Long-time variation of magnetic structure in CeIr3Si2

CeIr3Si2 shows successive magnetic transitions at TN1=4.1 K and TN2=3.3 K. At T < TN2 it shows three-step metamagnetic transitions below H=1.43 T. In this non-diluted compound a long-time variation of magnetic structure has been detected by means of magnetc susceptibility and time-resolved neutron scattering measurements. When a sample is rapidly cooled below TN2, the magnetic Bragg peaks corresponding to the intermediate temperature phase (TN2 < T < TN1) are observed. The amplitude of these Bragg peaks gradually decreases with time. On the other hand, another group of Bragg peaks corresponding to the low temperature phase (T < TN2) gradually grow with time. The characteristic time for these variations follows the Arrhenius law with an activation energy Ea/kB =4 K. This is the first observation of long-time variation of magnetic structure in non-diluted uniform magnets.