Room-temperature ferromagnetism in undoped $\mathrm{GaN}$ and $\mathrm{CdS}$ semiconductor nanoparticles

Room-temperature ferromagnetism has been observed in undoped $\mathrm{GaN}$ and $\mathrm{CdS}$ semiconductor nanoparticles of different sizes with the average diameter in the range $10–25\mathrm{nm}$. These nanoparticles were synthesized by simple routes and thoroughly characterized by various techniques. Magnetization measurements at room temperature show that these nanoparticles are ferromagnetic with a saturation magnetization of $\sim {10}^{-3}\mathrm{emu}∕\mathrm{gm}$, which is comparable to that observed in nanoparticles of nonmagnetic oxides. On the other hand, agglomerated particles of $\mathrm{GaN}$ and $\mathrm{CdS}$ exhibit diamagnetism or ferromagnetism with small saturation moments. Furthermore, the saturation magnetic moment decreases with the increase in particles size, suggesting that ferromagnetism is due to the defects confined to the surface of the nanoparticles, while the core of the particles remains diamagnetic. The observation of ferromagnetism is consistent with the prediction that Ga vacancies in $\mathrm{GaN}$ give rise to a ferromagnetic ground state.

Figure 1

(Color online) (a) XRD pattern of $\mathrm{GaN}$ nanoparticles annealed at different temperatures, $850°\mathrm{C}$, $950°\mathrm{C}$, and the bulk indicates the agglomerated particles (for details see text). (b) XRD pattern of $\mathrm{CdS}$ nanoparticles synthesized with different mercaptoethanol concentrations: (a) $30\mathrm{mM}$ as prepared, (b) $30\mathrm{mM}$, (c) $15\mathrm{mM}$, (d) without mercaptoethanol. The samples (b), (c), and (d) were annealed at $150°\mathrm{C}$ in ${\mathrm{N}}_2$ atmosphere.

Figure 2

(Color online) (a) and (b) are the FESEM image of $\mathrm{GaN}$ nanoparticles annealed at $850°\mathrm{C}$ and $950°\mathrm{C}$, respectively. (c) FESEM image of the compressed particles (bar) annealed at $950°\mathrm{C}$. (d) Room-temperature magnetization versus field curves for the three samples. The nanoparticles show ferromagnetism (magnetization corrected for the core diamagnetism), while the bulk exhibits diamgnetism.

Figure 3

(Color online) (a) Langevin fit to the magnetization data for $\mathrm{GaN}$ particles annealed at $850°\mathrm{C}$ and $950°\mathrm{C}$. The lines are fit to the data. (b) EPR spectrum measured at $300\mathrm{K}$ for $\mathrm{GaN}$ particles annealed at $850°\mathrm{C}$ and bulk sample. The $850°\mathrm{C}$ annealed sample shows EPR signal with $g=2.0047$, while no signal is observed for bulk sample.

Figure 4

(Color online) (a) TEM image of $\mathrm{CdS}$ nanoparticles of $15\mathrm{mM}$ mercaptoethanol capped, inset is the electron diffraction (ED) pattern of single particle and (b) TEM image of particles prepared without capping, showing agglomeration of particles. (c) UV absorption spectrum for $\mathrm{CdS}$ nanoparticles, the shift in the peak position is in agreement with particle size variation. (d) Room-temperature magnetization data for the various nanoparticles showing the highest saturation moment of $4\times {10}^{-3}\mathrm{emu}∕\mathrm{g}$ for the $30\mathrm{mM}$ sample.