Development of ferromagnetism in Pd nanoparticles with reduction in size

Bulk fcc Pd is a paramagnet just missing the Stoner criterion for ferromagnetism (N(E$_{F})$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 $\approx $ 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 $\chi $ of bulk Pd, $\chi $ for the 7 and 6 nm NPs are enhanced by an order of magnitude. For the 50 nm NPs, $\chi $ 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 T$_{C} \quad >$ 300K. Origin of this ferromagnetism in terms of surface magnetism and lattice expansion is discussed. [1] W. Gerhardt et al, Phys. Rev. B \underline {24}, 6744 (1981); [2] T. Shinohara et al, Phys. Rev. B \underline {91}, 197201 (2003); Jeon et al, J. Appl. Phys. \underline {103}, 09413 (2008); Litran et al, Phys. Rev. B \underline {73}, 054404 (2006).