Ferromagnetism in ZnO

Ferromagnetism in ZnO

002

Zaheer Ahmed Khan, Anshu, Budhi Singh and Subhasis Ghosh School of physical sciences, Jawaharlal Nehru University, New Delhi Email: zaheer.jnu@gmail.com

Abstract

We report the growth of high quality Cu,Ag, Ga and In doped ZnO thin films by magnetron sputtering. High level of Cu, Ga doping has been achieved at higher growth temperature. Room temperature ferromagnetism, with magnetic moment decreasing with increasing Cu and Ag content, has been observed in the thin film, whereas no RTFM has been observed in Ga and In doped ZnO.

Introduction

The possibility of achieving ferromagnetic properties in semiconductor has resulted great

enthusiasm for an emerging field of electronics

with the host

insulating matrix and negates the presence of any secondary phases of either metallic Cu or oxides of Cu. FWHM of (002)

peak of all the films are comparable to thin films grown by pulse laser

deposition (PLD) [4]. The ionic radii of

100

101

102

ZnCuO (10%)

Intensity (a.u.)

ZnCuO (2.5%)

ZnO

30 35 40 45 50

ZnAgO 10%

ZnAgO 5%

002

011 Ag O

ZnAgO 2.5%

2

ZnAgO 1%

ZnAgO 0.5%

100

101

102

110

103

004

ZnAgO 0.1%

20 30 40 50 60 70 80

2

Figure 1 XRD spectra of ZnO:Cu

and ZnO:Ag

known as spintronics,[1] which aim at using both spin and charge of the electron for promising spin based devices. Dietl [2] predicted that transition metal doped ZnO might display Curie temperatures above room temperature. Recently, it is being attempted to dope ZnO with nonmagnetic ions for achieving ZnO based DMS. In view of conflicting

Cu+2 (73 pm) and Zn+2 (74 pm) [5] are almost same, so one would expect XRD peak shift little at higher doping. Figure 1 shows that in addition to the (002) and (004) peaks, there are smaller peaks corresponding to (100), (101), (102), (110) and

(103) orientations, which are due to deviation from c-axis orientation in

results regarding room temperature ferromagnetism (RTFM) in ZnO and stability of observed magnetism, it is highly desirable to study RTFM in ZnO. In this paper, we report on magnetism in ZnO doped with non magnetic impurities.

Experimental details

ZnO thin films were deposited on quartz and silicon substrates using RF magnetron sputtering at substrate temperatures varying from 100oC to 600°C. The target was synthesized by conventional solid state reaction route using

undoped ZnO, but

Normalized Intensity

these peaks

disappear upon doping of Cu. It has been shown that this

enhancement of c axis orientation is due to the moderate quantity of Cu+1

atoms which exist as

10%

4%

2%

1%

0.5%

0.1%

0.01%

32 34 36 38

2

Figure 2 XRD spectra of ZnO:In

commercially available (Aldrich, USA) ZnO, Ag2O, CuO, Ga2O3, and In2O3 powders all (99.999%) pure, which were mixed in stoichiometric ratio, grounded for sixteen hours and sintered at 700 °C for twelve hours. Argon and oxygen were used as sputtering gases. X-ray diffraction (XRD) studies were performed using PANalytical Xpert Pro system. Data was collected for 2 values from 20 to 80 degrees using Cu (K =

1.54 Ã…) operating at 45 KV and 40mA current. Magnetic measurements were performed using a Quantum Design USA, Ever Cool MPMS XL-7 SQUID magnetometer with the thin film samples parallel to applied field at 300K and 10K.

Results and discussions

XRD of the Cu doped ZnO (ZnO:Cu) thin films showed pronounced c-axis orientation resulting in strong (002) peak at 34.4o and (004) peak at 72o respectively corresponding to wurtzite ZnO structure (SG: P63mc)[3], as shown in Figure

1. The absence of any peak corresponding to CuO planes indicate homogenous mixing of dopant atom

interstitials that share the oxygen with Zn atoms improving the c-axis orientation. Hence doping of Cu enhances the crystallinity and c-axis orientation of the host thin films.XRD of the Ag doped ZnO (ZnO:Ag) thin films showed c-axis orientation for doping below 1% resulting in strong (002) peak at 34.4o and (004) peak at 72o, respectively which indicate wurtzite ZnO structure, as shown in Figure

1. The presence of Ag2O (011) peak at around 38 degree in samples with doping concentration of 1% or more indicate secondary phase formation and inhomogeneous mixing of dopant atom with the host insulating matrix. The large ionic radius of Ag+1 (128 pm) as compare to Zn+2 (74 pm) [5]is probably responsible for secondary phase formation. Ag2O phase drives the c-axis oriented thin films to misoriented thin films. Recently it has been shown that c-axis orientation of host ZnO deteriorates and impurity peaks start to appear in ZnO thin films with 2% Ag.

