JOURNAL OF APPLIED PHYSICS 107, 09D903 共2010兲
Modifications in magnetic properties of BiMn2O5 multiferroic using swift heavy ion irradiation D. K. Shukla,1,a兲,b兲 Ravi Kumar,2,c兲 S. Mollah,1 R. J. Choudhary,3 P. Thakur,4 S. K. Sharma,5 N. B. Brookes,4 and M. Knobel5 1
Department of Physics, Aligarh Muslim University, Aligarh 202002, India 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, 13.083970 Sao Paulo, Brazil 2
共Presented 19 January 2010; received 31 October 2009; accepted 29 December 2009; published online 21 April 2010兲 We report the near edge x-ray absorption fine structure 共NEXAFS兲 and x-ray magnetic circular dichroism 共XMCD兲 studies at the Mn L3,2 edge of pulsed laser deposited pristine thin films of multiferroic BiMn2O5. These investigations are furthermore testified for BiMn2O5 thin films irradiated through 200 MeV Ag15+ ions with fluence value 5 ⫻ 1011 ions/ 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. 关doi:10.1063/1.3360356兴 I. INTRODUCTION
Multiferroic materials have attracted enormous research interest in recent years.1–12 These materials show coexistence of at least two out of elastic, electric, and magnetic orders due to concurrence among elasticity, charge, and spin degrees of freedom. These are vital for assorted potential applications 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 discovered multiferroic materials, RMn2O5 共where R = rare earth, Y and Bi兲 family reveals antiferromagnetism 共antiferromagnetic transition temperature, TN ⬃ 39 K兲 along with ferroelectricity 共ferroelectric transition temperature, TC ⬃ 35 K兲 as well as coupling between them. The said properties are the prerequisite for application purpose.6–12 Member BiMn2O5 from this family, has been widely studied,8–12 whose magnetic structure is commensurate with a propagation vector q = 共1 / 2 , 0 , 1 / 4兲 and is ferroelectric too, below TN. Geometrical structure of BiMn2O5 consists of mixed valence Mn-sites 共Mn3+ / Mn4+兲, where Mn4+O6 octahedra share edges to form infinite chains along the c-axis. These chains are linked by pairs of Mn3+O5 pyramids and BiO8 polyhedra that exhibit five different nearest neighbor interactions among Mn3+ / Mn4+ ions showing the way to frustrated magnetic structure.6–12 In our earlier study,10 it is found that the thin film of BiMn2O5 共synthesized on LaAlO3, LAO兲 is maga兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected] b兲 Present address: Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronen-Synchrotron DESY, 22605 Hamburg, Germany. c兲 On extraordinary leave from Inter University Accelerator Center, New Delhi-110067. 0021-8979/2010/107共9兲/09D903/3/$30.00
netically frustrated due to substrate induced strain and demonstrates 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 functionality. When swift heavy ion 共SHI兲 passes through a target material, it either excites or ionizes the atoms by inelastic collisions or displaces them by elastic collisions. Elastic collisions 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 literature that the electronic energy loss 共Se兲 due to inelastic collisions is able to generate point/cluster defects if it is less than the threshold value of electronic energy loss 共Seth兲. Nonetheless, the grater value of Se compared with Seth 共⬃14.25 keV/ nm, present case兲 may generate columnar amorphization. Stress/strain developed by created defects and amorphization is responsible for modification in diverse properties of the materials.13–15 In this paradigm, the ion beam treatment on highly sensitive geometrical/magnetic structure of BiMn2O5 共Refs. 8–12兲 may offer various possibilities 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 consequence on their magnetic properties by performing the near edge x-ray absorption fine structure 共NEXAFS兲 and x-ray magnetic circular dichroism 共XMCD兲 studies of the pristine and the irradiated BiMn2O5 thin films. Owing to the statistical nature of energy deposition process, stopping and range of ions in matter calculation16 based on Monte Carlo simulation is used to plan the irradiation energy 共⬃200 MeV兲. II. EXPERIMENTAL
Thin film of BiMn2O5 共thickness ⬃200 nm兲 has been deposited on c-axis oriented single crystal of LaAlO3 共LAO兲
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substrate by pulsed laser deposition 共PLD兲 under optimized conditions using single phased target of BiMn2O5 prepared by solid state reaction route.12 Then 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+ ion beam of fluence value 5 ⫻ 1011 ions/ cm2 by 15 UD tandem accelerator at the InterUniversity Accelerator Centre, New Delhi, India. The irradiation was performed under high vacuum condition 共base pressure 2 ⫻ 10−6 Torr兲. 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 uniformly scanned over 1 cm2 area using a magnetic scanner. Fluence value was determined by measuring the charge falling 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 L3,2 edge were performed at the European Synchrotron Radiation Facility 共ESRF兲 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 fluorescence yield modes, ensuring equally surface and bulk sensitivities. Spectra were normalized to incident photon flux. Base pressure of the experimental chamber was better than 3 ⫻ 10−10 Torr. III. RESULTS AND DISCUSSION
