Magnetic anisotropy and magnetization variation with temperature

Memory in nanomagnetic systems: Superparamagnetism versus spin-glass behavior

“…The value of R in the dense system is about eight times larger than that in the noninteracting case, implying that the magnitude of the FC magnetization memory effect does depend on the interparticle dipolar interaction. This is in quite good agreement with the experimental results of Telem-Shafir et al 16 In the noninteracting case, no memory effect is seen during a ZFC process ͑see Fig. 8͒ below ͑T b ͒, since the occupation probabilities of up and down particles are both equal to 0.5 ͑two-state model͒.…”

“…Time-dependent magnetization measurements suggest that dense nanoparticle samples may exhibit glassy dynamics due to dipolar interparticle interaction; 16,[26][27][28] disorder and frustration are induced by the randomness in the particle positions and anisotropy axis distributions. As discussed within a simple mean field theory picture, adapted to the two-state model of Chakraverty et al, 9 the random dipolar interaction can be accounted for in terms of a local, self-consistent field that has the form…”

“…The FCM of the dense system does not increase but stays almost constant below ͗T b ͘ which is the primary indicator for the glassy state. 10 In order to have a better understanding of glassy relaxation, time-dependent magnetization studies under various heating and cooling protocols were performed by Sasaki et al 10 16 In a double memory experiment ͑DME͒ under FC protocol the system is cooled under a field of 50 Oe. The field is cut off during the intermittent stops of the cooling at T = 30 K and at T = 40 K for 3000 sec at each stop.…”

“…A discernible experimental signature of superparamagnetism versus spin-glass behavior seems to be the magnitude of the field-cooled ͑FC͒ magnetization memory effect that is significantly larger for the interacting glassy systems than the one in noninteracting superparamagnetic particles. 16 Therefore, invoking spin-glass physics in interpreting data on the slow dynamics of nanomagnets can sometimes be like "killing a fly with a sledge hammer," especially if a simpler interpretation on the basis of superparamagnetism is available. We explore such situations in this paper.…”