Multiferroicity in Mn-deficient Ca3CoMnO6

J. Appl. Phys. 114, 027006 (2013); https://doi.org/10.1063/1.4811814 114, 027006

© 2013 AIP Publishing LLC.

Multiferroicity in Mn-deficient Ca3CoMnO6:

The consequence of Fe substitution

Cite as: J. Appl. Phys. 114, 027006 (2013); https://doi.org/10.1063/1.4811814

Submitted: 11 January 2013 . Accepted: 04 February 2013 . Published Online: 10 July 2013 L. Lin, Y. L. Xie, M. F. Liu, Y. J. Guo, Z. B. Yan, and J.-M. Liu

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Multiferroicity in Mn-deficient Ca

₃

CoMnO

₆

: The consequence of Fe substitution

L. Lin,¹Y. L. Xie,¹M. F. Liu,¹Y. J. Guo,^1,2Z. B. Yan,¹and J.-M. Liu^1,3,a)

1Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

2Department of Physics, Jiangsu Institute of Education, Nanjing 210013, China

3Institute for Advanced Materials, South China Normal University, Guangzhou 510006, China

(Received 11 January 2013; accepted 4 February 2013; published online 10 July 2013)

We present careful experiments on the multiferroicity in Ca3CoMn0.92O6by Fe substitution of Mn.

It is revealed that a proper Fe substitution modulates the Co/Mn spin order, favoring the ferroelectricity. The multifold interactions between the intra-chain spins and inter-chain spins are analyzed. It is suggested that the Mn deficiency and Fe substitution can break the inter-chain interactions to some extent, resulting in the delicate competition between these mutual interactions.

The modulation of the ionic (charge) disorder and spin frustration order is the core physics for improving the ferroelectric performance.V^C 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4811814]

I. INTRODUCTION

Multiferroics, especially the type II single phase multi- ferroics, have been retrieving continuous attentions in recent years due to the intense coupling between the magnetic and ferroelectric (FE) orders.^1–3Since the discovery of the well known colossal magnetoelectric coupling in TbMnO₃, a va- riety of “frustrated magnets,” e.g., Ni₃V₂O₈,⁴ CoCr₂O₄,^5,6 MnWO₄,⁷ Ba₃NiNb₂O₉,⁸ LiCuVO₄,⁹ other orthorhombic perovskite ReMnO₃,^10,11 and CaMn₇O₁₂,^12,13 have been synthesized, and the intrinsic interplay between the various degrees of freedom is carefully studied. Generally, for the magnetism-driven ferroelectricity, the specific spin order state is a necessary ingredient, e.g., the spiral spin order induced multiferroicity in (Tb,Dy)MnO₃ via the inverse Dzyaloshinskii-Mariya (DM) interaction or the symmetric exchange strictionmechanism.^2,3On the other hand, collin- ear antiferromagnetic (AFM) order can also contribute to the ferroelectricity as identified in quasi-1D Ising chain compound Ca₃CoMnO₆.¹⁴

For Ca₃CoMnO₆, all the Co^2þions are located in the tri- gonal prismatic sites, while the Mn^4þions occupy the octahe- dral sites.^14,15 Along thec-axis, the Co/Mn ions alternately establish ionic (charge) order and form an up-up-down-down (""##) spin order with Co (S¼1/2) and Mn (S¼3/2) at low temperature (T),^16–19shown schematically in Fig.1(a). Each spin chain is surrounded by six equally spaced chains and forms a triangular lattice on theabplane, favoring the geo- metric frustration. Basically, due to the exchange striction associated with the superexchange interaction between the nearest neighbor spins along the chain, the bonds between the parallel aligned spins will be shortened, while the anti- parallel spins should be stretched, thus generating the ferroe- lectricity along thec-axis. Therefore, both the Co/Mn ionic order (charge order) and the long range ""## Co/Mn spin

order (spin LRO) play an important role in the ferroelectricity of Ca₃CoMnO₆.^20,21

The physics underlying the multiferroicity of quasi-1D Ising chain Ca₃CoMnO₆is interesting, due to the complex interactions between the different ions as shown in Fig.1(b).

