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A way to enhance the magnetic moment of multiferroic bismuth ferrite

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2010 J. Phys. D: Appl. Phys. 43 242001

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IOP PUBLISHING JOURNAL OFPHYSICSD: APPLIEDPHYSICS

J. Phys. D: Appl. Phys. 43(2010) 242001 (5pp) doi:10.1088/0022-3727/43/24/242001

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A way to enhance the magnetic moment ofmultiferroic bismuth ferrite

Z X Cheng1, X L Wang, Y Du and S X Dou

Institute for Superconducting and Electronic Materials, University of Wollongong, NSW, Australia

E-mail:[email protected]

Received 4 February 2010, in final form 29 April 2010Published 3 June 2010

Online atstacks.iop.org/JPhysD/43/242001

Abstract

Transition metals, including Co, Cr, Ni and Mn, were doped into multiferroicBi0.8La0.2Nb0.01Fe0.99O3samples that were fabricated by a solid state reaction. X-raydiffraction results show that only the Mn-doped sample did not contain any impurity phases,while the other transition metal dopants destroyed the phase stability of the pureBi0.8La0.2Nb0.01Fe0.99O3phase and caused the formation of second phases. All the transitionmetal doped Bi0.8La0.2Nb0.01Fe0.99O3samples show significant enhancement in their magneticmoment at room temperature in comparison with the BiFeO3and Bi0.8La0.2Nb0.01Fe0.99O3samples without transition metal doping. This is explained by the formation of localferrimagnetic ordering or ferromagnetic ordering in the transition metal dopedBi0.8La0.2Nb0.01Fe0.99O3, according to the electron configurations of the dopant transitionmetal ions.

1. Introduction

Multiferroic materials have simultaneous ferroelectric andmagnetic orders, and enable a coupling interaction betweenthese two orders. This coupling interaction, also called themagnetoelectric (ME) effect, produces various possibilities forthe realization of mutual control and detection of electricalpolarization and magnetism [1, 2]. As a potential candidatefor practical application among the limited choices offered bythe single phase multiferroic materials, BiFeO3 (BFO) hasbeen attracting enormous attention, due to its multiferroicproperties at room temperature (RT) (Curie temperature,TC = 1103K, Neel temperature, TN = 643K) [36].However, BiFeO3 is antiferromagnetic (AFM) and orderedwith a G-type spin configuration along the [1 1 1]cor [0 0 1]hdirections in its pseudocubic or rhombohedral structure, whilehaving a superimposed incommensurate cycloid spin structurewith a periodicity of 620 along the [110]h axis at RT,which cancels the macroscopic magnetization and inhibitsthe observation of the linear ME effect [7]. Thus, onlyvery weak ferromagnetism is usually observed in a pure

BiFeO3 sample due to the partial destruction of the spiral1 Author to whom any correspondence should be addressed.

periodicity in grains with limited dimensions. To makesamples showing large magnetic moment that will producelarge magnetoelectrical coupling, doping methods have beenadoptedtomodifyBiFeO3. For example, La substitutionforBican improve the magnetic moment in BiFeO3by suppressingits cycloid structure, as well as promoting better ferroelectricproperties due to the improved phase stability of BiFeO3and the reduced formation of impurity phases [8]. Doping

with transition metals has been proved to be very effectivein improving the ferroelectric property of samples throughimproving their insulating property [9]. However, informationon how the magnetic properties are modified by the transitionmetal doping is very inadequate. In addition, the magneticproperties of bismuth ferrite are very sensitive to very tinyamounts of ferromagnetic (FM) impurity, which is usually outof the detection range of the common x-ray diffraction (XRD)techniques. Therefore, the so-called observed enhancement ofthe magnetic moment after doping is sometimes a result of theappearance of a strongly FM second phase material.

In this work, a new concept for the doping mechanismof transition metals in bismuth ferrite is proposed, which has

been experimentally tested. A variety of transition metalswere doped into La and Nb co-doped BiFeO3, an optimized

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J. Phys. D: Appl. Phys. 43(2010) 242001 Fast Track Communication

composition which shows very good multiferroic properties interms of highly improved magnetic moment and ferroelectricpolarization [10]. A systematic study of the doping effects onthe structure and magnetic properties was carried out, and itwas found that the transition metal dopants increase the RTmagnetic moments and change the magnetic behaviour of the

co-doped BiFeO3 significantly.

2. Experiments

Bi0.8La0.2Nb0.01Fe0.94TR0.05O3bulk material, with TR = Co,Cr, Ni or Mn, was fabricated by the traditional solid statereaction method. The starting materials were Bi2O3, La2O3,Nb2O5, Fe3O4, and transition metal oxides or carbonates inthe form of MnCO3, Cr2O3, NiO and Co3O4, all with a purityof 99.99% and from Aldrich. These materials were weighedout according to the molecular mole ratio with 5% extrabismuth for compensation of bismuth loss at high sinteringtemperatures. They were then mixed, pressed into pellets andsubsequently sintered in air for 3 h at 1073 K. The ceramicpellets were then crushed, ground, pressed into pellets againand sintered finally at 1273 K for 1 h.

