From spin induced ferroelectricity to dipolar glasses Spinel chromites

Journal of Solid State Chemistry 195 (2012) 41–49

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From spin induced ferroelectricity to dipolar glasses: Spinel chromites and mixed delafossites A. Maignan a,n, C. Martin a, K. Singh a, Ch. Simon a, O.I. Lebedev a, S. Turner b,c a

Laboratoire CRISMAT, UMR 6508 CNRS/ENSICAEN, 6 bd du Marećhal Juin, F-14050 CAEN Cedex 4, France EMAT, University of Antwerp, B-2020 Antwerp, Belgium c Canadian Centre for Electron Microscopy, McMaster University, Hamilton, ON L8S4M1, Canada b

a r t i c l e i n f o

abstract

Article history: Received 16 December 2011 Accepted 30 January 2012 Available online 9 February 2012

Magnetoelectric multiferroics showing coupling between polarization and magnetic order are attracting much attention. For instance, they could be used in memory devices. Metal-transition oxides are provided several examples of inorganic magnetoelectric multiferroics. In the present short review, spinel and delafossite chromites are described. For the former, an electric polarization is evidenced in the ferrimagnetic state for ACr2O4 polycrystalline samples (A ¼ Ni, Fe, Co). The presence of a Jahn–Teller cation such as Ni2 þ at the A site is shown to yield larger polarization values. In the delafossites, substitution by V3 þ at the Cr or Fe site in CuCrO2 (CuFeO2) suppresses the complex antiferromagnetic structure at the benefit of a spin glass state. The presence of cation disorder, probed by transmission electron microscopy, favors relaxor-like ferroelectricity. The results on the ferroelectricity of ferrimagnets and insulating spin glasses demonstrate that, in this research field, transition-metal oxides are worth to be studied. & 2012 Elsevier Inc. All rights reserved.

Keywords: Oxides Multiferroic Spinel Delafossite Chromite

1. Introduction Among the multiferroics, transition metal oxides are providing a large amount of examples. This is especially true if one considers the multiferroics in which it is the magnetic ordering that induces electric polarization so that the magnetization and electric polarization are coupled (type II multiferroics according to the classification given in the review by Khomskii [1]). The pioneering work on TbMnO3 [2] and TbMn2O5 [3] gave a new start to this field. As discussed in Ref. [1], these type II multiferroics are classified in two large families, compounds with complex non collinear antiferromagnetic (AF) structures and those with collinear structures (Ca3CoMnO6 [4], CdV2O4 [5] y). In the present short review the CuMO2 delafossites M¼ Fe, Cr, V [6–20] and the ACr2O4 spinel chromites [21–34] are taken as illustration of multiferroics with complex non collinear antiferromagnetic and collinear structures, respectively. Both structural types are well known for their geometric frustration with triangular network of M cations in the AMO2 delafossite which structure (Fig. 1(c)) can be described by a 1:1 regular stacking of CdI2-type layers made of edge shared MO6 octahedra interconnected by A þ cations in dumbbell coordination. In contrast to the 2D magnetism of the delafossite, the magnetism of the ACr2O4 normal spinels exhibits a 3D behavior. This is related to its

n

Corresponding author. Fax: þ33 0 2.31.95.16.00. E-mail address: [email protected] (A. Maignan).

0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2012.01.063

structure (Fig. 1(a) and (b)) in which defective planes of edgeshared CrO6 octahedra alternate with mixed layers of AO4 tetrahedra and CrO6 octahedra. In the AB2O4 spinel structure, a pyrochlore network, made by the B cations, is responsible for the geometric frustration [35]. In the following first section, we report on the magnetic, dielectric and pyroelectric properties of NiCr2O4, a normal spinel in which the Ni2 þ Jahn–Teller (JT) cation occupies the tetrahedral site. These results are compared to two other chromite spinels, tetragonal FeCr2O4 and cubic CoCr2O4 [34], i.e., with a JT- and a non JT-cation at the A site, respectively. In the second section, the results achieved for the mixed delafossite CuM0.5V0.5O2 (M¼Fe3þ , Cr3 þ ) will be described [36, 37]. Concerning their magnetic properties, these mixed delafossites behave like spin glasses whereas their dielectric and pyroelectric behaviors are characteristic of relaxor-type ferroelectrics. The properties of these dipolar glasses will be reviewed and compared to the spin induced ferroelectric delafossite such as CuCrO2 and AgCrO2. The search for defective regions responsible for the relaxor behavior in the CuM0.5V0.5O2 (M¼Fe, Cr) samples, by using transmission electron microscopy, reveals the existence of different types of defects in these mixed delafossites.

