Zhaolong Yang

Ferromagnetism in freestanding MoS2 nanosheets

Freestanding MoS2 nanosheets with different sizes were prepared through a simple exfoliated method by tuning the ultrasonic time in the organic solvent. Magnetic measurement results reveal the clear room-temperature ferromagnetism for all the MoS2 nanosheets, in contrast to the pristine MoS2 in its bulk form which shows diamagnetism only. Furthermore, results indicate that the saturation magnetizations of the nanosheets increase as the size decreases. Combining the X-ray photoelectron spectroscopy, transmission electron microscopy, and electron spin resonance results, it is suggested that the observed magnetization is related to the presence of edge spins on the edges of the nanosheets. These MoS2 nanosheets may find applications in nanodevices and spintronics by controlling the edge structures.

Singly-charged oxygen vacancy-induced ferromagnetism in mechanically milled SnO2 powders

The possibility of inducing long-range ferromagnetic order with non-transition metal ions has become a very exciting challenge in recent years. In order to elucidate the room temperature ferromagnetism of SnO2 powders, the magnetic properties of SnO2 powders that have been mechanically milled and subsequently annealed have been investigated. The results indicate that saturation magnetization of the samples increases with milling time, where a high saturation magnetization of 0.0012 emu g−1 can be obtained for the sample milled for 20 h, and saturation magnetization for the sample decreases gradually after annealing in air. Electron spin resonance results show large numbers of singly-charged oxygen vacancies on the surfaces of the SnO2 powders. Combined with X-ray photoelectron spectroscopy and room temperature photoluminescence results, this suggests that the observed ferromagnetism is related to the singly-charged oxygen vacancies.

A series of unexpected ferromagnetic behaviors based on the surface-vacancy state: an insight into NiO nanoparticles with a core–shell structure

Antiferromagnetic nanoparticles as ultimate low-dimensional materials potentially give novel magnetic properties that differ from their bulk form due to strong quantum and surface effects. Herein, we propose that the observed anomalous ferromagnetic behavior of NiO nanoparticles is due to the formation of a ferromagnetic particle shell that is oxygen-vacancy related. A novel self-consistent estimation of the saturation magnetization further confirmed our proposal. The samples, synthesized by a thermal decomposition method, exhibit diversely anomalous ferromagnetic behavior, such as hysteresis curves, large coercivities, exchange bias and spin-glass behavior. A large saturation magnetization of 0.536 emu g−1 exists in the 6 nm NiO sample and it is found to decrease with increasing crystal size. Neither impurity element nor change of valence state has been found in the samples, through X-ray photoelectron, X-ray diffraction or selected-area electron diffraction measurements. Remarkably, a large amount of oxygen vacancies exists, which was verified by X-ray photoelectron spectrum fitting results and Raman spectroscopy. A post-hydrogen-annealing process strongly elevates ferromagnetic ordering, as a result of surface defect enhancement. Moreover, our estimation of the saturation magnetization based on first principle calculation results is in agreement with the experimental conclusion, which reveals the significance of surface states in mediating anomalous magnetic properties in low-dimensional antiferromagnetic materials.

Ferromagnetism in sphalerite and wurtzite CdS nanostructures

Room-temperature ferromagnetism is observed in undoped sphalerite and wurtzite CdS nanostructures which are synthesized by hydrothermal methods. Scanning electron microscopy and transmission electron microscopy results indicate that the sphalerite CdS samples show a spherical-like shape and the wurtzite CdS ones show a flower-like shape, both of which are aggregated by lots of smaller particles. The impurity of the samples has been ruled out by the results of X-ray diffraction, selected-area electron diffraction, and X-ray photoelectron spectroscopy. Magnetization measurements indicate that all the samples exhibit room-temperature ferromagnetism and the saturation magnetization decreases with the increased crystal sizes, revealing that the observed ferromagnetism is defect-related, which is also confirmed by the post-annealing processes. This finding in CdS should be the focus of future electronic and spintronic devices.

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.

Tuning unexpected room temperature ferromagnetism in heteroepitaxial PbTiO3 thin films fabricated by hydrothermal epitaxy: crystal quality

The unexpected room temperature ferromagnetism in well-crystallized lead titanate (PbTiO3) heteroepitaxial thin films is attributed to crystal quality, which is produced by mild hydrothermal epitaxy on strontium titanate (100) substrates. The morphological, structural and magnetic properties of these epitaxial films were examined by a variety of experimental techniques. In the growth process of PbTiO3 films with a perovskite structure, the nucleations appear as islands firstly and subsequent growth follows the layer-by-layer growth mode; simultaneously as increase of growth time the stress between films and substrates releases gradually, the lattices of films follow substrates at first and then the films obey their own lattices; the crystal quality is increasing during this growth process. Meanwhile the results of magnetic measurement reveal that our films have the unambiguous ferromagnetism, and the strength of the ferromagnetic component decreases monotonously as increasing crystal quality. In addition, the growth mechanism involved a dissolution–crystallization mechanism is exposed.

Cu vacancies modulated the room temperature ferromagnetism in Cu2O/Cu nanoparticle composites

The Cu2O/Cu nanoparticle composites have been fabricated by heating a mixture of copper nitrate and different amounts of glycine. Magnetic measurement results indicate that the Cu2O/Cu composites show clear ferromagnetism, while the pure Cu2O and Cu show diamagnetism only. Through analyzing the photoluminescence X-ray photoelectron spectroscopy results, we proposed that the observed tunable ferromagnetism for the samples is attributed to Cu vacancies, where the interface of Cu2O/Cu composites plays an important role in modulating the concentration of Cu vacancies.

Unexpected surface superparamagnetism in antiferromagnetic Cr2O3 nanoparticles

We report an unexpected superparamagnetic behavior of antiferromagnetic Cr2O3 nanoparticles. The Cr2O3 particle cores retain their original antiferromagnetic phase, while the surfaces of the particles become superparamagnetic. The X-ray diffraction results confirm that the sample has a corundum structure without any other phases. Through X-ray photoelectron spectroscopy characterization, the particle surfaces present three different oxidation states: Cr3+ (antiferromagnetic), Cr4+ (ferromagnetic), and Cr6+ (nonferromagnetic). A bimagnetic particle model with Cr3+ cores and higher Cr oxidation surface states is used to explain the experimental results. In addition, we observe that spin-flop transitions occur in the antiferromagnetic cores below the Neel temperature (292 K). The spin-flop transition field is uncommon compared with other research, this novel behavior is attributed to the presence of superparamagnetism in the surfaces through the exchange field. These findings reveal the significance of surface states in mediating the magnetic properties in antiferromagnetic materials.

Large magnetoresistance effect in nitrogen-doped silicon

In this work, we reported a large magnetoresistance effect in silicon by ion implantation of nitrogen atoms. At room temperature, the magnetoresistance of silicon reaches 125 % under magnetic field 1.7 T and voltage bias -80 V. By applying an alternating magnetic field with a frequency (f) of 0.008 Hz, we find that the magnetoresistance of silicon is divided into f and 2f two signal components, which represent the linear and quadratic magnetoresistance effects, respectively. The analysis based on tuning the magnetic field and the voltage bias reveals that electric-field-induced space-charge effect plays an important role to enhance both the linear and quadratic magnetoresistance effects. Observation as well as a comprehensive explanation of large MR in silicon, especially based on semiconductor CMOS implantation technology, will be an important progress towards magnetoelectronic applications.