Manifestation of Memory and Butterfly in the Photon Echo on Erbium Ions in LuLiF 4 and YLiF 4

The memory effect in the form of hysteresis has been detected in the measured dependence of the intensity of the photon echo in YLiF 4 and LuLiF 4 samples with Er 3+ impurity ions on the orientation, strength, and variation direction of the magnetic field. The prehistory of the location of a sample in the magnetic field with a certain direction and strength is written and stored for no less than 6 h at a temperature of 2 K. The effect crucially depends on the orientation of the optical axis of the sample with respect to the external magnetic field.