If you mean why do you need waveguide for ferro-magnetic resonance, it is because there are places in the waveguide where the magnetic field rotates clockwise or anticlockwise as the wave passes. This does not happen in free space or coaxial cable, for instance, where the magnetic field just reverses direction twice a cycle.

Under a constant magnetic field bias, the magnetic moment of the material rotates clockwise or anticlockwise and depending on the bias this can be at microwave frequencies. The rotating microwave field in waveguide can couple to the rotating magnetic moment. This can result in different effects in opposite sides of a waveguide, or for waves travelling in opposite directions.

There are other ways to get a rotating microwave magnetic field.

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There is no need for a waveguide..but it can be done in a waveguide..the most frequently used configuration is the YIG filter layout with two orthogonal (half turn) loops..and MANY other versions of that it had eben been proposed to use a "naked"tiny YIG sphere and to inject it into the human body for temperature measurement during RF hyperthermia