- Email: [email protected]
Direct evidence for charge ordering and electronic phase separation in BixSr1−xMnO3 at room temperature
ARTICLE IN PRESS
Physica B 370 (2005) 172–177 www.elsevier.com/locate/physb
Direct evidence for charge ordering and electronic phase separation in BixSr1xMnO3 at room temperature A. Guptaa,, S.B. Samantaa, V.P.S. Awanaa, H. Kishana, A.M. Awasthib, S. Bhardwajb, A.V. Narlikarb, C. Fronterac, J.L. Garcia-Munozc a
Superconductivity and Cryogenics Division, National Physical Laboratory, K.S. Krishnan Marg, New Delhi 110-012, India b UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, MP, India c Institut de Ciencia de Materials de Barcelona, CSIC, Campus Universitari de Bellaterra, E-08193 Bellaterra, Spain Received 2 February 2005; received in revised form 22 August 2005; accepted 13 September 2005
Abstract We present a study of charge ordering and electronic phase separation (EPS) phenomenon in BixSr1xMnO3, for an exhaustive range of x (0.25pxp0.75), by STM/STS at room temperature (RT) and specific heat measurements at high temperatures (350–650 K). Atomically resolved STM images of the samples, in real space, show the presence of stripelike charge-ordered (CO) phase coexisting with charge-disordered (CD) phase. The STM images further reveal that the fraction of CO phase increases with an increase in x. The conductance spectra of these phases measured at nano level by STS are discussed. The transition to CO phase above RT is corroborated by specific heat measurements in all samples, giving a TCO(x) phase diagram for this system. r 2005 Elsevier B.V. All rights reserved. PACS: 75.47.Lx; 71.30.Ph Keywords: Bi-based manganites; Charge-ordering; Electronic phase separation
1. Introduction Discovery of huge negative magnetoresistance (MR) in rare earth-based manganites Ln1xAx MnO3 (Ln—rare earths, A—alkaline metals Ca, Sr, Ba, etc.) has attracted the attention of Corresponding author. Tel./fax: +91 11 25748709.
E-mail address: [email protected] (A. Gupta).
condensed matter physicists at large [1,2]. The phenomenon mainly centers on the Mn ion valance state through A cation substitutions and the ionic radii of the A cation in these compounds. It is now well-known that, for certain concentration ratios of trivalent (concentration of A ¼ 0) and tetravalent (concentration of A6¼0) Mn cation along with some other favorable conditions, the mobile charges can get ordered. This gives rise to
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.09.028
ARTICLE IN PRESS A. Gupta et al. / Physica B 370 (2005) 172–177
‘charge-ordered’ (CO) phase in manganites that is insulating, and antiferromagnetic/paramagnetic in nature. Interestingly, the CO phase of Mn3+/ Mn4+ cations can get melted with the application of magnetic field and result in an insulator to metal transition leading to a huge MR [3,4]. CO state is also seen for various other transition metal oxides, viz. La1/3Sr2/3FeO3 (Fe2+, Fe3+, Fe5+) and RE2BaCoO5 (Co2+, Co3+) etc. [5,6]. However, these CO compounds do not give rise to sufficient MR like manganites. Besides the observation of CO phase in manganites, there have been reports [7–9] of electronic phase separation (EPS). Percolation through EPS has also been identified as a possibility for appearance of MR [8]. CO and EPS have been evidenced by various techniques like electrical transport, magnetization, specific heat, neutron diffraction and transmission electron microscopy [10,11]. Observing and identifying these phases directly in real space is a big scientific challenge. Scanning tunneling microscope (STM) and scanning tunneling spectroscopy (STS) are beyond doubt the best techniques to achieve this goal. To our knowledge there are only scarce STM/STS reports on these materials. For instance, only very recently, CO and EPS were resolved in manganites at the nano level by STM/ STS [12,13]. Extremely high CO temperature much above room temperature (RT) in Bi-based manganites, indicating their technical and academic importance, was reported by Garcia-Munoz et al. (see Ref. [14]). We present here the evidence of CO phase and EPS, in a hitherto less-studied BixSr1xMnO3 system for a large range of x, at the nano level in real space at RT by STM/STS studies. The onset of a transition to CO phase could be further traced by specific heat measurements above RT on the same samples.
