Coexistence of ferromagnetism and antiferromagnetism in the L0.08Ca1.92MnO4 series

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Journal of Magnetism and Magnetic Materials 284 (2004) 172–180 www.elsevier.com/locate/jmmm

Coexistence of ferromagnetism and antiferromagnetism in the L0.08Ca1.92MnO4 series C. Autreta, C. Martina,, R. Retouxa, A. Maignana, B. Raveaua, G. Andre´b, F. Boure´eb, Z. Jirakc a

Laboratoire CRISMAT, ENSICaen et Universite´ de Caen, CNRS UMR 6508, 6, boulevard du Mare´chal Juin, 14050 Caen Cedex, France b LLB CEA Saclay 91191, Gif Sur Yvette Cedex, France c Institute of Physics ACSR, Cukrovarnicka 10, 162 53 Prague 6, Czech Republic Received 28 April 2004 Available online 22 July 2004

Abstract The present study is devoted to the L0.08Ca1.92MnO4 series (with L=La, Pr, Ho and Y) which exhibits, at low temperature, the highest ferromagnetic components in the magnetization curves, among the electron-doped manganites belonging to the first members of the Ruddlesden–Popper family. We show that, in the whole series, coexistence of ferromagnetism and antiferromagnetism is induced, independent of the size of the L cation and that the holmium-based compound has a particular behavior with the presence of 2D antiferromagnetism instead of 3D for the other cations. r 2004 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 61.14.Rq; 71.30.+h Keywords: Manganites; Ruddlesden popper phases; Phase separation; Neutron diffraction

1. Introduction The perovskite manganites Ln1
xAExMnO3 (Ln is a trivalent lanthanide and AE a divalent alkaline earth) have attracted a considerable attention, in recent years, due to the discovery of colossal magnetoresistance (CMR) [1,2]. This Corresponding author. Tel.:+33-2-31-45-26-37; fax: +33-

2-31-95-16-00. E-mail address: [email protected] (C. Martin).

effect is associated with the interplay between the charges, spins and orbitals, which results in very rich phase diagrams [3,4]. In the case of Ca-based compounds (AE=Ca), the right part of the phase diagram (i.e. x40:5) is particularly interesting because electron-doped manganites exhibit unusual transport and magnetic properties [5,6]. To study the effect of the dimensionality on the structural and magnetic properties, Ruddlesden– Popper phases [7], obtained by introducing a rocksalt layer between simple or multiple slabs of

0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.06.035

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perovskite, are thus good candidates, and particularly the Mn4+-rich compounds [8]. In fact, recent investigations of the CaO(Pr0.08Ca0.92MnO3)n series, with n ¼ 1, 1, 2 and 3, have evidenced ferromagnetic interactions in the whole system but with an anomalous behavior for n ¼ 1, which has been attributed to the low dimensionality of this compound [9]. The Ln2
xCaxMnO4 oxides, first synthesized by A. Daoudi et al. [10], are in fact of prime interest. Among these compounds, a particular attention has been paid for the Ca2MnO4 parent compound. At room temperature, Ca2MnO4 was first reported to be body-centered tetragonal structure ( (I4/mmm) with cell parameters a  a  3:67 A p

(ap being the parameter of the perovskite unit cell) ( [7]. In 1985, Leonowicz et al. have and c  12:08 A evidenced, on a Ca2MnO4 single crystal, a disorder on the equatorial oxygen in the perovskite block layer, leading to the group I41/acd with lattice p ( [11]. Later parameters a  b  ap 2 and c  24 A on, Takahashi et al. [12] and Fawcett et al. [13] have confirmed this space group by X-ray powder diffraction. The doubling of the c parameter ( (compared to I4/mmm with a  ap and c  12 A) is due to a rotation of octahedra in opposite directions along the c-axis. Other recent structural and magnetic studies have shown a complex state for a polycrystalline sample of Ca2MnO4 with the coexistence of two crystalline phases I41/acd and Aba2 (observed by electron diffraction) and with two antiferromagnetic structures below E115 K (characterized by neutron diffraction) [14]. Both observed space groups correspond to the same cationic composition and differ mainly by the positions of the oxygens in the basal plane of the octahedra. The magnetic state is also more complex than the one previously described by Cox et al. [15] and Tezuka et al. [16]. The substitution of L for Ca creates Mn3+ species in the Mn4+ matrix and this modifies the behavior of Ca2MnO4. By this way, a new phase diagram can be established. By progressive introduction of Mn3+ in the Mn4+ matrix, first, ferromagnetism associated with CMR is induced and then charge ordering is observed, even at room temperature [8,17,18]. The ferromagnetism observed for the electron-doped region (near

