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Uniform superparamagnetic iron oxide nanoparticles were obtained by coprecipitation under synthesis conditions that guarantee diffusion-controlled growth. Study of nanoparticle crystal structure formation by HRTEM showed that at the earlier stage of the reaction some nanoparticles consist of crystalline core and amorphous surface layer, whereas resulting particles display a high degree of crystalline order. This result suggests that nanoparticles are formed from fusion of noncrystalline primary particles of iron (hydr)­oxide. Slow addition of iron salts to excess ammonia restricts the amount of primary particles; as a result, their diffusion is the limiting step of the reaction, which provides the formation of uniform nanoparticles. Importantly, 5 min reaction product shows the same polydispersity and heating efficiency as the final product. Thus, monodispersity determines the particle properties and facilitates the control of heat generation for a given amplitude and frequency of AMF. Magnetic dipole interactions between single nanoparticles lead to the formation of dense aggregates (multicore particles) at the beginning of the reaction. The dispersions of separated multicore particles with hydrodynamic size of about 85 nm shows higher heating efficiency than dispersion of as-prepared nanoparticles. The increase of aggregate size leads to a decrease of heating efficiency to the value of as-prepared nanoparticles due to a demagnetizing effect.

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Aggregates of superparamagnetic nanoparticles, so called multicore particles get much attention due to collective magnetic behaviour. Despite the fact that saturation magnetization and coercivity of multicore particles are lower than for single particles of comparable size, they can generate large amount of heat in alternating magnetic field. This makes them promising for magnetic hyperthermia. However, correlation between internal magnetic structure of multicore particles and their heating ability in alternating magnetic fields are not clear yet. Detailed experimental investigations are required to determine the optimal sizes of multicore particles and the alternating magnetic field parameters to obtain maximal heat. In this study, we demonstrated how hydrodynamic size of multicore particles influences alternating magnetic field energy absorption. Dense aggregates composed of bare magnetic iron oxide nanoparticles of 13 nm were obtained by coprecipitation. Further peptization allowed to gain aqueous dispersions of multicore particles with various hydrodynamic size, varing from 85 to 170 nm, due to electrostatic stabilization. Multicore particles dispersions have saturation magnetization of 40 A m 2 /kgFe 3 O 4 and coercivity of 79.6 A/m regardless of their size. Dispersion of 85 nm multicore particles is stable and provides specific loss power of 42 W/gFe. Further increase of hydrodynamic size leads to low stability and loss of the ability to generate heat in alternating magnetic field.