1. Solid State Communications 151 (2011) 33–36
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Structural and magnetic characterization of rhombohedral Ga1.2Fe0.8O3 ceramics
prepared by high-pressure synthesis
N. Wanga
, F.S. Wena
, L. Lia
, Y.F. Lüa
, S.C. Liua
, Y.F. Lub
, Z.Y. Liua,∗
, B. Xua
, J.L. Hea
, D.L. Yua
, Y.J. Tiana
a
State Key Laboratory of Metastable Materials Science & Technology, Yanshan University, Qinhuangdao 066004, China
b
Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China
a r t i c l e i n f o
Article history:
Received 10 June 2010
Received in revised form
15 September 2010
Accepted 25 October 2010
by J.F. Sadoc
Available online 2 November 2010
Keywords:
A. Gallium iron oxides
A. Wide band gap semiconductor
D. Morin temperature
D. Spin reorientation
a b s t r a c t
The rhombohedral α-Ga1.2Fe0.8O3 ceramics have been synthesized by using a high pressure technique
at a pressure of 5 GPa and a temperature of 800 °C from orthorhombic ε-Ga1.2Fe0.8O3 ceramics, which
were identified to be isostructural with α-Fe2O3 and α-Ga2O3. The low temperature magnetism has been
studied for α-Ga1.2Fe0.8O3, the saturation magnetization is 0.26 µB/Fe at 5 K, and the Morin temperature
has not been found. Moreover, it is most probable that the spin reorientation of α-Ga1.2Fe0.8O3 has been
found at 50 K resulted from the change of magnetic dipole anisotropy and single-ion anisotropy with
temperature.
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
There is currently considerable interest in spintronics materials.
Recently, M2O3-type (M = Ga [1,2], In [3], Fe [4,5], etc.) oxides
have attracted extensive attention. Among these materials, on one
hand, the hematite (α-Fe2O3) with an R¯3c corundum structure is
a charge-transfer insulator with a band gap of 2.2 eV [6]. Spins in
the hematite are slightly canted away from an antiferromagnetic
orientation on the basal plane (111), giving place to the weakly
ferromagnetic state, between the Morin temperature TM ≈ 260 K
and the Néel temperature TN ≈ 956 K, but spins are oriented along
the [111] axis and perfectly antiferromagnetic below TM [7–9].
There has been substantial effort in doped hematite with various
elements, such as Ti, and so on, which are promising for application
to spintronics devices [10,11]. On the other hand, it is well-known
that Ga2O3 can exist in several forms, including α, β, γ , δ, and
ε polymorphs [12]. The β-Ga2O3 has been recognized as a stable
phase as wide-band-gap semiconductor (Eg = 4.9 eV) used
for wide applications [13]. The thermodynamically metastable α-
Ga2O3 with an R¯3c corundum structure can be prepared from β-
Ga2O3 [14]. Moreover, the band gap of the α-Ga2O3 polymorph
is 5.3 eV [15]. Ga2O3 is also promising for spintronics materials
proved by the results that Mn-doped γ -Ga2O3 shows room
∗ Corresponding author. Tel.: +86 03358057047; fax: +86 03358074545.
E-mail address: liuzy0319@yahoo.com (Z.Y. Liu).
temperature ferromagnetism [1,2]. The latest study found the band
gaps varied between 2.2 and 5.3 eV according to a corundum-
structured α-(Ga1−xFex)2O3 thin film on a sapphire substrate with
a different composition, the ferromagnetic properties have been
found at 110 K, and the α-(Ga1−xFex)2O3 film have been proved to
be useful as spintroncs materials [16]. However, the basic magnetic
properties of corundum-structured α-(Ga1−xFex)2O3 are absent
for spintronics devices application. In the previous reports, doped
hematite had been obtained by hydrothermal techniques, but the
detailed investigation of the magnetic properties of these materials
was absent [17,18]. In this letter, we report the structural and
magnetic characterization of bulk α-Ga1.2Fe0.8O3 prepared by the
high pressure and high temperature method in detail, especially,
spin reorientation is found with temperature.
2. Experiments
The α-Ga1.2Fe0.8O3 ceramics were prepared in two steps: first
a mixture of Ga2O3 (99.99%) and Fe2O3 (99.99%) in the required
molar ratio was pressed into bulk samples using a cold press
method and heated without pressure in air at 1350 °C for 10 h,
and in a second step the as-prepared bulk samples were loaded
in a high pressure apparatus at 800 °C and a pressure of 5 GPa
for 1 h. The X-ray diffractions were performed using a Rigaku
diffractometer with Cu K α radiation, and the Rietveld analysis
was used to determine the crystal structure of high-pressure
samples. Raman measurements were carried out on a RENISHAW
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2010.10.029

2. 34 N. Wang et al. / Solid State Communications 151 (2011) 33–36
micro-Raman spectrometer with a 514.5 nm Ar ion excitation
source under air ambient conditions. The hysteresis loops M(H)
under applied fields up to 5 T and the magnetization as a
function of temperature M(T) under an applied magnetic field
of 0.12 T were recorded in the 5–400 K range using a vibrating
sample magnetometer (VSM) attached onto a Physical Property
Measurement System (Quantum Design, PPMS-9).
