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Ferromagnetic Properties of Y-Doped AlN Nanorods Weiwei Lei,†,‡ Dan Liu,†,§ Xin Chen,† Pinwen Zhu,† Qiliang Cui,*,† and Guangtian Zou† State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China, Max-Planck-Institute of Colloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany, and Department of Earth System Sciences, Yonsei UniVersity, Seoul 120-749, Korea ReceiVed: March 16, 2010; ReVised Manuscript ReceiVed: August 16, 2010
Room-temperature ferromagnetism has been observed in Y-doped AlN (AlN:Y) nanorods. Our first-principle calculations have demonstrated that the ferromagnetism in AlN:Y is from Al vacancies and that the introduction of nonmagnetic rare-earth element Y into AlN can significantly reduce the formation energy of Al vacancy which leads to high Al vacancies responsible for the observed ferromagnetism in AlN:Y nanorods. These findings illustrate an efficient way to reduce the formation energy of cation vacancy by doping nonmagnetic elements, such as Y, leading to ferromagnetism in semiconductors. Introduction
Experimental Section
Dilute magnetic semiconductors (DMSs) are potential candidates for future electronic devices because they combine charges and spins into a single-semiconductor medium.1,2 DMSs are usually produced by doping semiconductors with magnetic transition metals (TMs) such as Mn, Fe, Cr, Co, and Ni.3-6 Although these reports show the appearance of a ferromagnetic phase, the mechanism responsible for the ferromagnetism is still unclear. Moreover, the mismatch and rather limited solubility of magnetic impurities in the host semiconductor have been unsatisfactory. Thus, fabricating new classes of DMSs to avoid these problems are highly desired. As a versatile wide band gap (6.2 eV) functional semiconductor, AlN has been studied extensively for its wide-range applications in flexible pulse-wave sensors, field emitters, piezoelectric devices, etc.7-9 Recently, many efforts have been made to study the synthetic process and magnetic properties of AlN based DMS by doping TMs.10-12 Using ab initio calculations, Wu et al.13 found that Al vacancies in AlN could result in a ferromagnetic ground state with a magnetic moment of 3.0 µB. They further predicted that sufficient Al vacancy concentration is hardly achieved in thermal equilibrium due to its high formation energy. Recently, we reported nonmagnetic rare-earth element Sc doped AlN (AlN:Sc) nanoprisms showing roomtemperature ferromagnetic behavior.14,15 This nonmagnetic element doping induced ferromagnetism deserves further investigation, because it can lead to a better understanding of the origin of ferromagnetism in DMSs and may open up a new class of DMSs without having to dope magnetic elements. In this paper, we show experimentally that it is possible to induce room-temperature ferromagnetic behavior in nonmagnetic rare-earth element Y doped AlN (AlN:Y) nanorods. Our theoretical calculations demonstrate that the origin of the observed ferromagnetism in AlN:Y is from Al vacancies. We also confirm that the doping Y can reduce the formation energy of Al vacancy and result in high Al vacancies in AlN:Y, leading to a large magnetic moment.
The AlN:Y nanorods were synthesized in an improved arc discharge plasma setup.16 Al (purity 99.99%), Y (purity 99.99%), and N2 gas (purity 99.99%) were used as sources. The N2 pressure was 40 kPa. The input current was maintained at 120 A, and the voltage was a little higher than 30 V. After a reaction for 5 min, a large number of white cotton-like particles deposited on the substrate. Phase, structural, and chemical composition analyses of the AlN:Y nanorods were carried out by X-ray diffraction (XRD) with Cu KR radiation, scanning electron microscopy (SEM), transmission electron microcopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). The magnetic properties were measured using a vibrating sample magnetometer (VSM) at room temperature. For the theoretical studies, we employed the all-electron projector augmented wave approach17 within the Perdew-BurkeErnzerh of parametrization of a generalized gradient approximation (GGA), as implemented in the Vienna ab initio simulation package code.18 A 3 × 3 × 2 AlN supercell containing 72 atoms was chosen for the calculations. The plane-wave kinetic energy cutoff was set at 520 eV. The 4 × 4 × 3 k-point meshes for the Brillouin zone sampling were constructed using the MonkhorstPack scheme.19 The geometries of the supercells with and without Y doping were fully optimized without using any symmetry constraint. For relaxed structures, the atomic forces were less than 0.02 eV/ Å.
* Corresponding author. E-mail:
[email protected]. † Jilin University. ‡ Max-Planck-Institute of Colloids and Interfaces. § Yonsei University.
