1. technology and computing

Magnetic-Semiconductors in Fe-Ti-Oxide Series and Their Potential
Applications
R .K. Pandey*1,2, P. Padmini1, L. F. Deravi3, N.N. Patil4, P. Kale1, J. Zhong1, J. Dou2, L. Navarrete2, R. Schad2, and M.
Shamsuzzoha5
1
2
Electrical and Computer Engineering, Physics and Astronomy, 3 Chemistry, 4 Computer Science, 5 Central Analytical
Facilities (CAF)
The University of Alabama, Tuscaloosa, AL, USA
C. O’Brien6 and W. J. Geerts6
6
Department of Physics
Texas State University, San Marcos, TX, USA
*Email: [email protected] and [email protected]
Abstract
Multifunctional nature of the Fe-Ti-oxides make them
attractive candidates for novel applications in which their
coupled semiconductor, magnetic, dielectric and optical
properties can be exploited. In particular, they appear to
be good candidates for the emerging technologies such
as spintronics, magneto-electronics, and radhard
electronics. In this paper we deal mostly with
pseudobrookite (Fe2TiO5), both pure and Mn-substituted
variety. Materials processing, structural, magnetic and
semiconducting properties have been discussed in-depth.
The temperature and magnetic field dependence on the
non-liner I-V characteristics are established opening the
possibilities for fabricating magnetically switched
devices.
1. Introduction
Iron titanates, Fe-Ti-oxides, are well known to
geophysicists and geologists and their unusual magnetic
properties have been of great interest to geophysicists for
a very long time. But they have rarely been studied for
their unique electrical, dielectric and magnetic properties
with potential applications in microelectronics and its
related technologies.
The
multifunctional
nature
(wide
band-gap
semiconductor, magnetic and dielectric properties) of
some members of this family has been recognized only
recently resulting in interest of a large group of
researchers to study this system. Particularly the coupled
magnetic and semiconductor properties of some
members of the ilmenite-hematite solid solutions (IH)
and Mn-doped pseudobrookite (Mn-PsB) are of special
interest both from the fundamental materials science
point of view and their potential impact to such emerging
technologies as spintronics, magneto-electronics and
radhard electronics.
Nature produces three stable compounds in the irontitanate system, namely, ilmenite (FeTiO3), pseudobrookite (Fe2TiO5) and ulv spinel (Fe2TiO4). Ilmenite is
the lead member of the group and most viable as a source
of TiO2 pigment. Its very interesting geomagnetism has
been extensively studied by geologists. In the recent past,
after its discovery on the moon in one of the earlier space
explorations, it drew attention of space scientists because
of the possibility (or, dream) of colonizing the moon
some day and setup civilization there. It is also a very
prominent raw material on earth and is used in shielding
nuclear radiation. When mixed with hematite, it assumes
the character of solid solutions with interesting electrical
and magnetic properties [1,2]. Many members of this
series are wide band gap semiconductors with band gap
greater than 2.0 eV as well as ferromagnetic with the
Curie point well above the room temperature. This
uniqueness of properties makes them extremely
attractive candidates for research leading to some novel
applications in microelectronics and perhaps also
spintronics. In the last few years this system has been
extensively studied by our group leading to some
pioneering publications. The interested readers are
referred to references [3-8].
The focus of this paper is not IH rather the lesser known
member of the Fe-Ti-oxide, namely, pseudobrookite
(PsB) and Mn-doped pseudobrookite (Mn-PsB). Like the
IH solid solution it is also a potential candidate for
spintronics, magneto-electronics and radhard electronics
and microelectronics in general. Compared to the IH this
material is less complicated, very stable even at high
temperatures and can be processed with greater ease and
reliability in air as ceramic, epitaxial and high quality
thin films and most importantly as bulk single crystals.
In comparison, it is almost impossible to grow single
crystals of IH by any known single crystal growth
method. The phase diagrams [9,10] point to the
possibility of dissociation of IH at high temperatures at
which the crystals must be grown. From the phase
diagram [11] one infers that PsB melts congruently at
about 1550 C. This advantage makes Mn-PsB perhaps a
superior class of material than IH solid solutions for
developing applications on large scale.
PsB crystallizes in orthorhombic structure with the lattice
constant of a = 0.97, b = 0.99 and c = 0.37 nm [11]. Its
crystal structure has been extensively studied and the
general agreement is that it is the most stable member of
the Fe-titanate family and is in equilibrium with
atmospheric oxygen [12-14].
