Coexistence of ferromagnetism and charge ordering in Pr0.5Ca0.5Mn1−xCrxO3

PERGAMON

Solid State Communications 114 (2000) 429–433 www.elsevier.com/locate/ssc

Coexistence of ferromagnetism and charge ordering in Pr0.5Ca0.5Mn1⫺xCrxO3 R. Mahendiran, M. Hervieu, A. Maignan, C. Martin, B. Raveau* Laboratoire CRISMAT, ISMRA, Universite´ de Caen, 6 Boulevard du Mare´chal Juin, Caen Cedex-14050, France Received 11 February 2000; accepted 17 February 2000 by C.N.R. Rao

Abstract Magnetotransport and magnetization measurements of the Cr-doped manganites Pr0.5Ca0.5Mn1⫺xCrxO3 …x ˆ 0:03; 0:05† show several anomalous features: large magnetic field induced magnetic moments and negative magnetoresistance (MR) with hysteresis, strong deviation of susceptibility from Curie–Weiss law below 240 K in the paramagnetic insulating phase, large differences between zero field cooled and field cooled magnetization and hysteresis in MR and magnetization in the ferromagnetic metallic phase. On the basis of magnetic measurements we suggest that Cr 3⫹ replaces Mn 3⫹ with opposite spin. In agreement with our magnetic and transport measurements, we show that charge ordering coexists with ferromagnetism using electron microscopy. Electron microscopy work together with susceptibility data suggests that charge and orbital ordering occur at different temperatures. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: D. Magnetically ordered materials; D. Galvanomagnetic effects; D. Electronic transport; D. Scanning and transmission electron microscopy; D. Exchange and superexchange

Ferromagnetism and charge ordering are two mutually exclusive phenomena in manganites of the type R1⫺xAxMnO3 where R is a trivalent rare earth ion and A is a divalent cation. While ferromagnetism requires the delocalization of eg electron over the background of localized parallel t32g core spins along the Mn 3⫹ –O–Mn 4⫹ network by double exchange interaction [1], charge ordering is realized by the localization of eg electron and hole at the Mn 3⫹ and Mn 4⫹ sites, respectively, and ordering of these ions in a particular pattern [2]. As charges order at the temperature TCO, antiferromagnetic (AF) superexchange interaction among the localized t32g spins starts to dominate over the ferromagnetic (FM) double exchange interaction resulting in a long range AF (CE type) ordering at a temperature TN. Thus, the FM metallic Nd0.5Sr0.5MnO3 is converted into a charge ordered AF insulator below Tco ˆ TN ˆ 148 K, and collapse of the insulating state under an external magnetic field was found to have its origin in the field induced structural transition [3]. For the smaller cation size compounds like Pr0.5Ca0.5MnO3, FM metallic state ceases to exist, * Corresponding author. Tel.: ⫹33-231-95-1212; fax: ⫹ 33-23195-1600. E-mail address: [email protected] (B. Raveau).

charge and AF ordering are decoupled …Tco ˆ 230 K; TN ˆ 170 K† and more than 30 T magnetic fields are needed to destroy the charge ordered state [4,5]. Surprisingly, Cr doping as small as 2% at the Mn sites in the same compound was found to induce FM metallic state and colossal MR [6,7]. This study opens up the possibility of converting insulating antiferromagnets into conducting ferromagnets with CMR property. However, little is known about how Cr destroys charge, AF ordering and the nature of the metallic state. The present communication is indented to clarify this point. Using extensive MR and magnetization data we show that Cr 3⫹ substitution brings electronic phase segregation in both para and FM states. We show the direct evidence for the presence of the charge ordered region below TC by electron diffraction and possible decoupling of charge and orbital ordering in the insulating phase. The preliminary characterizations of Pr0.5Ca0.5Mn1⫺xCrxMnO3 samples …0 ⱕ x ⱕ 0:1† including powder neutron diffraction study on x ˆ 0:05 have been already reported [6,7]. We focus on the new results in the x ˆ 0:05 compound in this communication, but some results on x ˆ 0:03 are also included to reinforce our conclusions. Fig. 1(a) shows the temperature dependence of the resistivity r…T† of x ˆ 0:05 under various external magnetic

