J O U R N A L O F C H E M I S T R Y Materials Ferromagnetism and magnetoresistance in monolayered manganites Ca2-xLnxMnO4 A. Maignan,a C. Martin,a G. Van Tendeloo,b M. Hervieua and B. Raveaua aLaboratoire CRISMAT, UMR 6508 associe�e au CNRS, ISMRA et Universite� de Caen, 6 Boulevard du Mare�chal Juin, 14050 Caen Cedex, France bEMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerpen, Belgium Received 10th July 1998, Accepted 26th August 1998 Ferromagnetism (Tc) and negative magnetoresistive properties (maximum at 30 K) are observed in the monolayered manganites Ca2-xLnxMnO4 (Ln=Pr, Sm, Gd, Ho and 0<x0.20) despite their pure bidimensional character.A detailed structural study was carried out using X-ray diVraction, electron diVraction, and high resolution electron microscopy.This shows that they exhibit an orthorhombic cell, with a#b#apÓ2, c#12 A° and Aba2 or Abma as possible space groups. In all these oxides, (001)-type twinning is observed, on a unit cell scale, creating in this way a local periodicity of 24 A° . In the x=0.08 doped samples, it is observed that the microstructural state is strongly dependant on the synthesis process but it does not aVect the magnetotransport properties.Introduction Experimental The manganites Ca2-xLnxMnO4 were investigated for Ln= Numerous investigations performed recently on the Pr, Sm, Gd, Ho and for 0x0.20, such that the electron manganites Ln1-xAxMnO3 with the perovskite structure have concentration is not too high. The compounds were prepared shown their great ability to develop ferromagnetic metallic from stoichiometric mixtures of CaO, Ln2O3 or Pr6O11 and properties, allowing colossal magnetoresistance (CMR) to be MnO2 first heated at 1000 °C for 12 h, pressed into bars, generated.Such properties originate from double exchange sintered at 1200 °C, then at 1500 °C for 12 h and finally cooled (DE) interactions between Mn3+ and Mn4+ species.1–5 down to room temperature at a rate of 1 °Cmin-1.A second An important challenge is to modify the magnetoresistive process was used for one of the compounds, Ca1.92Pr0.08MnO4, properties of the manganites by introducing rock salt type in order to check the influence of the thermal treatment on layers between the octahedral layers, leading to layered mangathe microstructural state and the magnetic properties.It partly nites (Ln,A)n+1MnnO3n+1. In this respect, the study of the diVers from the above one only by the cooling rate: from 1500 n=1 member of this series, which exhibits the well known to 800 °C at 5 °Cmin-1 and then quench to room temperature. K2NiF4 structure is of great interest since it exhibits isolated The purity and homogeneity of the sample were checked by [MnO2]2 layers.Consequently, its bidimensionality should X-ray diVraction (XRD) and electron diVraction (ED), reduce the 3d-bandwidth with respect to the 3D manganites, coupled with energy dispersive spectroscopy (EDS). The XRD so that the hole mobility is decreased. From these considerpattern was collected by means of a Philips diVractometer ations, a weakening of the DE interactions leading to the using Cu-Ka radiation, in the angular range 102h/°110, disappearance of CMR properties can be expected.The study by steps of 0.02°. of La1-xSr1+xMnO4 performed by Rao et al.6,7 is of great The electron diVraction study was carried out with JEOL interest. It shows that this compound is insulating and does 200CX and 2010 electron microscopes, working at 200 kV.not exhibit any ferromagnetic ordering but shows a spin glass The high resolution electron microscopy study was carried out transition around 20 K. Recently Moritomo et al.8,9 confirmed with a TOPCON electron microscope, having a point reso- the existence of a spin glass phase for this compound and lution of 1.8 A°. The samples were prepared by crushing the showed the absence of magnetoresistance properties, in crystals in alcohol and the small flakes were deposited on a agreement with its bidimensional character.holey carbon film, deposited on a Cu grid. The three Although the above results strongly support the absence of microscopes are equipped with EDS