X-ray diffraction patterns 共not shown here兲 of irradiated sample17 suggest that the irradiation amplifies the lattice parameters and diminishes the grain size of the film. It is to be noted that the pristine film suffers from strain which is induced 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 materials 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.22 According to this model, most of the energies of the incident ions are transferred to the host electrons during electron slowing down regime 共Se Ⰷ Sn兲. This brings the increase in temperature in the electronic subsystem to far above its melting temperature 共thermal spike兲. The rapid heating is localized around traveling ion path in material and is followed by a fast thermal quenching 共⬃1013 – 1014 K / s兲. Therefore, the flow of SHI through BiMn2O5 causes rapid thermal quenching after massive heating. This instigates higher energetic configuration giving rise to the proliferation of unit cell volume 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.17 To 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
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FIG. 1. 共Color online兲 Normalized Mn L3,2 edge NEXAFS spectra of pristine as well as 200 MeV Ag15+ ions irradiated BiMn2O5 thin film at room temperature along with the reference spectra of MnO and MnO2.
this context, element specific characterizations like NEXAFS and XMCD facilitates to comprehend the upshot of irradiation 共observed change in magnetic properties兲. Figure 1 shows the normalized Mn L3,2 edge NEXAFS spectra of the pristine and the irradiated 共with 5 ⫻ 1011 ions/ cm2兲 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 multiplets, namely L3共2p3/2兲 and L2共2p1/2兲 with ⬃11 eV energy apart. Each of these two regions further split into t2g and eg orbital features because of the crystal-field effect of neighboring ions. These spectra demonstrate valance-specific multiplet structure with a chemical shift due to change in oxidation state. Detailed description of Mn L3,2 edge NEXAFS spectra of pristine thin film along with reference spectra of MnO and MnO2 are presented elsewhere,10 reveal that Mn has +3.5 state as the ratio of Mn3+ / Mn4+ is 1 in BiMn2O5. SHI irradiation induced modifications can be directly observed in both the peaks L3 as well as L2 共Fig. 1兲. However, it is more prominent in L3 edges 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 L3 region displays evolution of new peak at ⬃640 eV 共marked by arrow in Fig. 1兲. This exactly matches with the intense L3 peak of MnO which has domination of t2g states. 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 L2 peak toward lower energy also vindicates the increased spectral weightage of Mn3+. Evolu-
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magnetically frustrated PLD grown thin film of multiferroic BiMn2O5 as a target. Interestingly, irradiation induced ferrimagnetism 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 findings spur that the SHI irradiation can be used to customtailor the properties of oxide multiferroics for technological application. ACKNOWLEDGMENTS
Authors are thankful to Dr. A. Roy, Director, InterUniversity Accelerator Centre 共IUAC兲, New Delhi, India. D.K.S. is thankful to CSIR, New Delhi, India for providing financial support. Department of Science and Technology 共DST兲, Government of India, is acknowledged for supporting this work under Project No. S2/SR/CMP-0051/2007. S.K.S. is thankful to FAPESP 共Process No. 2006/06792-2兲 for providing financial support. M.K. acknowledges the John Simon Guggenheim Memorial Foundation for the Guggenheim Fellowship 2009–2010. 1
FIG. 2. 共Color online兲 Mn L3,2 edge XMCD spectra at 100 and 300 K for pristine and 200 MeV Ag15+ ions irradiated BiMn2O5 thin film at a magnetic field of five tesla.
tion of Mn2+ content at the cost of Mn4+ with irradiation may be accepted as a direct reason of increasing magnetic moment. However, in order to acquire in depth information about magnetic contribution from Mn2+, XMCD is performed 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 presents 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 NEXAFS collected for the photon including parallel and antiparallel helicity with the applied magnetic field 共5 T兲 gives the XMCD signal. In the spectra for the irradiated sample 共Fig. 2兲, 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 共Fig. 1兲. IV. CONCLUSIONS
In conclusion, a modeled irradiation experiment has been performed using 200 MeV Ag15+ as projectile ion and
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