Novel Insights of Enhanced Magnetization and Magnetoelectric Coupling in the Nanocomposite of BiFeO3 with Lanthanum Strontium Manganite

Novel insights on the magnetic, ferroelectric and magnetoelectric coupling of the nano composites of BiFeO3 (BFO) and La0.7Sr0.3Mn0.95Fe0.05O3 (LSMFO) with the latter exhibiting colossal magnetoresistance (CMR) behavior are provided in this study for the first time. Appreciable magnetoelectric coupling is observed in the composite and is seen to be significantly varying at temperatures below the blocking temperature of LSMFO. The local structure and magnetic properties of both the BFO and LSMFO particles in the composite are investigated in a detailed manner using Mössbauer spectroscopy studies specially devised to address the problem concerned. Enhanced values of the effective magnetic fields are observed at the Fe sites associated with BFO particles which are in contact with LSMFO particles. Interestingly, an appreciable increase in the magnetic hyperfine fields at Fe sites corresponding to LSMFO is also observed in the nanocomposites. New insights are derived based on the results of Mössbauer studies toward comprehending the magnetic and magnetoelectric coupling effects as deduced in these composites.


Introduction
There has been a resurgence of research interest in enhancing the magnetoelectric coupling effects in BiFeO 3 toward realization of magnetoelectric memory and storage devices with enhanced efficiency. [1,2]Striking optimal ways of inducing weak ferromagnetic ordering partially in BiFeO 3 (BFO) in terms of size effects, suitable doping leading to an appreciable increase in the DOI: 10.1002/apxr.202200101magnetoelectric coupling still remain as a challenging problem of research and technological interest.[18][19] Most of the studies reported on FM multilayer thin films and BFOAFM-based nanocomposites have addressed in detail regarding the magnetic properties, in particular the coercivity and not the ferroelectric properties.[22] Nanoparticles of BiFeO 3 have been reported to exhibit weak ferromagnetic properties, [23][24][25] while quite a few research work have demonstrated that the nanoparticles of BiFeO 3 prepared through selective methods display ferroelectric ordering in addition to weak ferromagnetic properties. [23,24]Importantly, an appreciable magnetoelectric coupling effects have been observed in such nanoparticles of BiFeO 3 . [24]Weak ferromagnetic ordering as reported in the case of the nanoparticles of BiFeO 3 , is ascribed due to the non-cancellation of spins at the surface of these nanoparticles.Though there are some results reported on the observation of enhanced magnetoelectric coupling effect in the composites of BiFeO 3 with ferromagnetic oxides, detailed understanding of these results leading to magnetoelectric coupling effects is yet to emerge. [9,15,26]As the magnetoelectric coupling effect is established in the case of nano BiFeO 3 , it is quite likely that the coupling effects could be significantly enhanced in the case of nanocomposite of BiFeO 3 with suitably chosen ferromagnetic oxides.
Among the ferromagnetic oxides, manganites exhibiting colossal magnetoresistance (CMR) might be a good choice as these oxides exhibit a strong correlation between spin, charge, orbital, and lattice degrees of freedom [27][28][29][30] leading to the scope of significantly improving the multifunctionality of the devices made in combination with BiFeO 3 .33][34] In this study the ferromagnetic nanoparticles of La 0.7 Sr 0.3 MnO 3 (LSMO) are chosen as a constituent of the composite of nano BiFeO 3 particles due to the following reasons.A) Crystal structure of LSMO is rhombohedral (R3c) and the same as that of BFO, which would enable the formation of coherent interface between the nanograins of LSMO and BFO.This is a very important condition with respect to enhancing magnetic interactions among the particles, B) LSMO is a half metallic system in which there is a strong correlation between charge, spin, and orbital properties providing an option for us to explore new multifunctional devices, C) system of the nanoparticles of LSMO interestingly exhibits metal to insulator transition with the transition temperature and the value of the resistance are strongly dependent upon the application of magnetic field (Colossal Magnetoresistance effect) which is tunable based on the size. [35,36]Any detailed exploration of the magnetoelectric coupling in the nanocomposites of BFO-LSMO could lead to the realization of devices with enhanced multifunctionality.And D) the LSMO particles are reported to exhibit magnetostriction and strain effects.This is likely to result in significantly improved magnetoelectric coupling if nanocomposite of BFO is made with LSMO particles.
The present study proposes to investigate the magnetic, ferroelectric, and magnetoelectric coupling effects in the nanocomposites of BiFeO 3 (BFO) and La 0.7 Sr 0.3 MnO 3 (LSMO).Importantly, this study aims at providing an atomic scale understanding of these properties using Mössbauer spectroscopy.In order to comprehend the magnetic and magnetoelectric coupling effects as inferred from the magnetization and ferroelectric investigations, it is important to analyze the magnetic properties of both the manganite and ferrite particles based on the values of the hyperfine parameters deduced at Fe sites.As it is important to understand the magnetic properties of the nanoparticles of both of the LSMO and BFO of the composites in this study, about 5% of iron atoms, appreciably enriched with 57 Fe, are doped at Mn sites in LSMO, henceforth will be referred as LSMFO.Unlike the bulk LSMO, the nanoparticles of LSMFO exhibits ferromagnetic metal to paramagnetic insulating transition. [35,36]Further this study also would elucidate the rationale of the magnetoelectric coupling effect and its variation with respect to varying external magnetic field and temperatures chosen on the basis of the magneto transport properties of the dispersed LSMFO nanoparticles.

