Reversible post-synthesis tuning of the superparamagnetic blocking temperature of γ-Fe₂O₃nanoparticles by adsorption and desorption of Co(ii) ions

Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles

“…Namely, M S decreases and H C rises as the amount of hard phase is increased [106,107,249,260,293]. However, in certain cases, if the difference in M S between the counterparts is small (e.g., hard Co-ferrite and soft Mnferrite both with M S ~ 80 Am 2 /kg (80 emu/g) at room temperature), although M S remains rather constant with the relative amount of hard and soft phase, H C does evolve according to the hard/soft volume ratio [101,102,143,[251][252][253]262,292]. There exist some systematic studies fixing the core size and methodically increasing the shell size both for conventional hard/soft and inverse soft/hard systems [93,99,101,102,114,125,[127][128][129]132,190,243,248,249,253,263].…”

“…For instance, Salazar-Alvarez et al [143] studied in -Fe 2 O 3 nanoparticles the relative changes in the intensity of diffraction peaks corresponding to (surface) octahedral vacancies during their substitution with Co(II) ions. The surface increase was correlated with the changes in blocking temperature and corroborated with X-ray photoelectron spectroscopy.…”

“…Specifically, for the nanoparticles to be magnetically stable at room temperature they need to have a high K u , this implies that the field available for conventional write-heads may not be sufficient to write the information. One possible way to circumvent this problem may be to use materials with lower K u (i.e., soft) and tune their effective anisotropy by coupling them to high K u (i.e., hard) [143] or by using antiferromagnets [399]. An alternative approach would be to reduce the effective anisotropy of high K u (i.e., hard) nanoparticles by coupling them with low K u (i.e., soft) materials, for example in a core/shell structure, similarly to what has been proposed for continuous thin film media [386,389,390].…”

“…For example, the previous mentioned one-pot synthesis of FePt/Fe 3 O 4 core/shell nanoparticles where the use of an excess of iron precursor in the synthesis of FePt nanoparticles leads the formation of an iron oxide shell [135][136][137][138] [143,144].…”

“…When analyzing the magnetic properties observed experimentally for diverse hard-soft core/shell nanoparticles a spread of different behaviors can be found [91][92][93][94][95][98][99][100][102][103][104]106,107,110,111,114,115,[121][122][123][125][126][127][128][129][130][131][132]136,140,141,143,175,. First, it should be pointed out that given that we are dealing with nanoparticles the core and shell sizes are rather small (i.e., usually smaller than 2 H ), thus, most of the systems exhibit smooth hysteresis loops typical of strongly exchange coupled hard-soft counterparts.…”