The intra-chain nearest neighboring (NN) ferromagnetic (FM) interactionJ1(J1>0) and the next nearest neighboring (NNN) antiferromagnetic interaction J2Mn or J2Co

(J2Mn

, J2Co<0) are the strongest. For the inter-chain interactions, the NN Co-Mn interactionJ4, the Co-Co NN and NNN inter- actionsJ3CoandJ⁰3Co, as well as the Mn-Mn NN and NNN interactions J3Mn andJ⁰3Mn may be necessarily considered.

For our discussion, the difference between the Co-Co and Mn-Mn interactions may be ignored. The balance between those interactions is so delicate that even small ionic order defect and disruption of some interactions can bring remark- able response in the multiferroic behavior.^21,22

In particular, recent works confirmed that the spin LRO could be survived only in the Co/Mn compensated Ca₃Co_1þxMn_1xO₆ with reduced ionic order, while the LRO is abruptly lost in the stoichiometricx¼1 state, meet- ing the best Co/Mn ionic order,²⁰thus leading to a suppres- sion of polarization P.²¹ However, in this case, the enhancement remains limited and the ferroelectric transi- tion pointTFEremains still low,TFE16 K. Meanwhile, in order to stabilize the spin LRO, some frustrated interactions can be disrupted by introducing local Co/Mn chemical mis- match, e.g., deficiency of Mn or Co ions in Ca₃CoMnO₆, thus leading to enhanced ferroelectricity and transition tem- perature.²³Apart from the above addressed efforts, an alter- native approach intending to modulate the Co/Mn spin order and ionic order is to introduce other magnetic ion by substitution of Mn/Co ions based on the Mn-deficiency sys- tem. Along this line, similar ion radius and bigger differ- ence in spin moment between the substituting and substituted species are the pre-requisites, so that the com- peting interactions can be controlled through creation of regions with smaller or larger moments.

a)Author to whom correspondence should be addressed. Electronic mail:

liujm@nju.edu.cn.

0021-8979/2013/114(2)/027006/5/$30.00 114, 027006-1 VC2013 AIP Publishing LLC

In this work, we start from the Mn-deficient Ca₃CoMn_0.92O₆and present careful experiments on the mul- tiferroic behaviors of Fe-substitution of Mn in this Mn-deficient Ca₃CoMn_0.92xFe_xO₆ (CCMFO). The main motivation for this study includes two aspects. First, it was demonstrated that the Mn-deficient Ca₃CoMn_0.92O₆ favors the spin non-frustration, which may beneficial to the ferroe- lectricity.²³However, over Mn-deficiency would damage the Co/Mn ionic order and thus the ferroelectricity. In order to suppress the spin frustration without damaging seriously the ionic order, magnetic substitution of Mn or Co is an interest- ing alternative. Second, Fe^3þdoes have similar ionic radius to Mn^4þ but different moment from that of Mn^4þ and it would be a proper choice to investigate the Fe substitution of Mn in Ca₃CoMn_0.92O₆. It will be shown that Fe-substitution does have positive impact on the multiferroicity of CCMFO.

II. EXPERIMENTAL DETAILS

A series of CCMFO with 0x0.13 were synthesized by conventional solid-state reaction. For details, stoichiomet- ric mixtures of CaCO₃, Co₃O₄, MnCO₃, and Fe₂O₃ were thoroughly ground, calcined at 850C for 24 h, pressed into pellets, and then calcined at 950C for 24–48 h. It is noted that all the samples were prepared under the same condition, and a careful chemical composition analysis was checked via X-Ray fluorescence spectrometer (XRF) to make sure that the measured chemical composition is consistent with the nominal values within60.1% measuring uncertainty.

The crystal structure of the as prepared samples was checked by powder X-ray diffraction (XRD) using Cu Ka radiation at room temperature. The T dependence of the dielectric constantewas probed by HP4294A impedance an- alyzer in connection with the physical property measurement

system (PPMS) (Quantum Design, Inc.). For measuring the polarization P, pyroelectric current method was used by the electrometer (Keithley 6514) connected to PPMS and details of the procedure and data calibration can be found in Refs.

24and25. The magnetizationMwas probed using the super- conducting quantum interference device magnetometer (SQUID) (Quantum Design, Inc.).

