Nanoparticles have found a wide application due to their superior electrical and electronic properties. Ferrite nanoparticles being a class of special oxides have also been applied in a wide range of applications ranging from millimeter wave integrated circuitry, to the power handling techniques, permanent magnets up to the magnetic recording media (Valenzuela, 2005, 2012). Due to the evolving data storage techniques in recent times, there has been reduction in bit’s dimension towards the nanoscale     (Larson     et     al.,     2004;     Sbiaa      and Piramanayagam, 2007; Gu et al., 2016). Utilizing the optical and electrical properties of materials, optical recording has been developed (Gu et al., 2016). For instance, plasmonic has data storage on metal nano-structures which has a sharp resonance at a specified wavelength which depends fully on the size and shape of the nanostructure (Mansuripur, 2011).

Mansuripur (2011) notes that, the electronic data storage has been depending on static and dynamic random access  memories  (SRAM  and  DRAM),  but  of recent the Flash Memory has emerged as a contender in storage technology. Despite of the introduction of the flash disk there has been an increase in challenges such as low capacity, have slow data rates and they retain data for a short period of time which is due to charge leakage. These have therefore created a field for nano-volume materials to solve these challenges. Among the likely candidates of materials that have been used as core in memory devices and continued to elicit much research interests in this area of applications are the ferrites (Priya et al., 2019; Flores-Martinez et al., 2019). Lot of research is devoted to recognize the possibilities to improve the electrical properties in order to make it suitable for applications in electronic devices and microwave industries. The advocacy of nanomaterials has been supported by the idea that as the size of the magnetic materials approaches nanoscale, there is an increase in surface area and this therefore has led to novel phenomena (Yao et al., 2007). Ferrites nanomaterials have also exhibited a high signal-to-noise ratio and thermal stability (Ozawa et al., 2017; Tada et al., 2017), properties that should be ideal to data storage devices.

Al doped cobalt ferrites has been reported as suitable for high frequency applications and magnetic memory devices (Priya et al., 2019) among other applications that requires higher electrical conductivity like in electrodes for supercapacitors (Bhujun et al., 2016). Aluminum doping has shown improved electrochemical performance (Priya et al., 2019). Kumar and Kar (2011) reported Al doped cobalt ferrites as a potential candidate for soft magnet applications such as magnetic recording. Their study observed that aluminum in cobalt ferrites reduces saturation magnetization, coercivity, remanent magnetization and magneto crystalline anisotropy constant, which reveal that the material becomes soft magnetic material with the Al³⁺ substitution. Waghmare et al. (2017), reported that incorporation of non-magnetic Al³⁺ ions in cobalt ferrite resulted in decrease of saturation magnetization and coercivity because replacement of Fe³⁺ by Al³⁺ ions weakens the sub lattice interaction and lowers the magnetic moment of the unit cell. Furthermore, more studies have shown improvements of properties that are important in data memory by doping Zn ions to cobalt ferrites (Jadhav et al., 2010; Somaiah et al., 2012: Jamil et al., 2017; Nayeem et al., 2017).  Most of these studies have focused on optimization of the magnetic properties (Pankhurst et al., 2003; Yao et al., 2007).

This study sought to investigate the effect of a double substituted ferrite, the aluminum and zinc doped cobalt ferrite. The intent has also been refocused to investigate the effect of such substitution to electronic properties specifically the work function, energy band gap and optical properties. The study used sol-gel synthesis method, which enables use of low temperatures and less time consuming besides being one  of  the  methods  that has been widely used in production of materials at nanoscale level (Kahn and Zhang, 2001; Krishna et al., 2012), and the ease of varying the stoichiometry of the samples.

Synthesis

Sol-gel method was used to synthesize Zn_1-xCoFe_2-xAl_xO₄ (x =0.0, 0.2, 0.4, 0.6, 0.8 & 1.0). All the chemicals were of analytical reagent grade (AR) with 99% purity and supplied by Sigma Aldrich Laboratory Supplies. The varied stoichiometry of the Ferric Nitrate nanohydrate (Fe(NO₃)₃.9H₂O), Cobalt nitrate hexahydrate (Co(NO₃)₂∙6H₂O), Aluminum nitrate nonahydrate-Al(NO₃)₃∙9H₂O and Zinc nitrate hexahydrate-Zn(NO₃)₂∙6H₂O were calculated into their masses as shown in Table 1.

