Room-temperature ferromagnetism in Li-doped $p$-type luminescent ZnO nanorods

We have observed ferromagnetism in Li-doped ZnO nanorods with Curie temperature up to 554 K. Li forms shallow acceptor states in substitutional zinc sites giving rise to $p$-type conductivity. An explicit correlation emerges between increase in hole concentration with decrease in magnetization and Curie temperature in ZnO:Li. Occurrence of ferromagnetism at room temperature has been established with observed magnetic domain formation in ZnO:Li pellets in magnetic force microscopy and prominent ferromagnetic resonance signal in electron paramagnetic resonance spectrum. Magnetic ZnO:Li nanorods are luminescent, showing strong near UV emission. Substitutional Li atoms can induce local moments on neighboring oxygen atoms, which when considered in a correlated model for oxygen orbitals with random potentials introduced by dopant atom could explain the observed ferromagnetism and high Curie temperature in ZnO:Li nanorods.

Figure 1

XRD profile of ZnO:Li powder with 2 and 10% Li concentration showing monophasic hexagonal wurtzite structure.

Figure 2

TEM images of ZnO:Li $\left(2\text{at}\text{.}\%\right)$ at two different magnifications showing well-developed transparent nanorods.

Figure 3

(Color online) Observed magnetic moment with variation in magnetic field for ZnO:Li nanocrystalline powder at room temperature. ZnO:Li $\left(2\text{at}\text{.}\%\right)$ exhibits typical ferromagnetic hysteresis loop. For 5% Li-doped samples, the $M\text{−}H$ curve indicates super paramagnetic behavior. A decrease in magnetization was observed with further increase in Li concentration. Diamagnetic characteristic was shown by undoped ZnO prepared under similar conditions.

Figure 4

(Color online) atomic force microscope (AFM) images of ZnO:Li (a) $2\text{at}\text{.}\%$ and (b) $10\text{at}\text{.}\%$ pellets.

Figure 5

(Color online) Magnetic domains visible in magnetic force microscope (MFM) image $\left(3\mu \times 3\mu \right)$ of ZnO:Li (a) $2\text{at}\text{.}\%$ and (b) $10\text{at}\text{.}\%$ pellets. On the right-hand vertical color scale, pink signifies maximum magnetic force.

Figure 6

EPR spectra of ZnO: Li $\left(2\text{at}\text{.}\%\right)$ sample. The vertical line shows $g$ value of TCNE (tetracyanoethylene) $\left(g=2.0028\right)$ which was used as a standard. The broad spectra centered at $g=2.0034$ observed at room temperature correspond to ferromagnetic resonance (FMR).

Figure 7

(Color online) Photoluminescence (a) excitation spectra showing excitation peak at 244 nm. The inset shows the excitation spectra at 600 nm emission showing a sharp peak due to excitonic transition and (b) emission spectra showing strong peak at 398 nm.