There are a number of ways that reciprocity principles in optics may be affected by the presence of a static magnetic field (Potton, 2004). A classic example is Faraday rotation in which a plane polarised electromagnetic beam propagating through a suitable medium is rotated in the presence of a static magnetic field along the direction of propagation. The handedness of this rotation depends on the propagation direction, a nonreciprocal effect usefully applied to the construction of optical isolators (Dötsch et al., 2005). Nonreciprocal effects of this type are closely related to the idea that magnetic fields break time reversal symmetry. Similar nonreciprocal phenomena can occur, in various guises, on reflection off a semi-infinite sample. We discuss such behaviour in the present chapter, in the context of reflection off antiferromagnetic materials. In contrast to nonreciprocal phenomena based on the Faraday effect, our interest is in the Voigt geometry, in which the static magnetic field is perpendicular to the direction of propagation. We consider the well established phenomena of nonreciprocity in the intensity and phase of oblique incidence radiation, but concentrate mainly on recent developments on nonreciprocal power flow and finite beam effects.
We restrict discussion to the simple two dimensional geometry shown in Figure 1. Radiation is reflected, in the
Now compare Figure 1(a) to Figure 1(b), in which the sign of the incident angle has been reversed. In the absence of the magnetic field (B0 = 0), we can consider Figure 1(b) as the mirror reflection of Figure 1(a) through the
there is no symmetry operation that leads us from Figure 1(a) to Figure 1(b), and the two figures are not equivalent. Nonreciprocal behaviour is thus, in principle, possible. Whether or not it occurs in practice, however, depends on the material properties of the sample.
In the present chapter we consider nonreciprocity associated with reflection off a simple uniaxial antiferromagnet. In this case the static field represented by B0 in Figure 1 is an external field, since an antiferromagnet has no intrinsic macroscopic magnetic field. We consider a geometry in which the anisotropy associated with the spin directions, along with the external field B0, is perpendicular to the plane of incidence. This is equivalent to putting the anisotropy along
In considering nonreciprocity in the intensity and phase of the reflected beam, it is sufficient to simply consider the effect of interchanging the incident and reflected beams (i.e. reversing the sign of
2. Antiferromagnet permeability
The crucial parameter that determines the nonreciprocal optical properties of antiferromagnets is the magnetic permeability in region of the magnon (or spin wave) frequencies (Mills & Burstein, 1974), which typically lie in the terahertz range. We think of an antiferromagnet as two interpenetrating sublattices having opposite spin directions. Waves consisting of spins precessing in opposite directions in the two sublattices are then possible, and magnons of this type can interact with electromagnetic radiation. Their resonant frequencies are linked not only to the anisotropy field
The scalar quantity
In this study, we are interested in propagation of electromagnetic waves (strictly speaking polariton waves, since the waves include a contribution from the precessing spins in addition to that of the electromagnetic radiation) within the
the polaritons follow the familiar dispersion relation
In the presence of an external field B0 along the anisotropy axis, the two sublattices are no longer equivalent. This leads to two effects. Firstly, there are now two resonances instead of one and, secondly, the permeability tensor is no longer diagonal, but gyromagnetic. It thus takes the form (Mills & Burstein, 1974):
The diagonal elements
It is straightforward to see that
3. Nonreciprocity in reflection of plane waves
3.1. Reflected intensity
As discussed in the introduction, we can regard reflectivity
We are interested in reflection from vacuum in s polarisation. The complex reflection coefficient
with a corresponding transmission coefficient
Let us first consider how this sign change affects the complex reflection coefficient
At bulk region frequencies (
The overall reflectivity is given by
and is therefore reciprocal.
At reststrahl region frequencies (
Simulated oblique incidence (
When the damping is nonzero, the above symmetry arguments do not apply, and the reflectivity
3.2. Reflected phase
The complex reflection coefficient
Although thermodynamic arguments show that, in the absence of damping, the reflected intensity R, and hence the amplitude ρr, should be reciprocal (Remer et al., 1984), such arguments cannot be applied to the reflected phase ϕr. A detailed discussion of nonreciprocity in the reflected phase on reflection off antiferromagnets is given in Dumelow et al. (1998). Here we summarise the main results.
where m is an arbitrary integer. We include the term 2πm since it is convenient to plot the phase outside the range −π < ϕr< π.
