Phase diagram of low doping manganites

Physica B 289}290 (2000) 85}88 Phase diagram of low doping manganites R. De Renzi!,*, G. Allodi!, G. Amoretti!, M. Cestelli Guidi!, S. Fanesi!, G. Guidi!, F. Licci", A. Caneiro#, F. Prado#, R. Sanchez#, S. Osero!$, A. Amato% !Dipartimento di Fisica e Unita% INFM, Viale delle Scienze, 7A, 43100 Parma, Italy "Istituto MASPEC-CNR, Viale delle Scienze 37A, 43100 Parma, Italy #Centro Atomico Bariloche, Bariloche, Argentina $San Diego State University, S. Diego, CA, USA %Paul Scherrer Institut, Villigen, Switzerland Abstract We present a study performed on several LaMnO compositions. Detailed analysis around x"0.09 detects phase 3`x@2 separation between a higher transition, ¹ , and a lower one ¹ , with coexisting #uctuating and magnetically ordered # N clusters (the Nèel point, ¹ , being the onset of an inhomogeneous non-collinear antiferromagnetic order detected by N neutrons). The ordered clusters give rise to an extremely fast, static Kubo}Toyabe muon depolarization, while mixed static and dynamic local "elds are detected inside the #uctuating clusters. Below ¹ precession frequency lines are N resolved, despite the huge broadening introduced by localized Mn4` ordered moments. A phase diagram is obtained and brie#y discussed in the context of electronic phase separation. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 75.30.Kz; 75.20.Hr; 76.75.#i; 75.30.Vm Keywords: Magnetism; Manganites; Spin-glass relaxation; Phase separation Low hole doping in insulating La manganites, which corresponds to a small fraction of Mn4` (3k ) replacing the dominant Mn3` (4k ), leads to B B the disappearance of antiferromagnetic order, to the appearance of a ferromagnetic state, and eventually to the insurgence of metallic properties [1]. In the intermediate insulating compositions neutron scattering detects two magnetic transitions above a certain threshold charge concentration. The two critical points are interpreted as a higher Curie-like temperature (although only a small fraction of the moment is seen, hence we refer to this as ¹ , for # critical) and a lower Nèel temperature, ¹ . The N * Corresponding author. Fax: #39-0521-905-223. E-mail address: roberto.derenzi@"s.unipr.it (R. De Renzi). magnetic behaviour at low-to-intermediate doping, where ferromagnetic and antiferromagnetic orders were long known to coexist [2], is of great interest for the understanding of a system in which polaronic and magnetic excitations are strongly coupled. The interplay of many competing interactions is predicted to produce electronic phase separation, which may explain the observed coexistence of different magnetic structures. Powders of LaMnO with x"0, 0.04, 0.08, 3`x@2 0.1, 0.12, 0.14 were prepared according to standard procedures (the chemical formula is a useful simpli"cation, since charge doping appears to be due to coupled cation vacancies, (LaMn) , [3], rather 1~x@6 than to oxygen excess). For the x"0.09 sample the Mn4` content, x, was determined by titration [4]. 0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 2 6 8 - 4 86 R.D. Renzi et al. / Physica B 289}290 (2000) 85}88 Two other batches of powders were obtained starting from a common x"0 specimen [5], by thermogravimetrically controlled annealing in an oxygen atmosphere. lSR experiments were carried out at PSI (Villigen, CH) on the GPS spectrometer and at ISIS (Chilton, UK) on the EMU and MUSR instruments. The latter is a pulsed muon facility, optimal for measurements at long times relative to muon implantation, but the time-width of the muon pulse sets an upper cut-o! on the observation of both high precession frequencies and fast decays (roughly 10 MHz and 20 ls~1, respectively). The complementarity of the two facilities is fully exploited with manganites, since fast and slow relaxing components often coexist. The end member LaMnO is