Magnetic properties of fully oxygenated bare CuO nanoparticles have been investigated using magnetization, X-ray diffraction, neutron diffraction, and Raman scattering measurements. The Langevin field profile is clearly revealed in the isothermal magnetization of 8.8 nm CuO nanoparticle assembly even at 300 K, revealing a 172 times enhancement of the ferromagnetic responses over that of bulk CuO. Surface magnetization of 8.8 nm CuO reaches 18% of the core magnetization. The Cu spins in 8.8 nm CuO order below 400 K, which is 1.7 times higher than the 231 K observed in bulk CuO. A relatively simple magnetic structure that may be indexed using a modulation vector of (0.2, 0, 0.2) was found for the 8.8 nm CuO, but no magnetic incommensurability was observed in bulk CuO. The Cu spins in 8.8 nm CuO form spin density waves with length scales of 5 chemical unit cells long along the crystallographic a- and c-axis directions. Considerable amounts of electronic charge shift from around the Cu lattice sites toward the interconnecting regions of two neighboring Cu–Cu ions, resulting in a stronger ferromagnetic direct exchange interaction for the neighboring Cu spins in 8.8 nm CuO.

The origin of the concomitant memory effect is still a controversial issue and poor evidence in the observation of nanocrystal systems. We report on a type of concomitant memory effect driven by first-field-induced unidirectional magnetic anisotropy at the interface of ferromagnetic CrO 2core and antiferromagnetic Cr 2 O 3 -shell nanorods, with the effect becoming less significant in pure CrO 2 nanorods. To corroborate the results, a core−shell anisotropic energy model was used to determine the coherent rotation of magnetization and exchange coupling constant, giving direct access to the anisotropic interfacial properties of the core−shell nanoscale system without spatially resolved magnetic measurement. The outcome of this study will be useful for the future development of thermal memory devices.

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