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Fig. 1. (a) Temperature dependence of the resistivity (r ) of x ˆ 0:05 under various magnetic fields (H). The arrow indicates the data taken during cooling. (b) FC and ZFC dc magnetization at 2 mT. Inset: Field dependence of reduced magnetization j (see the text for the details). (c) Inverse of the real (x 0 ) part of ac x measured with 0.3 mT and v ˆ 10 Hz for x ˆ 0:03 and x ˆ 0:05: TCO and TC refer to the charge ordering and FM Curie temperatures, respectively.

fields (H). As T decreases from 300 K, r…T; H ˆ 0† shows a spontaneous insulator–metal (I–M) transition around 140 K with strong hysteresis between cooling and warming and concomitant changes in magnetization (M) shown in

Fig. 1(b) during warming. The value of r decreases and the I–M transition shifts up in T with increasing value of H. All the above behaviors resemble the paramagnetic insulator–ferromagnetic metal transition in the well-studied La0.7Ca0.3MnO3-type compounds [8]. However our magnetic and MR results are in contrast with the simple interpretation of paramagnetic insulator–ferromagnetic metal transition. We find unusually large differences between zero field cooled (ZFC) and field cooled (FC) magnetization (M) as shown in the main panel of Fig. 1(b) for H ˆ 2 mT ( ˆ 20 G). The fractional change (j ) in magnetization at 5 K expressed as j ˆ …M FC ⫺ MZFC †=MZFC ; shown in the inset of Fig. 1(b), vanishes only above 0.05 T for x ˆ 0:05 and is as large as 0.3 for x ˆ 0:03 even at 5 T (not shown here). The real part of the inverse ac susceptibilities, (x 0 ) ⫺1, (see Fig. 1(c)) of both x ˆ 0:03 and 0.05 shows strong deviation from the Curie–Weiss fit (dark lines) below T ⬇ 240 K: This is an important observation and its implication will be discussed later. Magnetization (M) and magnetoresistance …MR ˆ ‰r…H† ⫺ r…0†Š=r…0†† isotherms also show features uncharacteristic of a true paramagnetic state. We show M and MR isotherms for x ˆ 0:05 in Fig. 2(a) and (c). The corresponding quantities for x ˆ 0:03 are shown in Fig. 2(b) and (d). We can see from Fig. 2(a) that M is linear with H at 225 K as expected of a paramagnet, but as the temperature lowers, M shows a very clear change from linearity at low fields to a rapid increase with intermediate H before tending to show saturation at 5 T. This trend is clearly visible in the temperature range 200–125 K which covers both the paramagnetic …T ⬎ 140 K† and the FM regions. Similar trends are found in x ˆ 0:03 (Fig. 2(b)) with pronounced hysteresis. Such large field induced magnetic moments in the paramagnetic state are indeed unexpected. As the temperature lowers further, we find qualitative changes in M(H) with a large

Fig. 2. Magnetization per formula unit (M) for (a) x ˆ 0:05; and (b) x ˆ 0:03: MR for (c) x ˆ 0:05 and (d) x ˆ 0:03:

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Fig. 3. Magnetization (M) at 5 T as a function of composition (x) at 5 K. The black dots are experimental points and the line is to guide the eye. Magnetic moment M calculated assuming Cr 3⫹ spin is opposite to Mn 3⫹ spin (thick line) or parallel Mn 3⫹ spin (thin line) which it replaces.

increase at low fields as that of a ferromagnet. However, even at low temperatures M(H) shows significant hysteresis, larger for x ˆ 0:03; persisting up to the highest field of 5 T. It should be also noted that M at 5 K is smaller than its value at 100 K in the metallic state x ˆ 0:03; suggesting the absence of long range ferromagnetism in this compound. Both compositions show unusually large MR (80–100%) in the paramagnetic phase (Fig. 2(c) and (d)) accompanying the observed field induced magnetic moments. The values of MR decrease below 150 K for x ˆ 0:05 and 125 K for x ˆ 0:03: Some of the notable features of the low temperature MR …T ˆ 5 K† are the absence of rapid drop at low fields,

Fig. 4. A schematic spin configuration in ac-plane without (top) and with (bottom) Cr 3⫹. The dashed zig-zag lines illustrate interchain FM coupling. The region enclosed by circle in the bottom figure represents the FM clusters formed by two Cr 3⫹ ions (shaded squares). Mn 3⫹-eg orbital ordering is also shown as lobes. Cr 3⫹ substitution weakens orbital ordering as shown in the bottom figure.

