Memory effect versus exchange bias for maghemite nanoparticles

Abstract

a b s t r a c tWe studied the temperature dependence of memory and exchange bias effects and their dependence on each other in maghemite (γ-Fe 2 O 3 ) nanoparticles by using magnetization studies. Memory effect in zero field cooled process in nanoparticles is a fingerprint of spin-glass behavior which can be due to i) surface disordered spins (surface spin-glass) and/or ii) randomly frozen and interacting nanoparticles core spins (super spin-glass). Temperature region (25-70 K) for measurements has been chosen just below the average blocking temperature (T B ¼75 K) of the nanoparticles. Memory effect (ME) shows a nonmonotonous behavior with temperature. It shows a decreasing trend with decreasing temperature and nearly vanishes below 30 K. However it also decreased again near the blocking temperature of the nanoparticles e.g., 70 K. Exchange bias (EB) in these nanoparticles arises due to core/shell interface interactions. The EB increases sharply below 30 K due to increase in core/shell interactions, while ME starts vanishing below 30 K. We conclude that the core/shell interface interactions or EB have not enhanced the ME but may reduce it in these nanoparticles.

Room Temperature Magnetic Memory Effect in Nanodiamond/γ-Fe2O3 Composites

“…However, from careful repeated experiments, we confirmed that ZFC MME is absent in the MND sample. The observed results differ significantly from the literature in which both FC and ZFC MME were reported from γ-Fe 2 O 3 and Fe/γ-Fe 2 O 3 nanostructure, confirming SG state [ 4 , 5 , 23 ]. However, the obtained MME from MND partially agrees with weakly interacting γ-Fe 2 O 3 -alginate nanocomposite from which only FC MME (but with decreasing magnetization) without ZFC MME was reported in the low-temperature region far below RT [ 6 ].…”

“…The FC M(H a ) loop exhibits a loop shift in either direction depending on the direction of the cooling field with a similar EB field H EB of 11 Oe. The obtained value of the H EB is much smaller than the reported value of around 77 Oe under a cooling field of 50 kOe from 6.1 nm SG like γ-Fe 2 O 3 nanoparticles [ 23 ]. The origin of a relatively small EB field from the MND sample possibly lies at the interfacial spin-exchange interaction between vacancy-induced short-range magnetic clusters and compensated ferrimagnetic γ-Fe 2 O 3 .…”

“…Besides this, a monotonic increase in the magnetization with the decrease of temperature can be seen from the FC curve. Contrary to the above in a few reports, a plateau below a critical temperature (in the low-temperature region) has been reported from pure γ-Fe 2 O 3 and Fe/γ-Fe 2 O 3 nanostructures ascribed to collective freezing of the system leading to the appearance of an SG-like phase [ 4 , 5 , 23 , 24 ]. Whereas, a similar temperature dependency of the FC magnetization curve was reported from dextran or SiO 2 coated γ-Fe 2 O 3 nanoparticles and γ-Fe 2 O 3 -alginate nanocomposite retaining in SPM state, possibly due to reduced or absence of interparticle interactions [ 6 , 25 , 26 ].…”

“…The finite-size effect in γ-Fe 2 O 3 nanoparticles can lead to exchange bias (EB) coupling between the ferromagnetically aligned core spins and disordered frozen surface spins, resulting in surface SG behavior [ 23 ]. To investigate the EB phenomenon, FC M(H a ) loop measurements were carried out at 10 K with a cooling field of ±20 kOe ( Figure 2 c).…”

Existence of two spin dynamics in the temperature and magnetic field dependence of the magnetization curves of ferrimagnetic Co1.75Fe1.25O4 and its composite with BaTiO3

“…In addition to intrinsic spin disorder and frustration in magnetic materials, the interfacial magnetic exchange interactions in hetero-structured systems have shown many novel magnetic phenomena, e.g., blocking of magnetic domains, domain wall pinning, exchange bias, training effect, and memory effect [6][7][8][9][10][11][12][13]. The exchange bias effect was primarily modeled for FM and AFM layers of thin films [3][4].…”