| Summary: | <p> Magnetic relaxation describes the process by which a magnetic system prepared in a non-equilibrium state returns to an equilibrium distribution. Thin film samples of iron phthalocyanine (FePc) are deposited onto heated substrates. Substrates are made of either silicon, or smooth gold on silicon. FePc molecules self-assemble into small crystalline structures. Due to the planar shape of the molecules, iron chains are formed. The length of these chains depends on the deposition temperature of the sample. Here, FePc thin films are saturated in an applied magnetic field of 3 T at low temperatures. The magnetic field is then reduced to 0 T at a rate of either 54 Oe/s or 100 Oe/s. The change of the magnetization at zero-field and constant temperature is recorded over a time interval of 5000 s. A series of 200 nm thick FePc samples are prepared at varying deposition temperatures onto silicon substrates. Based on the separation distance between iron chains, the inter-chain interactions—probably based on dipole interactions—is expected to be small. The intra-chain interactions are modified by the grain size. Using the stretched exponential model, a non-vanishing asymptotic remnant magnetization is found. The value of this asymptote is shown to decay exponentially with measurement temperature, and vanish near 4.5 K. The dynamic response has a peak which becomes higher in temperature with larger grain size up to 180°C, where we expect a phase transition in the thin film morphology. Above 3.2 K, the relaxation time appears activated, but the data is inconclusive at this moment. From these results, we find that both static and dynamic magnetic responses play an important role in FePc thin films in the measured temperature range of 2.5 K to 4.0 K. Asymptotic remnant magnetization, the static variable, is only non-zero below 4 K and importantly depends on the grain size as larger grains tend to make the inter-chain interactions more important.</p>
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