Discrete breathers in an one-dimensional array of magnetic dots

The dynamics of the one-dimensional array of magnetic particles (dots) with the easy-plane anisotropy is
 investigated. The particles interact with each other via the magnetic dipole interaction and the whole system is
 governed by the set of Landau–Lifshitz equations. The spatially...

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Published in:Физика низких температур
Date:2015
Main Authors: Pylypchuk, R.L., Zolotaryuk, Y.
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Language:English
Published: Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2015
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Cite this:Discrete breathers in an one-dimensional array of magnetic dots / R.L. Pylypchuk, Y. Zolotaryuk // Физика низких температур. — 2015. — Т. 41, № 9. — С. 942–948
 . — Бібліогр.: 46 назв. — англ.

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Digital Library of Periodicals of National Academy of Sciences of Ukraine
_version_ 1860112109048692736
author Pylypchuk, R.L.
Zolotaryuk, Y.
author_facet Pylypchuk, R.L.
Zolotaryuk, Y.
citation_txt Discrete breathers in an one-dimensional array of magnetic dots / R.L. Pylypchuk, Y. Zolotaryuk // Физика низких температур. — 2015. — Т. 41, № 9. — С. 942–948
 . — Бібліогр.: 46 назв. — англ.
collection DSpace DC
container_title Физика низких температур
description The dynamics of the one-dimensional array of magnetic particles (dots) with the easy-plane anisotropy is
 investigated. The particles interact with each other via the magnetic dipole interaction and the whole system is
 governed by the set of Landau–Lifshitz equations. The spatially localized and time-periodic solutions known as
 discrete breathers (or intrinsic localized modes) are identified. These solutions have no analogue in the continuum
 limit and consist of the core where the magnetization vectors precess around the hard axis and the tails
 where the magnetization vectors oscillate around the equilibrium position.
first_indexed 2025-12-07T17:34:27Z
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fulltext © Roman L. Pylypchuk and Yaroslav Zolotaryuk, 2015 Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 9, pp. 942–948 Discrete breathers in an one-dimensional array of magnetic dots Roman L. Pylypchuk 1 and Yaroslav Zolotaryuk 2 1 Physics Department, Ludwig-Maximilians-Universität, Theresienstrasse 37, 80333 München, Germany 2 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kyiv, 03680, Ukraine E-mail: yzolo@bitp.kiev.ua Received April 16, 2015, published online July 24, 2015 The dynamics of the one-dimensional array of magnetic particles (dots) with the easy-plane anisotropy is investigated. The particles interact with each other via the magnetic dipole interaction and the whole system is governed by the set of Landau–Lifshitz equations. The spatially localized and time-periodic solutions known as discrete breathers (or intrinsic localized modes) are identified. These solutions have no analogue in the con- tinuum limit and consist of the core where the magnetization vectors precess around the hard axis and the tails where the magnetization vectors oscillate around the equilibrium position. PACS: 63.20.Pw Localized modes; 63.20.Ry Anharmonic lattice modes; 75.10.Hk Classical spin models. Keywords: magnetic dots, antiferromagnets, Landau–Lifshitz equations. 1. Introduction The artificially manufactured periodic arrays of mag- netic particles have received much attention in the litera- ture during the last two decades [1–8]. Apart from their technological importance as candidates for the high- density magnetic storage media [1,2,5], these arrays appear to be a good testing ground for studying various nonlinear magnetic wave phenomena [9–11]. Many of these phenomena, such as magnetic solitons, domain walls, vortices are well studied and documented in scientific literature [12,13]. Owing to the fact that they are well described by the continuum version of the Landau- Lifshitz (LL) equation, these solutions can be treated ana- lytically. In some cases, if the underlying system is integrable, the inverse scattering method has been applied [14]. The anharmonic localization in lattices occupies a special place among other nonlinear wave phenomena. The discrete breathers (DBs) (also know as intrinsic localized modes) [15–21] are time-periodic and spatially localized excitations. Unlike their continuum counterparts that nor- mally exist only in the integrable systems [22], discrete breathers can exist in discrete media that are not necessari- ly described by the integrable equations. Discrete breathers owe their existence to the fact that the spectrum of the lin- ear waves is bounded and as a result all the resonances with the linear excitations can be avoided if the breather frequency and/or the system parameters