Experimental Section
Polycrystalline nanoparticles of BiFeO 3 (BFO) and La 0.67 Sr 0.33 Mn 0.95 Fe 0.05 O 3 (LSMFO) were synthesized separately by solgel method.Tartrate-based solgel method was followed for the synthesis of nanoparticles of BiFeO 3 . [32]Salts of Bi(NO 3 ) 3 • 5 H 2 O and Fe(NO 3 ) 3 •9H 2 O of stoichiometric amounts were dissolved in 2 n HNO 3 solution by means of rigorous stirring and the tartaric acid (TA) was used as chelating agent in this synthesis procedure.TA was added to the solution keeping the molar ratio of TA to the total metal nitrates as 1:1.In the present case the gel solution was heat treated at 363 K for 4 h mainly toward dehydration.Subsequently the temperature was raised to 423 K and was held for 3 h to enhance the rate of evaporation leading to the formation of dried powder.The dried powder calcined at 773 K for 2h to yield phase pure of BiFeO 3 nanoparticles.
Synthesis of the nanoparticles of La 0.67 Sr 0.33 Mn 0.95 Fe 0.05 O 3 (LSMFO) was done by citrate autocombustion method.Stoichiometric amounts of La(NO 3 ) 3 .6H 2 O, Sr(NO 3 ) 2 , Mn(NO 3 ) 2 .4H 2 O, and Fe(NO 3 ) 3 .9H 2 O were dissolved in DI water.Citric acid was added to the solution in the molar ratio of CA to total metal nitrates as 1:1.Ethylene glycol was used as complexing agent.The pH value was adjusted to 7 by adding NH 4 OH solution.The resulting solution was heat treated at 363 K for 3 h for dehydration.Subsequent to dehydration in terms of evaporation of water, the temperature was increased to 523 K to enable the combustion process.The resultant fluffy black powder was ground and calcined at 823 K for 6 h to get rid of organic residues.The calcined powder was pressed into pellets and sintered at 1073 K for 2 h.The phase pure powders of BFO and LSMFO in proper weight percentage were ground in agate mortar with isopropanol as grinding medium.The mixed powder were pressed into pellets and sintered at 673 K for 2 h (cf. Figure 1).
Phase purity and crystal structure information were derived from the X-ray diffraction (XRD) analysis using PANalyt-icalX'Pert PRO X-ray diffractometer.The patterns were acquired using Co K source with diffracted angular range of 2 from 20°to 90°with a step size of 0.015.Micro-Raman spectrometer (WiTec Alpha 300 RA) with 532 nm Ar+ ion Laser was used to analyze the phonon modes of both the BFO and LSMFO particles.For these studies the laser power was optimized to 1 mW on the sample surface after taking into consideration the (S/N) ratio and sample degradation.Microstructural and compositional studies were carried out using Scanning Electron Microscopy (SEM) (Zeiss sigma 300).Magnetic characterization was done using vibrating sample magnetometer (VSM) from M/s. Cryogenic Inc. Magnetization as a function of temperature was obtained by two different experimental protocols, zero-field cooled (ZFC) and field cooled (FC) with applied field of 0.05 T. Magnetic hysteresis loops were also obtained at 5 K and room temperature.Temperature-dependent resistivity studies were carried out between the temperature range of 4 and 298 K using a commercial 15 T cryogen-free system from Cryogenic Ltd., UK.Ferroelectric studies were performed using ferroelectric tracer (Marine India).Magnetoelectric coupling studies were carried out by recording ferroelectric loop with the application of external magnetic field.Ferroelectric loop was recorded at low temperature (at 100 K) by keeping the sample holder in liquid nitrogen bath besides the measurements done at room temperature.
Mössbauer spectrometer was operated in constant acceleration mode and in transmission geometry using 57 Co source dispersed in Rh matrix.Each Mössbauer spectrum was acquired in 1024 channels.The values of isomer shifts presented in this study were given with respect to that of -Fe absorber at 300 K. Spectra were fitted to Lorentzian line shapes of line width (Γ) using a nonlinear least squares program to obtain hyperfine parameters such as isomer shift ( i ), quadrupole splitting (∆ i ) and magnetic hyperfine fields (B hf ) experienced by relative fractions f i of distinct 57 Fe absorber atoms.For carrying out Mössbauer measurements under the application of an external magnetic field parallel to the direction of -ray, the sample was sandwiched between two rare earth-based ring magnets each of magnetic field strength close to 0.3 Tesla.Sample-magnet assembly was placed such that the magnetic lines of force were perpendicular to the direction of the -ray.