Equation 24 shows that, in the bulk regions, the reflected phase is nonreciprocal even in the absence of damping. This is also the case in the reststrahl regions, but the phase does not follow a simple symmetry relation of the type given by this equation.
The amplitude and phase for reflection off MnF2 in the absence of damping are shown in Figures 4(a) and 4(c) respectively. The conditions are the same as those used in Figure 2. Note that we have shown the phase as varying within the range π to 3π in order to show that it changes continuously with frequency. The amplitude is reciprocal, in agreement with Figure 2, but nonreciprocity in the reflected phase is quite marked in both the bulk and reststrahl regions, obeying Equation 24 in the bulk regions.
Figures 4(b) and 4(d) show reflection amplitude and phase respectively in the presence of damping. In line with the reflectivity results in the previous subsection, the reflection amplitude now shows slight nonreciprocity. The phase shows the same type of nonreciprocity as seen without damping, although the bulk region symmetry arguments of Equation 24 no longer apply.
3.3. Power flow
The nonreciprocal phenomena described in the Subsections 3.1 and 3.2 were analysed ten or more years ago, and concern the behaviour of a reflected plane wave. Recently we have started studying nonreciprocal behaviour within the antiferromagnet itself, in particular with respect to the direction of the internal power flow (Lima et al., 2009), represented by the time-averaged Poynting vector (Landau & Lifshitz, 1984),
We consider an angle of refraction in terms of the direction of the time-averaged Poynting vector S2 (which is not necessarily the same as the wavevector direction) in the antiferromagnet, as shown in Figure 5. The angle of refraction θ2, defined in this way, is given by
In s polarisation the E field is confined along z, so the Poynting vector is most easily represented in terms of the Ez field, making use of the conversion k × E = ωμ0μH. The resulting time averaged Poynting vector has components
The direction of power flow can thus be obtained by substitution into Equation 26.
We now investigate the above expressions in order to search for possible nonreciprocity in the power flow direction in the antiferromagnet, taking power flow to be nonreciprocal if
In order to consider power flow, we restrict ourselves initially to the case where there is no damping in the system (Γ = 0). The calculated values of θ2 for oblique incident reflection off MnF2 in this case are shown in Figure 6.
In the case of zero damping, as discussed previously, μ1, μ2, and μv are all wholly real. k2y is real in the bulk regions and imaginary in the reststrahl regions.
so that S2 is parallel to k2. Since kx is continuous and k2y positive it also follows that refraction must be positive, i.e. θ2 always has the same sign as θ1. Overall, therefore, power flow follows the type of behavior shown in Fig. 7(a). This is confirmed by the calculations of the θ2 shown in Figure 6 for the bulk regions. Note that in these regions radiation may, in principle, flow an infinite distance into the antiferromagnet, and is thus unaffected by the sample surface. Our result that power flow is reciprocal in this case is thus consistent with the idea that radiation should display reciprocal behavior in the interior of a sample.
We now consider the case when k2y is imaginary. In this case we get
very fact that the field is assumed to extend over an infinite plane means that energy flow along the surface does not violate causality.
We now turn to the case where damping is present. Now k2y should, in general, be complex, with the its imaginary part greater near the reststrahl regions. Since imaginary k2y results in nonreciprocal power flow, we should also expect such nonreciprocity in the case of complex k2y. In Fig. 10 we show the power flow directions both for normal and for oblique incidence, assuming Γ = 0.0007cm−1. There is now no distinct division between reststrahl and bulk regions, and nonreciprocal power flow occurs both inside and outside the nominal reststrahl regions. At normal incidence the power flow directions are now no longer restricted to θ2 = 0 and θ2 = ±90 . Since nonzero θ2 implies nonreciprocity, the associated fields must be some extent be bound to the sample surface in all regions for which θ2 ≠ 0 .
4. Reflection of a finite beam
The previous section discusses various phenomena associated with plane wave reflection off an antiferromagnet. However, when the plane wave is replaced by a finite beam, we predict additional effects concerning the profile and position of the reflected beam (Lima et al., 2008, 2009). Such effects are expected with either normal or oblique incidence radiation. However, we concentrate on normal incidence effects firstly for simplicity and secondly because they are more unexpected.
We examine reflection of finite beams in two ways. Firstly, we interpret the reflection using an angular spectrum analysis, in which the incident beam is considered as a Fourier sum of plane waves. We then give a power flow interpretation of the predicted effects.