In 1981, however, its monoclinic phase was discovered
which converts to the orthorhombic phase at about 927
ºC [15]. For the growth of the monoclinic phase a
relatively fast cooling rate of 10 ºCh-1 was used. The flux
contained a small amount of SrO. It is not clear whether
the growth of monoclinic phase was promoted by the
presence of alkaline earth oxide or was the result of fast
cooling. It was found that the monoclinic crystals were
magnetic, more precisely perhaps parasitic magnetism
instead of intrinsic ferromagnetism. In contrast, pure PsB
crystals are paramagnetic, or at best weakly magnetic, at
room temperature. They show strong anisotropic uniaxial
“spin-freezing” behavior [16,17]. The spin-glass
transition occurs between at 53 and 55 K. In PsB iron
ions exist predominately in 3+ state and are believed to
be the contributor to spin-glass freezing. From
M ssbauer-effect it was concluded that the spins of the
Fe3+ ions are parallel to the crystallographic c-axis [16].
Later in this paper we will show that Mn-doping
enhances its ferromagnetic property while still retaining
its semiconductor property. Both pure PsB and Mndoped PsB are n-type semiconductors. Because of the
coupled semiconductor-magnetic properties Mn-PsB is
an attractive material for spintronics, and magnetoelectronics. It is believed to have strong radiation
immunity like IH [5,7] which makes it also a suitable
candidate for radhard electronics.
2. Materials Processing
(a) Sample Processing: Three types of samples were
processed for this study – ceramic, films and bulk single
crystals.
Ceramic: In order to first understand the basic nature of
physical properties and processing chemistry dense,
homogeneous ceramic samples were processed using the
standard procedure [18]. Another reason for making high
quality ceramic discs is that we need targets for the
growth of films by the laser ablation method which will
be described later in this paper. The samples were mostly
1 inch diameter discs having the thickness between 3 to 4
mm. High purity powder of pseudobrookite was used to
press disks at 10,000 psi using an automatic uniaxial
press. All the initial pressing was done at room
temperature in the presence of air. These green ceramic
were sintered multiple times and finally annealed in Aratmosphere to obtain dense, and chemically
homogeneous disks showing the desired x-ray diffraction
patterns and low resistance.. The details for making
highly dense and homogenous ceramic samples of IH
have been recently reported in reference [19]. Because of
the chemical similarities with IH we adopted the same
processing protocol for making PsB and Mn-PsB
ceramic as for IH ceramic.
Since pure PsB is not magnetic it was decided to dope it
with Mn3+ ions (source being Mn2O3) to make it
ferromagnetic. First ceramic of PsB with varying
percentage of Mn was processed and characterized.
Samples having Mn-doping from 4 to 40 atomic percent
showed both n-type as well as ferromagnetic behavior at
room temperature. The highest magnetic moment was
obtained when a doping level of 40 percent was used.
Bulk single crystal: Growth of single crystals of PsB was
undertaken because of the limited literature available that
deal with the magnetic and electrical properties of PsB as
well as for studying the feasibility of potential
applications.
High temperature solution growth method was used for
crystal growth using PbO⋅V2O5 flux. The charge to flux
ratio was maintained at 7.5:92.5 by weight for all growth
experiments as originally reported in [20]. Czochralski
growth was reported in [21]. Our first few runs produced
high quality crystals but small in size. Larger crystals, 812 mm × 2-3 mm × 3-5 mm, were grown by seeding the
melt and introducing the growth procedure as shown in
Figure 1. Once the growth runs were over, crystals were
harvested by dissolving the flux in 25% HNO3 acid.
Platinum crucibles were used because of the very
aggressive nature of molten flux and growth taking place
in air at high temperatures. For the growth of Mn-PsB
crystals, the charge was prepared with 40% Mn. The
growth was initiated under the same general protocol as
for pure PsB. Good quality crystals were obtained and
presence of Mn was confirmed by EDAX analysis.
