Abstract
The profile, radius, and energy of chiral skyrmions, found in magnetic materials with the Dzyaloshinskii–Moriya (DM) interaction and easy-axis anisotropy perpendicular to the film, have been previously calculated in the asymptotic limits of small and large skyrmion radius, as functions of the model parameter. We extend the asymptotic analysis to the case of an external field or a combination of anisotropy and external field. The formulae for the skyrmion radius and energy are then modified, by the use of fitting techniques, into very good approximations through almost the entire range of skyrmion radii, from zero to infinity. We include a study of the effect of the magnetostatic field on the skyrmion profile in two cases. We compare the profile of magnetic bubbles, stabilized without the chiral DM interaction to that of a chiral skyrmion.
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1. Introduction
Chiral magnetic skyrmions are topological magnetic configurations that are stabilized in materials with the Dzyaloshinskii–Moriya (DM) interaction [1–3]. They have been observed in a number of experiments and the detailed features of individual skyrmions have been resolved experimentally to an impressive degree for isolated skyrmions [4–7] and in a skyrmion lattice [8–10]. The experimental works have mapped the profile of the skyrmion, i.e. the magnetization as a function of the distance from its center. The skyrmion profile forms the foundation for the study of the statics and of dynamical behaviors of skyrmions and the subsequent derivation of quantitative results. The profile enters in formulae for dynamical phenomena, for example, skyrmion translation and oscillation modes [11, 12] or antiferromagnetic skyrmion excitations [13], and it is crucial for quantitative calculations. Furthermore, it quantifies the localization of the entity which in turn establishes its particle-like nature.
Approximation formulae that capture basic features of the skyrmion profile have been proposed. For skyrmions of large radius, an ad-hoc ansatz based on explicit one-dimensional domain wall profiles [14] has been suggested and is widely used to examine structural and dynamic properties [4, 15–17]. The domain wall profile with an adjustable wall thickness [11, 18] and a related ansatz [6] have been employed as more flexible forms in order to capture the skyrmion features over a wider parameter range. For the case of a model with easy-axis anisotropy, accurate skyrmion profiles in the asymptotic sense have been obtained for skyrmions of small [19] and of large radius [20]. Similar methods were used for a model including the magnetostatic field [21, 22]. In the present paper, we give a comprehensive analysis of the skyrmion profile for the case of easy-axis anisotropy perpendicular to the film and external magnetic field. We use the methods that were developed for the case of easy-axis anisotropy only in [19, 20]. The above references focus on asymptotic formulae for skyrmions of small (close to zero) and large (tending to infinity) radius. Specifically, the results of the asymptotics for small radii R are valid for and those for large radii are valid for . The present paper gives a set of formulae for determining the skyrmion radius that are good approximations in the whole parameter range, i.e. including the intermediate radii, . The present approximation formulae are derived so as to be consistent with the asymptotic ones. We further present the results of a set of numerical simulations for the effect of the magnetostatic field on the axially-symmetric skyrmion. This is included for completeness and in order to be able to make a direct comparison between the profiles of chiral skyrmions (obtained in the bulk of the text) and bubbles. Beyond basic differences, the two entities share many features. A comprehensive study of the effect of the non-local magnetostatic field has been given in [17, 23] where skyrmions and bubbles are compared and furthermore bi-stability phenomena have been revealed.
We, finally, give a study of the skyrmion energy, made possible by the availability of a detailed analytical description of the skyrmion profile. We extend previous asymptotic analysis results for skyrmions of small radius and we give fitting formulae covering almost the entire range of radii. The analytical calculations and formulae will help focus the efforts for applications of skyrmions.
The paper is arranged as follows. Section 2 formulates the problem. Section 3 gives the far field for the skyrmion profile. Section 4 gives the formulae for the skyrmion profile and radius for the case of an external field. Section 5 gives the corresponding formulae for the case of easy-axis anisotropy. Section 6 studies the effect of the magnetostatic field on the skyrmion and gives comparisons with magnetic bubbles. Section 7 extends asymptotic analysis results and introduces fitting formulae for the skyrmion energy. Section 8 contains our concluding remarks. The
2. Formulation
We consider a thin film of a ferromagnetic material on the xy-plane with symmetric exchange, DM interaction, anisotropy of the easy-axis type perpendicular to the film, and an external field. The micromagnetic structure is described via the magnetization vector with a fixed length normalized to unity, . The energy is
where are the exchange, DM, and anisotropy parameters, respectively, and the external field is normalized to the saturation magnetization . The DM energy density may be chosen to have the bulk form or the interfacial form , where and the summation convention is invoked, while are the unit vectors for the magnetization in the respective directions.