2. NiCr2O4: Ferroelectricity induced by a collinear structure After the first reports on spin induced ferroelectricity in several transition metal oxides exhibiting non collinear spin structures,

A. Maignan et al. / Journal of Solid State Chemistry 195 (2012) 41–49

1.0

TS

11.65

ε'

Tc

0.8 0.6

11.60

0.4

χ (emu.mol-1)

42

0.2

11.55

0.0 20

40

60

80

100

120

T (K) 0.25

dε'/dT

0.20 Fig. 1. Perspective view of the ACr2O4 spinel, cubic (a), tetragonal (b) and of the delafossite AMO2 structure (c). The unit cells (black lines) and corresponding cell parameters are also given. The octahedra are occupied by trivalent cations (Cr, Fe, V) whereas the divalent (a,b) and monovalent (c) cations are in tetrahedral and dumbbell coordinations, respectively.

0.0071 TJT

0.15

0.0070

0.10

χ (emu.mol-1)

0.0072

0.0069

explained by the spin current model [38], ferroelectricity in crystal of the CoCr2O4 normal spinel was found to be related to its conical spin order at TS occurring well below its ferrimagnetic transition TC [22]. Nonetheless, the more recent report on the multiferroic behavior of CdV2O4 [5], in its collinear magnetic structure, motivated us to revisit polycrystalline FeCr2O4 and CoCr2O4 normal spinels [36]. A larger electric polarization (P) was measured for the former and the existence of P up to TC for both spinels, i.e., in the collinear magnetic state was evidenced. These spinels exhibit structural differences linked to the presence of a Jahn–Teller (JT) cation at the A site in FeCr2O4. Below TJT ¼140 K, the FeCr2O4 undergoes a tetragonal distortion whereas the CoCr2O4 structure remains cubic down to the lowest temperatures. As the difference in P could be connected somehow to the presence or absence of a JT cation at the A site, a polycrystalline sample of NiCr2O4, in which Ni2 þ is also a Jahn–Teller cation, has been prepared and characterized. 2.1. Sample preparation and structural characterization The synthesis of the NiCr2O4 sample was made by solid-state reaction at high temperature. A mixture of the NiO and Cr2O3 precursors was weighted in the 1:1 stoichiometry ratio. After mixing in an agate mortar, parallelepiped bars were obtained by pressing the powder. The bars were heated several times at 1200 1C for 12 h in air in order to obtain a single phase (i.e., without Cr2O3 impurity). Room temperature X-ray powder diffraction data collected for the reacted sample confirm that it is single-phased, crystallizing in the I41/amd tetragonal spacegroup. The structural refinements lead to unit cell parameters ˚ in agreement with those of the (a¼5.846(1) A˚ and c¼8.410(1) A) literature [27]. 2.2. Magnetic and dielectric behavior The temperature dependence of the dielectric permittivity (e0 ) and magnetic susceptibility (w) of NiCr2O4, both recorded upon warming, are shown in Fig. 2(a). All e measurements were made on thin parallel plates with capacitor geometry with electrodes of silver paste. The measurements were performed by using a

0.05 0.0068 270

280

290

300

310

320

T (K) Fig. 2. NiCr2O4. (a) T-dependent real part of the dielectric permittivity (e0 , left y-axis) and magnetic susceptibility (w, right y-axis). Both type of measurements are made upon warming (w: moH¼ 10 2 T after zero-field-cooling, zfc). The vertical dashed lines are for the ferrimagnetic transition (TC) and the setting of the collinear antiferromagnetic component (TS). (b) T dependence of de0 /dT (left y-axis) and w(right y-axis) revealing a change of slope at the Jahn–Teller transition (TJT, vertical dashed line).

