NMR spectroscopy in mixed valence manganites

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ELSEVIER

Journal of Magnetism and Magnetic Materials 196-197 (1999) 446-450

Journalof magnetism and magnetic materials

Invited paper

NMR spectroscopy in mixed valence manganites Cz. Kapusta a'*, P.C. Riedi b ~Department of Solid State Physics. Faculty oj Physics and Nuclear Techniques, ~:)ffversi~. ' of Mining and Metallurgy, 30-059 Cracow, Poland bDepartment of Physics and Astrononn,, University qf St. Andrews, St. Andre*~w. File, KY16 9SS. Scotland, UK

Abstract

A survey is given of the nuclear magnetic resonance (NMR) experiments in the manganese perovskites of the form RE~ -xA~MnO3 and REMnO3+a (RE = rare earth, A = Ca, St, Ba). The SSMn, *3~La and STSr spin-echo spectra in the frequency range 5-650 MHz are discussed and the microscopic information on the Mn valence states and the nature of magnetic interactions is analysed as a function of the hole doping. ~ 1999 Elsevier Science B.V. All rights reserved. Kevwords. N M R - spin echo; Magnetoresistance - colossal; Perovskites

The manganese perovskites derived from LaMnO3 exhibit very interesting magnetic and transport properties [1-4] including a 'colossal' magnetoresistance which makes them attractive for applications. Their complex magnetism originates from the competition of the superexchange and double-exchange interactions between manganese magnetic moments preferring antiferromagnetic and ferromagnetic order, respectively. The superexchange dominates in the compounds containing solely Mn 3+ or Mn 4+ ions, e.g. LaMnO3 and CaMnO3, leading to antiferromagnetic structures. Partial substitution of the trivalent rare earth with a divalent alkaline earth metal introduces holes at the Mn 3 + sites, i.e. produces a mixture of the Mn 3 + and Mn 4+ ionic states. Hopping of carriers between the Mn 3 + and Mn "*+ adjacent sites is the source of the double exchange. The ratio of the rare earth to alkaline earth content of 2 : 1 is optimum for the double-exchange mechanism and corresponds to the highest magnetic ordering temperatures Tc. Such optimally doped perovskites, where RE = La, exhibit a metallic conductivity below Tc and an 'activated' temperature dependence of the conductivity above Tc. For

*Corresponding author. Fax: + 48-12-6341247; e-mail: [email protected].

a review of the lattice, magnetic and transport properties of the manganites see e.g. [5,6] Nuclear magnetic resonance (NMR) allows the study of the properties of magnetic materials at the microscopic level via hyperfine interactions. In zero-field NMR, the hyperfine fields, probed by the resonant response of nuclear magnetic moments at individual atomic sites, provide information on the local magnetic states. Resonance occurs at a frequency v when 2nv = ;'Be, where ,, is the nuclear gyromagnetic ratio ;,/n = 10.553 M H z T 1 for SSMn and Be is the effective internal magnetic field, arising largely from hyperfine interactions. The 5SMn N MR measurements at 4.2 K have been reported for the Lal _,Ca~MnO3 [7,8], Lal .,Sr,MnO3 [9], Lal xPb.~MnO3 [10], Prl x(Ca,Srk-MnO3 [11], Lal xSrxMnO3 a [12], and LaMnO3+,~ [8] compounds. Two separate resonances observed for the compounds with a low divalent element doping x < 0.2 have been attributed to the two distinct ionisation states of manganese-Mn 3+ and Mn 4+ [7]. For some of the Pr~ ~(Ca,Sr)~MnO3 samples a line corresponding to Mn 2+ ions was also observed [I1]. The Mn 3~ and Mn ~+ lines shift in the applied magnetic field to lower frequencies which corresponds to the dominating contribution to Be from the polarisation of core electrons by the spin of the parent atom. This contribution is antiparallel to the magnetic moment and partly cancells

0304-8853/99/$ - see front matter ~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 85 3 ( 9 8 ) 0 0 8 0 6 - 3

Cz.

