Nd3+ crystal-field study of weakly doped Nd1−xCaxMnO3

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 3607–3610

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Nd3+ crystal-field study of weakly doped Nd1xCaxMnO3 S. Jandl a,, A.A. Mukhin b, V. Yu Ivanov b, A. Balbashov c, M. Orlita d a

Universite de Sherbrooke, De partement de Physique, 2500 Boulevard de l’Universite , Sherbrooke, Quebec, Canada J1K 2R1 General Physics Institute of the Russian Academy of Sciences, 38 Vavilov St., 119991 Moscow, Russia Moscow Power Engineering Institute, 14 Krasnokazarmennaya St., 105835 Moscow, Russia d Grenoble High Magnetic Field Laboratory, 25, Avenue des Martyrs, Boˆıte Postale 166, F-38042 Grenoble, France b c

a r t i c l e in fo

abstract

Article history: Received 7 April 2009 Received in revised form 11 June 2009 Available online 30 June 2009

Nd3+ crystal-field excitations in Nd1xCaxMnO3 (x ¼ 0.025, 0.05 and 0.1) single crystals are studied via infrared transmission as a function of temperature and external magnetic field. We report excitations associated with Nd3+ sites as detected in NdMnO3 and excitations due to Ca doping. The latter reveal phase separation between the usual A-type antiferromagnetic states and the insulating canted (ferromagnetic) spin states in the vicinity of doped Ca2+ ions. Both Nd3+ crystal-field levels could be described using calculated parameters for NdMnO3. Also, while oxygen stoichiometry and coherent Jahn–Teller distortions seem not to be affected by Ca doping, increased absorption bandwidths characterize the doped crystals. & 2009 Elsevier B.V. All rights reserved.

Keywords: Infrared transmission Crystal-field Doped manganites

1. Introduction Whenever RMnO3 manganites are doped with divalent cations A2+ resulting in R1xAxMnO3 (R ¼ lanthanides and A ¼ Ba, Sr, or Ca), the obtained Mn4+ ions weaken the impact of Jahn–Tellertype distortions in favor of double exchange interactions. In particular, simultaneous ferromagnetic and metallic attributes develop for x0.3 compounds [1–3] along with a colossal negative magnetoresistance observed near the concomitant paramagnetic insulator–ferromagnetic metallic phase transition [4]. In addition, Mn–O bond lengths are also affected, giving rise to temperaturedependent structural disorder in which the precise roles of the lattice, charge and orbital configurations are still debated [5]. The low-doping regime is of particular interest because it gives one a simple way to probe the physical mechanisms that are believed to play an important role in the large-doping regime. Recent theoretical and experimental studies have recourse to phase separation [6]. On the other hand, in the framework of the mean field theory, the low-doping regime is rather related to a canted antiferromagnetic homogeneous state [7]. It is worth noting that the low-doping regime of LaMnO3 is also associated with ferromagnetic insulating states [8] not explained by the double exchange magnetic interaction. They rather reflect orbital ordering [9] and important electron–phonon coupling [10,11]. In contrast with La1xCaxMnO3, Nd1xCaxMnO3 and Nd1xSrxMnO3 are likely to show a closer competition such as

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E-mail address: [email protected] (S. Jandl). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.06.078

orbital, charge ordering and antiferromagnetic superexchange [12,13]. At ToTN, the parent NdMnO3 (D2h16-Pbnm space group) is an antiferromagnetic compound characterized by static Jahn–Teller distortions [14]. Its A-type structure is related to a layered structure with MnO2 in-plane ferromagnetic interaction and MnO2 inter-plane antiferromagnetic interaction [15,16]. Raman active phonons of Nd1xCaxMnO3 [17,18] are sensitive to the magnetic evolution of the Mn3+ sublattice as a function of temperature. In particular, for x ¼ 0, 0.025 and 0.05, the most intense 607 cm1 Raman active B2g phonon softens below TN80 K, following the paramagnetic to canted antiferromagnetic phase transition. While magnetization measurements for the x ¼ 0.1 compound reveal an overall antiferromagnetic transition, its Raman B2g mode does not soften below TN. The latter fact suggests a disruption of long-range antiferromagnetism [18]. The lightly doped Nd1xCaxMnO3 (x ¼ 0.08 and 0.12) have been investigated by several means including X-ray, neutron powder diffraction, magnetization and AC magnetic susceptibility measurements [19]. These compounds exhibit a complex magnetic behavior at low temperatures. This behavior is well described in the framework of the magnetic phase separation model which predicts the simultaneous presence of antiferromagnetic and ferromagnetic phases (with opposite f–d exchange interaction terms). In the ferromagnetic phase, the neodymium magnetic moments start to be ordered near TN parallel to the moments of the manganese ions. On the other hand, the antiferromagnetic phase is related to magnetic moments aligned in the opposite direction. For the NdMnO3 and Nd1xSrxMnO3 (x ¼ 0.05 and 0.1) compounds we have previously reported the lifting of the Nd3+

