Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet $Ca3Mn2Ge3O12$

The effect of linearly polarized light illumination on the metamagnetic phase transition in the antiferromagnetic garnet $Ca3Mn2Ga3O12$ is studied. The crystal is exposed to light propagating both along the tetragonal axis [001] and along the [100] direction. In both cases, a change of the field $Ht$ of the metamagnetic phase transition is observed under illumination, and this change depends on the orientation of plane or polarization of the light with respect to the crystal axes. In the first case, $k‖H‖[001],$ the value of $Ht$ decreases on exposure to light with the polarization E‖[110] and increases on exposure to light with the polarization E‖[11̄0]. In the second case, k‖H‖[100], the value of $Ht$ decreases irrespective of the orientation of the plane of polarization of the light with respect to the crystal axes. However, the magnitudes of the change of $Ht$ are different for light with the polarization E‖[011] and with the polarization E‖[01̄1]. The change of the field of the metamagnetic phase transition in the second case is much larger than in the first case. A phenomenological theory of the photomagnetic effects observed in the antiferromagnetic garnet $Ca3Mn2Ge3O12$ is developed. It is shown that the effect of light illumination on the metamagnetic phase transition is related to the photoinduced magnetic moment in this antiferromagnet. The magnetic moment induced by linearly polarized light in the garnet $Ca3Mn2Ge3O12$ is detected experimentally by means of a SQUID magnetometer.