The structures of the obtained samples were examinedon a Philips 1700 XRD machine using Cu K radiation.The morphologies of the sample cross-sections wereexamined with a field emission scanning electron microscope(FESEM, JEOL 7500) operated at 30 kV. Magnetic propertymeasurements of the samples were conducted on a QuantumDesign Magnetic Property Measurement System (MPMS)and a 14 T Physical Property Measurement System (PPMS)equipped with a vibrating sample magnetometer (VSM). The

field cooled and zero fieldcooled magnetization measurements(FCZFC) at high temperatures were carried out at an appliedmagnetic field of 1000 Oe. The magnetic hysteresis loops(MH)were scanned between 5 T and 5 T.

3. Results and discussion

Figure1shows the XRD patterns on the logarithmic scale ofall the transition metal doped Bi0.8La0.2Nb0.01Fe0.94TR0.05O3samples in comparison with the XRD patterns of an undopedBiFeO3sampleandaBi0.8La0.2Nb0.01Fe0.99O3samplewithoutany transition metal doping, which were both prepared

using the same technique. It can be seen that all thedoped samples, including those co-doped with two or threeelements, show more pure phases than the undoped BiFeO3,which contains a significant amount of impurities suchas Bi25Fe2O39, Bi25FeO40 and Bi2Fe4O9. On comparingthe XRD patterns of all the doped samples, only theMn-doped Bi0.8La0.2Nb0.01Fe0.94TR0.05O3 and the undopedBi0.8La0.2Nb0.01Fe0.99O3 samples are found to be completelyin pure phase without any detectable impurity. This resultis in accordance with what has been reported on the dopingeffects of La, which can improve the crystallization andstabilize the bismuth ferrite phase, as LaFeO3 is stabler andeasier to form than BiFeO3 [8]. These XRD results also

indicate that Mn doping did not degrade the stabilization ofthe bismuth ferrite phase from the La and Nb doping, whereas

Figure 1.XRD patterns in log intensity of the transition metaldoped Bi0.8La0.2Nb0.01Fe0.94TR0.05O3samples, where TR is (A) Mn,(B) Ni, (C) Co and (D) Cr. XRD patterns of referencesamples: (E) pure Bi0.8La0.2Nb0.01Fe0.99O3with no transition metaldoping and (F) BiFeO3. The BiFeO3pattern is indexed according tothe R3C structure, and the impurity diffraction peaks are assigned toBi25Fe2O39, Bi25FeO40and Bi2Fe4O9.

other transition metal dopants lead to the reappearance ofthe impurity phase. This is in accordance with the materialstability of the compounds that these transition metals formwith bismuth and oxygen. BiFeO3 is a relatively stable

compound, which can be synthesized in ambient atmosphere.However, BiCoO3, BiCrO3, BiMnO3 and BiNiO3 can onlybe synthesized by a high pressure, high temperature method.Among them, BiMnO3is stabler than the other three bismuthcompounds.

In addition to the production of impurities caused bythe different dopants, distortion in the crystal structure wasalso observed for the samples doped with different transitionmetals. The diffraction pattern of BiFeO3, except forthe peaks from impurities (labelled in the XRD pattern),can be well indexed by a space group R3C structure.Bi0.8La0.2Nb0.01Fe0.99O3without transition metal doping and

its Co-doped counterpart have the same structure as theundoped BiFeO3. However, the structures of the Mn-, Ni-and Cr-doped samples were distorted, as evidenced by theapparent diffraction peak splitting of the (0 1 2) facets. Asimilar peak splitting was also observed for its higher orderdiffraction peak (0 2 4). Together with the fact that no extrapeaks were observed, particularly for the Mn-doped sample,the simultaneity of splitting happening on the (0 1 2) peakand on its higher order diffraction peak both prove that theobserved peak splitting is due to structural distortion ratherthan an impurity phase. This is a good indication that thosetransition metals were substituted into the crystal lattice.

