Magnetic properties of nanoscale multilayers. — The superparamagnetic nature of ultrathin Fe layers between Ag layers has been investigated for a long time, but the dependence of the superparamagnetic properties on the number of the alternating Fe and Ag layers in a multilayer stack has not yet been studied. Superparamagnetism of a nominally few monolayer thick magnetic layer appears due to the island growth mode of the layer and multilayers of such discontinuous layers are called granular multilayers. The effects of dipole interactions on the superparamagnetic behavior have long been intensively studied, but only a few granular multilayer systems were studied from this point of view experimentally.
We have studied the blocking temperature, TB, – the temperature above which the relaxation frequency of the magnetic moment exceeds the frequency characteristic to the measurement – in Fe/Ag granular multilayers with different number (n=1, 2, 5, 10, 20) of Fe/Ag bilayers. TB was determined from magnetization measurements in a superconducting quantum interference device (SQUID) and it is defined as the cusp temperature of the magnetization measured after cooling the sample from 300K to 5K in zero field. The average size of the Fe granules were estimated from Mössbauer spectroscopy measurements. The role of the dipolar interactions and a perpendicular magnetic anisotropy component in the observed unusually large variation of TB was investigated by Monte-Carlo (MC) simulations in collaboration with the Democritos Research Center, Athens.
Two representative FC/ZFC magnetization curves are shown for the n=1 and 10 samples in Fig. 1. The TB values measured in different applied fields are shown for all the samples in Fig. 2. As can be seen from the figures there is a large, almost an order of magnitude increase of TB as the number of bilayers is increased from 1 to 10. Upon further increasing the number of bilayers (n=20), no change of the blocking temperature is observed.
In case of a non-interacting particle assembly, the increase of TB can result from the increase of the average grain size and/or the anisotropy energy. Significant changes of these factors for our samples could be ruled out by the Mössbauer spectroscopy measurements. The Monte Carlo simulations indicated that the dipolar interactions between the neighboring layers and the out-of-plane anisotropy should play a decisive role in the observed large variation of the blocking temperature.
Figure 1. Magnetization of granular multilayers with the indicated number of Fe/Ag bilayers measured after cooling in 10 Oe field (full symbols) and in zero field (empty symbols).
Figure 2. Dependence of the temperature of the maxima of the measured ZFC curves (TB) on the measuring field for different number of Fe/Ag bilayers.