Enhancement of magnetization in Eu doped BiFeO3 nanoparticleslv钱包
Jian Liu, Liang Fang, Fengang Zheng, Sheng Ju, and Mingrong Shen
Citation: Appl. Phys. Lett. 95, 022511 (2009); doi: 10.1063/1.3183580
View online: dx.doi/10.1063/1.3183580
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Enhancement of magnetization in Eu doped BiFeO3nanoparticles Jian Liu,Liang Fang,a͒Fengang Zheng,Sheng Ju,b͒and Mingrong Shen c͒
Department of Physics and Jiangsu Key Laboratory of Thin Films,Soochow University,Suzhou215006,
People’s Republic of China
͑Received27April2009;accepted29June2009;published online14July2009͒
Bi1−x Eu x FeO3͑BEFO x͒nanoparticles have been synthesized by a sol-gel method.The samples maintain the pure rhombohedral structure up to x=0.1,while an impure pha of EuFeO3is obrved at x=0.15.The remenant magnetization of the BEFO x,which is significantly higher than tho in nonmagnetically rare-earth doped BiFeO3,is found to increa with the increa of Eu concentration.Ferromagnetic coupling between Eu3+and Fe3+ions,evidenced by our first-principles calculations,is found to be the main reason for such a magnetization enhancement.
In the mean time,the appearance of EuFeO3pha is demonstrated to be another factor for the further magnetization enhancement in BEFO x=0.15.©2009American Institute of Physics.
͓DOI:10.1063/1.3183580͔
Magnetoelectric͑ME͒multiferroics,which show simul-
taneous electric and magnetic orders,have been widely stud-
ied in recent years for their potential applications in data绷带包扎
storage,nsors,and spintronic devices.1–4Among all the
identified single pha ME multiferroics,BiFeO3͑BFO͒is one of few candidates exhibiting room temperature ferroelec-
tric͑Curie temperature T Cϳ1103K͒and G-type antiferro-magnetic͑Néel temperature T Nϳ643K͒properties.5Al-though this property is very promising in terms of practical applications,the antiferromagnetic ordering with a spatially modulated spin structure in bulk BFO,6does not allow a net magnetization as well as linear ME effect in it.Weak ferro-magnetic properties and the linear ME effect,however,have been obrved in BFO thinfilms,which is due to the het-eroepitaxial strain,oxygen deficiencies,enhanced anisotropy, or parasitic phas.5,6Recently,attentions have been paid to the realization of weak ferromagnetism in BFO nanowires and nanoparticles,7,8where the significant size effect on the modification of magnetic properties in BFO was evidenced. On the other hand,it has been demonstrated that the spatially modulated spin structure of BFO can be suppresd by rare-earth͑RE͒doping or alkaline-earth elements replacement of Bi,giving ri to improved magnetic properties in the doped BFO compounds.9–12
Although veral experiments were performed on the RE ͑e.g.,La3+,Nd3+,or Sm3+͒doped BFO comp
ounds,minor attention has been paid to europium͑Eu͒doped BFO.13,14 Unlike other RE cations,Eu3+ion is magnetically active and its coupling to Fe3+ion is expected to has an giant impact on the magnetization of BFO even in the lightly doping concen-tration.In this letter,we prepared Bi1−x Eu x FeO3͑x=0–0.15, abbreviated as BEFO x͒nanoparticles by a sol-gel method followed by rapid thermal annealing process.The micro-structure,chemical composition,and magnetic properties are studied.Our results showed that the remnant magnetizations of BEFO were at least an order of magnitude greater that tho obrved for the La,Nd,or Sm doped BFO at the same doping concentrations.Thisfinding was later supported by ourfirst-principle calculations,which demonstrates a ferro-magnetic coupling between Fe3+and Eu3+ions.
Pure and Eu doped BFO nanoparticles were prepared by a sol-gel method.The precursor solution was prepared by mixing appropriate amounts of Bi͑NO3͒3·5H2O, Eu͑NO3͒3·3H2O,and Fe͑NO3͒3·9H2O dissolved in
2-methoxyethanol.Tartaric acid in1:1molar ratio with re-spect to metal nitrates was added to the solution.The mixed gel was then dried and grinded into powders,followed by calcination at550°C for120min.After that,the powders were sintered for450s in air at820°C,and then sub-quently cooled rapidly to room temperature.The crystal structures of the sintered samples were measured by
x-ray diffraction͑XRD͒on an Rigaku D/MAX3C x-ray diffracto-meter using Cu K␣radiation.The measured voltage and cur-rent were40kV and150mA,respectively.The morpholo-gies of samples were investigated by transmission electron microscopy͑TEM͒using a FEI Model TecnaiG220instru-ment.Raman scattering spectra were recorded in backscatter-ing geometry with a JY-HR800Raman spectrometer using an Ar+lar͑514.5nm͒as excitation line.The room tem-perature magnetization-magneticfields͑M-H͒for all samples were carried out with a commercial superconducting quantum interference device͑SQUID͒system͑Quantum De-sign MPMS͒.