analysers. CMR eVect in K2NiF4 type structure, owing to its bidimen- Magnetization was measured with a vibrating sample sionality, the possibility of inducing ferromagnetism in such magnetometer.The samples were first zero field cooled down oxides, by decreasing the thickness of the rock salt layer is to 5 K and then a magnetic field of 1.45 T was applied. The worthwhile investigating. For this reason we have explored data were collected upon warming up to 300 K.AC susceptibil- the manganites Ca2-xLnxMnO4, synthesised for the first time ity was registered with an AC-DC Quantum Design SQUID by Daoudi and Le Flem;10 the small size of Ca2+ and Ln3+ magnetometer. The resistance measurements were performed cations (Ln=Pr, Nd, Sm, Gd) should indeed allow DE by a four-probe technique. Four contacts of indium were interactions to be enhanced.This investigation was also motiv- deposited on bars by using ultrasonic waves. The data were ated by the fact that the electron doped tridimensional perov- registered during cooling from 300 to 5 K in 0 and 7 T. skites Ca1-xLnxMnO3 exhibit CMR properties.11–13 In the present paper we show the existence of ferromagnetism and of negative magnetoresistive properties in the monolayered Results and discussion manganites Ca2-xLnxMnO4 in spite of their bidimensional Evidence for ferromagnetism and magnetoresistance eVect character. A detailed structural study shows an orthorhombic symmetry and indicates that the existence of extended defects The evolution of the resistivity of the manganites Ca2-xLnxMnO4 vs.temperature in absence of magnetic field does not aVect their magnetotransport properties.J. Mater. Chem., 1998, 8(11), 2411–2416 2411Fig. 1 T-dependent resistivity (r) ofCa2-xPrxMnO4 samples registered in earth magnetic field; x values are labelled on the graph. is very similar whatever the nature of the lanthanide Ln=Pr, Sm, Gd or Ho. They all exhibit a semiconducting behaviour as illustrated for the praseodymium phases Ca2-xPrxMnO4 (Fig. 1). In the 200–250 K temperature domain, no inflection, which would correspond to a signature of charge ordering is observed on the r(T) curves, in contrast to La0.5Sr1.5MnO4.14 The most important point is related to the shape of the r(T ) curves for low electron concentrations (x=0.05 to 0.08), at low temperature. For these low x values a kind of plateau is observed between 50 and 20 K, followed by an upturn at the lower temperature, suggesting a re-entrant transition from an insulating to a semimetallic state.The magnetization, registered under 1.45 T, vs. temperature [Fig. 2(a)] shows that the doping of Ca2MnO4 with electrons Fig. 2 (a) T-dependent magnetization (M) for the series Ca2-xPrxMnO4 (1.45 T); (b) ac-x curves registered with hac=10 Oe; induces significant ferromagnetic interactions.Starting from frequencies are labelled on the graph. the antiferromagnetic Ca2MnO4, the magnetic moment at 4.2 K increases as x increases, reaches 0.46 mB per mol of Mn for x=0.06, remains practically constant in the range for x=0.20 [Fig. 3(d)]. The magnetoresistivity of these samples has also been confirmed from the r(H)T curves.An example 0.06x0.10, and finally decreases rapidly for x>0.10, so that the ferromagnetic interactions for x=0.20 have disap- is given for Ca1.92Pr0.08MnO4 [inset of Fig. 3(b)]. These curves exhibit the reversibility of the magnetoresistance eVect. peared. Note, that the shape of the M(T) curve for x= 0.06–0.10, is diVerent from that observed for a classical Very similar results are observed for all diVerent lanthanides of the series Ln=Pr, Sm, Gd, Ho.For each, the highest ferromagnetic transition, i.e. it is very smooth, in agreement with the fact that the magnetic moment at low temperature is ferromagnetic interactions are observed in the range x= 0.08-0.10. They all exhibit a smoothM(T) curve as illustrated far below the theoretical value (3.1 mB for x=0.10) characteristic of a perfect ferromagnetic However, in a small ac- in Fig. 4 for the oxides Ca1.92Ln0.08MnO4. However, the most interesting result deals with the fact that the magnetic moment field of 10 Oe, the