Agilent 4284A LCR meter (1 Vac bias, 1 kHz rfo100 kHz), a cryostat from Quantum Design ensuring the T and magnetic field (H) control. The w(T) curve (zero-field-cooling) shows two magnetic transitions: ferrimagnetic transition with an inflection point transition at TC ¼75 K and a smaller change at TS ¼ 31 K, corresponding to the conical spin order [39]. These values agree with those of the literature [27, 39]. Correspondingly, dielectric anomalies are observed at TS and TC, the e0 (T) curve showing a shallow minimum at this latter temperature. The absolute value of e0 and the anomalies vs. T are also in good consistence with previous report [27]. The coincidence of the e0 change with the magnetic transition results from the coupling between spins and charges which is also confirmed by the negative magnetodielectric effect measured at 10 K (Fig. 3(a)). Interestingly, the [de0 /dT](T) derivative curve (Fig. 2(b)) reveals a change of slope at 310 K which is the temperature of the Jahn–Teller structural transition from cubic (T4 TJT) to tetragonal (ToTJT). This TJT value is larger than the value obtained by e0 (T) measurements for FeCr2O4, TJT ¼140 K [34]. It must be also pointed out that TC values are very similar for NiCr2O4 and FeCr2O4 but larger for CoCr2O4 (Fig. 3(b)). The H driven isothermal magnetization curves collected at 10 K (Fig. 4) confirmed that NiCr2O4 behaves like CoCr2O4 and FeCr2O4 with characteristic hysteresis loops of ferrimagnets: lack of M saturation and low remnant magnetization values (MR ¼0.2 mB/f.u.).


43

2.3. Electric polarization

0.03

Δε'/ε' %

0.00

-0.03

-0.06 T=10K

-0.09 -9

-6

-3

0 μ0H (T)

3

6

9

The temperature dependence of the polarization P was measured with an electrometer in coulombic mode. In order to align the dipoles, a poling static electric field (Epoling) was applied at 120 K prior to cooling down to 8 K. At the latter T, E was removed and then a waiting time was applied to limit the signal drift. Afterwards, P was measured as a function of T upon warming at 5 K min 1. As shown in Fig. 5(a) (Epoling ¼ þ300 kV m 1), for the first time in NiCr2O4, a polarization has been evidenced. First, in the instrument resolution, P remains rather constant, PE13 mC m 2, up to 50 K and then, P decreases showing a broad transition with an inflection point near TC ¼75 K. By poling within 300 kV m 1, the sign of P changes to give a symmetric P(T) branch. By increasing E up to 600 kV m 1, P reaches values of 33 mC m 2 at 8 K (Fig. 5(b)) which is very similar to the value reported for FeCr2O4 [34]. Thus, the P(T) curves for NiCr2O4 and FeCr2O4 are almost superimposed, with larger P values but lower transition temperature than that of CoCr2O4 (Fig. 5(b)). 2.4. Discussion

1.0

The present P measurements up to TC in NiCr2O4 confirm the previously reported spin induced ferroelectricity in FeCr2O4 [34]. This result is important as it gives another example of a multiferroicity in a collinear magnetic state. It shows that the conical spin order found in CoCr2O4 is not only responsible for ferroelectricity, as first suggested by the report on CoCr2O4 [22, 23]. This is reinforced by the fact that in NiCr2O4, the conical magnetic

χ (emu.mol-1)

0.8 0.6

FeCr2O4 CoCr2O4

0.4

NiCr2O4

15

0.2

-2

0.0

P (μC.m )

10

0

20

40

60 80 T (K)

100

120

140

Fig. 3. (a) NiCr2O4: T¼ 10 K magnetodielectric curve.(b) Comparison of the zfc w(T) curves of two chromite spinels having a JT cation at the A site (NiCr2O4 and FeCr2O4) with CoCr2O4 in which Co2 þ is a JT none active cation.

5 0 -5 -10 -15

20

0

40

80

100

Fe

-0.6 -0.9 -4

-3

-2

-1

0

1

2

3

4

5

μ0H (T) Fig. 4. T¼ 10 K magnetic field driven magnetization curves [M(H)] of ACr2O4 chromite spinels (A ¼ Ni, Fe, Co).

Interestingly, the coercive magnetic field values (Fig. 4) are very close for M¼Fe and Ni (moHC ¼1 T) and larger than that of CoCr2O4 (0.25 T).

4

24

Fe

18

3

Co

Ni

2

12

-2

-2

P ( μC.m )

-0.3

-5

ACr O

30

Co

0.0

5

36

Ni

0.3

P (μC.m )

ACr2O4

0.6

M (μB/f.u.)

60

T (K)

0.9

1

6 0

0 20

40

60

80

100

120

140

T (K) Fig. 5. (a) T dependence of the remnant electric polarization (P) of NiCr2O4 (Epoling ¼ 7300 kV m 1). (b) left y-axis: P(T) curves of NiCr2O4 (Epoling ¼ þ600 kV m 1) and FeCr2O4 (Epoling ¼ þ200 kV m 1); the P(T) curve of CoCr2O4 is also shown for sake of comparison (right y-axis; Epoling ¼ þ 120 kV m 1).