Results and Discussion
XRD patterns as obtained in BFO, LSMFO, and their composites subjected to mixing and subsequent annealing at 673 K are shown in Figure 2. The patterns as obtained in both of the BFO and LSMFO particles are indexed to rhombohedral R3c structure matching with the literature results. [24]Further it is observed that the patterns corresponding to the mixed and annealed powders of nanocomposites contain only the Bragg reflections corresponding to BFO and LSMFO phases.It can be seen that the intensity of the most intense peak of LSMFO lying around the 2 value of 38 degree, increases with the increase in the concentration of LSMFO in the BFO-LSMFO composite as shown in the right panel of Figure 2.
The mean crystallite sizes of BFO and LSMFO were calculated using Williamson-Hall plot as shown in Figure S1, Supporting Information.The mean crystallite sizes of BFO and LSMFO were deduced to be ≈60 and 30 nm, respectively.Based on the analysis of SEM Micrographs, log normal distribution of particle size were obtained (Cf. Figure 3) in the case of both of LSMFO and BFO and their mean sizes are deduced to be ≈40 and 65 nm, respectively.In addition, the strain effects were calculated from the Williamson-Hall plot for BFO, the composites in the as mixed condition and after annealing treatments, taking in to account the peaks corresponding to BFO and are shown in Figure S2, Sup-porting Information.The value of strain as obtained shows that the sintering process has resulted in an enhanced strain effect of the BFO particles in the composites subsequent to annealing treatment at 673 K.
Results of the elemental mapping of the BFO-LSMFO composites based on SEM studies are shown in Figures S3 and S4, Supporting Information.These results show the uniform distribution of metal ions throughout the investigated area.Table S1, Supporting Information, contains the composition of each of the elements in the composites in the area of investigation and also from spot analysis.The composition of the composites is found to be matching well with that of the compounds of BFO and LSMFO.
Variation of magnetization in terms of the field cooled (FC) and zero-field cooled (ZFC) conditions with temperature and M-H loop corresponding to LSMFO nanoparticles are shown in the top panel of Figure 4. ZFC and FC curves are observed to be overlapping with decreasing temperature up to 220 K below which these curves are seen to be bifurcating.ZFC exhibits a broad variation with the maximum occurring ≈130 K, while FC is seen to increase continuously below 130 K marking the magnetic interaction between the nanoparticles of LSMO whose mean size is deduced to be ≈30 nm.M-H loop shows a non-saturation behavior with the area almost close to zero.The value of coercive field is found to be quite low close to 10 G, while that of M r is ≈0.12 emu g −1 .Based on the ZFC-FC curves and M-H loop it is understood that the LSMFO particles of mean size close to 30 nm (as obtained from XRD) are exhibiting superparamagnetic behavior.Similar results of magnetization and M-H loop variations corresponding to LSMO particles of mean size close to 30 nm are reported. [36,37]These LSMO particles are deduced to be superparamagnetic based on the results of the field dependent variation of magnetization with temperature.The M-H loop obtained at 300 K shows almost zero coercive field and remanence which imply the SPM nature of the particles.In addition the temperature dependent magnetization in ZFC and FC modes show bifurcation of magnetization while decreasing temperature which is quite different from that of the paramagnetic state.The broad ZFC shows a distribution in anisotropy due to distribution in the particle size.
The observed variation of magnetization in LSMFO is explained mainly in terms of the competing Mn 3+ -O-Mn 4+ -based double exchange interactions leading to ferromagnetic ordering (FM) and Mn 3+ -O-Mn 3+ -based superexchange interactions resulting in antiferromagnetic (AFM) ordering.In addition the doping of Fe at Mn sites partially disrupts the Mn 3+ -O-Mn 4+ chains meant for double exchange (DE) interactions leading to decreased magnetization.LSMFO sample is shown to exhibit superparamagnetic behavior with a blocking temperature close to 130 K.These particles are understood to be having core-shell magnetic configuration with the core of the nanoparticles of LSMFO experiencing FM ordering while the shell of the LSMFO particles have disordered spins. [35]agnetization behavior of BFO particles in terms of ZFC-FC curves show the weak ferromagnetic behavior of the particles with the maximum value of FC magnetization is about 1/1000 times that of LSMFO particles.Weak ferromagnetic ordering as observed is understood to be arising due to non-cancellation of surface spins in the case of fine nanoparticles of BiFeO 3 . [23,24]ased on the magnetization results the superparamagnetic nature of LSMFO particles and the weak ferromagnetic behavior of BFO particles at room temperature are deduced.
Structural analysis and spin-lattice coupling in BFO-LSMO composites are studied using Raman spectroscopy.According to group theory, BFO has 18 optical phonon modes with irreducible representation Γ opt, R3c = 4A 1 + 5A 2 + 9E.The A1 (Transverse optical modes) and E (Longitudinal optical modes) are Raman and IR active but A2 modes are Raman inactive.So the irreducible form becomes [38][39][40][41][42] Γ Raman, R3c = 4A 1 + 9E. Figure 5 shows  the Raman spectra as obtained in BFO and the annealed composites viz., 95BFO-5LSMFO and 90BFO-10LSMFO whereas the Raman modes as obtained in these samples are listed in Table S2, Supporting Information.Spin-phonon coupling present in the system [43] could be deduced based on Raman mode analysis.Analysis of the Raman spectra of BiFeO 3 samples shows phonon mode anomalies due to different spin reorientation transition temperatures which give insights on spin-phonon coupling in this system. [44]The suppression of peak intensity of 136 cm −1 (A1 1TO) mode with respect to 168 cm −1 (A1 2TO) corresponding to Bi-O modes implies the enhanced coupling of magnetic, ferroelectric and/or structural order parameters. [25,43]Here, the value of the relative intensity I(A1 1TO)/I(A1 2TO) is deduced to be 2.72, 1.57, and 1.44 in BiFeO 3 , 95BFO-5LSMFO and 90BFO-10LSMFO, respectively.It implies that there are appreciable coupling of magnetic and ferroelectric ordering parameters in the case of BFO-LSMFO composites.
Having inferred the magnetoelectric coupling effects in the composites using Raman spectroscopy results, the variation of the ferroelectric polarization with the applied electric fields (P-E loop studies) was studied in BFO, 95BFO-5LSMFO, and 90BFO-10LSMFO samples while exposed to different values of the applied magnetic field at 300 K and are shown in the left panel of the Figure 6.The ferroelectric parameters are mentioned in Table S3, Supporting Information.while slight changes in P-E are seen with respect to varying magnetic field at 300 K in the case of 95BFO-5LSMFO, the observed changes are seen to be appreciably much higher in the case of 90BFO-10LSMFO.
LSMFO with 5% of Fe is shown to exhibit metal to insulation transition (MIT) ≈195 K (Cf. Figure S6, Supporting Information).Therefore it is chosen to look at the variation of P-E loop with the applied magnetic field suitably at a temperature in which there would be enhanced magnetization coupled with itinerancy of e g electrons due to Mn 3+ -O-Mn 4+ -based double exchange interactions.Panel shown on the right-hand side in Figure 6 shows the deduced variation of P-E loop with magnetic field in pristine and the composite of BFO samples at ≈100 K which is well below the MIT of the LSMFO.Based on these results appreciably higher magnetoelectric coupling effects are observed in the composite of BFO containing 10 wt% of LSMFO.The P-E loop is seen to vary significantly at 100 K as compared to that of the data obtained at 300 K.It is striking to see that the P-E loop variation is seen to show quite appreciable changes with the application of external magnetic field at 100 K in BFO-LSMFO composite samples while the effects are seen to be quite dominant in 90BFO-10LSMFO at 100 K with respect to magnetic field.It is important to discuss the possibility of magnetoresistance effects due to LSMFO particles contributing for the observed magnetoelectric coupling effects.It is already seen that the LSMFO nanoparticles exhibit ferromagnetic ordering with the Curie temperature ≈300 K. From the magnetoresistance results as obtained in LSMFO it is seen that values of magnetoresistance are 0.67% and 0.94% for the applied fields of 0.25 and 0.35 T, respectively at 300 K. while the variation in MR is deduced to be 12% and 13%, respectively at 100 K for the applied fields of 0.25 and 0.35 T. Hence the relative variation of the magneto resistance is only 12% as observed at 100 K while the magnetic field is increased from 0.25 to 0.35 T. On the other hand the magnetoresistance effects are expected to significantly smaller in the case of the composites of BFO-LSMFO.From the P-E loop results as obtained in 90BFO-10LSMFO sample it could be observed that the values of ΔPs in (%) at 100 K are noted to be high as 10.5% and 26.5% for the applied fields of 0.25 and 0.35 T, respectively.This would imply that the observed relative variation in the value of ΔPs in (%) at 100 K is close to 60%.Therefore we can very well rule out the possibility of any extrinsic effects such as magnetoresistance associated with LSMFO contributing for the appreciable changes in P-E loop with the application of magnetic field implying the magnetoelectric coupling effects.