ferromagnetic transition is sharp and this x¾(T) curve allows one to determine the Curie temperature at 4.2 K increases linearly as the size of the lanthanide decreases, starting from 0.46 mB for Ln=Pr going through TC=110 K [Fig. 2(b)]. Moreover, the strong frequency dependence of the data below 105 K, together with the existence of 0.56 mB and 0.64 mB for Ln=Sm and Gd, respectively, and reaching finally 0.70 mB for Ln=Ho. This size eVect strongly a peak at 105 K seems to indicate that the sample must be considered as a cluster glass rather than a ferromagnet, as supports our hypothesis that the thickness of the rock salt layer significantly influences the ferromagnetism in this one- already reported for the Ca1-xSmxMnO3 perovskite for x0.12.15 layered structure.Doping with thorium (Th4+) also indicates that the electron concentration plays a prominent role for the The above results, existence of a ferromagnetic component and evidence for a re-entrant transition on the r(T) curves, appearance of ferromagnetism.One indeed observes that the maximum magnetic moment of 0.44 mB, is obtained for suggest the possibility of finding magnetoresistive properties for 0.06x0.12. The r(T) curves (Fig. 3) registered under the oxide Ca1.96Th0.04MnO4 (Fig. 4), corresponding to the same electron concentration as the lanthanide based manga- 7 T support this viewpoint.The largest negative magnetoresistance is observed for x=0.06 and 0.08 samples [Fig. 3(a), (b)] nites Ca1.92Ln0.08MnO4, assuming the tetravalence of thorium. For Ca1.92Th0.08MnO4, the magnetic moment has already which exhibit practically identical r(T ) curves with a maximum resistivity ratio (RR=r0/r7T), RR=2.7 at 40 K i.e.a mag- decreased down to 0.05 mB. The increase of ferromagnetic interactions is not suYcient netoresistance MR=-64%, with MR=(RH-R0)/R0. For x= 0.12 [Fig. 3(c)] the negative magnetoresistance at 40 K is still to increase significantly the magnetoresistance as the size of the lanthanide decreases. This is illustrated by comparing the similar, MR=-50% at 40 K.This maximum magnetoresistive eVect for 0.06x0.12 is in agreement with the ferromagnet- r(T) curves of the manganites Ca1.92Ln0.08MnO4, registered under 0 and 7 T (with Ln=Sm and Ho in Fig. 5). One ism which is maximum for this composition range. Then, the magnetoresistance decreases rapidly as x increases, as shown observes similar shapes of the r(T) curves, characterised by a 2412 J.Mater. Chem., 1998, 8(11), 2411–2416Fig. 4 M(T ) curves for the oxides Ca1.92Ln0.08MnO4 with Ln=Pr, Sm, Gd and Ho and for Ca1.96Th0.04MnO4 (Th curve). Fig. 5 r(T ) curves for two Ca1.92Ln0.08MnO4 samples; (a) Ln=Sm and (b) Ln=Ho. Structure and defects—relations with magnetoresistance properties The main issue concerning the monolayered manganites Ca2-xLnxMnO4, is the origin of their magnetoresistance properties.Such properties may be intrinsic, or due to extended Fig. 3 r(T ) curves registered during cooling from 300 K down to 5 K defects or inhomogeneities.16 In order to answer this question in 0 and 7 T. The resistivity ratio r(T)H=0/r(T)H=7T is also shown a transmission electron microscopy (TEM) study is necessary. (right y-axis); (a) Ca1.94Pr0.06MnO4, (b) Ca1.92Pr0.08MnO4, (c) For comparison, the pure Ca2MnO4 has been studied first. Ca1.88Pr0.12MnO4 and (d) Ca1.80Pr0.20MnO4.Inset of (b): isothermal magnetoresistance [r(H) curves] registered for T=10 K, and T=50 K. Remarkably enough, we could not confirm the tetragonal structure previously described.17,18 We had to lower the symmetry to orthorhombic in order to index all diVraction patterns in a unit cell with lattice parameters a#5.2 A° , b#5.2 A° and re-entrant transition around 40 K, and an upturn at 20 K.The resistivity ratio r0/r7T at 40 K is very similar, i.e. is ca. 3 for c#12.1 A° . DiVraction evidence is presented in Fig. 6. Along the [001] zone [Fig. 6(a)], it is clear that the intensity of the Sm [Fig. 5(a)] and Ho [Fig. 5(b)]. Note however that the maximum around 40 K is more clearly observed for Sm reflections marked by two white triangles is not equal, violating the tetragonal symmetry; the