It is useful to consider the length scales that arise naturally,
is the exchange length, is the domain wall width, gives the pitch of the spiral solution for strong DM interaction, and is related to the radius of small skyrmions. Using as the unit of length, and assuming an external field perpendicular to the film, the scaled form of the energy is
where we have assumed that in the uniform magnetization of the film and we have used the scaled DM and anisotropy parameters
Static magnetization configurations satisfy the time-independent Landau–Lifshitz equation
The bulk DM form is and the interfacial one is .
Skyrmion solutions with axial symmetry can be described in polar coordinates using the spherical angles ( for the magnetization. We choose for the case of interfacial DM interaction and for the case of bulk DM interaction. In both cases, the angle satisfies
Existence, uniqueness, stability and minimality of axially symmetric skyrmions in the regime and , respectively, has been examined rigorously in [24, 25]. In the following, we will use the convention that in the skyrmion center and at spatial infinity . The skyrmion radius R will be defined to be at the radial distance where .
3. Far field
In the far field, where Θ is small, equation (6) reduces by linearization to
Under a scaling transformation this gives the modified Bessel equation. The appropriate solution, decaying for , is the modified Bessel function of the second kind [26], that also appeared in a similar context in [27]. We have
where C is an arbitrary constant, γ ≈ 0.57721 is the Euler–Mascheroni constant, and
The asymptotic form of for large values of r gives [26]
It is interesting to note that the result for the far field does not depend on the DM interaction. The exponential decay (10) is due to the anisotropy and the external field.
4. External field
We consider that we only have an external field and no anisotropy, κ = 0. Then, we may choose as the unit of length which leads to setting h → 1 in equation (6) and the DM parameter is
We initially focus on skyrmions of small radius. In this limit, it is convenient to work with the variable
which is the modulus of the stereographic projection of the magnetization vector. In the case, of exchange interaction only, we have the well-known axially-symmetric Belavin–Polyakov (BP) solution [28]
that represents a skyrmion of unit degree with a radius R. Inspired by the BP solution (13), we remove the singularity at the origin by defining the field
For comparison purposes, we note that the variable in [19] is related to the present by .
The equation for is given in
For h = 1 and κ = 0, equation (A13) reduces to
or
We solve equation (A1) (or equation (6)) numerically using a shooting method and we find the skyrmion profiles for various values of the parameter λ. Given the profile, we detect the skyrmion radius. In figure 1 we show the skyrmion radius found numerically as a function of the parameter. Formula (16) is given in entry (b) of the figure and it fits excellently the numerically found skyrmion radius for small R.
Isolated skyrmions exist in this model for while at a transition to a skyrmion lattice occurs [29]. Inverting relation (16) gives the leading order of approximation
Following [30], a modification of the denominator of equation (17),
leads to a very good fit of the numerical data over a wide range as shown in figure 1. Equation (18) is consistent with equation (16) at λ → 0, but the factor 1.8 is found only by fitting the numerical data. For larger λ, the linear relation
fits the numerical data. The combination of formulae (18) for and (19) for give a very good fit of the skyrmion radius for the entire range of the parameter space.
We now turn to the skyrmion profiles. Equation (A6) gives the profile at the skyrmion inner core and equation (8) gives the profile at large distances. Based on the above asymptotic results, we will give simple approximation formulae for the skyrmion profile.
The field around the skyrmion center is found by a Taylor expansion of the solution in equation (A6) close to r = 0 (applied for ) while the parameter λ is substituted from equation (15). The calculations are given in section 'Field at the skyrmion center' and lead to equation (A15). For the present case, this reduces to the approximation
and it is valid for r < R for small radius skyrmions.
In the far field, Θ and the field u in equation (12) are proportional to each other and both satisfy the modified Bessel equation (7). We will continue the discussion using the field u, instead of Θ, because it eventually gives a good fit for the skyrmion profile over a wider interval in r. For small R, we have , where the factor R follows from the asymptotic matching conditions. We write
Figure 2 shows the profile for a skyrmion of small radius, for λ = 0.4. Surprisingly, the far field formula (21), shown by the blue dashed line, gives a good approximation almost in the entire space. Close to r = 0, equation (20) is the correct approximation while equation (21) cannot be justified in this region. Note that the approximation becomes progressively less successful when applied for larger λ.
