ARTICLE IN PRESS
MOKE spectroscopy of sputter-deposited Cu-ferrite films
Š. Višňovskýa,, M. Veisa, E. Liškováa, V. Kolinskýa, Prasanna D. Kulkarnib,
N. Venkataramanib, Shiva Prasadb, R. Krishnanc
a
Faculty of Mathematics and Physics, Institute of Physics, Charles University of Prague, Ke Karlovu 5, 12116 Prague 2, Czech Republic
b
Indian Institute of Technology, Bombay, Mumbai 400 076, India
c
Laboratoire de Magnetisme et d’Optique de Versailles, CNRS, 78935 Versailles, France
Abstract
The copper ferrite CuFe2O4 films deposited at 50 and 200 W RF power on fused quartz substrates and subsequently
heat treated were investigated by polar magneto-optic Kerr effect (MOKE) spectroscopy at photon energies ranging
from 1.2 to 5 eV. CuFe2O4 films can be stabilized in two phases at room temperature depending on deposition
conditions and post-deposition thermal treatment, i.e., (a) cubic with high magnetization and (b) tetragonal with low
magnetization. The differences are due to cation redistribution and Jahn–Teller distortions and manifest themselves in
the MOKE spectra.
PACS: 75.50.Gg; 78.20.L
Keywords: Magneto-optical effects; Thin films sputtered; Ferrites spinel; Spectroscopy
It has been found that sputter-deposited Cu-ferrite
(CuFe2O4) films can be stabilized in two phases at room
temperature depending on deposition conditions and
post-deposition thermal treatment, i.e., (a) cubic with
high magnetization (rapidly cooled) and (b) tetragonal
with low magnetization (slowly cooled) [1,2]. The
differences in the crystallographic symmetry and magnetization are explained by cation redistribution and
Jahn–Teller distortions.
The copper ferrite is conventionally characterized by
the formula (Cu1x2+Fex3+)A [Cux2+Fe2x3+]BO2
4 .
Here A and B denote tetrahedral and octahedral sites,
respectively. The parameter 0oxo1 characterizes the
degree of inversion in the spinel structure, x ¼ 0
corresponds to the normal spinel and x ¼ 1 corresponds
to the inverse spinel. In polycrystalline Cu-ferrite rapidly
cooled from the temperature of 760 1C or higher to the
room temperature, the magnetic moment nB in mB (Bohr
magneton) per formula unit at the temperature T ¼ 0 K
is 2.3 mB ðx 0:84Þ and the symmetry is cubic. A slow
cooling from TX760 1C results in the tetragonal
symmetry with nB ¼ 1:3 mB ðx 0:96Þ: The number of
ferric cations Fe3+ increased and the number of Cu2+
reduced in tetrahedral A sites compared to rapidly
cooled samples [3].
The oxygen tetrahedra and octahedra with central
Fe3+ each contribute in a different way to MOKE
spectra. Below the absorption edge their contributions
are of the opposite sign. The reduction of symmetry
from a cubic to a tetragonal one produces splitting of
triplet electronic states, which should be seen in
magneto-optic Kerr effect (MOKE) spectra. The
MOKE spectroscopy can therefore be employed as a
sensitive tool in the diagnostics of the film preparation
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196
Table 1
Sample parameters
Sample
1
2
RF power
(W)
50
50
Annealing
temperature (1C)
Cooling process
—
650
As-deposited
Rapidly cooled to
80 K
Rapidly cooled to
80 K
Slowly cooled
Slowly cooled
3
50
850
4
5
50
200
850
850
below the absorption edge expected near 2.5 eV and
produced by strong transitions originating from the
ferric ions [8,9]. The structure in the spectra is richer in
the sample rapidly cooled from 650 1C (Fig. 2) with a
weak manifestation of the interference at low photon
energies. Smeared out structures in Figs. 1 and 2 may be
due to residual stresses in the film.
The sample rapidly cooled from 850 1C shows the
features typical for ferrimagnetic spinels [8–11]. It shows
a pronounced structure and high amplitudes in the
MOKE spectra consistent with higher magnetization in
the cubic phase. This sample displays well-developed
interference oscillations below 2.5 eV, which confirm a
good interface quality. We observe that the annealing at
850 1C improves the film crystallinity (Fig. 3).
The importance of high annealing temperatures
is confirmed in slowly cooled samples in Figs. 4 and
5. They display remarkable optical quality and
well-defined MOKE interference at lower photon
energies. The well-resolved structure above 2.5 eV shows
0.04
Kerr effect (deg)
0.03
0.02
0.01
0
-0.01
-0.02
-0.03
rotation
-0.04
ellipticity
-0.05
1
1.5
2.5
2
3
3.5
4
4.5
5
E (eV)
Fig. 1. Magneto-optic polar Kerr rotation and ellipticity
spectra in the as-deposited sample at 50 W of RF power.