4.2. Angular spectrum analysis
Here we summarise the angular spectrum analysis used in describing reflection of a finite beam normally incident on an antiferromagnet (Lima et al., 2008), using a two dimensional model in which the incident beam, centred at x = 0, is considered as an angular spectrum of plane waves propagating in the xy plane. It can thus be represented in the form
where ψ(kx) is a distribution function representing the shape of the beam. At the sample surface, which we place at y = 0, the electric field of the incident beam is
The electric field of the corresponding reflected beam is given, at the surface, by
Here r(kx) represents the complex reflection coefficient for the relevant plane wave component, and is given by Equation 14. It is convenient to consider this complex coefficient in terms of amplitude ρr(kx) and phase ϕr(kx), in the form of Equation 21, i.e.,
If the beam is sufficiently wide, there will be a narrow distribution of kx values centred, at normal incidence, around kx = 0. We can thus substitute Equation 38 into Equation 37 and expand ρr(kx) and ϕr(kx) as a Taylor series around kx = 0. If we ignore terms in
The second term on the right hand side of Equation 39 can, in practice, normally be ignored (Lima et al., 2008). In fact, in the absence of damping, it is identically zero since reciprocity in ρr implies
The reflected field is thus given, to a good approximation, by the first term on the right hand side of Equation 39. This term is simply the reflection coefficient r(0) for a normally incident plane wave multiplied by an integral which gives the profile of the field along x. This integral is identical to that of the incident beam (Equation 36) except that x has been replaced by X, given by Equation 42. Thus the shape of the reflected beam is the same as that of the incident beam, but it has been shifted along the surface of the sample by a distance Dr equal to
as shown in Figure 11(a). In the case of reciprocal reflected phase (i.e. ϕ(−kx) = ϕ(kx)),
Before discussing the lateral shift described by Equation 43 in any detail, we note that the behaviour of the reflected beam is in some ways similar to that of an oblique incidence finite beam which undergoes total internal reflection when passing from an optically denser to a less dense medium. Such a beam also suffers a lateral displacement Dr upon reflection, as shown in Figure 11(b). This displacement was observed experimentally by Goos & Hänchen (1947) and is normally referred to as a Goos-Hänchen shift (Lotsch, 1970). Goos-Hänchen shifts have also been reported in the case of external reflection in specific instances such as reflection off metals (Wild & Giles, 1982, Leung et al., 2007), and the lateral displacement discussed here can be considered as a type of a normal incidence Goos-Hänchen shift. Equation 43 is, indeed, a normal incidence version of the classical expression commonly used to describe Goos-Hänchen shifts (Artmann, 1948), and the angular spectrum analysis used above in deriving the equation is basically the same as that previously used to describe Goos-Hänchen shifts in the case of total internal reflection (Horowitz & Tamir, 1971, McGuirk & Carniglia, 1977).
We now apply Equation 43 to the specific case of reflection off an antiferromagnet. In the absence of damping, the reflection coefficient r can easily be resolved into its real and imaginary parts, allowing explicit evaluation of this equation. This gives
in both the bulk and the reststrahl regions. Since the sign of μ2 depends on the sign of B0, it is immediately obvious that the direction of the lateral displacement will be reversed if the external field direction is reversed. In the presence of damping, Equation 43 can be evaluated numerically, but the results are found to be almost identical to those with Γ = 0 (Lima et al., 2008).
The calculated values of Dr, ignoring damping, are compared with the reflectivity spectrum at upper reststrahl region frequencies in Figure 12. A lateral shift is predicted in both the bulk and the reststrahl regions. Similar shifts are predicted around the lower reststrahl region, but of opposite sign (Lima et al., 2008). There is a divergence in Dr at the reflectivity minimum just below the reststrahl region. At this frequency, the assumptions made in deriving Equation Dr clearly do not apply.
In order ot verify the shifts shown in Figure 12, reflection of a particular incident beam profile may be modelled. For simplicity, we have considered a gaussian beam whose focal plane is the surface of the sample (Lima et al., 2008, 2009). The incident beam can thus be represented by Equation (35) with (Horowitz & Tamir, 1971)
where 2g represents the beam width at the focal plane. At the sample surface the incident beam profile is thus represented by Equation 36 and the reflected beam profile by Equation 37. The integrals in these two equations can be evaluated numerically, and the profiles of the corresponding E fields thus obtained.