Based on the results of the resistivity studies on the 5% Fe doped LSMO sample it is deduced that the metal to insulator transition occurs ≈195 K. Enhanced double exchange interaction at 100 K in LSMFO nanoparticles results in a significantly high exchange magnetic interaction between LSMFO and BiFeO 3 nanoparticles.This is also understood to result in a significantly higher magnetoelectric coupling effect in the case of BiFeO 3 -LSMFO mixture.It is important to prove that the magnetic exchange interactions as exerted by LSMFO on BiFeO 3 nanoparticles are appreciably high.In the following the Mössbauer results obtained in BFO-LSMFO composites are discussed in a detailed manner to elucidate the manifestation of the magnetic interactions of LSMFO particles on the weak ferromagnetic particles of BiFeO 3 .This is done based on the discussion of Mössbauer results corresponding to BiFeO 3 , LSMFO, and the composites of BFO-LSMFO, which are prepared by the mixing and annealing of the nanoparticles of BiFeO 3 and LSMFO pertaining to this study.
Mössbauer spectrum (Cf Figure 7) as obtained in the BiFeO 3 nanoparticles could be deconvoluted into four components in which the two dominant components are due to Fe atoms associated with FeO 6 octahedra oriented along <111> subjected to varying magnitude of rhombohedral distortion resulting in the variation of Fe-O-Fe angle.Any variation in the Fe-O-Fe angle leads to changes in the magnitude of super exchange interactions along <111> leading to two distinct Fe sites with equal fractions of Fe atoms experiencing different values of hyperfine fields and quadrupole splitting [45,46] which are similar to the bulk polycrystalline BiFeO 3 experiencing antiferromagnetic interactions.Close to 5% of Fe atoms experiences weak ferromagnetic magnetic interactions with hyperfine field values close to 44 Tesla which is understood to be due to the fraction of Fe atoms associated with the shell of BiFeO 3 .A small fraction of Fe atoms (≈4%) experiencing pure quadrupole interaction could be understood to be associated with Bi 25 FeO 40 .Based on the results of magnetization and M-H loop as obtained in this study and in comparison with the results as reported [36,47] on LSMFO particles of almost same mean size, it is deduced that these particles are of superparamagnetic nature as discussed earlier.These particles are characterized by fluctuation of spins at a rate close to or higher than (1/) where  is the mean lifetime of I = 3/2 state of 57 Fe resulting in superparamagnetic relaxation of spins which is typically of 10 ns.Accordingly, the MS as obtained in LSMFO particles could be fitted with a doublet (Cf.Table 1).This result also implies the high quality of the LSMFO sample with Fe atoms doped homogeneously in the matrix marking the absence of any clustering.
It is seen that with the application of an external magnetic field of 0.3 Tesla there is an increase in the fractions of Fe atoms exposed to magnetic hyperfine fields, which is understood to be due to enhanced magnetic interactions between the LSMFO particles similar to the reported results in the case of the superparamagnetic particles of Fe 3 O 4. [48] MS results corresponding to the composites of 95BFO-5LSMFO and 90BFO-10LSMFO obtained at room temperature are discussed in the following.Detailed MS investigations are carried out at 84 K in the composite 90BFO-10LSMFO, which is observed to exhibit significant magnetoelectric coupling effects as established in the present study based on the results of the P-E loop variation at low temperature and under the application of magnetic fields discussed earlier.MS studies are done in the composite of 95BFO-5LSMFO and 90BFO-10LSMFO subsequent to rigorous mixing in isoproponal with application of 0.3 T magnetic field, c) LSMFO and d) LSMFO with application of 0.3 T magnetic field, e) BFO-LSMFO mixed powder with 5 wt% manganite, f) BFO-LSMFO mixed powder with 5 wt% manganite with application of 0.3 T, g) BFO-LSMFO mixed powder sintered at 673 K 2 h with 5 wt% manganite, h) BFO-LSMFO mixed powder sintered at 673 K 2 h with 5 wt% manganite with 0.3 T field, i) BFO-LSMFO mixed powder with 10 wt% manganite, j) BFO-LSMFO mixed powder with 10 wt% manganite with application of 0.3 T, and k) BFO-LSMFO mixed powder sintered at 673 K 2 h with 10 wt% manganite are shown.While l) shows the Mössbauer spectrum obtained in 90BFO-10LSMFO sintered at 673 K 2 h with the application of magnetic field of 0.3 T. solution followed by selective annealing treatments (Cf Figure 7) and the results are tabulated (Cf 1).
MS as obtained in 95BFO-5LSMFO could be deconvoluted into six components (Cf. Figure 7 and Table 1).Important observations of the results with respect to bismuth ferrite and the manganite of the composite are discussed and compared with that of the pristine BiFeO 3 .
The hyperfine parameters such as the magnetic hyperfine fields and quadrupole splitting corresponding to fractions referred as f 1 and f 2 of Fe sites associated with FeO 6 remain same as the case of the pristine BiFeO 3 .It is important to note that the fraction f 3 of Fe atoms which is understood to be associated with weak ferromagnetic interactions in the case of the pristine BiFeO 3 , is observed to experience appreciably higher value of the magnetic hyperfine fields in the case of 95BFO-5LSMFO as compared to that of BFO.This is understood in terms of the appreciable increase in the hyperfine field at Fe sites associated with shell of the BiFeO 3 particles which are in contact with the LSMFO particles and exposed to significantly high ferromagnetic exchange interactions.Detailed discussion on the origin of the magnetic interaction is deferred to the end of this section.
Based on the fact that the bulk of the BiFeO 3 particles exhibit AFM ordering while LSMFO particles are ferromagnetically ordered and taking into account the values of the hyperfine fields corresponding to other fractions it is deduced that the remaining fractions of iron atoms (Cf Table 1) might be due to Fe sites associated with LSMFO particles of the composites.In the case of LSMFO as mixed with BiFeO 3 , it can be seen that there is a decrease in the relative fractions of Fe atoms experiencing superparamagnetic spin fluctuations leading to appearance of the components exposed to low values of hyperfine fields, similar to the case of the MS results as obtained in LSMFO under the application of magnetic field.Hence the relative decrease in the fractions associated with superparamagnetic spin fluctuations leading to occurrence of hyperfine field components are understood due to enhanced magnetic interactions between LSMFO and BiFeO 3 particles.It can be noted that with the application of a moderate field of 0.3 Tesla there is a reduction in the superparamagnetic fraction due to increasing magnetic interactions due to partial spin polarization of surface spins of the particles.Analogously in the case of BFO-LSMFO composite, as each LSMFO particle is dominantly sharing interface with the surrounding BFO particles there is an effective enhancement in the magnetic exchange interactions at the interface.This is understood to result in a reduction of superparamagnetic fractions of LSMFO leading to appreciable fractions of Fe atoms associated with LSMFO particles experiencing magnetic interactions with high values of hyperfine fields as shown in the table.