2. Experimental Polycrystalline samples of BixSr1xMnO3 (where 0.25pxp0.75) were synthesized by standard solid-state reaction route as described elsewhere [14,15]. The STM/STS studies are carried out with a commercial set-up (Nanoscope-II) at RT (about 300 K) under ambient conditions. For
173
all the samples, just before the study, fresh surfaces were prepared by peeling off the material from the chosen surface by high-grade emery paper. The samples were subsequently tapped vigorously on a table to get rid of any loose powder. Treatment with any kind of chemical was totally avoided. All the studies reported in this paper were performed with the probing tip (Pt–Ir) scanning the basal plane (i.e. Mn–O plane) that can be observed [12] in the Bi-based manganite samples. The images reported here were obtained at a low tunneling current of 0.05 nA under bias voltages which raised from about 10 to 650 mV. All the STM images presented in the paper were the most representative ones of at least 15–20 images obtained, each from at least three or four different regions of each sample. Specific heat measurements in the temperature range 350–650 K were also carried out on the same samples using a modulated differential scanning calorimeter (MDSC) 2910 (TA instrument). A heating rate of 10 K/min with a modulation of 71.5 K in 60 s was maintained and argon gas was flown at a rate of 44 cm3/s.
3. Results and discussion In Figs. 1a–c we show typical STM images of the atomically resolved basal Mn–O plane (i.e. along [0 0 1] direction) in BixSr1xMnO3 with x ¼ 0:25, 0.50 and 0.75, respectively. In all the images, the observed lattice parameter (0.4 nm) matches with that expected (ap) for pseudo-cubic LnMnO3 pervoskite. However, based on the intensity of the brightness, in all the images we can delineate regions with Mn atoms arranged in two differentsized unit cells. One is marked by small cap ‘x’ with dimensions of 0.4 nm 0.4 nm and the other is marked by capital ‘X ’ with dimensions of 0.4 nm 0.8 nm in the figures. Interestingly, in some of the bigger (‘X ’-type) unit cells faint atoms are visible, whereas in some cases the atoms are missing. The variation in brightness at the atomic sites occurs due to the extreme sensitivity of STM to variation of local density of states (LDOS) via variation in the valence charge on the Mn cation, which can vary from +3 to +4. A large change in
ARTICLE IN PRESS 174
A. Gupta et al. / Physica B 370 (2005) 172–177
LDOS near E F (i.e., Fermi energy) when the sample is cooled below its Curie temperature was reported by STS in Ref. [13]. The ‘x’-type unit cells (lattice parameter ap) may be identified with charge-disordered regions in the sample due to fluctuating valent Mn3+/Mn4+ cations; whereas the ‘X ’-type unit cells (lattice parameter doubled in one direction with length 2ap) with CO regions of Mn4+ cations. Figs. 1a–c give a direct evidence of coexistence of CO and CD phase, as it is also supported by the conductance spectra measured in these regions (see below). There are two important observations we would like to underline here. Firstly, the presence of three kinds of entities— bright, faint and missing atoms in the STM images—is not consistent with the ordering of only two Mn sites either with a valence of 3+ or 4+. Secondly, the CO in the present work does not match with the checkerboard pattern [12] and conforms more to stripe-like order of bright Mn atoms with intermittent row of faint/missing Mn atoms. It is important to note that in Fig. 1a, for the x ¼ 0:25 sample, barring a small CO region, most of the region is still CD. However, for samples with higher values of x ¼ 0:50 and 0.75, we observe an increase in CO regions with respect to CD regions, see Figs. 1b and c. As shown in Fig. 1b, towards the upper right corner of the picture, most of the observed unit cells belong to the CD geometry, and rest of the picture is nearly full of unit cells with CO geometry. In Fig. 1c for x ¼ 0:75, the STM image clearly shows that barring a few unit cells revealing the CD phase, nearly all the unit cells correspond to the CO phase. We may thus conclude that with increasing Bi content in the BixSr1xMnO3 system the CO fraction grows at the expense of the CD fraction. In Fig. 2, we plot normalized conductance dln I/ dln V ( ¼ [dI/dV]/[I/V], where I is the tunneling current and V is the bias voltage) as a function of
Fig. 1. STM images (4 nm 4 nm) of the observed basal Mn–O plane in BixSr1xMnO3 samples with (a) x ¼ 0:25, (b) x ¼ 0:50, and (c) x ¼ 0:75. The unit cells marked by small cap ‘x’ and capital ‘X’ are representative of two different-sized unit cells, corresponding to the CD and CO regions, respectively. See text for explanation. The solid lines are only a guide to the eye.
