Huiqiang Bao

Observation of glassy ferromagnetism in Al-doped 4H-SiC.

Glassy ferromagnetism is observed in diluted magnetic semiconductor Al-doped 4H-SiC. We propose a possible explanation for the origin of ferromagnetism order that is the coeffect of sp(2)/sp(3) configuration along with the structural defects. This result unambiguously demonstrates the existence of intrinsic ferromagnetism order in nonmagnetic sp systems.

Observation of spin-glass behavior in antiperovskite Mn3GaN

The dc magnetization, ac susceptibility, and isothermal remanent magnetization measurements are carried out for antiperovskite Mn3GaN. Strong indication for a frozen state with freeing temperature of Tf at ∼133K is found, determined from the difference between field-cooled and zero-field-cooled magnetizations. The ac susceptibility measurements show a peak around Tf, with the peak position shifting as a function of the driving frequency f and magnetic field H, respectively. Relaxation effects are observed after switching the external magnetic field below Tf. These findings consistently demonstrate that Mn3GaN exhibits a canonical spin-glass state at low temperature.

Magnetic properties of Mn-doped 6H-SiC

We report the synthesis and characterizations of low Mn-doped (<10−3 molar fraction) 6H-SiC. Raman scattering studies show an unusual shift in Raman peak with altering Mn contents. The magnetic properties measurement shows the typical ferromagnetic order was established at as low Mn-doped concentration as 10−4 molar fraction at around 250 K. It is speculated that the defects-related effects other than the Mn content play a more important role to determine the magnetic ordering.

Experimental observation of ferromagnetism evolution in nanostructured semiconductor InN

Recently, undoped semiconductors have been pushed to the frontiers of diluted magnetic semiconductors (DMSs) since no spurious ferromagnetism (FM) signal from metal segregation occurs. However, some aspects still remain untouched in undoped semiconductors nanostructures, including: 1) the evolution of spin moments during the fabrication process; 2) possible intrinsic relationship between the magnetic properties and morphological features, the latter being also dependent on formation process. In this article, we synthesized undoped InN nanostructures and observed a gradual magnetic transition from diamagnetic to FM, and an obvious morphology evolution from tube-shape to wire-shape products with the increase of nitridation time. It is speculated that increasing surface defects such as N vacancies in the morphology transition process are the intrinsic causes for the enhanced FM order with increase of nitridation time. The results provide some useful clues in understanding the true magnetic origin in nanostructured DMSs and reported properties inconsistencies in nanostructured materials.

Structural and magnetic properties of (Al, Fe)-codoped SiC

In this study, as a first attempt, we choose Al and transition metals (TMs), Fe, as codoping atoms to synthesize (Al, TM)-codoped SiC. X-ray diffraction and Raman analysis showed that a series of single-phase codoped 4H-SiC samples were obtained and no trace of any other impurity phases, such as Fe3Si and polytypes of other types of SiC, was found. Measurement of magnetic properties showed that codoping by Al and Fe elements changed the original glassy ferromagnetism (FM) features in Al-doped SiC and induced a robust room temperature FM order that gradually dominated (Al, Fe)-codoped 4H-SiC with increase in Fe content. The contribution of Al elements to the magnetic properties of (Al, Fe)-codoped 4H-SiC can be neglected and the magnetic origin should be ascribed to be induced by Fe doping. The only major role of Al is to stabilize the codoped crystal structure as 4H- single phase. As the first investigation on (Al, TM)-codoped SiC, this study opens a new pathway to obtain TM-doped SiC-based diluted magnetic semiconductors with a high Tc via the codoping strategy.

Temperature dependence of the A1(LO) and E2 (high) phonons in hexagonal InN nanowires

The frequencies and dampings of the zone-center optical phonon modes of A1(LO) (longitudinal-optical) and E2 (high) in wurtzite InN nanowires have been investigated by micro-Raman scattering in the temperature range from 80 to 300 K. Our results reveal that the phonon frequencies decrease and the linewidths broaden with increasing temperature. The obtained experimental data of the frequencies and linewidths at various temperatures can be well described by an empirical model which takes into account the contribution of the thermal expansion of lattice and symmetric decay of phonons into two and three identical phonons with lower energy. The results show that decay into two phonons is the probable channel for the A1(LO) mode and three-phonon decay dominates the E2 (high) mode.

Observation of symmetrically decay of A1(longitudinal optical) mode in free-standing GaN bulk single crystal from Li3N flux method

We report Raman analysis of A1(LO) (longitudinal optical) and E2(high) phonon lifetimes in a bulk GaN single crystal and their temperature dependence from 77 K to 770 K. Both the A1(LO) and E2(high) phonons in GaN were observed decaying primarily into two phonons of equal energy [Klemens model, P. G. Klemens, Phys. Rev. 148, 845 (1966)]. This is a rare example of a high-quality free-standing GaN bulk single crystal displaying abnormal A1(LO) phonon decay. These results will have significant impact on designing and understanding of GaN-based high-speed, high-power electric devices.