ARTICLE IN PRESS S. Jandl et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3607–3610

ground-state Kramers doublet degeneracy (14 cm1) as a consequence of Mn3+–Nd3+ interactions below TN (75 K) [20,21]. The Nd3+ crystal-field (CF) Hamiltonian parameters have been calculated using the measured CF levels, the Kramers doublet exchange splittings, the g-tensor components and abinitio methods. The study of Nd1xSrxMnO3 CF excitations revealed that new ferromagnetic domains and phase separation are generated as a result of Sr doping [21]. Compared to Nd1xSrxMnO3, the radius of the A-site cation in Nd1xCaxMnO3 is reduced (1.17 A˚ for Ca2+ vs. 1.236 A˚ for Sr2+). This implies a decrease of the Mn–O–Mn angle (from 1651 to 1571) and an ˚ The latter increase of the Mn–O distance (from 1.936 A˚ to 1.945 A). consequences have several impacts including, for instance, the x ¼ 0.5 compounds charge ordering transition effective temperatures [22]. In this paper, we present a study of Ca lightly doped NdMnO3 CF excitations as a function of doping, temperature and external magnetic field. We aim to determine whether magnetic phase separation is present in Nd1xCaxMnO3 and if there are similarities between the ferromagnetic domain CF excitations in both Nd1xCaxMnO3 and Nd1xSrxMnO3 compounds.

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2. Experiments The Nd1xCaxMnO3 (x ¼ 0, 0.025, 0.05 and 0.1) single crystals (1mm, 2mm, and 200 mm) were grown using the floating zone method described in Ref. [23]. The infrared transmission measurements as a function of temperature were obtained in the 1800–5000 cm1 range with a Fourier transform interferometer BOMEM DA3.002 equipped with an InSb detector, quartz-halogen and globar sources and a CaF2 beamsplitter. For measurements under external magnetic field, with 1 cm1 resolution, a Bruker Instruments model 113 Fourier transform spectrometer, equipped with tungsten and globar light sources, was used to collect and analyze the spectra. The samples were placed in the bore of a superconducting magnet and in a helium bath cryostat at 1.8 K, with the magnetic field parallel to the incident radiation. A composite Si bolometer mounted directly beneath the sample was used to measure the intensity of the transmitted light.

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Frequency (cm-1) Fig. 1. (a) Nd0.95Ca0.05MnO3 Raman active phonons at T ¼ 80 K (a) and T ¼ 4.2 K (b). Phonon softening is indicated by a vertical line. * indicates laser plasma lines. (b) Nd0.9Ca0.1MnO3 Raman active phonons at T ¼ 80 K (a) and T ¼ 4.2 K (b). Absence of phonon softening is indicated by a vertical line. * indicates laser plasma lines.

3. Results and discussion

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Prior to the CF absorption measurements, we used microRaman spectroscopy in order to check the samples stoichiometries by monitoring the Nd1xCaxMnO3 phonon frequencies as a function of temperature [18]. Both Ag and B2g phonons, of stretching and bending types, related to octahedral distortions were detected. On the other hand, broad bands due to large noncoherent Jahn–Teller distortions resulting in disordered-induced phonon density of states were absent. This indicates the overall persistence of coherent Jahn–Teller distortions at low Ca doping. Softening of the 607 cm1 phonon below TN80 K, as observed in NdMnO3, Nd0.975Ca0.025MnO3 and Nd0.95Ca0.05MnO3, disappears in Nd0.9Ca0.1MnO3 (Fig. 1(a) and (b)). Granado et al. [24] and Laverdie re et al. [25] have studied, respectively, La1xMn1xO3 and RMnO3 single crystals and have also reported the softening of the 607 cm1 Raman active phonon. By scaling the frequency shift to the normalized square of the sublattice magnetization, they have associated this softening with spin–phonon coupling caused by phonon modulation of the nearest neighbors exchange integral. Absence of phonon softening in Nd0.9Ca0.1MnO3 is a clear indication of long-range antiferromagnetism disruption at this doping concentration.