ARTICLE IN PRESS A. Gupta et al. / Physica B 370 (2005) 172–177
0.4/div
dli/dlv
O
−
+
O
(a)
100.71mV/div
0.4/div
dli/dlv
O
− (b)
175
nature with a finite LDOS at E F . However, the existence of a shoulder at a bias voltage of around 7450 mV (see Fig. 2a) shows that the CD region is not highly metallic. These results probably indicate the presence of mixed valent ‘metal’ in the CD region of the sample, where the charge on Mn sites keeps fluctuating with time. In contrast, the CO region shows a U-shaped spectra along with a zero conductance at zero bias (see Fig. 2b) that is characteristic of an ‘insulator’ (with a gap of around 450 meV) and the LDOS at E F to be zero. In the CO regions of samples with varying Bi content no systematic variation of gap was observed, and the values varied between 300 and 600 meV from region to region. However, the identification by STS of ‘insulating’ and ‘metallic’ nature of the coexisting CD and CO unit cells in the STM images reveal EPS at the nano level. Fig. 3 shows molar specific heat (C P ) as a function of temperature for BixSr1xMnO3 samples with different x. We note that all samples show a peak in CP(T), much above RT, that shifts to higher temperature with increasing x. Except in case of the sample with highest Bi content x ¼ 0:75, where a small accompanying anomaly appears at around 525 K that could be due to compositional variation in the sample. In the
+
O 100.71mV/div
Fig. 2. Normalized conductance STS spectra (dln I/dln V) against bias voltage for Mn–O basal plane in (a) CD region and (b) CO region.
bias voltage. The shown spectra in these figures are most representative of the several spectra for all samples taken in the regions with CD unit cells (with lattice parameter 0.4 nm) and CO unit cells (with doubled lattice parameter 0.8 nm in one direction), respectively. However, the exact point in these regions where the STS is done cannot be specified. In the CD region, the observed V-shaped STS spectra along with a finite conductance at zero bias (see Fig. 2a) reveal features of the ‘metallic’
Fig. 3. Molar specific heat as a function of temperature in BixSr1xMnO3 samples with varying x. The solid lines represent the fitted background. The data for x ¼ 0:75 was lowered by 10 J/mol K for clarity of the graph. Inset—TCO as a function of x in BixSr1xMnO3 samples.
ARTICLE IN PRESS 176
A. Gupta et al. / Physica B 370 (2005) 172–177
inset of Fig. 3 we show the characteristic peak temperature (TCO) as a function of x. We may identify TCO with an onset of CO in these samples. There are many reasons for such a conjecture. Firstly, best to our knowledge, there are no TCO measurements reported for such an exhaustive range of x in the BixSr1xMnO3 system. Secondly, the observed values of TCO 500 and 600 K for samples with x ¼ 0:50 and 0.75, respectively, match perfectly with those reported originally based on susceptibility and NPD data [15]. Thirdly, in case of Ca-based analogous BixCa1xMnO3 system, with an overall lower (by 150 K) TCO, a nearly similar monotonic increase of TCO with increase in Bi content was reported [16]. Fourthly, in the BixSr1xMnO3 system, besides the CO transition, another magnetic transition (Neel point) is expected at a much lower temperature with respect to RT and thus also TCO. For instance, in the x ¼ 0:5 sample, the transition occurs at TN 150 K (5TCO500 K [14]). Thus, the observed CP(T) peak at a temperature much greater than RT in case of the samples with x ¼ 0:25 and 0.33, where no data could be measured above 500 K (see Fig. 3), along with STM/STS results may be taken as a proof of CO in them. Finally, we compare the CP(T) peaks attributed to CO transition in our Bi(Sr)-based manganite samples with those observed in other rare earthbased manganites. A closer look at the Cp(T) results in our samples (see Fig. 3) reveal clear trends. In order to analyze these results for all samples, a background line (solid lines in Fig. 3) was fitted to the Cp(T) data, above and below the peak, with a polynomial function. With an increase in Bi content from x ¼ 0:25 to 0.75, the peak width is seen to rise from 50 to 170 K, the jump in Cp(TCO) increases from 3 to 4.5 J/ mole K and the total change in entropy (DS) increases from 0.2 to 0.7 J/mole K. As reported in Refs. [17–20], for other rare earth manganites, the Cp(T) peaks at TCO can vary from being very sharp to totally smeared. In case of Pr(Ca)-based manganites, the reported jump in Cp(TCO230 K) is around 20–30 J/mole K and the width of the peak is 60 K [17]. In case of single crystalline Pr(Ca)-based sample the peak in Cp(TCO230 K) was found to be extremely sharp, with a width
of 5 K [18]. In La(Sr)-based polycrystalline samples, with two different Sr content, the Cp(TCO120 and 140 K) showed very sharp and large anomalies (jump in Cp125 and 60 J/mole K) with a width of 5 K [19]. The total entropy change associated with CO transition in these reports [17–19] is DS1.6–2 J/mole K. However, it was observed that, with increasing Sr content in La(Sr)-based [19] and Ga addition in Pr(Ca)-based [20] manganites, the jump at TCO becomes nearly negligible. Similar observation is reported in [21], where for Pr(Ca)-based samples with lower Ca content, the jump in Cp(T) peak decreases significantly. On comparison with these other manganites, we find that our samples with high values (xX0:45) of Bi content show transitions broader than those reported, and the values of DS(x) in our samples are midway between those reported. We should mention that the former finding needs to be seen in the light of extremely high TCO (bRT) in our samples. If the CO transition at TCO is a first-order transition, then both CO and CD phases should coexist at the transition. However, such coexistence is not expected at T5TCO. For instance, in our sample with x ¼ 0:75, TCO (600 K) is much higher than RT (300 K), no CD phase is expected to remain at RT, that is contrary to the observation. Whether such a contradiction results due to nano level inhomogeneity in the sample or is it a representative of true EPS in a homogenous sample is an ongoing academic debate. The latter has been favored in many recent works [7–9].
