Tuning of net magnetic moment in BiFeO3 multiferroicsby co-substitution of Nd and Mn

Pawan Kumar, Manoranjan Kar n

Department of Physics, Indian Institute of Technology Patna, Patna 800013, India

a r t i c l e i n f o

Available online 5 April 2014

Keywords:Crystal structureXRDRietveldMultiferroicsCycloid spin structure.

a b s t r a c t

The structural and magnetic properties of Bi1�xNdxFe1�xMnxO3 ceramics prepared by the tartaric acidmodified sol–gel technique have been studied to understand the effect of structural modification on themagnetic Properties of BiFeO3. The co-substitution of Nd and Mn at Bi and Fe sites respectively in BiFeO3

significantly suppress the impurity phases (Bi25FeO40, Bi2Fe4O9 etc.). The Rietveld analysis of X-raydiffraction (XRD) patterns indicates the existence of compositional driven crystal structure transforma-tion from rhombohederal (R3c space group, higher crystal symmetry) to the orthorhombic (Pbnm spacegroup, lower crystal symmetry) with the increase in substitution concentration due to excess chemicalpressure (lattice strain). The quantitative crystallographic phase analysis has been carried out by Rietveldanalysis of all the XRD patterns. Magnetic measurements reveal that co-substituted BiFeO3 nanoparticlesfor x¼0.050 have enhanced remnant magnetization about 21 times as compared to pure one. Theremnant magnetization reaches a maximum value at the morphological phase boundary (x¼0.050) andfurther increase (x40.050) in substitution concentration results in the reduction of remnant magnetiza-tion due to the appearance of complete antiferromagnetic ordering in the orthorhombic structurebecause of the significant contribution from the crystallographic phase of Pbnm space group (asobtained from the quantitative crystallographic phase contribution by the Rietveld analysis).

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Bismuth ferrite (BiFeO3) is probably the most promising multi-ferroics which exhibits the coexistence of ferroelectric order andG-type canted antiferromagnetic order well above room tempera-ture (TC¼1103 K and TN¼643 K) [1]. These materials are interest-ing due to their fascinating technological applications, suchas multistate memory devices, spintronic devices, magneticallymodulated transducers, ultrafast optoelectronic devices and sen-sors [2–5]. The crystal structure for polar phase of BiFeO3 (BFO)(at room temperature) is described by rhombohederally distortedperovskite structure with R3c space group. The structural distor-tions in the rhombohedral phase can be described in terms of theantiphase tilting of the adjacent oxygen octahedra (a� a� a� tiltsystem in Glazer's notation [6]) with respect to ideal perovskite,and the polar ionic displacements along the ⟨1 1 1⟩c direction ofthe parent cubic cell ([0 0 1]hex direction in the hexagonal setting).The electric polarization in this compound arises due to asymme-try in charge of the Bi 6S2 lone pair while the magnetization arises

from the exchange interaction between magnetic cation at theB-site (Fe). The R3c symmetry allows the existence of weakferromagnetic moment due to the Dzyaloshinky–Moriya interac-tion but the cycloid spin structure with the periodicity of �62 nmprevents net magnetization which leads to net zero magnetization[7–10].

Recently, the large magnetic moment that will produce largemagetoelectric coupling constant at room temperature throughA- site and/or B-site substitution in BFO have been reported [11–16].Direct evidence of cycloid suppression in A-site substituted sampleshas been given via nuclear magnetic resonance (NMR) measure-ments which has been correlated with the structural transition fromrhombohedral to orthorhombic crystal structure [11,15,16]. Thisstructural phase transition (in morphotropic phase boundary(MPB)) significantly enhances the magnetization as well as magne-toelectric interaction. However, it has limited technological applica-tions because BiFeO3 is generally quite difficult to be prepared inphase pure ceramics because of its narrow temperature range ofphase stabilization. However, several attempts have been made toprepare phase pure by chemical route and the solid state routefollowed by leaching with nitric acid [17,18]. The nitric acid leachingis normally used to eliminate above impurity phases which leads tothe formation of coarser powders and its poor reproducibility.