While only a slight increase in the hyperfine fields as experienced by Fe sites associated with FeO 6 in the case of pristine BiFeO 3 is seen, in the case of the mixed powder when subjected to a moderate field of 0.3 Tesla, an appreciable increase in the value of hyperfine field corresponding to Fe atoms corresponding to the shell of BFO is observed.The values of hyperfine fields and other parameters corresponding to Fe atoms associated with LSMFO remain almost same.Interesting observation is that the results as obtained in the mixture under the application of external magnetic field are seen to be similar to that of the mixture subsequent to annealing at 673 K.This clearly implies that due to moderate annealing at 673 K for 2 h the LSMFO particles which are almost randomly and homogeneously distributed in the system comprising mainly of BFO get bonded with reduced surface roughness leading to enhanced magnetic interactions while largely retaining the particle size distribution.The above results clearly imply that the enhanced magnetic interactions could in principle be realized due to strong exchange coupling between BFO and LSMFO.MS results as obtained in 90BFO-10LSMFO show that the hyperfine parameters corresponding to f 1 and f 2 associated with FeO 6 octahedra while remaining the same as that of the bulk BFO, there is an appreciable increase in the value of the hyperfine field experienced by the Fe atoms associated with BFO as compared to that of 95BFO-5LSMFO.Also it is important to note the values of hyperfine fields as experienced by the Fe atoms associated with LSMFO particles are larger than that of 95BFO-5LSMFO.Effective magnetic interactions between BFO and LSMFO particles have been established at atomic scale based on the hyperfine parameters deduced at Fe sites by means of detailed Mössbauer studies done on BFO, 95BFO-5LSMFO and 90BFO-10LSMFO and the systematic comparison of these results.
Based on the results of the Mössbauer studies it is important to understand the aspects of magnetization corresponding to BiFeO 3 particles which are dominantly exhibiting antiferromagnetic ordering, the shells of BiFeO 3 particles which are in contact with LSMFO exhibiting weak ferromagnetic ordering and LSMFO particles exhibiting ferromagnetism.Effective magnetization of these are studied in terms of the mean hyperfine field which is obtained as <B hf > = ∑ (f i * B hf (i)) /∑ f i, whereas f i are the fractions of iron atoms associated with sites i, corresponding to Fe atoms occupying the zones or particles of interest.
Based on the P-E results carried out in BFO and BFO-LSMFO composites as discussed earlier (Cf Figure 6), it was deduced that 90BFO-10LSMFO exhibits significantly higher magnetoelectric coupling at low temperature.Hence in the following the results obtained in 90BFO-10LSMFO at 84 K under different conditions will be discussed.Toward this the results of Mössbauer studies carried out at 84 K in the case of BFO and LSMFO will be discussed first, which will be followed with the discussion of the Mössbauer results obtained in 90BFO-10LSMFO at 84 K (Cf Figure 8, Table 2 and Figure 9, Table 3).
Mössbauer spectrum as obtained in BFO at 84 K (Cf Figure 8) could be deconvoluted into four components similar to that of the results obtained at room temperature.Because of the increase in Fe-O-Fe-based superexchange interactions there is an increase in the values of the hyperfine fields at Fe sites corresponding to FeO 6 while only a slight increase in the hyperfine field associated with the weak ferromagnetic component is observed.MS results obtained in BFO at 84 K under the application of the external magnetic field shows that there is an appreciable increase in the hyperfine field as experienced by the fraction f 3 corresponding to the weak ferromagnetic component.In order to understand the magnetic interactions in the composite in particular with respect to BFO and LSMFO particles which are in contact, it is quite important to comprehend the magnetic properties in BFO and LSMFO particles.This requires that the hyperfine parameters need to be obtained in LSMFO at 84 K.
Mössbauer studies were carried out 84 K in LSMFO particles, and the results are tabulated.MS spectra could be deconvoluted into six components with the fractions of Fe atoms exposed to magnetic interactions of different magnitudes in terms of the values of hyperfine fields ranging up to 54 Tesla.Occurrence of different fractions of Fe sites is understood to be strongly dependent upon the size of LSMFO the particles and their distribution.Also this is dependent upon the magnetic interactions between particles.An important observation is an appreciable increase in the Table 2. Hyperfine parameters as experienced by Fe atoms associated with BiFeO 3 and LSMFO at 300 K and 84 K with and without the presence of magnetic field.values of the hyperfine fields with the application of the external magnetic field as deduced based on the Mössbauer results.The hopping of e g electrons by double exchange interaction mechanism (Mn 3+ -O-Mn 4+ ) mainly contributes for the enhanced magnetization in the case of the nanoparticles of LSMFO with the other parameters of the system such as the mean size, distribution and surface defects of the LSMFO particles playing a critical role.MS obtained at 84 K (Cf Figure 9) in the composite 90BFO+10LSMFO shows an appreciable increase in the hyperfine fields associated with FeO 6 octahedra of BiFeO 3 .These values of the hyperfine fields are almost same and only slightly higher than that of pristine BiFeO 3 at LT (Cf.Table 2).On the other hand the hyperfine fields associated with Fe sites of the shell of BFO are significantly higher of the order of 51.15 Tesla as compared to the value of 47.7 Tesla corresponding to pristine BFO.This result elucidates a much stronger ferromagnetic interaction at the shell of the 90BFO-10LSMFO composites as compared to that of the pristine BFO at 84 K.It is striking to see that there is an appreciable increase in the values of hyperfine fields associated with the Fe sites corresponding to LSMFO in the composite.With an additional increase in the value of the external magnetic field similar trend of increasing hyperfine fields at Fe sites associated with the shell of BFO and LSMFO are observed.Results obtained based on the Mössbauer spectrum obtained at 300 K subsequent to that of the low temperature results, indicate an enhanced magnetization in terms of higher value of hyperfine fields corresponding to the Fe sites associated with shell and changes in the hyperfine parameters, as compared to the results obtained at 300 K prior to low temperature measurements.This result elucidates the memory effects associated with the magnetoelectric coupling in the composite.These results are shown in the Table 3, while the Figure 10 displays mainly the variations in the values of the mean hyperfine fields associated with BFO, weak ferromagnetic component of BFO which are in contact with LSMFO particles and that of LSMFO.
Main results of this study are elucidated in Figure 10 in terms of the variation of the mean hyperfine fields at Fe sites corresponding to antiferromagnetically ordered BiFeO 3 , weakly ferromagnetic shells of BiFeO 3 which are in contact with LSMFO particles and those of LSMFO particles.It is known that the net mag-Table 3. Hyperfine parameters as obtained based on the analysis of Mössbauer spectra corresponding to 90BFO-10LSMFO with and without application of magnetic field at 84 K.  are mainly dictated by the increase in the magnetization due to Mn 3+ -O-Mn 4+ -based double exchange interaction although the Fe sites do not take part in those interaction.Based on the results of the present study it is important to note that there is an enhanced magnetization at the LSMFO particles in the composite compared to the pristine LSMFO which remain superparamagnetic at room temperature as deduced based on the Mössbauer studies.These results point to an important finding that the enhanced magnetization at the shell of BFO is mainly due to an enhanced double exchange interactions in the LSMFO particles This is essentially understood due to enhanced magnetic interactions between BFO-LSMFO as dominantly contributed due to significantly increased double exchange interactions at LSMFO particles below blocking temperatures of the superparamagnetic LSMFO.But it is important to note that there has been a strongly enhanced exchange coupling effects induced increase in the magnetic field at the shell of BFO of the BFO-LSMFO composites even at RT as discussed earlier.Based on these results it can be noted that there are synergetic effects of magnetic exchange cou-pling induced enhanced magnetization as observed both in the case of BFO shell and the LSMFO in the case of the composites.In several studies on the multilayer thin films of LSMO-BFO an enhanced magnetization well within the atomic layers of BFO lying on both the sides of LSMO has been reported which is mostly understood based on orbital reconstruction. [5,49]It is seen from the results of XRD analysis that with enhanced doping of LSMFO in the nanocomposite matrix composed dominantly of BiFeO 3 , an enhanced strain effects at the BiFeO 3 particles have been elucidated.Based on the similar strain effects as established using TEM studies in the BiFeO 3 corresponding to BFO-LSMO multilayers, [5,49] it was proposed that the d x

