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A MONOCLINIC MANGANITE, La_0.9Mn0_3-δ, WITH COLOSSAL MAGNETORESISTANCE PROPERTIES NEAR ROOM TEMPERATURE

A. Maignan, C. Michel, M. Hervieu and B. Raveau
Laboratoire CRISMAT, URA 1318 associd au CNRS, ISMRA et Universite de Caen, 6,
Boulevard du Marechal Juin, 14050 Caen, Cedex, France (Received 1 August 1996 by D. Van Dyck)

The investigation of the La-Mn-O system has allowed manganites La0.90MnO3_ with colossal magnetoresistance properties, near room temperature, to be isolated. These, phases exhibit indeed a ferromagnetic metallic to paramagnetic insulating transition, with Tc ranging from 230 K to 260 K, depending on the hole carrier density ( = 0.05-0.06). A resistance ratio of 6 in a magnetic field of 7 T is observed near room temperature (260 K) for the first time in the La-Mn-O system. The CMR La0.9MnO3_ phases, can be indexed in a classical rhombohedrai cell, but their electron diffraction and X-ray powder srudyo shows that thejr symmetry isa monoclinic 121 a with d = 7. 790(1) A, b = 5.526(1) A, c = 5.479(1) A, 0 = 90.78°. Decreasing slightly, the oxygen content leads to orthorhombic phases and kills the CMR properties. Copyright © 1996 Elsevier Science Ltd

     The lanthanum manganite LaMnO_{3^_r} has been the object of numerous studies after the pioneering work on the magnetic order in this compound performed by Wollan and Koehler [1-2]. "LaMnO₃", that exhibits a Neel temperature ranging from 100 K to 141 K [1, 3, 4] has been used as a model for the study of the relationship between transport properties and magnetism in such material [5, 6].
     The discovery of colossal magnetoresistance (CMR) properties in manganese oxides with the perovskite structure [7] has renewed the interest of the scientific community for the La-Mn-O system. In a recent study, C.N.R. Rao et at. [8] have evidenced CMR properties for the perovskite La_0.96Mn_0.96O₃. The latter indeed exhibits a Curie temperature of 200 K, with a resistance ratio smaller than two. In fact, the crystal chemistry of the "La-Mn-O" perovskites is much more complex than expected forty years ago. Several recent studies [8-10] show that there exist at least four forms La_1-xMnO_3+y — two orthorhombic, one rhombohedral and one cubic forms — depending on the oxygen stoichiometryr but also on the cationic deficiency.
     Taking into consideration the previous results, it appears clearly that it should be possible to enhance the CMR properties of the "LaMnO" perovskites, by varying the hole carrier density, i.e. by changing the: Mn(III) ratio. For this reason we have explored the lanthanum deficient manganites. We report herein on a new CMR perovskite La_0.90MnO_3- with a monoclinic symmetry, that exhibits a T_c of 260 K and a magnetoresistance ratio of six, in a magnetic field of 7 T at this temperature.
     In order to control the mixed valence Mn(III)-Mn(IV), we have tried to synthesize the ideal compound La_0.9MnO₃, which should exhibit the same Mn(III) : Mn(IV) ratio as the manganite La_0.7Sr_0.3MnO₃, where T_c reaches 369 K [11]. (Nevertheless, in the case of the La-Mn-O system, the oxidation of Mn(III) into Mn(IV) is difficult to optimize, in contrast to the La-Sr-Mn-O system in which the "O₃" stoichiometry is easily reached. The method of synthesis — temperature, oxygen pressure and cooling speed — influences dramatically the nature of the so obtained perovskite. We present here, the results acquired for four different La_0.9MnO₃ samples, that were all prepared from the same corresponding mixture of La₂O₃ and Mn₂O₃ in the ratio La:Mn = 0.9 calcined at 950°C for 12 h, reground and pressed in the form of bars of 2 x 2 x 10 mm3. The same heating speed of 300°Ch~l was used for their synthesis. Samples A₁ and B₁ were heated at 1300°C and 1400°C for 12 h respectively, then cooled at 30O°Ch~l down to 800°C and quenched from 800°C to room temperature. Samples A₂ and B₂ were heated at 1300°C and 1400°C for 12 h respectively, cooled at 300°Ch-l down to 800°C; then annealed at 1000°C for 12 h and slowly cooled at 5°Ch-1 down to room temperature. All the thermal treatments were performed in air.

