Multiferroics and Magnetoelectrics: A Comparison ... - ACS Publications

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Multiferroics and Magnetoelectrics: A comparison between some chromites and cobaltites. K. R. S. Preethi Meher, Christine Martin, Vincent Caignaert, Francoise Damay, and Antoine Maignan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4020546 • Publication Date (Web): 16 Aug 2013 Downloaded from http://pubs.acs.org on August 27, 2013

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Chemistry of Materials

Multiferroics and magnetoelectrics: a comparison between some chromites and cobaltites K.R.S. Preethi Meher1, C. Martin1, V. Caignaert1, F. Damay2 and A. Maignan1* 1

Laboratoire CRISMAT, UMR 6508 CNRS/ENSICAEN, 6 bd du Maréchal Juin 14050 CAEN Cedex 4 - France 2

Laboratoire Léon Brillouin, UMR12, CEA Saclay, CEA-CNRS, 91191 Gif sur Yvette Cedex - France

Abstract Oxides containing magnetic 3d transition metals offer a large family of structures with frustrated magnetic networks. Some of those incommensurate antiferromagnetic structures are responsible for the local breaking of inversion symmetry so that these oxides are called spin induced ferroelectrics. As listed in the introduction of this short review, the number of these multiferroics keeps on increasing. As for applications the coupling between these ferroisms is needed, some magnetic oxides, despite a lack of ferroelectric ground state, exhibit large magnetoelectric coupling with a magnetic field induced polarization. Thus, they are classified as “magnetoelectrics”. In the present review, we focus on recently studied systems showing ferroelectric-like behaviors or large magnetoelectric coefficients. This will be illustrated by chromites with the comparison between ceramics of CuCrO2 and AgCrS2: they exhibit different antiferromagnetic

ground

states,

centro-

and

non

centrosymmetric.

For

the 1

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magnetoelectrics, the example of the cobaltite, CaBaCo4O7, a polar ferrimagnet, is taken to illustrate the existence of large magnetoelectric coefficients in crystals. Then, the existence of spin induced ferroelectricity in centrosymmetric orthochromites, RECrO3 with RE=Lu or Er, is discussed. Through this selection of potential multiferroics or magnetoelectrics, it is clear that many magnetic materials in different forms (bulk/crystal/thin film) remain to be studied.

Keywords: multiferroic, magnetoelectric, cobaltite, chromite, oxide, antiferromagnetism, ferrimagnetism.

* Antoine Maignan Laboratoire CRISMAT, UMR 6508 ENSICAEN/CNRS, ENSICAEN, UCBN 6 boulevard du Maréchal Juin, 14050

Caen cedex 4 - France

[email protected] Tel: +33(0)2.31.45.26.04 Fax: +33(0)2.31.95.16.00

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1- Introduction The strong coupling between the orbital/charge/spin degrees of freedom in transition metal oxides is driving their fascinating physical properties among which the revisited multiferroicity [1]. For the latter, two types of ordering for spins and electric dipoles should coexist in the same material and, in principle, this opens the possibility to make four state memories if the magnetization (electric polarization) can be tuned by an external electric (magnetic) field. Multiferroics can be classified in two categories. The first one corresponds to the compounds with spontaneous polarization (TC ferroelectric) appearing above the magnetic ordering temperature [TC or TN ferro(antiferro) magnetic] such as BiFeO3 [2]. In contrast, for the second category (also called type II or “improper” ferroelectrics), the polarization starts to develop below or at the magnetic ordering temperature. Though these type II multiferroics exhibit smaller polarization P values than type I ferroelectrics, the magnetoelectric coupling between the magnetization M and P is much higher. From the materials view point, most of these “improper” ferroelectrics are 3d transition metal oxides as examplified by different types of compounds going from vanadates to cuprates: spinel vanadates CdV2O4 [3], and FeV2O4 [4], chromites with the spinels ACr2O4 (A=Cu [5], Fe [6], Ni [7]) and the A’CrO2 delafossites (A’= Li, Cu, Ag [8]),

orthomanganites

RMnO3

with

R=Tb

and

Dy

[9,10],

RMn2O5

with

R=Tb, Gd…[11-13], CaMn7O12 a quadruple perovskite [14], MnWO4 [15], ferrites, with GaFeO3 [16], orthoferrites such as GdFeO3 [17], CuFeO2 [18] and the corresponding doped delafossites of CuFe1-xMxO2 formula (M= Al, Ga, Rh) [19-21], m-, y-, and z-type hexaferrites [22-25], ordered perovskites RBaCuFeO5 [26-27], the mixed manganite cobaltite Ca3CoMnO6 [28], a chain compound with Co/Mn ordering in the magnetic



