In polar coordinate, H. Suhl [H. Suhl,
Phys. Rev. 97, 555 (1954)] derived the
following expression for the condition of resonance:
^2\right)^{1/2})
This equation is based on the LL equation of motion. In the original
Equation, there is a factor
^2) ,
which can be dismissed since
the damping constant
 . The total magnetic energy is composed of:
+ The Zeeman energy
+
The uniaxial anisotropy and magnetocrystalline anisotropy energies,
which takes the form [ref. Yu et al., J. Appl. Phys. 85 (8) (1999)]:
 \mathrm{sin}^2\beta-\frac{3}{4}K_1\mathrm{sin}^4\beta)
where
 is the angle of the magnetization versus the direction of easy magnetization.
+
The magnetostatic energy:
The figure below shows the geometry of the different axis and the
definition of the angles.
The components of the vector magnetization are:
)
and that of the bias field are
)
.
Note that the field vector is set to remain in the film plane.
From the Equation of Smit Suhl and the magnetic energy density, one get:
+4\pi M_s)\times)
+2(1-2\mathrm{sin}^2\phi)\frac{K_1+K_u}{M_s}-\frac{9K_1}{M_s}\mathrm{cos}^2\phi \mathrm{sin}^2\phi+\frac{3K_1}{M_s}\mathrm{sin}^4\phi)]^{0.5})
+ FOR THE FIELD ALONG THE EASY AXIS [001]:
In that configuration, 
and the condition of resonance is:
\times(H+2\frac{K_1+K_u}{M_s})]^{0.5})
+ MAGNETIZATION DIRECTION EQUILIBRIUM
The direction of equilibrium of the magnetization in the film plane is
given by: 
with  .
Hence:
=\frac{K_1+K_u}{M_s}\mathrm{sin}2\phi \ -\frac{3 K_1}{2 M_s}\mathrm{sin}^2\phi \mathrm{sin}2 \phi)
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