Magnetization Reversal in CsNi^IICr^III(CN)₆ Coordination Nanoparticles: Unravelling Surface Anisotropy and Dipolar Interaction Effects

Abstract

5402www.MaterialsViews.com wileyonlinelibrary.com their post-modifi cation. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] We have reported an alternative surfactant-free synthesis that led, by adjusting the physico-chemical para meters, to the formation of stable colloidal aqueous solutions of nanocrystals with a narrow size distribution. [ 25,26 ] This stability originates from the negatively charged surface and has been exploited to set a versatile simple synthetic route for core-multishell heterostructured nanoparticles. [ 17 ] The method is based on a seed-mediated growth in solution of a shell on the as-obtained colloids either using the same PBA coordination network or another one. [ 17,[25][26][27][28][29] It was quite remarkable to observe that the size distribution of the resulting objects was of the order of 10%. Then, knowing that the cell parameter of the PBA networks is about 1 nm, which corresponds to two atomic layers ( Figure 1 ), such a size distribution corresponds to a control during the growth in solution over one monolayer. [ 17 ] The control of the monodispersity and the absence of stabilizing agents in these anionic objects allowed their organization in thin fi lms, [ 30,31 ] their use for surface nanopatterning, [ 32 ] and is essential for the analysis of their magnetic behaviour (see below).In a fi rst communication, [ 27 ] we have reported a series of cubic-like coordination nanoparticles of the ferromagnetic network CsNi II Cr III (CN) 6 ( T C (bulk) = 90 K) with sizes ranging from 6 to 30 nm and a narrow distribution (10-20%), using the seedmediated growth process. The study of the magnetic properties of these particles dispersed in a polymer (polyvinylpyrrolidone, PVP, for instance) allowed us determining experimentally the single domain limit size that was found to be around 15-16 nm. Below this size, as expected, a uniform reversal of the magnetization was observed. These results were in line with a Néel-Brown thermal activated magnetization reversal process with a relaxation governed by an anisotropy energy proportional to the volume of the particles, with no detectable surface anisotropy effect. [33][34][35] However, upon decreasing their size surface effects may become dominant and contribute greatly to the magnetic anisotropy of nanoparticles. [ 36,37 ] As evidenced in thin fi lms and nanoparticles, surface anisotropy constant is found higher by many orders of magnitude than that of the bulk. [ 1,38,39 ] Specifi cally in PBA nanoparticles, the role of surface anisotropy cannot be neglected because the coordination sphere of the divalent metal ions (Ni II ions here) present on the surface can be different from those belonging to the

The remanence ratio in CoFe2O4 nanoparticles with approximate single-domain sizes

“…The highest K for CoFe 2 O 4 NPs at 10 K reaches ~10 7  erg/cm 3 , much larger than 1.8–3.0 × 10 6  erg/cm 3 for the bulk CoFe 2 O 4 ; this is consistent with the previous reports that in thin films and nanoparticles, surface anisotropy constant is found higher by many orders of magnitude than that of the bulk [15, 16]. In narue the enhanced K value results from the interaction between the surface spins of different particles [7], and between the spins from the surface and core in a single NP [6, 13].…”

“…Below a certain temperature, the canted surface spins freeze into a spin-glass-like state and the hysteresis loops obtained after a field cooling (FC) shift, as a consequence of the unidirectional anisotropy resulting from the coupling between the disordered surface layer and core spins [6, 13, 14]. Furthermore, the interaction between the surface spins of different particles enhances the effective anisotropy [7, 15], making the surface anisotropy constant many orders higher than that of the bulk material [16]. …”

Simultaneous effects of surface spins: rarely large coercivity, high remanence magnetization and jumps in the hysteresis loops observed in CoFe2O4nanoparticles

“…21,22 (4) The interaction between the surface spins of different particles affects the surface anisotropy, increasing the effective anisotropy. 24 Apart from surface spins, the dipolar interaction between nanoparticles is another non-negligible energy. 24 Apart from surface spins, the dipolar interaction between nanoparticles is another non-negligible energy.…”

Coupling Nanostructured CsNiCr Prussian Blue Analogue to Resonant Microwave Fields

“…From previous studies, we expect that the magnetocrystalline anisotropy of CsNiCr NPs is related only to Ni(II) ions and it is estimated to be rather weak (0.5 kJ m −3 ) . The surface anisotropy contribution is expected to represent the dominating term for small NPs, especially for 6 nm NPs where 40% of the atoms are located at the surface .…”

“…The demagnetizing field is expected to be more relevant for the smallest NPs due to their slightly elongated shape, while larger NPs have essentially cubic shape . This term, however, is also affected by inter‐particle dipolar interactions ( B inter ) and thus strongly depends on the degree of dilution of the NPs in the non‐magnetic matrix …”