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Physica B

http://dx.doi.org/10.1016/j.physb.2014.03.0800921-4526/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ91 612 2552013; fax: þ91 612 2277383.E-mail address: mano@iitp.ac.in (M. Kar).

Physica B 448 (2014) 90–95

Hence, we have adopted the chemical route of synthesis for uniformparticle size and better reproducibility.

Weak ferromagnetic ordering induced due to Nd substitution atA-site in BiFeO3 has been observed byWu et al. [19] and Yuan et al.[20]. The Mn substitution at Fe site of BiFeO3 is reported to inhibitthe grain growth which resulted in reduced particle size [21]and improve the magnetic as well as electric properties [22,23].Chauhan et al. had reported the structural phase transition in 15%Mn doped BiFeO3 sample due to the distortion in the rhombohe-dral structure with increasing Mn substitution which resulted insignificant enhancement in magnetization [24]. Khomchenko et al.have reported the structural transition and weak ferromagneicbehavior in Mn substituted Bi0.92Nd0.08FeO3 ceramics preparedby conventional solid-state reaction [25]. Takeshi et al. havealso reported that Nd substitution suppresses the formation ofsecondary phases and Mn substitution reduces the leakage currentof co-substituted BiFeO3 [26]. Nd3þ and Mn3þ has been chosen assubstituent ions because it is magnetically active and couldenhance the magnetic properties through the coupling betweenNd3þ and Fe3þ , Nd3þ and Mn3þ and Fe3þ and Mn3þ .

However, to the best of our knowledge, a systematic study ofthe influence of crystal structural transition on magnetic proper-ties has not been carried out on Nd and Mn co-substituted BFOceramics. Due to the fact that co-substitution of Nd and Mn caneliminate the formation of impurity phases and enhance themagnetization of co-substituted BiFeO3 ceramics, we have under-taken a study on Nd and Mn co-substituted BiFeO3 prepared by thetartaric acid modified sol–gel method and carry out three phaseRietveld analysis to study the crystallographic phases and itscorrelation of different crystallographic symmetry with the phy-sical properties.

2. Materials and methods

Bi1�xNdxFe1�xMnxO3 with x¼0.000, 0.025, 0.050, 0.075 and0.100 (BNFM-0, BNFM-025, BNFM-050, BNFM-075 and BNFM-100,respectively) were prepared by a tartaric acid modified sol–geltechnique. The detailed preparation procedure has been discussedin our previous publication [27]. Here, the starting materials weretaken as bismuth nitrate, iron nitrate, neodium oxide, manganeseacetate and tartaric acid (purityZ99.0%) and carefully weighted instoichiometric proportion. The molar ratio of metal nitrates totartaric acid was taken as 1:2. The resulting materials werethoroughly grinded and annealed at 700 1C for 3 h.

The crystallographic phases of all the samples were determinedby the powder X-ray diffraction (XRD) study using 18 kW Cu-rotating anode (Rigaku TTRX III diffractometer, Japan) with CuKαradiation (λ¼1.5418 Å) operating in the Bragg–Brentano geometryin a 2θ range of 10–120o at a scan step of 0.01o with counting timefor each step was 2.0 s. The microstructural properties of thesamples were investigated by using Field Emission ScanningElectron Microscopy (FE-SEM), Hitachi S-4800, JAPAN, operatingat an accelerating voltage of 10 kV and equipped with energy-dispersive X-ray spectroscopic capability. Room temperature mag-netization versus magnetic field (M–H) measurement for all thesamples were carried out by Magnetic Properties MeasurementSystem (MPMS), Quantum Design Inc., USA in the maximumapplied magnetic field of 780 kOe.

