2, University of Nebraska, Lincoln, Nebraska, United States
Our group has been studying the magnetic and structural properties of novel magnetic nanoparticles for the past few years. Our specific focus is on nanometer-length-scale and real structure control of new or metastable structures as a means of creating materials with high magnetization, high spin polarization, large magnetocrystalline anisotropy and high ordering temperatures using non-equilibrium fabrication techniques. In the last couple of years our work has been focused on nanoclusters made by the cluster beam deposition (Co2Ge, Mn5Si3, Fe5Si3, Co3Si) [1-2] and L10 FePt (CoPt) made by chemical synthesis. These types of materials may have potential impact in ultra-strong permanent magnets and extremely high density magnetic recording media.
Our experimental and theoretical results show unique and interesting properties in nano-size particles which are drastically different from bulk. For example, in Co2Ge and Mn5Si3 nanoparticles, high Curie temperatures of around 820 K and 590 K, respectively were found as compared to bulk systems which are paramagnetic at room temperature. The enhancement of Curie temperature in Co2Ge is believed to be due to both size effects and surface segregation of Co atoms while in the case of Mn5Si3 is attributed to a different electronic structure in the nanoparticle surface. Also, Fe5Si3 nanoparticles show a higher Curie temperature (by about 50% of bulk) and enhanced magnetization which are attributed to a large spin polarization at the cluster surface. In Co3Si nanoparticles, a large coercivity of 4.3 kOe at room temperature was obtained, despite the easy-plane anisotropy in bulk, which can be explained by the combined effects of exchange interactions between the particles and their field alignment during deposition. L10 FePt and CoPt nanoparticles were fabricated at low temperatures (350 oC) by liquid phase synthesis using Bi doping without the need of post annealing. FePt and CoPt nanoparticles exhibit coercivities of 13.6 kOe and 1.4 kOe, respectively. Surface segregation of Bi atoms was found to enhance the low temperature ordering of the L10 structure. The Curie temperature of the fcc and L10 phase particles in both systems show an unexpected trend; increasing ordering in FePt leads to an increase in Curie temperature, while an increase in ordering in CoPt leads to decrease in Curie temperature.
This work is supported by DOE DE-FG02-04ER46152 and DE-FG02-90ER45413
 B. Das, B. Balasubramanian, P. Manchanda, P. Mukherjee, R. Skomski, G. C. Hadjipanayis, and D. J. Sellmyer, Nano Letters 16 (2), 1132 (2016)
 Balamurugan Balasubramanian, Priyanka Manchanda, Ralph Skomski, Pinaki Mukherjee, Shah R. Valloppilly, Bhaskar Das, George C. Hadjipanayis, and David J. Sellmyer, applied physics letters 108, 152406 (2016)