Effect of dipolar and exchange interactions on magnetic blocking of maghemite nanoparticles K. Nadeem a,n , H. Krenn a , T. Traussnig b , R. W¨ urschum b , D.V. Szabo ´ c , I. Letofsky-Papst d a Institute of Physics, Karl-Franzens University Graz, Universit¨ atsplatz 5, A-8010 Graz, Austria b Institute of Materials Physics, University of Technology Graz, A-8010 Graz, Austria c Institute for Materials Research III, Karlsruhe Institute for Technology (KIT), 76021 Karlsruhe, Germany d Institute for Electron Microscopy, University of Technology Graz, Steyrergasse 17, A-8010 Graz, Austria article info Article history: Received 21 September 2010 Received in revised form 6 December 2010 Available online 5 March 2011 Keywords: Nanoparticle Interparticle interaction Dipolar Exchange abstract Magnetic interparticle interactions compete with the magnetic blocking of ultrafine magnetic nanoparticles. We have prepared maghemite (g-Fe 2 O 3 ) nanoparticles by microwave plasma synthesis as a loose powder and in compacted form. In ZFC/FC measurements, blocking temperature of the compacted sample C is larger than that of the powder sample P. The frequency dependence of AC susceptibility of the sample C shows a large shift of blocking temperature with increasing frequency. Vogel–Fulcher law gives a large value of T 0 for the sample C. To get evidence of a possible spin-glass freezing in both samples, scaling law fitting is applied to the AC susceptibility data. The value of the exponent (zv) of the critical slowing down dynamics fits to the spin-glass regime for both samples. For the sample P, spin-glass freezing occurs on the surface of individual nanoparticles, while in the sample C surface spin-glass freezing is concomitant with a superspin-glass formation as a consequence of coupling between particles. The sample C also shows an enhancement of coercivity due to dipolar interactions among the nanoparticles. Exchange interactions are attributed only to touching nanopar- ticles across their interfaces. All these measurements indicate the presence of strong interparticle dipolar interactions in the compacted sample C. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Maghemite (g-Fe 2 O 3 ) nanoparticles have been investigated intensively over the last years due to their potential applications in industry [1]. Nanoparticle magnetism is strongly influenced by interparticle dipolar (long range) or/and exchange (short range) interactions [2,3]. Maghemite (g-Fe 2 O 3 ) is one of the ferrimagnetic materials ordered according to the inverse spinel structure with vacancies at the octahedral sites [4]. In spinel ferrite structure, oxygen forms an FCC-lattice with cations distributed over tetrahedral (A) and octahe- dral (B) sites. The unit cell of a spinel ferrite consists of 32 oxygen, 16 trivalent iron and 8 divalent transition metal ions like in nickel ferrite (NiFe 2 O 4 ) or cobalt ferrite (CoFe 2 O 4 ). The spins at the tetrahedral and octahedral sites are anti-parallel to each other. In maghemite (g-Fe 2 O 3 ), Fe 3 þ ions occupy both tetrahedral (A) and octahedral (B) sites, the latter being only partly occupied by iron. The unit cell of maghemite (g-Fe 2 O 3 ) is cubic with composition (Fe 3 þ 8 ) A [Fe 3 þ 40/3 ‘‘&’’ 8/3 ] B O 32 [4], where brackets ( ) and [ ] represent tetrahedral and octahedral sites, respectively, and ‘‘&’’ represents iron vacancies at octahedral sites. Due to these vacancies and competing interactions among spins located on different sublattices together with broken bonds, surface spins of maghemite nanopar- ticles experience frustration and disorder, which are ingredients for a possible spin-glass state. For magnetic nanoparticles, the blocking temperature is the temperature up to which the particle’s magnetic moment keeps alignment to its anisotropy ‘‘easy’’ axis during experimental observation times. The blocking temperature of a nanoparticle depends on its surroundings and on interparticle interac- tions [5,6]. Consistently, the energy barrier of individual nano- particles is strongly influenced by exchange and dipolar interactions. Kechrakos and Trohidou compared the influence of both exchange and dipolar interactions and their dependence on the particle concentration using Monte Carlo simulations [7]. Below the percolation limit both exchange and dipolar interac- tions raise the average blocking temperature due to enhanced interparticle interactions. Garcı ´a-Otero et al. [8] have also reported an increase of blocking temperature with increasing nanoparticle concentration studied by Monte Carlo simulations, as well as diluted iron-based nanoparticles dispersed in paraffin were experimentally studied by Vargas et al. [9]. Nunes et al. [10] Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.02.041 n Corresponding author. Tel.: þ43 316 380 1625; fax: þ43 316 380 9816. E-mail address: kashif.nadeem@edu.uni-graz.at (K. Nadeem). Journal of Magnetism and Magnetic Materials 323 (2011) 1998–2004