44


3. CuMO2 delafossites: Two different origins for ferroelectricity?

5.25 10kHz

CuCrO2

50kHz

5.20

Several type II multiferroics have been reported in the triangular lattice antiferromagnets ABO2 crystallizing in the delafossite structure. They are related to the delafossite itself, CuFeO2, or to CuCrO2 [6–20]. For the former, the paraelectric collinear antiferromagnet ground state, can be transformed into a ferroelectric incommensurate non collinear antiferromagnetic state under application of an external magnetic field or by chemical substitutions at the Fe site such as in CuFe1 xAlxO2 (0.014rx r0.030 [11–13]), and CuFe1 xRhxO2 (0.02rx r0.15 [18]). The electric and magnetic ground state of CuCrO2 is a ferroelectric properscrew order [7–10]. Thus, for all these phases, ferroelectricity is induced by magnetic ordering so that they belong to the class of type II multiferroics. In the following, for this structural type, we will review an other type of ferroelectricity reminiscent of the relaxor behavior encountered in Pb(Nb5 þ /M2 þ )O3 perovskites [41–43]. Ferroelectricity in that case is explained in terms of micro- or nano-polar regions created by the compositional disorder induced by the presence of different species at the B-site of the ABO3 perovskites.

100kHz

5.15

5.10

ε'

5.05

6.75

10 kHz 50 kHz 100 kHz

AgCrO2

6.70

3.1. Magnetic and dielectric measurements

6.65

6.60 5

10

15

20

25

30

35

T(K) Fig. 6. e0 (T) curves for the ACrO2 delafossites [A ¼Cu (top panel), Ag (bottom panel)] collected at different frequencies (f). A clear peak is observed at TN. No curve shift with f at the transition can be evidenced.

structure reported for CoCr2O4 does not exist below TS, but rather a collinear antiferromagnetic component superimposed to the ferrimagnetic structure [39]. For these chromites, the comparison between tetragonally distorted spinels by the JT active A2 þ cation (Ni, Fe) and cubic CoCr2O4 phase supports the fact that the JT distortion amplifies the polarization of these phases. The P values of the formers are larger by an order of magnitude than those exhibited by CoCr2O4. To explain the existence of a polar state in the ferrimagnetic structures of AB2O4 phases, one may probably consider the rather complex magnetic situation, with different A2 þ B3 þ , A2 þ A2 þ , B3 þ B3 þ magnetic interactions, which different exchange pathways could be responsible for local symmetry breaking. This mechanism was proposed to explain the polarization in the CdV2O4 spinel [5]. It must be also pointed out that the values of P for these type II multiferroics (Pmax 35 mC m 2) are smaller, by several orders of magnitude, than the values found in type I ferroelectrics such as PZT. Thus the atomic displacements induced by the magnetic ordering in the chromite spinels are expected to be much smaller. In conclusion, this short review on chromite spinels demonstrates that the search for new type II multiferroics should not be limited to noncollinear antiferromagnetic incommensurate structures but must be extended to insulating ferrimagnets and antiferromagnets. As also supported by the recent report of ferroelectricity at RT in orthoferrites such as SmFeO3 [40], the coexistence of different types of magnetic interactions seems to be a ingredient required to sort possible candidates.

In great contrast to the metallic behavior of the PdCoO2 or PtCoO2 delafossites, for which the network of Pd(Pt) ensures the charge delocalization, the delafossites such as CuCrO2 or AgCrO2 are insulators as can be seen from their e0 (T) curves shown in Fig. 6 in the region of their Neél temperatures (TN ¼24 K). The existence of a dielectric peak at the AF ordering temperature indicates a spin-charge coupling. The evidence below TN for a maximum polarization of 30 mV m 2 [8] indicates that these polar incommensurate antiferromagnets [10] belong to the class of spin induced ferroelectrics (type II). Following the pionnering work made by Doumerc and coworkers [20] on the V3 þ substitution for Fe3 þ or Cr3 þ in the CuMO2 delafossites, two polycrystalline samples were prepared: CuFe0.5V0.5O2 and CuCr0.5V0.5O2 [36, 37]. The possibility to synthesize samples with such compositions is not obvious if one considers that the CuVO2 delafossite cannot be stabilized and also ¨ because, according to Mossbauer studies [20] in mixed Fe/V delafossite, Fe is trivalent implying that a trivalent oxidation state for V. This creates an interesting magnetic situation with a mixture of S ¼5/2 (Fe3 þ ) and S ¼1 (V3 þ ) spins in the CdI2-type slab of the delafossite structure. The synthesis of CuFe0.5V0.5O2 was made in silica ampoules at 800 1C starting from bars obtained by pressed powders of mixtures of Fe2O3, V2O5 and Cu2O precursors weighted in the appropriate ratios. In the Cr case, a preparation in two steps had to be performed. First, the bars of pressed powders obtained from the Cu2O, Cr2O3 and V2O5 precursors set in a close evacuated silica tube, were reacted at 1100 1C. A second reaction in the same conditions (ampoulas and temperature) was then necessary to obtain a pure phase (without traces of VOx oxides). The structural refinements form the X-ray diffraction patterns in the R3¯m space group of the delafossite lead to unit cell parameters consistent with the previous reports [20]. The T dependence of the magnetic susceptibility, collected in zerofield-cooling (zfc) and field-cooling (fc) modes in a SQUID magnetometer, show curves typical of spin glasses with a cusp in the zfc and rather flat T-dependent fc branches (Fig. 7). When measured in acsusceptibility, the characteristic temperatures (Tf) of cusp maximum shift up with the frequency increase. The analysis with a power law yields to parameters typical for spin glasses (Fig. 8). The deduced spin glass temperatures TSG are 19.9 K and 10.6 K for CuFe0.5V0.5O2 and CuCr0.5V0.5O2, respectively. These magnetic characterizations