The incident and reflected beam intensities can, in general, be well represented by |E|2. In Figure 13 we show the resulting intensity profiles along x for the three frequencies marked in Figure 12. It is seen that, although the modelled incident beam is very narrow (g = λ, the free space wavelength), all the reflected beams are displaced along the surface in excellent agreement with Equation 44. It is also observed that damping does not noticeably affect this displacement. Explicit simulations have also confirmed the prediction of Equation 44 that beam displacement is, to a very good approximation, independent of beam width (Lima et al., 2008).
It is useful not only to consider the incident and reflected fields at the surface, but also the overall E field distribution in the xy plane. We conveniently consider the electric fields in terms of an incident field Ei(x,y) and a reflected field Er(x,y) in the region x < 0 (vacuum) and a transmitted field Et(x,y) in the region x > 0 (antiferromagnet). Ei(x,y) is given by Equation 35, with Er(x,y) and Et(x,y) given by
respectively. The resulting profiles at the three frequencies indicated in Figure 12 are shown in Figures 14 and 15. Figure 14 shows the profile in the absence of damping and Figure 15 shows the profile when damping is included. The left hand panels show the incident and transmitted fields, whilst the right hand panels show the reflected field.
In all cases the reflected fields are displaced along x in accordance with Figures 12 and 13. In addition, we see that the transmitted field is also displaced. This result can be anticipated from the fact that the phase ϕt of the complex transmission coefficient t is nonreciprocal. An analysis equivalent to that used in determining Dr then gives a lateral displacement Dt of the transmitted field profile given by
It is noticeable that, in the case of the bulk frequency B, near the top of the reststrahl band, there is a much larger displacement in the transmitted field than in that of the reflected beam. In the absence of damping, the field decays away from the interface in the reststrahl region (Figure 14(a)), but propagates into the sample, perpendicular to the surface, in the bulk regions (Figures 14(b) and 14(c)) . When damping is included, the reststrahl region behaviour is largely unchanged, but there may now be significant decay in the bulk regions. At frequency B, just above the top of the reststrahl region, this decay is fairly small, but at frequency C, near the bottom of the reststrahl region, it is similar to that in the reststrahl region itself.
4.3. Power flow analysis
4.3.1. Reststrahl region
In the above analysis we have considered the displacement of the reflected beam as an interference effect. It is also useful, however, to consider this effect in terms of power flow. In the reststrahl regions, in the absence of absorption, energy conservation principles can be used in a straightforward way to analyse the lateral shift (Lima et al., 2009). The analysis is similar to that used by Renard (1964) in the case of the conventional Goos-Hänchen shift for total internal reflection.
We make use of the result of subsection 3.3 that, in the reststrahl regions and in the absence of damping, a normally incident plane wave reflected off an antiferromagnet induces power flow parallel to the surface within the antiferromagnet, as shown in Figures 9(b) and 9(c). We consider this to be the behaviour in the centre portion of a wide finite incident beam, such as that represented in Figure 16. The central portion of this incident beam lies between x2 and x3, and it gradually decays away to zero between x2 and x1 and between x3 and x4. The internal energy flux associated with plane wave reflection in the central portion is represented by P2. Energy conservation therefore requires that there is a net flux P1 entering the antiferromagnet near one edge of the beam and a net flux leaving near the other edge. This is equivalent to a lateral shift Dr of the reflected beam with respect to the incident beam. Thus P1 enters in the region between x1 and x2 + Dr and P3 leaves in the region between x3 and x4 + Dr. We consider energy flow within a slice, of thickness Δz, in the xy plane. Within this slice we have P1 = P2 = P3.
P1 is the difference between the incident and reflected flux between X2 and x2 + Dr, and can be written as
where Si(x) and Sr(x) are the intensities, represented in terms of time averaged Poynting vectors along y, of the incident and reflected waves respectively. Since we expect the reflected beam to have the same shape as the incident beam, we can write
so that the first and last terms on the right hand side of Equation 49 cancel. Thus only the second term contributes to P1, and the integral is only performed between x2 and x2 + Dr, in the central portion of the beam. Within this interval Si(x) has a constant value, which we denote as Smax. Equation 49 thus becomes
Using Equation 25 and a standard application of Maxwell’s equations, Smax can be expressed in terms of the incident field Emax within the central region of the beam as
The flux P2 within the antiferromagnet is given by
S2x(y) can be expressed in terms of the electric field Ez(y) within the antiferromagnet using Equation 27. Ez(y) itself can be related to the field Emax of the incident beam by the equation
so P2 may also be obtained as a function of Emax. On putting P1 = P2 in the above equations, and solving for Dr we get exactly the same result as obtained using the angular spectrum analysis (Equation 44). Thus simple energy conservation principles may be used to predict the Goos-Hänchen shift in this case.
