Swift heavy ion irradiation induced magnetism in magnetically frustrated BiMn
2
O
5
thin films
D. K. Shukla,
1,
* Ravi Kumar,
2
S. Mollah,
1
R. J. Choudhary,
3
P. Thakur,
4
S. K. Sharma,
5
N. B. Brookes,
4
and M. Knobel
5
1
Department of Physics, Aligarh Muslim University, Aligarh 202002, India
2
Centre for Materials Science and Engineering, National Institute of Technology, Hamirpur 177005, India
3
UGC-DAE Consortium for Scientific Research, Indore 452001, India
4
European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France
5
Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas (UNICAMP), Campinas 13.083-970, SP, Brazil
Received 18 May 2010; revised manuscript received 24 August 2010; published 23 November 2010
The swift heavy ion SHI irradiation induces weak ferrimagnetism FM in magnetically frustrated poly-
crystalline BiMn
2
O
5
thin films. This is manifested from irradiation induced higher energetic configuration that
accounts for evolution of the Mn
2+
state in the Mn
3+
/ Mn
4+
network. Basically, this is the root of large
magnetic moment in the irradiated samples. X-ray diffraction and Raman-scattering data of the samples
indicate considerable modifications in the crystal structure after the SHI irradiation. FM in the irradiated
samples and magnetically frustrated behavior of the pristine sample is apparent from dc magnetization mea-
surements. Element specific characterizations such as near-edge x-ray absorption fine structure spectroscopy at
O K and Mn L
3,2
edges along with x-ray magnetic circular dichroism at Mn L
3,2
edge show the evolution of the
Mn
2+
at disbursement of the Mn
4+
. The microscopic origin behind the induced weak FM is found to be the
increased orbital moment in the irradiated thin films.
DOI: 10.1103/PhysRevB.82.174432 PACS numbers: 77.55.Nv, 81.15.Gh, 61.80.Jh, 75.70.-i
I. INTRODUCTION
Field of multiferroics has created enormous curiosity
among materials research communities and technologists
during last few years.
1–21
Multiferroic materials exhibit con-
currence of more than one ferroic properties magnetic/
ferroelectric/ferroelastic in same phase, regardless of their
contrasting origin, and offer the possibilities for variety of
applications such as nonvolatile memories, capacitors, trans-
ducers, actuators, data storage multiple state memories, mag-
netic field sensors, etc.
1–9
Among recently discovered multi-
ferroic materials, RMn
2
O
5
where R =rare earth, Y, and Bi
family demonstrates antiferromagnetism T
N
39 K to-
gether with ferroelectricity T
C
35 K and coupling be-
tween them which is a prerequisite for application
purpose.
10–21
BiMn
2
O
5
from above family has been studied
widely,
12,15
whose magnetic structure is commensurate. It is
ferroelectric at all temperatures below T
N
and T
C
with a
propagation vector q =
1
2
,0,
1
4
. It is recently reported
14
that
thin film of the BiMn
2
O
5
is magnetically frustrated and its
magnetic property is highly influenced by the strain. Funda-
mentally, thin films of the BiMn
2
O
5
exhibit the magnetically
frustrated spin-glass-type behavior, which is not acceptable
for multiferroic applications.
14
This issue is an obstacle in
realization of multiferroic-based memory devices of these
materials. In fast moving technology of information storage
system, chip-based memory and logic devices are impera-
tive. Dependence on magnetic thin-film element for func-
tionality and nonvolatile information storage will have added
features due to simultaneous ferroelectricity. Recently, the
remarkable pronouncements coming out of the ion beam
treatment of magnetic materials carried out on permalloy or
CoPtCr, Fe-Pt, Ni-Fe, etc., having special relevance for ap-
plication in magnetic information storage have revealed the
prospects to custom tailor the magnetic properties e.g., mo-
ment, coercivity, anisotropy, magnetoresistance, etc. which
are crucial for application purposes.
22–25
The geometrical/
magnetic structure of the BiMn
2
O
5
offers various possibility
of engineering its magnetic property by the ion beam.
12,16
When swift heavy ion SHI passes through a material, it
either excites or ionizes the atoms of the target by inelastic
collisions or displaces them by elastic collisions. Elastic and
inelastic collision processes are dominant, respectively, in
the low- and high-energy regimes. It is evident from the
literature that the electronic energy loss S
e
owing to inelas-
tic collision is capable of generating point/cluster defects if it
is less than the threshold value of electronic energy loss
S
eth
. If S
e
is greater than S
eth
14.25 keV / nm, for the
present case, the energetic ions may create columnar amor-
phization. Stress/strain developed on account of the created
defects or amorphization is responsible for the modification
of the materials Refs. 26–28 and references therein.
The present paper reports the 200 MeV Ag
15+
ion irradia-
tion induced weak ferrimagnetism FM in the magnetically
frustrated polycrystalline BiMn
2
O
5
thin films. Due to statis-
tical nature of energy deposition process, stopping and range
of ions in matter SRIM calculation
29
based on Monte Carlo
simulation is used to plan the irradiation energy
200 MeV. Consideration of ion fluences 1 10
11
,5
10
11
and 1 10
12
ions / cm
2
has been decided as an ex-
tended approximation of the single-ion impact.
II. EXPERIMENTAL DETAILS
Thin film of the BiMn
2
O
5
thickness 200 nm was de-
posited on c-axis-oriented single crystals of the LaAlO
3
LAO substrate by pulsed laser deposition PLD under op-
timized conditions using single phase target of the BiMn
2
O
5
synthesized by solid state reaction route.
14,16
Thin film of the
BiMn
2
O
5
was cut into four pieces, each of 5 5 mm
2
size.
In order to keep the growth conditions uniform for all the
samples, these were used for irradiation and further studies.
PHYSICAL REVIEW B 82, 174432 2010
1098-0121/2010/8217/1744329 ©2010 The American Physical Society 174432-1