Magnetic behaviour of nanosized zinc ferrite under heavy ion irradiation

Abstract

Present investigation reports the effect of 100 MeV oxygen beam on the magnetic properties of zinc ferrite nanoparticles. The nanoparticles for this study were synthesized by using the nitrates of zinc and iron in the matrix of citric acid and by sintering the precursor at 500 and 1000 °C. Both these samples were irradiated by 100 MeV oxygen beam with two different fluence 1 × 10¹³ and 5 × 10¹³ ions/cm². Besides the presence of cubic spinel phase, ZnO phase appears after the irradiation in both the samples. A decrease in average particle size was observed in the irradiated samples as estimated by X-ray diffraction pattern. The magnetization versus applied field curves show the decrease in magnetization with the fluence of the beam, which is attributed to the ZnO phase. The thermal magnetization curve for the sample ZF500 shows almost constant value of blocking temperature after irradiation whereas for ZF1000 it increases from 18 K to 32 K at a fluence of 5 × 10¹³ ions/cm².

Introduction

With the advent of nanoscience, spinel ferrites have attracted much attention as they exhibit improved physical properties with respect to their bulk counterparts [1]. These materials are preferred for the high frequency applications due to their high resistivity [2]. The magnetic properties of nanoferrites are very much affected by the method of synthesis apart from the particle size [3], [4], [5], [6]. Zinc ferrite, having normal spinel structure in bulk form, exhibits the magnetic ordering below the Néel temperature ∼10 K. The magnetic properties of this system show strong dependence on the state of chemical disorder and the cation site occupancy in the materials [7]. The structural formula is generally written as (Zn_1−xFe_x)_A[Zn_xFe_2−x]_BO₄, where round and square brackets denote A and B-sites, respectively; x shows the degree of inversion. Thermal and mechanical treatments mostly result in the appearance of the disorder of Zn and Fe ions over the A and B-sites, leading to significant variation in the magnetic properties [8]. Recently, it has been reported that the magnetic properties of inverted zinc ferrite arises due to co-existence of ferrimagnetic and antiferromagnetic ordering [9]. Generally, the dominant mechanism for the magnetic properties of this system is superexchange magnetic interaction between metal ions through oxygen ions. This interaction is also limited to nearest neighbours and sensitive to the site occupancy of metal ions. From the application point of view, zinc ferrites are widely used as catalysts [10], as a re-generable absorbent material for desulphurisation of hot coal gases [11], etc. In the nanoregime, the modification of surface properties will have a profound bearing in determining these properties. Moreover, since it exhibits magnetization at room temperature in nanoregime it may be potential material for other applications too.

Tailoring of magnetic, electrical and optical properties of these nano ferrites by the swift heavy ions (SHI) is a work of great importance. It not only increases the applicability of these ferrites but also elucidates the interaction of swift heavy ions with magnetic nanomaterials. During the last decades a lot of work has been done on the interaction of SHI with ferrites. In their study authors generally use mixed ferrite spinel and concluded that magnetic properties of these systems when irradiated by swift heavy ions may be modified due to breaking of ferrimagnetic ordering, surface state pinning, cation inversion and creation of defects [12], [13], [14], [15], [16], [17].

Regarding the interaction of SHI with materials various models are proposed by the researchers. Thermal spike model [18] is one and most popular among the several damage models to explain the observed change in materials especially, in insulators. According to this, the materials behave like a thermodynamical system by the introduction of swift heavy ion. The energy of the swift heavy ion is converted into heat raising the temperature of the system of the order of 10⁴ K. This local temperature of the system decreases with cooling rate 10¹¹–10¹² K/s giving rise to production of several types of defects, depending upon the amount of energy lost in the medium. This causes change in the microstructure, leading to change in physical and chemical properties of the target.

With the best of our knowledge there is only one report available regarding the interaction of SHI with zinc ferrite. It was observed that the ion beam induces magnetization in zinc ferrite because of a site exchange of Zn and Fe leading to a new magnetic phase [19]. In the present investigations we have studied the magnetic properties of zinc ferrite irradiated by 100 MeV oxygen beam. We have recorded the magnetization as a function of applied field at room temperatures (RT) and at low temperature (10 K) for the pristine and irradiated specimen in order to observe the change in the magnetic properties of the system under investigation. Further, thermal magnetic measurements in zero-field cooled (ZFC) and field cooled (FC) mode were carried out to understand the underlying physics responsible for the change in magnetic properties.