnetic behavior in the form of magnetization loops,caused by a time varying magnetic field. The resulting mac-roscopic magnetic behavior as well as the magnetic dipoleconfiguration at each time point is used to study magneticmemory material properties included in the micromagneticmodel.II. MAGNETIC MEMORY BEHAVIORHysteresis modeling handles the problem of how to con-struct predict the transition curves, which correspond toany changes of the magnetic field H. In spite of the variety ofcharacteristics among different magnetic materials some gen-eral features are observed in their magnetization processes.These features have been described already in 1905,2 and areknown as Mandelung’s rules. Considering the hysteresiscurves in Fig. 1, these experimentally established rules canbe stated as follows:1 The path of any transition reversal curve is uniquelydetermined by the coordinates of the reversal point, fromwhich this curve emanates.2 If any point 4 of the curve 3-4-1 becomes a new reversalpoint, then the curve 4-5-3 originating at point 4 returnsto the initial point 3 “return-point memory”.3 If the point 5 of the curve 4-5-3 becomes the newestreversal point and if the transition curve 5-4 extendsbeyond the point 4, it will pass along the part 4-1 ofcurve 3-4-1, as if the previous closed loop 4-5-4 did notexist at all “wiping-out property”.© 2006 American Institute of Physics01-108G101-2 Van de Wiele, Dupré, and Olyslager J. Appl. Phys. 99, 08G101 2006III. MICROMAGNETIC MODELThe conventional theory of ferromagnetic materials isbased on the assumption, following Landau and Lifshitz,1that the magnetization of magnetic dipoles m varies with theposition, but that it has a fixed temperature-dependent mag-nitude m=Ms below Curie temperature. The evolution ofm is governed by the Landau-Lifshitz LL equationmt=G1 + 2mHeff −G1 + 2mMs mHeff 1with  and G the damping constant and the gyromagneticconstant, respectively. The static micromagnetic equilibriumcondition is usually formulated as mHeff=0.For the present study we considered a two-dimensionalmodel of a thin ferromagnetic sheet with thickness d. Here,we assume all quantities being invariant in the z direction,while keeping the three-dimensional character of the LLequation 1. The x axis is chosen orthogonal to the sheet.Consequently, for a static micromagnetic equilibrium, the zcomponent of m and Heff must be zero in order to avoidmagnetic charges at infinity on the z direction. In equilib-rium, the walls appearing in the domain structure will be ofNéel type, resulting in large magnetic charges in the wallsand high corresponding magnetostatic fields. To overcomethese high magnetostatic fields, large external magnetic fieldsmust be applied in order to run through hysteresis loops.The effective field Heff is given byHeff = Hexch + Hani + Hms + Ha 2with the exchange and anisotropy fieldsHexch =2A0Ms2xex + yey + zez , 3Hani = −10Ms anixex +aniyey +anizez 4with A the exchange stiffness and the anisotropy energyani=K11222+2232+3212+K2122232.Here, i is the direction cosine of the m vector withFIG. 1. Transition curves illustrating Mandelung’s rules.respect to the ith crystallographic preferable direction cubicstructure, while  j j=x, y, or z refer to the direction cosineof m with respect to the x ,y ,z-coordinate system. K1 andK2 are anisotropy constants.The magnetostatic field follows from  ·Hms=− ·mand Hms=0 using two-dimensional Green’s functions:Hms = −12S  mx,my,0 − 2 − 2 m ·  −  −  − 4 d.5Here, =xex+yey and =xex+yey. S is the area for which0xd, −	y +	. The applied field Ha is the quasi-static unidirectional field, enforced along the y direction.For the space discretization, we considered a polycrystalstructure of the thin ferromagnetic sheet consisting of a largenumber of interacting basis cells and periodic in the y direc-tion. Each cell contains one single magnetization vector mand has predefined easy axes for the magnetization, as, e.g.,shown in Fig. 2. Notice that two preferable directions arealways lying in the xy plane, while the third is along the zdirection. This is favorable as the z component of m must bezero for a static micromagnetic equilibrium in order to avoidmagnetic charges at infinity in the z direction. The spacederivatives appearing in Eqs. 3 and 4 are approximatedby a classical finite difference method. Using the