Dynamic Monte Carlo simulation on behavior of phase transition of a random magnetic system in finite observation time

Random magnets generally exhibit gradual phase transitions more or less. The origin of the phenomena has been controversial for a long time: intrinsic phenomena of disordered magnets or non-equilibrium effect due to finite observation time. We now support the latter, but there have not been clear evidences experimentally and theoretically. We show that the behavior of phase transition of a simple random magnetic system differs in the observation time by using a dynamic Monte Carlo simulation. The target of the simulation is experiments of the line width of NMR spin-echo spectra, a type of the order parameter, on MnxCd1−x (HCOO)2·2(NH2)2CO. The calculated results indicate that, as the averaging time becomes shorter, the phase transition becomes more gradual. This tendency is most pronounced around the percolation concentration. The calculated results coincide well with the characteristic features of the experimental results. This coincidence supports that the smearing behavior of the order parameter is a non-equilibrium effect, though Ising model employed in the simulation is different with Heisenberg system of the target substance.