Kapusta, P.C. Riedi / Journal of Magnetism and Magnetic Materials 196-197 (1999) 446-450

with the applied field. The Mn 2÷ line shifts to higher frequencies which indicates that the M n 2 + moments are antiparallel to the bulk magnetisation. A single line found in the optimally doped compounds with 0.2 < x < 0.4 was attributed to a fast hoping of the carriers between the Mn 3+ and Mn 4+ sites, resulting from the double-exchange interaction [7]. The hopping time Zh, is much shorter than the period of the Larmor precession of the Mn nuclear spins, ZL, which is about 10 - 9 S, SO the nuclear spins see a 'motionally' averaged hyperfine field corresponding to an averaged Mn3+/Mn 4+ state. For a value of Zh comparable to or larger than rL the line broadens or, eventually, splits into separate Mn 3+ and Mn 4+ resonances as it is observed with decreasing the doping level x [7]. Such an effect of a line broadening related to the temperature variation of Zh has recently been observed for Pr0.,Bao.3MnO3 [13]. Also, doping of Fe for Mn in Lao.TPbo.3MnO3 as little as 3% already leads to distinct Mn 4+, Mn3+/Mn 4+ and Mn 3 + lines [10]. The temperature dependence of the N M R spectra up to the magnetic ordering temperature were reported for several La~_~Ca~MnO3 compounds with partial substitution of Tb and Y for La [14]. A single resonance line was observed for all compounds, including a semicond u c t i n g L a o . , ~ s T b o . 2 / C a o . 3 3 M n O 3 , which shows that the occurrence of the double-exchange interaction alone is not sufficient for the material to attain metallic conductivity. In view of these N M R results, the spin-glass state which was attributed to a competition of the F M and A F M interactions [5] could be described as a cluster glass, consisting of clusters of spins coupled by the double-exchange interaction. A striking result of Ref. [14] was that the 'magnetic signal' corresponding to a nonvanishing hyperfine field at the Mn nucleus existed for some of the Tb- and Y-doped compounds far above the spin-glass temperature or Curie temperature. The hyperfine field which reflects the magnitude of the local magnetisation decreased only slightly with temperature, keeping for some compounds more than 80% of its low temperature value at Tc or Tg (Fig. 1), whereas in normal ferromagnets it tends to zero on approaching Tc and no zero-field signal is observed above Tc. It indicated the presence of unusual magnetic correlations extending to the temperatures much higher than Tg or Tc and corresponding to the clusters of Mn spins coupled by the double-exchange interaction. The effect was attributed of the existence of magnetic polarons with the lifetime Zc greater than 10- 5 s which is a necessary condition to obtain the N M R spin-echo signal in a magnetic field produced by correlated magnetic moments. The changes of Be with temperature correspond to the temperature variation of the value of the manganese magnetic moment on the N M R time scale for #M~. The reduced values of Bo, B¢(T)/B,(4.2 K) [14], plotted as

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Fig. 1. The temperature dependence of the reduced hyperfine field Be(T)/Be(4.2K) for selected compounds (after Ref. 1-14]).