very weak one belongs to a 90° [Fig. 5(a)] than for Ho [Fig. 5(b)] or for Pr [Fig. 3(b)]. J. Mater. Chem., 1998, 8(11), 2411–2416 2413along complex [hk0] zones [such as the zone in Fig. 7(a)] where additional weak reflections are clearly present in one row out of two.The pattern of Fig. 7(a) is again not compatible with a tetragonal symmetry. It can be interpreted on the basis of the superposition of the [1290] and [2190] zones, suggesting that twinning domains are systematically present in the crystallites. The 420 and 240 reflections are superposed due to the pseudotetragonal character of the cell (a#b); the weaker reflections [see the rows of dots indicated by arrows in Fig. 7(a)] are generated by the perovskite cell distortion (a#b#apÓ2). The A-type lattice implies the existence of the 120 and 122 reflections in the [2190] zone, the 121 being forbidden but the 211 is observed in the [1290] zone. In the bright (BF) and dark (DF) field images, given in Fig. 7(b) and (c), respectively, the basic (002) lattice fringes, separated by 6 A° are clearly resolved. Dark field imaging using several of these reflections produces images, with the 6 A° spaced fringes but where superimposed darker and lighter bands are visible. The width of these bands is hardly a few unit cells wide; they are the signature of the two orthorhombic variants, rotated 90° around the c-axis.Sometimes however, as in the area indicated A in Fig. 7(c), the twinning is locally periodic every 12 A° , creating in this way a local unit cell of 24 A° . The fact that this twinning, every 12 A° , is often on a unit cell scale, generating a doubling of the c parameter (24 A° ) is to compare with the similar supercell ‘apÓ2×apÓ2×2c’ observed by Leonowicz et al.17 on single crystals.Despite that the electron diVraction allows one to reject the hypothesis of the formation of such a double cell in our sample, the DF observations show a close relationship Fig. 6 Ca2MnO4: (a) [001], (b) [110] and (c) [100] ED patterns. between the two structures. In fact, by applying the above periodic twinning mechanism to the orthorhombic Aba2 cell (present work) with a#b#apÓ2, c#12 A° , we can create a P- oriented domain.The weak reflections h00 and 0k0 (h, k= type orthorhombic double cell with a#b#apÓ2, 2c#24 A° 2n+1) are present in this section due to double diVraction; and space group Pba2. If we consider the atomic positions of this is clear from the [100] section in Fig. 6(c). The [110] zone the as-built structure, it appears that simply by constraining is shown in Fig. 6(b). All these diVraction data show that the the y coordinate to the particular value y=1/2-x, we will conditions limiting the reflection are hkl5k+l=2n, 0kl5k=2n generate a tetragonal cell with the space group I41/acd, which and h0l h=2n and are compatible with space group Aba2 is identical with that previously proposed.17,18 Structure (no. 41) or Abma (no. 64). calculations were therefore carried out in the Aba2 and Further convincing evidence for the orthorhombic symmetry comes from tilting about the c* axis and dark field imaging I41/acd space groups from the powder XRD data for Fig. 7 Ca2MnO4: (a) enlarged ED pattern showing that the [1290] and [2190] zones are systematically superimposed, as the result of twinning phenomena; (b) bright and (c) dark field corresponding images. 2414 J. Mater. Chem., 1998, 8(11), 2411–2416Fig. 8 Ca1.92Pr0.08MnO4 sample: [110] ED pattern and HREM image, showing the characteristic contrast of the n=1 member of the (Ca,Ln)n+1MnnO3n+1 series. Most of the crystallites of the slowly cooled sample are defect free. Fig. 9 Dark field images of the ‘800 °C-quenched’ Ca1.92Pr0.08MnO4 sample: (a) showing that the existence of twinning phenomena is not Ca1.92Sm0.08MnO4 [a=5.2206(11), b=5.2195(11) and c= dependant on the thermal process (by selecting the weak extra 12.0055(4)A° for Aba2].Both lead to reasonable R values, reflections); (b) showing the existence of numerous defects and of the which attest to the validity of the model, but none allows an strain field associated with the pancake-like defects.accurate refinement