Figure1shows the XRD patterns of BFO and BEFO nanoparticles.It is obrved that the BFO sample has a rhombohedral structure15with no trace of other impurity phas.The average crystallite size of BFO sample estimated from XRD pattern by means of the Scherrer equation is ϳ80nm,which implies the nanostructure characteristic of BFO and can be confirmed by the measurement results from TEM images.As shown in int͑a͒of Fig.1,the pure BFO powder exhibits a well-defined cubic morphology with an average particle size about100nm,which is slightly larger than the crystallite size obtained by the Scherrer equation. Such phenomenon has also been obrved in other reported works and can be ascribed to the agglomeration of the particles.16Moreover,the BFO pha keeps pure with x up to 0.1,indicating the incorporation of Eu3+ions into the BFO crystal structure.However,
some other peaks͑indicated by star symbols͒can be obrved at BEFO x=0.15sample,which
a͒Electronic mail:lfang@suda.edu.
b͒Electronic mail:jusheng@suda.edu.
c͒Electronic mail:mrshen@suda.edu.
APPLIED PHYSICS LETTERS95,022511͑2009͒
0003-6951/2009/95͑2͒/022511/3/$25.00©2009American Institute of Physics
95,022511-1
are attributed to the new pha of EuFeO 3͑EFO ͒.This phe-nomenon demonstrates that the solution limitation is near 10%for Eu doping in BiFeO 3.It can be noted from the int ͑b ͒in Fig.1that the XRD peaks move to large angles as the Eu concentration increas,revealing that Eu 3+ions may oc-cupy the Bi site in BFO becau the ion radius ͑1.07Å,CN 8͒for Eu 3+is smaller than that ͑1.17Å,CN 8͒of Bi 3+ion.17In addition,the TEM images of all the doped samples show that the particle sizes are about 100nm and have no signifi-cant difference.
黄金手镯The structural pha change in BEFO is also investigated by the Raman measurements.In order to clarify the Raman active modes in the samples,the measured spectra were fitted and decompod into individual Lorentzian components.As can be en in Fig.2,three peaks with strong scattering intensities and the six peaks with medium scattering intensi-ties can be obrved in our pure BFO powders,which agree well with tho of the BFO thin films and ceramics.18,19However,E -1,A 1-1,A 1-2,and A 1-3and E -2modes,which
have been demonstrated governed by Bi–O covalent bonds,shift gradually to high mode frequencies with the increa of Eu doping concentrations,indicating the dopant Eu is enter-ing into the Bi-site of BFO.At x =0.15,the A 1-1,A 1-3,and E -2modes nearly disappear,demonstrating an abrupt change of Bi–O covalent bonds.Moreover,a new peak at 300cm −1can be obrved,which may result from EFO pha in the compounds.Such peak has also been reported to exist in NdFeO 3single crystal,and can be assigned as A g mode.20The oxidation states of Fe for all the samples were deter-mined by x-ray photoelectron spectroscopy ͑XPS ͒͑PHI-550͒analysis ͑data not shown here ͒.From the XPS spectra of Fe 2p ,no obvious differences were obrved for the samples.Two peaks at 710.6and 724.1eV correspond to Fe 3+,while no peak due to Fe 2+can be found.21
Figure 3shows the room temperature magnetization-magnetic field ͑M -H ͒curves of BEFO x samples f国产轿车销量排行榜前十名
桌面共享
or the maximum magnetic field ͑H m ͒of 45kOe.The pure BFO nanoparticles display a low level of spontaneous magnetiza-tion instead of the antiferromagnetic characteristic in bulk BFO,which is consisted with other reports and attributed to the fact that the spiral spin order is partially destroyed in the BFO nanoparticles.8,22As expected,the Eu doped samples exhibit a hysteresis M -H behavior ͓Figs.3͑b ͒and 3͑c ͔͒,however,becau of the limitation of the maximum field of SQUID,the hysteresis loops behaved as minor loops.This phenomenon was also reported in some nanoparticle systems,such as PtMn and CaMnO 3−␦,23,24and might be attributed to antiferromagnetic core and ferromagnetic sur-face of the nanoparticles.Moreover,similar as other RE ions doped BFO compounds,remnant magnetization ͑M r ͒and co-ercive field ͑H c ͒of BEFO samples increa with the increa of Eu doping concentrations,as shown in Fig.3͑d ͒.