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but generally seen in polycrystalline manganites [8–13], and unusual large hysteresis in MR persisting up to 7 T. These results suggest that both the paramagnetic and FM phases are anomalous in these compounds. Now we try to understand the origin of the observed anomalous behaviors. The trivalent chromium is isoelectro3 †: Upon Cr 3⫹ substitution, Mn 3⫹ content nic to Mn 4⫹ …t2g decreases. Such a change can bring metallic like behavior at low temperature as seen in La0.5Ca0.5MnO3 [8,14,15]. But, Pr1⫺xCaxMnO3 compounds are charge ordered insulators over a wide x range …0:3 ⱕ x ⱕ 0:7† [4,5] and hence the decrease of Mn 3⫹ contents alone is not responsible for bringing the FM metallic like behavior in this compound in the absence of external magnetic field. We propose the following model based solely on experimental facts: as Cr 3⫹ replaces the isovalent Mn 3⫹ ions, it goes with reversed spin. This can be inferred from the saturation magnetic moment at 5 T and 5 K for x ˆ 0:05: If Cr 3⫹ replaces Mn 3⫹ without altering the direction of spin, the expected moment is …3:5 ⫺ x†mB which is equal to 3.45m B for x ˆ 0:05; a value higher than the experimental result, m…obs† ˆ 3:0mB : If Cr 3⫹ has reversed spin, then the expected magnetic moment is …3:5 ⫺ 7x†mB ˆ 3:15mB ; a value closer to the observed one. Our experimental results shown in Fig. 3 suggest such a possibility exists over a wide x range (0.06–0.3) and is further supported by the independent work on magnetic dichroism in x ˆ 0:05 [16]. We do not have a clear explanation for the spin reversal behavior of Cr 3⫹, but it might be due to the FM superexchange interaction between the empty eg orbital of Cr 3⫹ and the half filled eg orbital of Mn 3⫹ [17]. We illustrate our model through Fig. 4 which shows the spin and eg ⫺ d2z orbital ordering for x ˆ 0 (top) and two Cr 3⫹ ions (bottom). The CE-type AF ordering found in the parent compound …x ˆ 0† below 170 K [4,5] consists of FM zig-zag chains coupled antiferromagnetically in the ac-plane as shown in the top diagram of Fig. 4. These planes are stacked along baxis but with opposite spins. A metallic conduction along the FM zig-zag chain is inhibited by the strong repulsive Coulomb interaction among the charge carriers and interchain AF coupling. As Cr 3⫹ ions, denoted by the squares in the bottom of Fig. 4, replace two of the Mn 3⫹ sites, it breaks the AF coupling between the chains and forms local FM clusters as shown in the circled region. In this scenario, Cr 3⫹ is not directly involved in double exchange, but it promotes itinerancy of eg electron between the surrounding Mn ions by polarizing them through positive exchange interaction [17]. An indirect evidence for the non-participation of Cr 3⫹ in the double exchange mechanism is TC ( ⬇ 140– 150 K) remains unaffected over a wide x range [6,7]. It should be mentioned that we show ‘two’ Cr 3⫹ in the acplane only for illustration and this number neither corresponds to the actual doping level x nor all the substituted Cr 3⫹ are confined to a single two-dimensional ac-plane. However, such a simple schematic diagram serves our purpose: Upon Cr 3⫹ substitution, the long range charge ordered AF phase of the parent compound …x ˆ 0† breaks

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Fig. 5. Temperature evolution of the modulation (qa ⴱ) vector. The sizes of the dots indicate intensity of q. Too and TCO are temperature corresponding to orbital and charge ordering respectively (see the text). TCO was determined from (x 0 ) ⫺1 in Fig. 1(c).