2
-y 2 orbital is energetically preferred than d 3z 2 −r 2 orbital. [5]A strong hybridization between the d 3z Such an orbital reconstruction is understood to produce significant enhancement in the canting of spins leading to an appreciable increase in the effective magnetization at Fe sites of the shells of BFO which are in contact with LSMFO (Cf Figure S7, Supporting Information).Thus the magnetic exchange coupling is contributed due to the interaction of moments of Mn and Fe across the interface.In addition the presence of oxygen vacancies if any in very small concentration at the shells of nanoparticles of BFO which are in contact with LSMO would result in an occurrence of Mn 4+ in LSMO particles at the interface.This would lead to an enhanced Mn 3+ -O-Mn 4+ -based double exchange interaction at LSMFO which in turn would also result in an increase in the magnetization at the shell of BiFeO 3 particles that are in contact with the LSMFO particles.Such an enhanced magnetization in this BFO dominant system leads to appreciable magnetoelectric coupling effects as reported in this study.Strain associated with LSMFO particles [50] is understood to play an important role for the appreciable magnetoelectric coupling effects as observed in the composite.
Summarizing, the present work elucidates the magnetic and ferroelectric properties of the composites of BFO-LSMFO which are deduced to exhibit ferromagnetic and magneto-electric coupling effects.Importantly, the local structure and magnetic properties of both the BFO and LSMFO particles are investigated in a detailed manner based on Mössbauer studies which imply the ferromagnetic ordering of the shells of the BFO particles which are in contact with LSMFO particles based on the values of the hyperfine fields deduced at Fe sites associated with BFO.In the composites the LSMFO particles are observed to exhibit partially enhanced ferromagnetic properties contrary to the superparamagnetic nature of the pristine LSMFO particles.Enhanced magnetization in the LSMFO particles and thereby the BFO particles which are in contact with the LSMFO particles in the composites is comprehended based on the orbital reconstruction [5,50] due to Fe 3+ -Mn 3+ -based hybridization at the interface or at the grain boundary separating the grains of BFO and LSMFO.
This results in an appreciable increase in the Fe 3+ -O-Mn 3+ superexchange interactions resulting in ferromagnetic ordering at the interface or at the coherent grain boundary separating LSMFO and BiFeO 3 resulting in an enhancement of the interface magnetism.This in is understood to result in the enhanced magnetization at the shell of BFO particles that are in contact with LSMFO and in the LSMFO particles.Strain associated with LSMFO particles and Fe 3+ -O-Mn 3+ -based superexchange interactions resulting in ferromagnetic ordering are understood to play an important role for the observed magnetoelectric coupling effects.

