Neutron powder diffraction of carbon-coated FeCo alloy nanoparticles John Henry J. Scott a) Microanalysis Research Group, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Krishna Chowdary Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Zafer Turgut Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Sara A. Majetich Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Michael E. McHenry Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Neutron powder diffraction is used to study the order–disorder transformation in carbon-coated Fe x Co 1 -x nanoparticles produced using a radio frequency plasma torch. The nanoparticles, nominally Fe 50 Co 50 , are produced from alloy powder and acetylene precursors by gas-phase nucleation from the plasma. The resulting nanoparticles undergo an order–disorder transformation near 730 °C, passing from an ordered B 2 CsCl structure to a disordered A 1 body-centered-cubic structure upon heating, similar to the transformation seen in bulk equiatomic FeCo. Although it is very difficult to quench the disordered state in bulk samples, the extreme cooling rates present in the plasma reactor produce metastable disordered nanoparticles. Neutron powder diffractograms acquired during a heating–cooling cycle at 27, 500, 710, 800, 710, and 400 °C indicate the particles relax to their equilibrium ordered state upon heating first, disorder as they pass through the transformation temperature, and reorder upon cooling. © 1999 American Institute of Physics. S0021-89799937008-0 INTRODUCTION For many decades FeCo alloys have played an important role in applications requiring soft magnetic materials. Be- cause of their combination of low coercivity, large perme- ability, and very large saturation induction these alloys have seen wide use in transformer cores, electrical generators, electrical motors, and pole pieces. More recently, the need for soft magnetic materials capable of performing at tem- peratures above 600 °C has led to renewed interest in FeCo alloys, particularly FeCo–C nanocomposites. Unfortunately, FeCo undergoes an order–disorder transformation at ap- proximately 730 °C, forming an ordered alloy at room tem- perature even in severely quenched samples. 1 The ordered state displays decreased resistivity and decreased ductility compared to the disordered material. The change in conduc- tivity results in degraded performance in high frequency ap- plications requiring low eddy current losses, while the me- chanical embrittlement makes the ordered alloy difficult to cold roll and machine. In this work, the order–disorder trans- formation in C-coated FeCo nanoparticles is characterized by neutron powder diffraction. EXPERIMENT The FeCo nanoparticles used in this study were synthe- sized using a 50 kW rf plasma reactor operated at 6 A, 8 kV, and 3 MHz. The plasma was used to vaporize Fe 50 Co 50 alloy starting material consisting of 5–10 m powder produced by inert gas atomization. Acetylene gas was injected into the plasma to coat the nanoparticles with carbon and to reduce subsequent oxidation during handling in air. The synthesis protocols are described elsewhere 2,3 along with characteriza- tion of the powder using transmission electron microscopy TEM and x-ray diffraction XRD. Neutron powder diffraction was conducted at the Na- tional Institute of Standards and Technology NIST Center for Neutron Research using the high-resolution powder dif- fractometer on BT-1. A Cu311 monochromator and 15 ft Soller collimators produced 1.540 Å neutrons incident on the sample. The nanoparticle powder was held in a vacuum fur- nace by a sample holder constructed from a Nb sheet. A bank of 32 parallel detectors was used to acquire constant wave- length diffractograms at the following temperature points of a heating–cooling profile: 27, 500, 710, 800, 710, and 400 °C. Patterns were acquired from 27.5° to 124° in 2 in steps of 0.05°. RESULTS AND DISCUSSION FeCo superlattice reflections ( h +k +l =odd) are seen easily in neutron diffraction because of the difference in scat- tering lengths of Fe and Co. Figure 1 shows the 100 super- lattice line from diffraction patterns acquired at six different a Electronic mail: johnhenry.scott@nist.gov JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 8 15 APRIL 1999 4409 0021-8979/99/85(8)/4409/3/$15.00 © 1999 American Institute of Physics Downloaded 23 Jan 2001 to 128.2.132.154. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html.