a layer antifer3 romagnet, where the muon senses two local "elds, almost entirely of dipolar origin: their values at zero temperature are 0.9510(3) and 0.6290(2) T, the lower one corresponding to the stable muon site [6]. The precession frequencies in these two "elds may be observed at ¹/¹ "0.5 in the Fourier spectra of the N muon polarization in Fig. 1. The "gure also shows the spectra of x"0.04 and 0.08 samples: charge doping produces a very pronounced broadening of the frequency peaks. The inset illustrates the best "t of the x"0.04 data which shows how the precession pattern, observed only in the PSI data, decays after the "rst few hundred ns. The broadening is generically understood in terms of localized ordered Mn4` ions and of the Fig. 1. Fourier spectra at low temperature in LaMnO . 3`x@2 Inset: corresponding polarization "t at early times. Fig. 2. Fast Kubo}Toyabe component in the x"0.10 sample. The dashed line indicates the slow S component. The decay of polarization for t'50 ns is entirely due to the latter. consequent spin frozen static disorder. However, distinct frequency peaks may be observed only below a certain charge doping dependent temperature ¹ (x) } a single broad peak is detected below N 50 K even in a ferromagnetic La Ca MnO 0.85 0.15 3 sample. Hence, the detection of a non-zero average local "eld is characteristic of the low-temperature ordered phase detected by neutrons [7], in general an inhomogeneous, non-collinear magnetic structure, whose short-range nature goes from canted antiferromagnetic to canted ferromagnetic. Above ¹ (x)1 the precession pattern of Fig. 1, inset, canN not be distinguished any more and it is replaced by a very fast static Gaussian Kubo}Toyabe relaxation, such as that shown in Fig. 2. The plot shows the early time polarization } measured at PSI } for the x"0.1 sample at ¹"105 K'¹ "90 K. N A second feature is apparent: a sizeable fraction of the muon polarization, indicated by the dashed line, displays a much slower relaxation. We label it S (&slow', for the time being). Actually the transition to the paramagnetic state, at a higher temperature ¹ (x), is signalled by the disappearance of the KT # fraction and the attainment of the full asymmetry by the S fraction. The S fraction is easily measured at ISIS up to 14 ls after the muon implantation. Sample x"0.09 1 ¹ (x) should rather be called ¹@ for ferromagnetic samples N C like La Ca MnO . 0.85 0.15 3 R.D. Renzi et al. / Physica B 289}290 (2000) 85}88 was extensively studied to this purpose. Fig. 3a shows a comparison between the muon polarization above (open symbols) and below ("lled symbols) ¹ "135 K. Here the fast initial KT # depolarization is seen as a missing fraction, due to the ISIS high frequency cut-o!. The S fraction is not a normal paramagnetic fraction, as it could be due to a distribution of ¹ values from local inhomogeneities in the cation N composition. Its relaxation has two time regimes and the initial faster one is of static origin, as it is proven in Fig. 3b by the progressive quenching e!ect of moderate longitudinal applied magnetic "elds. The solid lines in these plots, as well as those in panel (a), come from two component "ts, where the S fraction is adjusted to the spin-glass relaxation function described in details in Ref. [8]. It is an extension of the Uemura [9] model, with the following zero "eld expression: A G B a2t2 1 4 G(a , j, b; t)" e~(jt)b#2 1! 4 [a2t2/2b#jt]1~b 3 4 b a2t2 , (1) exp ! 