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Wavenumber (cm-1) Fig. 2. 4I9/2-4I11/2 Nd3+ CF transitions in Nd1xCaxMnO3 at T ¼ 300 K: x ¼ 0 (a), x ¼ 0.025 (b), x ¼ 0.05 (c) and x ¼ 0.1 (d).

ARTICLE IN PRESS S. Jandl et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3607–3610

Fig. 4 bands (1) and (3) do not show any new excitations in the doped samples. On the other hand, bands (2) and (4) show additional excitations at 1987 and 2040 cm1. Their intensities increase with doping, while their local deformations-dependent frequencies remain unchanged for the three doping concentrations, i.e. x ¼ 0.025, 0.05, and 0.1. These new CF excitations, associated with Nd3+ perturbed sites in the vicinity of the Ca2+doping ions, are different from those observed in Nd0.9Sr0.1MnO3 at 1992 and 2046 cm1 [21]. This reflects differences in the local Ca2+ and Sr2+ deformations (Mn–O distances and Mn–O–Mn angles). The detection of these new CF excitations in Nd1xCaxMnO3 and Nd1xSrxMnO3 stresses that the Nd3+ sites close to the Ca2+- or Sr2+-doping ions pertain to domains where distorted antiferromagnetic structure with additional canting or even ferromagnetic order is developed [26], rather than usual A-type antiferromagnetic ordering. The additional excitations are confirmed in the absorption measurements at T ¼ 1.8 K in the bands (2) and (4) as shown in Fig. 5. At such a low temperature, the ground-state Kramers doublet excited level is depleted in line

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In an ideal perovskite, the Nd3+ ions occupy centrosymmetrical sites preventing the CF excitations from being infrared active. In Nd1xCaxMnO3, the Jahn–Teller distortions decrease the Nd3+ site point symmetry allowing one to observe optical transitions. Fig. 2 shows the 4I9/2-4I11/2 Nd3+ CF transitions at T ¼ 300 K for Nd0.975Ca0.025MnO3, Nd0.95Ca0.05MnO3 and NdMnO3, Nd0.9Ca0.1MnO3. Five of the six expected NdMnO3 4I11/2 CF levels are clearly observed in the 1950–2250 cm1 range absorption bands at 1969, 2027, 2082, 2148 and 2219 cm1 (with 72 cm1 uncertainty). Transition from the thermally populated first excited ground-state level at 67 cm1 is associated with the 1902 cm1 band. The excitation band broadenings range from 40 to 70 cm1 reflecting the high-temperature anharmonicities and shortened lifetimes. For the x ¼ 0.025 doping sample the frequencies of the absorption bands are not significantly affected. However, they are slightly shifted for x ¼ 0.05 and 0.1 dopings towards 1974, 2031, 2081, 2153 and 2228 cm1, with band broadenings between 45 and 90 cm1. The absence of additional broad CF excitations in the doped samples indicate that oxygen stoichiometry (as well as overall homogeneities) seem not to be affected by doping. Below TN80 K, the degeneracy of the Kramers doublet ground state is lifted due to the Nd–Mn exchange interaction (for instance see Fig. 3 where each Nd0.975Ca0.025MnO3 4I9/2-4I11/2 absorption band is splitted below T ¼ 80 K). In the Nd1xCaxMnO3 compounds, the related line splittings are not masked by band broadening originating in distortions triggered by the Jahn–Teller effect and MnO6 octahedra rotation. Indeed, Fig. 4 clearly shows the lifting of the degeneracy (see bands (1)–(2) and (3)–(4)). The lifting typically reaches 14 cm1 at 10 K for the two NdMnO3 lowest 4 I11/2 CF levels at 1966–1980 cm1 and 2015–2029 cm1. Moreover, because of less important Nd–Mn exchange interactions, the Kramers doublet splittings are found to decrease as doping increases: 13, 11 and 9 cm1 at T ¼ 10 K for Nd0.975Ca0.025MnO3, Nd0.95Ca0.05MnO3 and Nd0.9Ca0.1MnO3, respectively. A splitting of 10 cm1 has also been reported for Nd0.9Sr0.1MnO3 [21], confirming that even light Ca or Sr dopings oppose local antiferromagnetism as observed in metallic (0.3Zx40.5) doped manganites (in which antiferromagnetic interactions are ultimately suppressed). Interestingly, even though Raman measurements corroborates long-range the antiferromagnetism suppression in Nd0.9Ca0.1MnO3, observation of a Kramers doublet splitting in that compound indicates that short-range antiferromagnetism persists.