T max
Initial
Heating
T
First
Soaking
Cycle @
1350 C
for 12-15
hours
Second
Soak @
1325 oC
for 2-4
hours
Range of
Crystallization:
1325 – 850 oC
Very slow
cooling @
0.5-1 C per
hour
100000
Rapid
Cooling
Time
Fig.1 Crystal growth protocol, T = Temperature ( C)
Film Growth: Films were grown by the PLD method
using the home-made ceramic targets. The growth
parameters were established by growing multiple films
under different conditions. The following parameters
were found to produce smooth textured films on (100)
MgO substrates varying in thickness between 50 to 150
nm (depending upon the duration of growth) with good
semiconductor and magnetic properties: 248 nm
wavelength (Lambda Physique Excimer Laser) with a
maximum power rating of 500 mJ per pulse; substrate
temperature between 625 and 675 ºC, laser power energy
500 mJ, chamber maintained at
10-5 Torr, and
substrate rotation rate of 5.0 rpm. Films of PsB have also
been grown using the solgel technology [22,23]. No
published literature is available that deals with the
growth of Mn-substituted PsB films. We might be the
first group to grow highly textured films of Mn-PsB with
high enough quality as to study multifunctional
properties.
3. Properties and Discussion
Film and crystal samples were characterized by x-ray
diffraction (XRD) for crystal structure, energy dispersed
x-ray diffraction (EDAX) for chemical composition and
general surface morphology, and transmission electron
microscopy (TEM) for atomic periodicity. All samples
were orthorhombic with lattice parameters in good
agreement with reported values [11]. Films had very
smooth surfaces with well pronounced texturing and the
2000
18.15
(020)
80000
Instensity (cps)
T < T max: Super
saturation
surface roughness varied from 2 to 5 nm as determined
from atomic force microscopy (AFM). They were
transparent with faint gold color. The bulk crystals,
however, were invariably black and no amount of
annealing would change their color.
Figure 2 shows the characteristic x-ray diffraction peaks
and the rocking curve of pure PsB crystals. The well
pronounced n(010) peaks and rocking curve width of
0.040º confirm the high quality of the crystals. The WDS
analysis showed no trace of impurities or the presence of
second phase. The TEM diffraction pattern in Figure 3
showed perfect atomic periodicity confirming the
excellent quality of the crystal.
1500
Intensity(cps)
.
1000
60000
0.040 deg
40000
36.74
(040)
500
0
10
20000
56.41
(060)
20
30
40
50
60
2Θ(deg)
70
80
90
0
-0.10
-0.05
0.00
0.05
0.10
ω (deg)
Fig.2 XRD pattern (left) and rocking curve (right) of
PsB crystal.
Fig.3 TEM micrograph of PsB crystal
The Seebeck coefficient measurements confirmed that
both the PsB and Mn-PsB crystals are n-type
semiconductors and Mn-PsB crystals displayed magnetic
hysteresis loops. No well defined loops were found for
the PsB crystals.
Table I summarizes some of the properties of bulk
crystals of the two types grown by seeded high
temperature solution growth method. It is to be noted
that both the resistivity and the activation energy is
remarkably smaller for Mn-doped samples than for pure
PsB. Mn-doping, or more precisely substitution, makes
PsB more conductive and reduces its activation energy.
Consequently, this material should be appropriate for
device fabrication. Because of these advantages,
emphasis was put on the growth of highly textured thin
film of Mn-PsB. Its physical properties should serve as
the bench-mark for technologists and process engineers.
Various combinations of temperature and argon pressure
inside the growth chamber were tried to find the
optimum conditions that would yield the best samples
with maximum magnetic moment, saturated magnetic
loop and electrical conductivity.
In Figure 4 XRD spectrum is presented for the films
grown at 450 C under different values of chamber
pressure and atmosphere. We find that 50 mT of Ar
pressure produces the best crystallinity and texturing
with the MgO substrate.
(g) MgO substrate
(f) Vac
(e) 50mT Ar
(d) 100mT Ar
(c) 200mT Ar
(b) 450C Vac
(a) 300mT Ar
2500
Intensity
2000
1500
(g)
(cm2V-1s-1)
Bandgap (eV)
(f)
30
o
650 C vacuum
o
650 C 50m τ Ar
20
o
650 C 100m τ Ar
(d)
o
M em u/g
(b)
(a)
0
30
35
40
2 theta (deg)
45
50
Table I: Important Parameters for PsB and Mn-PsB Bulk
Crystals
Seebeck
coefficient
( V⋅K-1)
Mobility
Mn-PsB
n-type
n-type
2.197
0.294
-35
?
