Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing

In the last decade, transition-metal-doped ZnO has been intensively investigated as a route to room-temperature diluted magnetic semiconductors (DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition, or secondary phases. In order to clarify this question, we have performed a thorough characterization of the structural and magnetic properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal that Co or Ni nanocrystals (NCs) are the major contribution of the measured ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed and they exhibit superparamagnetic properties. The Co or Ni NCs are crystallographically oriented with respect to the ZnO matrix. Their magnetic properties, e.g., the anisotropy and the superparamagnetic blocking temperature, can be tuned by annealing. We discuss the magnetic anisotropy of Ni NCs embedded in ZnO concerning the strain anisotropy.

Figure 1

(Color online) RBS random (ran.) and channeling (ch.) spectra: (a) Co implanted ZnO and (b) Ni-implanted ZnO (the fluence for Co and Ni ions is indicated). The yield of channeling spectra progressively decreases with increasing fluence.

Figure 2

(Color online) SR-XRD $2\theta -\theta$ scan revealing the existence of Co or Ni precipitates in (a) Co-implanted ZnO and (b) Ni-implanted ZnO.

Figure 3

(Color online) XRD pole figure revealing the crystallographical orientation relationship between $\mathrm{Co}∕\mathrm{Ni}$ NCs and ZnO matrix. (a) $\mathrm{Co}\left(10\underset{̱}{1}1\right)$ (in square) together with the tail of $\mathrm{Zn}\mathrm{O}\left(10\underset{̱}{1}2\right)$ (in circle). The data points out of squares and circles are due to the background. (b) Ni(200) (in square) together with the tail of $\mathrm{Zn}\mathrm{O}\left(10\underset{̱}{1}2\right)$ (in circle).

Figure 4

(Color online) Hysteresis loops measured at $5\mathrm{K}$ for Co-implanted ZnO with different fluences (for the sample with the fluence of $0.8\times {10}^{16}{\mathrm{cm}}^{-2}$ only a half loop is shown). Inset: hysteresis loop measured at $300\mathrm{K}$ for the sample with the highest fluence.

Figure 5

(Color online) Magnetization curves at $50\mathrm{Oe}$ after ZFC (open symbol) and FC (solid symbol) for the Co-implanted ZnO. With increasing fluence, the Co NCs is growing in size, resulting in a higher blocking temperature.

Figure 6

(Color online) Hysteresis loops measured with the field along ZnO[0001] (out of plane) and $\left[10\underset{̱}{1}0\right]$ (in plane) for $\mathrm{Co}∕\mathrm{Ni}$-implanted ZnO with the fluence of $8\times {10}^{16}{\mathrm{cm}}^{-2}$ measured at $300\mathrm{K}$. (a) Co-implanted ZnO and (b) Ni-implanted ZnO. (c) and (d) show the schematic geometry for magnetization measurements.

Figure 7

(Color online) RBS random (ran.) and channeling (ch.) spectra for Co-implanted ZnO with a fluence of $4\times {10}^{16}{\mathrm{cm}}^{-2}$ after thermal annealing at different temperatures. The lattice damage induced by implantation is progressively reduced by increasing the annealing temperature.

Figure 8

(Color online) XRD $2\theta \text{−}\theta$ scans: (a) Co-implanted ZnO crystals with different annealing temperatures. The wide range XRD pattern for one of the samples (inset) reveals that no other crystalline phase (e.g., CoO) could be detected. The arrow points to the peak shoulder coming from the fcc-Co(111) diffraction in the sample of $823\mathrm{K}\mathrm{ann.}$ (b) Ni-implanted ZnO crystals with different annealing temperatures. The inset shows a comparison between as-implanted and $823\mathrm{K}$ annealed samples.

Figure 9

(Color online) XRD $\varphi$ scans for $\mathrm{hcp}\text{−}\mathrm{Co}\left(10\underset{̱}{1}1\right)$ ($\theta =23.78°$ and $\chi \sim 61.9°$), fcc-Co(002) ($\theta =25.76°$ and $\chi \sim 54.8°$), and $\mathrm{Zn}\mathrm{O}\left(10\underset{̱}{1}1\right)$ ($\theta =18.13°$ and $\chi \sim 61.6°$) reveal the in-plane orientation relationship for Co NCs with respect to ZnO.

Figure 10

(Color online) (a) $2\theta \text{−}\theta$ scans for $\mathrm{Zn}\mathrm{O}\left(10\underset{̱}{1}1\right)$ and $\mathrm{hcp}\text{−}\mathrm{Co}\left(10\underset{̱}{1}1\right)$; those small sharp peaks are from artificial noise in the measurements. (b) $2\theta \text{−}\theta$ scan for fcc-Co(002).

Figure 11

(Color online) (a) ZFC/FC magnetization curves at $50\mathrm{Oe}$ for the samples after implantation and annealing at different temperatures. Solid symbols are FC curves, while open symbols are ZFC curves. [(b)–(d)] $M\text{−}H$ curves measured at $5\mathrm{K}$ for all samples: along $\mathrm{Zn}\mathrm{O}\left[10\underset{̱}{1}0\right]\parallel \mathrm{hcp}\text{−}\mathrm{Co}\left[10\underset{̱}{1}0\right]\parallel \mathrm{fcc}\text{−}\mathrm{Co}\left[112\right]$ (solid symbols) and $\mathrm{Zn}\mathrm{O}\left[0001\right]\parallel \mathrm{hcp}\text{−}\mathrm{Co}\left[0001\right]\parallel \mathrm{fcc}\text{−}\mathrm{Co}\left[111\right]$ (open symbols). Implantation or annealing temperature is shown.

Figure 12

(Color online) (a) ZFC/FC magnetization curves at $50\mathrm{Oe}$ for the samples after implantation and annealing at different temperatures. Solid symbols are FC curves, while open symbols are ZFC curves. (b) $M\text{−}H$ curves measured at $5\mathrm{K}$ for all samples along $\mathrm{Zn}\mathrm{O}\left[10\underset{̱}{1}0\right]\parallel \mathrm{fcc}\text{−}\mathrm{Ni}\left[112\right]$. [(c) and (d)] $M\text{−}H$ curves measured at $5\mathrm{K}$ for all samples along $\mathrm{Zn}\mathrm{O}\left[10\underset{̱}{1}0\right]\parallel \mathrm{fcc}\text{−}\mathrm{Ni}\left[112\right]$ (solid symbols) and $\mathrm{Zn}\mathrm{O}\left[0001\right]\parallel \mathrm{fcc}\text{−}\mathrm{Co}\left[111\right]$ (open symbols) after 823 and $923\mathrm{K}$ annealing, respectively. Implantation or annealing temperature is shown.

Figure 13

(Color online) XRD $2\theta \text{−}\theta$ scan of Ni(200) and Ni(111).

Figure 14

(Color online) The sample annealed at $923\mathrm{K}$ shows a vertical shift along the magnetization axis after field cooling $(H=2000\mathrm{Oe})$ due to the exchange coupling between FM and AFM materials. The field is along the ZnO[0001] direction. The inset shows the temperature dependence of the vertical shift.