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chains, the cobaltites Ba2CoGe2O7 [29], Ca2CoSi2O7 [30], LiCoPO4 [31], CaBaCo4O7 [32], the nickelate Ni3V2O8 [33], and the cuprates with the CuO tenorite [34] and LiCuVO4 [35]. Although, far from being exhaustive, this list outlines the important contribution of magnetic oxides to the field of the so-called “multiferroics”. In order to explain how the local centrosymmetry is broken by the magnetic ordering, serveral mechanisms have been proposed. The first one is based on the spin current model [36-37]. Two neighboring non collinear spins, Si and Sj, create a spin current js= SixSj, from which the corresponding polarization P is derived according to the relation P∼rijxjs (Eq. 1), where rij is the unity vector connecting these spins. It explains why many of the quoted phases exhibiting cycloidal spiral antiferromagnetic (AF) structures are multiferroics. In that case, the wave vector Q of the helix is perpendicular to the spin rotation axis, and P develops in the rotation plane of the spins, according to P∼QxSixSj (Eq. 2). Such complex AF structures often result from magnetic frustration and thus lattices with geometric frustration (triangular, kagome, pyrochlore…) provide good basic ingredients to look for multiferroic properties. However, the case of the proper screw AF structure found in some multiferroic triangular delafossites, such as CuCrO2 [38], crystallizing in the R-3m space group, shows the limit of the spin current model [39]. In this delafossite, the spins rotation in a perpendicular plane to Q implies P=0 according to Eq. 2. A model taking into account the d-p hybridization has been proposed to explain the existence of P in this R-3m phase [39]. Another mechanism for some compounds relies on the exchange striction [40]: an asymmetric exchange in the spin chain made of alternating magnetic cations, as in Ca3CoMnO6, corresponding to ↑↑↓↓ “up-up-down-down” spins creates a net


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polarization along the chains due to the fact that the strictions for the ↑↓ and ↑↑ bonds are note equivalent. This short review focuses on recent new systems for which ferroelectricity and/or large magnetoelectric effects have been reported: (i) two examples are chosen, belonging to the class of compounds for which the paramagnetic state is already polar [for a review, ref. 41]. The ferrimagnetic cobaltite CaBaCo4O7 [32,42] and the polar antiferromagnetic sulfide AgCrS2 [43-46], the latter being compared to the non polar antiferromagnetic delafossite oxide CuCrO2 [8,38]. (ii) the recent report of “improper” ferroelectricity (below TN) in crystals of orthoferrites [47] and ceramics of orthochromites [48] cannot be explained by considering the aforementioned models based on the spin current and exchange striction as shown by the symmetry analysis in ref. [49]. This motivated to re-visit two bulk orthochromites, LuCrO3 and ErCrO3 [50]. All the following results show the complexity of this topic, theoretically or experimentally. Part of this lies certainly in the fact that polarization measurements are uneasy. This outlines the need for more efforts to grow crystals or epitaxial thin-films to confirm or not the intrinsic multiferroic nature of these new materials.

2- The triangular lattice antiferromagnets (TLA) CuCrO2 and AgCrS2 After the report of the multiferroic properties in crystals of CuFeO2 induced either by external magnetic field application [18] or chemical substitution at the Fe-site [19-21],



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the AMO2 delafossites have been revisited by several groups. In particular, spin induced ferroelectricity was reported for polycrystalline CuCrO2 [8]. In this structure, the CrO6 edge-shared octahedra form CdI2-type layers which are interconnected by Cu+ cations in dumbbell coordination (Fig. 1a). Its space-group is R-3m, i.e. centrosymmetric for both the paramagnetic and antiferromagnetic states, TN being ∼25K. In the CrO2 layers, the Cr3+ S=3/2 magnetic cations form a triangular network. To explain the ferroelectricity appearing in the ordered magnetic state, the local symmetry must be polar. However, the proper screw type of spin ordering in the ferroelectric state of iron-based delafossites cannot be explained by the spin current model or Dzialoshinskii-Moriya interaction as the vector rij is parallel to SixSj which yields P=0 from Eq.2. This is why Arima [39] proposed a hybridization model based on the single-ion anisotropy rather than spin-spin interactions to explain the polarization. This model is also valid in the case of CuCrO2 as the proper screw (Fig. 2a) incommensurate antiferromagnetic structure reported by M. Poienar et al [38] could not explain the in-plane polarization. In the sulfide analogs, the AgCrS2 compound was worth studying [51]. Its space group is polar at room temperature (R3m) in contrast to the centrosymmetric R-3m space group of the CuCrO2 delafossite. This comes from structural differences (Fig. 1a & 1b). In AgCrS2, first, the Ag+ cations and Cr3+ cations are off-centered along the layer stacking direction in their distorted tetrahedral and octahedral coordinations. The AgCrS2 sulfide undergoes a polar to polar transition at TN (=42K), the antiferromagnetic ferroelectric phase crystallizing in the Cm space group [44,45]. The AgCrS2 exhibits a “4L” AF structure with double ferromagnetic stripes (Fig. 2b). The R3m to Cm structural transition at TN goes with a magnetoelastic transition which is believed to be responsible for atomic


