Exchange bias behavior of monodisperse Fe 3 O 4 /g-Fe 2 O 3 core/shell nanoparticles Yosun Hwang a , S. Angappane a,1 , Jongnam Park b , Kwangjin An b , T. Hyeon b , Je-Geun Park c, d, * a Department of Physics, SungKyunKwan University, Suwon 440-746, Republic of Korea b National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea c FPRD & Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea d Center for Strongly Correlated Materials Research, Seoul National University, Seoul 151-747, Republic of Korea article info Article history: Received 28 February 2011 Received in revised form 9 September 2011 Accepted 9 November 2011 Available online 18 November 2011 Keywords: Fe 3 O 4 nanoparticles Exchange bias behavior Core-shell structure abstract We have carried out systematic studies on well-characterized monodisperse Fe 3 O 4 /g-Fe 2 O 3 core/shell nanoparticles of 2e30 nm having a very narrow size distribution and possessing a uniquely mono-layer of surface g-Fe 2 O 3 . This unique core-shell structure, probably having a disordered magnetic surface state, leads us to three key observations of unusual magnetic properties: i) a very large magnetic exchange anisotropy reaching over 7 Â 10 6 erg/cm 3 for the smaller particles, ii) exchange bias behavior in the magnetization data of the core/shell Fe 3 O 4 /g-Fe 2 O 3 nanoparticles, and iii) the temperature dependence of the coercive field following an unusual exponential behavior. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Over the past years, we have seen an increasingly large amount of works done on magnetic nanoparticles. With their immense potential applications in very diverse fields of science and tech- nology, a growing number of new nanoparticles have been continuously synthesized and studied [1,2]. Magnetic properties of such nanoparticles are often found to show distinctively different behavior from those of their bulk counterparts. For example, ferromagnetic nanoparticles are found to show enhanced magne- tization and magnetic anisotropy, which make them useful for various technological and medical applications in addition to being of greater fundamental interest [3,4]. Some of ferromagnetic nanoparticles exhibit an interesting property called exchange bias, i.e. a displacement of hysteresis loops along the magnetic field axis. It was first reported for fine Co particles, and subsequently attributed to exchange interaction at the interface between ferromagnetic (FM) Co core and antiferro- magnetic (AFM) CoO shell [5]. Such exchange bias at the FM/AFM interface is also expected to give rise to an enhanced coercivity [6]. Despite the original discovery, the exchange bias phenomenon has so far been more systematically studied for thin film systems simply because it is easier to prepare FM/AFM combinations in films with a greater control of the interface than practically possible with nanoparticles, at least until recently [7,8]. However, advances made in the field of magnetic nanoparticle synthesis over last few years [9] have prompted renewed interest in nanoparticles in general, and exchange bias systems in particular. Of further interest, some selective chemical treatments can be easily done on the surface of nanoparticles: e.g., oxidation, nitra- tion, and sulfation, so opening up a new window of opportunities of tailoring the magnetic properties of nanoparticles by chemical methods. Among Fe-related systems, most works have been so far focused on exchange bias behavior in g-Fe 2 O 3 [10], g-Fe 2 O 3 coated Fe nanoparticles [11] and Fe 3 O 4 core (ferri)eFeO shell (AFM) systems [12]. However, there has been so far no report of an exchange bias phenomenon in the core/shell Fe 3 O 4 /g-Fe 2 O 3 nano- particles, in which the surface of Fe 3 O 4 nanoparticles has been naturally modified to g-Fe 2 O 3 shell during the chemical synthetic process. That these Fe 3 O 4 nanoparticles have unique bio- compatibility with human body adds further motivation to the systematic studies of the nanoparticles, making our results re- ported here of more than just academic interest. We report here extensive studies on novel magnetic properties of the core/shell Fe 3 O 4 /g-Fe 2 O 3 nanoparticles by using well- * Corresponding author. Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea. Tel.: þ82 2 880 6613; fax: þ82 2 884 3002. E-mail address: jgpark10@snu.ac.kr (J.-G. Park). 1 Present address: Centre for Soft Matter Research, Jalahalli, Bangalore 560-013, India. Contents lists available at SciVerse ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.11.011 Current Applied Physics 12 (2012) 808e811