III. RESULTS AND DISCUSSION

First, we identify the crystalline structure of the as- prepared samples, and the detected XRD reflections are all well consistent with the K₄CdCl₆ rhombohedral structure (space groupR-3C),^26,27as presented in Fig.2. No impurity phases are detected up tox¼0.13. The inset shows the local amplified (113) and (300) peaks, indicating a slight shift of these peaks toward the high angle side with increasingx, due to the slightly smaller Fe ion than Mn ion.

Subsequently, the major concern goes to the effect of the Fe substitution. The measuredP(T) curve for a series of samples is shown in Fig.3(a). At a first glance and shown in the inset of Fig. 3(a),P is enhanced with increasing xuntil x¼0.03 at which P reaches the maximal 18lC/m². Beyond this point, P begins to decay with xbut a value as large as 12lC/m²is reserved even atx¼0.13. These results do suggest that a proper Fe substitution of Mn favors the spin LRO, thus enhancing the ferroelectricity. For the mag- netic properties, the measured v(T) data for these samples under zero field cooling (ZFC) conditions are shown in Fig.3(b), indicating the slight shift of the broad peak towards the high-Tside with increasingx, accompanied with the slight enhancement ofv. It is suggested that the Fe-substitution does induce the spin fluctuations, obviously due to the difference in moment between Fe spin and Mn spin and the possible relaxa- tion of the frustrated structure, although the underlying physics can be complicated.

Next, our attention is focused on the magnetic field (H) modulation ofP. Taking samplex¼0.05 as an example, the measured P(T) curves under various H are presented in

FIG. 2. XRD spectra for CCMFO with 0x0.13. The inset shows the (113) and (300) peaks for the samples with differentx.

FIG. 1. (a) Schematic drawing of ferroelectric origin in Ca3CoMnO6. The

""##spin order and alternating ionic order are the two keys to induce the ferroelectricity. (b) The multifold magnetic interactions in Ca3CoMnO6.

027006-2 Linet al. J. Appl. Phys.114, 027006 (2013)

Fig. 3(c). The response of P to H is quite weak over the wholeT-range, and this is reasonable since the ferroelectric- ity generated via the exchange striction mechanism is usually highly robust against H. For details, the measured P is slightly suppressed with increasingH, due to the suppression of the ""## spin configuration. For other samples, similar results are obtained.

To shed light on the multiferroic behavior of the Fe substitution in Mn-deficient CCMFO, one is allowed to con- sult to the involved multifold competing interactions. We look at the spin frustration in CCMFO with the multifold interactions. To proceed, we employ the Mont Carlo (MC) simulation to decide the ground state given for different interactions. All the spins are considered to be Ising spin, and a 6624 lattice matrix along thea, b, and cdirec- tion is applied, with the angle between theaaxis andbaxis 60. It should be noticed that the selected size of the lattice is reasonable, because our attention is only paid to look for

the ground state. The Hamiltonian of the Ising model is written as H¼Hin þ Hbe þ Hothers, where Hin, Hbe, and Hothersrepresent the intra-chain, inter-chain, and other inter- action energy.

Here, again it is mentioned that for the simplicity, the difference between the Co-Co and Mn-Mn interactions is ignored, and only the Mn-Mn interactionsJ3andJ⁰3are dis- cussed. The intra-chain and inter-chain interactions are writ- ten as

Hin¼ X

u

X

hi;ji

J1Su;iSu;jþX

½k;l

J2Su;kSu;l

;

Hbe¼ X

hu;vi

X

½i;j

J3Su;iSu;jþX

½k;l

J⁰3Su;kSu;lþX

½m;n

J4Su;mSu;n

;

whereuandvare the chain index subscripts,i,j,k,l,m,n are the site indices. The interactionsJ1,J2,J3,J⁰3, andJ4are labeled in Fig. 1(b). In the above formula, the J>0 repre- sents FM interaction, while J<0 indicates the AFM interaction.

It is known that the strongest intra-chain interactionJ1

andJ2determine the""##spin order state. The MC simula- tion shows that this ground state can still be maintained if only J3or J4is available, as shown in Figs.4(a)–4(d). For the cases in Figs.4(a)and4(b), no matter the nearest inter- chain Co-Mn interactionJ4is FM or AFM, the directions of the local electric dipole moments between the adjacent chains are always opposite, suggesting thatJ4prefers to sup- press the macroscopic polarization. On the contrary, if the lattice includes the J3 interaction, the ground state is pre- sented in Figs.4(c)and4(d), showing that the local polariza- tions from the adjacent chains tend to align in the same direction, no matter that theJ3is FM or AFM.