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shifts explaining the spin induced polarization below TN [44] (Fig. 3). This had been explained in a recent theoretical paper showing that the “4L” AF structure results from nearest-neighbor FM interaction combined with AF exchange between third neighbors [46]. This comparison shows that type II or “improper” ferroelectricity can be achieved for both non polar and polar layered AF, CuCrO2 and AgCrS2. For the latter, the magnetoelastic coupling at TN plays an important role to explain the spin induced polarization.

3- The ferrimagnetic magnetoelectric CaBaCo4O7 cobaltite The so called “114” cobaltite according to the cation ratios belongs to a family of oxides in which two types of layers of corner shared CoO4 tetrahedra are stacked along c forming 1:1 alternating triangular and kagomé layers of mixed-valent cobalt cations (Fig. 4a). The high spin Co2+/Co3+ cations are AF ordered below TN ∼110K in YBaCo4O7 [52] but the magnetic order is ferrimagnetic for CaBaCo4O7 below TC ∼70K (Fig. 4b) [53] with a crystallographic orthorhombic polar space group, Pbn21, for both ferrimagnetic and paramagnetic states. Similarly to that of some boracites exhibiting linear magnetoelectric effects [54-56], the magnetic point group is reduced to mm2’ [32],. These compounds belong to the class of magnetic points groups, where all three vectors, polarization P, magnetization M and ferrotoroidic moment T, are allowed and mutually orthogonal. From the mm2’ space group, the coupling between Pz along the polar axis , and the magnetic field H can be anticipated according to the equation [32,42]: Pz=α32Hy+(β311Hx2+β322Hy2+β333Hz2)/2 (Eq. 3), 7 ACS Paragon Plus Environment


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where αij and βijk are the coefficients of the linear and bilinear magnetoelectric susceptibility tensor. From equation 3, applying H along the easy magnetization axis , the polarization along is reduced to: Pz=α32Hy+β322Hy2/2 (Eq. 4). α32 is predicted only for the magnetic ordered state whereas only β322 remains for T>TC. From this local symmetry analysis, it turned out that crystals are necessary to validate or invalidate the predictions. Millimeter size crystals have been grown by using a mirror furnace starting from the melting of CaBaCo4O7 feeding rods prepared by solid state reaction by mixing CaCO3, BaCO3 and Co3O4 precursors at 1100°C. Plate shape crystals were obtained with the thinnest dimension corresponding to the (polar) axis. A capacitor with a thin plate parallel geometry was prepared for polarization and capacitance measurements performed with an electrometer (pyroelectric current) and a LCR meter, respectively. A clear peak in the dielectric permittivity (ε’) is measured at TC=64K (Fig. 5a) suggesting the existence of a polarization in that direction. A second smaller peak coexists at ∼68K but no corresponding change in the magnetic properties was detected suggesting that its origin is not magnetic. The ε’ value goes through a maximum as the applied magnetic field H increases, with a shift of the characteristic T at the peak. This demonstrates the existence of a magnetodielectric coupling near TC. As a peak in ε’ at the paraelectric to ferroelectric transition is an indication of a possible polarization, pyroelectric measurements were performed. However, no P reversal was made possible by switching with the maximum poling electric field of our experimental set-up (±1.1kV.cm-1). This contrasts with the switching observed for the corresponding ceramics [32]. Such a 8 ACS Paragon Plus Environment