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Standard image High-resolution imageWe conclude the section by giving a description of the complete skyrmion profile over the full spatial range. A validity condition for the near field is [19]. Using equation (16) and , we have that equation (20) is valid for
The form of equation (20) indicates that, in the regime where condition (22) is valid, the skyrmion profile is close to the BP profile. The exponentially decaying behavior (10) is valid for . Between the regimes of validity of the BP profile and the exponentially decaying profile, the modified Bessel function (21) is still a good approximation.
The results of this section are relevant for materials presenting a small anisotropy, e.g. as a result of compensation between perpendicular magnetocrystalline anisotropy and the magnetostatic field.
5. Easy-axis anisotropy
We consider the case of easy-axis anisotropy and no external field, h = 0. Then, we may choose as the unit of length which leads to setting κ → 1 in equation (6) and
This section is based on the work presented in [19, 20].
5.1. Small radius
For skyrmions of small radius, it is convenient to work in the variables u(r) of equation (12) and of equation (14) (see
or
This asymptotic formula was derived in [19]. It is shown in figure 3(a) that it matches excellently the numerically found skyrmion radius for small R in the range .
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Standard image High-resolution imageInverting formula (25) gives, to the lowest order of approximation, equation (17). A modification of the denominator of equation (17),
as obtained in [30], leads to a very good fit of the numerical data over the very wide range , as shown in figure 3(a).
Using asymptotic results from
Figure 4 shows formulae (20) and (21) compared to numerically calculated skyrmion profiles for two values of the parameter.
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Standard image High-resolution imageWe finally mention that including the magnetostatic field is expected to be modeled as an easy-plane anisotropy for skyrmions of small radius and should thus not change qualitatively the present results [22]. This is supported also by the results presented in section 6.
5.2. Large radius
At the parameter value , the skyrmion radius diverges to infinity and a phase transition occurs from the uniform to the spiral state. When the skyrmion is large, an asymptotic series for the profile is given in negative powers of R [19],
where are functions of r − R. The leading order contribution satisfies
This is the standard one-dimensional wall profile.
We now give approximation leading order formulae for the core and the far field of the skyrmion. The field at the core is [20]
where is the modified Bessel function of the first kind. For the far field, we have
that simplifies to
in the region far from the domain wall. The profile at the skyrmion domain wall region (28) is matched to the skyrmion core and to the far field to all asymptotic orders in [20].
Figure 5 shows a numerically calculated skyrmion profile and this is compared to the leading order formulae (28)–(30).
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Standard image High-resolution imageThe relation between the parameter λ and the radius R is derived during the asymptotic process. It is given as the series [19],
where
Retaining terms up to and solving for R, we obtain
This is shown in figure 3 by the green line. The fitting formula (26) and the asymptotic result (34) for large radius give a very good approximation of the numerically found radii for the entire parameter space.
One could go on to consider the case when both perpendicular anisotropy and an external field are present. Section 3 and the
6. Two cases including the magnetostatic field
The magnetostatic field is known to be crucial for stabilizing magnetic bubbles [31] which are cylindrical domains that share topological features with skyrmions. Magnetic bubbles, even ones with high skyrmion numbers, as well as topologically trivial bubbles, with skyrmion number zero, have been observed and studied [17, 23]. When the DM interaction is present, the magnetostatic field is usually considered to be of secondary importance for skyrmion generation and stability. However, it has been shown that the study of the combined effect of DM interaction and magnetostatic field is necessary for a proper understanding of the relation between chiral skyrmions and magnetic bubbles, and it has revealed unusual phenomena such as bi-stability [17, 23]. A detailed calculation of the skyrmion energetics including the magnetostatic field [17] leads to the understanding of skyrmion and bubble stability. In this section, we present the results of numerical simulations for the profile of skyrmions when the magnetostatic field is included. We give two examples aiming to make a comparison between bubble and skyrmion profiles in specific cases.
We first summarize the results on the stability of cylindrical domains [32] that will be useful in the interpretation of the numerical results in this section. Easy-axis anisotropy perpendicular to the film is necessary in order to have stable magnetic bubbles in a material. The magnetostatic field is favoring the expansion of the bubble and it thus has a demagnetization effect. The bubble domain wall tends to shrink in order to minimize the bubble size, however, the demagnetizing effect is typically stronger and thus no energy balance can be achieved. The stabilization of bubbles is actually obtained by the addition of an external (bias) field perpendicular to the film. The bubble domain wall is primarily of Bloch type as this is favored by the magnetostatic interaction. Both Bloch chiralities are energetically equivalent. The structure of the bubble wall is though complicated by the effects of the film boundaries [33, 34].