It should be noticed that the interactions between the intra-chain identical ions should include theJ3andJ⁰3, where J⁰3is a little bit smaller thanJ3due to the relatively longer Mn-Mn distance. Given the lattice structure, the adjacent chain shifts up to 2/3 Co-Mn interaction, with the NN and NNN Mn-Mn distance 2.1 A˚ and 2.4 A˚ , respectively.

Therefore, both the J3 and J⁰3 interaction should have the same sigh (J3>0,J⁰3>0 orJ3<0,J⁰3<0). From this point, there is also a competition between the J3and J⁰3. The J⁰3

prefers to weaken J3. If the J3 and J⁰3 have the opposite signs, as shown in Fig.4(e), their roles should be compatible with no competition. However, given the ionic location, the same sign of J3 and J⁰3 may be more reasonable. On the whole, because of the balance of J⁰3, the contribution ofJ3

would be weakened, while the influence ofJ4becomes criti- cal, resulting in the anti-parallel local polarizations. This is the partial reason for small polarization in Ca₃CoMnO₆.

If we further consider the intra-chain AFM interaction J2 (J2<0), it is seen that the J2, along with the superex- change pathJ3andJ30will form a trigonal path, as shown in Figs. 4(e) and4(f). Due to the strong AFM J2, this path is also frustrated, because theJ3andJ⁰3should be of the same type, allowing the competition between the J2and J3 (J⁰3).

However, the key point here is that one J2 must compete with six J3 (J⁰3) whose sum may be sufficient to compete

FIG. 3. (a) MeasuredP(T) for differentx. The inset reveals the measured P(x) atT¼2 K. (b) MeasuredMas a function ofTunderH¼100 Oe for dif- ferentx. (c) MeasuredP(T) data forx¼0.05 under differentH.

with one J2, resulting in much interesting and complex competition.

Basically, for Ca₃CoMnO₆, if ratio Co/Mn¼1.0, the

""##spin LRO order would be destroyed due to the compe- tition between theJ2,J3, andJ⁰3. Given one Mn deficiency, as shown in Fig.4(g), the surrounding interactionJ3(J⁰3) is broken, as indicated by the dashed lines, while the J2

remains less affected. At the same time, the Mn vacancy can damage the adjacent sixJ3, suppressing the competition. In this case, the""##spin LRO order will be gradually devel- oped, resulting in an enhanced polarization.²³Based on this scenario, one understands that the Fe substitution of Mn makes the situation more complicated, because of more probable competing interactions, e.g., the Co-Fe interaction J4Co-Fe, Mn-Fe interactionsJ3Mn-Fe

(J⁰3Mn-Fe), and the intra- chain interactionsJ1Co-Fe andJ2Fe-Mn. As discussed earlier, a small amount of Fe substitution can weaken theJ3to some extent, meanwhile strengthen the J2, thus leading to an enhanced polarization. On the other hand, too much Fe sub- stitution would undoubtedly change theJ1, which is strong enough to drive the spins from the""##configuration into

the """# or """" configuration. The M-T data shown in Fig. 3(b)do show increasingM with increasingx. In brief, the ferroelectric behavior depends on the dedicate balance between the multifold inter-chain and intra-chain interac- tions and more profound investigation is definitely needed.

IV. CONCLUSION

In summary, we have reported our careful experiments on the multiferroicity Mn-deficient Ca₃CoMn_0.92O₆ by par- tial Fe-substitution of Mn. It has been revealed that the Fe- substitution of Mn ion further improves the polarization and the maximal effect is obtained at a substitution level x¼0.03. The complicated intra-chain and inter-chain inter- actions induced by the ionic disorders and magnetic substitu- tion are responsible for the modulated multiferroicity.

ACKNOWLEDGMENTS

This work was supported by the National 973 Projects of China (Grant Nos. 2011CB922101 and 2009CB623303), the Natural Science Foundation of China (Grant Nos.

11234005 and 11074113), and the Priority Academic Program Development of Jiangsu Higher Education Institutions, China.

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