Results and Discussion Figure 1a displays the powder XRD patterns of the AlN:Y nanorods and undoped AlN nanoparticles. The peaks can be readily indexed to wurtzite-structure (space group: P63mc (186)) AlN (JCPDS file No. 08-0262), showing only a single phase. No peaks corresponding to metallic Al or aluminum yttrium alloy could be found in the XRD patterns. All peaks of AlN:Y nanorods show a clear shift toward lower angles relative to those of the pure AlN nanoparticles, suggesting an increase of lattice constants. The increase of lattice constants after doping Y is due to the substitution of Al3+ (0.51 Å) by larger Y3+ (0.89 Å), which agrees well with our calculations and previous reports.20,21 The chemical composition of the nanorods was determined by EDS spectrum (Figure 1b). Only peaks of the elements Al, Y, and N are present in the EDS spectrum. The spectrum reveals
10.1021/jp102375e 2010 American Chemical Society Published on Web 08/30/2010
Ferromagnetic Properties of Y-Doped AlN Nanorods
J. Phys. Chem. C, Vol. 114, No. 37, 2010 15575
Figure 1. (a) XRD pattern of AlN:Y nanorods and undoped AlN nanoparticles. (b) EDS spectrum of the as-synthesized AlN:Y nanorods.
Figure 2. (a) Low magnification SEM image of the as-synthesized AlN:Y nanorods. (b) A high magnification SEM image of typical AlN:Y nanorods with chain-like structure. (c) TEM image of the AlN:Y nanorods. (d) HRTEM image of a AlN:Y nanorod. Inset presents its corresponding FFT image.
Figure 3. Magnetization hysteresis loops of the AlN:Y nanorods measured at room temperature.
that the doping of Y ions in AlN corresponds to a concentration close to 0.96%.
A characteristic SEM image of the as-synthesized AlN:Y (Figure 2a) shows a large amount of nanorods forming a network, homogeneously distributed over a large area. A typical chain-like structure is shown in Figure 2b. The AlN:Y nanorods have a predominantly hexagonal prism-like structure and that they tend to adhere to one another due to static charges,22 forming chains by self-assembly. Such static electricity interactions may alter the long-range interactions between nanorods. The nanorods were 150-200 nm in length and 100-150 nm in diameter. The low magnification TEM image of the AlN:Y nanorods is shown in Figure 2c. It clearly reveals the nanorods form chains by approaching each other. Figure 2d shows the HRTEM image obtained from one of these nanorods with the inset showing the fast Fourier transform (FFT). No secondary phase (nitride) or clustering of Al or Y in the nanorods is detected. FFT confirms the wurtzite structure of the nanorods. The spacing of 0.255 nm measured from the lattice fringe corresponds to the d-spacing of the (0002) plane, suggesting a growth direction along the [0001] of the nanorod.
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Figure 4. (a) Top view of the 72-atom AlN supercell containing a Y atom; (b) top view of the 72-atom AlN supercell containing an Al vacancy; and (c) top view of the 72-atom AlN supercell containing an Al vacancy plus a Y atom.
The magnetization versus magnetic field curve of the AlN:Y nanorods measured at room temperature shows a hysteresis loop, revealing its ferromagnetic behavior, as shown in Figure 3. The saturation magnetization of the AlN:Y nanorods is 0.05 emu g-1, and its coercive field is about 101 Oe. The temperature dependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetization for the as-prepared AlN:Y nanorods is measured (Figure S1). The ZFC and FC curves separate from each other in the whole temperature range of 2-350 K, and no indication of order-disorder phase transition is observed, which further confirm the ferromagnetism of the AlN:Y nanorods and the Currie temperature higher than 350 K. For the ZFC curve, an observable feature around 55 K is due to the magnetic transition in oxygen adsorbed on the surface of samples during cooling.23 It is noted that small increase in the FC magnetization with decreasing temperature indicate strong interparticle coupling.24 It is confirmed by our SEM results (Figure 2), the AlN:Y nanorods form chains by static electricity interactions to create anisotropic coupling. To fully understand the mechanism of the ferromagnetism in AlN:Y nanorods, we calculated the magnetic properties of three supercell systems: a) with an Al atom replaced by a Y atom (Al35YN36), b) with an Al vacancy created by removing one Al atom (Al35N36), and c) with an Al vacancy plus one substituting Y atom (Al34YN36). In the first-principle calculations, using the atomic models illustrated in Figure 4, the calculated lattice constants, a and c, of wurtzite AlN unit cell are 3.128 and 5.016 