a function of the reduced temperature in Fig. 1, show that Be (#Mn) does not vanish at Tc and has nearly 90% of its low temperature value at Tc for L a o . 5 7 T b o . l C a o . 3 3 M n O 3 and at 3Tg for Lao.45Tbo.z2Cao.33MnO3. It is worth noting that the samples with lower magnetic ordering temperatures exhibit slightly faster decrease of Be (#Mn) with increasing temperature, indicating the smaller strength of the double exchange interaction. The key N M R parameters such as the linewidth, the spin-lattice relaxation time Ta and the spin-spin relaxation time Tz do not show any significant changes related to a critical behaviour on crossing Tc in contrast to the N M R signal intensity which approximately follows the temperature dependence of the bulk magnetisation indicating a first-order type of the transition in these compounds. However, in a recent paper a second-order transition has been deduced for Laa_xNaxMnO3 0.1 < x < 0.2 from a study of the N M R spectra and the relaxation time T2 [15]. Since these compounds have Tc near 300 K it can be inferred that the partial substitution of a rare earth element or yttrium for La in the optimally doped Laa-xCaxMnO3 or Lal-xSr~MnO3 which decreases the magnetic ordering temperature also changes the magnetic transition from second order to first order. A 55Mn N M R study of the antiferromagnetic Pro.67Cao.33MnO3 in the magnetic field has been carried out recently [16]. At zero field the compound is insulating and exhibits charge ordering [17]. In a magnetic field of 5 T it undergoes a first-order transition from the insulating to the metallic state [18]. The N M R spectrum of the zero-field-cooled sample at 4.2 K consists of three peaks at 320, 390 and 440 MHz which can be attributed to the M n 4 +, M n 3 +/Mn 4+ and M n 3 + sites, respectively (Fig. 2). The application of a magnetic field up to 5 T changes the relative intensities of the lines enhancing the Mna+/Mn 4+ line with respect to the Mn 3+ and the Mn 4+ lines which reflects a rise of the population of the

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Cz. Kapusta, P.C. Riedi / Journal oJ Magnetism and Magnetic' Materials 196-197 (1999) 446-450 m,

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Fig. 2. The SSMn spin-echo spectra of Pro.~,;Ca0.33MnO3 in applied magnetic field at 3 K.

Fig. 3. The 55Mn spin-echo spectra of LaMnO3 +,~corresponding to the antiferromagnetic regions in the applied magnetic field at3K.

DE-coupled Mn sites with the increasing magnetic field. Surprisingly, the lines shift only slightly to lower frequencies, an order of magnitude less than expected from the SSMn gyromagnetic ratio which indicates an unusual screening of the magnetic field in the charge ordered state. At a field 6 T a single-line spectrum exhibiting the full shift expected for the 55Mn gyromagnetic ratio and the Mn moments parallel to the magnetisation appears which indicates a transition to the DE-controlled metallic state. The single-line pattern persists after removal of the field (top spectrum) and after warming the sample up to 50 K which reflects a large hysteresis of the first-order M - I transition and a large stability of the magnetic field-induced metallic state. A systematic SSMn NMR study of LaMnO3+6 has been reported in Ref. [8]. The resonances corresponding to Mn in the AF- and FM-coupled regions have been observed. From the analysis of the spectra and the relaxation times a microscopic phase separation into AF and FM microdomains has been concluded. The resonance corresponding to the AF domains obtained for LaMnO3.o2 and LaMnO3.15 in Ref. [16] at 250260 MHz exhibits a shift in the applied field to higher frequencies about a half of that expected for the 55Mn gyromagnetic ratio, Fig. 3. LaMnO3.o2 is an antiferromagnetic insulator, whereas LaMnO3.15 was found to be a semiconducting ferromagnet [19]. Recently, a cluster glass behaviour was observed for the latter compound [4]. Following Ref. [8], this NMR signal can be

attributed to the AF domains aligning their magnetic moment directions perpendicular to the applied field, possibly with some canting in the applied field. The zero-field spectrum of LaMnO3.o2 and LaMnO3.1s at 300-500 MHz consists of three lines centred at 315, 380 and 430 MHz, Fig. 4. The lowermost one exhibits a full shift expected for the 55Mn gyromagnetic ratio in the magnetic field and is attributed to the Mn 4+ moments which are parallel to the bulk magnetisation. The 380MHz line, attributed to the Mn3+/Mn 4+ state, shows a much smaller shift, indicating that the DE coupled moments are not strictly parallel to the magnetisation. The uppermost line, attributed to the Mn 3 + sites shifts slightly to higher frequencies which indicates a predominantly antiparallel arrangement of the Mn 3" moments with respect to the magnetisation. Similarly to Pro.6~Cao.33MnO3, the population of the DE-coupled Mn3+/Mn ~+ ions rises with increasing magnetic field whereas the population of Mn 3+ and Mn 4+ ions falls. Very similar behaviour of the three line spectrum has been observed for LaMnO3.15, although the intensity of the spectrum was about seven times bigger than for LaMnO3.o2 indicating that the population of the Mn 3+ Mn3+/Mn 4+ and Mn 4+ states corresponding to these resonance lines is related to the number of cation vacancies created by the oxygen excess. The NMR experiments on 139La have been reported for Lal -xCaxMnO3 and LaMnO3 ~~ [20], Lal xNaxMnO3 [21], Lao.sCao.sMnO3 [22] and