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Ca2MnO4) is not conventional, in agreement with the bidimensional character of these compounds [8]. The shape of the magnetization curves is very smooth and the maximal moment value, registered at low temperature in E1 T, is far below the theoretical one. Moreover, the frequency dependence of the susceptibility suggests that these samples have to be considered as cluster glass, similarly to the analogue perovskite compounds [5,6]. Based on our previous paper [8], we have selected the L0.08Ca1.92MnO4 series, because for this Mn valency the low-temperature magnetization is maximum. We report herein the results of the structural study (by electron microcopy and neutron diffraction) in connection with the transport and magnetic properties, for four compounds (L=La, Pr, Ho and Y).

2. Experimental section The manganites L0.08Ca1.92MnO4 (La, Pr, Ho, Y) were prepared by solid state reaction of La2O3 or Pr6O11 or Ho2O3 or Y2O3, CaO and MnO2, weighted in stoichiometric proportions, as previously reported for Ca2MnO4 [14]. After the first step at 900 1C, powders were pressed in the form of bars under 1 ton/cm2 and finally sintered in air at 1350 1C for 48 h. The X-ray powder diffraction data were registered at room temperature with a Philips diffractometer using Cu Ka radiation. Neutron powder diffraction experiments were carried out at the Leon Brillouin Laboratory (CEA/Saclay). The room temperature neutron powder diffraction (NPD) patterns were registered on the high-resolution 3T2 instrument in the range ( 61o2yo125.61 by 0.051 step (l ¼ 1:2251 A). The temperature-dependent diffraction patterns were recorded on the G4.1 diffractometer ( Samples were first cooled at 1.5 K (l ¼ 2:4266 A). and patterns were registered during warming from 1.5 K to room temperature. Crystalline and magnetic structures were refined with the Rietveld method using the Fullprof program [19,20]. In order to check the homogeneity of the samples, numerous crystallites were characterized

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technique on sintered bars with 2  2  10 mm3 dimensions. Resistivity data were measured with decreasing temperature from 400 to 5 K in magnetic fields of 0 and 7 T.

using electron diffraction (ED). The samples were prepared by crushing the bars in n-butanol and the small crystallites in suspension were deposited on a holey carbon film, supported by a nickel grid. The ED was performed on JEOL 200CX and JEOL 2010 electron microscopes (tilt7601) equipped with EDS analyzers (energy dispersive spectrometer) and tilting rotating sample holders. EDS analyses of the different samples show that the cationic compositions of the crystallites are identical to the nominal ones in the limit of the errors. Magnetization M(T) curves were collected, using a superconducting quantum interference device (SQUID), upon warming (from 5 K to 400 K) after a magnetic field of 1.45 T was applied. Transport and AC magnetic susceptibility [w0 (T) and w00 (T)] measurements were performed with a quantum design physical property measurements system (PPMS). Resistivity and magnetoresistance measurements were performed with the four-probe

3. Results and discussion 3.1. Evidence for ferromagnetism and CMR The temperature-dependent magnetizations curves (1.45 T, zero-field cooling) (Fig. 1) show that the four samples exhibit weak ferromagnetism (FM), below a temperature close to 120 K. The transition temperature is approximately the same whatever be the compound, irrespective of the L nature. These M(T) curves are not typical of a ‘‘classical’’ ferromagnet : the transition is broad and the moments are far below the theoretical one which should reach about 3.08 mB per formula. The

0.7 35000

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Ho

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T (K) Fig. 1. The temperature dependence of magnetization under 1.45 T (zfc) for the oxides L0.08Ca1.92MnO4 with L=La, Pr, Ho and Y. Inset: w0 (T) [left y-axis] and 1/w0 (T) [right y-axis] curves of Y0.08Ca1.92MnO4, hAC=10 Oe, the frequency values of the excitation field are labelled on the graph.

