Si9+ ion-irradiation induced modification of structural and magnetic properties of Zn-nanoferrite

Abstract

Swift heavy ion (SHI) irradiation is an important tool to understand ion matter interaction, and to study variation in structural, magnetic, properties. Ion irradiation affects the matter, present in their path by elastic, inelastic collisions and respectively exhibit nuclear energy losses (S_n) and electronic energy losses (S_e). Simulation authenticates that in MeV range, S_e dominates over S_n in SHI. Effect of irradiation on matter depends on ion species, ion energy and flounce. Spinel ferrites has cubic structure with fcc lattice comprising of two inter-penetrating sub-lattices: sub-lattice A (tetrahedral), sub-lattice B (octahedral) with ~75% empty interstitial sites, therefore energy imparted by ionic irradiation causes cation movement among A and B sites which affects structure and magnetic properties [1, 2] . Spinel structure has shown resistance for SHI as reported in [2].  As reported earlier [2], SHI irradiation is also known to alter cationic distribution in ferrites, would reflect change in magnetic properties. Focus of literature available on ZnFe₂O₄ [2], and references therein, is on exploring structural, magnetic properties. However, literature infrequently reports correlation between magnetic properties, cation distribution, role of ion irradiation to tailor magnetic properties. Therefore, in this work we use x-ray diffraction (XRD); scanning electron microscopy, energy dispersive x-ray spectroscopy (SEMEDS), ); magnetic studies to probe the effect of 120 MeV ²⁸Si⁹⁺ ion irradiation of ZnFe₂O₄ by monitoring structural changes, cation distribution, magnetic properties and correlation among them. ZnFe₂O₄ (ZNF) samples were prepared utilizing Nitrate-Citrate pre-cursors, as described in [2]. Formed dry gel samples were annealed at 450 ^oC/3h, and were used for characterization. SRIM (The Stopping and Range of ions in matter calculation code) is used to calculate penetration depth of ion (R_P) in ZNF. The R_P dependence of S_e ,S_n (electronic, nuclear energy loss) for ZnF can be seen in figure 1(a). For 120 MeV ²⁸Si⁹⁺ ion irradiation on ZnF theoretical S^e is 446 eV/Å and S_n is 0.3374 eV/Å with R_P 22.76 µm. To avoid ion implantation sample thickness was selected such that it is less than R_P. ²⁸Si⁹⁺ ion irradiation (using 15 UD pelletron accelerator at Inter University Accelerator Center, New Delhi) was done with ion flounce value of 1×10¹², 1×10¹³, 1×10¹⁴ ions/cm². XRD, SEM-EDS, magnetic data was analysed to get structural parameters, cationic distribution, particle size distribution, and magnetic parameters. XRD (fig. 1b) confirms the formation of spinel nano ferrite (grain diameter: 14.47 nm - 14.84 nm), for pristine, irradiated samples). Fig. 1b (inset) shows XRD of pristine sample, dependence of a_exp. on irradiation dose, shows linear decrease, described by equation: a_exp. = 0.84 – 5.6 x10^-4 [Irradiation Dose], showing strong correlation between irradiation dose and a_exp.. Reduction of a_exp., shows irradiation induced shrinkage of spinel unit cell, and is consistent with obtained dislocation density values (range between 0.79 X 1015 - 1.16 X10¹⁵ lines/m²). Irradiation shows: i) increase of Oxygen parameter, which is a

measure of disorder in the sample (range between 0.3830 – 0.3837), ii) increase of degree of inversion (range between 0.25 – 0.38), ascribed to changes in cationic distribution, iii) reduction of Fe³⁺ ions on B-site, with concurrent increase on A-site, while Zn²⁺ ions remain more populated on A-site than on B-site, iv) modification of A–O–B, B–O–B, A–O–A super-exchange interaction, which will have effect on magnetic properties, v) increase of coercivity (range between 1452.3 Oe – 2605.4 Oe), is consistent with obtained anisotropy value (range between 0.411´10³ erg/cc - 1.22 ´10³ erg/cc), while saturation magnetization range between 4.1 Am²/kg - 8.02 Am²/kg, ascribable to irradiation induced modification of disorder in the specimen. EDX confirms the uniform allocation of elements, creation of the preferred ferrite phase. Fig. 1(d, e) shows particle size distribution (inset: SEM image) for pristine, irradiated (1´10¹⁴ ions/cm²). Sample mean particle size (14.21 nm, 22.58 nm), distribution width (10.36 nm, 25.19 nm) of pristine, irradiated samples shows irradiation induced changes. It is worth noting that, distribution width is an important indicator, as it provides the basis to assess the particle homogeneity, and their lower values correspond to less scattered particle size, and volume. SEM images show particle agglomeration with different shapes, non-uniform particle size dispersal due to magnetic nature of ferrites. Present results clearly show strong correlation between structural properties, cationic distribution, and magnetic properties.