We may examine explicitly the power flow behaviour in the xy plane using the type of calculation used in obtaining Figures 14 and 15. Figure 17(a) shows the overall power intensity and flux lines (Lai et al., 2000) for reflection off MnF2 at frequency A, corresponding to the upper reststrahl region. The behaviour is similar to that predicted from Figure 16 (although in the present case the shift is negative). Thus, for x > −0.01 cm, the incident intensity is greater than the reflected intensity, so that the overall power flow is to the right. At x −0.01 cm, the incident and reflected beams cancel, whereas for x < −0.01 cm, the reflected beam dominates and the overall power flow is to the left. Flux continuity is thus preserved in the manner illustrated in Figure 16.
Figure 17(b) shows power flow when damping is included. In this case the flux continuity argument no longer applies (hence we have not attempted to show continuous flux lines), but the overall behaviour is not significantly changed.
It is interesting to note that the power flow explanation of the normal incidence Goos Hänchen shift gives a very clear example of the breaking of time reversal symmetry by a static magnetic field. In the example shown in Figure 16, there is power flow along the positive x direction within the antiferromagnet, and the reflected beam is thus displaced in this direction. If, however, time reversal were applied to the reflected beam, so that it became a new incident beam, without reversing the direction of B0, the flow of energy within the antiferromagnet would not retrace its path along the negative x direction, but would once again flow along positive x, and there would be a further displacement of the reflected beam in this direction.
4.3.2. Bulk regions
The above power conservation principles used in analysing the normal incidence Goos- Hänchen shift in the reststrahl regions are based on the principle that all the incident energy will be reflected back from the antiferromagnet surface. This is clearly not the case in the bulk regions. Furthermore, in the absence of damping, the power flow resulting from normally incident plane waves is transmitted normal to the surface, as seen in Figure 9(a). The reststrahl region analysis requires power flow parallel to the interface, so one might expect that there would be no lateral shift of the reflected beam in the bulk regions. The angular spectrum analysis, however, predicts that such a shift should occur in both the reststrahl and the bulk regions, so it is important to understand, in terms of energy flow, how such a shift is possible in the bulk regions.
In order to analyse the power flow, we once again take the example of reflection of a Gaussian beam and examine the power flow profile in the xy plane. The resulting overall power intensity and flux at the two bulk frequencies B and C are shown in Figure 18. Before analysing the lateral shift in any detail, it is worth noting that the profile of the power intensity within the antiferromagnet is different from that of the electric field amplitudes
shown in Figures 14 and 15. This is because the power intensity depends on the magnetic field component of the electromagnetic field as well as its electric field component. The magnetic field profile is in fact very different from that of the electric field (Lima et al., 2009), leading to a different power intensity profile.
At frequency B, ignoring damping (Figure 18(a)), overall power flow is to the right both in the vacuum region y < 0 and within the antiferromagnet y > 0, and remains perpendicular to the surface. There is flux continuity across the interface, as expected, and the overall profile is slightly displaced in the positive x direction with respect to the incident beam, which is centred at x = 0. We can understand this if we recall that the reflected beam is slightly displaced in the negative x direction and is less intense than the incident beam (see Figure 13(b)). The overall flux for y < 0, within the vacuum region, is the incident beam minus the reflected beam, so it is shifted in the positive x direction with respect to the pure incident beam. Flux continuity requires that this displacement is transferred to the transmitted beam, i.e. in the bulk regions the transmitted beam is displaced along x in the direction opposite to the displacement of the reflected beam. The behaviour in the presence of damping (Figure 18(b)) is not appreciably different.