XRD patterns were analyzed employing the Rietveld refine-ment technique with the help of Fullprof package [28]. Thepatterns for all the samples could be refined using the R3c, Pbnmand I23 space groups. All the occupancy parameters were takenas fixed at their corresponding composition during refinement.Other parameters, such as, Zero correction, scale factor, halfwidth parameters, lattice parameters, atomic fractional position

coordinates, thermal parameters were varied during refinement.Background was refined by the sixth order polynomial whereaspeak shape by the pseudo-Voigt function.

3. Results and discussion

The XRD patterns for all the samples (annealed at 700 oC) havebeen shown in Fig. 1. It is important to note that Bi25FeO40

impurities peaks intensity are significant than that of Bi2Fe4O9 inthe respective XRD patterns of all the samples. These resultsconfirm the formation of BNFM nanoparticles with negligibleimpurities. The Goldschmidt tolerance factor (t) in ABO3 (BiFeO3)structure is defined as

t ¼ ð⟨rA⟩þroÞffiffiffi

2p

ð⟨rB⟩þroÞð1Þ

Here ⟨rA⟩ and ⟨rB⟩ are the average radius of A and B site cationsrespectively and ro is the ionic radius of oxygen. The tolerancefactor is used to quantify the structural stability of perovskitecompounds. The value of t is 1 for an ideal cubic structure. Whenthe value of “t” is smaller than one, the compressive strain acts onthe Fe–O bonds and hence on Bi–O bonds which infers that theoxygen octahedral must buckle in order to fit a bigger cation intoa smaller space for minimum lattice strain. This induced latticedistortion suppresses the rhombohedral phase and leads to theevolution of lower symmetric crystallographic phase such asorthorhombic or tetragonal. Reflections (1 0 4) and (1 1 0) arepartially separated in the BNFM-025 sample as shown in Fig. 2.

20 30 40 50 60 70

x = 0.025

*

* Bi25FeO40

(220

)(2

08)

(300

)(2

14)

(018

)

(122

)(1

16)

(024

)

(202

)(0

06)

(113

)(110

)(1

04)

(012

)

Inte

nsity

(A.U

.)

2θ (Degree)

x = 0.075

x = 0.100

x = 0.050

Fig. 1. XRD patterns of Bi1�xNdxFe1�xMnxO3 for x¼0.025–0.100 annealed at700 oC.

31.4 31.6 31.8 32.0 32.2 32.4 32.6 32.8 33.0

(110

)

(110

)

(104

)

2θ (Degree)

Inte

nsity

(A. U

.)

x = 0.025x = 0.050x = 0.075x = 0.100

Fig. 2. Shift in the XRD patterns of Bi1�xNdxFe1�xMnxO3 for x¼0.025–0.100.

P. Kumar, M. Kar / Physica B 448 (2014) 90–95 91

On increasing the Nd and Mn content, the doublets merge to givea single peak, which is clearly visible in BNFM-050 and highersubstitutions. For example, the doubly split peaks of (1 0 4)and (1 1 0) merge into a broadened peak of (1 1 0) for 42.5%substitution percentage. These results correspond to the latticedistortion in rhombohedral structure of BNFM-050 occurring withthe increase in the substitution concentration which leads torhombohedral to orthorhombic crystal phase transition. The unitcell volume decrease with the increase in the substituent concen-tration upto x¼0.075 due to increased lattice strain because ofionic size mismatch between Nd3þ (1.12 Å) and Bi3þ (1.17 Å),which can be inferred from the shift of the XRD peak alongpositive x-axis in Fig. 2. However, it increases with the substitutionpercentage (x40.075) due to decreased lattice strain because ofdominance of orthorhombic structure which supports our quanti-tative Rietveld analysis. It also shows the decreasing peak splittingwith the increase in the substitution concentration which suggeststhe existence of the structural transition.