Cz. Kapusta, P.C. Riedi /Journal of Magnetism and Magnetic Materials 196-197 (1999) 446-450 I

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Lao.TSr0.3MnO3 [23]. The zero-field NMR signal at 20-30 MHz corresponding to a positive hyperfine coupling was observed for the Lal-xCaxMnO3 and LaMnO3 +6 compounds including the antiferromagnetic ones [20]. This was attributed to the presence of FM regions also in the AF materials, earlier reported for La0.5Cao.5MnO3 [-22]. The measurement of the paramagnetic frequency shift of 139La in some Lal-xCaxMnO3 and LaMnO3+6 compounds [20] has shown that it follows the Curie-Weiss law. Its extrapolation to zero distinguished between the FM regions, which gave the intercept at a positive temperature, and the AF regions for which the intercept gave a negative temperature. Nuclear relaxation measurements gave an indication that the FM regions are influenced by diffusing, AF-correlated excitations, whereas the AF regions experience spin fluctuations from diffusing, FM-correlated excitations. Consistently with the Mn NMR results [8] an electronic phase separation connected with small magnetic JT polaron has been concluded to occur in these compounds. The STSr NMR spectrum has been reported for Lao.vSr0.3MnO3 [23]. The spectrum consists of a resonance line centred at 7 MHz of a 1 MHz linewidth. The shape of the line is asymmetric with a smaller slope towards high frequencies, similar to the La resonance. The corresponding B~ amounts to 3.8 T, slightly larger than for La where it equals 3.4 T. These values are more

449

than an order of magnitude larger than expected for the dipolar field and indicate a nonvanishing polarisation of electrons at the La and Sr sites. The existence of a spin polarised 5d band has been found in the recent X-MCD measurements of Lao.TSr0.3MnO3 [23]. The above discussion shows that NMR enables us to distinguish the ionisation states of manganese on the microscopic level and provide the information on the rate of carrier hopping between Mn sites. From measurements in an applied magnetic field the directions of magnetic moments corresponding to the individual Mn ionisation states with respect to the bulk magnetisation could be determined. It also made it possible to study the evolution of the populations of the individual ionisation states with applied field and showed an increase of the number of the DE coupled Mn ions at the expense of the static Mn 3+ and Mn 4+ states with the increasing field. The magnetic-field-induced first-order transition from the charge-ordered AF state to a metallic state in Pro.67Cao.33MnO3 could be observed at the atomic level and an unusual screening of the magnetic field in the charge ordered AF state has been found. Microregions of ferromagnetically and antiferromagnetically coupled Mn moments coexist in most of the investigated manganites. The FM regions are influenced by diffusing, AF correlated excitations, whereas the AF regions experience spin fluctuations from diffusing, FM correlated excitations. Double-exchange-coupled ferromagnetic clusters with a lifetime exceeding 10 5 s occur in the optimally doped compounds at temperatures considerably exceeding the magnetic ordering temperatures, thus indicating a first order type of magnetic ordering transition and the DE nature of the magnetic polarons here. This work was supported by the Engineering and Physical Sciences Research Council, U K and the State Committee for Scientific Research, Poland, Grant No. 2P03B-139-15. The authors gratefully acknowledge the collaboration with Prof. M.R. Ibarra, Prof. J.M.D. Coey, Dr. G.J. Tomka, Dr. J.M. De Teresa and the members of OXSEN group.

