Daqiang Gao

Ferromagnetism in ultrathin VS2 nanosheets

Two-dimensional nanosheet crystals have potential applications in nano-electronic devices. Prompted by the recently observed ferromagnetism in pristine VX2 (X = S and Se) monolayers by first-principle calculations, we provide the experimental evidence for room temperature ferromagnetism in ultrathin VS2 nanosheets by observing their clear hysteresis and ferromagnetic resonance. The first-principle calculation results indicate that unusual hybridization of the V 3d and S 3p orbitals occurs in the monolayer VS2, and the relative transfer of electrons between the V and S atoms plays an important role in the transition of the spin moments. According to the previous work, the stable ferromagnetism in ultrathin VS2 nanosheets is considered as a competitive effect between through-bond and through-space interactions, where the through-bond interactions dominate. This finding opens a new path to explore the spintronics in pristine two-dimensional nanostructures.

Room temperature ferromagnetism of Cu doped ZnO nanowire arrays

We report the room temperature ferromagnetism of Cu doped ZnO nanowire arrays embedded in anodic aluminum oxide template. The nanowires about 100 nm in diameter were fabricated by electrodepositing Zn2+ and Cu2+ into the template and postoxidation annealing in air atmosphere. Magnetization measurements indicated that the Zn1−xCuxO (x=0.05 and 0.07) nanowires exhibit room temperature ferromagnetism and the Zn0.93Cu0.07O nanowires annealed in vacuum reveals an enhancement of the ferromagnetism. X-ray diffraction pattern, x-ray photoelectron spectroscopy, and Raman spectra measurements showed that all the samples possess a typical wurtzite structure and no other impurity phases were observed.

Intrinsic ferromagnetism in hexagonal boron nitride nanosheets

Understanding the mechanism of ferromagnetism in hexagonal boron nitride nanosheets, which possess only s and p electrons in comparison with normal ferromagnets based on localized d or f electrons, is a current challenge. In this work, we report an experimental finding that the ferromagnetic coupling is an intrinsic property of hexagonal boron nitride nanosheets, which has never been reported before. Moreover, we further confirm it from ab initio calculations. We show that the measured ferromagnetism should be attributed to the localized π states at edges, where the electron-electron interaction plays the role in this ferromagnetic ordering. More importantly, we demonstrate such edge-induced ferromagnetism causes a high Curie temperature well above room temperature. Our systematical work, including experimental measurements and theoretical confirmation, proves that such unusual room temperature ferromagnetism in hexagonal boron nitride nanosheets is edge-dependent, similar to widely reported graphene-based materials. It is believed that this work will open new perspectives for hexagonal boron nitride spintronic devices.

Coexistence of ferromagnetism and superconductivity in YBCO nanoparticles

Nanoparticles of superconducting YBa(2)Cu(3)O(7-δ) were synthesized via a citrate pyrolysis technique. Room temperature ferromagnetism was revealed in the samples by a vibrating sample magnetometer. Electron spin resonance spectra at selected temperatures indicated that there is a transition from the normal to the superconducting state at temperatures below 100 K. The M-T curves with various applied magnetic fields showed that the superconducting transition temperatures are 92 K and 55 K for the air-annealed and the post-annealed samples, respectively. Compared to the air-annealed sample, the saturation magnetization of the sample by reheating the air-annealed one in argon atmosphere is enhanced but its superconductivity is weakened, which implies that the ferromagnetism maybe originates from the surface oxygen defects. By superconducting quantum interference device measurements, we further confirmed the ferromagnetic behavior at high temperatures and interesting upturns in field cooling magnetization curves within the superconducting region are found. We attributed the upturn phenomena to the coexistence of ferromagnetism and superconductivity at low temperatures. Room temperature ferromagnetism of superconducting YBa(2)Cu(3)O(7-δ) nanoparticles has been observed in some previous related studies, but the issue of the coexistence of ferromagnetism and superconductivity within the superconducting region is still unclear. In the present work, it will be addressed in detail. The cooperation phenomena found in the spin-singlet superconductors will help us to understand the nature of superconductivity and ferromagnetism in more depth.

Transforming from paramagnetism to room temperature ferromagnetism in CuO by ball milling

In this work, we experimentally demonstrate that it is possible to induce ferromagnetism in CuO by ball milling without any ferromagnetic dopant. The magnetic measurements indicate that paramagnetic CuO is driven to the ferromagnetic state at room temperature by ball milling gradually. The saturation magnetization of the milled powders is found to increase with expanding the milling time and then decrease by annealing under atmosphere. The fitted X-ray photoelectron spectroscopy results indicate that the observed induction and weaken of the ferromagnetism shows close relationship with the valence charged oxygen vacancies (Cu1+-VO) in CuO.

Origin of the unexpected room temperature ferromagnetism: formation of artificial defects on the surface in NaCl particles

The unexpected room temperature ferromagnetism in pure sodium chloride (NaCl) particles with different crystal size synthesized by breaking at different times is attributed to surface defects, which provides a novel opportunity to further understand the origin of ferromagnetism in the traditional “nonmagnetic” inorganic non-metallic materials. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy suggest that breaking progress does not change the samples body, but drastically reduces the size of the samples, whats more, it is found to enhance the strength of the ferromagnetic component with decreasing the samples size through magnetism measure; the first-principle calculation results confirm the experimental conclusion. Ferromagnetism originates from surface effect, probably the long range ferromagnetic interactions between the surface Cl vacancies.