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moment measured at low temperature in 1.45 T increases as the size of the lanthanide decreases [21], starting from 0.2 mB for L=La to 0.6 mB for Ho. This could be explained by considering that the size of the L cations has an effect on the thickness of the rock-salt layer and consequently small L favor the ferromagnetism. An exception from this trend is the Y0.08Ca1.92MnO4 compound which exhibits a moment smaller than the one observed for the Ho compound. An hypothesis to explain this phenomenon could be the fact that Y is non-magnetic. As it will be shown later, a higher FM component for Ho than Y is also shown by NPD. The weak FM is also probed by ACmagnetic susceptibility measurements, as shown in the inset of Fig. 1 for Y0.08Ca1.92MnO4. Within the low value of the excitation AC magnetic field (hAC ¼ 10 Oe) the transition towards a magnetized state is very abrupt starting below about 115 K. After a first w0 maximum at 110 K a second broad maximum is found at 20 K such as w0 (20 K)4 w0 (110 K). For all temperatures below the first w0 maximum at 110 K, the w0 values become frequency-dependent with w0 values which decrease as f increases. Such w0 (T) curves characterized by two w0 maxima, sharp and broad, at high-and lowtemperatures respectively, together with frequency-dependent values, are similar to those of the cluster glass phases reported for the electron-

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doped perovskite manganites, such as Sm0.1Ca0.9MnO3 [5,6]. This type of w0 (T) curves suggests that ferromagnetic clusters develop below temperature of the sharp w0 rise but that the lack of long-range magnetic ordering is responsible for the dynamic effects of the resulting frozen domains embedded in the AFM matrix. But in contrast to the electron-doped perovskites, the magnetoresistive effect is weak for the studied Ruddlesden– Popper phases. This is illustrated in Fig. 2 which shows the r(T) curves registered under 0 and 7 T, from 400 to 5 K, for the four samples. The maximal r0/r7 T resistivity ratio reaches only 2.5 corresponding to negative magnetoresistance. Nevertheless, this effect exists for a large range

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ρ (Ω.cm)

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Pr 0

100

200

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T (K) Fig. 2. Resistivity curves under 0 (solid lines) and 7 T (dotted lines) from 400 K down to 5 K for the L0.08Ca1.92MnO4 series with L=La, Pr, Ho and Y.

Fig. 3. Calculated (with characteristics of G41 diffractometer) neutron powder diffraction patterns, in a selected 2y range chosen in order to show the difference between the I41/acd (upper) and I4/mmm (lower part) space groups.

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Fig. 4. G41 data of Ho0.08Ca1.92MnO4 : comparison between the 1.5 and 300 K patterns (a), selected patterns showing the progressive character of the magnetic transitions (b) and the whole thermodiffractograms (c).

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of temperature. The appearance of MR for To110 K is in agreement with the existence of FM interactions below this temperature as shown in the M(T) curves (Fig. 1). 3.2. Structural and magnetic characterizations The room temperature structures were refined by using the high-resolution 3T2 diffractometer data. These NPD patterns arepindexed in the ( I41/acd space group with a  ap 2 and c  24 A as previously reported [11–14,16]. A close inspection of these patterns shows a broadening of the peaks, which differentiate the I41/acd from I4/mmm space groups. To clearly see which are the peaks involved by the difference between both space groups, simulated patterns are given in Fig. 3. The broadening is particularly important in the case of the Ho sample, as illustrated in Fig. 4a. This phenomenon was previously observed and explained for Ca2MnO4 [14] and is taken into account for the calculations [20]. It is linked to the coexistence of two close crystallographic phases, with the same cationic composition, and will be discussed in the next section (electron microscopy). The lattice parameters and selected Mn–O distances resulting from the refinements are given in Table 1. The lattice volume is larger for the substituted phases than for Ca2MnO4, in agreement with the introduction of Mn3+ in the Mn4+ matrix. The substitution on the Ca site induces an increase of the a parameter and a decrease of the c one. This c-axis compression indicates a t32g dz2 d1x2
y2 configuration for the Mn3+ cations. In the same way, the substitution tends to decrease the elongation of the MnO6 octahedra