4. Conclusions We presented the STM/STS studies and hightemperature specific heat measurements in the BixSr1xMnO3 system for an exhaustive range of x (0.25pxp0.75). The STM images and STS give evidence of ‘insulating’ CO phase coexisting with a ‘metallic’ CD phase at room temperature. The observed CO in real space conforms to a striped order rather than a chekerboard pattern. With STM, we could identify three Mn entities, that raise further questions regarding the often-sited presence of only two entities Mn3+/Mn4+ ions in
ARTICLE IN PRESS A. Gupta et al. / Physica B 370 (2005) 172–177
CO manganites. The specific heat measurements show characteristic peaks that were identified with CO transition. Based on the peak positions, a TCO(x) phase diagram is proposed, where the TCO values are shown to lie much above the room temperature for all the studied values of x.
Acknowledgements The authors (from NPL) thank Professor Vikram Kumar for his continued interest and encouragement. References [1] R. von Helmholt, J. Wecker, B. Holzapfel, L. Schultz, K. Samver, Phys. Rev. Lett. 71 (1993) 2331. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura, Science 270 (1995) 961. [4] J.B. Goodenough, J. Appl. Phys. 81 (1997) 5330. [5] J.Q. Li, Y. Matsui, S.K. Park, Y. Tokura, Phys. Rev. Lett. 79 (1995) 297. [6] E. Suard, F. Fauth, V. Caignaert, I. Mirebeau, G. Baldinozzi, Phys. Rev. B 61 (2000) R11871. [7] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034.
177
[8] M. Uehera, S. Mori, C.H. Chen, S.-W. Cheong, Nature 399 (1999) 560. [9] P. Levy, F. Parisi, G. Polla, D. Vega, G. Leyva, H. Lanza, R.S. Freitas, L. Ghivelder, Phys. Rev. B 62 (2000) 6437. [10] W. Bao, J.D. Axe, C.H. Chen, S.-W. Cheong, Phys. Rev. Lett. 78 (1997) 543. [11] T. Asaka, S. Yamada, S. Tsutsumi, C. Tsuruta, K. Kimoto, T. Arima, Y. Matsui, Phys. Rev. Lett. 88 (2002) 97201. [12] Ch. Renner, G. Aeppli, B.-G. Kim, Yeong-Ah Soh, S.-W. Cheong, Nature 416 (2002) 518. [13] A. Biswas, S. Elizabeth, A.K. Raychaudhuri, S.L. Bhatt, Phys. Rev. B 59 (1999) 5368. [14] J.L. Garcia-Munoz, C. Frontera, M.A.G. Aranda, A. Llobet, C. Ritter, Phys. Rev. B 63 (2001) 64415. [15] C. Frontera, J.L. Garcia-Munoz, C. Ritter, C. Ritter, L. Manosa, X.G. Capdevila, A. Calleja, Solid State Commun. 125 (2003) 277. [16] H. Woo, T.A. Tyson, M. Croft, S.-W. Cheong, J.C. Woicik, Phys. Rev. B 63 (2001) 134412. [17] M.R. Lees, O.A. Petrenko, G. Balakrishnan, D.McK. Paul, Phys. Rev. B 59 (1999) 1298. [18] A.K. Raychaudhuri, A. Guha, I. Das, R. Rawat, C.N.R. Rao, Phys. Rev. B 64 (2001) 165111. [19] R. Klingeler, J. Geck, R. Gross, L. Pinsard-Gaudart, A. Revcolevschi, S. Uhlenbruck, B. Buchner, Phys. Rev. B 65 (2002) 174404. [20] V. Hardy, A. Maignan, S. Hebert, C. Martin, Phys. Rev. B 67 (2003) 24401. [21] H.D. Yang, H.L. Huang, P.L. Ho, W.L. Huang, C.W. Huang, S. Mollah, S.J. Liu, J.-Y. Lin, Physica B 329–333 (2003) 801.
