Properties of Ni–Zn ferrite thin films deposited using spray pyrolysis

Abstract

Nanocrystalline, homogeneous spinel Ni_1 − xZn_xFe₂O₄ thin films were deposited on glass substrates by using spray pyrolysis of metal nitrate aqueous solutions. The thickness of deposited films was below 500 nm, but crystallite size was under 30 nm. It has been shown that the DC resistivity, dielectric loss and optical band gap of deposited films are influenced by the zinc content. High DC resistivity and low dielectric losses of thin Ni_1 − xZn_xFe₂O₄ films are explained by mixed n-p conductivity and nanograin structure of spray pyrolysis deposited coatings which are changing with the ratio of Ni/Zn.

Introduction

Spinel ferrites with general formula AB₂O₄ are a large class of oxides with remarkable application possibilities from simple permanent magnets [1] to microwave applications [2], magnetic recording [3], gas sensors [4], catalysts [5] and photocatalysts [6]. Generally, spinel ferrites have been applied for the last 70 years [7]. From all ferrites the Ni–Zn spinel ferrites are important as microwave materials, due to its high resistivity and low coercivity [1], [7]. At the same time, Ni–Zn ferrites are structure sensitive and it is not easy to produce point-defect free, stoichiometric Ni–Zn ferrites for high resistivity applications.

Since the miniaturization tendency of the electronic components started in the 1990s, the spinel ferrites have been prepared in the form of nanostructured thick and thin films. Properties of ferrite films are depending on preparation route, due to its strong influence on type of the film (epitaxial or polycrystalline), microstructure, particle size, chemical homogeneity and cationic distribution between tetrahedral and octahedral sub-lattice sites [8].

For deposition of spinel ferrite polycrystalline thin films different physical and chemical vapor deposition techniques are used [9], [10], [11], [12]. The given methods are complicated, and expensive; and require special equipment and sometimes high processing temperatures above 500 °C [13]. As a low temperature method involving preparation of the ferrite film by using solution processing sol–gel is combined with spin- or dip-coating [14]. However, sol–gel method usually requires many processing steps and expensive reagents, while commercial technology requires high quality and low cost ferrite films [7].

An alternative method to obtain spinel ferrite thin films is spray pyrolysis [15], [16], [17]. Generally, spray pyrolysis has been applied to deposit a wide variety of thin and thick metal oxide films since the 1970s [18]. Despite that, Langlet et al. by using spray pyrolysis obtained very homogenous ferrite thin films with good reproducibility [19], nevertheless for spinel ferrite thin film growth this method is used rarely. Also, only a few works can be found on Ni–Zn ferrites obtained by spray pyrolysis, where focus on the investigation of microstructural, optical and magnetic properties was held [15], [16]. At the same time, regarding practical applications, it is also important to characterize electric and dielectric properties of the Ni–Zn ferrites obtained by the spray pyrolysis method.

The aim of the present work is to deposit nanostructured Ni_1 − xZn_xFe₂O₄ thin films on glass substrates by using spray pyrolysis and to investigate their structural, microstructural, electric, dielectric and optical properties.