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and the rotation of the octahedra in the basal plane remains strong. These results show a clear evolution, with respect to Ca2MnO4, due to the Mn3+ for Mn4+ substitution but they do not evidence a significant L-size effect. Two main characteristic features are observed by electron diffraction (Figs. 5 and 6). Figs. 5(a and b) show the experimental and calculated (by using the I41/acd space group and atomic positions refined from neutron diffraction data) ED patterns, respectively, of the La-substituted compound recorded along the ½2 1 0 zone axis. Fig. 5a, representative of the observations of the La-, Prand Y-substituted compounds, shows that these compounds present a quite low rate of defects. This diffraction pattern is characteristic of quite large regular domains with no twinning phenomena. The superposition of diffuse streaks along c* is very smooth and the spots can be indexed in the ( Here, tetragonal I41/acd symmetry with c  24 A. there are only a few defects compared to the Hosubstituted compound, as shown in Fig. 6. Diffuse streaks parallel to c* are clearly evidenced (arrows

Fig. 5. (a) Experimental ½2 1 0 ED pattern for La0.08Ca1.92MnO4 and (b) simulated ½2 1 0 ED pattern using the structure parameters determined by neutron diffraction (I41/acd space group).

Table 1 Cell parameters and calculated Mn–O distances and Mn–O–Mn angle at 300 K from NPD data (3T2 diffractometer) Ln

La

Pr

Ho

Y

Ca2MnO4 [14]

a (A˚) c (A˚) V (A˚3) Mn–Oap (A˚) (  2) Mn–Oeq (A˚) (  4) Mn–Oeq–Mn (1)

5.2172(2) 24.0580(1) 654.85 1.929(3) 1.866(1) 162.8(1)

5.2169(1) 24.0487(6) 654.51 1.935(2) 1.864(1) 163.36(6)

5.2162(1) 23.9890(3) 652.71 1.941(1) 1.864(1) 163.18(5)

5.2089(2) 24.0281(9) 651.94 1.927(2) 1.862(1) 162.84(9)

5.18684(4) 24.1228(2) 648.98 1.9447(6) 1.8571(4) 161.84(2)

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Fig. 6. (a) Experimental ½2 1 0 ED pattern for Ho0.08Ca1.92MnO4 showing diffuse streaks and spots, along the stacking direction and (b) corresponding dark field image where ‘‘pancakes’’ and twinning phenomena are clearly evidenced. A and B domains indicated by white arrows show the different periodicity observed in the grains.

on the diffraction pattern in Fig. 6a) and are related to the existence of nanotwinning phenomena described and explained in the Ca2MnO4 study [14]. Two types of diffraction patterns are superimposed, the first one with spots relative to the tetragonal I41/acd-type structure and the second one presenting a lot of diffuse streaks representative of the orthorhombic distortion of the structure. This ED pattern corresponds to the dark field image, given in Fig. 6b, where ‘‘pancakes’’ and twinning phenomena are clearly evidenced. A and B domains indicated by white arrows show the different periodicity observed in the grains. This kind of pictures was previously described in Ref. [14], where it was shown that the microstructure of these compounds depends on the synthesis conditions. Unfortunately, for the Ho-based sample, even by changing the synthesis conditions, no free defect sample was obtained.

