Properties of polycrystalline nanoparticles with uniaxial and cubic types of magnetic anisotropy of individual grains

Abstract

The influence of the crystal structure inhomogeneities on the magnetic properties of cobalt nanoparticles with different aspect ratio and spherical nanoparticles of chromium dioxide, cobalt ferrite and magnetite has been studied by means of numerical simulation. The polycrystalline nanoparticles are modeled by means of subdivision of the nanoparticle volume into tightly bound single-crystal granules with randomly distributed directions of the easy anisotropy axes. The probability of appearance of quasi uniform and vortex states in sufficiently large assemblies of polycrystalline nanoparticles of various types have been calculated depending on the nanoparticle diameter. It is shown that the subdivision of a nanoparticle into single-crystal granules with different orientations of the easy anisotropy axes substantially reduces the effective single-domain diameters for particles with uniaxial type of anisotropy of individual granules. However, for particles with cubic type of magnetic anisotropy the influence of the crystal structure inhomogeneities on the equilibrium properties of the particles is not so important even for magnetically hard cobalt ferrite nanoparticles. It is practically absent for magnetically soft magnetite nanoparticles.PACS 75.50.Tt; 75.40.Mg

Low-Dimensional Assemblies of Magnetic MnFe2O4 Nanoparticles and Direct In Vitro Measurements of Enhanced Heating Driven by Dipolar Interactions: Implications for Magnetic Hyperthermia

“…Including the Brukhatov-Kirensky relation 35 to account for the thermal dependence of Keff was also unsuccessful (see the Supporting Information and Figure S6 for details on the fitting procedure). These deviations suggest that the MNPs are not single-domain particles but possess a more complex magnetic structure, since above and close to the single-to multi-domain transition, a non-linear spin configuration should provide a minimum total energy while keeping the saturation magnetization close to the bulk value 36,37 ; it has consequently been proposed to use MNPs with sizes close to the single-to multidomain transition to improve local heating in biological media. 38 , 39 Based on the above considerations, we performed ferromagnetic resonance (FMR) experiments to measure the particles' magnetic field anisotropy and estimate an effective magnetic anisotropy constant Keff of the oriented nanoparticles.…”

A shape visualization of a magnetic anisotropy energy density of single-domain magnetic nanoparticles

“…A total energy of a magnetic anisotropy of a particle in a general case is the sum of an energy of magnetocrystallographic anisotropy and a shape anisotropy energy. In this case, it takes place [15] an effective combined magnetic anisotropy of a nanoparticle,…”

Application of Magnetosomes in Magnetic Hyperthermia

“…Unfortunately, the popular chemical methods for the synthesis of magnetic nanoparticles of iron oxides [ 1 , 20 , 21 , 22 ] in most cases give assemblies with a wide distribution of nanoparticles in size and shape. Moreover, the obtained nanoparticles turn out to be polycrystalline, which leads to a low saturation magnetization of particles in comparison with the corresponding single-crystal three-dimensional samples [ 23 ]. For such assemblies it is hardly possible to obtain sufficiently high SARs under the above restrictions on the frequency and amplitude of AC magnetic field [ 24 , 25 ].…”