Ferromagnetism in CuO–ZnO multilayers
C. Sudakar,
1,a͒
K. Padmanabhan,
1
R. Naik,
1
G. Lawes,
1
B. J. Kirby,
2
Sanjiv Kumar,
3
and
V. M. Naik
4
1
Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201, USA
2
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg,
Maryland 20899, USA
3
NCCCM, Bhabha Atomic Research Centre, ECIL Post, Hyderabad 500062, India
4
Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, Michigan 48128, USA
͑Received 24 March 2008; accepted 15 June 2008; published online 28 July 2008͒
We investigated the magnetic properties of CuO–ZnO heterostructures to elucidate the origin of the
ferromagnetic signature in Cu doped ZnO. The CuO and ZnO layer thickness were varied from 15
to 150 nm and from 70 to 350 nm, respectively. Rutherford backscattering experiments showed no
significant diffusion of either Cu in ZnO or Zn in CuO layers. Magnetic measurements indicate
ferromagnetism at 300 K, which depends on the CuO particle size, but not on the CuO–ZnO
interfacial area. Polarized neutron reflectometry measurements show that the observed magneti-
zation cannot be accounted for solely by spins localized near the CuO–ZnO interface or in the CuO
layer. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2959186͔
Since the discovery of room temperature ferromagnetism
͑RT-FM͒ in ͑Zn,Co͒O,
1
ZnO has been identified as a prom-
ising host semiconductor material for magnetic applications,
and has since been shown to exhibit RT-FM when doped
with many other transition metal elements, including V, Cr,
Fe, Co, and Ni.
2,3
Remarkably, FM was recently reported in
ZnO doped with nonmagnetic Cu ions.
4,5
However, the ori-
gin of the FM in Cu doped ZnO is unclear,
6
and we have
previously
7
discussed the possible role of CuO planar nano-
clusters in promoting a net moment. This is consistent with
the suggestion that the puzzling FM in certain diluted mag-
netic semiconducting ͑DMS͒ oxides originate from second-
ary phase nanocrystals that are crystallographically coherent
within the host oxide matrix.
8
If the concentration of dopant
ions exceeds the solubility limit, spinodal decomposition
leads to regions with lower and higher densities of magnetic
ions.
8,9
Such mechanisms are believed to be responsible for
Co metal clusters in ͑Zn,Co͒O,
10
the ZnMnO metastable
phase in ͑Zn,Mn͒O,
11
CuO nanoplanar clusters in ͑Zn,Cu͒O,
7
and the Cr rich ͑Zn,Cr͒Te metallic nanocrystals embedded in
a Cr deficient ͑Zn,Cr͒Te matrix.
9
These secondary phases
have traditionally been ruled out as the origin of the FM
moment, as they order antiferromagnetically ͑AFM͒. How-
ever, it has recently been argued that uncompensated spins at
the surface of Zn rich CoO,
12
and Co rich ͑Zn, Co͒O,
13
lead
to FM, and that AFM nanoparticles exhibit clear FM signa-
tures. It is important to clarify that how finite-size and
surface/interface effects in AFM can lead to FM signals in
order to understand the origin of FM in DMS oxides.
In this letter, we report the magnetic properties at the
interface of antiferromagnetic CuO and diamagnetic ZnO
thin layers by systematically analyzing ͑i͒ CuO ͑150 nm͒
on sapphire͑0001͒, ͑ii͓͒ZnO͑350 nm͒ /CuO͑150 nm͒ /
ZnO ͑350 nm͔͒ trilayer on sapphire ͑0001͒, ͑iii͒ and ten lay-
ers of ͓CuO͑ϳ15 nm͒ /ZnO͑ϳ70 nm͔͒ on sapphire ͑0001͒
with the top and bottom layers being ZnO. We find no evi-
dence for diffusion of either Cu into the ZnO layers or of Zn
into the CuO layers using Rutherford back scattering ͑RBS͒.
The FM magnetization in all samples ranged from
2to5kA/m of CuO. The surface disordered state of the
CuO nanoparticles seems to play a major role in determining
the FM moment of the samples. Our polarized neutron re-
flectometery ͑PNR͒ measurements suggest that the FM ap-
pears to be distributed over a larger region than just the
ZnO / CuO interfaces or even just the CuO layers.
We deposited the CuO and CuO–ZnO thin film hetero-
structures by rf reactive sputtering on ͑0001͒ sapphire
͑
␣
-Al
2
O
3
͒ single crystal substrates. Highly pure Zn ͑99.99%͒
and Cu ͑99.999%͒ targets were used as the sputtering sources
for ZnO and CuO, respectively. The samples were sputter
deposited in high purity Ar+O
2
atmosphere at a total pres-
sure of 1 mTorr with an optimized oxygen partial pressure of
0.2 mTorr. The first sample we studied was a 150 nm thick
pure CuO film. The second sample was prepared by first
depositing a 350 nm thick ZnO film on the sapphire sub-
strate, then depositing a 150 nm thick CuO layer on top of
this initial film, then finally covering this CuO layer with
another 350 nm thick ZnO layer. This three layer
ZnO / CuO / ZnO structure will be referred as ZCZ in the fol-
lowing discussion. The third sample was prepared by depos-
iting a ϳ70 nm thick ZnO layer on sapphire followed by a
ϳ15 nm thick CuO layer. This ZnO͑70 nm͒ / CuO͑15 nm͒
stack was repeated ten times, followed by a final deposition
of a 70 nm thick ZnO layer to prepare the multilayer struc-
ture. This sample is referred as ͓ZCZ͔
10
in the following text.
All samples were deposited at 300 K and annealed at 773 K
in air after deposition. These three sample geometries were
chosen to control the area of the CuO–ZnO interface, while
maintaining a fixed amount of CuO and ZnO ͑for ZCZ and
͓ZCZ͔
10
͒ in the different multilayers. The interface between
the ZnO and CuO layers is increased by an order of magni-
tude in the ͓ZCZ͔
10
compared to ZCZ.
Figure 1͑a͒ shows the x-ray diffraction ͑XRD͒ spectra of
the CuO thin film, ZCZ and ͓ZCZ͔
10
multilayer heterostruc-
tures. The CuO ͑150 nm͒ thin films on sapphire are highly
textured with ͑111
¯
͒ planes. Bulk CuO crystallizes in the
monoclinic space group C2 / c.
14
The structure consists of
CuO ladders stacked along ͓110͔ and ͓1
¯
10͔ that intersect
a͒
Author to whom correspondence should be addressed. Electronic mail:
csudakar@gmail.com.
APPLIED PHYSICS LETTERS 93, 042502 ͑2008͒
0003-6951/2008/93͑4͒/042502/3/$23.00 © 2008 American Institute of Physics93, 042502-1