Magnetization Reversal in Chemically Synthesized Hexagonal Cobalt Microplatelets B. K. Mahato, A. Ganguly, B. Rana, and A. Barman* Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata -700 098, India ABSTRACT: We report the magnetization reversal modes in hexagonal cobalt microplatelets prepared via the solvothermal chemical synthesis method. Experimental results do not reflect any effect of anisotropy due to the shape of an individual platelet and the shape of the overall geometry of the ensemble of platelets. Micromagnetic simulations on a single hexagonal cobalt platelet demonstrate different kinds of reversal modes depending upon the direction of the applied magnetic field, whereas for a group of three platelets the reversal modes of a single platelet are manifested in a slightly modified form due to the weak magnetostatic interactions among themselves. In this case, a poor qualitative agreement is observed with the experimental hysteresis loops. A much better qualitative agreement with the experimental loops is observed for a random ensemble of seven platelets, and a strongly collective magnetization reversal is observed in this case. ■ INTRODUCTION Magnetic micro- and nanoparticles are potential candidates for modern and future magnetic technology. Discovery of a number of unusual and interesting magnetic properties in confined magnetic particles has enhanced the research interest in such systems. The key reason behind that is the increment of the surface potential energy with the decrease in the size of the particles. Most of the technological applications including magnetic data storage, 1 magnetic resonance imaging, 2 mag- nonic crystals, 3 and biomedicine and biotechnology 4,5 demand the understanding and manipulation of the quasistatic and ultrafast magnetic processes of an ensemble of magnetic nanoparticles as well as that of the single nanoparticle. By changing the shape, size, and chemical composition of the microparticles and nanoparticles, one may manipulate their magnetic properties and magnetization reversal mechanisms to a large extent. 6,7 Therefore, the synthesis of magnetic micro- and nanoparticles with high crystallinity of the desired crystal structure and with controlled shape and size have become one of the key issues of the modern materials science and nanoscience. A large volume of literature exists on the synthesis and characterization of magnetic properties of magnetic micro- and nanoparticles of different shapes. 8-11 There are few reports about the details of the magnetization reversal of cluster and chains of single domain spherical magnetic nanoparticles, where the shape anisotropy due to the individual nanoparticle is absent and the reversal is driven primarily by the overall geometry of the clusters or chains. 12-14 Simulations of magnetization reversal of nanoparticles with magnetocrystalline and shape anisotropies 15 and the influence of surface anisotropy on the magnetization reversal of maghemite nanoparticles with ellipsoidal shape 16 have been reported. Magnetization reversal mechanisms of single hexagonal shaped magnetic platelets have been reported by Nakatani et al. in the 1990s using micromagnetic simulations. 17,18 However, extension of such works to an ensemble of platelets and comparison of the simulation results with the experimental results have been missing in the literature. Here, we have presented the magnetization reversal modes of chemically synthesized hexagonal cobalt microplatelets. Experimentally measured hysteresis loops show the collective reversal of the platelets. After the formation, uncoated magnetic microplatelets do not remain well separated; rather, they form agglomerates. An effort has been made here to understand the effect of the neighboring platelets in the agglomerates on the individual platelets. Micromagnetic simulations show that the normal reversal modes of isolated single platelets are significantly modified in the ensemble, influencing the overall reversal behavior. ■ EXPERIMENTAL METHODS To synthesize hexagonal Co microplatelets, a homogeneous transparent solution is prepared by dissolving 1 g of CoCl 2 , 6H 2 O into 30 mL of ethylene glycol (EG) by rigorous stirring for 30 min. Following that, 3 mL of ethylene diamine (EDA) is added dropwise to the above solution at room temperature with continuous stirring by a magnetic stirrer for another 30 min. EDA is a small organic molecule to control the nucleation Received: July 19, 2012 Revised: September 20, 2012 Published: September 24, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 22057 dx.doi.org/10.1021/jp307144j | J. Phys. Chem. C 2012, 116, 22057-22062