Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles

Abstract

We report systematic investigations of structural, vibrational, resonance and magnetic properties of nanoscale NiO powders prepared by ball milling process under different milling speeds for 30 hours of milling. Structural properties revealed that both pure NiO and as-milled NiO powders exhibit face centered cubic structure, but average crystallite size decreases to around 11 nm along with significant increase in strain with increasing milling speed. Vibrational properties show the enhancement in the intensity of one-phonon longitudinal optical (LO) band and disappearance of two-magnon band due to size reduction. In addition, two-phonon LO band exhibits red shift due to size-induced phonon confinement effect and surface relaxation. Pure NiO powder exhibit antiferromagnetic nature, which transforms into induced ferromagnetic after size reduction. The average magnetization at room temperature increases with decreasing the crystallite size and a maximum moment of 0.016 μB/f.u. at 12 kOe applied field and coercivity of 170 Oe were obtained for 30 hours milled NiO powders at 600 rotation per minute milling speed. The change in the magnetic properties is also supported by the vibrational properties. Thermomagnetization measurements at high temperature reveal a well-defined magnetic phase transition at high temperature (TC) around 780 K due to induced ferromagnetic phase. Electron paramagnetic resonance (EPR) studies reveal a good agreement between the EPR results and magnetic properties. The observed results are described on the basis of crystallite size variation, defect density, large strain, oxidation/reduction of Ni and interaction between uncompensated surfaces and particle core with lattice expansion. The obtained results suggest that nanoscale NiO powders with high TC and moderate magnetic moment at room temperature with cubic structure would be useful to expedite for spintronic devices.

Size-dependent magnetic properties of branchlike nickel oxide nanocrystals

“…This phenomenon has been reported in many literatures and can be ascribed to the size-induced phonon confinement effect in corroboration with the strength of the two-phonon coupling and surface relaxation. 5,14 This confirms that the size of NiO nanocrystals has strongly effect on their magnetic properties.…”

“…Vibration modes at around 400 cm 1 , 530 cm 1 , 730 cm 1 , 900 cm 1 , 1090 cm 1 , and 1450 cm 1 had been reported in the literatures and were attributed to one-phonon(1P) transverse optical (TO), (1P) longitudinal optical (LO), two-phonon (2P) TO, 2P TO+LO, 2P LO of vibrational origin and two-magnon (2M) band associated with Ni 2+ -O 2--Ni 2+ superexchange interaction, respectively. 5,14,20 And the 2M band, which supports the existence of antiferromagnetic (AFM) state through Ni 2+ -O 2--Ni 2+ superexchange interaction in bulk NiO, is not observed in this work. The disappearance of 2M intensity indicates that the decrease of AFM spin correlations.…”

“…In addition, the development of NiO was focused to obtain ferromagnetism even above room temperature to facilitate integration of spintronic devices. 5 And anomalous behavior of NiO would be achieved due to an uncompensated number of spins in the two sublattices when its size is reduced to nano-dimensions. [5][6][7] The magnetic performance of such nanomaterials is related to their chemical stoichiometry, size, and morphology, which strongly depend on the synthesis route.…”

Vacancy-Induced Ferromagnetic Behavior in Antiferromagnetic NiO Nanoparticles: A Positron Annihilation Study

“…So, if we subtract this AF contribution from the overall magnetic behavior, we will extract the other phase. As done previously, 21,24 we obtain the saturation magnetization (M s ) in the same way. For the NiO nanoparticles annealed at 100 and 400…”

“…It has been reported that the FM might come from the surface and finite size effects, or a net magnetic moment probably arises from uncompensated spins. Several models, such as the two-sublattice model, 3 multi-sublattice model, 10 and core-shell model 9,[12][13][14][18][19][20][21][22][23][24] have been used to illustrate the anomalous properties of NiO nanoparticles. However, different groups reported that * Electrochemical Society Fellow.…”