Physica B 370 (2005) 172–177 www.elsevier.com/locate/physb
Direct evidence for charge ordering and electronic phase separation in BixSr1xMnO3 at room temperature A. Guptaa,, S.B. Samantaa, V.P.S. Awanaa, H. Kishana, A.M. Awasthib, S. Bhardwajb, A.V. Narlikarb, C. Fronterac, J.L. Garcia-Munozc a
Superconductivity and Cryogenics Division, National Physical Laboratory, K.S. Krishnan Marg, New Delhi 110-012, India b UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, MP, India c Institut de Ciencia de Materials de Barcelona, CSIC, Campus Universitari de Bellaterra, E-08193 Bellaterra, Spain Received 2 February 2005; received in revised form 22 August 2005; accepted 13 September 2005
Abstract We present a study of charge ordering and electronic phase separation (EPS) phenomenon in BixSr1xMnO3, for an exhaustive range of x (0.25pxp0.75), by STM/STS at room temperature (RT) and specific heat measurements at high temperatures (350–650 K). Atomically resolved STM images of the samples, in real space, show the presence of stripelike charge-ordered (CO) phase coexisting with charge-disordered (CD) phase. The STM images further reveal that the fraction of CO phase increases with an increase in x. The conductance spectra of these phases measured at nano level by STS are discussed. The transition to CO phase above RT is corroborated by specific heat measurements in all samples, giving a TCO(x) phase diagram for this system. r 2005 Elsevier B.V. All rights reserved. PACS: 75.47.Lx; 71.30.Ph Keywords: Bi-based manganites; Charge-ordering; Electronic phase separation
1. Introduction Discovery of huge negative magnetoresistance (MR) in rare earth-based manganites Ln1xAx MnO3 (Ln—rare earths, A—alkaline metals Ca, Sr, Ba, etc.) has attracted the attention of Corresponding author. Tel./fax: +91 11 25748709.
E-mail address: [email protected] (A. Gupta).
condensed matter physicists at large [1,2]. The phenomenon mainly centers on the Mn ion valance state through A cation substitutions and the ionic radii of the A cation in these compounds. It is now well-known that, for certain concentration ratios of trivalent (concentration of A ¼ 0) and tetravalent (concentration of A6¼0) Mn cation along with some other favorable conditions, the mobile charges can get ordered. This gives rise to
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.09.028
ARTICLE IN PRESS A. Gupta et al. / Physica B 370 (2005) 172–177
‘charge-ordered’ (CO) phase in manganites that is insulating, and antiferromagnetic/paramagnetic in nature. Interestingly, the CO phase of Mn3+/ Mn4+ cations can get melted with the application of magnetic field and result in an insulator to metal transition leading to a huge MR [3,4]. CO state is also seen for various other transition metal oxides, viz. La1/3Sr2/3FeO3 (Fe2+, Fe3+, Fe5+) and RE2BaCoO5 (Co2+, Co3+) etc. [5,6]. However, these CO compounds do not give rise to sufficient MR like manganites. Besides the observation of CO phase in manganites, there have been reports [7–9] of electronic phase separation (EPS). Percolation through EPS has also been identified as a possibility for appearance of MR [8]. CO and EPS have been evidenced by various techniques like electrical transport, magnetization, specific heat, neutron diffraction and transmission electron microscopy [10,11]. Observing and identifying these phases directly in real space is a big scientific challenge. Scanning tunneling microscope (STM) and scanning tunneling spectroscopy (STS) are beyond doubt the best techniques to achieve this goal. To our knowledge there are only scarce STM/STS reports on these materials. For instance, only very recently, CO and EPS were resolved in manganites at the nano level by STM/ STS [12,13]. Extremely high CO temperature much above room temperature (RT) in Bi-based manganites, indicating their technical and academic importance, was reported by Garcia-Munoz et al. (see Ref. [14]). We present here the evidence of CO phase and EPS, in a hitherto less-studied BixSr1xMnO3 system for a large range of x, at the nano level in real space at RT by STM/STS studies. The onset of a transition to CO phase could be further traced by specific heat measurements above RT on the same samples.
