Abstract
We report on the generation of large inverse remanent magnetizations in nano-sized core/shell structure of Au/Ni by turning off the applied magnetic field. The remanent magnetization is very sensitive to the field reduction rate as well as to the thermal and field processes before the switching off of the magnetic field. Spontaneous reversal in direction and increase in magnitude of the remanent magnetization in subsequent relaxations over time were found. All of the various types of temporal relaxation curves of the remanent magnetizations are successfully scaled by a stretched exponential decay profile, characterized by two pairs of relaxation times and dynamic exponents. The relaxation time is used to describe the reduction rate, while the dynamic exponent describes the dynamical slowing down of the relaxation through time evolution. The key to these effects is to have the induced eddy current running beneath the amorphous Ni shells through Faraday induction.
Highlights
Magnetic materials in the form of nano-sized particles or amorphous structures can generate many novel behaviors
The memory effect in magnetic systems refers to the development of magnetization that is sensitive to changes in external condition, whereas the aging effect reflects the strong dependence of the relaxation of the magnetization on the waiting time before relaxation begins
It shows that the magnetic energy of the NP during measurement is smaller than the μp Ha expected for systems with an instantaneous response to Ha, but reveals behavior of a slow spin response as the wait time for each magnetization measurement is short
Summary
Magnetic materials in the form of nano-sized particles or amorphous structures can generate many novel behaviors. Memory and aging effects that reflect the characteristics of slow spin dynamics [1,2] have been observed [3,4,5,6,7,8] in magnetic nanoparticles (NPs). The magnetic relaxation of the superspin will collectively freeze at low temperatures, with the freezing temperature strongly dependent on the particle or domain size and concentration [11,12,13,14]. The broad distribution of the relaxation time and the random distribution of the particle position in the magnetic NP assembly can be a good source for the development of slow spin dynamics [17,18]. The low coercivity, low Materials 2016, 9, 426; doi:10.3390/ma9060426 www.mdpi.com/journal/materials
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