Largely Enhanced Ferromagnetism in Bare CuO Nanoparticles by a Small Size Effect

Abstract

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.

Accelerated mapping of electronic density of states patterns of metallic nanoparticles via machine-learning

“…Nanoparticles (NPs) are of great scientific interest because they often show unexpected physical and chemical properties resulting from their quantum confinement effect 1,2 or high surface area 3,4 . This leads to various applications of NPs, such as quantum dots, 5-7 magnetic 8,9 or bio- [10][11][12][13] materials, and catalysis 3,[14][15][16][17][18][19][20][21] . As a key feature to determine the properties of NPs, an electronic structure such as electronic density of states (DOS) has been usually considered, where the electronic structure significantly depends on the sizes and shapes of the NPs although the elements constituting the NPs are identical.…”

“…As a key feature to determine the properties of NPs, an electronic structure such as electronic density of states (DOS) has been usually considered, where the electronic structure significantly depends on the sizes and shapes of the NPs although the elements constituting the NPs are identical. 9,17,20,[22][23][24][25][26] First-principles density functional theory (DFT) calculations have been mainly utilized to predict DOS patterns of NP structures. In particular, the plane-wave (PW) basis technique has been employed for metallic NP systems despite its extremely high computational cost.…”

Observation of Cu Spin Fluctuations in High-Tc Cuprate Superconductor Nanoparticles Investigated by Muon Spin Relaxation

“…As reported in CuO with the particle size of 8.8 nm, the appearance of magnetism could be linked to the charge redistribution in both the 4s and 3d electrons of Cu ion. It is proposed that the charge redistribution was triggered by the lattice periodicity disruption at the surface [14]. Further theoretical investigation on the distribution of the charge carrier in HTSC nanoparticles is needed, especially the electronic charge of Cu and O ions.…”

“…An antiferromagnet CuO was also reported to behave ferromagnetically, showing a hysteresis loop at 8 K and 300 K when reducing the particle size to 25 nm [12][13][14]. In addition, the magnetic transition temperature, T N , was reported to be drastically decreased to 12 K from that observed in the bulk form of about 229 K when reducing the particle size down to 2-3 nm.…”

Metal Deposition Induced by the Step Region of Si (111)-(7 × 7) Surface

“…This is partially due to the difficulties in obtaining stable, regular, and high-density atomic structures. Additionally, as the size of the metal deposition decreases to a certain extent (around 10 nm), some unique phenomena follow, such as the small-size effect, surface effect, and so on [9,10].…”