3. Results and discussion
The X-ray diffraction patterns with Rietveld refinement for as-
prepared and high-pressure samples are shown in Fig. 1. As shown
in Fig. 1(a), it is observed that all diffraction peaks of the as-
prepared sample can be indexed to orthorhombic ε-Ga1.2Fe0.8O3
with space group Pc21n with a = 8.7354 Å, b = 9.3773 Å and
c = 5.0761 Å, which is consistent with the value reported by
other groups [19]. On the other hand, as shown in Fig. 1(b), the
X-ray pattern of the high-pressure sample exhibits a significant
difference from that of ε-Ga1.2Fe0.8O3. All the observed peaks of
the high pressure sample can be indexed by reflections of the
R¯3c rhombohedral structure which is identified to be isostructural
to α-Fe2O3, and no impurity phase is found, which is noted
as α-Ga1.2Fe0.8O3. The lattice parameters of α-Ga1.2Fe0.8O3 are
determined as a = 5.0136 Å, c = 13.5592 Å (those of α-Ga2O3
are a = 4.9825 Å, c = 13.433 Å [20], and those of α-Fe2O3 are
a = 5.038 Å, c = 13.722 Å [21], respectively), which are smaller
than those of α-Fe2O3, and larger than those of α-Ga2O3 due to
the ionic diameter of Ga3+
(0.062 nm) being smaller than that of
Fe3+
(0.067 nm). In ε-Ga1.2Fe0.8O3, there are four nonequivalent
cation sites which are categorized into Fe1, Fe2, Ga1, and Ga2 [19].
The Fe1, Fe2, and Ga2 sites are surrounded by distorted oxygen
octahedra with the non-centrosymmetric structure, but the Ga1
site is located inside a nearly regular tetrahedron. Due to a similar
ionic radius, the distribution of Fe ions is disordered on the four
cation sites. According to previous report [19], the occupancies of
Fe ions for ε-Ga1.2Fe0.8O3 on the four sites of Fe1, Fe2, Ga1, and
Ga2 are 0.73, 0.62, 0.08, and 0.18. From the Rietveld analysis of
our XRD data, the Fe populations on each site are 0.70, 0.58, 0.11,
and 0.21, respectively [22]. In α-Fe2O3, however, all Fe ions are
distributed over 2/3 of the cation sites which are surrounded by
octahedra of oxygen [23]. In the case of the substitution of Fe ions
by divalent or tetravalent ions, these dopant ions will enter into
an interstitial site and replace different amounts of Fe ions in order
for the conservation of charge [18]. However, the Ga ions only enter
into the substitutional sites. Thus, the occupation of Fe ions is 0.60
for α-Ga1.2Fe0.8O3.
In order to characterize the structure of α-Ga1.2Fe0.8O3
comparing with those of α-Fe2O3 and α-Ga2O3, the Raman spectra
of α-Fe2O3, α-Ga2O3 (data from Ref. [14]) and α-Ga1.2Fe0.8O3
are shown in Fig. 2. The spectra for α-Fe2O3, α-Ga2O3 and α-
Ga1.2Fe0.8O3 were deconvoluted with Lorentzian peaks and the
obtained peak parameters are given in Table 1. It is clearly
observed that all the resolved Raman modes of α-Ga1.2Fe0.8O3 can
be indexed to those of α-Fe2O3 and α-Ga2O3, which obviously
ascertains the rhombohedral structure with space group R¯3c of
α-Ga1.2Fe0.8O3. In addition, the shift of Raman modes shows
that the substitution of Ga on Fe sites leads to the local
distortion of the structure around cation sites because of a smaller
radius. Moreover, the peak at 660 cm−1
(marked by dotted line)
indicates the formation of the lattice disorder in the structure
of α-Ga1.2Fe0.8O3, which has been observed that in the mixed
compound of Fe2O3–Cr2O3 [24] and the hematite film doped with
Ti [25]. Therefore, these XRD patterns and Raman spectra confirm
the formation of pure α-Ga1.2Fe0.8O3 ceramics.
Fig. 3(a) shows the hysteresis loops at different temperatures.
The magnetic hysteresis loops are not saturated in our polycrys-
talline ceramics even under magnetic fields up to 5 T. In order to
Fig. 1. The measured (°) and calculated (solid line) powder XRD patterns: (a)
orthorhombic ε-Ga1.2Fe0.8O3, (b) rhombohedral α-Ga1.2Fe0.8O3. Below the XRD
patterns is the difference between the measured and calculated profiles. The
vertical bars mark all possible Bragg reflections.
