Appl Phys A
DOI 10.1007/s00339-011-6549-z
Optical and electronic properties of NiFe
2
O
4
and CoFe
2
O
4
thin
films
R.C. Rai · S. Wilser · M. Guminiak · B. Cai ·
M.L. Nakarmi
Received: 17 July 2011 / Accepted: 20 July 2011
© Springer-Verlag 2011
Abstract We report the optical and electronic properties of
the inverse spinel ferrite NiFe
2
O
4
and CoFe
2
O
4
thin films
deposited on single crystal sapphire by electron beam de-
position. We carried out variable temperature (78–500 K)
transmittance measurements on the thin films to investigate
the optical properties and electronic structures of these fer-
rites. The absorption spectra of both NiFe
2
O
4
and CoFe
2
O
4
thin films show insulating characters with Ni (Co) d to d on-
site transitions below 3 eV. The energy bands above 3 eV
are mainly due to the O 2p to Fe 3d charge transfer tran-
sitions. The observed electronic transitions have been as-
signed based on the first principles calculations and compar-
isons with structurally similar Ni and Co-containing com-
pounds. The Co
2+
d to d transition in the CoFe
2
O
4
thin film
shows a strong temperature dependence, likely due to the
spin-charge coupling effect.
1 Introduction
Spinel ferrites, such as NiFe
2
O
4
and CoFe
2
O
4
, have at-
tracted a great deal of interest because of their potential
applications in the upper microwave and lower millimeter
wave ranges, spin filters, and heterostructures devices [1–
4]. In particular, NiFe
2
O
4
(NFO) and CoFe
2
O
4
(CFO) have
R.C. Rai ( ) · S. Wilser · M. Guminiak
Department of Physics, SUNY College at Buffalo, Buffalo, NY
14222, USA
e-mail: rairc@buffalostate.edu
Fax: +1-716-8784421
B. Cai · M.L. Nakarmi
Department of Physics, Brooklyn College of the CUNY,
Brooklyn, NY 11210, USA
been used to fabricate superlattice structures and multifer-
roic heterostructures that demonstrate magnetoelectric cou-
pling between ferromagnetic and ferroelectric thin film lay-
ers [5–9]. The thin films of these inverse ferrites display rela-
tively weaker physical properties compare to the bulk coun-
terparts [10–12]. The coupling of spin, charge, and lattice
degrees-of-freedom in these ferrites is expected to be signif-
icant due to the exchange interactions between the two dif-
ferent types of magnetic ions residing on different crystallo-
graphic sites [2, 13]. Consequently, the physical properties
of these ferrites could be strongly influenced by such cou-
pling processes. Understanding the optical properties, elec-
tronic structures, and coupling effects of these ferrites is es-
sential not only to improve thin film quality but also to de-
velop potential applications.
Inverse spinel ferrites have a chemical formula AB
2
O
4
(where A = Ni, Co, and B = Fe) in which A
2+
[Ni
2+
(3d
8
)
and Co
2+
(3d
7
)] ions reside on octahedral sites and Fe
3+
(3d
5
) ions reside equally among tetrahedral (A-sites) and oc-
tahedral sites (B sites). Oxygen ions are found in fcc close-
packed structures. The unit cell contains 32 O-atoms in a cu-
bic lattice with 8 T
d
(FeO
4
) and 16 O
h
(AO
6
and FeO
6
) oc-
cupied sites. Both NFO and CFO are in high-spin states with
the electronic configurations of t
6
2g
e
2
g
and t
5
2g
e
2
g
, respectively.
Similarly, NFO and CFO are ferrimagnetic insulators with
high Curie temperatures T
C
= 850 and 790 K, respectively
[3, 8]. Optics is a local probe that can provide insight into the
local crystal structures, such as tetrahedral and octahedral
environments, and the corresponding electronic structures
of inverse spinel ferrites. In particular, electronic absorption
spectroscopy is useful in the investigation of the electronic
transitions arising due to different local structures, and the
analysis of these transitions using ligand field theory can re-
veal valuable information about the electronic structures of
this family.