0.92 for T >500 K
?
-10
-500 0
0
500 0
4
1 10
Fig. 5 Room temperature magnetic moment of Mn-PsB
films as a function of growth temperature and chamber
pressure.
4
1.6
3
1.2
1
0.8
2.5
2
1.5
1
0.6
0.4
(1kHz)
(10kHz)
(100kHz)
(1MHz)
3.5
R.T = 0.3 -cm
Eac ~ 0.012eV
capacitance (pF)
0.17
0
field O e
1.4
0.50
0.19 for T < 500 K
[24]
0.46 for T > 500 K
[24]
o
650 C 300m τ Ar
-30 4
-1 10
log Res (ohm-cm)
Activation
energy (eV)
Pure PsB
o
650 C 200m τ Ar
-20
Fig.4 X-ray diffraction pattern of Mn-PsB films grown
by PLD at 450º C on MgO substrate
Semiconductor
nature
Resistivity
(k ⋅cm)
450 C 50m τ Ar
10
(c)
500
2.3
The bandgap of Mn-PsB film was determined by optical
absorption method and it was found to be 2.3 eV which
is in good agreement with the value reported in reference
[21]. for PsB crystal. The value of 0.92 eV as reported
by [24] appears to be too low. Being basically an
insulator its bandgap cannot be comparable to narrow
bandgap semiconductors.
Figure 5 presents the magnetic loops for Mn-PsB films
grown at different conditions of temperature and
pressure. The best film with the highest value of
magnetic moment is achieved for growth taking place at
450 ºC at 50 mT of Ar-pressure. This film has the
magnetic moment of 25 emu/g with the coercivity of just
6.5 Oe. The loop saturates around 2 kOe.
(e)
1000
(intrinsic)
6.3 for T <500 K
(extrinsic)
0.92 [29]
0.5
3
3.5
4
4.5 5
5.5
-1
1000/T (K )
6
6.5
0
-5
0
Voltage (V)
5
Fig.6 Temperature dependence of resistivity (left) and
frequency dependence of capacitance (right).
methods. The results are almost identical. Here in Figure
7 the MR measured with the 2-point probe method is
shown. It decreases as the field increases and the curve is
parabolic which is typical of all semiconductors. The
solid curve is best fit of the experimental curve
(fluctuating).
Resistance (KOhm)
The temperature (T) dependence of resistivity ( ) and
capacitance C as a function of frequency ( ) is shown in
Figure 6 for the Mn-PsB film. The nature of the vs. T-1
curve confirms the semiconductor nature of the film and
its almost perfect linearity points to the high quality of
the film. From this curve we get the room temperature
value of the resistivity to be 0.3 ⋅cm and the activation
energy of approximately 12 meV. These values also
point to the device quality of the film. It is believed that
the conduction in PsB appears to be polaron mediated
[24]. It is argued that the conductivity in this material is
due to the small polaron band conduction or small
polaron hopping conduction. Since the polaron band
conduction is possible only at low temperatures it is
assumed that the conduction in PsB is due to polaron
hopping. Very small value of the activation energy of 12
meV at room temperature could not be due to the
intrinsic conduction. Such low values of the activation
energy points to the presence of point defects and
interstitials in the material [24]. The nature of the
capacitance (C) vs. voltage (V) dependence, Fig.6, is
typical of insulators. Also, the strong frequency
dependence of C is typical of polar and non-polar
dielectrics. The relative dielectric constant ( r) at room
temperature is found to be approximately 7000. For pure
PsB crystals it is reported that the r remains practically
constant between 300 to 500 K and then it rises fast and
reaches its maximum value around 900 K and then
saturates [24]. This type of behavior puts pseudobrookite
in the special class of dielectrics known as non-linear
(possibly even polar) dielectrics and warrants more
investigation. It is important to point out that
conductivity, thermal power and dielectric constant show
anomalous behavior around 500 K for PsB crystals [24].
The nature of this transition point is not well understood
and warrants further studies.
Since Mn-substituted PsB film with 40 atomic %
Mn3+ shows excellent n-type conductivity and
magnetization we decided to study its magnetic field
dependence of resistivity and current-voltage (I-V)
characteristics. Two experiments were designed: first, to
determine the classical magneto-resistance; and second,
to study the I-V relationship as function of magnetic field
and temperature. The motivation for the second study
comes from the non-linear I-V behavior found in IH
samples and their prospects for fabricating “varistor”
devices [4,5,7]. Radiation hardness of these devices have
also been dealt with in references [5,7].