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discrepancy, maybe due to a too low applied voltage, emphasizes that this cobaltite could rather be considered as a polar magnetoelectric rather than as a “multiferroic”. This is confirmed by the giant magnetoelectric effect observed by measuring P(H) loops near TC (T=65K in Fig. 5b). From the P(H) curves (inset of Fig. 6), the α32 and β322 coefficients (Fig. 6) were extracted with α32 reaching 764ps/m (SI units) at 60K a value comparable to that for TbPO4 single crystals [57]. Above TC, as predicted by Eq. 4 for H along and

along , α32 becomes ∼0 whereas β322 changes of spin at ∼ TC (Fig. 6). This T-dependence of α32 and β322 is very similar to that reported for boracites [58,59]. The largest α32 values found in the present CaBaCo4O7 crystal, much greater than those of boracites, is accompanied by large P change under magnetic field. This yields a huge magnetoelectric effect near TC with a ∆P variation of ∼9mC.m-2 induced by 9T (Fig. 5b), one of the highest reported value among magnetoelectrics. Nonetheless, these results are still not completely understood, as according to Eq. 4, P should change of sign as H becomes negative which is not the case (Fig. 5b). As magnetoelastic coupling is found near TC for this cobaltite, it might be that this effect results from strains rather than the magnetoelectric coupling predicted by the symmetry analysis. The results obtained on crystals show that the ferrimagnetic oxides could provide good candidates to obtain large magnetoelectric effects in the vicinity of their magnetic ordering temperature. As the origin of both ∆ε and ∆P changes at TC is associated to an abrupt change in the unit-cell parameters, it is believed that magnetoelastic effects, as for AgCrS2, are also responsible for the giant magnetoelectric coefficient and polarization variations.



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In addition to the fact that the large magnetoelectric effect is reached for the CaBaCo4O7 ferrimagnet, another interesting feature would be the possibility to get ferroelectricity in compounds containing CoO4 tetrahedra. This was already reported for another non centrosymmetric compound, Ba2CoGe2O7 (SG P-421m) with CoO4 tetrahedra and whose magnetic symmetry allows also the existence of spontaneous polarization, magnetization and toroidal moment [60].

4- Orthochromites: ferroelectricity in orthorhombic perovskites? In the orthoferrites and orthomanganites, several compounds with spin induced ferroelectricity have been reported: in the case of manganites, the centrosymmetry is broken by the cycloid magnetic structure (TbMnO3 and DyMnO3) [9,10] or by the presence of two types of exchange striction in the collinear E-type AF structure (HoMnO3) [61]. In the REFeO3 orthoferrites (RE= Rare Earth), it is the coupling between the RE magnetic ordering and the Fe3+ magnetic network which is responsible for multiferroic behavior at low temperature as in GdFeO3 below about 2.5K [17]. More recently, new reports have attracted our attention, with multiferroic behaviors at higher T, for orthoferrites [47] and orthochromites [48] (Fig. 7): below TN∼670K in SmFeO3 and TN∼133K in ErCrO3. These results obtained on crystals and ceramics for the former and the latter, respectively, still needed to be reproduced. In particular, as discussed in a comment [49], for the Pbnm space group, the magnetic ordering of the trivalent Fe (or Cr) magnetic moments cannot break the inversion symmetry. Although, due to the low T ordering of the RE3+ magnetic moments, the RE-Metal exchange


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striction could not be invoked, but in ref. [48] for the orthochromites, ferroelectricity was found only for paramagnetic RE cations at the A-site. This motivated us to revisit this system [50]. Two orthochromites were synthesized, (1400°C in air by mixing the RE2O3 and CrO3 precursors), ErCrO3 and LuCrO3, i.e. with paramagnetic and diamagnetic RE cation at the A-site, respectively. This difference of Asite cation impacts the magnetic susceptibility (Fig. 8). Though the TN values are close, TN=110K and 133K for LuCrO3 and ErCrO3, respectively, the χ values are larger for the latter. This is explained by the presence of the paramagnetic Er3+ cations. Moreover, there exists a spin reorientation (SR) in ErCrO3 with a χ decrease below TSR, not seen in LuCrO3 (Fig. 8). This SR results from anisotropy of magnetic interactions between RE3+ and Cr3+ cations. In contrast to the previous report suggesting the need for a magnetic RE cation to generate a ∆P change below TN, our ∆P(T) measurements obtained by direct measurement of charge with an electrometer, reveal the existence of significant ∆P changes (∼100µC.m-2) at TN for both LuCrO3 and ErCrO3 (Fig. 9). These values are of same order of magnitude as in ref. [48]. Moreover, this ∆P is switchable by changing the sign of the poling field. For LuCrO3, and in a less pronounced manner for ErCrO3, the closing of the ∆P curve at TN is not complete, the curves showing a second broad transition leading to the merging of the branches near 180K-200K. This second transition reflects the beginning of the leakage contribution. The losses remain small and frequency independent at 100K (Fig. 10), but for higher T (150K in Fig. 10), they increase a lot for low frequencies indicating the presence of leakage currents. This strongly supports that the second transition observed in the ∆P(T) curves results from this leaky behavior.