0.05

χ(emu.mol )

0.04

μ H=10 T

CuFe

0.20

V

O

0.15

zfc

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zfc

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CuCr

V

χ(emu.mol )

fc fc

O 0.05

0.01 0

10

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50

45

observed in the T dependence of the losses (tand, Fig. 9(c) for Cr0.5V0.5) which exhibits a maximum in the region where e0 (f) effect is maximum. Such f effect on e0 is very different in the case of the spin induced ferroelectric parent compound (Fig. 6): as f increases, the e0 magnitude and the characteristic T of the peak are kept unchanged. It must be also emphasized that the T region of the tand, maximum in Fig. 9(c) (30 K at 10 kHz) is beyond the Tf value measured for the same frequency by ac magnetic w (Fig. 8(b)). This strongly suggests that the feature on the dielectric permittivity is not resulting from spin freezing. In fact, these large f effects on e0 are reminiscent of the relaxor ferroelectrics such as the substituted ABO3 perovskites [41–43].

60

T(K) Fig. 7. w(T) curves (zfc and fc; moH¼ 10 2 T) for CuCr0.5V0.5O2 (left y-axis) and CuFe0.5V0.5O2 (right y-axis).

8.2

8.0

CuFe

V

O

T =19.90K zv=6.06

-6

CuFe0.50V0.50O2

ε'

-4

7.8

τ =4.49x10 s

10kHz 50kHz

lnτ

p=0.027

100kHz

-8

7.6

-10

0

30

60

90

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150

T(K)

-3.2

-4

-3.0

-2.8

CuCr

V

-2.6

O

-2.4

-2.2

-2.0

16

T =10.60K

14

zv =9.12

-6

τ =2.13x10 s

CuCr0.50V0.50O2 ε'

-12 -3.4

lnτ

p=0.037

12 10kHz

-8

50kHz

10

100kHz

-10 8

-2.0

-1.8

-1.6 -1.4 ln[(T -T )/T ]

-1.2

Fig. 8. Analysis with a power law of the time [T¼ 1/(2pf)] vs. freezing temperature (Tf) with frequency f of the ac excitation magnetic field. CuFe0.5V0.5O2 (top panel); CuCr0.5V0.5O2 (bottom panel).

0.09 CuCr0.50V0.50O2 0.06 tanδ

-12

0.03 demonstrate that the mixed B site occupation favors a disordered magnetic situation contrasting with the AF ordering for the pristine compounds, CuFeO2 and CuCrO2 [44, 7, 10]. Both CuM0.5V0.5O2 ceramics are rather insulating in agreement with the mixed occupation of trivalent cations at the B site. Accordingly, dielectric permittivity as a function of T was collected for CuFe0.5V0.5O2 and CuCr0.5V0.5O2 at different frequencies (Fig. 9(a) and (b)). For both compounds, there exists a large T region (450 K) of frequency dependence where the curves shift towards higher T as f increases. A similar shift with f is also

0.00 0

20

40

60

80

100

T(K) Fig. 9. e0 (T) curves at different frequencies for CuFe0.5V0.5O2 (top panel) and CuCr0.5V0.5O2 (middle panel) showing a T region with large shift with f. The tand(T) curve is given for the latter (bottom panel).