The crystallite size has been calculated using Scherrer's formulausing Full Width at Half Maximum (FWHM) obtained from theRietveld refinement. [29]. The crystallite size decreases with theincrease of substituent ion concentration upto x¼0.050. Thisimplies the development of chemical pressure (lattice strain)inside the lattice due to size mismatch of host and substitutedcations which lead to local structural disorder and reduces the rateof nucleation as an effect decrease the crystallite size. However,

with the further increase in substitution percentage, it increasesdue to relaxation in the lattice strain beyond the MPB region,where the orthorhombic phase becomes dominant crystallo-graphic phase.

The Rietveld refinement of XRD patterns for BNFM-025 andBNFM-050 samples have been shown in Fig. 3. The refined latticeparameters and fractional atomic positions of the atoms areenlisted in Table 1. The rhombohedral structure of BiFeO3 corre-sponds to a Glazer tilt system of a� a� a� which induces asuperlattice peak at (1/2 1/2 1/2) reciprocal lattice position [30].The rhombohedral phase shows the characteristic doublet ofhighest intensity peaks in XRD profile and presence of superlattice reflection at 2θ¼39o [31]. However, the characteristicdoublet of highest intensity peaks and presence of super lattice

Fig. 3. Rietveld refined XRD patterns of Bi1�xNdxFe1�xMnxO3 for x¼0.025 and 0.075. Here, three rows of Bragg positions are for R3c, Pbnm and I23 space groupsrespectively.

Table 1Magnetic parameters of Bi1�xNdxFe1�xMnxO3 (for x¼0.000 [25] and x¼0.025–0.100) ceramics annealed at 700 1C.

Bi1�xNdxFe1�xMnxO3 Crystallitesize (nm)

Mr

(emu/g)MS at 80 kOe(emu/g)

HC

(kOe)

x¼0.000 96.0 0.0086 0.6470 0.1712x¼0.025 46.8 0.1541 0.9242 11.687x¼0.050 26.6 0.1874 1.0278 11.133x¼0.075 29.3 0.1806 1.1254 10.273x¼0.100 30.1 0.1508 1.1514 09.957

P. Kumar, M. Kar / Physica B 448 (2014) 90–9592

reflection at 2θ¼39o is not prominent for the higher substitutionwhich clearly indicates the presence of lower crystallographicsymmetry. Rietveld fitting with the different structural models(R3c, R3cþPbnm, R-3cþPbnm, R3cþPbnmþ I23, R3cþPbnmþPbam, etc.) showed that the observed XRD patterns of all thesamples is a result of the superposition of three spectral contribu-tions (R3cþPbnmþ I23), where third space group as I23 has beenused to quantify the impurity phase of Bi25FeO40. The respectivecontribution of three crystallographic phases has been enlisted inTable 2.

Fig. 4 shows the typical FE-SEM image of the sample Bi0.95Nd0.05Fe0.95Mn0.05O3. The image shows that the particles arehomogeneously distributed. The average size is found to be�25 nm, which is close to the crystallite size obtained from XRDpatterns. The compositions of samples are determined using theEnergy-Dispersive X-ray Spectroscopy (EDS) analysis. The typicalEDS pattern of sample Bi0.95Nd0.05Fe0.95Mn0.05O3 has been shownin Fig. 4. The EDS pattern reveals the presence of Bi, Fe, Nd, Mn andO elements in the sample. No extra peaks have been traced whichindicate that there is no contamination in the sample. We have

observed that the ratio of the elements is close to the empiricalformula.

The magnetization versus magnetic field (M–H) curves at roomtemperature of all samples with a maximum applied magneticfield of 780 kOe have been shown in Fig. 5. Magnetic hysteresisloop shows the linear magnetic field dependence of magnetizationfor BiFeO3 which indicates that it is antiferromagnetic material(M–H loop shown in our earlier publication) [27]. However, thesimultaneous substitution of Nd and Mn induces weak ferromag-netism. The presence of unsaturated hysteresis loops and smallremnant magnetization indicates the canting of the antiferromag-netic spin structure due to size mismatch and/or ferrimagneticspin structure due to different magnetic nature of substituted Mnand Nd compare to that of Fe ions.

