ε-Fe2O3: An Advanced Nanomaterial Exhibiting Giant Coercive ...

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APPLIED PHYSICS LETTERS 99, 253108 (2011)
Room-temperature ground magnetic state of e-Fe 2 O 3 : In-field Mössbauer
spectroscopy evidence for collinear ferrimagnet
Jiri Tucek, 1 Shin-ichi Ohkoshi, 2,a) and Radek Zboril 1
1 Faculty of Science, Departments of Experimental Physics and Physical Chemistry, Regional Centre
of Advanced Technologies and Materials, Palacky University, Slechtitelu 11, 783 71 Olomouc, Czech Republic
2 Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
(Received 4 September 2011; accepted 24 November 2011; published online 22 December 2011)
e-Fe 2 O 3 is a remarkable iron(III) oxide polymorph exhibiting a large room-temperature (RT)
coercive field, coupled magnetoelectric properties, and millimeter-wave ferromagnetic resonance.
Despite great application potential, its room-temperature ground magnetic state is still under
scrutiny. Employing in-field 57 Fe Mössbauer spectroscopy, we unambiguously demonstrate that at
room temperature, e-Fe 2 O 3 behaves as a collinear ferrimagnet, hence excluding any canting of
sublattice magnetizations. When exposed to an external magnetic field, e-Fe 2 O 3 can be modeled as
a two-sublattice ferrimagnetic nanomaterial with the highest coercivity among all currently known
ferrimagnetic (nano)materials. VC 2011 American Institute of Physics. [doi:10.1063/1.3671114]
The e-Fe 2 O 3 is one of 4 crystalline polymorphs of
iron(III) oxide; it exists only in a nanosized form and its natural
abundance is very limited. 1,2 This form has been known
since 1934, 2 and it remained quite unexplored for a long period
of time. Recently, Jin et al. 3 found that the e-phase
exhibits a giant room-temperature (RT) coercive field of
2 T, a rather surprising discovery for a (nano)material,
where the magnetic behavior is driven solely by Fe 3þ ions
with a half-filled d-shell. In addition to that, e-Fe 2 O 3 has
been recently reported to have coupled magnetoelectric
properties 4 and to show a millimeter-wave ferromagnetic
resonance, 5 raising thus significantly its application potential.
The e-Fe 2 O 3 is well recognized nowadays as promising
candidate for assembly of recording media with high coercivity.
Potential applications have been foreseen in electric/
magnetic field tunable devices and components in communication
systems for suppression of electromagnetic (EM) interference
and stabilization of EM transmittance. 2,4,5
Conditio sine qua non to the use of e-Fe 2 O 3 in high-tech
applications requires solution of problems linked to its synthesis
and purity (e.g., low yields, a lack of precise control of
the resulting product size, and presence of polymorphous
admixtures). 2
Apart from limitations arising from synthesis, there are
certain open questions of the e-Fe 2 O 3 magnetic behavior that
need to be unveiled. 2 Among several, one issue remains crucial.
It is necessary to gain a correct description of the nature
and type of e-Fe 2 O 3 magnetic regime at room temperature.
Presently, e-Fe 2 O 3 is thought to behave either as a collinear
ferrimagnet 2,3,6–9 or as a canted antiferromagnet. 2,10,11 The
two different hypothesis describing the RT ground magnetic
state of e-Fe 2 O 3 are affected by significant presence of Fe 2 O 3
polymorphous admixtures which prevents the correct experimental
data interpretation. In fact, the discrepancies in the
data analyses arised from the use of techniques (e.g., magnetization
measurements) being unable to distinguish various
a) Authors to whom correspondence should be addressed. Electronic
addresses: ohkoshi@chem.s.u-tokyo.ac.jp and zboril@prfnw.upol.cz.
magnetic regimes. However, till now, there has been a lack
of focused study on e-Fe 2 O 3 with a pure specimen using infield
57 Fe Mössbauer spectroscopy (IFMS) carried out at
300 K. In case of Fe 3þ -containing compounds like e-Fe 2 O 3 ,
IFMS may provide information on a material’s magnetic
structure comparable to those derived from neutron powder
diffraction. 12
In this work, we address the issue of e-Fe 2 O 3 RT ground
magnetic state employing IFMS. To meet the study scope,
for e-Fe 2 O 3 preparation, we chose a synthetic procedure that
combines the reverse-micelle and sol-gel techniques as
reported by Jin et al. 3 This procedure allows assembly of homogeneous
single-phased specimen of e-Fe 2 O 3 origin (nanorods
having length of 80 to 140 nm and width of 15 to
40 nm), hence without any experimentally detectable
admixtures of other Fe 2 O 3 polymorphs. 2 The synthetic
approach employs SiO 2 matrix and Ba 2þ ions as stabilizing
and phase-purity agents, respectively. 3 In all previous IFMS
studies, the amount of a-Fe 2 O 3 and/or c-Fe 2 O 3 nanoparticles
as extra components of the main e-Fe 2 O 3 phase brought extra
free parameters in the fitting procedure of the magnetic
response, thus providing unclear results. 6–11
The macroscopic magnetic response of the prepared e-
Fe 2 O 3 /SiO 2 nanocomposite was monitored by measuring the
RT hysteresis loop (Fig. 1) using a superconducting quantum
interference device (SQUID) magnetometer (Quantum
Design). The sample magnetization values were recalculated
with regard to the molar ratio of Fe 2 O 3 /SiO 2 (i.e., Fe 2 O 3 /
SiO 2 ¼ 0.22, Ref. 3) and known SiO 2 magnetic response at
300 K (paramagnetic behavior) under an external magnetic
field (B ext ). The profile of the measured RT hysteresis loop
resembles that one observed for e-Fe 2 O 3 : (1) a wide hysteresis
loop area, (2) coercive field of 2 T, and (3) magnetization
non-saturation in high B ext . 2 Using the law of approach
to saturation, 13 we obtain a saturation magnetization value of
20 Am 2 /kg that is close to that derived from previous studies.
3,14 This value is expected when a net magnetization
value of 0.30 l B per Fe is taken for e-Fe 2 O 3 . 2 In absence of
B ext , the phenomenon of distribution in spin orientation
0003-6951/2011/99(25)/253108/3/$30.00 99, 253108-1
VC 2011 American Institute of Physics
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