are chosen appro- priately [23,24]. In magnetic systems DBs have been stud- ied rather extensively [20,25–33]. In particular, experi- mental observation of breathers in antiferromagnets has been reported [34,35]. The bulk of these studies was performed for the Hei- senberg magnets, where the interaction between the neigh- bouring spins occurs via the exchange interaction. For the Heisenberg models the exchange interaction usually domi- nates over the single-ion anisotropies, so the breather solu- tions in the underlying models can be treated as the weakly discrete modes. Also, interesting solutions that have no analogue in the continuum limit has been discussed [29,30,36,37]. The weakness of the interspin interaction comparing to the anisotropy (or they should be at least of the same order) is necessary for these excitations. In the magnetic dots arrays the interaction between the dots is of the dipole-dipole kind, which decays as 3r (with r being the distance between the interacting magnets). Thus, if the array is manufactured with the spacing large enough to guarantee the fulfilment of the necessary non-resonance conditions, it is highly probable that DBs will exist in such a system. To investigate the DB existence in the magnetic dot array is the main aim of this article. Discrete breathers in an one-dimensional array of magnetic dots Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 9 943 The paper is organized as follows. In the first section the model of interacting magnetic dipoles is presented. The next section is devoted to the numerical studies of discrete breathers. Discussion and conclusions are given in the last section. 2. The model 2.1. The Hamiltonian and equations of motion We consider the one-dimensional array of N immobile equidistant magnetic particles that interact as magnetic dipoles. The Hamiltonian of this model consists of the dipole- dipole interaction energy between all dots and the magnet- ic anisotropy term for each dot [38]: 2 3 3 =1 ( , ) 3( , )( , )1 = . 2 | | N N zn m mn n mn m n m n n H D a m n M M ν M ν M M (1) Here D is the anisotropy constant, a is the distance be- tween the adjacent dots, ,mn xν e and =nM ( , , )x y z T n n nM M M 2 2(| | = )n MM is the magnetic dipole momentum of the nth particle. In this article the easy- plane anisotropy is considered with the plane of the array (xy) being the easy plane, thus, < 0.D It is convenient to introduce the new dimensionless var- iables in which the total magnetic dipole momentum is normalized to unity: 2= / , = / | | , 2 | | .n n M H H D M t D Mtμ M (2) In the new dimensional variables the dynamics of the magnetic moment of the nth dot is described by the dis- crete version of the LL equation [38]: = [ ] , n nn Hμμ μ ( ) ( ) ( ) = ,x y zx y zn n n n μ e e e (3) where the dot denotes differentiation with respect to time. Now the system of coupled dipoles has only one parame- ter: 3=1/ (2 | | ).D a This parameter appears as a prefactor in the dipole-dipole term of the dimensionless energy .H It can be treated either as a measure of the dis- creteness of the system or as the ratio of the dipole-dipole and exchange energies. 2.2. Dispersion law Before embarking on studies of the nonlinear vibrations of the array it is useful to recall the dispersion law of the linear waves (magnons). The magnon spectrum can be found when Eq. (3) is linearized around the obvious ground state, where the dipoles are lying in the easy plane and are oriented tail-to-tail: ( ) ( ) ( )=1, = = 0.x y z n n n Consider first the infinite array. Then the dispersion law is well-known and reads [7,11] 2 3 3 =1 =1 cos( ) 2 cos( ) 2 ( ) = 2 2 1 .L n n nq nq q n n (4) Typical curves for the dispersion law for the different val- ues of the coupling constant are given in Fig. 1(a). If the array is finite the magnon band becomes discrete. It con- sists of the set of modes ( ) , n L = 1, 2, , .n N The de- pendence of these modes as a function of the coupling con- stant is given in Fig. 1(b). Strictly speaking, the discrete translational invariance is lost for the finite linear array and we can speak about it only in the approximate sense if is small. As a result, there is a mode that is placed below the band (see Fig. 1(b)). If the periodic boundary conditions are applied the translational invariance of the array holds exactly. Fig. 1. (Color online) Dispersion law (4) for = 0.01 (curve 1), = 0.03 (curve 2) and = 0.06 (curve 3) (a). Frequencies of the magnon modes as a function of for the finite chain with N = 11 (blue) and N = 31 (red). Thick black lines demonstrates L(0) and L( ) from Eq. (4) (b). Roman L. Pylypchuk and Yaroslav Zolotaryuk 944 Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 9 3. Discrete breathers and their properties In this section we report the results of the studies of dis- crete breathers with the help of numerical simulations. 