Fig. 5 clearly indicates the significant enhancement in max-imum magnetization (MS) on Nd and Mn co-substitution. Since theaverage particle size of co-substituted BiFeO3 nanoparticles is�25 nm which is less than spiral ordering of 62 nm. Therefore,the primary cause for the observed weak ferromagnetism inco-substituted samples could be attributed to the size effect of

Table 2Structural parameters and atomic coordinates obtained by the Rietveld refinement of XRD patterns for Bi1�xNdxFe1�xMnxO3 for x¼0.025, 0.050, 0.075 and 0.100.

Space group R3c Pbnm

Value of a, b, c in Åx¼0.025 a¼b¼5.5735(4), c¼13.8162(5) a¼5.6497(8), b¼5.6027(0), c¼8.2620(4)x¼0.050 a¼b¼5.5681(8), c¼13.7587(1) a¼5.6811(5), b¼5.4494(0), c¼8.0706(2)x¼0.075 a¼b¼5.5645(1), c¼13.7516(7) a¼5.5498(2), b¼5.5279(5), c¼7.8928(5)x¼0.100 a¼b¼5.5784(5), c¼13.9236(4) a¼5.5625(8), b¼5.5363(0), c¼7.8960(4)

I23 (%) Volume R3c (%) Volume Pbnm (%) Volume

Crystallographic phase contributions (%) and unit cell volume (Å3)x¼0.025 06.61 1056.74 90.62 371.60 02.77 261.53 0%x¼0.050 04.38 1056.65 76.39 369.58 19.23 249.85 4.17%x¼0.075 07.25 1056.16 45.51 368.87 47.24 242.14 43.36%x¼0.100 09.37 1056.19 14.27 375.24 76.36 242.16 69.48%

Reliability indices for BNFM-050RP (%), RWP (%) 14.2, 18.9χ2 1.37RBragg (%) 6.38 10.41

Fractional atomic coordinates for BNFM-050Bi/Nd 6a (0, 0, 0) 4c (0.9976(7), 0.0232(5), 0.2500(3))Fe/Mn 6a (0, 0, 0.2218(0)) 4a (0.5000(2), 0.0000(5), 0.0000(1))O(1) 18b (0.4498(7), 0.0271(2) 0.9580(0)) 4c (0.0659(0), 0.4797(0), 0.2501(0))O(2) 8d (0.7362(0), 0.2731(0), 0.0312(6))

Fig. 4. FE-SEM image and EDS pattern of Bi0.95Nd0.05Fe0.95Mn0.05O3ceramics.

P. Kumar, M. Kar / Physica B 448 (2014) 90–95 93

the suppression of spiral order. The enhancement in magneticproperties by substitution of smaller Nd3þ ions for Bi3þ can resultfrom a combination of the factors: (a) increase in spin canting due tothe change in bond angle of Fe–O–Fe, (b) suppression of cycloid spiralspin structure due to ionic size mismatch and (c) magnetic interactionbetween Nd3þ and Fe3þ , Nd3þ and Mn3þ and Fe3þ and Mn3þ . Thebuckling of the Fe–O–Fe bond angle has a crucial role in the magneticproperties of BFO because it controls Fe3þ–O–Fe3þ super-exchangeinteractions. Since the super-exchange interaction Fe3þ–O–Fe3þ isantiferromagnetic, the enhancement of magnetization on dopingis due to canting of the antiferromagnetic sublattices [32]. Nd andMn co-substitution has been found to induce more buckling in theFe–O–Fe bond angle, which in turn increases canting of spins and,hence, enhances the magnetization.