2. Experimental Polycrystalline samples of BixSr1xMnO3 (where 0.25pxp0.75) were synthesized by standard solid-state reaction route as described elsewhere [14,15]. The STM/STS studies are carried out with a commercial set-up (Nanoscope-II) at RT (about 300 K) under ambient conditions. For
173
all the samples, just before the study, fresh surfaces were prepared by peeling off the material from the chosen surface by high-grade emery paper. The samples were subsequently tapped vigorously on a table to get rid of any loose powder. Treatment with any kind of chemical was totally avoided. All the studies reported in this paper were performed with the probing tip (Pt–Ir) scanning the basal plane (i.e. Mn–O plane) that can be observed [12] in the Bi-based manganite samples. The images reported here were obtained at a low tunneling current of 0.05 nA under bias voltages which raised from about 10 to 650 mV. All the STM images presented in the paper were the most representative ones of at least 15–20 images obtained, each from at least three or four different regions of each sample. Specific heat measurements in the temperature range 350–650 K were also carried out on the same samples using a modulated differential scanning calorimeter (MDSC) 2910 (TA instrument). A heating rate of 10 K/min with a modulation of 71.5 K in 60 s was maintained and argon gas was flown at a rate of 44 cm3/s.
3. Results and discussion In Figs. 1a–c we show typical STM images of the atomically resolved basal Mn–O plane (i.e. along [0 0 1] direction) in BixSr1xMnO3 with x ¼ 0:25, 0.50 and 0.75, respectively. In all the images, the observed lattice parameter (0.4 nm) matches with that expected (ap) for pseudo-cubic LnMnO3 pervoskite. However, based on the intensity of the brightness, in all the images we can delineate regions with Mn atoms arranged in two differentsized unit cells. One is marked by small cap ‘x’ with dimensions of 0.4 nm 0.4 nm and the other is marked by capital ‘X ’ with dimensions of 0.4 nm 0.8 nm in the figures. Interestingly, in some of the bigger (‘X ’-type) unit cells faint atoms are visible, whereas in some cases the atoms are missing. The variation in brightness at the atomic sites occurs due to the extreme sensitivity of STM to variation of local density of states (LDOS) via variation in the valence charge on the Mn cation, which can vary from +3 to +4. A large change in
ARTICLE IN PRESS 174
A. Gupta et al. / Physica B 370 (2005) 172–177
LDOS near E F (i.e., Fermi energy) when the sample is cooled below its Curie temperature was reported by STS in Ref. [13]. The ‘x’-type unit cells (lattice parameter ap) may be identified with charge-disordered regions in the sample due to fluctuating valent Mn3+/Mn4+ cations; whereas the ‘X ’-type unit cells (lattice parameter doubled in one direction with length 2ap) with CO regions of Mn4+ cations. Figs. 1a–c give a direct evidence of coexistence of CO and CD phase, as it is also supported by the conductance spectra measured in these regions (see below). There are two important observations we would like to underline here. Firstly, the presence of three kinds of entities— bright, faint and missing atoms in the STM images—is not consistent with the ordering of only two Mn sites either with a valence of 3+ or 4+. Secondly, the CO in the present work does not match with the checkerboard pattern [12] and conforms more to stripe-like order of bright Mn atoms with intermittent row of faint/missing Mn atoms. It is important to note that in Fig. 1a, for the x ¼ 0:25 sample, barring a small CO region, most of the region is still CD. However, for samples with higher values of x ¼ 0:50 and 0.75, we observe an increase in CO regions with respect to CD regions, see Figs. 1b and c. As shown in Fig. 1b, towards the upper right corner of the picture, most of the observed unit cells belong to the CD geometry, and rest of the picture is nearly full of unit cells with CO geometry. In Fig. 1c for x ¼ 0:75, the STM image clearly shows that barring a few unit cells revealing the CD phase, nearly all the unit cells correspond to the CO phase. We may thus conclude that with increasing Bi content in the BixSr1xMnO3 system the CO fraction grows at the expense of the CD fraction. In Fig. 2, we plot normalized conductance dln I/ dln V ( ¼ [dI/dV]/[I/V], where I is the tunneling current and V is the bias voltage) as a function of
Fig. 1. STM images (4 nm 4 nm) of the observed basal Mn–O plane in BixSr1xMnO3 samples with (a) x ¼ 0:25, (b) x ¼ 0:50, and (c) x ¼ 0:75. The unit cells marked by small cap ‘x’ and capital ‘X’ are representative of two different-sized unit cells, corresponding to the CD and CO regions, respectively. See text for explanation. The solid lines are only a guide to the eye.