Figure2shows an MH loop at RT and the FCZFC

magnetization up to 900 K for the Mn-doped sample. Thehysteresis loop, which has a significant linear contribution,

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Figure 2. Magnetic properties of Bi0.8La0.2Nb0.01Fe0.94Mn0.05O3compound in comparison with Bi0.8La0.2Nb0.01Fe0.99O3, which has noadditional transition metal doping: (a)MHloop at RT; (b) FCZFC magnetization curve; (c) temperature dependence of inverse magneticsusceptibility; (d) temperature dependence of inverse magnetic susceptibility of Bi0.8La0.2Nb0.01Fe0.99O3compound for comparison, withthe inset showing the MHloop at RT.

shows a maximum magnetic moment of 0.55 emu g1

at 5 Tand a large coercive field of 1.79T at RT. The FCZFCmagnetization measurement shows that the sample has anAFM transition, with a ferrimagnetic or ferromagnetic featurestarting from 635 K. Through fitting the part of the FC curveabove the transition temperature using the CurieWeiss law,a CurieWeiss temperature of 1273 K was obtained. Thehysteresis and the magnetic phase transition behaviour of thissample are very different from those of the undoped BiFeO3,which has TN = 643K and shows almost standard AFMbehaviouratRTwithoutanyhysteresis(intheformofastraightline before saturation at a high magnetic field). In addition,the AFM transition temperature of 635 K and the CurieWeiss

temperature of 1273 K of the Bi0.8La0.2Nb0.01Fe0.94Mn0.05O3compound are both much lower than TN = 796K andthe CurieWeiss temperature, N = 2170 K, of the parentcompound Bi0.8La0.2Nb0.01Fe0.99O3, while the magneticmoment is several times higher.

Figure3shows a hysteresis loop of the Bi0.8La0.2Nb0.01Fe0.94Cr0.05O3 sample at RT, and the FC and ZFCmagnetization at high temperatures. Similar to the Mn-dopedsample, this sample has an AFM transition with strongferromagnetic/ferrimagnetic features, with TN = 675K anda CurieWeiss temperature of 1805 K. The Cr-doped sampleshows a magnetic moment of 0.5 emu g1 at 5T in a perfecthysteresis loop with less of a linear component from the AFM

contribution in comparison with the Mn-doped sample, alongwith a slightly smaller coercive field of 1.67 T.

Figure 3.FCZFC magnetization curve of theBi0.8La0.2Nb0.01Fe0.94Cr0.05O3sample measured at 1000 Oe. Upperright inset is the temperature dependence of the inverse magneticsusceptibility. Lower left inset is the RT MHloop.

In figure 4, the magnetization hysteresis loop of theBi0.8La0.2Nb0.01Fe0.94Ni0.05O3sample at RT and the FCZFCmagnetization curves show that the magnetic ordering startsat around 653 K. However, this sample has another magnetictransition at a temperature above 800 K. The magnetizationbehaviourundertheappliedmagneticfieldisalsoverydifferent

from thoseof itsMn- andCr-dopedcounterparts, as this sampleshowsa muchhigher magneticmoment at5 T, upto 1 emu g1,

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Figure 4. FCZFC magnetization curve of theBi0.8La0.2Nb0.01Fe0.94Ni0.05O3sample measured at 1000 Oe. Inset isMH loop measured at RT.

Figure 5. FCZFC magnetization curve of theBi0.8La0.2Nb0.01Fe0.94Co0.05O3sample measured at 1000 Oe. Inset isRTMHloop.

with a much reduced AFM contribution and a very smallcoercive field of 410Oe.

The Bi0.8La0.2Nb0.01Fe0.94Co0.05O3 sample shows anearlysaturated hysteresis loop with a momentof 1.14 emug1

at an applied magnetic field of 5 T and a coercive field of1700 Oe at RT (figure5). The observed magnetic moment

is the largest among these transition metal doped samples.However, similar to the Ni-doped sample, this sample hasanother magnetic transition above the AFM-like transitiontemperature of 668 K.

As can be seen from the above measurements, all thesamples with transition metal doping show much enhancedmagnetization at RT in comparison with the pure BiFeO3sample and the La and Nb co-doped BiFeO3 sample, whichwere both fabricated using the same techniques. Due to thecontroversy over the cause of the large magnetic moment,whether it is from an impurity or intrinsic, such as hasbeen observed in room temperature FM diluted magneticsemiconductors, the XRD patterns of all the studied samples

were re-examined. The results show that the impurities ofBi25Fe2O39 and Bi25FeO40 in some of these samples are

paramagnetic or only FM at very low temperatures, whileBi2Fe4O9 is paramagnetic at RT [11, 12]. In addition,the magnetization measurements above RT indicate that nodetectable magnetic ordering transition from impurities whichcould contribute to MHloops measured at RT was presentbelow the AFM transition temperatures of the samples.