Conclusion
The present work elucidates that the nanocomposites of BFO-LSMFO exhibit ferromagnetic and ferroelectric properties resulting in a strong magneto-electric coupling effects.Importantly the local structure and magnetic properties based on the hyperfine parameters at Fe sites in both the BFO and LSMFO particles and their interface are investigated in a detailed manner using Mössbauer studies.These results imply the ferromagnetic ordering of the shells of the BFO particles which are in contact with LSMFO particles.Enhanced magnetization at the interface of the BFO-LSMFO particles is mainly understood due to orbital reconstruction resulting in a transfer of e g electron leading to Mn 3+ -O-Fe 3+ -based ferromagnetic exchange interactions.This is understood to have resulted in an appreciably higher values of the hyperfine fields at Fe sites associated with both the BFO and LSMFO particles in the composite, in contrast to that of the BFO and LSMFO.Correlation of the results based on the Mössbauer studies carried out at 84 K in LSMFO and the studies carried out in 90BFO-10LSMFO composite at 300 K prior to and after measurements at 84 K might imply the magnetoelectric couplingbased enhanced magnetization at the shells of BFO in contact with LSMFO which could be utilized in several applications like magnetoelectric coupling-based memory devices.Strain associated with LSMFO particles is understood to play a key role for the observed magnetoelectric coupling effects along with Fe 3+ -O-Mn 3+ -based superexchange interactions leading to ferromagnetic ordering in the BFO-LSMFO composites.