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Fig. 4. I9/2- I11/2 Nd CF transitions in Nd1xCaxMnO3 at T ¼ 10 K: x ¼ 0 (a), x ¼ 0.025 (b), x ¼ 0.05 (c) and x ¼ 0.1(d). * indicates the new CF excitations that are due to the doping.

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Wavenumber (cm-1) Fig. 3. 4I9/2-4I11/2 Nd3+ CF transitions in Nd0.975Ca0.025MnO3 at T ¼ 300 K (a), 60 K (b) and 10 K (c). * * indicates the ground-state Kramers doublet splitting.

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Wavenumber (cm-1) Fig. 5. 4I9/2-4I11/2 Nd3+ CF transitions in Nd1xCaxMnO3 at T ¼ 1.8 K: x ¼ 0 (a), x ¼ 0.025 (b), x ¼ 0.05 (c) and x ¼ 0.1 (d). * indicates the new CF excitations that are due to the doping.

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strengthens the role of local probe the rare-earth CF levels play in manganite compounds. Finally, under applied external magnetic field, Zeeman splittings are observed in NdMnO3 and Nd0.975Ca0.025MnO3, while they are masked in Nd0.95Ca0.05MnO3 and Nd0.9Ca0.1MnO3. The latter results are due to doping-induced band broadening and possible twining.

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We acknowledge the financial support from the National Science and Engineering Research Council of Canada, the Fonds Que be cois de la Recherche sur la Nature et les Technologies, and the Program of Quantum Phenomena in Condensed Matter of the Russian Academy of Sciences and Russian Foundation for Basic Researches. References

3+

Fig. 6. I9/2- I11/2 Nd CF transitions in Nd1xCaxMnO3 at T ¼ 1.8 K under applied magnetic field B ¼ 12 T: x ¼ 0 (a), x ¼ 0.025 (b), x ¼ 0.05 (c) and x ¼ 0.1 (d); (1–10 ) and (2–20 ) correspond to gx and gy factors Zeeman splittings, respectively.

with the vanishing of bands (1) and (3) and the simultaneous amplification of bands (2) and (4). While the regular sites absorption bands are well resolved at such low temperatures, the perturbed site absorption bands tend to saturate for the various doping concentrations. This suggests an important increase of their oscillator strengths between 10 and 1.8 K. Under 12 T applied external magnetic field, Zeeman splittings of the 4I11/2 CF levels in Nd1xCaxMnO3 are observed (see Fig. 6). The (1–10 ) and (2–20 ) excitations are related to Zeeman splittings associated with the gx and gy factors in NdMnO3, as observed in the twinned samples [20,21]. These bands are amplified and their bandwidths increase with doping. No resolvable Zeeman splittings of the CF levels associated with perturbed Nd3+ could consequently be detected.

4. Conclusion The infrared transmission study of the Nd3+ ions CF excitations in Nd1xCaxMnO3 (x ¼ 0.025, 0.05 and 0.1) single crystals has been compared to the transmission in NdMnO3. Our study reveals the presence of a magnetic phase separation in the doped samples. As a consequence of doping, we report the detection of two sets of CF levels, as already observed in Nd1xSrxMnO3. One is associated with unperturbed sites related to the NdMnO3 antiferromagnetism with its typical Zeeman splitting below TN, and a second one is linked to perturbed sites in the vicinity of the Ca2+ cations where local A-type antiferromagnetism is suppressed. While the energy differences between the two sets are within the uncertainty values of the CF parameters that describe the NdMnO3 CF Hamiltonian and predict the CF levels, their detection

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