46


150 CuCr0.5V0.5O2 100

1.5 1.0

50

P (µC.m-2)

P (µC.m-2)

CuFe0.5V0.5O2

0.5 0.0 -0.5 -1.0 -1.5 5

10

15

20

25

30

T(K)

0 0

50

100

150

T (K) Fig. 10. P(T) curve of CuCr0.5V0.5O2 obtained by poling from 120 K with Epoling ¼ þ 135 kV cm 1. Inset: P(T) curve of CuFe0.5V0.5O2.

3.2. Electric polarization of the CuM0.5V0.5O2 delafossites As ferroelectricity could be expected for relaxors, polarization measurements were made with the same technique as described in Section 2.2. A similar experimental process as for the spinel chromites was used for the P measurements. The first measurements were made for CuFe0.5V0.5O2 with a poling field of 7600 kV m 1. A small P value at the lowest T was measured, P¼1.3 mC m 2 (inset of Fig. 10). As this value is smaller by two orders of magnitude than in polycrystalline parent compounds (for instance in CuFe0.92Rh0.08O2, P¼110 mC m 2 [18]), more measurements with different conditions were made on CuCr0.5V0.5O2. As expected for a relaxor, its polarization has been found to depend on the poling T. In Fig. 10, a P maximum value of 150 mC m 2 is reached when the sample is poled at 120 K, with a transition temperature at the midpoint of 120 K. Such a value is much larger than P¼21 mC m 2 measured in the pristine CuCrO2 polycrystalline sample [45], and also is the transition temperature, 120 K against 24 K (TN) in CuCrO2. These results demonstrate that the mechanism responsible for the ferroelectric behavior in the CuM0.5V0.5O2 delafossites is not induced by the magnetic ordering. In addition, the transition temperature in Fig. 10, much larger than the spinglass temperature TSG (10.6 K), rules out a relation between the spin glass and the relaxor ferroelectric behaviors. To our knowledge, it was the first time that such a behavior is reported in a delafossite. 3.3. Local structural informations As in relaxor ferroelectrics, short-scale polar regions are supposed to be responsible of their peculiar behavior, a transmission electron microscopy (TEM) study was made for both CuCr0.5V0.5O2 and CuFe0.5V0.5O2. Its first objective was to measure local chemical composition and to look for possible local ordering phenomena between the Fe(Cr) and V cations which, despite similar ionic radii, might have been favored by their 1:1 ratio. The second objective was to look for structural defects which could be at the origin of the relaxor ferroelectric behavior. These observations were all made at room temperature. 4. Experimental The verification of the crystal structure of CuCr(Fe)0.5V0.5O2 and the identification of possible ordering – disordering of the

Cr(Fe)–V cations and possible defect structures were carried out by several TEM transmission electron microscopy (TEM) techniques, including electron diffraction (ED), high resolution TEM (HRTEM), high-angle annular dark field scanning TEM (HAADFSTEM) and spatially resolved electron energy-loss spectroscopy (EELS). The combination of these TEM techniques allows a characterization of the crystal and chemical structure of the material to be made. High resolution transmission electron microscopy and electron diffraction experiments were carried out on Tecnai G2 30UT microscope operated at 300 kV, with a 0.17 nm point resolution. HR-HAADF-STEM and STEM-EELS experiments were carried out on a FEI Titan 80–300 ‘‘cubed’’ microscope at EMAT and a similar instrument at the Canadian Centre for Electron Microscopy (CCEM), both equipped with an aberration-corrector for the imaging lens as well as for the probe forming lens and a high resolution energy filter (GIF) for spectroscopy. Data for Figs. 12(a) and 14 were acquired at EMAT, using a microscope acceleration voltage of 300 kV. For imaging, an inner collection semi-angle b of 50 mrad was used. Spectroscopy experiments were performed on a GIF-QUANTUM spectrometer – acquiring simultaneous ADF and EELS signals. The STEM convergence semiangle used was 21.5 mrad; the collection semi-angle b was 100 mrad. To generate the atomic resolution elemental maps, the acquired EELS data was PCA treated and reconstructed using three principal components. After reconstruction, the EELS edges were background subtracted, and maps generated using a 20 eV integration window. Data for Fig. 12(b) and (c) were acquired at the CCEM using a microscope acceleration voltage of 200 kV. For imaging, an inner collection semi-angle b of 60 mrad was used. Spectroscopy experiments were performed on a GIF-TRIDIEM spectrometer – acquiring simultaneous ADF and EELS signals. The STEM convergence semi-angle used was 18.5 mrad; the collection semi-angle b was 70 mrad. To generate the atomic resolution elemental maps, the EELS edges were background subtracted, and maps generated using a integration window; no PCA data treatment was performed. For all STEM experiments the beam current was kept close to 40 pA for imaging and 80 pA for spectroscopy. Proper acquisition times were chosen to avoid beam damage and provide the best combination of signal-to noise ratio, spatial and energy resolution.