The normalized XRD profile of Ga doped ZnO (GZO) films with different Ga content from

0.01% to 10% grown at 600 oC is (not shown here). All the films show pronounced c-axis orientation resulting in strong 2 (002) peak corresponding to hexagonal wurtzite structure. The absence of Ga2O3 phases in XRD patterns of GZO films marked by arrows in Figure 2 indicate homogeneous mixing of Ga2O3 with ZnO. The normalized XRD profile of In doped ZnO (IZO) thin films grown at 600 0C substrate temperature is shown in Figure 2. The absence of In2O3 phase in the x-ray diffraction pattern of IZO films indicates homogenous mixing of In2O3 with

M 1.73 µB according to relation M= g

µB for g = 2, S = 1/2 [6].Observed magnetic moment value of 1.51µB/Cu atom in our films is quite close to above estimated magnetic moment value for Cu2+ ions, implying that for low doping, Cu ions in our thin films are predominantly in magnetically active Cu2+ state confirmed by XPS studies. Tiwari et al. [6], have observed a saturation magnetic moment of 1.45 µB/Cu atom for 5% ZnO:Cu thin films. The consistent drop in moment per Cu atom of dopant at higher Cu concentration

could be due to either (i) antiferromagnetic

ZnO. XRD

results shows that, in IZO thin films as In content increases the diffraction peak 2 (002)

shifts slightly towards

2

ZnO (Cu 10%)

ZnO (Cu 7.5%)

ZnO (Cu 5%)

ZnO (Cu 2.5%)

Moment ( /Cu)

B

1 ZnO (Cu 1%) ZnO (Cu 0.5%) ZnO (Cu 0.1%) ZnO (Cu 0.05%)

0

-1

2.0

Moment ( /Cu)

B

1.5

1.0

0.5

0

B

0.1 1 10

1.0

Moment ( /Ag)

0.6

0.2

ordering between Cu ions occurring at shorter separation or (ii) breaking of FMO due to doping induced disorder or alloy disorder in case of samples with higher Cu content. The Bohr magnetron value of 1.51B/Cu atom is more than that in PLD grown epitaxial layers [4] .

ZnO

The moment per Ag atom at 300 K decreased with increasing Ag concentration as shown in inset of Figure 3. Observed maximum

Moment (X10-2 emu/gm)

lower -2

Dopant Content (%)

magnetic moment

0.2

diffraction angle. This

behavior indicates that

-10 -5 0 5 10

Field (KO )

e

Figure 1 Room temperature magnetisation of ZnO:Cu samples. Inset show variation of magnetic moment with dopant concentration

value of 1.17µB/Ag atom in our films is less than above estimated magnetic

0.1

0

-0.1

-0.2

the incorporation of In into ZnO increase the c-axis lattice constant and there is a residal stress developed within the IZO films. This may be due to larger ionic radius of In3+ (0.094nm) than that of Zn2+ (0.074 nm).

Magnetic measurements on undoped, ZnO:Cu, ZnO:Ag, ZnO:Ga and ZnO:In films were

moment value for

Ag2+ ions,

implying compared to ZnO:Cu less no of Ag ions are in magnetically active

state. This is not

0.4

ZnO (Ga 1% )

0.2

0

-0.2

-0.4

-5.0 -2.5 0 2.5 5.0

Field (KOe)

Figure 2 MH of ZnO and ZnO:Ga

performed in the temperature range of 10300 K using a SQUID magnetometer. All the measurements were corrected for substrate effects. The magnetization as a function of applied field at 300K for undoped ZnO shows diamagnetic behavior. The magnetization as a function of applied field at 300 K for ZnO:Cu is shown in Figure 3. All the RF sputtered ZnO:Cu and ZnO:Ag samples grown at 600°C show FMO at room temperature. The moment per Cu atom at 300 K decreased with increasing dopant concentration as shown in inset of Figure 3. In order to understand the observed high magnetic moment of the films, it is essential to have an insight into the possible electronic configurations of Cu ions inside ZnO host. Cu atoms in its unionized state have an outer shell electronic configuration of [Ar] 3d104s1 and, hence Cu+ and Cu2+ ions are expected to possess 3d10 and 3d9 configurations, respectively. In 3d10 configuration, all the d electrons are paired and, hence Cu+ ion inside ZnO host does not possess any magnetic moment. On the other hand, in the case of Cu2+ ions (Cu on Zn site) with d9 configuration, one unpaired electron is available. This will give rise to a spin angular momentum of 1/2 which can result in a net magnetic moment of

surprising because size of Ag2+ is higher than that

of Zn2+, whereas size of Cu2+ is very close to that of Zn2+. Magnetisation measurement of ZnO:Ga and ZnO:In samples revealed diamagnetic character as shown in Figure 4.

Conclusions

In conclusion, we have shown that stable room temperature ferromagnetism can be observed in ZnO:Cu and ZnO:Ag thin films grown at higher growth temperature without post growth annealing. It is important to optimize growth conditions and concentration of dopant to have maximum magnetization in ZnO:Cu and ZnO:Ag thin films. It is further observed that the moment per dopant atom decreases with increasing dopant concentration. The vacant d orbital plays an important role in observation of RTFM in Ag and Cu doped ZnO.

References

[1] S. A. Wolf et al., Science 294, 1488 (2001).

1. T. Dietl, Nature Mater. 2, 646 (2003).

2. Z.A.Khan et.al., Appl. Phys. Lett. 99, 42504 (2011)

3. D. Chakraborti,et.al, Appl. Phys. Lett. 90, 6250(2007)

4. CRC Handbook of Chemistry and Physics, David R. Lide (CRC, Boca Raton, 1991).

[6] A. Tiwari, Appl. Phys. Lett. 92,062509 (2008).