Magnetic memory effect in chelated zero valent iron nanoparticles N. Ghosh n , B.K. Mandal, K. Mohan Kumar School of Advanced Sciences, VIT University, Vellore 632014, Tamilnadu, India article info Article history: Received 3 December 2011 Received in revised form 12 June 2012 Available online 3 July 2012 Keywords: Magnetic nanoparticle Magnetization Memory Magnetic relaxation abstract We report the study of nonequilibrium magnetic behavior of air stable zero valent iron nanoparticles synthesized in presence of N-cetyl-N,N,N-trimethyl ammonium bromide chelating agent. X-ray photoelectron spectroscopy study has suggested the presence of iron oxides on nZVI surfaces. Zero- field-cooled and field-cooled magnetization measurements have been carried out at 20–300 K and 100 Oe. For field-cooled measurements with 1 h stops at 200, 100 and 50 K when compared with the warming cycle, we found the signature of magnetic memory effect. A study of magnetic relaxation at the same temperatures shows the existence of two relaxation times. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Magnetic nanoparticles have attracted much attention of con- temporary scientific community due to their potential applications in data storage and biomedicine industries and the interesting physics underlying the various exotic phenomena exhibited by them [1–3]. Most of such works have focused on ferromagnetic and ferrimagnetic nanoparticles because of their high magnetic moments that make them industrially valuable. Often magnetic nanoparticles are covered with a protective shell when they are used in technological applica- tions. For example, iron–iron oxide nanoparticles are used in mag- netic recording tapes [4, 5]. In this context zero valent iron nanoparticles (nZVI) can be mentioned which are very promising materials for environmental remediation technology because of their unique physiochemical properties, especially its high surface area over iron filings. However, stability of nZVI particles in air is difficult to achieve, since they are highly sensitive to oxygen and oxide formation. In order to stabilize these nanoparticles many researchers have employed the nZVI synthesis with a wide variety of stabilizing agents, surfactants and capping agents [6]. Comparatively better result is achieved by synthesizing nZVI in presence of different chelating agents [7]. From technological point of view it would be quite interesting to explore the existence of magnetic memory effect in chelated nZVI considering their robust stability criteria in air. In this article, we report our investigation of magnetic memory effect in nZVI synthesized in the presence of N-cetyl-N, N, N-trimethyl ammo- nium bromide (CTAB) chelating agent. We have studied nonequili- brium magnetic behavior of nZVI with respect to time at certain temperatures. 2. Experiment In order to synthesize nZVI at ambient temperature, 0.1 M FeSO 4 in 150 ml of Milli-Q water and appropriate amount of CTAB have been mixed in a 3 necked round bottom flask by propeller mixing. Then 0.5 M NaBH 4 containing 100 ml distilled water is added dropwise into the mixture solution and slowly the solution turns to black color. The black colored particles are washed thrice with ethanol and finally filtered, dried, and pulverized [7]. X-ray photoelectron spectroscopy (XPS) study has been carried out to get information on surface composition of nZVI samples. The dc magnetic measurements of nZVI samples (in powder form) have been performed in 20–300 K temperature range by using a commercial (Lakeshore VSM 7410) Vibrating Sample Magnet- ometer (VSM). Initially magnetization (M) of nZVI is measured at zero field cooled (ZFC) and field cooled (FC) conditions at applied magnetic field (H) of 100 Oe. For investigation of mag- netic memory effect, FC magnetization measurement is done at H ¼ 100 Oe down to 20 K with stops of 1 h at 200 K, 100 K and 50 K. The sample is warmed again up to 300 K in the presence of same field and magnetization is recorded. In order to do magnetic relaxation measurement, the sample is cooled down to 200 K in the absence of any field. Then a field H ¼ 1 T is applied in 25 s and it took again around 25 s to remove the field. After removing the field, the magnetization is recorded as a function of time (t ¼ 1.4 h). Later, the sample is further cooled at H ¼ 0 T and the same measurement has been performed at 100 and 50 K. 3. Results and discussions The results of XPS study on nZVI are described in Fig. 1. The inset shows the magnified version of XPS spectra in the 700– Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.06.026 n Corresponding author. Tel.: þ91 416 2202352; fax: þ91 416 2243092. E-mail addresses: nilotpal@vit.ac.in, ghosh.nilotpal@gmail.com (N. Ghosh). Journal of Magnetism and Magnetic Materials 324 (2012) 3839–3841