5. Results The ED patterns, collected along four main zone axes of both CuCr0.5V0.5O2 and CuFe0.5V0.5O2 are highly similar. They reveal excellent crystallinity and can be unambiguously indexed based upon the hexagonal R3 m structure with corresponding unit cell parameters as reported in Ref. [20]. As an example, Fig. 11 shows ED patterns collected for CuCr0.5V0.5O2. No superstructure or modulations due to possible ordering of Cr(Fe)–V cations are detected on any of the ED patterns. TEM imaging of microcrystals demonstrates the presence of preferential lamella-type structures in both delafossites, which are quite often observed for this type of layered materials. The lamellas are stacked along the c-axis, and are caused by low-angle boundaries that result in small misorientation between neighboring lamellas. Nevertheless, the structure within a single lamella is mostly defect free. A high resolution HAADF-STEM image along the most informative and relevant (11 20) zone axis is shown in Fig. 12 for both compounds and demonstrates a perfect sequence of alternating Cu and Cr(Fe)–V layers. There is no evidence from HAADF imaging or from diffraction of segregation or ordering of Cr–V or Fe–V within a single lamella.


Fig. 11. Electron diffraction patterns of CuCr0.5V0.5O2 along the four main zone axis orientations.

47

ordering can be distinguished. To further investigate a possible local ordering of Cr(Fe) and V, atomic resolution elemental maps were acquired using spatially resolved EELS. To acquire this atomic resolution data, the so-called spectrum imaging technique was adopted. In this technique, a small electron probe is finely scanned over the sample (in our case a probe of 1 A˚ in diameter ˚ is scanned at steps of 0.5 A), acquiring EELS data at each position. In the case of CuCr0.5V0.5O2, the V–L2,3, Cr–L2,3 and Cu– L2,3 edges were used to generate atomic resolution elemental maps. In the case of CuFe0.5V0.5O2, the V–L2,3, Fe–L2,3 and Cu–L2,3 edges were used. The atomic resolution elemental maps confirm the results from HAADF-STEM; no ordering of Cr(Fe) or V layers can be detected, not even on a local scale. All interlayers are homogeneously mixed Cr(Fe)–V. A quantification of the Cr(Fe)/V content in the top Cr(Fe)/V layer yielded 47 75 at% V; 5375 at% Cr and 5475 at% V; 46 75 at% Fe. These values fit well to the nominal compositions. One of the important questions related to the properties of these delafossites is the possible presence of defects or chemical or/and structural inhomogeneities. After careful examination, several types of structural defects were found in the material. The first type (mentioned previously) are low angle boundaries along the c-axis between lamellas. Certainly, these low angle boundaries imply local distortions of the atom bonding along the boundary which may result in compositional difference along this boundary from the bulk, and segregation of one or another type of the atoms, Cr(Fe)/V. The second type of defect is a twinned structure along the caxis. Fig. 13(a) shows a low magnification TEM image and the

Fig. 12. (a) [11–20] HAADF-STEM image of a defect free CuCr0.5V0.5O2 crystal and corresponding ADF signal and STEM-EELS elemental maps indicating the Cu–Cr/V stacking giving as inset; (b) – HAADF-STEM image acquired along the same zone axis for a CuFe0.5V0.5O2 crystal; (c) ADF signal and STEM-EELS elemental maps indicating the Cu–Fe/V stacking.

As HAADF-STEM is a Z-contrast technique (with so-called mass-thickness sensitivity approximately proportional to Z2), an ordering of Cr and V might be apparent in high resolution images, even though Cr and V only differ by Z¼1. However, as is evident from the HAADF-STEM image (Fig. 12(a) and (b)), no specific

Fig. 13. Low magnification (a) and high resolution (b) bright field TEM images of a twinned structure region and the corresponding SAED pattern. Note the streaks along the c-axis in the SAED pattern due to the presence of multitwins. (c) Enlargement of the twinned region marked by a white rectangle in (b). The corresponding twin model is overlaid onto the image (big blue atoms represent Cu, red is Cr/V and small blue is oxygen). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 15. [11–20] HRTEM image of coherent 135 GB and corresponding FT pattern. Proposed structural model is given as inset. Fig. 14. High resolution [11–20] HAADF-STEM image of a twinned lamella-type CuCr0.5V0.5O2 crystal. FFT pattern taken from region A and B at opposite sides of the twin boundary (marked by white arrows) and a corresponding twin boundary structural model are given as insets.