Fig. 2. Room temperature Raman spectra (open circles), together with their fitted
spectra (red solid line) and the decomposed active modes (black solid line) of α-
Fe2O3, α-Ga1.2Fe0.8O3 and α-Ga2O3 ceramics. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this
article.)
calculate the saturation magnetization, we make use of the law
of approach to saturation at 5 K [26], M = Ms(1 − A/H), where
Ms is the saturation magnetization, A is the coefficient of magnetic
hardness, and H is the applied magnetic field. In Fig. 3(b), we have
shown the M versus 1/H plot by taking the high field part of the
first quadrant of M–H loop at 5 K. The linear fit for the high field
part is shown by a continuous line (black color) and its extrapo-
lation to the origin. So when H tends infinity it leads to the satu-
ration magnetization, Ms = 0.26 µB/Fe. Comparing with α-Fe2O3,
which is of antiferromagnetic order below 260 K, the increase of
saturation magnetization is attributed to the substitution of Ga on
Fe sites. According to reference [7,8,27], when x is equal to 0.95
for α-(Ga1−xFex)2O3, the Morin temperature disappears, and the
α-Ga1.2Fe0.8O3 is consistent with the trend.
As shown in Fig. 3(c), the values of Hc increase with decreasing
temperature in the range from 400 to 5 K, and the value of Hc
is 0.08 T at 5 K. Especially, the obvious increase in coercivity
with decreasing temperature can be observed below 50 K (marked
by dotted line). In order to understand more deeply the origin
of the rapid increase of coercivity, the magnetization (M) vs
temperature (T) curve of α-Ga1.2Fe0.8O3 ceramics under an

3. N. Wang et al. / Solid State Communications 151 (2011) 33–36 35
Table 1
Raman modes of α-Fe2O3, α-Ga1.2Fe0.8O3 and α-Ga2O3 ceramics.
Modes Space group A1g (cm−1
) Eg (cm−1
) Eg (cm−1
) Eg (cm−1
) Eg (cm−1
) A1g (cm−1
) Eg (cm−1
)
α-Fe2O3 R¯3c 223 244 291 297 408 495 609
α-Ga1.2Fe0.8O3 R¯3c 218 240 287 310 414 529 684
α-Ga2O3 R¯3c 217 240 286 328 432 573 688
Fig. 3. M vs H curves of α-Ga1.2Fe0.8O3 at various temperatures (a); M vs 1/µ0H plot to calculate the saturation magnetization at 5 K (b); temperature dependence of
coercivity field (µ0Hc ) (c); temperature dependence of magnetization (M) at an applied field of 0.12 T (d).
external field of 0.12 T was measured, as plotted in Fig. 3(d). It
is obvious that the magnetization also drops at 50 K (marked
by dotted line), which is equal to the temperature of coercivity.
We then go to the physical origin of the transition at 50 K. It is
known that α-Fe2O3 shows a magnetic transition from weakly
ferromagnetic to antiferromagnetic at the Morin temperature due
to the spin reorientation phenomenon in which the easy axis of
magnetic anisotropy changes. Therefore, for α-Ga1.2Fe0.8O3, the
large coercivity arises from the large magnetic anisotropy, and it is
attributed to the change of magnetic dipole anisotropy and single-
ion anisotropy with temperature [28]. Here, most probably, we
interpret the spin reorientation in α-Ga1.2Fe0.8O3 at 50 K. However,
careful magnetization and spin structure characterization are
needed for understanding α-Ga1.2Fe0.8O3. On the basis of the
aforementioned analysis, the magnetic results encourage the
rhombohedral gallium iron oxides for application as spintronics
materials.
4. Conclusions
In conclusion, the rhombohedral α-Ga1.2Fe0.8O3 ceramics have
been synthesized by using a high pressure technique at a pressure
of 5 GPa and a temperature of 800 °C from orthorhombic ε-
Ga1.2Fe0.8O3, which are isostructural to α-Fe2O3 and α-Ga2O3.
The Raman spectra prove the formation of the lattice disorder in
the structure of α-Ga1.2Fe0.8O3. The low temperature magnetic
properties of α-Ga1.2Fe0.8O3 are studied, and the saturation
magnetization is 0.26 µB/Fe at 5 K. The clear increase in coercivity
with decreasing temperature can be observed below 50 K, and the
magnetization also dropped at 50 K for M–T curves. According to
experimental results, it is most probable that the spin reorientation
of α-Ga1.2Fe0.8O3 have been found at 50 K resulting from the
change of magnetic dipole anisotropy and single-ion anisotropy
with temperature.
Acknowledgements
We thank the Natural Science Foundations of China (Grants
No. 50672082, No. 50871096, and No. 50821001), HeBei Province,
China (Grant No. E2009001636), PCSIRT (Grant No. IRT0650), and
National Science Fund for Distinguished Young Scholars (Grant No.
51025103) for support of this work.
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