The weighed amount of metal nitrates were dissolved in 10 ml of pure water and stirred viorously using a magnetic stirrer to form a salt solution. 2 M Citric acid solution was then added to the salt solution in the ratio 1:1. after heating for 10 min. A red-brownish gel was formed after vigorous stirring for 3 h at 90°C. Further heating continued at 95°C until the gel underwent self autocombustion, leading to formation of a loose massive powder. The product was grinded to fine powder using an agate motor and pestle and  calcined at 650°C for 4 h to remove any organic impurity. 

Characterization

The XRD analysis of the synthesized nanoparticles was done by a powder XRD (Bruker D2 Phaser Diffractometer) using a CuKα radiation at λ=1.54060 Å operating at a voltage of 30 kV and a current of 10 mA and at a 2𝜃 range for 10-90° with a sweeping rate of 2 °/min.  Scanning Kelvin Probe Microscopy (SKPM) was used to study the work function of the samples. The tip voltage was set between 2.9515 - 3.000 V and the peak-to-peak voltage V_ptp was 1325.7 mV which was kept constant with mean peak-to-peak voltage V_ptp of 1422.6 mV. Scanning took place for 200 points in steps of 1 where the backing potential V_b+ was in the range ±5000. UV-Visible 1800 from Shimadzu was used to obtain the absorbance of the samples. Part of the sintered powder of the samples of Al_xZn_1-xCo Fe₂O₄ ferrites were dissolved in pyridine and ethylene glycol in the ratio 1:10 under magnetic stirrer and stirred for 10 minutes for further UV-vis characterization.

XRD analysis

The phase and purity of the synthesized Zn_1-xCoFe_2-xAl_xO₄ samples were examined by the XRD. The observed XRD patterns are shown in Figure 1

Then normalized diffraction peaks at 2θ values 30.405, 35.69, 43.473, 53.620, 57.4281 and 62.766 are indexed to (220), (311), (400), (422), (511) and (440) reflection planes of aluminum doped zinc cobalt ferrite respectively. The patterns match well with the JCPDS card No. 22-1086 and confirms a spinel of cubic structure which further shows pure samples were synthesized as no additional peak was recorded. The sharp  peaks  indicate a high degree of crystallization. The doping cations therefore, were well incorporated in the precursors. For the spinel cubic structure, the lattice parameter, a, was determined from the intense peak (311) reflection on the XRD pattern by the use of the equation, _. The crystalline size D was determined from the formula, .

Evidently the particle size and the lattice parameter decreases with increase of Aluminum concentration. This may be due to the substitution of Al³⁺ ions that are smaller than the Fe⁺³ ions (Table 2). It could also be as a result of cation redistribution and migration of some Al³⁺ ions from the tetrahedral to octahedral sites, orchestrating some Zn²⁺ and Co²⁺ to migrate from octahedral to tetrahedral sites. This would further cause a smaller bond between the cations, and hence reduced lattice constant and smaller cubic spinel structure. Similar behavior has been also reported for CoAl_0.2Fe_1.8O₄ (Chae et al., 2004; Ansari et al., 2016).

SKPM Line and area scans

The SKPM images of the line and area scans in Figure 2 and 3 respectively shows that a low work function, in the range of 300-680 meV, which changes with concentration of the dopant Aluminum. This could be attributed to the change of electron distribution on the surface of the samples. There is an increase in the number of electrons as the concentration of the dopant increases. Much of the values are having averagely a lower work function which is an important factor in application in electronics.

The electronic and optical properties of Aluminum doped zinc cobalt ferrite nanoparticles were studied using the Scanning Kelvin Probe Microscopy and the UV-vis spectroscopy. The work function from the area and line scans from the kelvins probe was found to be reduced from 680 to 300 meV with sample of x=0.6 recording the lowest work function. Likewise, the absorption from the UV-vis was also found to be in the range of 304-340 nm in wavelength  range.  The  energy  absorbed  when   the ultra violet radiations are directed to the sample was found to range from 1.85-4.08 which was attributed to re alignment of the aluminum, ferric and cobalt ions as well as zinc and oxygen ions in the sample to A and B sites. The addition of aluminum did not cause a significant fracture in the alignment of the ferrite particles; the increase in the size of the hole as the aluminum content increases can be attributed to the larger ionic radius of the aluminum ions. The hole density was maintained making the ferrite a better candidate for application in the storage, recording, transmission and memory devices. It is therefore clear aluminum doping in ferric nanoparticles bring about change in the energy absorbed, band gap and the work function as well as making the ferrite applicable in a wide range of electronic devices.

The authors have not declared any conflict of interests.