The magneto-resistance (MR) for the Mn-PsB film was
measured using both the 2-point and 4-point probe
32.67
32.66
32.65
32.64
32.63
32.62
32.61
32.60
32.59
32.58
32.57
295 K
2 Point probe MR
Poly. (2 Point probe MR)
-12
-9
-6
-3
0
3
6
9
12
H (KGauss)
Fig.7 Magneto-resistance of Mn-PsB film on MgO.
Having established the MR effect we proceeded to
examine the effect of temperature and magnetic field on
the I-V characteristics of the Mn-substituted PsB film
(0.6 Fe2TiO5.0.4Mn2O3). The initial I-V relations using
the 4-point probe method at varying temperatures looked
very linear. This technique was employed in order to
eliminate any effect of contact resistance due to the Ag
contacts and the interface. However, to exploit any
possible non-linearity we took the best linear fit for the IV graph up to four significant digits, subtracted the linear
component and examined the non-linear component of
the data set. Figure 8 shows the average I-V behavior at
300 K(a) and 441 K(b).. The magnetic field applied were
H= 1, 0, and –1 Tesla. We observe that the non-linear
component is three orders of magnitude smaller than the
original linear curve and here the magnetic field effect is
quite pronounced which is practically indistinguishable
in its linear counterparts. It is important to realize that
the sample experiences no Joule heating (=i2R) because
of the low currents applied and thereby we eliminate any
thermal effects that could cause the non-linearity or field
dependence.
From Figure 8 we see clearly that the non-linearity in
the I-V behavior is both magnetic field and temperature
dependent. The dependence on temperature is quite
understandable and is of no particular interest to
technology. However, magnetic field dependence is of
interest because non-linear devices based on Mn-PsB
could be switched magnetically.
140
I (uA)
100
10
0
Acknowledgments: We thank the U.S. Office of Naval
Research (Grant # N00014-03-1-0358) and the U.S.
Department of Energy (Grant #DE-FG02-03ER46039 )
for their sponsorships. We also thank the staff of MINT
Center and Dr. Mike Bersch of Central Analytical
Facilities (CAF) at the University of Alabama for their
technical support.
-1 0
-2 0
-6 0 0
60
I (nA)
applications. Spintronics, magneto-electronics, radhard
electronics are its other possible applications.
20
-4 0 0
-2 0 0
0
200
V (m V )
400
600
20
-2 0
-6 0
(a)
-1 0 0
1 T e s la
- 1 T e s la
300 K
0 T e s la
-1 4 0
-6 0 0
-4 0 0
-2 0 0
0
200
400
600
V (m V )
300
References
I (uA)
60
200
30
0
-3 0
-6 0
-7 0 0
-5 0 0
-3 0 0
I (nA)
-1 0 0
100
300
500
700
V (m V )
100
0
-1 0 0
(b)
-2 0 0
1 T e s la
- 1 T e s la
0 T e s la
441 K
-3 0 0
-8 0 0
-6 0 0
-4 0 0
-2 0 0
0
200
400
600
800
V (m V )
Fig. 8 Temperature and magnetic field dependence of
non-linear I-V characteristics of Mn-PsB film: (a) 300 K
and (b) 441 K. The inserts are the original linear curves.
4. Summary
Only recently it has been realized that many members of
the family of Fe-Ti-oxides possess fascinating properties
that could be exploited for many novel applications.
These are very robust materials, relatively less expensive
to process and their raw materials are abundant. From the
point of electronic technology some solid solutions of
ilmenite-hematite (IH) and Mn-substituted
pseudobrookite (Mn-PsB) are good candidates for
emerging technologies of spintronics and radhard
electronics. The paper deals primarily with Mn-PsB and
shows its superiority, at least from application point of
view, to the more established IH system. Like many IH it
is also a wide bandgap semiconductor with bandgap of
the order of 2.3 eV, magnetic moment approaching a
value of 25 emu/g with the coercivity of just 6.5 Oe. Its
dielectric constant is about 7000 which makes it also a
very good material for gate-oxide and capacitor
applications. Its well pronounced magneto-resistance and
magnetic field dependence of the non-linearity of I-V
characteristics add additional dimensions to its potential
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