Figure 1 .
Figure 1.Flow chart explaining the steps followed to the synthesis of BiFeO 3 -La 0.67 Sr 0.33 Mn 0.95 Fe 0.05 O 3 composite.

Figure 3 .
Figure 3. Particle size distribution fitted with log normal function for LSMFO and BFO nanoparticles.

Figure 4 .
Figure 4. Temperature dependence of Magnetization of a) LSMFO and c) BFO nanoparticles are shown in the left panels.While Magnetic Hysteresis loops as obtained at 300 K in b) LSMFO and d) BFO nanoparticles are shown in the right panel.

Figure 6 .
Figure 6.Ferroelectric polarization studies on BiFeO 3 and nanocomposites.Ferroelectric P-E loop as obtained with and without the magnetic field for a) BiFeO 3 , c) nanocomposite of BiFeO 3 with 5 wt% manganite, and e) nanocomposite of BiFeO 3 with 10 wt% manganite are shown in the left panel.While ferroelectric P-E loop with and without the application of external magnetic field at 100 K for b) BiFeO 3 , d) nanocomposite of BiFeO 3 containing 5 wt% manganite and f) nanocomposite of 90BFO-10LSMFO are shown in the right panel.

Figure 8 .
Figure 8. Mössbauer spectra obtained in the systems as mentioned viz., a) BiFeO 3 at 300 K, b) BiFeO 3 at 84 K, and c) BiFeO 3 at 84 K with B ext = 0.3 T. d) LSMFO at room temperature e) LSMFO at 84 K and f) LSMFO at 84 K with 0.3 T.

Figure 9 .
Figure 9. Mössbauer spectra obtained in the composite samples subjected to annealing at 673 K under different conditions viz., a) 90BFO-10LSMFO at 300 K, b) LSMFO at 300 K, c) 90BFO-10LSMFO at 84 K, d) 90BFO-10LSMFO at 84 K with B ext = 0.3 T, e) 90BFO-10LSMFO at 84K with B ext = 0.4 T, f) 90BFO-10LSMFO at 300 K after measurement at 84 K with the application of external magnetic field B ext = 0.4 T and g) 90BFO-10LSMFO at 300 K with B ext = 0.3 T after the measurement.

Figure 10 .
Figure 10.Variation of the mean value of the magnetic hyperfine field in Tesla deduced at Fe sites associated with weak ferromagnetically ordered shells of BiFeO 3 particles that are in contact with LSMFO particles (<Bhf> BFO-WFM ) and LSMFO particles (<Bhf> LSMFO ) with different treatments as deduced based on the results of Mössbauer studies done in 90BFO-10LSMFO under different conditions/treatments mentioned as follows.a) as mixed powder, b) mixed powder under B ext = 0.3 T, c) annealed at 673 K, d) annealed under B ext = 0.3 T. Whereas e-g) correspond to Mössbauer spectrum in annealed 90BFO-10LSMFO sample as obtained at 84 K with B ext = 0, 0.3 and 0.4 Tesla, respectively.Enhanced magnetization at the shells of BFO particles in contact with LSMFO is understood due to Fe 3+ -O-Mn 3+ -based superexchange interactions at the interface resulting in ferromagnetic interactions.

2 −r 2 −r 2 and d x 2 −y 2
orbitals of Mn and Fe via the 2p orbital of oxygen atoms, accompanied by the orbital reconstruction results in the population of d 3z 2 bonding orbitals of Fe and Mn, respectively as these are the lowest lying energy states.This results in the occurrence of d x 2 −y 2 orbital ordering at the interface of the BFO nanoparticles which are in contact with LSMFO.

Table 1 .
Hyperfine parameters deduced by the analysis of Mössbauer spectra obtained at 300 K in pristine BFO, LSMFO and the nanocomposites such as 95BFO-5LSMFO and 90BFO-10LSMFO.