Synthesis, morphology and microstructure of pomegranate-like hematite (a-Fe 2 O 3 ) superstructure with high coercivity Marin Tadic a,⇑ , Nada Citakovic b , Matjaz Panjan c , Boban Stanojevic d , Dragana Markovic a , Ðor - de Jovanovic e , Vojislav Spasojevic a a Condensed Matter Physics Laboratory, Vinca Institute, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia b Military Academy, Generala Pavla Jurisica Sturma 33, University of Belgrade, 11000 Belgrade, Serbia c Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia d Vinca Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, University of Belgrade, Serbia e Center for Solid State Physics and New Materials, Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia article info Article history: Received 30 April 2012 Received in revised form 6 July 2012 Accepted 9 July 2012 Available online 27 July 2012 Keywords: Hematite (a-Fe 2 O 3 ) Iron oxide Coercivity Self-assembly nanoparticles Magnetic properties Morin temperature abstract We found novel and superior magnetic properties of the hematite (a-Fe 2 O 3 ) that originate from an inter- nal microstructure of particles and strong inter-particle interactions between nanocrystal sub-units. The hematite particles were synthesized by thermal decomposition of iron (III) nitrate without any template or surfactant. The purity, size, crystallinity, morphology, microstructure and magnetic features of the as- prepared particles were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy (RS) and SQUID magnetometry. An XRD study reveals a pure phase of a-Fe 2 O 3 whereas TEM shows a-Fe 2 O 3 spheres with a diameter of about 150 nm. RS also shows high quality and purity of the sample. Moreover, TEM and HRTEM images show a pomegranate-like superstructure and evidence that the spherical particles are composed of individual well-crystallized nanoparticle sub-units (self-assembled nanoparticles) with a size of about 20 nm. Mag- netic measurements display hysteretic behavior at the room temperature with remanent magnetization M r = 0.731 emu/g, saturation magnetization M S = 6.83 emu/g and coercivity H C = 4350 Oe, as well as the Morin transition at T M = 261 K. These results and comparison with those in the literature reveal that the sample has extremely high coercivity. The magnetic properties of the sample are discussed in relation to morphology, internal microstructure, surface effects and exchange and dipole–dipole interactions. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Nanosized iron oxides have been one of the most active research topics in materials science in the past decade [1–26]. There is currently great interest in nanosized a-Fe 2 O 3 due to both its fundamental interest and potential technological applications [1,25–93]. Hematite (a-Fe 2 O 3 ) crystallized in the rhombohedral system space group R-3c with n-type semiconducting properties (2.1 eV band gap) [1]. It is the most stable iron oxide with a high resistance to corrosion, low cost, that is biocompatible, environ- mentally friendly and non-toxic. Such qualities emphasize hema- tite as a prominent candidate for applications such as pigment, catalyst, sensor, environmental pollutant cleanup agent, electrode material, biomedical material and magnetic material [1,26– 28,32,49,50,59, 84,92,93]. Despite extensive research in both bulk and nanosized form, some of its features are still not fully explored and understood. One of these unknowns are its magnetic proper- ties [27–36,39–48,53–56,60–81,83–90]. In bulk hematite the Neel temperature is at T N  960 K whereas the Morin transition takes place at T M  263 K [1]. Below T M the material is a uniaxial antifer- romagnet. Above T M , the material is weakly ferromagnetic [1]. As the particle size decreases, the Morin temperature is reduced, and tends to vanish for particles smaller than about 8–20 nm [27–30,33,81,89,90]. If the particles become small enough, the direction of the magnetic moment in a single domain fluctuates due to thermal agitation, leading to superparamagnetic behavior above the blocking temperature T B , and to spatial freezing of these moments below T B [27,28]. Consequently, nanoparticle hematite is an interesting material for fundamental research of magnetic properties, because it can display three critical temperatures: the Neel temperature, the Morin temperature and the blocking tem- perature [1,27–36,39–46,48,53–56,59–81,83–91]. It is well-known that the magnetic properties of a-Fe 2 O 3 are sensitive to the size, inter-particle interactions, morphology and microstructure of the samples [1,27–36,39–46,48,53–56,59–81,83–91]. For example, 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.047 ⇑ Corresponding author. E-mail address: marint@vinca.rs (M. Tadic). Journal of Alloys and Compounds 543 (2012) 118–124 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom