Nanocrystalline Ferrites Prepared by Mechanical Activation and Mechanosynthesis

Abstract

At present, the high‐energy milling method becomes widely used for the preparation of nanocrystalline materials due to its relative simplicity and availability. In this research report, selected examples are presented of the preparation of nanoscale spinel ferrites by mechanical activation and mechanosynthesis. Nanosized particles of mechanochemically prepared spinel ferrites are structurally and magnetically disordered due to the non‐equilibrium cation distribution and non‐collinear spin arrangement. As a consequence, they exhibit interesting physical and chemical properties markedly different from those of their bulk counterparts. Several interesting features are involved in the work, e.g., superparamagnetism, an enhanced chemical reactivity, an irreversible magnetic behaviour, an enhanced magnetic ordering temperature, and an altered (reduced or enhanced) saturation magnetization in comparison with the corresponding bulk materials. Taking into account the considerable relevance of the thermal stability of nanoscale complex oxides to nanoscience and nanotechnology, results on the response of mechanochemically prepared spinel ferrites to changes in temperature will also be presented.

Direct determination of the cation disorder in nanoscale spinels by NMR, XPS, and Mössbauer spectroscopy

“…This observation is consistent with the results of the EXAFS investigations of nanoscale milled Zn-containing spinels [6,7] It should be noted that the mean free path of photoelectrons in zinc oxides for the Zn 2p 3/2 signal is about 2 nm, leading to the value of the information depth of the XPS measurements of about 6 nm [5]. Thus, taking into account similar values of the information depth and the particle size of the milled ZnFe 2 O 4 (≈8 nm) [1], it could be stated that the results of the XPS measurements reflects not only the surface structural disorder but also the interior structure of ZnFe 2 O 4 nanoparticles.…”

“…'Recoil' software [2] was used for the quantitative evaluation of the Mössbauer spectra. The average canting angle, Ψ , was calculated from the ratio of the intensities of lines 2 and 1, I 2,1 , according to Ψ = 90 • − arcsin[(3I 2,1 /2)/(1 + 3I 2,1 /4)] 1 …”

“…Although numerous papers have been published on the mechanically induced structural disorder in spinels in the past years (see, for example, [1] and references cited therein), this is the first report which quantitatively describes the change in the cation distribution when the particle size is reduced to nanometer level for spinels with different cation arrangement in the bulk state, i.e., for partly inverse spinel MgAl 2 O 4 (0 < λ < 1), normal spinel ZnFe 2 O 4 (λ = 0), and fully inverse spinel NiFe 2 O 4 (λ = 1). The important impact of the present work from the methodology point of view is the first application of solid-state nuclear magnetic resonance spectroscopy (NMR) to the study of the mechanically induced structural disorder on a local atomic scale in spinels.…”

Mechanochemical reactions and syntheses of oxides

“…The chemical reactions in oxides induced by mechanical action can be categorized along the same lines that are commonly used in solid state chemistry and in chemistry, in general; they are either of homogeneous or of heterogeneous type. 5,13,29 Homogeneous processes shall be exemplified here by the mechanically triggered formation of nonequilibrium structural (atomic) and magnetic (spin) configurations in complex oxides and by mechanically induced phase transformations in binary oxides. Heterogeneous processes shall be illustrated with the specific case of mechanically induced formation reactions (the so-called mechanochemical syntheses or mechanosyntheses) of complex oxides.…”

Simultaneous formation of ferrite nanocrystals and deposition of thin films via a microwave-assisted nonaqueous sol–gel process

“…cation) or by changing the crystal morphology, i.e., size and shape. Magnetic spinels of the Fe 3 O 4 and MFe 2 O 4 type have been successfully produced by various methods ranging from mechanical to gas phase and solution routes [9][10][11][12]. The latter include co-precipitation, thermal decomposition, hydro-and solvothermal, and reverse micelle approaches [13][14][15][16][17][18][19][20][21][22][23][24][25].…”