The static Landau–Lifshitz equation has the form
where the bulk DM interaction term has been chosen, hm is the magnetostatic field normalized to the saturation magnetization, and is used as the unit of length. The parameters are given here again for convenience
We often consider the substitution in order to take into account the fact that the magnetostatic field is equivalent to easy plane anisotropy for ultra-thin films. This substitution is equivalent to setting and in equation (35). After these substitutions, the field hm would represent only the deviation of the magnetostatic field from its approximation as an easy-plane anisotropy term.
We find solutions of equation (35) using an energy relaxation algorithm [34]. Our method works in cylindrical coordinates (r, z) and it is confined to axially symmetric configurations only. The magnetostatic field is calculated using a conjugate gradient method. We consider an infinite film achieved by employing a long enough numerical mesh in the r direction and assuming no magnetic charges on the lateral boundary. We place the film of thickness df so that its central plane is at z = 0, i.e. the film extends over . We find that solutions for the magnetization satisfy the parity relations .
We first explore the case of easy axis anisotropy only and no bias field. We consider a thin film with thickness and parameter values . We discretize space with lattice spacings and in the r and z directions respectively. The numerical mesh extends to r = 20. Figure 6 shows the cylindrical components of the magnetization for a skyrmion solution as functions of r at various levels of z. The skyrmion radius is R ≈ 1.5, a significant increase in comparison to the skyrmion radius R = 0.42 found for λ = 0.33 when the magnetostatic field is neglected. Furthermore, the profile acquires a nonzero mr component, which is an odd function in the direction perpendicular to the film.
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Standard image High-resolution imageWhen the parameter λ is increased, the skyrmion radius increases. In the presence of the magnetostatic field, we find no static skyrmions for λ > 0.35. This should be compared to the critical value when the magnetostatic field is neglected. This finding suggests that skyrmions with a large radius cannot be obtained (without a bias field) because they are destabilized by the magnetostatic field.
It is interesting to compare the chiral skyrmion profiles with magnetic bubble ones that are stabilized without the DM interaction. We choose a film with thickness , extending over , and the parameter values (and set λ = 0). The lattice spacings are and the numerical mesh extends to r = 40. The energy minimization algorithm converges to a static bubble with radius R ≈ 8. Figure 7(a) shows the three components of the magnetization as functions of r at various levels of z. The stray field at the bubble domain wall induces large values for mr close to the film boundaries. Correspondingly, the component mφ depends strongly on z. As a result, the bubble wall is of a hybrid character; Bloch at the center and tilted toward Neél type closer to the boundaries. The hybrid wall structure has been shown to persist when DM interaction is included [35, 36]. The component mz also has a dependence on z that results in the bubble domain wall width to depend on z and to the bulging shape of the bubble [33, 34].
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Standard image High-resolution imageFigure 7(b) shows the spherical angle Θ for the bubble by blue solid lines at various z-levels. For comparison, a red dashed line shows the chiral skyrmion profile that is obtained for a DM parameter λ = 0.6316 and no magnetostatic field. For that value of λ, the chiral skyrmion has a similar radius as the bubble in the figure. The issue of the differences between a chiral skyrmion and a bubbles was earlier debated in [37, 38]. We finally note that our comparison between a chiral skyrmion and a bubble is only possible for large radii as magnetic bubbles are not stabilized for small radii [32].
7. Energy of a skyrmion
The skyrmion radius, studied in the previous sections, is a length scale that determines the balance of the individual interaction terms and thus provides information about the skyrmion energy. The energetics of the model is important not only as a means to understand the skyrmions as energy minima but also in order to build arguments concerning their stability and excitations. In the regime of small radii, the energy asymptotics has been examined rigorously in [21, 25, 39].
We consider the case of easy-axis anisotropy and no external field so that the energy is given in equation (3) for , and it is measured in units of . A useful result for the energy components of a skyrmion is obtained by employing a standard scaling argument [29],
7.1. Small radius
For small skyrmion radius, asymptotic analysis gives [19]
while can be obtained from equation (37). Result (38) can be used to produce asymptotic series for the energy terms as we will now demonstrate.
We consider a specific DM parameter λ0 and the energy for any configuration Θ. This is written as
where we have set
so that all terms are integrals over Θ that do not contain parameters.
We confine ourselves to the configurations that are the profiles corresponding to static skyrmions for any parameter value λ (that may be different from λ0). The virial relation (37) holds,
and it is used in equation (39) to obtain
for the energy of profile in a system with parameter value λ0.