periodicityin the y direction, the magnetostatic field Eq. 5 is calcu-lated by fast Fourier transforms in order to save computa-tional time.3For the time discretization the quasistatic applied fieldHa is approximated with a piecewise constant time function.It is assumed that at the moment the applied field Ha jumpsfrom a constant value to the next one, the material is in staticmicromagnetic equilibrium. Using the LL equation 1, themagnetization dynamics in each basis cell is computedthrough time stepping until a new static micromagnetic equi-librium is obtained corresponding with the new constantvalue for the applied field. The magnetization dynamics isevaluated analytically at ti , ti+1 by introducing, in each ba-sis cell, a local u ,v ,w system, with an axis along Heffti.In the local u ,v ,w system at time step ti, one has Heffti=aeu and mti=uieu+viev+wiew. At the next time step ti+1= ti+t, we obtain mti+1=ui+1eu+vi+1ev+wi+1ew from Eq.1 withui+1 = MseacMstMs + ui − e−acMstMs − uiacMst −acMst, 6FIG. 2. Scheme of the micromagnetic model of an thin ferromagnetic sheet.Periodical structure in the y direction is shown.e Ms + ui + e Ms − ui08G101-3 Van de Wiele, Dupré, and Olyslager J. Appl. Phys. 99, 08G101 2006vi+1 = Ms2vi cosat − wi sinateacMstMs + ui + e−acMstMs − ui, 7wi+1 = Ms2vi sinat + wi cosateacMstMs + ui + e−acMstMs − ui, 8where Hefft=Heffti for ti t ti+1, a=GHeffti / 1+2,and c= /Ms. In order to improve the rate of convergence, is chosen to be 1.IV. EVALUATION OF MEMORY BEHAVIOR FOR THEMICROMAGNETIC MODELFor the evaluation of the magnetic memory behaviorduring the magnetization processes in the thin ferromagneticsheet, several numerical experiments were performed. Con-sider the data 0Ms=2.16 T, A=1.510−11 J m−3, K1=0.48105 J m−3, K2=−0.50105 J m−3, G=−2.21105 mA−1 s−1 pure iron see Ref. 3, p 518. We divideone period of the polycrystal structure into N=64 basiscells—squares of 10 nm—and choose the microstructuralfeatures as shown in Fig. 2. Numerical data are given belowfor the case when the magnetic field of Fig. 3 is applied. Theapplied magnetic field is approximated by a piecewise con-stant function, considering 2800 equidistant time intervals. Ineach time interval, the applied field takes the constant valueHa,k. At the beginning of each time interval k, where theapplied magnetic field jumps from Ha,k to Ha,k+1, micromag-netic dynamics are calculated by time stepping equation 1,using t=10−12 s. After 45 steps in an average way, thenext micromagnetic equilibrium is reached. Figure 4 showsthe state of the magnetic dipoles in the sheet at the timepoints of local extrema or at the time points when minorloops are closed. Notice the recovery of the microstructuralstate of time point c when reaching time point e and the stateof time point a when reaching time point f . This recoveryguarantees the same value for the macroscopicmagnetization—the average of the dipole magnetization vec-tors in one period of the crystallographic structure—and con-sequently the closing of the minor loops.FIG. 3. The applied quasistatic magnetic field Ha along the y direction.V. CONCLUSIONSFor the presented micromagnetic model, we observe thatwhen a time-varying magnetic field with local minima andmaxima is applied, numerical experiments reveal that 1 mi-nor hysteretic loops in the resulting macroscopic magnetiza-tion are indeed closed return-point memory, 2 the closedminor loops are completely erased from the memorywiping-out property, and 3 the magnetic domain structureat the starting point of a minor loop is recovered exactlywhen this loop is closed.This work was supported by the Belgian Fund of Scien-tific Research by the projects G.0309.03 and G.0322.04.1L. D. Landau and E. M.Lifshitz, Electrodynamics of Continuous MediaPergamon Press, Oxford-London-New York-Paris, 1960.2E. Mandelung, Ann. Phys. 17, 861 1905.3G. Bertotti, Hysteresis in Magnetism Academic, New York, 1998.FIG. 4. The micromagnetic equilibria in the time points a, b, c, d, e, and fof Fig. 3. The arrows show the orientation of the local magnetic dipoles mwhile the gray scale gives the energy density high density: white, lowdensity: black.

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Memory properties in a Landau-Lifshitz hysteresis model for thin ferromagnetic sheets

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