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We believe that these results are important: (i) they confirm the possibility to evidence a polarization change below TN in polycrystalline samples of orthochromites by the technique of direct measurement of charge, (ii) they show that the presence of a paramagnetic rare-earth cation at the A-site is not a necessary condition to obtain this type of evidence for ferroelectricity. Consequently, the explanation based on the RE-Cr exchange striction enhancing the distortion caused by the poling field [48] does not hold any longer. Other origins are now to be looked for to explain the existence of ∆P change near TN. In the absence of evidence for a magnetic field effect on ∆P (up to 14T), and without P(E) loops, it is too early to claim that these materials are “improper” ferroelectrics, i.e. the magnetic ordering breaks the inversion symmetry responsible for the polarization appearance. At this stage, crystal growth is necessary to reproduce these results.

5- Conclusion Beside the existing review papers (to quote some of them, ref. [1,41,62,63,64]) on the rather well understood multiferroic materials, this short review aims at pointing out that the physical properties –multiferroicity, magnetoelectricity- in other class of magnetic oxides (or sulfides) are still to be studied or understood. Up to date, the principal limitation for application of the magnetoelectric effect was the magnetic ordering temperatures well below room temperature. With the hexaferrites [2225], the magnetoelectric effect in low magnetic field at 300 K has been achieved [for a review, ref. 64]. Interestingly, polycrystalline samples were used to demonstrate this magnetoelectric effect in ferromagnetic hexaferrites [24]. The large magnetoelectric


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coupling in the ferrimagnet CaBaCo4O7 reinforces the assumption that searching for ferrimagnetic compounds could lead to the discovery of large magnetoelectric effects at high T. Another promising route, yet to confirm, is opened by the results obtained for orthoferrites and orthochromites. At this stage crystals of orthochromites have to be grown to test the reproducibility of the ferroelectric measurements. Finally, though not discussed here, it should be mentioned that all insulating magnetic oxides that we have tested in the laboratory exhibit magnetodielectric effects, but only a few of them show some ferroelectricity signatures. Starting from materials with polar groundstate should help in the the quest for multiferroics and magnetoelectrics. References: 1.

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Page 18 of 31

Figure captions: Fig. 1: Schematic representation of the delafossite structure of CuCrO2 (a) and AgCrS2 (b). The monovalent cations separating two successive CdI2-type layers of edge-shared CrO6 octahedra are in dumbbell (a) or tetrahedral (b) coordinations. Fig. 2: (a) Helicoidal incommensurate antiferromagnetic structure refined from the neutron diffraction data of CuCrO2, (b) AgCrS2: “4L” AF structure. Fig. 3: AgCrS2: unit cell volume (V) as a function of T (top panel) and (bottom panel) comparison of the magnetic susceptibility (χ) (b-right axis) and electrical polarization (P) (b-left y-axis). Fig. 4: CaBaCo4O7: Schematic representation of the crystallographic (top panel, a) and ferrimagnetic structures (bottom panel, b). Fig. 5: CaBaCo4O7 single crystal. Magnetic field (H) effect on (a) the dielectric permittivity (ε’) as a function of T and (b) the electrical polarization (P) vs H at 65K. Fig.6:

T-dependence of the linear (α32) and bilinear (β322) magnetoelectric coefficients from the susceptibility tensor calculated by fitting the P(H) curves (inset).

Fig. 7: Schematic structure of the orthochromites RECrO3. Fig. 8: T-dependent magnetic susceptibility of LuCrO3 (a) and ErCrO3 (b). Fig. 9: Relative variation of the electric polarization (∆P) as a function of T for LuCrO3 (solid symbol) and ErCrO3 (open symbol) collected upon warming at 2K.min-1 after subjecting to poling electric fields mentioned in the figure. 18 ACS Paragon Plus Environment

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Fig. 10: Loss factor (tanδ) as a function of frequency for LuCrO3 at different 100 K and 150 K.



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Figure 1

b

a


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Figure 2

b

a



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Figure 3


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Figure 4



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Figure 5a


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Figure 5b



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Figure 6


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Figure 7

c

a

b



Figure 8

6

ZFC FC

(a) 4 2

TN

3

χ (emu/g.Oe) X10

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0 -2 20 15

(b)

10

TSR

TN

5 0 0

50

100

150

200

Temperature (K)


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Figure 9



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Figure 10


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TOC graphic

30 25

ε'

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0T 9T

20 15 55

60

65

70 T(K)

75

80

85


Multiferroics and Magnetoelectrics: A Comparison ... - ACS Publications

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