3.1. Spontaneous localization Discrete breathers are spatially localized excitations that are periodic in time, i.e. =1 =1{ ( )} = { ( )}N N n n n nt t Tμ μ , = 2 /T , where is the breather frequency. The con- cept of the anti-continuum limit [17,18] is important for constructing the DB solutions. In the current model it can be implemented by setting = 0 . Thus, the dipole-dipole interaction between the dots is absent and each of them can be excited independently. If a particular dot with the num- ber 0=n n is excited, the magnetization vector will per- form precession around the hard axis with the frequency ( ) 0 = z n . Projection on the xy plane demonstrates the following dynamics: 0 0 ( ) ( ) = x y n n i 2 ( )1 e .i t Similarly, several dots located in the arbitrary places of the array, can be excited. In this article we will restrict ourselves to the configura- tions that consist of the precessing core of rN dipoles. Such an initial state can be represented as follows: 2 2 (0) 2 2 = , ,0 0 1 cos 1 cos 1 1 1 1 1 1 = 0 , 0 , , 0 , 1 sin , , 1 sin 0 , 0 , 0 0 0 0 0 0 0 n n n Nr m . (5) _______________________________________________ First of all we report on the simple numerical experi- ment that demonstrates the phenomenon of dynamical lo- calization of the magnetic dot magnetization. We take the anti-continuum configuration (0)m from Eq. (5) as the initial condition and integrate the LL equations numerical- ly. The fourth order Runge–Kutta method was used. The precision of the method was tested by monitoring the con- servation of the total energy and the dipole moment. We have observed different results that depend on the value of . If this constant is sufficiently small, the localized state Fig. 2. (Color online) Contour plots of the temporal evolution of the ( )z n (a), (c) and ( )1 x n (b), (d) components of the magnetization in the array of N = 100 magnetic dots with the initially excited one [(a), (b), = 0.039] and three [(c), (d), = 0.0045] dipoles. Discrete breathers in an one-dimensional array of magnetic dots Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 9 945 persists for rather long times. Otherwise, for larger values of the initially excited dipoles fall into the easy plane and localization disappears. In Fig. 2 the contour plots of the dynamical evolution of the array magnetization are demonstrated for = 1rN [panels (a), (b)] and = 3rN [panels (c), (d)] initially excited dots. Here the dipoles were initially excited with ( ) = 0.9z n . As one can see, energy stays with the initially excited central dots remain in the excited state for rather long time, with the respective magnetization vectors precessing around the hard axis. The lifetime of the localized excitation exceeds the period of one rotation 2 /0.9 7T by several orders of magni- tude. Thus, the phenomenon of dynamical localization is established. At this point we wish to know whether a local- ized mode is an exact periodic solution that can be attribut- ed to the excitations known as discrete breathers [19]. Be- low we investigate these excitations in more detail. 3.2. Breather periodic orbits In this subsection we show that time-periodic localized modes are indeed exact solutions of the LL equation. Nu- merically this task can be performed in the following way. Define the evolution operator 0 0 1 2 ˆ : ( ) ( ), = col( , , , ) ,T NI t t T μ μ μm m m (6) which stands for the integration of the LL equations (3) along the time interval 0 0[ , ].t t T The fixed points of the 3N-dimensional map 2 0 0 0( ) ( ) | ( ) | 1= 0, =1,2, , ,nt T t t T n Nn nμ μ μ (7) will be the periodic solution with the period T. This map is complemented by the term 2 0| ( ) | 1,n t Tμ which is necessary to ensure that the normalization condition holds after each iteration step. We start from the anti-continuum Fig. 3. (Color online) Dynamics of the central out-of-plane and neighbouring magnetization vectors of the DB orbit on the unit sphere for: (a) blue curve (central dot, n = 16), red curve (n = 15), other parameters = 0.022, = 0.5, N = 31; (b) same as (a) but for = 0.048, = 0.75; (c) same as (a) but for N = 20 dots with Nr = 2, blue curves correspond to the dots n = 10 and n = 11, red curve : n = 9; (d) N = 21 dots with Nr = 3, = 0.5, = 0.016, pink curve corresponds to n = 11, blue curves n = 10 and n = 12, red curve: n = 9. Roman L. Pylypchuk and Yaroslav Zolotaryuk 946 Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 9 limit (5), “turn on” the dipole-dipole interaction by setting > 0 and show that the spatially localized excitation per- sist. Then can be increased gradually and the breather periodic orbit can be followed until it ceases to exist. In case of successful choice of the initial configuration 0( )tm convergence with the desired precision takes place after several iteration steps. The numerical scheme based on the Newton method [39] has been designed [40] for finding DB periodic oribits. It has been shown to work successful- ly in the number of models [41], including magnetic lattic- es [29,30]. Below we report the main results. Starting from the anti-continuum approximation (5) and using the method, described above, we have managed to detect the breather periodic orbits for the different values of and , number of dots in the array N, and number of the precessing dipoles, .rN The phase in the initial condition (0)m does not seem to play any role as we have achieved conversion to the same solutions for the different values of . The dynamics of the magnetization vector nμ of these periodic orbits is shown in Fig. 3. The struc- ture of DB's in all these figures is the same: it consists of the core of few dipoles that precess around the hard axis (although due to the interaction the precession trajectory is tilted towards the x axis) and the weakly oscillating tails. If the coupling constant is increased, the precessing tra- jectory is tilted stronger toward the x axis as the in-plane dipoles interact stronger with the precessing dipole (com- pare Figs. 3(a) and 3(b)). Note, that the oscillations beyond the precessing core appear to be rather weak (shown by the red trajectories). Next we estimate the existence area of DBs on the param- eter plane ( , ). We remind that in order to exist, the breather frequency together with its multiples should not resonate with the linear waves of the system. In the anti- continuum limit ( = 0) the allowed range of the breather frequencies is 0 < < 1. Since the precession frequency coincides with the z component of the magnetization vector, cannot exceed 1. If the coupling is on, the allowed breath- er frequencies lie in the range [0, ] ( ) < < 1maxq L q . Thus, the existence area of DBs in the ( , ) parameter plane coincides approximately with the upper left triangle in Fig. 2(b) with the edges, given by = 1 , = 0 and ( ) = N L . We have managed to track numerically the DB periodic orbit starting from = 0 up to the critical values when the Newton method ceases to exist. We have found the orbits to persist into the magnon spectrum. In that case the breather tails do not decay asymptotically as 0, .n N In- stead, we observe a bound breather-magnon state. However, these solutions appear to be unstable. The asymptotic behaviour of the breather tails is given in Fig. 4. The decay law is close to the power law if we are not far from the anti-continuum limit. Indeed, we observe almost power law decay for = 0.018 with ( ) 6 01 | |x n n n and ( , ) 3 0| | .y z n n n The power-law decay is in ac- cord with other models that possess long-range interac- tion [42–44]. As increases and the dipole-dipole inter- action becomes more prominent. At this point we notice that the decay law becomes faster than the power law [see Fig. 4(b)]. This can be attributed to the fact that the array does not possess the discrete translational invariance. Although the absence of this symmetry is felt rather weakly, it becomes more and more pronounced as the coupling constant increases. Moreover, it should mani- fest itself in the strongest way at the edges of the array, since Fig. 4. (Color online) Spatial decay of the breather profile [1 – | n| (x) (+), ( )y n (), ( )z n ()] on the log-log scale for the array of N = 31 dots with = 0.75 and = 0.018 (a) and = 0.048 (b). The solid lines in the panel (a) approximate the decay of the magnetization (see text for details). Discrete breathers in an one-dimensional array of magnetic dots Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 9 947 the dipoles at the edges interact only with the dipoles to the left (to the right), while the dipoles in the middle of the array interact symmetrically with all their neighbours. We remind, that the discrete translational invariance is possible only when the periodic boundary conditions are imposed. Suppose we are not looking for the breather periodic orbit. Instead, we are simply interested in the details of the time evolution of the initial configuration (5) on the large time scale. Then we obtain the quasiperiodic localized so- lution. Its spatial structure will be the same as for the breathers, discussed in the previous paragraphs. The time evolution of the magnetization components appears to be quasiperiodic, as shown in Fig. 5. Here we have excited initially = 5rN dipoles with the precession frequency = 0.75. As the course of evolution the localized struc- ture persisted, but the temporal evolution exhibits two fre- quencies: the precession frequency 0.75 and the much lower envelope frequency. Within of one modulation peri- od the magnetic moment can encompass the hard axis ap- proximately ten times. It is not possible to trace the quasiperiodic breather solution with the method used in this section for the periodic breathers. The problem of the quasiperiodic breather existence is an interesting problem on its own [45,46] and will be pursued independently. 