Modifying the morphology and magnetic properties of magnetite nanoparticles using swift heavy ion irradiation Shubha Gokhale a,⇑ , Subhalakshmi Lamba a , Neha Kumari a , Bhupendra Singh a , D.K. Avasthi b , S.K. Kulkarni c a School of Sciences, Indira Gandhi National Open University, New Delhi 110068, India b Inter University Accelerator Centre, P.O. Box 10502, New Delhi 110067, India c Indian Institute of Science Education Research, Dr. Homi Bhabha Road, Pune 411008, India article info Article history: Received 21 February 2014 Received in revised form 28 April 2014 Accepted 28 April 2014 Available online 24 May 2014 Keywords: Magnetite nanoparticles SHI induced modifications Magnetic anisotropy abstract Magnetite (Fe 3 O 4 ) nanospheres of 8–11 nm diameter synthesized using a chemical co-precipitation method were deposited as thin films on different substrates using spin coating. The thin films were irradiated with Ag ions at 100 MeV energy. Comparison of unirradiated, as synthesized Fe 3 O 4 nanopartic- ulate thin film and ion irradiated film shows that irradiation causes dramatic changes in the morphology, structure and magnetic properties. Monte Carlo simulations carried out on this system indicate that the origin of the changes in the magnetic properties lies in the enhanced magnetic anisotropy energy density and reorientation of magnetic easy axis. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Magnetic nanoparticles find wide applications in technology area like memory devices, sensors and as therapeutic tools [1–3]. Magnetite (Fe 3 O 4 ) particles are a preferred choice due to their non-toxic and stable nature in comparison with other magnetic nanoparticles like cobalt or nickel [4]. The magnetic properties of nanoparticles can be tailored by modifying their shape. In recent years ion beam irradiation has emerged as a powerful tool to impart shape modification in the nanoparticles [5–7]. In the pres- ent work we have investigated the effect of swift heavy ion (SHI) irradiation on magnetite nanoparticles. There have been some reports earlier on the effects of ion beam irradiation on the semiconductor/metal/metal oxide nanoparticles embedded in a matrix like silica [8–10]. Here we report for the first time the effect of ion beam irradiation on free standing magnetite nanoparticles, dispersed on a substrate in the form of a thin film. We have used 100 MeV silver ion beam dose of 1  10 14 ions/cm 2 in our investigations. The magnetite nanoparticles were synthe- sized by a chemical route and characterized using transmission electron microscopy (TEM) along with EDS for morphology, vibrat- ing sample magnetometer (VSM) for magnetization properties and X-ray diffraction (XRD) for crystal structure determination. The irradiated sample results were compared with pristine (un-irradi- ated) samples. Pristine spherical magnetite particles of 8–11 nm diameter with good crystallinity suffered sputtering and melting due to interaction with the ion beam. Ion irradiation gives rise to shape modification in form of increased diameter in the plane of the thin film. This observation indicates that we have been able to achieve the shape modification of nanoparticles without embed- ding or encapsulating them with amorphous material like silica. After ion beam irradiation the crystallinity and the magnetic prop- erties of the sample are found to be modified. To understand the observed post-irradiation changes in the magnetic properties, we have performed Monte Carlo simulations on the arrays of magne- tite particles in a thin film like geometry (2d + h system). It reveals that the marked difference observed between in-plane and out-of- plane magnetization behaviour of the irradiated particles may be attributed to the increased anisotropy energy density and orienta- tion of the magnetization easy axis in the out-of-plane direction. 2. Experimental Magnetite particles were synthesized using an aqueous route. 20 ml solution of 0.2 M FeCl 3 was mixed with appropriate quantity of ammonia solution. This solution was stirred at 80 °C for 2.5 h. In another container 10 ml solution of 0.2 M FeSO 4 was mixed with ammonia solution and ultrasonicated at room temperature for 5 min. After cooling the FeCl 3 solution to room temperature, the two solutions were mixed and ultrasonicated for 35 min. The product in the form of black precipitate was washed twice with water and finally with ethanol. After this the sample was dried using vacuum dryer to obtain black powder of Fe 3 O 4 nanoparticles. http://dx.doi.org/10.1016/j.nimb.2014.04.020 0168-583X/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +91 1129572816; fax: +91 1129532167. E-mail address: sgokhale@ignou.ac.in (S. Gokhale). Nuclear Instruments and Methods in Physics Research B 333 (2014) 64–68 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb