Modifications in magnetic properties of BiMn2O5multiferroic using swift
heavy ion irradiation
D. K. Shukla,1,a?,b?Ravi Kumar,2,c?S. Mollah,1R. J. Choudhary,3P. Thakur,4S. K. Sharma,5
N. B. Brookes,4and M. Knobel5
1Department of Physics, Aligarh Muslim University, Aligarh 202002, India
2Centre for Materials Science and Engineering, National Institute of Technology, Hamirpur 177005, India
3UGC-DAE Consortium for Scientific Research, Indore 452001, India
4European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France
5Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas (UNICAMP), Campinas, 13.083-
970 Sao Paulo, Brazil
?Presented 19 January 2010; received 31 October 2009; accepted 29 December 2009;
published online 21 April 2010?
We report the near edge x-ray absorption fine structure ?NEXAFS? and x-ray magnetic circular
dichroism ?XMCD? studies at the Mn L3,2edge of pulsed laser deposited pristine thin films of
multiferroic BiMn2O5. These investigations are furthermore testified for BiMn2O5thin films
irradiated through 200 MeV Ag15+ions with fluence value 5?1011ions/cm2. Though the pristine
film is primarily antiferromagnetic in nature, irradiation induces ferrimagnetism in it. Element
specific characterizations, NEXAFS and XMCD demonstrate the evolution of Mn2+state piloting to
magnetic signal associated with it. © 2010 American Institute of Physics. ?doi:10.1063/1.3360356?
Multiferroic materials have attracted enormous research
interest in recent years.1–12These materials show coexistence
of at least two out of elastic, electric, and magnetic orders
due to concurrence among elasticity, charge, and spin de-
grees of freedom. These are vital for assorted potential ap-
plications in nonvolatile memories, capacitors, transducers,
actuators, high density data storage, multiple state memories,
magnetic field sensors, logical memory based devices, etc.
Apart from numerous prospective functions, they are rich
source of fundamental physics too. Among recently discov-
ered multiferroic materials, RMn2O5?where R=rare earth, Y
and Bi? family reveals antiferromagnetism ?antiferromag-
netic transition temperature, TN?39 K? along with ferro-
electricity ?ferroelectric transition temperature, TC?35 K?
as well as coupling between them. The said properties are the
prerequisite for application purpose.6–12Member BiMn2O5
from this family, has been widely studied,8–12whose mag-
netic structure is commensurate with a propagation vector
q=?1/2,0,1/4? and is ferroelectric too, below TN. Geo-
metrical structure of BiMn2O5consists of mixed valence
Mn-sites ?Mn3+/Mn4+?, where Mn4+O6 octahedra share
edges to form infinite chains along the c-axis. These chains
are linked by pairs of Mn3+O5pyramids and BiO8polyhedra
that exhibit five different nearest neighbor interactions
among Mn3+/Mn4+ions showing the way to frustrated mag-
netic structure.6–12In our earlier study,10it is found that the
thin film of BiMn2O5?synthesized on LaAlO3, LAO? is mag-
netically frustrated due to substrate induced strain and dem-
onstrates spin-glass like behavior. Nevertheless, by further
tailoring the strain or defects present in the as grown film,
one can improve its magnetic property for more attractive
When swift heavy ion ?SHI? passes through a target ma-
terial, it either excites or ionizes the atoms by inelastic col-
lisions or displaces them by elastic collisions. Elastic colli-
sions are dominant at low energy regime, whereas inelastic
collision process dominates at high-energy regime where the
elastic collisions are insignificant. It is evident from the lit-
erature that the electronic energy loss ?Se? due to inelastic
collisions is able to generate point/cluster defects if it is less
than the threshold value of electronic energy loss ?Seth?.
Nonetheless, the grater value of Secompared with Seth
??14.25 keV/nm, present case? may generate columnar
amorphization. Stress/strain developed by created defects
and amorphization is responsible for modification in diverse
properties of the materials.13–15In this paradigm, the ion
beam treatment on highly sensitive geometrical/magnetic
structure of BiMn2O5?Refs. 8–12? may offer various possi-
bilities of engineering the magnetic property of this material.
Present article deals with the effect of 200 MeV Ag15+
ion irradiation on ionic state of the Mn ions and its conse-
quence on their magnetic properties by performing the near
edge x-ray absorption fine structure ?NEXAFS? and x-ray
magnetic circular dichroism ?XMCD? studies of the pristine
and the irradiated BiMn2O5thin films. Owing to the statisti-
cal nature of energy deposition process, stopping and range
of ions in matter calculation16based on Monte Carlo simu-
lation is used to plan the irradiation energy ??200 MeV?.
Thin film of BiMn2O5?thickness ?200 nm? has been
deposited on c-axis oriented single crystal of LaAlO3?LAO?
a?Author to whom correspondence should be addressed. Electronic mail:
b?Present address: Hamburger Synchrotronstrahlungslabor HASYLAB at
Deutsches Elektronen-Synchrotron DESY, 22605 Hamburg, Germany.
c?On extraordinary leave from Inter University Accelerator Center, New
JOURNAL OF APPLIED PHYSICS 107, 09D903 ?2010?
0021-8979/2010/107?9?/09D903/3/$30.00 © 2010 American Institute of Physics
substrate by pulsed laser deposition ?PLD? under optimized
conditions using single phased target of BiMn2O5prepared
by solid state reaction route.12Then the film was cut into two
pieces of size 10?5 mm2. One part of it was kept pristine.
The other was irradiated at room temperature with
200 MeV Ag15+
?1011ions/cm2by 15 UD tandem accelerator at the Inter-
University Accelerator Centre, New Delhi, India. The irra-
diation was performed under high vacuum condition ?base
pressure 2?10−6Torr?. Incident angle of the ion beam was
kept slightly away from the surface normal to the sample to
avoid the channeling effects. The beam current was kept 0.1
pnA to avoid heating of the film. The ion beam was uni-
formly scanned over 1 cm2area using a magnetic scanner.
Fluence value was determined by measuring the charge fall-
ing over the sample surface under the secondary electron
suppressed geometry. Ladder current was measured with a
current integrator and a scalar counter. NEXAFS and XMCD
measurements at the Mn L3,2edge were performed at the
European Synchrotron Radiation Facility ?ESRF? ID08
beamline by employing an APPLE II type undulator giving
?100% linear/circular polarization. All scans were collected
simultaneously in both total electron yield and total fluores-
cence yield modes, ensuring equally surface and bulk sensi-
tivities. Spectra were normalized to incident photon flux.
Base pressure of the experimental chamber was better than
III. RESULTS AND DISCUSSION
X-ray diffraction patterns ?not shown here? of irradiated
sample17suggest that the irradiation amplifies the lattice pa-
rameters and diminishes the grain size of the film. It is to be
noted that the pristine film suffers from strain which is in-
duced by the substrate. However, the strain is released and
the film relaxes after irradiation because of the transfer of
irradiation energy into the film. Similar observations have
been reported earlier on different materials’ thin films.18–21
Hence, all the Se-dependent effects induced in these materi-
als are probably related to the same basic energy transfer
processes between the incident ions and the target atoms.
This can be explained by thermal spike model.22According
to this model, most of the energies of the incident ions are
transferred to the host electrons during electron slowing
down regime ?Se?Sn?. This brings the increase in tempera-
ture in the electronic subsystem to far above its melting tem-
perature ?thermal spike?. The rapid heating is localized
around traveling ion path in material and is followed by a
fast thermal quenching ??1013–1014K/s?. Therefore, the
flow of SHI through BiMn2O5causes rapid thermal quench-
ing after massive heating. This instigates higher energetic
configuration giving rise to the proliferation of unit cell vol-
ume as well as shrinkage in grain size.
Magnetization measurements insinuate that the films’
magnetic property changes from spin glass to ferrimagnetic
behavior after the irradiation.17To look into the change in
magnetic properties, we have investigated the effect of SHI
irradiation on Mn ions present in the film. These ions are
mainly responsible for the magnetic property of system. In
this context, element specific characterizations like NEXAFS
and XMCD facilitates to comprehend the upshot of irradia-
tion ?observed change in magnetic properties?.
Figure 1 shows the normalized Mn L3,2edge NEXAFS
spectra ofthe pristineand
?1011ions/cm2? thin films at room temperature along with
the references MnO and MnO2. Spin-orbit interaction of the
Mn 2p core states splits the spectrum into two broad multi-
plets, namely L3?2p3/2? and L2?2p1/2? with ?11 eV energy
apart. Each of these two regions further split into t2gand eg
orbital features because of the crystal-field effect of neigh-
boring ions. These spectra demonstrate valance-specific mul-
tiplet structure with a chemical shift due to change in oxida-
tion state. Detailed description of Mn L3,2edge NEXAFS
spectra of pristine thin film along with reference spectra of
MnO and MnO2are presented elsewhere,10reveal that Mn
has +3.5 state as the ratio of Mn3+/Mn4+is 1 in BiMn2O5.
SHI irradiation induced modifications can be directly ob-
served in both the peaks L3as well as L2?Fig. 1?. However,
it is more prominent in L3edges since it is more susceptible
to local environment than L2. The inflection point of Mn L3
edge shifts toward higher energy as valence number of Mn
increases from +2 in MnO to +4 in MnO2. After irradiation,
the L3region displays evolution of new peak at ?640 eV
?marked by arrow in Fig. 1?. This exactly matches with the
intense L3peak of MnO which has domination of t2gstates.
This indicates that some part of Mn ions are transferred into
Mn2+and ratio of Mn3+/Mn4+of pristine film is changed
from 1. With the evolution of Mn2+, the peak at ?643 eV
due to Mn4+diminishes confirming that evolution of Mn2+is
at expense of Mn4+. In this scenario, spectral signature of
Mn3+, which is left undistributed will have more obvious
dominancy in this multiplet spectrum and is clearly observed
at ?641.5 eV. Shift in L2peak toward lower energy also
vindicates the increased spectral weightage of Mn3+. Evolu-
FIG. 1. ?Color online? Normalized Mn L3,2edge NEXAFS spectra of pris-
tine as well as 200 MeV Ag15+ions irradiated BiMn2O5thin film at room
temperature along with the reference spectra of MnO and MnO2.
09D903-2Shukla et al.J. Appl. Phys. 107, 09D903 ?2010?
tion of Mn2+content at the cost of Mn4+with irradiation may
be accepted as a direct reason of increasing magnetic mo-
ment. However, in order to acquire in depth information
about magnetic contribution from Mn2+, XMCD is per-
formed at 100 and 300 K.
The well established XMCD method is one of the most
reliable techniques for directly measuring the moments in a
valence shell. Figure 2 demonstrates the normalized XMCD
spectra for pristine and irradiated samples measured at a
magnetic field of five tesla. Each upper and lower panel pre-
sents the XMCD spectra at 300 K and 100 K, respectively,
for the same sample. Although there is a little dependence of
the NEXAFS signal on the polarization of the incident x-ray
photon, a clear reproducible difference between the NEX-
AFS collected for the photon including parallel and antipar-
allel helicity with the applied magnetic field ?5 T? gives the
XMCD signal. In the spectra for the irradiated sample ?Fig.
2?, the XMCD signal of Mn2+at energy ?640 eV is clearly
observed. This is in accordance with the spectral position of
Mn2+observed in NEXAFS spectra for irradiated sample
In conclusion, a modeled irradiation experiment has
been performed using 200 MeV Ag15+as projectile ion and
magnetically frustrated PLD grown thin film of multiferroic
BiMn2O5as a target. Interestingly, irradiation induced ferri-
magnetism is observed in an originally antiferromagnetic
thin film of BiMn2O5. Element specific observations such as
NEXAFS and XMCD illustrate the evolution of Mn2+in a
network of Mn3+/Mn4+at the expanse of Mn4+. These find-
ings spur that the SHI irradiation can be used to custom-
tailor the properties of oxide multiferroics for technological
Authors are thankful to Dr. A. Roy, Director, Inter-
University Accelerator Centre ?IUAC?, New Delhi, India.
D.K.S. is thankful to CSIR, New Delhi, India for providing
financial support. Department of Science and Technology
?DST?, Government of India, is acknowledged for supporting
this work under Project No. S2/SR/CMP-0051/2007. S.K.S.
is thankful to FAPESP ?Process No. 2006/06792-2? for pro-
viding financial support. M.K. acknowledges the John Simon
Guggenheim Memorial Foundation for the Guggenheim Fel-
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FIG. 2. ?Color online? Mn L3,2edge XMCD spectra at 100 and 300 K for
pristine and 200 MeV Ag15+ions irradiated BiMn2O5thin film at a mag-
netic field of five tesla.
09D903-3Shukla et al.J. Appl. Phys. 107, 09D903 ?2010?