0.04
0.03
Kerr effect (deg)
process including cation distribution among A and B
sites. Previous magneto-optic (MO) investigations in the
photon energy range between 0.5 and 2.5 eV of sputterdeposited Cu-ferrite films were performed by Šimša et al.
[4]. Veis et al. studied polar and longitudinal MOKE
azimuth rotation spectra between 2 and 5 eV on sputterdeposited Cu-ferrite films stabilized in the cubic phase
[5]. Kim et al. published MOKE spectra of CuFe2O4
films with thickness of about 0.5 mm deposited on
Al2O3(0 0 1) substrates prepared by a sol–gel method
[6]. The aim of the present work is to extend Ref. [5]. It
compares the MOKE spectra of the films stabilized in
cubic or tetragonal phase, covering a wider spectral
range. The films are characterized by complex polar
MOKE spectra.
The samples were CuFe2O4 films deposited at 50 or
200 W RF power on polished fused quartz substrates.
The film thickness was in the region of 0.1 mm. Five
samples were selected, as-deposited, annealed for 2 h at
temperatures between 650 and 850 1C and subsequently
rapidly quenched to the liquid nitrogen temperature or
slowly cooled to the room temperature. The information
on the sample preparation including RF power,
temperature of annealing and subsequent cooling
process are collected in Table 1. The polar MOKE
spectra were measured at photon energies ranging from
1.2 to 5 eV using azimuth modulation and compensation
technique in the applied magnetic field sufficient for the
sample saturation.
The polar MOKE is given as a complex ratio of the
elements of Jones reflection matrix [7] as rps/rpp. The
real and imaginary parts of rps =rpp correspond to
azimuth rotation and ellipticity, respectively. The angle
of light incidence, y; was set to y ¼ 12:21 using a beam
focused on the area of 2 2 mm2. The reduced area
allowed beam sweeping across the film and checking for
the film homogeneity. The polar MOKE signal varied by
5%, approximately, when the beam was moved from the
central part to the edge of the film.
Fig. 1 shows polar MOKE rotation and ellipticity
spectra in the as-deposited sample. The lines in the
spectra are broad and little structure is resolved. No
interference effects are observed at photon energies
0.02
0.01
0
-0.01
-0.02
-0.03
-0.04
1.2
rotation
ellipticity
1.7
2.2
2.7
3.2
3.7
4.2
E (eV)
Fig. 2. Magneto-optic polar Kerr rotation and ellipticity
spectra in the sample deposited at 50 W of RF power,
subsequently annealed at 650 1C and rapidly cooled from
650 1C to the liquid nitrogen temperature.
ARTICLE IN PRESS
197
0.06
0.04
Kerr effect (deg)
0.02
0
-0.02
-0.04
-0.06
-0.08
rotation
ellipticity
-0.1
-0.12
1
1.5
2
2.5
3
3.5
4
4.5
5
E (eV)
Fig. 3. Magneto-optic polar Kerr rotation and ellipticity
spectra in the sample deposited at 50 W of RF power,
subsequently annealed at 850 1C and rapidly cooled from
850 1C to the liquid nitrogen temperature.
0.25
correspondence with spectra on natural faces of bulk Lior Mg-ferrite single crystals [12]. The negative MOKE
rotation peak centered near 4 eV in rapidly cooled
samples of cubic symmetry splits in slowly cooled
samples of tetragonal symmetry.
The present spectra demonstrate that MOKE spectroscopy provides a valuable tool for characterization of
ferrimagnetic films even at submicron thicknesses. In
combination with spectroscopic ellipsometry the observed interference may be used for the precise
determination of the film thickness. In the absence of
MOKE spectra of bulk CuFe2O4, the completely
characterized films would provide a sound basis for a
detailed explanation of the electron structure of this
spinel.
This work was partially supported by Grant Agency
of Czech Republic (202/03/0776) and Grant Agency of
Charles University (314/2004/B-FYZ/MFF).
Kerr effect (deg)
0.2
0.15
References
0.1
0.05
0
-0.05
-0.1
-0.15
rotation
ellipticity
-0.2
1
1.5
2
2.5
3
3.5
E (eV)
4
4.5
5
Fig. 4. Magneto-optic polar Kerr rotation and ellipticity
spectra in the sample deposited at 50 W of RF power,
subsequently annealed at 850 1C and slowly cooled from
850 1C to the room temperature.
0.06
Kerr effect (deg)
0.04
0.02
0
-0.02
-0.04
-0.06
rotation
ellipticity
-0.08
-0.1
1
1.5
2
2.5
3
3.5
4
4.5
5
E (eV)
Fig. 5. Magneto-optic polar Kerr rotation and ellipticity
spectra in the sample deposited at 200 W of RF power,
subsequently annealed at 850 1C and slowly cooled from
850 1C to the room temperature.
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