into charge ordered domains within which spins are ordered antiferromagnetically …x ⱕ 0:03† or fluctuating …0:04 ⬍ x ⬍ 0:15† and FM metallic domains. The sizes of the CO/AF domains decrease with increasing x and we verified the true paramagnetic state in x ˆ 0:15 by means of the Arrot plot (not shown here). Ordering of charges around T ˆ 240 K in the paramagnetic phase of the parent compound …x ˆ 0† leads to a prominant maximum x (minimum in x ⫺1) [4,5] and deviation from the Curie–Weiss law. We see a clear minimum in x ⫺1 around 240 K in x ˆ 0:03 (Fig. 1(c)) but it is only smeared out in x ˆ 0:05: These observations lead us to conclude that charge ordering still exists in the paramagnetic phase. As T lowers below 175 K, FM clusters increase in size and eventually percolate around 140 K giving rise to metallic behavior. The threshold value for metallic conduction in these compounds, xc ˆ 0:02 [6,7], is much lower than xc ⬇ 0:15 predicted for manganites [18]. This is because I–M transition in our compound is determined by the volume fraction and percolation of FM clusters. As a consequence of the percolative nature of the I–M transition, pockets of charge ordered and AF regions remain down to the lowest temperatures. The main contribution to MR at low temperatures …T ⬍ 50 K† comes from the majority FM phase in accordance with the described mechanisms [8–13]. Inhomogenities in the form of AF or charge ordered regions at the grain boundary or interface between FM domains can lead to the hysteresis behavior [13]. As the temperature increases FM phase fraction decreases and in the temperature range 150–200 K the majority phase is charge ordered with isolated, noninteracting FM clusters embedded in it. Magnetization of the charge ordered AF matrix …x ⱕ 0:03† increases linearly with increasing H for low fields but as H just crosses the threshold field for meta-magnetism, M shows a rapid increase. This is responsible for the observed field induced magnetic moments in the paramagnetic state and as a consequence charge ordering is destroyed and a large decrease in resistivity occurs. Neutron diffraction study on x ˆ 0:05 [7] did not suggest a long range AF ordering in the insulating phase

…T ⬎ 150 K†; but short range ordering or AF spin fluctuations cannot be ruled out. The direct evidence for the existence of charge ordered domains within the FM region …T ⬍ 140 K† is given by electron diffraction (ED). Ordering of charges appears as superlattice reflections in ED pattern at positions q from the main Bragg peaks [19]. We have observed regions with and without superlattice reflections in x ˆ 0:05: In Fig. 5 we show the temperature evolution of the charge modulation vector qa ⴱ. The value of qaⴱ ˆ 0:42 at 92 K is substantially lower than the parent compound …qaⴱ ⬇ 0:5† [19]. As T increases, we find subtle variations in the intensity whose origin will not be addressed in this short communication and also in values: qa ⴱ decreases down to 0.385 at 150 K, only diffusive streaks are observed between 150–165 K as shown by the shaded region of the figure and finally no satellites of measurable intensity are detected above 170 K. This last observation appears to be in contradiction to (x 0 ) ⫺1 which showed the signature of charge ordering in the majority phase around 240 K and above in discussions on M and MR. These discrepancies can be understood if we consider the fact that ED is more sensitive to the local lattice distortions induced by orbital ordering accompanying charge ordering rather than to charge ordering itself [20]. Although charge ordering is believed to be primarily caused by strong near neighbor Coulomb interaction among the charge carriers [21], a long range charge ordering at half doped manganites is always accompanied by the co-operative JahnTeller orbital ordering [2,4,5]. However, short range charge ordering occurring over a few nanometer size will be too weak to be detected by structural changes but can be detected by tunneling spectroscopy [22]. We suggest that the charge ordering still occurs in x ˆ 0:05 around 240 K albeit in smaller discontinuous regions compared to x ⬍ 0:05; but Mn 3⫹-eg orbitals in these regions which are fluctuating above 155 K order below this temperature creating subtle variations in the lattice parameters and Mn–O bond lengths [7]. Arulraj et al. [22] observed similar structural variations in their ‘incipient’ charge ordered compound (NdLa)0.5Ca0.5MnO3 close to TC and attributed it to ordering of ‘local Jahn-Teller’ distortions. Thus, electron microscopy study combined with the susceptibility data suggest that charge ordering …TCO ˆ 240 K† and orbital ordering …TOO ˆ 150 K† are decoupled in our compound. In conclusion, our experimental results suggest that charge ordering exists in both the high temperature insulating and the low temperature FM metallic phase of Pr0.5Ca0.5Mn1⫺xCrxO3 and substituted Cr 3⫹ has a spin opposite to that of Mn 3⫹ which it replaces. Our microscopy work combined with magnetization study suggests that orbital ordering and charge ordering are decoupled. After the completion of our work, we have been made aware of some recent results [23–25] related to our work, but at low doping insulating phases.

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