Development of ferromagnetism in Pd nanoparticles with reduc - tion in size

Submitted for the MAR09 Meeting of The American Physical Society Development of ferromagnetism in Pd nanoparticles with reduction in size MOHINDAR SEEHRA, JAMES RALL, West Virginia University, J. LIU, C. ROBERTS, Auburn University — Bulk fcc Pd is a paramagnet just missing the Stoner criterion for ferromagnetism (N(EF )I > 1) [1]. Several groups have reported weak ferromagnetism in 2-4 nm Pd nanoparticles (NPs) [2]. We report systematic development of weak ferromagnetism in Pd NPs with reduction in size. Magnetic measurements (M vs. T) are compared for bulk Pd with those of size D ≈ 50nm, 7nm, and 6nm. The samples of size D = 7 nm and 6 nm were prepared by an aqueous seed-mediated growth and characterized by TEM and x-ray diffraction with the latter showing expansion of the lattice with decrease in size. Compared with the low-field magnetic susceptibility χ of bulk Pd, χ for the 7 and 6 nm NPs are enhanced by an order of magnitude. For the 50 nm NPs, χ follows nearly the Curie law. The hysteresis loops (M vs. H) for the 7 and 6 nm NPs shows a decrease in coercivity and remanence from 2K to 300K suggesting TC > 300K. Origin of this ferromagnetism in terms of surface magnetism and lattice expansion is discussed. [1] W. Gerhardt et al, Phys. Rev. B 24, 6744 (1981); [2] T. Shinohara et al, Phys. Rev. B 91, 197201 (2003); Jeon et al, J. Appl. Phys. 103, 09413 (2008); Litran et al, Phys. Rev. B 73, 054404 (2006). Mohindar Seehra West Virginia University Date submitted: 01 Dec 2008 Electronic form version 1.4