It can also be noted that the M r value of BEFO x =0.01sample is about 0.13emu/g under 45kOe,which is signifi-cantly greater than tho of nonmagnetically RE doped BFO.For example,the measured M r values at H m of 60kOe are 0.067,0.015,and 0.071emu/g for Bi 0.85La 0.15FeO 3,Bi 0.9Nd 0.1FeO 3,and Bi 0.875Sm 0.125FeO 3ce-ramics,respectively.9–11Obviously,such phenomenon cannot simply be explained bad on the suppression of the spin structure due to the Bi-site substitution.9–11We propo that the unique distribution of Eu 4f electrons which results in
the
FIG.1.͑Color online ͒͑a ͒XRD patterns of BEFO x samples.Int ͑a ͒shows the TEM image of undoped BFO.Int ͑b ͒shows the magnified patterns of ͑104͒and ͑110͒diffraction
peaks.
FIG.2.͑Color online ͒Raman scattering spectra ͑measured spectra:open circles and fitted spectra:thick solid lines ͒and the decompod active modes ͑thin solid lines and arrows ͒for BEFO x samples.The intensities of the BEFO x =0.15spectra have been amplified by three times for better
illustration.
玻璃品牌
金庸武功FIG.3.͑Color online ͓͒͑a ͒–͑c ͔͒Magnetization hysteresis loops of BEFO x samples.͑d ͒Eu concentration dependent remnant magnetization and coer-cive field.
Eu 3+ions magnetically active can cau such magnetic en-hancement.In order to examine this point,first-principle cal-culations were utilized to investigate the local electronic structures as well as magnetic interactions in BEFO.Our ab initio calculations are performed using the accurate full-potential projector augmented wave method,as implemented in the Vienna ab initio simulation package ͑V ASP ͒.25They are bad on the density functional theory with the generalized gradient approximation ͑GGA ͒.The on-site Coulomb inter-action is included in the GGA+U approach with U =5.0eV and J =1.0eV for Fe 3d electrons.The fully re-laxed crystal structure,which shows a R 3c rhombohedral symmetry,is in good agreement with experiment.25In the mean time,antiferromagnetic configuration is the ground state,with 0.1569eV/f.u.lower than the ferromagnetic one.For the effect of Eu doping,we build up a 2ϫ2ϫ2super-cell,with 80atoms therein.One of Bi atoms is replaced by Eu,which corresponds to 6.25%doping.The on-site Cou-lomb interaction is included in the GGA+U approach with U =6.0eV and J =1.0eV for Eu 4f electrons.It is found that Eu 3+relaxes clo to three Fe 3+ions and a ferromagnetic coupling between Eu 3+and three Fe 3+is obrved,leading to a pure magnetic moment of the system which is 6B.This calculation strongly supports that the magnetically active Eu 3+ions coupled with Fe 3+ions can produce enhanced magnetism in BEFO compound.Besides the contribution of unique distribution of Eu 4f electrons,Eu 3+has a smaller ion radius than tho of other ions radius ͑La 3+:1.16Å,CN 8;Nd 3+:1.11Å,CN 8;and Sm 3+:1.08Å,CN 8͒,1
7which can lead to larger lattice distortion when substitute Bi ion and may also contribute partly to the enhanced magnetization.
The appearance of EFO pha might be another reason responsible for the obrved high M r and H c in BEFO x =0.15.To prove this assumption,BiEuO 3͑BEO ͒and EFO samples were prepared using the same method for BFO.Figure 4shows the XRD results.Both samples have the polycrystal-line structure with pure phas,and the BEO sample has a cubic structure,26while the pattern of EFO can be well in-dexed to orthorhombic structure.27The int of Fig.4shows the M -H curves of the BEO and EFO samples.The plot for the BEO shows a linear decrea with magnetic field,indi-cating a paramagnetic-ordered state at room temperature.However,EFO exhibit ferromagnetic behavior from the M -H curve,which is similar to that reported in the literatures.28Thus the existence of the EFO pha in BEFO x =0.15may further enhance the magnetic moment in the compounds.
However,the total understanding of the enhancement of magnetic moment needs further study by using some unique techniques,such as photoelectron emission microscope.
In summary,Eu doped BFO nanoparticles were synthe-sized by a sol-gel method followed by rapid thermal anneal-ing process.XRD and Raman studies indicated that all the samples showed the rhom
bohedral structure up to the mod-erated doping level of x =0.1,while a impure pha of EFO appeared at x =0.15,illustrating that the soluble limitation is around 10%for Eu 3+doping into BFO.An enhancement in magnetization for BEFO samples was obrved with the in-crea of Eu doping concentrations,which was ascribed mainly to the contribution of the magnetically active charac-teristic of Eu 3+ions.The appearance of europium orthofer-rite might be another reason responsible for the obrved high magnetization values in BEFO x =0.15.This study demon-strated that Eu 3+ions were more effective to increa the M r of BFO than other RE ions such as La 3+,Nd 3+,or Sm 3+.This rearch was supported by the Natural Science Foundation of Jiangsu Education committee of China ͑Grant No.08KJB140008͒and the National Natural Science Foun-dation of China ͑Grant No.50702036͒.
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FIG.4.͑Color online ͒XRD patterns of BEO and EFO samples.Int shows the magnetization hysteresis loops of BEO and EFO samples.