The temperature dependence study was performed using the G4.1 diffractometer, which has a high flux and a good resolution at low angles to solve the magnetic structures. The temperature dependence is similar for all the samples : the NPD patterns do not show any structural change on cooling these samples down to 1.5 K as shown for the Ho sample (Fig. 4c). As the temperature decreases, the volume decreases like the a parameter, whereas the c parameter increases. The temperature dependence of the lattice constants is exemplified in Fig. 7 for the Pr compound. At TN (around 110 K) a kink is observed in the a parameter (and consequently on the V) vs T curves but no structural effect allows to determine TC. This study shows definitely the coexistence of FM and AFM for all the compounds, below TC, TN always being higher than TC. TN is close to 110 K whatever be the sample but its determination as well the one of TC are

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Vol (Å )

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Fig. 7. Temperature dependence of the cell parameters, with the a and c corresponding to the left and right y-axis, respectively, and of the volume (in inset) for Pr0.08Ca1.92MnO4.

difficult, because the transitions are broad (Figs. 4(b and c)). At low temperature, the parent compound, i.e. Ca2MnO4, is AFM and its structure was refined with two magnetic structures, corresponding to the (0 0 0) propagation vector and the K2NiF4 model [14]. Both models are schematized in Fig. 8, the Mn magnetic moments are along the stacking c-axis. All the title compounds exhibit the same kind of complex AFM structures at low temperature. Depending on the samples, the AFM peaks in the NPD patterns are more or less well defined, the ratio between the two AFM types varies. Moreover, particularly in the La case, a third AFM phase should be taken into account (with the appearance of a new peak of small intensity around 191 in 2y). In the same way, the AFM peaks (with Ca2MnO4 type) exhibit different shapes depending on the composition: clearly 2D for the Ho sample (Fig. 4b), with a typical Warren profile [22] and 3D for the other compounds (even if a broadening of the peaks has to be included in the calculations). But the more important point is that, in contrast to Ca2MnO4, the substituted samples exhibit, in addition to AFM, a FM component, detected with the peak around 231 in Fig. 4. As for AFM, the shape of this peak is different from one compound to the other: it is clearly characteristic of 3D FM for Ho but is broader for La, Pr and Y indicating short-range FM interactions. For the Ho sample,

k1 = (000)

k 2NiF4

Fig. 8. Drawing of the two AFM structures observed for Ca2MnO4 [14].

in the FM phase, the Mn magnetic moment is refined to E0.8 mB in the basal plane. Considering the 3D FM and 2D AFM characters of the Ho compound, it seems that the magnetic super-exchange interactions are very strong within the (a, b) planes and weak between these planes along the c-axis, whereas the ferromagnetic interactions due to the double exchange seem to be isotropic. This may be linked to the disorder evidenced along the stacking axis by electron microscopy in agreement with the broadening of few crystallographic peaks on the NPD patterns.

4. Conclusion This study clearly shows that the L for Ca substitution in Ca2MnO4, at low level, induces a coexistence of ferromagnetism with different types of antiferromagnetic structures. The so-obtained

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strong competition between FM (Mn moment in the (a, b) plane) and AFM (Mn moment along the c-axis) appears whatever be the L cation, showing that the introduction of a small amount of Mn3+ in the Mn4+ matrix is the key parameter. It must also be emphasized that the L size does not lead to important changes on the structural and magnetic properties, in contrast to the Ln0.5Sr1.5MnO3 system for which charge ordering is observed for large Ln size and not for smaller cations [23]. Finally, the Ho-substituted compound exhibits a particular behavior with the evidence of 2D AFM with 3D FM. The high level of disorder observed by electron microscopy for this compound thus seems to favor FM at the expense of AFM.

Acknowledgements The authors are grateful to Pr M. Hervieu for many fruitful discussions. C. Autret acknowledges the European Union (Scootmo contract HPRN_CT_2002_00293) for financial support. References [1] C.N.R. Rao, B. Raveau, Colossal and Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific, Singapore, 1998. [2] Y. Tokura (Ed.), Colossal Magnetoresistive Oxides, Gordon and Breach Science Publishers, New York, 1999. [3] P. Schiffer, A.P. Ramirez, W. Bao, S.-W. Cheong, Phys. Rev. Lett. 75 (1995) 3336.

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