From Bulk to Nanoscale Magnetism and Exchange Bias in CuO Nanoparticles

Published

November 1, 2001

Author(s)

A Punnoose, H Magnone, M Seehra, John E. Bonevich

Abstract

Detailed studies of the temperature (5 K to 350 K) and magnetic field variations (up to H = 50 k0e) of the magnetization of CuO nanoprticles of size range 37 nm to 6.6 nm are reported. These particles are synthesized by the sol-gel method in combination with high temperature annealing, followed by structural characterization by x-ray diffraction and high resolution transmission electron microscopy. With decrease in particle size d from 37 to 10 nm, the unit cell volume and the b-axis increased and the bulk Neel temperature T(subN) decreased according to Ψ(sub m) = -deltalnT(subN)/deltalnb = 30. For particles with d < 10 nm, there is a more rapid lattice expansion and the magnetic susceptibility Chi varied as l/d, accompanied by a weak ferromagnetic component and hysteresis loops. For the 6.6 nm particles for which detailed studies are reported, there is a rapid increase in the coercivity H(subc) and the remanence M(subr) below 40 K accompanied by an exchange bias H(subE) for the FC (field-cooled) samples in H - 50 kOe. From 10 K to 40 K, H(subE) decreases monotonically to zero. However above 40 K, a symmetic hysteresis loop is observed with H(subc) decreasing weakly towards zero as temperature increases toward 350 K. The hysteresis loop and the 1/d variation of Chi are interpreted in terms of uncompensated surface Cu(superscript2+) spins whereas the transition at 40 K is suggested to be T(subN) of the spins in the core of 6.6 nm particles. Similarities to the hysteresis loops observed in permalloy/CoO system are noted.

Citation

Physical Review B (Condensed Matter and Materials Physics)

Volume

64

Issue

No. 17

Pub Type

Journals

Keywords

high resolution TEm, magnetism, nanoscale materials, sol-gel, x-ray diffraction

Citation

Punnoose, A. , Magnone, H. , Seehra, M. and Bonevich, J. (2001), From Bulk to Nanoscale Magnetism and Exchange Bias in CuO Nanoparticles, Physical Review B (Condensed Matter and Materials Physics) (Accessed April 20, 2024)