We take the derivative of the energy expression (42) with respect to λ,
and require that . This gives the condition
An explicit calculation of the terms of equation (44) utilizing equation (38), shows that terms of order with should necessarily be included in the energy asymptotic series for the condition (44) to be satisfied. Specifically,
where are constants (with from equation (38)). Substituting these in equation (44) obtains
In order that equation (46) be satisfied to all orders in , it should hold
For m = 1, this gives (since )
We thus have
More information would be required to obtain explicit values for with . Comparing the second of equation (45) with numerical data, we propose the following formula,
Using formulae (49) and (50), the energy is
This is in agreement with rigorous results in [25] where an error term is given. The numerical data for the energy are fitted well with formulae (49)–(51), for parameter values .
Figure 8 shows the numerically calculated skyrmion energy as a function of λ. A modification of formula (51) is found to fit the numerical data very well almost in the entire parameter range,
This formula is plotted in figure 8. For λ close to the critical value formula (55) (of the next subsection) should be used instead.
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Standard image High-resolution imageEquation (51) gives
Equation (53) gives a good approximation only in the range R < 0.5. It is remarkable that the numerical data are fitted well over a large range of R by a quite significant modification of equation (53),
that is plotted in figure 9.
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Standard image High-resolution imageWe finally note that the techniques used in this subsection can be readily applied to the case of skyrmions of small radius in applied external field.
7.2. Large radius
For large skyrmion radius, asymptotic analysis gives [20]
or (cf equation (34))
In the range , the numerical data of figure 8 are fitted well with formula (55).
Figure 9 shows the skyrmion energy as a function of its radius. For large R, the data are fitted very well with the formula
which is a sharpening of equation (56) and it is plotted in the figure. We further note that each one of the energy components is fitted very well, for , by the forms
8. Concluding remarks
We have presented fitting formulae of skyrmion profiles and energy that cover almost the entire range of the parameter values and of skyrmion radii. These are based on formulae obtained through asymptotic analysis [19, 20]. The analysis is extended to include the effect of an external magnetic, field in the
We have further studied the effect of the magnetostatic field on the skyrmion profile in two specific cases. It tends to increase the skyrmion radius while it appears to destabilize skyrmions of large radius. This indicates that attention should be focused on skyrmions of small radius while larger radius skyrmions should be relevant for very thin films where the magnetostatic field is equivalent to an anisotropy term. We compared the profile of magnetic bubbles stabilized without the chiral DM interaction to that of a chiral skyrmion. We found that the profiles are similar in the two cases, the main difference between the two being the variation of the bubble profile across the film thickness. The present study of the effect of the magnetostatic field is confined to specific cases, and it is consistent with more extensive studies that study extensive details of this fairly complicated issue [17, 23].
Finally, we have produced asymptotic series expressions for all skyrmion energy terms using an argument for the minimization of the energy. This has resulted in numerical expressions that sharpen previous asymptotic results. We also give approximate formulae that cover almost the entire range of the parameter values and of skyrmion radii.
The results of section 3 for the far field, the formulae of
The obtained formulae can be used for comparisons with experimental results and for a variety of investigations where the details of skyrmion features are important, notably, in skyrmion dynamics. We also hope to motivate work that may prove that some of the fitting formulae, or variations of them, can be obtained by accurate calculations.
Acknowledgment
The publication of the article in OA mode was financially supported by HEAL-Link.
Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.
Appendix: Asymptotic analysis for skyrmion of small radius
Appendix. Near field
We will mainly work with the field defined in equation (14). It satisfies the equation
The ensuing analysis will follow the techniques developed in [19].
An assumption which is found to be consistent with our results is to neglect the term containing in equation (A1),
In the near field, we replace in equation (A2) with its initial value . The approximation is valid for the range of r over which . The equation obtained in this way,
is integrable by the integrating factor
We obtain
which integrates to
We finally have
The constant of integration has been judiciously chosen to eliminate the fourth-order singularity r−4 on the right.
We make the change of variables and . Then equation (A4) becomes
This is integrated to give
or
where is the dilogarithm function and the constant of integration has been chosen so that .
Keeping the dominant terms for large τ we have
which, in the original variables, gives the near field
The skyrmion radius is found by the condition . In the case that the skyrmion radius R is small, this is given by
Appendix. Asymptotic matching
The solution of the equation (7) for the far field is
where ξn is given in equation (9) and the solution is valid for any . For small values of the radial coordinate, we keep only the first three terms in the series,
The far field for the field is derived from the latter formula noting that for small values of , as seen from equations (14) and (12). We find
or
Matching the near field in equation (A8) with the far field in equation (A11) order by order we have and
or
Combined with equation (A9), relation (A12) gives implicitly the skyrmion radius as a function of the parameters,