of the positions of the oxygen atoms located in the [MnO2] plane. Considering the complexity of the twinning system [Fig. 7(c)], it appears indeed impossible to get significant results from powder XRD data. Clearly, it prepared following the same steps of the process as the other compounds, but was ‘quenched’ from 800 °C to room tempera- seems that Leonowicz et al.,17 obtained a diVerent distortion of the Ca2MnO4 structure due to the fact that they worked at ture. The EDS analyses showed that the actual composition is also very close to the nominal one.This ‘800 °C-quenched’ much lower temperature (900 °C instead of 1500 °C)and used a flux (CaCl2). Ca1.92Pr0.08MnO4 sample exhibits exactly the same magnetotransport properties as the ‘slowly cooled sample’, whose Doping Ca2MnO4 with praseodymium or with gadolinium (x=0.08) and slowly decreasing the temperature (see microstructure is very regular, if one excepts the twinning phenomena.The dark field images show that the formation Experimental section), produces the same remarkable microstructure. The overall bright field images show an even contrast of (001)-type twins is not dependant on the cooling rate [Fig. 9(a)]. However their microstructure shows a diVerent with a very few extended defects, and the [110] HREM images exhibit the characteristic centring of the n=1 members of the behaviour [Fig. 9(b)], involving the formation of ‘pancakelike’ defects along the [110] direction. The term ‘pancake-like’ (Ln,A)n+1MnnO3n+1 series, as the undoped Ca2MnO4 (Fig. 8). Only some occasional intergrowth defects corresponding to refers to the dimensions of the defect, which is large along two directions (in the present case, a few nanometers) and n=3 or n=4 members are detected. The dark field image, recorded according to the same conditions confirms the pres- small along the perpendicular direction. Along [110], it is clear that the internal structure of the defective area is perovskite ence of numerous (001)-type twins, similar to the undoped material [Fig. 7(c)]. Apparently the orthorhombic symmetry like. The width of the defects is variable but never larger than two or three perovskite units [white triangle in Fig. 10(b)], is maintained. EDS analyses over relatively large areas (ca. 100 nm) confirm, in the limit of accuracy of the technique, corresponding to the local formation of n=2 and n=3 members of the (Ln,A)n+1MnnO3n+1 series. In a number of cases that the cation ratio (Ca/Ln#24 and Mn/Ca#0.52) is in agreement with the nominal Ca1.92Ln0.08MnO4 composition. there is even no change in the periodicity of the basic Ca2MnO4 stacking, only a diVerence in intensity [see arrows in Fig. 10(a)]. The absence of 3D perovskite and the very small number of intergrowth defects clearly show that the ferromagnetism The dark field images, taken with a smaller objective aperture [Fig. 9(b)] clearly indicate however that there is a strain field and especially the magnetoresistance are intrinsic properties of the monolayered manganites Ca2-xLnxMnO4.associated with most of the precipitates. These results clearly show that the presence of pancake-like The possible influence of the presence of extended defects such as intergrowth members upon the properties of such defects, correlated to the local formation of n=2 members of the series, and the associated strain fields do not aVect the samples has also been studied. For this study, we considered a second type of preparation.A Ca1.92Pr0.08MnO4 sample was magnetotransport properties of these oxides. J. Mater. Chem., 1998, 8(11), 2411–2416 2415The authors gratefully acknowledge the University of Caen, for supporting this work through a position of Associate Professor. References 1 C. Zener, Phys. Rev., 1951, 82, 403. 2 P. W. Anderson and H.Hasegawa, Phys. Rev., 1955, 100, 675. 3 G. H. Jonker and J. H. Santen, Physica, 1950, 16, 337. 4 P. G. de Gennes, Phys. Rev., 1960, 118, 141. 5 J. B. Goodenough, Magnetism and the chemical bond, John Wiley and Sons, New York–London, 1963. 6 C. N. R. Rao, P. Ganguly, K. K. Singh and R. A. M. 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