References [1] J.H. Van Santen, G.H. Jonker, Physica 16 (1950) 599. [2] S. Jin, H.M. O'Bryan, T.H. Tiefel, M. McCormack, W.W. Rhodes, Appl. Phys. Lett. 66 (1995) 382. [3] E.O. Wollan, W.C. Koehler, Phys. Rev. 100 (1955) 545. [4] C. Ritter, M.R. Ibarra, J.M. de Teresa, P.A. Algarabel, C. Marquina, J. Blasco, J. Gracia, S. Oseroff, S.-W. Cheong, Phys. Rev. B 56 (1997) 8902. [-5] A.P. Ramirez, J. Phys.: Condens. Matter 9 (1997) 8171. [6] M.R. Ibarra, J.M. De Teresa, J. Magn. Magn. Mater. 177-181 (1998) 846.

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[7] G. Matsumoto, J. Phys. Soc. Japan 29 (19701 615. [8] G. Allodi, R. de Renzi, G. Guidi, F. Licci, M.W. Pieper. Phys. Rev. B 56 (1997) 6036. [9] A. Anane, C. Dupas, K. Le Dang, J.P. Renard, P. Veillet, A.M. de Leon-Guevara, F. Millot, L. Pinsard, A.V. Revcolevschi, J. Phys.: Condens. Matter 7 (1995) 7015. [10] L.K. Leung, A.H. Morrish, Phys. Rev. B 15 (1977) 2485. [1 l] G.J. Tomka, P.C. Riedi, Cz. Kapusta, G. Balakrishnan, D.McK. Paul, M.R. Lees, J. Barrat, J. Appl, Phys. 83 (1998) 7151. [12] A.M. de Leon-Guevara, P. Berthet, F. Millot, A. Revcolevschi, A. Anane, C. Dupas, K. Le Dang, J.P. Renard, P, Veillet, Phys. Rev. B 56 (19971 6031. [13] M.M. Savosta, P. Novak, Z. Jirak, P. Hejtmanek, M, Marysko, Phys. Rev. Lett. 79 (1997) 4278. [14] Cz. Kapusta, P.C. Riedi, G.J. Tomka, W. Kocemba, MR. Ibarra, J.M. De Teresa, M. Viret, J.M.D. Coey, J. Phys.: Condens. Matter, submitted.

[15] M.M. Savosta, V.A. Borodin, P. Novak, Z. Jirak, P. Hejtmanek, M. Marysko, Phys. Rev. B 57 [1998) 13379. [16] Cz. Kapusta, P.C. Riedi et al., unpublished. [17] Z. Jirak, S. Krupicka, Z. Simsha, M. Dlouha, S. Vratislava, J. Magn. Magn. Mater. 53 (1985) 153. [18] Y. Tomioka, A, Asamitsu, Y. Moritomo, Y. Tokura, J. Phys. Soc Japan 64 (1995) 3626. [19] L. Ranno, M. Viret, A. Marl R.M. Thomas, J.M.I). Coey, J. Phys.: Condens. Matter 8 (1996) L33. [20] G. Allodi, R. de Renzi, G. Guidi, Phys. Rev. B 57 (1998) 1024. [21] M.K. Gubkhin, T.A. Khimich, E.V. Kleparskaya, T.M. Perekalina. A.V. Zalessky. J. Magn. Magn. Mater. 154 [1996) 351. [22] G. Papavassiliou, M. Fardis, A. Simopoulos, G. Kallias, M. Pissas, D. Niarchos, N. loanidis, C. Dimitropoulos, J. Dolinsek. Phys. Rev. B 55 (1997) 15000. [23] Cz. Kapusta, R. Mycielski, W. Kocemba, E. Goering, D. Ahlers, K. Attenkofer, P. Fischer, G. Schuetz. HASYLAB/DESY. Annual Report, 1996, p. 321.