Magnetic thin films of yttrium iron garnet (YIG) grown from liquid phase epitaxy show very regular strip domains. The width of the domain is a function of the applied magnetic field. For certain substitution for YIG, the magnetization of the domains is practically parallel to the direction of the strip domain. The magnetizations of adjacent domains lying on either side of the magnetic wall are anti-parallel. This leads to the realization of waveguides in the strip domains. It is shown that due to the value of Faraday specific rotation. theta =263 degrees /cm, lambda 0=1152 mu m, for YIG, the different calculations for the propagation of plane monochromatic waves may be limited to the first power of Theta . Under these conditions, the modes are only circularly polarized. The width of strips required to guide these waves is calculated.