ARTICLE IN PRESS A. Gupta et al. / Physica B 370 (2005) 172–177
0.4/div
dli/dlv
O
−
+
O
(a)
100.71mV/div
0.4/div
dli/dlv
O
− (b)
175
nature with a finite LDOS at E F . However, the existence of a shoulder at a bias voltage of around 7450 mV (see Fig. 2a) shows that the CD region is not highly metallic. These results probably indicate the presence of mixed valent ‘metal’ in the CD region of the sample, where the charge on Mn sites keeps fluctuating with time. In contrast, the CO region shows a U-shaped spectra along with a zero conductance at zero bias (see Fig. 2b) that is characteristic of an ‘insulator’ (with a gap of around 450 meV) and the LDOS at E F to be zero. In the CO regions of samples with varying Bi content no systematic variation of gap was observed, and the values varied between 300 and 600 meV from region to region. However, the identification by STS of ‘insulating’ and ‘metallic’ nature of the coexisting CD and CO unit cells in the STM images reveal EPS at the nano level. Fig. 3 shows molar specific heat (C P ) as a function of temperature for BixSr1xMnO3 samples with different x. We note that all samples show a peak in CP(T), much above RT, that shifts to higher temperature with increasing x. Except in case of the sample with highest Bi content x ¼ 0:75, where a small accompanying anomaly appears at around 525 K that could be due to compositional variation in the sample. In the
+
O 100.71mV/div
Fig. 2. Normalized conductance STS spectra (dln I/dln V) against bias voltage for Mn–O basal plane in (a) CD region and (b) CO region.
bias voltage. The shown spectra in these figures are most representative of the several spectra for all samples taken in the regions with CD unit cells (with lattice parameter 0.4 nm) and CO unit cells (with doubled lattice parameter 0.8 nm in one direction), respectively. However, the exact point in these regions where the STS is done cannot be specified. In the CD region, the observed V-shaped STS spectra along with a finite conductance at zero bias (see Fig. 2a) reveal features of the ‘metallic’
Fig. 3. Molar specific heat as a function of temperature in BixSr1xMnO3 samples with varying x. The solid lines represent the fitted background. The data for x ¼ 0:75 was lowered by 10 J/mol K for clarity of the graph. Inset—TCO as a function of x in BixSr1xMnO3 samples.
ARTICLE IN PRESS 176
A. Gupta et al. / Physica B 370 (2005) 172–177
inset of Fig. 3 we show the characteristic peak temperature (TCO) as a function of x. We may identify TCO with an onset of CO in these samples. There are many reasons for such a conjecture. Firstly, best to our knowledge, there are no TCO measurements reported for such an exhaustive range of x in the BixSr1xMnO3 system. Secondly, the observed values of TCO 500 and 600 K for samples with x ¼ 0:50 and 0.75, respectively, match perfectly with those reported originally based on susceptibility and NPD data [15]. Thirdly, in case of Ca-based analogous BixCa1xMnO3 system, with an overall lower (by 150 K) TCO, a nearly similar monotonic increase of TCO with increase in Bi content was reported [16]. Fourthly, in the BixSr1xMnO3 system, besides the CO transition, another magnetic transition (Neel point) is expected at a much lower temperature with respect to RT and thus also TCO. For instance, in the x ¼ 0:5 sample, the transition occurs at TN 150 K (5TCO500 K [14]). Thus, the observed CP(T) peak at a temperature much greater than RT in case of the samples with x ¼ 0:25 and 0.33, where no data could be measured above 500 K (see Fig. 3), along with STM/STS results may be taken as a proof of CO in them. Finally, we compare the CP(T) peaks attributed to CO transition in our Bi(Sr)-based manganite samples with those observed in other rare earthbased manganites. A closer look at the Cp(T) results in our samples (see Fig. 3) reveal clear trends. In order to analyze these results for all samples, a background line (solid lines in Fig. 3) was fitted to the Cp(T) data, above and below the peak, with a polynomial function. With an increase in Bi content from x ¼ 0:25 to 0.75, the peak width is seen to rise from 50 to 170 K, the jump in Cp(TCO) increases from 3 to 4.5 J/ mole K and the total change in entropy (DS) increases from 0.2 to 0.7 J/mole K. As reported in Refs. [17–20], for other rare earth manganites, the Cp(T) peaks at TCO can vary from being very sharp to totally smeared. In case of Pr(Ca)-based manganites, the reported jump in Cp(TCO230 K) is around 20–30 J/mole K and the width of the peak is 60 K [17]. In case of single crystalline Pr(Ca)-based sample the peak in Cp(TCO230 K) was found to be extremely sharp, with a width
of 5 K [18]. In La(Sr)-based polycrystalline samples, with two different Sr content, the Cp(TCO120 and 140 K) showed very sharp and large anomalies (jump in Cp125 and 60 J/mole K) with a width of 5 K [19]. The total entropy change associated with CO transition in these reports [17–19] is DS1.6–2 J/mole K. However, it was observed that, with increasing Sr content in La(Sr)-based [19] and Ga addition in Pr(Ca)-based [20] manganites, the jump at TCO becomes nearly negligible. Similar observation is reported in [21], where for Pr(Ca)-based samples with lower Ca content, the jump in Cp(T) peak decreases significantly. On comparison with these other manganites, we find that our samples with high values (xX0:45) of Bi content show transitions broader than those reported, and the values of DS(x) in our samples are midway between those reported. We should mention that the former finding needs to be seen in the light of extremely high TCO (bRT) in our samples. If the CO transition at TCO is a first-order transition, then both CO and CD phases should coexist at the transition. However, such coexistence is not expected at T5TCO. For instance, in our sample with x ¼ 0:75, TCO (600 K) is much higher than RT (300 K), no CD phase is expected to remain at RT, that is contrary to the observation. Whether such a contradiction results due to nano level inhomogeneity in the sample or is it a representative of true EPS in a homogenous sample is an ongoing academic debate. The latter has been favored in many recent works [7–9].
