Metal-semiconductor transition in NiFe2O4 nanoparticles due to reverse cationic distribution by impedance spectroscopy

Abstract

We have investigated the magnetic and electrical response of the sol-gel synthesized NiFe 2 O 4 nanoparticles. Changes in the impedance plane plots with temperature have been discussed and correlated to the microstructure of the material. Thermally activated hopping carriers between Fe 3þ-Fe 2þ and Ni 2þ-Ni 3þ ions have been determined for a decrease in the resistance of the sample and a change in the conduction mechanism around 318 K. The mixed spinel structure and broken exchange bonds due to small size effects are due to the canted spin structure at the surface of the nanoparticles. The magnetization is found to be influenced by the surface spin canting and anisotropy. We have established the semiconducting to metallic transition (SMT) temperature to be around 358 K in terms of localized and delocalized e g electrons along with a transition from less conductive [Fe 3þ-O 2À-Fe 3þ ] and [Ni 2þ-O 2À-Ni 2þ ] linkage to more conductive [Fe 3þ-Fe 2þ ] and [Ni 2þ-Ni 3þ ] linkage at the octahedral B site. A decrease in the dielectric constant with temperature has been discussed in terms of the depletion of space charge layers due to the repulsion of delocalized e g electrons from the grain boundary planes. The anomalies in tangent loss and conductivity data around 358 K are discussed in the context of the SMT. V

Electrical conductance and complex impedance analysis of La0.6Pr0.1Ba0.3Mn1−xNixO3 nanocrystalline manganites

“…In contrast, these values decrease with temperature, above T MS , which indicates the presence of thermally activated conduction mechanism in this system which is similar to a semiconducting behavior. Such behavior has been reported in other ceramics [37,38]. In the other hand, the obtained R g and R gb values are lower for x = 0 than for x = 0.1 which makes the La 0.6 Pr 0.1 Ba 0.3 Mn 0.9 Ni 0.1 O 3 sample lesser conductor as compared with La 0.6 Pr 0.1 Ba 0.3 MnO 3 sample.…”

Cation distribution and enhanced surface effects on the temperature-dependent magnetization of as-prepared NiFe2O4 nanoparticles

“…Moreover, the mixed cationic distribution of the as-prepared sample, as deduced from Mössbauer measurement, is expected to give a higher value of net magnetic moment, i.e., 4.6l B compared with that of inverse spinel NiFe 2 O 4 , i.e., 2l B . However, the as-prepared sample shows a reduced value of magnetization which is ascribed to the weakening of the A-B exchange interactions due to spin canting effect that dominates over the effect of site exchange of the cations in the surface shell [26]. Therefore, in the view of the above discussion, one can believe that the small value of magnetization is an outcome of deteriorating of the A-B exchange interaction as well as prominent surface spin disorder.…”

“…The lattice parameter is calculated using interplanar spacing (d) and Miller indices (hkl) values and is found to be 8.339 Å , which is less than its bulk counterpart. This reduction in lattice parameter may be attributed to the increased degree of inversion, more surface energy and surface tension which can lead to the distortion of the lattice parameter [23,26]. Figure 1b shows transmission electron microscopy (TEM) image of the as-prepared NiFe 2 O 4 nanoparticles.…”

Magneto-resistive coefficient enhancement observed around Verwey-like transition on spinel ferrites XFe2O4 (X = Mn, Zn)

“…Numerous cations can be placed in the A and B sites to tune the magnetic and electronic properties of the material; also, are impacted by crystallite and particle size, crystal morphology, and the phase purity. [2][3][4][5][6][7][8] In these systems, the conduction phenomena have been described by the Verwey's hopping mechanism, where the mobility of electrons and holes affects the electrical conductivity. Here, the hopping probability depends on the separation of the ions and on the activation energy, which at the same time is dependent on temperature.…”

“…Their unique electronic, magnetic, and physical properties may be used in technological applications. [1][2][3][4] The spinel ferrite structure is described by the formula (A) [B] 2 O 4 ; where oxygen atoms form a close packed lattice with transition metal ions located in the interstitial tetrahedral (A) sites and the octahedral (B) sites. The different ferrite systems can have normal or mixed spinel structure depending cation distribution.…”