Therefore, the observed enhancements in the magneticmoments are regarded as intrinsic for the Mn- and Cr-dopedsamples in the detection limit of the common XRD technique.However, it could not be determined whether the magnetictransitions above TNfor the Co- and Ni-doped samples wereintrinsic or from impurities. Doping with transition metalssignificantly shifted the transition temperature from 796 Kfor the Bi0.8La0.2Nb0.01Fe0.99O3sample down to between 635and 675 K, if the higher temperature transitions in the Ni-and Co-doped samples are not counted. This indicates thatthe transition metal doping decreases the magnetic interactionbetween magnetic ions, which is well in accordance with themuch lower temperatures for magnetic ordering that occur inBiTRO3compounds. The lower CurieWeiss temperatures ofthe Mn- and Cr-doped samples also indicate that the magneticinteractions in these two samples are much weaker than in theBi0.8La0.2Nb0.01Fe0.99O3sample with transition metal doping.In addition, the magnetic moment per magnetic ion wascalculated through fitting the CurieWeiss law to the Mn- andCr-dopedsamples. SpinvaluesofS = 2.51andS = 2.82wereobtained per Fe/Mn and per Fe/Cr. These values indicate thatallthe transition metalsare in thehigh spin states, although theyare slightly higher than the theoretical values of the high spinstates of Fe3+ (S =2.5), Mn3+ (S =2)and Cr3+(S =3/2),which indicates that other non-magnetic ions contribute to the

magnetic moment.Considering the magnetic structure of BiFeO3, its cycloid

spin structure has a periodicity of 620 along the [1 1 0]haxis, with AFM ordering for the two adjacent [1 0 0]h Mnchains at RT (shown in figure6(a)). One chain of Mn with acomplete cycloid period contains about 64 iron ions [13]. Thelow doping level of 5% will introduce about 3 other transitionmetals into an entire chain containing 64 ions. It has beenshown above that Fe3+ in BiFeO3is only in the spin state withS = 5/2, and the effective moment, eff = 5.92B, whereB is the Bohr magneton. Depending on the 3d electronconfiguration of the dopant transition metal, for example,

3d

4+

for Mn

3+

in the high spin state with quantum numberS = 2 and an effective momenteff = 4.90B, 3d3+ for Cr3+

with S = 3/2 and eff = 3.87B, and 3d7+ for Ni3+ withS = 1/2 and eff = 1.73B, a local FM ordering structurewill form at the dopant ion position and thus produce a netmoment accordingto the superexchangemechanism[14], eventhoughthe overall AFMordering structure of theBiFeO3is notchanged (shown as the FM part in figure6(b)). For anotherdopant transition metal ion, Co3+ in the high spin state withS = 2 and eff = 4.90B, a local ferrimagnetic structurewill form according to the superexchange mechanism, andsimilarly, a net moment will be produced (shown in the AFMpart in figure6(b)). It is possible that the dopant transition

metals in BiFeO3are in the valence states or spin states otherthan what has been listed above, which will produce different

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Figure 6. (a) Two chains of Mn3+ with complete spin cycloidperiods and AFM ordering in BiFeO3result in zero moment. (b)Doping with transition metals will result in local ferrimagneticordering (labelled AFM) or local FM ordering (labelled FM),depending on the 3d electron configurations of the dopant transitionmetals, which will result in non-zero moment.

(This figure is in colour only in the electronic version)

electron configurations. However, the doping effects of allthese transition metals will result in a local enhancement inmagnetic moment accordingto the superexchangemechanism,whatever local magnetic ordering is present. The low dopinglevel reduces the possibility of a symmetrical arrangementof dopant ions in the cycloid chain, or of any ordering ofthe ions and thus a cancellation of the local net moment,because the large distances between dopant ions will resultin a random arrangement. The observed non-AFM transitionfeatures atTNfor all the samples are good indications that theferrimagnetic/ferromagnetic transitions occur simultaneouslywith the AFM transitions. The above discussion can be

supported by the fact that the grain size of the samples islarger than 62 nm along their [11 0]hdirection. Otherwise, thedoping effect will be mainly the particle size effect, becausea grain size of less than 62nm will have an incompleteperiod of cycloid structure and thus a net moment. The SEMobservations show that doping with different transition metals

does change the grain morphology; however, the grain sizes inall the samples are much larger than 62nm. For example, thegrain size of the Mn-doped sample is more than 100nm, whilethe grain sizes of the other transition metal doped samples areeven larger(SEM imagesnot shown here). Hence themagneticproperty enhancement caused by the transition metal doping is

due to the formation of ferromagnetic/ferrimagnetic orderingrather than the grain size effect.

4. Conclusion

In summary, a new doping mechanism to form a localferrimagnetic/ferromagnetic structure for improving themagnetic moment in bismuth ferrite has been proposed,which was tested by examining transition metal dopedBi0.8La0.2Nb0.01Fe0.94TR0.05O3. All the samples show muchimproved magnetic moment in comparison with the referencesample with no transition metal doping. In addition, noimpurities were observed in the Mn-doped sample accordingto both the magnetic and the XRD measurements, indicatinggood phase stability of the Mn-dopedBi0.8La0.2Nb0.01Fe0.99O3sample.

Acknowledgments

Z X Cheng thanks the Australian Research Council for supportthrough a Future Fellowship (FT0990391). The authors alsothank Dr Tania Silver for critical reading of the manuscript.

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