At frequency C in the absence of damping (Figure 18(c)) the situation is similar to that at frequency B except that there is now a resultant power to the left for x < −0.08 cm−1. This is related to the fact that the reflected beam is more intense than the incident beam at these values of x (see Figure 13(c)). This also means that there must be power flow to the left within the antiferromagnet. This presents an apparent problem since there is no obvious source for the associated energy. Nevertheless, detailed calculations of power flow several centimeters into the antiferromagnet show that a portion of the incident energy does indeed return to the left in this region (Lima et al., 2009). This is easier to see when damping is present (Figure 18(d)), in which case some of the incident energy returns to the left without penetrating a long distance into the antiferromagnet. In fact, in this case, the overall power flow is somewhat similar to that observed in the reststrahl regions. This is another illustration of the concept that, in the presence of damping, the bulk and reststrahl regions can be thought of as merging into one another.
In the above examples, frequency B (Figures 18(a) and 18(b)) can be regarded as representative of most of the bulk frequencies. Frequency C (Figures 18(c) and 18(d)), in contrast, displays rather peculiar behaviour particular to frequencies between the reststrahl region and the reflection minimum (see Figure 12).
5. Conclusions and future prospects
In this chapter we have examined various nonreciprocal effects associated with reflection of terahertz radiation off antiferromagnets. Of these effects, only nonreciprocity in the reflectivity has, to our knowledge, been investigated experimentally at the time of writing (Remer et al., 1986, Brown et al., 1994).
A simple, if slightly indirect, way of observing the nonreciprocal reflected phase has been suggested (Dumelow & Camley, 1996, Dumelow et al., 1998), and uses the configuration shown in Figure 19. Here, a dielectric layer is deposited onto the surface of an antiferromagnet and the overall reflectivity off the overall structure measured. In this setup, there is reciprocal reflection from the vacuum/dielectric interface, but the phase of the radiation reflected from the dielectric/antiferromagnet interface is nonreciprocal. Interference between these partial waves is thus nonreciprocal, leading to a nonreciprocal
overall reflectivity which depends on the dielectric layer thickness, as shown in Figure 20. This is true even when the reflectivity off the pure antiferromagnet is close to reciprocal, as is the case for MnF2.
The discussion of nonreciprocity in the power flow is concerned with power flow behaviour within the interior of an antiferromagnet. Obviously it is not straightforward to measure this experimentally. It appears more reasonable to investigate the effect of this nonreciprocal power flow on the radiation interacting with a finite sized sample of a given shape. The analysis presented in this chapter does not extend to this type of system, since the antiferromagnet is considered to be infinite along x. However, other techniques such as the finite difference time domain (FDTD) method should help clarify the expected behaviour.
The lateral shift predicted in the case of reflection of a finite beam off an antiferromagnet should in principle be measurable given a suitable coherent source such as a far infrared laser (Rosenbluh et al., 1976), backward wave oscillator (Dobroiu et al., 2004), or YIG oscillator with frequency multiplied output (Kurtz et al., 2005). In order to observe the normal incidence shift, a beam splitting arrangement appears necessary. It is also, however, important to consider the effect at oblique incidence, both theoretically and experimentally. In this case the effect should be observable directly without the use of a beamsplitter.
In this chapter we have only discussed phenomena in the Voigt configuration with the external field aligned along the anisotropy axis, deliberately avoiding the more complicated configurations in which the external field makes an angle with the anisotropy field (Almeida & Mills, 1988), or in which these axes are not perpendicular to the plane of incidence. However, we should point out that theoretical works on the reflected amplitude and phase do exist for more complex geometries (Stamps et al., 1991, Dumelow et al., 1998), and, in the case of reflectivity, there is some experimental work (Abraha et al., 1994, Brown et al., 1995).
Finally, we stress that, although we have concentrated on reflection off antiferromagnets in this chapter, the basic priciples involved stem from the form of the permeability tensor given in Equation 8. However, there are other types of material, such as ferromagnets or ferrimagnets, that also have a gyromagnetic permeability of this form. We therefore expect similar phenomena for these materials, although some of the symmetry arguments have to be looked at in a slightly different way since, in general, such materials have their own internal macroscopic magnetic field. One can also have a dielectric tensor of this form, such as that associated with magnetoplasma excitations. In this case, p-polarisation radiation should give results similar to those presented here for s-polarisation reflection off antiferromagnets (Remer et al., 1984).
This work was partially financed by the Brazilian Research Agency CNPq (projects Universal 482238/2007-0, CT-ENERG 554889/2006-4, and CNPq-Rede NanoBioestruturas 555183/2005-0).