Å, respectively, in good agreement with the experimental values. On the basis of this unit cell, we constructed a 3 × 3 × 2 supercell containing 72 atoms, which was sufficient for studying ferromagnetism.25,26 For the first system (Al35YN36), there is some increase in the lattice constants, about 0.021 Å for a and 0.020 Å for c after structural optimization, which can be attributed to the fact that the ionic radius of Y is larger than that of Al.25,26 This result corresponds well with our XRD patterns. The calculated total magnetic moment is 0 µB. For the second (Al35N36), the calculated total magnetic moment is 3.0 µB. (This value is in good agreement with the result of a similar calculation by Wu.13) For the last one (Al34YN36), the resulting total magnetic moment is also 3.0 µB. The spin-resolved densities of states (DOS) of the three systems as well as that of bulk AlN are shown in Figure 5. For bulk AlN (Al35N36) (Figure 5a) and Al35YN36 (Figure 5b), the DOS of the majority spins and minority spins are both symmetric, and there is no resultant spin polarization. However, the DOS of the majority spins and minority spins of Al35N36 and Al34YN36 are changed dramatically near the Fermi level, as shown in Figure 5, panels c and d, respectively. It is clear that some localized unoccupied bands appear above the Fermi level where there were no majority spin states, which indicates that the Al vacancy induced a high spin polarization with a magnetic moment of 3.0 µB. No obvious magnetic moments
Figure 5. Spin-resolved density of states of (a) the 72-atom bulk AlN supercell, (b) the 72-atom AlN supercell with one Al atom substituted by a Y atom, (c) the 72-atom AlN supercell containing an Al vacancy, and (d) the 72-atom AlN supercell containing an Al vacancy plus a Y atom. Fermi level is set to zero. Positive (negative) values correspond to the majority (minority) spin.
are found from other atoms. Hence, we believe that the doping of Y atoms in AlN can significantly reduce the formation energy of Al vacancies resulting in high Al vacancies. To confirm our hypothesis, we calculated the formation energies of two systems: Al35N36 and Al34YN36. The defect formation energy in neutral state is defined as13 tot Ef ) Edefect - Etot 0 + n-µ- - n+µ+
(1)
tot where Edefect is the total energy of the supercell containing the defect, E0totis the total energy of the host supercell, n- and n+ are the number of Al atoms substituted by the defect and the number of defect atoms introduced to the supercell, respectively, and µ- and µ+ are the corresponding chemical potentials. For Al35N36, n- ) nAl ) 1 and n+ ) 0, we calculated the formation energy of an Al vacancy (under N-rich condition) to be 6.405 eV. (This result is very similar to that calculated by Wu13). This formation energy is so high that the formation of sufficient Al vacancies is difficult to achieve at thermal equilibrium.13 However, for Al34YN36, n- ) nAl ) 2 and n+ ) nSc ) 1, the calculated formation energy is 4.876 eV under N-rich condition, which is much smaller than that of Al35N36. These findings are
Ferromagnetic Properties of Y-Doped AlN Nanorods in good agreement with our experimental observations, and further corroborate our hypothesis that ferromagnetism in this type of systems comes from inducing high Al vacancies by doping Y atoms. Conclusions In conclusion, we have experimentally shown that AlN:Y nanorods exhibit room-temperature ferromagnetism. The results of our calculations confirm that the formation energy of Al vacancy is significantly reduced due to the doping of Y. The small formation energy of Al vacancy can induce high Al vacancy concentration which results in ferromagnetism. All results indicate that the doping of nonmagnetic element, such as Y, in semiconductors can induce magnetic properties, and that it is a viable method for developing a novel class of DMSs. Acknowledgment. The authors are grateful to Prof. Yanming Ma and Mr. Keh-Jim Dunn for many useful discussions. This work was supported by Natural Science Foundation of China (No. 50772043), The Graduate Innovative Fund of Jilin University (20092003, 20091011, and MS20080217), and National Basic Research Program of China (Nos. 2005CB724400 and 2001CB711201). Supporting Information Available: Temperature dependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetization of nanorods. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sharma, P.; Gupta, A.; Rao, K. V.; Owens, F. J.; Sharmaa, R.; Ahuja, R.; Guillen, J. M. O.; Johansson, B.; Gehring, G. A. Nat. Mater. 2003, 2, 673. (2) Ohno, H. Science 1998, 281, 951.
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