In the present study, the Mr value of BNFM-050 nanoparticles issignificantly larger than that of nonmagnetically substituted (e.g.,La, Ba) BiFeO3 [33,34] It indicates that magnetically active Nd playsan important role in enhancement of magnetization. It has beenfound in intermetallic compounds of rare-earth and 3d transitionmetals that the rare earth and transition element sublatticescouple parallel in the case of a light rare earth and antiparallelin the case of a heavy rare earth [33]. Ferromagnetic couplingbetween Eu3þ and Fe3þ has also been observed by first-principlestudy on Eu-doped BFO nanoparticles. [35] Therefore, it is pro-posed that the 3d moments of Fe3þ align ferromagnetically withthe 4f spin-moment of the Nd3þ and the ferromagnetic exchangeinteraction between Nd3þ–Fe3þ further enhances the magnetiza-tion in Nd and Mn co-substituted BiFeO3 nanoparticles. The

contribution in magnetization from the secondary phases ofBi24FeO39 and Bi2Fe4O9 can be ruled out because these phasesare paramagnetic at room temperature [36,37]. In the present case,magnetic impurity as Fe2O3 has not been detected form X-RayDiffraction (XRD) pattern analysis. Moreover, ferromagnetichysteresis loop for BiFeO3 has not been observed which hasmaximum impurity phases among all samples.

The remnant magnetization (Mr) value increases sharply for theco-substituted samples as compared to the pure one for the lowersubstitution concentration (xr0.05) due to suppression of thespin cycloid structure of the BiFeO3 matrix. The Mr valueapproaches saturation near the MPB region (at around x¼0.050).This indicates that the spin cycloid structure is completely brokenfor this composition. The Mr starts to decrease for xZ0.05, whichmay be due to the appearance of perfect antiferromagneticordering in the orthorhombic structure. Moreover, magnetichysteresis loop for the x¼0.10 shows linear magnetic field depen-dence of magnetization indicating the significant contributionfrom the antiferromagnetic ordering.

4. Conclusions

Bi1�xNdxFe1�xMnxO3 ceramics were prepared by the tartaricacid modified sol–gel technique. It is important to note thatimpurities phase formation has been suppressed in the presentsamples. The co-substitution of Nd and Mn has resulted in struc-tural transition from rhombohedral symmetry (space group R3c) to

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

Magnetic Field (kOe)

Mag

netiz

atio

n (e

mu/

g)x = 0.025

Magnetic Field (kOe)

Mag

netiz

atio

n (e

mu/

g)

x = 0.050

-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

1.2

Magnetic Field (kOe)

Mag

netiz

atio

n (e

mu/

g)

x = 0.075

-80 -60 -40 -20 0 20 40 60 80

-80 -60 -40 -20 0 20 40 60 80

Magnetic Field (kOe)

Mag

netiz

atio

n (e

mu/

g)

x = 0.100

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

-80 -60 -40 -20 0 20 40 60 80

-80 -60 -40 -20 0 20 40 60 80-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

1.2

Fig. 5. M–H loops of Bi1�xNdxFe1�xMnxO3 for x¼0.025–0.100.

P. Kumar, M. Kar / Physica B 448 (2014) 90–9594

orthorhombic (Pbnm) as indicated by the XRD spectra analysis. Thecrystallographic phase percentage has been quantified by theRietveld analysis of all XRD patterns. Co-substitution with Nd andMn leads to the presence of unsaturated hysteresis loops and smallremnant magnetization which indicates the canting of the anti-ferromagnetic spin sub-lattices due to size mismatch and/or ferri-magnetic spin structure due to different magnetic nature ofsubstituted Mn and Nd compare to that of Fe ions. There is asignificant enhancement in the remnant magnetization for thesample Bi0.95Nd0.05Fe0.95Mn0.05O3 (at morphotrophic phase bound-ary consistent with the Rietveld analysis) and further increase in thesubstitution percentage leads to the decrease of the Mr due toappearance of complete antiferromagnetic ordering (in the orthor-hombic crystal structure). The enhanced magnetic properties will beinteresting for magnetic field sensor applications.

Acknowledgment

The authors gratefully acknowledge Dr. Dhanbir Singh Ranaand his research group at IISER Bhopal for extending the MPMSfacility. Authors are thankful to Department of Science andTechnology, Ministry of Science and Technology, New Delhi forfinancial support.

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