Strongly exchange coupled inverse ferrimagnetic soft/hard, MnxFe3−xO4/FexMn3−xO4, core/shell heterostructured nanoparticles

Abstract

Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of Mn x Fe 3Àx O 4 /Fe x Mn 3Àx O 4 with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness ($0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft/hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft/hard core/shell nanoparticles can be fine tuned to match specific applications.

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

“…XMCD element selective hysteresis loops carried out at the Mn and Fe . The element selective hysteresis loops show that the coercivities of the core and the shell are almost the same; however the approach to saturation of the hard shell is slower than the one of the soft core [105,107]. Interestingly, for Fe 3 O 4 /Mn 3 O 4 nanoparticles it can be obviously observed that the XMCD signal for the Mn-and Feedge are inverted (see Fig.…”

“…11d, the switching field distribution of MnFe 2 O 4 /FeMn 2 O 4 nanoparticles is noticeably asymmetric, evidencing a tail towards high fields. This has been attributed to the presence of exchange coupled soft and hard phases in the nanoparticles [105].…”

“…Another interesting basic property of hard-soft core/shell nanoparticles is that they have also been reported to exhibit exchange bias [94,105,107,122,247,259,263,279,307]. Namely, the hysteresis loops exhibit a loop shift in the field axis, H E .…”

“…This method is based in a two-step synthesis process, where pre-made nanoparticles are used as seeds for the posterior deposition of the shell. From a thermodynamic point of view, this approach is controlled by the heterogeneous nucleation of the shell precursor avoiding, thus, the homogeneous nucleation of new nanoparticles [105,107,133,134]. In comparison with the surface treatment approach this method allows an independent control of the core (pre-made nanoparticles) and the shell during the synthesis, resulting in a controllable size ratio between the core diameter and shell thickness and allowing the growth of shells with excellent crystallinity.…”

“…However, given the great versatility of many chemical routes in controlling materials, crystallinity, homogeneity, sizes or even shapes, less attention has been paid to the synthesis of core/shell nanoparticles using physical approaches [17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Some examples of standard chemical routes used for the synthesis of hard-soft core/shell nanoparticles are coprecipitation [89][90][91], thermal decomposition [92][93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108][109], metal reduction [110,111], microwave-assisted methods [112,113] and electrodeposition [114,115].…”