4. Conclusions We presented the STM/STS studies and hightemperature specific heat measurements in the BixSr1xMnO3 system for an exhaustive range of x (0.25pxp0.75). The STM images and STS give evidence of ‘insulating’ CO phase coexisting with a ‘metallic’ CD phase at room temperature. The observed CO in real space conforms to a striped order rather than a chekerboard pattern. With STM, we could identify three Mn entities, that raise further questions regarding the often-sited presence of only two entities Mn3+/Mn4+ ions in
ARTICLE IN PRESS A. Gupta et al. / Physica B 370 (2005) 172–177
CO manganites. The specific heat measurements show characteristic peaks that were identified with CO transition. Based on the peak positions, a TCO(x) phase diagram is proposed, where the TCO values are shown to lie much above the room temperature for all the studied values of x.
Acknowledgements The authors (from NPL) thank Professor Vikram Kumar for his continued interest and encouragement. References [1] R. von Helmholt, J. Wecker, B. Holzapfel, L. Schultz, K. Samver, Phys. Rev. Lett. 71 (1993) 2331. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura, Science 270 (1995) 961. [4] J.B. Goodenough, J. Appl. Phys. 81 (1997) 5330. [5] J.Q. Li, Y. Matsui, S.K. Park, Y. Tokura, Phys. Rev. Lett. 79 (1995) 297. [6] E. Suard, F. Fauth, V. Caignaert, I. Mirebeau, G. Baldinozzi, Phys. Rev. B 61 (2000) R11871. [7] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034.
177
[8] M. Uehera, S. Mori, C.H. Chen, S.-W. Cheong, Nature 399 (1999) 560. [9] P. Levy, F. Parisi, G. Polla, D. Vega, G. Leyva, H. Lanza, R.S. Freitas, L. Ghivelder, Phys. Rev. B 62 (2000) 6437. [10] W. Bao, J.D. Axe, C.H. Chen, S.-W. Cheong, Phys. Rev. Lett. 78 (1997) 543. [11] T. Asaka, S. Yamada, S. Tsutsumi, C. Tsuruta, K. Kimoto, T. Arima, Y. Matsui, Phys. Rev. Lett. 88 (2002) 97201. [12] Ch. Renner, G. Aeppli, B.-G. Kim, Yeong-Ah Soh, S.-W. Cheong, Nature 416 (2002) 518. [13] A. Biswas, S. Elizabeth, A.K. Raychaudhuri, S.L. Bhatt, Phys. Rev. B 59 (1999) 5368. [14] J.L. Garcia-Munoz, C. Frontera, M.A.G. Aranda, A. Llobet, C. Ritter, Phys. Rev. B 63 (2001) 64415. [15] C. Frontera, J.L. Garcia-Munoz, C. Ritter, C. Ritter, L. Manosa, X.G. Capdevila, A. Calleja, Solid State Commun. 125 (2003) 277. [16] H. Woo, T.A. Tyson, M. Croft, S.-W. Cheong, J.C. Woicik, Phys. Rev. B 63 (2001) 134412. [17] M.R. Lees, O.A. Petrenko, G. Balakrishnan, D.McK. Paul, Phys. Rev. B 59 (1999) 1298. [18] A.K. Raychaudhuri, A. Guha, I. Das, R. Rawat, C.N.R. Rao, Phys. Rev. B 64 (2001) 165111. [19] R. Klingeler, J. Geck, R. Gross, L. Pinsard-Gaudart, A. Revcolevschi, S. Uhlenbruck, B. Buchner, Phys. Rev. B 65 (2002) 174404. [20] V. Hardy, A. Maignan, S. Hebert, C. Martin, Phys. Rev. B 67 (2003) 24401. [21] H.D. Yang, H.L. Huang, P.L. Ho, W.L. Huang, C.W. Huang, S. Mollah, S.J. Liu, J.-Y. Lin, Physica B 329–333 (2003) 801.