Pressure dependence of two-magnon Raman scattering in NiO

Plasmon induced enhancement of surface optical phonon modes and magnon properties of NiO nanoparticles: Raman spectral probe

In the present work, the influence of Ag-induced plasmons on the surface optical (SO) phonon modes of NiO nanoparticles was extensively studied using room temperature Raman spectroscopy. Remarkable intensity enhancements were observed for the rarely reported SO phonon modes compared to the other first-order phonon modes of NiO nanoparticles. The occurrence of SO modes was further studied using an approximate dielectric continuum (DC) model and a difference between the calculated and experimental SO frequencies was observed, which can be attributed to the presence of one magnon background over the first order phonon modes. The experimental and theoretical SO frequencies became closer at higher Ag concentration and the second-order magnon (2M) and phonon bands disappeared in the NiO:Ag samples. The absence of magnon and higher order phonon modes in the NiO:Ag samples indicates changes in the magnetic properties of the nanomaterials, which has been further supported by the vibrating-sample magnetometer (VSM) measurements.

Utilization of Zn-containing electric arc furnace dust for multi-metal doped ferrite with enhanced magnetic property: From hazardous solid waste to green product

One-step solid state reaction method was proposed for the first time to realize the transformation of the Zn-containing EAFD from hazardous solid waste to multi-metal doped ferrite with enhanced magnetic property. The effect of Zn-containing EAFD to NiCl₂·6H₂O mass ratio (R_ZE/N, g·g^-1) on the phases transformation was investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman spectroscopy. The as-synthesized samples were treated by toxicity characteristic leaching procedure (TCLP). It is shown that the TCPL played a key role in determining both the purity and toxicity of the obtained ferrites. The pure metal doped Ni-Zn ferrite with higher saturation magnetization (Ms, 56.8 emu·g^-1) and lower coercivity (Hc, 58.5Oe) was gained under the optimum conditions. And the pure ferrite was a green product according to the TCLP and EN12457 standards. Moreover, the evaluation of environmental impact and the recovery ratio of the dust were also discussed.

Pressure induced structural and spin state transitions in Sr3Fe2O5

The effect of pressure on the structural, electronic and magnetic properties of the two-legged spin ladder structure Sr₃Fe₂O₅ was investigated, using density functional theory within the generalized gradient approximation (GGA) + U method.

Reduction Dynamics of Doped Ceria, Nickel Oxide, and Cermet Composites Probed Using In Situ Raman Spectroscopy

The redox properties of gadolinium doped ceria (CGO) and nickel oxide (NiO) composite cermets underpin the operation of solid oxide electrochemical cells. Although these systems have been widely studied, a full comprehension of the reaction dynamics at the interface of these materials is lacking. Here, in situ Raman spectroscopic monitoring of the redox cycle is used to investigate the interplay between the dynamic and competing processes of hydrogen spillover and water dissociation on the doped ceria surface. In order to elucidate these mechanisms, the redox process in pure CGO and NiO is studied when exposed to wet and dry hydrogen and is compared to the cermet behavior. In dry hydrogen, CGO reduces relatively rapidly via a series of intermediate phases, while NiO reduces via a single-step process. In wet reducing atmospheres, however, the oxidation state of pure CGO is initially stabilized due to the dissociation of water by reduced Ce(III) and subsequent incorporation of oxygen into the structure. In the reduction process involving the composite cermet, the close proximity of the NiO improves the efficiency and speed of the composite reduction process. Although NiO is already incorporated into working cells, these observations suggest direct routes to further improve cell performance.

Electronic structures and magnetism of SrFeO2 under pressure: a first-principles study

We have studied the electronic structures and magnetism of SrFeO2 under pressure by first-principles calculations in the framework of density functional theory (DFT) with GGA+U and HSE06 hybrid functionals, respectively. The pressure-induced spin transition from S = 2 to S = 1 and the antiferromagnetic-ferromagnetic (AFM-FM) transition observed in experiment are well reproduced by taking the site repulsion U and its pressure dependence into account. The electronic structure and its change with the pressure can be qualitatively understood in an ionic model together with the hybridization effects between the Fe 3d and O 2p states. It is found that the pressure leads to a change in Fe 3d electronic configuration from (d(z(2)))(2)(d(xz)d(yz))(2)(d(xy))(1)(d(x(2)-y(2)))(1) under ambient conditions to (d(z(2)))(2)(d(xz)d(yz))(3)(d(xy))(1)(d(x(2)-y(2)))(0) at high pressure. As a result, the spin state transits from S = 2 to S = 1 and both the antiferromagnetic intralayer Fe-O-Fe superexchange interaction and the interlayer Fe-Fe direction exchange coupling at ambient pressure become ferromagnetic at high pressure according to the Goodenough-Kanamori (G-K) rules. Additionally, our calculations predict another spin transition from S = 1 to S = 0 at pressures of about 220 GPa.

Insulator-metal transition in highly compressed NiO

The insulator-metal transition was observed experimentally in nickel monoxide (NiO) at very high pressures of ~240 GPa. The sample resistance becomes measurable at about 130 GPa and decreases substantially with the pressure increase to ~240 GPa. A sharp drop in resistance by about 3 orders of magnitude has been observed at ~240 GPa with a concomitant change of the resistance type from semiconducting to metallic. This is the first experimental observation of an insulator-metal transition in NiO, which was anticipated by Mott decades ago. From simple multielectron consideration, the metallic phase of NiO forms when the effective Hubbard energy U(eff) is almost equal to the estimated full bandwidth 2W.

Growth mechanism and magnon excitation in NiO nanowalls

The nanosized effects of short-range multimagnon excitation behavior and short-circuit diffusion in NiO nanowalls synthesized using the Ni grid thermal treatment method were observed. The energy dispersive spectroscopy mapping technique was used to characterize the growth mechanism, and confocal Raman scattering was used to probe the antiferromagnetic exchange energy J2 between next-nearest-neighboring Ni ions in NiO nanowalls at various growth temperatures below the Neel temperature. This study shows that short spin correlation leads to an exponential dependence of the growth temperatures and the existence of nickel vacancies during the magnon excitation. Four-magnon configurations were determined from the scattering factor, revealing a lowest state and monotonic change with the growth temperature.PACS: 75.47.Lx; 61.82.Rx; 75.50.Tt; 74.25.nd; 72.10.Di.

Ultrafast manipulation of antiferromagnetism of NiO

Photoexcitation of antiferromagnetic NiO leads to ultrafast reorientation of Ni2+ spins due to change of the magnetic anisotropy. Recovery of the magnetic ground state occurs as coherent oscillation of the antiferromagnetic order parameter between hard- and easy-axis states manifesting itself as quantum beating. The coherence time is approximately 1 ns with the beating frequency being determined by the anisotropy energy.

Pressure-tuned collapse of the Mott-like state in Ca(n+1)Ru(n)O(3n+1) (n=1,2): Raman spectroscopic studies

We report a Raman scattering study of the pressure-induced collapse of the Mott-like phases of Ca3Ru2O7 (T(N)=56 K) and Ca2RuO4 (T(N)=110 K). The pressure dependence of the phonon and two-magnon excitations in these materials indicate (i) a T approximately 0 pressure-induced collapse of the antiferromagnetic (AF) insulating phase above P(*) approximately 55 kbar in Ca3Ru2O7 and P(*) approximately 5-10 kbar in Ca2RuO4, (ii) a remarkable insensitivity of the exchange interaction to pressure in both systems, and (iii) evidence for persistent AF correlations above the critical pressure of Ca2RuO4, suggestive of phase separation involving AF insulator and ferromagnetic metal phases.