In this paper the magnetostatic energy in a ferromagnetic thin film with two imperfect, rough surfaces has been calculated. It has been shown that the surface roughness gives in some cases a considerable contribution to an effective perpendicular anisotropy. Within the model assumed it has been proved that the value of magnetostatic energy and, as a consequence, the anisotropy in a thin imperfect film does not depend on the relative location of the two rough surfaces.
PACS numbers: 75.70.-i
1. Introduction
Discovery of the phenomenon of perpendicular anisotropy has gained much interest in the past few years. It has opened new possibilities for memory applications. Ferromagnetic thin films have been widely investigated for use as a data storage medium in magneto-optic or magneto-recording systems [1, 2].
The perpendicular magnetic anisotropy is attributed to the interfacial surface anisotropy, which is a straightforward consequence of the fact that surface atoms are located in a different environment than the bulk ones. It has been found that at least three mechanisms give rise to the surface anisotropy. Those are the single-ion mechanism, dipole-dipole interaction and also the effect caused by the surface roughness.
The importance of the interfacial surface roughness in the phenomenon of magnetic anisotropy was suggested by Bruno [3]. In this paper we examine only the surface roughness, as a factor which contributes to the dipolar magnetic anisotropy. We derived formulas determining the magnetostatic energy and the surface anisotropy as functions of some geometrical parameters of an imperfect ferromagnetic thin film. The calculation of the magnetostatic energy and the surface anisotropy has been performed within the macroscopic theory of magnetism. (We have applied the continuous medium approximation.)
2. Model of surface roughness
In this paper we shall adopt the model of surface roughness proposed by Bruno [3]. The difference between the model of Bruno and the actual model used in the calculation is that we consider a ferromagnetic film having two rough surfaces and finite thickness t. The thickness t may be considered an average, i.e. the experimentally measured distance between surfaces of the film. The following parameters also characterize our probe: the average deviation from the plane of the surface the average lateral size d of ``craters'' or ``terraces'', i.e. the flat areas on the surface, and the phase difference between ``craters'' on one surface and the other (cf. Fig. 1).
Fig.1. Representation of a thin film with surface roughness showing the characteristic parameters t, , d and .
3. Calculation of magnetostatic energy
Calculation of the magnetostatic energy is based on the standard Fourier
series method proposed first by Kittel [4]. The magnetostatic energy is
given by the following expression:
Fig.2. Decomposition of the magnetization distribution in a thin film with two rough surfaces.
Arrows indicate
direction of magnetization of each specific region in space. A picture in
the right upper corner of Fig. 2 shows the top and the bottom view of the
film. The right column of Fig. 2 shows the top view of each of the
component parts forming together our rough ferromagnetic film. The gray
regions are ``terraces'' and the white ones are ``craters''. The signs +
and - indicate the direction of magnetization in each region. Let us
denote the magnetizations of the corresponding parts as ,
and , demagnetizing fields as ,
and , distributions of magnetic poles on surfaces
of each probe as , and and,
eventually, scalar potentials of demagnetizing fields as ,
and correspondingly (Fig. 2). Equation (1) transforms
now as follows:
In the same way we can expand the scalar potential
of the
magnetic field
Fig.3. Graph of the magnetostatic energy as a function of the average deviation from the plane of the surface.
4. Surface anisotropy
As we already know the magnetostatic energy of a thin film with
magnetization normal to the surface, we can easily derive the value of
magnetostatic energy of this film with magnetization parallel to the plane
of the surface. To do this we apply the sum rule presented by Yafet et
al. [5]. By the symmetry reasons we have that
(the magnetostatic energy with
magnetization parallel to the x axis equals the magnetostatic energy with
magnetization parallel to the y axis). The energies EMx,
EMy and EMz satisfy the following equation:
Fig.5. The difference between the magnetostatic energies as a function of .
Fig.6. The surface anisotropy constant KS as a function of the average deviation from the plane of a surface.
The difference between magnetostatic energies with magnetization parallel
and normal to the plane of the surface can be written in the following
form:
5. Conclusion
In this paper we have derived formula describing magnetostatic energy in a ferromagnetic thin film with two rough surfaces magnetized with saturation magnetization normal to the plane of the surface. The value of magnetostatic energy depends on the average distance between the surfaces. Still it has been found that it does not depend on the parameter , i.e. the phase difference between ``craters'' on the higher surface and ``craters'' on the lower surface. In other words, relative location of the two surfaces does not influence the value of magnetostatic energy. Also the values of energy anisotropy have been calculated. In case of our thin film the difference between magnetostatic energies ( ) remains a negative number within the range of parameters t, d and used. It means that the easy direction magnetization remains parallel to the surface of the thin film for the examined class of probes.
Acknowledgments
I would like to thank Prof. Henryk Szymczak from the Institute of Physics of the Polish Academy of Sciences, who proposed the subject of this paper, stimulated my work on the problem and served me with friendly advice. I also wish to thank Polish Children's Fund, which helped me to develop my skills and knowledge in physics by inviting me to take part in Research Workshop in Physics held in the Institute of Physics of the Polish Academy of Sciences in Warsaw.