4 #jt 2b A C D BH 87 where j is the dynamical relaxation rate, a the 4 width of the static "eld distribution, and b gives the shape of the "eld distribution (b"1 for a Gaussian "eld distribution from concentrated magnetic moments; b"0.5 for a Lorentzian "eld distribution, from diluted moments). Up to longitudinal "elds of 50 mT all "ts converge with the same values of the parameters b, j, a , as well as the S and 4 KT signal amplitudes, A . This holds true in the S,KT entire interval ¹ (¹(¹ . N # The other two panels in Fig. 3 show the temperature dependence of two "t parameters: in panel (c), the relative weights, w "A /(A #A ) and S S S KT w "A /(A #A ), of the S and of the fast KT KT S KT Kubo}Toyabe fractions; in panel (d), the width of the internal "eld distribution inside S (a is 2pc 4 k times the second moment *B). This width develops as an order parameter, while the weight transfer from w to w indicates a corresponding build up S KT of the KT volume fraction, as in a "rst-order nucleation process. The magnitude of a (¹) } a hun4 dred Oe, in "eld units } proves the electronic origin of the static disordered magnetic "elds in the S volume fraction, but it is much smaller than the Fig. 3. Sample x"0.09: (a) P (t) at ¹"140 K'¹ and at ¹"130 K (¹ . The missing signal (2 KT) is the fast relaxing part of k # # 3 a Kubo}Toyabe relaxation. (b) Field decoupling of the static part of the S relaxation. At 200 mT also the fast KT fraction begins to show the decoupling action of the "eld. (c) Temperature dependence of the two fractions, w and w . (d) Temperature dependence of the KT S internal frequency width of the S fraction, responsible for the static initial relaxation of panel (b). 88 R.D. Renzi et al. / Physica B 289}290 (2000) 85}88 Fig. 4. The phase diagram } ¹ (triangles), ¹ (circles) } # N includes LaMnO data ("lled symbols) and Ca-doped single 3`x@2 crystal data (open symbols). Lines are guides to the eye. magnitude of the KT width * } a few kOe, again in "eld units } which is clearly due to local ordered Mn moments. In our view a possible explanation of the S internal "elds is that those Mn moments which are immediate neighbours of the muon are #uctuating, and the static dipolar "eld is due to close enough clusters of ordered &frozen' moments. This may happen, for instance, if the KT fraction does actually nucleate in nanoclusters inside the S fraction. We are therefore led to propose a superparamagnetic picture for the phase between ¹ (x) and # ¹ (x): S clusters of #uctuating moments coexist N with KT clusters of already blocked moments. The nucleation process is possibly driven by local #uctuations of the density of carriers, as in electronic phase separations [10,11]. A similar lSR analysis holds for all the measured doping concentrations and also for two La Ca 1~x x MnO single crystals [6]. The two transition tem3 peratures thus identi"ed } ¹ , quite precisely mea# sured from the development of a fast KT fraction, and ¹ , more loosely determined by the appearN ance of resolved frequency lines } map a phase diagram inside the macroscopically insulating phase, which is shown in Fig. 4. In conclusion, we have shown that implanted muons independently detect an intermediate phase in low doping manganites, mapping its phase diagram. They further indicate that this may be a region where electronic phase separation is realized. Discussions with C. Bucci are gratefully acknowledged. The work of the Parma group was partially supported by a MURST Co"n '97 grant on &Polaroni magnetici nelle manganiti'. References [1] A. Millis, Nature 392 (1998) 147. [2] E.O. Wollan, W.C. Koehler, Phys. Rev. 100 (1955) 545. [3] J.A.M. Roosmalen, E.H.P. Cordfunke, R.B. Helmolt, J. Solid State Chem. 110 (1994) 100. [4] F. Licci, G. Turilli, P. Ferro, J. Magn. Magn. Mater. 164 (1996) L268. [5] M. Causa et al., J. Magn. Magn. Mater. 196}197 (1999) 506. [6] R. De Renzi et al., Physica B 289}290 (2000), these proceedings. [7] M. Hennion et al., Phys. Rev. Lett. 81 (1998) 1957. [8] R. De Renzi, S. Fanesi, Physica B 289}290 (2000), these proceedings. [9] Y.J. Uemura et al., Phys. Rev. B 31 (1985) 546. [10] V.J. Emery, S.A. Kivelson, Physica C 209 (1993) 597. [11] S. Yunoki, A. Moreo, E. Dagotto, Phys. Rev. Lett. 81 (1998) 5612.