4. Discussion and conclusions Discrete breathers (intrinsic localized modes) have been demonstrated to exist in the one-dimensional array of magnetic dots that interact as magnetic dipoles. We have focused on the arrays with the easy plane anisotropy. DBs are time periodic and spatially localized solutions of the Landau–Lifshitz equation. The structure of the breather solution is as follows: several dipoles in the core of the breather rotate around the hard axis and the rest perform small amplitude oscillations while lying in the easy plane. It should be noted that this type of breathers has no ana- logue in the continuum limit. The breather frequency should not resonate with the linear modes of the array (magnons). It appears that the area of breather existence is limited from below by the maximal frequency of the magnon band and by the value = 1 (in the dimensionless units) from above. In terms of the structure and the existence conditions the solutions obtained in this article are similar to the DBs in classical ferromagnetic Heisenberg chains with the easy- plane anisotropy, obtained earlier [29,30]. The difference is that in the Heisenberg chains the ground state is degen- erate while for the array of magnetic dipoles it is con- strained to the state ( ) = 1.x n Another difference appears due to the long-range dipole-dipole interaction, and it man- ifests itself in the asymptotic behaviour of the magnetiza- tion away from the breather core. Also, we believe that experimental observation of DBs in the arrays of magnetic particles seems to be much more easier as compared to the previously studied Heisenberg models. While in both mod- els (Heisenberg and magnetic dots) the breathers exist if the interaction is considerably weaker than the anisotropy, such a situation is rather rare for the Heisenberg lattices, where the exchange interaction usually dominates over the anisotropy energy. In the case of magnetic dots the interac- tion can be chosen sufficiently weak by increasing the dis- tance between the particles. 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institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
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language English
last_indexed 2025-12-07T17:34:27Z
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publisher Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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spelling Pylypchuk, R.L.
Zolotaryuk, Y.
2018-01-05T17:25:26Z
2018-01-05T17:25:26Z
2015
Discrete breathers in an one-dimensional array of magnetic dots / R.L. Pylypchuk, Y. Zolotaryuk // Физика низких температур. — 2015. — Т. 41, № 9. — С. 942–948&#xd; . — Бібліогр.: 46 назв. — англ.
0132-6414
PACS: 63.20.Pw, 63.20.Ry, 75.10.Hk
https://nasplib.isofts.kiev.ua/handle/123456789/128076
The dynamics of the one-dimensional array of magnetic particles (dots) with the easy-plane anisotropy is&#xd; investigated. The particles interact with each other via the magnetic dipole interaction and the whole system is&#xd; governed by the set of Landau–Lifshitz equations. The spatially localized and time-periodic solutions known as&#xd; discrete breathers (or intrinsic localized modes) are identified. These solutions have no analogue in the continuum&#xd; limit and consist of the core where the magnetization vectors precess around the hard axis and the tails&#xd; where the magnetization vectors oscillate around the equilibrium position.
We thank V.P. Kravchuk for useful discussions. One of&#xd; the authors (Y.Z.) acknowledges the financial support from&#xd; the Ukrainian State Grant for Fundamental Research&#xd; No. 0112U000056.
en
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
Физика низких температур
К 80-летию уравнения Ландау–Лифшица
Discrete breathers in an one-dimensional array of magnetic dots
Article
published earlier
spellingShingle Discrete breathers in an one-dimensional array of magnetic dots
Pylypchuk, R.L.
Zolotaryuk, Y.
К 80-летию уравнения Ландау–Лифшица
title Discrete breathers in an one-dimensional array of magnetic dots
title_full Discrete breathers in an one-dimensional array of magnetic dots
title_fullStr Discrete breathers in an one-dimensional array of magnetic dots
title_full_unstemmed Discrete breathers in an one-dimensional array of magnetic dots
title_short Discrete breathers in an one-dimensional array of magnetic dots
title_sort discrete breathers in an one-dimensional array of magnetic dots
topic К 80-летию уравнения Ландау–Лифшица
topic_facet К 80-летию уравнения Ландау–Лифшица
url https://nasplib.isofts.kiev.ua/handle/123456789/128076
work_keys_str_mv AT pylypchukrl discretebreathersinanonedimensionalarrayofmagneticdots
AT zolotaryuky discretebreathersinanonedimensionalarrayofmagneticdots