Structure, magnetic and optical properties, and Hall effect of Co- and Fe-doped SnO
2
films
Hyun-Suk Kim, Lei Bi, G. F. Dionne,* and C. A. Ross
†
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
Han-Jong Paik
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,
Daejeon, 305-701, Republic of Korea
Received 14 March 2008; revised manuscript received 31 May 2008; published 26 June 2008
Magnetically doped SnO
2
is a promising dilute magnetic semiconductor and may also be applicable in a
variety of magneto-optical applications. Epitaxial films of Sn
1-x
Co
x
O
2
x 0.2 and Sn
1-x
Fe
x
O
2
x 0.4 were
grown by pulsed-laser deposition on R-plane Al
2
O
3
substrates. Structural, magnetic, and magnetotransport
measurements consistently point to a source of magnetism within the host lattice rather than from an impurity
phase. The films are strained and their magnetic anisotropy is consistent with the presence of substantial
amounts of magnetoelastic high-spin Fe
2+
, or high- or low-spin Co
2+
. Sn
0.9
Co
0.1
O
2
films have a reasonably
high Faraday rotation of 570°/cm, and the refractive index n and extinction coefficient k at 1550 nm wave-
length are 1.957 and 0.0102, respectively.
DOI: 10.1103/PhysRevB.77.214436 PACS numbers: 78.20.Ls, 75.80.+q, 75.50.Pp, 85.70.Sq
I. INTRODUCTION
Dilute magnetic semiconductors DMS, in which ferro-
magnetic behavior above room temperature is induced in
nonmagnetic semiconductors by doping with magnetic
transition-metal TM ions, are of great interest as potential
semiconductor-compatible magnetic components for practi-
cal spintronics applications.
1–3
Another promising applica-
tion of DMS is to utilize the Zeeman effect of the localized
magnetic moments of the transition-metal ions in high con-
centrations to create Faraday rotation of linearly polarized
light. For example, paramagnetic Cd
1-x
Mn
x
Te x
=0.2–0.24 is a good candidate for magneto-optical devices
operating in the wavelength range of 600–800 nm.
4
Re-
cently, ferromagnetism above room temperature has been re-
ported in SnO
2
heavily doped with Co or Fe.
5,6
Ogale et al.
5
described Sn
1-x
Co
x
O
2
x 0.3, which exhibits ferromag-
netism with a Curie temperature T
C
as high as 650 K, and
with a magnetic moment reported as 7.5
B
/ Co. Subse-
quently, Coey et al.
6
reported room-temperature ferromag-
netism T
C
610 K in Sn
0.86
Fe
0.14
O
2
with a moment of
1.8
B
/ Fe. Because SnO
2
is an attractive optical material
with a large optical band gap 3.6 eV,
7
DMS based on
SnO
2
could be useful for a variety of applications requiring
combined magnetic and optical functionality. However, there
has been no report of the magneto-optical properties of TM-
doped SnO
2
at communication wavelengths. Furthermore,
select TM ions in oxide hosts can introduce large magneto-
elastic effects through spin-orbit-lattice interactions in an oc-
tahedral crystal field.
8–10
In this study, we report on the structural, magnetic, and
magneto-optical properties of epitaxial Sn
1-x
Co
x
O
2
x
0.2 and Sn
1-x
Fe
x
O
2
x 0.4 films fabricated by pulsed-
laser deposition. Their magneto-optical properties were char-
acterized using Faraday rotation and optical absorption at
1550 nm wavelength. The origin of the correlated magnetic,
optical, and magneto-optical properties will be discussed.
II. EXPERIMENT METHODS
The Sn
1-x
Co
x
O
2
x =0, 0.02, 0.04, 0.1, 0.2, and 0.3 and
Sn
1-x
Fe
x
O
2
films x =0, 0.1, 0.2, 0.3, and 0.4 were grown by
pulsed-laser deposition PLD using ceramic targets. The tar-
gets were prepared using standard solid-state reaction tech-
niques. Prescribed amounts of SnO
2
, Fe
2
O
3
, and CoO pow-
ders were mixed and pressed into disks with a diameter of 1
in, which were sintered at 1150 °C for 24 h in air. The sin-
tered targets were ablated by a KrF excimer laser Lambda
Physik LPX200 operating at a wavelength of 248 nm and 10
Hz. The substrate temperature, oxygen pressure, and laser
energy density were 700 °C, 2 10
-6
Torr, and 1.5 J / cm
2
,
respectively. The films were cooled in the same oxygen pres-
sure as used during deposition at a rate of 10 °C / min.
A typical thickness of the films was 500 nm, measured by
Tencor P-10 profilometer. In order to investigate the struc-
ture, both two-dimensional 2DRef. 11 and conventional
one-dimensional 1D x-ray diffraction XRD measure-
ments were carried out using a Bruker AXS D8 DISCOVER
with GADDS General Area Detector Diffraction System
and a RIGAKU RU300, respectively. The chemical compo-
sition of the films was characterized by wavelength disper-
sive spectroscopy WDS. This showed that the Co and Fe
contents in the films, given above, are about 1.6 times higher
than those in the targets, which is attributed to evaporative
loss of Sn.
5,6
X-ray photoelectron spectroscopy XPS was
used to analyze the elemental valence states in Sn
1-x
Co
x
O
2
and Sn
1-x
Fe
x
O
2
films. The microstructure and roughness of
the TM-doped SnO
2
films were investigated by high-
resolution transmission electron microscopy HRTEM and
atomic force microscopy AFM. The magnetic properties
were measured using a quantum design SQUID magnetome-
ter and an ADE vibrating sample magnetometer VSM.
Electrical resistivity, carrier concentration, and Hall-effect
measurements were carried out using van der Pauw geom-
etry. The magneto-optical behavior was investigated by mea-
suring the Faraday rotation of 1550 nm wavelength light
passing perpendicular to the film plane with the applied field
PHYSICAL REVIEW B 77, 214436 2008
1098-0121/2008/7721/2144367 ©2008 The American Physical Society 214436-1