corresponding selected area ED (SAED) pattern of a CuCr0.5V0.5O2 crystal exhibiting a dense configuration of twins of few unit cells wide along the c-axis. HRTEM image (Fig. 13(b)) clearly shows that these defects are not stacking faults, which could be expected in these layer structures, but are definitely permutation twins with a [0 0 0 1] twin plane. The contrast variations suggest varying chemical compositions from one layer to another, namely a deviation from the perfect crystal Cr/V ratio. The existence of such non-stochiometric layers would lead to local differences in c-parameter and the appearance of streaks along the c-axis in the ED pattern (inset in Fig. 13(a)). Another type of the defect present in this sample is twinning along the (0 0 0 1) plane. Such defects are quite rare for hexagonal structures and can be defined as a rotation twins along [0 0 0 1] axis. A HR-HAADF-STEM image of such a twin is shown in Fig. 14. The FFT patterns from lamellas A and B clearly demonstrate two different orientations of the lamellas with following relationship: (11–20) CuCr0.5V0.5O2//(10–10) CuCr0.5V0.5O2 and [0 0 0 1] CuCr0.5V0.5O2//[0 0 0 1] CuCr0.5V0.5O2. The twin interface is heteroepitaxial and free of secondary phases. A structural model of the twin interface is given as inset and implies strong distortions at the interface due to the structural mismatch between both orientations. Fig. 15 shows a high resolution TEM image of unusual coherent 135 GB and corresponding FT pattern. Two CuCr0.5V0.5O2 grains are rotated with respect to each other on angle close to 1351 and doing so create a coherent heteroepitaxial boundary along the (1 1 0 1) coincidence plane. The epitaxial relationships between two grains (denoted as A and B) can be determined from FT patterns and are as follows: (11–20)A//(11–20)B, [1 1 0 7]A//[0 0 0 1]B The corresponding model based upon the HRTEM image is given as inset. Obviously, such a GB entails strong local distortions of the atom bonding along the boundary, and in particular within the MO6 octahedra. Such distortion of the MO6 octahedra might lead to local preferential occupation or favor V3þ –V3 dimerization as in CdV2O4 [5]. These atomic defects may be implied in the formation of short-scale polar regions, which dimensions are expected to increase as T decreases below RT.

6. Discussion In this section, for the CuMO2 delafossite with (M/V) mixed occupancy at the M site, a new type of ferroelectricity has been evidenced. Consequently the delafossite can exhibit either spin induced or relaxor-type ferroelectricity. It must be pointed out that these two types rely, on the one hand, on ordered crystallographic and magnetic structures, as in CuCrO2, where Cr3 þ triangular lattice explains the complex antiferromagnetism, whereas, in the other hand, the relaxor behavior is the result of disordering at the M site in the CuM0.5V0.5O2 delafossites. For the latter, this disordering is confirmed by the spin glass behavior and the lack of evidence for Fe3 þ (Cr3 þ )/V3 þ ordering by TEM. 7. Concluding remarks The possibility to combine several functionalities in a same material and, moreover, to achieve a coupling between them is very challenging for the Solid State Chemists and Physicists. In that respect, the magnetoelectric coupling of type II multiferroics opens the route to different types of applications. Since the first reports of spin induced ferroelectricity in manganites, several type II multiferroics oxides have been reported. The present short review sheds light on polycrystalline spinel chromites and CuMO2 delafossites. Two main conclusions can be made: (i) oxides with collinear magnetic structures are worth studying to find magnetoelectric coupling; the recent report of a room temperature multiferroics behavior in canted AF orthoferrites [40] provides support to this research direction. (ii) the relaxor-like ferroelectric behavior in spin glass delafossites opens new perspectives to the field; by tailoring the magnetic disorder, this type of ferroelectricity can be induced. The role of the magnetic V3 þ cation and its natural tendency to dimerization might be an important factor to create local symmetry breaking. More work is now needed to study the possible coupling which might exist between electric polarization and structural defects in these spin and dipolar glasses.

Acknowledgments This work was partly funded by the Agence National de la Recherche [ANR-08-BLAN-0005-01]. S.T. gratefully acknowledges


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