Pingyun Li

Size-dependent ordering and Curie temperatures of FePt nanoparticles

The analytic models for size-dependent ordering and Curie temperatures of FePt nanoparticles have been proposed in terms of the size-dependent melting temperature. It is found that the order-disorder transition temperature TO and Curie temperature TC decrease with decreasing the particle size D, and the drop becomes dramatic once the size decreases to about 3 and 6 nm below for TO and TC, respectively. Moreover, the suppression in TC(D) is nearly twice as large as that in TO(D) when D is in the range of 5–20 nm. The accuracy of the developed model is verified by the recent experimental and computer simulation results.

Thickness and grain size dependent mechanical properties of Cu films studied by nanoindentation tests

The hardness and the elastic modulus of Cu films with thickness (t) and grain size (d) have been investigated by nanoindentation tests. The d and the indentation depth increase linearly with the increase in t. The hardness rises with the decrease in t, whereas the elastic modulus is independent of t and it is about 20% less than conventional coarse-grained Cu. The enhanced hardness is attributed to the smaller d and the indentation depth. The analysis of load–displacement curves indicates that the scope of the critical shear stress for different thick Cu films ranges from 3.2 to 4.1 GPa, which is similar to the theoretical shear stress of single crystalline Cu. The present results are explained by the dislocation mediated mechanism even if d reaches about 16.4 nm for the Cu film with t = 180 nm.

Size-dependent Curie transition of Ni nanocrystals

The mechanical spectroscopy and magnetization measurements are performed on Ni nanocrystals from room temperature to 650 K. It is found that the peak temperatures of internal friction are in agreement with the corresponding Curie temperatures of Ni nanocrystals obtained from the magnetization-temperature curves, showing that the traditional mechanical spectroscopy can also be employed to investigate the Curie transition of ferromagnetic nanocrystals. Moreover, the analytical model for size-dependent Curie temperature is proposed in terms of a size-dependent melting temperature model. The Curie temperature drops with decreasing grain size in Ni nanocrystals, which agrees with the corresponding experimental results.

Facile synthesis of superparamagnetic Ni–Fe ultrafine nanoalloy nanoparticles with equilibrium ordered phase structure via a sol–gel process

Ni-Fe nanoalloy nanoparticles with an average grain size of 4 nm in diameter have been prepared by a sol-gel method under a hydrogen atmosphere where ethanol and oleic acid have been used as solvent and surfactant, respectively. X-ray diffraction (XRD) and selected area electron diffraction (SAED) examinations of the nanoparticles show the occurrence of (111), (200), (220) and (311) diffraction peaks and rings, meaning that the nanoparticles have a face-centered-cubic phase structure. Moreover, a superlattice diffraction peak and a diffraction ring/spot can also be observed in XRD and SAED results, indicating the formation of an equilibrium ordered L1(2) phase structure. The as-prepared Ni-Fe nanoalloy particles show typical superparamagnetic behavior at room temperature and the blocking temperature of the nanoparticles is determined to be about 50 K.

Rolling deformation induced reduction of rate sensitivity and enhancement of hardness in nanocrystalline NiFe alloys

Both hardness (H) and rate sensitivity (m) of nanocrystalline NiFe alloys were studied by nanoindentation testing. It was found that H increases, and m decreases after rolling in the alloys. It is interesting that the decrease in m by rolling is totally contrary to the conventional coarse grain alloys. The dislocation density is remarkably enhanced by rolling deformation, which leads to the hardening behaviour of the samples. The dislocation absorbed at the grain boundary (GB) and/or sub-GB and grain growth by rolling are responsible for the reduced m of the rolled alloys.

Chemical ordering phase transitions in Ni–Fe nanoalloys

The chemical ordering phase transitions in Ni75Fe25 and Ni70Fe30 nanoalloys are investigated by differential scanning calorimetry (DSC), mechanical spectroscopy (MS), vibrating sample magnetometer (VSM) measurements and thermodynamical calculation. An internal friction peak occurs at 646 K in the Ni75Fe25 nanoalloy with an average grain size of 23 nm diameter during MS measurement. An exothermic peak appears during the DSC tests of nanoalloys. Associated with the results of thermodynamical prediction and VSM measurements, both the exothermic peak and the internal friction peak are convinced to be originated from chemical ordering phase transition. Compared with inefficacy of electron diffraction and x-ray diffraction, it is an effective route of employing DSC, MS, VSM and thermodynamical prediction in investigating the chemical ordering phase transitions in Ni–Fe nanoalloys.