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For lower frequencies, we can find examples.

One practical example may be at very low/zero frequency. Magnetic permeable materials distort static (or almost-static) magnetic fields. Technically, we can call this refraction.

Oscillating magnetic fields (radio waves) are routinely used to measure thepermeability of materials; what is measured is the effect on the field by the sample, that again it is basically the index of refraction.

Besides lack of transparency, I believe that at the frequencies you are asking the permeability of ferromagnets becomes a mess, because the large magnetic domains are "slow" at reacting to the magnetic field. The technique of oscillating magnetic field as before can be used also to assess the size and properties of the magnetic domains, proving that high-frequency permeability differ from the static one.

I think that a transparent fluid/solid loaded with superparamagnetic nanoparticles can show visible refraction effect, but I can't find the source of this belief and not sure up to which frequency it may go; Infrared seems too much. That's my candidate metamaterial.

Techniques like Neutron Scattering,used to observe the spin structure may be seen as electron-scale refraction?

For lower frequencies, we can find examples.

One practical example may be at very low/zero frequency. Magnetic permeable materials distort static (or almost-static) magnetic fields. Technically, we can call this refraction.

Oscillating magnetic fields (radio waves) are routinely used to measure thepermeability of materials; what is measured is the effect on the field by the sample, that again it is basically the index of refraction.

Besides lack of transparency, I believe that, as you wrote, at high frequency the permeability of ferromagnets becomes a mess, because the large magnetic domains are "slow" at reacting to the magnetic field. The technique of oscillating magnetic field as before can be used also to assess the size and properties of the magnetic domains, proving that high-frequency permeability differ from the static one.

I think that a transparent fluid/solid loaded with superparamagnetic nanoparticles can show visible refraction effect, but I can't find the source of this belief and not sure up to which frequency it may go; Infrared seems already too much. That's my candidate metamaterial.

Magnons / spin waves should be the way to go to understand the high frequency limit.

One practical example may be at very low/zero frequency. Magnetic permeable materials distort static (or almost-static) magnetic fields. Technically, we can call this refraction.

Oscillating magnetic fields (radio waves) are routinely used to measure thepermeability of materials; what is measured is the effect on the field by the sample, that again it is basically the index of refraction.

Besides lack of transparency, I believe that at optical frequency the permeability of ferromagnets becomes a bit messy, because the large magnetic domains are "slow" at reacting to the magnetic field. The technique of oscillating magnetic field as before can be used also to assess the size and properties of the magnetic domains, proving that high-frequency permeability differ from the static one.

I think that a transparent fluid/solid charged with superparamagnetic nanoparticles can show visible refraction effect, but I can't find the source of this belief.

Techniques like Neutron Scattering,used to observe the spin structure may be seen as electron-scale refraction?

For lower frequencies, we can find examples.

One practical example may be at very low/zero frequency. Magnetic permeable materials distort static (or almost-static) magnetic fields. Technically, we can call this refraction.

Oscillating magnetic fields (radio waves) are routinely used to measure thepermeability of materials; what is measured is the effect on the field by the sample, that again it is basically the index of refraction.

Besides lack of transparency, I believe that at the frequencies you are asking the permeability of ferromagnets becomes a mess, because the large magnetic domains are "slow" at reacting to the magnetic field. The technique of oscillating magnetic field as before can be used also to assess the size and properties of the magnetic domains, proving that high-frequency permeability differ from the static one.

I think that a transparent fluid/solid loaded with superparamagnetic nanoparticles can show visible refraction effect, but I can't find the source of this belief and not sure up to which frequency it may go; Infrared seems too much. That's my candidate metamaterial.

Techniques like Neutron Scattering,used to observe the spin structure may be seen as electron-scale refraction?

One practical example may be at very low/zero frequency. Magnetic permeable materials distort static (or almost-static) magnetic fields. Technically, we can call this refraction.

Oscillating magnetic fields (radio waves) are routinely used to measure thepermeability of materials; what is measured is the effect on the field by the sample, that again it is basically the index of refraction.

Besides lack of transparency, I believe that at optical frequency the permeability of ferromagnets becomes a bit messy, because the large magnetic domains are "slow" at reacting to the magnetic field. I think that a transparent fluid/solid charged with superparamagnetic nanoparticles can show visible refraction effect, but I can't find the source of this belief.

Techniques like Neutron Scattering,used to observe the spin structure may be seen as electron-scale refraction?

One practical example may be at very low/zero frequency. Magnetic permeable materials distort static (or almost-static) magnetic fields. Technically, we can call this refraction.

Oscillating magnetic fields (radio waves) are routinely used to measure thepermeability of materials; what is measured is the effect on the field by the sample, that again it is basically the index of refraction.

Besides lack of transparency, I believe that at optical frequency the permeability of ferromagnets becomes a bit messy, because the large magnetic domains are "slow" at reacting to the magnetic field. The technique of oscillating magnetic field as before can be used also to assess the size and properties of the magnetic domains, proving that high-frequency permeability differ from the static one.

I think that a transparent fluid/solid charged with superparamagnetic nanoparticles can show visible refraction effect, but I can't find the source of this belief.

Techniques like Neutron Scattering,used to observe the spin structure may be seen as electron-scale refraction?