     The cationic composition of the bars, at the end of the thermal treatments, determined by TEM coupled with EDS analysis was found to be identical to that of the nominal one before heating. The X-ray analysis performed with a Philips diffractometer with CuKa radiation shows that three'of these samples — A₁, A₂ and B₂ — exhibit practically identical X-ray powder patterns. Figure 1(a) shows one of these patterns obtained for A₂. Such a pattern can be indexed in the classical rhombohedral cell, i.e. a = 5.479(1)A, a = 60.61°. However the investigations of this phase by electron diffraction shows that there exist additional weak reflexions that make that its actual symmetry is not rhombohedral R3c, but monoclinic I2/a. The X-ray powder pattern was then indexed with the following parameters a = 7.790(1) A, b = 5.526(1) A, c = 5.479(1) A, 0 = 90.78(1)°. In order to check the validity of this indexation, a structural study was performed. For this study, Mn₃O₄, whose main diffraction peak (20-36.1°) was visible on the XRD pattern [Fig. l(a)J, was introduced as secondary phase in the calculations. Location of the different atoms in the crystallographic sites of the space group I2/a was: Mn in 4a (000), La in 4e (1/4, y, 0) with y ~ 1/2, O(l) in 4e with y = 0 and O(2) in 8f(xyz) with x = 0, y - 1/4, z-1/4. Only isotropic thermal factors were considered, those of oxygens being fixed to 1.0 A₂. The sample was assumed to be oxygen stoichiometric. Refinement of the positional parameters, B factors for lanthanum and manganese and occupancy of the La site allowed the R_p and R_i, agreement factors to be lowered to 15.2% and 7.2% respectively. The different refined parameters are given in Table 1, the experimental and calculated XRD patterns are plotted in Fig. 1(a). It is interesting to note that the refined occupancy of the lanthanum crystallographic site leads to the formula La_0.91(1)MnO₃, i.e. very close to the nominal composition.
     The interatomic bond distances are given in Table 2. In spite of the lack of accuracy in the oxygen location, the distances are quite comparable with those observed in other variants of La_1-xMnO₃ series (see for example [9]).
    The difference between the cell parameters of the three samples — A₁,A₂ and B₂ — though it exists, is very small, i.e. less than l%o. This small difference is in agreement with the oxygen content of each of these samples, determined by microthermograyimetric analysis, which varies only slightly with the thermal treatment, according to the formulation La_0.9MnO_3-, 5 ranging from 0.05 to 0.06.
     In contrast, the sample B₁ exhibits a significantly different X-ray pattern [Fig. 1(b)]. The latter was indexed in an orthorhombic cell with the following parameters a = 5.644(1)A, 6 = 7.724(1)A, c = 5.529(1)A. This difference is also in agreement with the microthermogravimetric measurements that show a higher 5 value of 0.10.
     The d.c. resistivity measured between 4.2K and 300 K, in the absence of a magnetic field is presented on Fig. 2 for these four samples. The examination of this figure allows the following features to be emphasized:
          (i) -The three monoclinic samples, A₁,A₂ and B₂ display a transition from a metallic or semimetallic to a semiconducting state, characteristic of the CMR perovskite.
          (ii) The transition temperature T_max, that corresponds to Tc, is significantly higher than that observed for La_1-xMn_1-xO₃. One indeed obtains T_max values ranging from 230 K to 260 K, whereas the highest value observed for La_1-xMn_1-xO₃ was of 200 K for x = 0.04 [8]. This effect of the lanthanum deficiency upon the increase of T_max is in agreement with the neutron diffraction data of the rhombohedral phase La_0.88MnO_2.92, for which a T_c of 248 K was evidenced recently [9].
          (iii) The variation of T_c between the three monoclinic samples A₁, A₂ and B₂ from 230K to 260K is easily explained in termS Of hole doping. The Sampfe A₁ that exhibits the highest T_c (260 K) should display a nearly optimized Mn(IV)/Mn(III) ratio, whereas the A₂ sample that has been annealed at lower temperature, may contain more oxygen and consequently is overdoped with respect to A₁ leading to a decrease of Tc (250 K). In a similar way, the B₂ sample that has been synthesized at higher temperature (1400°C), although annealed at 1000°C, has not been fully reoxygenated and consequently may be underdoped with respect tO A₁, leading to a lower T_c (230 K). In any case, it clearly appears that T_c is very sensitive to the oxygen stoichiometry in agreement with the previous results showing the strong influence of the Mn(IV)/Mn(III) ratio upon Tc for the manganites Pr_1-x(Ca,Sr)_xMnO₃ [12]. The variation of oxygen stoichiometry that induces such a variation of TC is of course, too low to be detected by microthermogravimetry.
          (iv) The orthorhombic sample B₁ is a semiconductor, in agreement with its lower oxygen content, showing its significantly undoped character with respect to the monoclinic samples.
     The magnetization curves versus temperature, registered with a SQUID magnetometer in a magnetic field of 1.45T, for the monoclinic zero field cooled samples show that these phases exhibit a clear transition from a ferromagnetic to a paramagnetic state as T increases. This behaviour is illustrated in Fig. 3 for the samples .4 1 and A₂; one observes that Tcs defined as the inflexion point of the transition coincides with T_max and that the saturation magnetization (M_s) ranges from 3.5 B to 4.0B, idicating that these materials are good ferromagnets. In contrast, the B₁ sample was found to be a poor ferromagnet with M_s = 1 B and TC = 130 K.
    The transition from the ferromagnetic metallic state to the paramagnetic insulating state observed for the monoclinic samples clearly indicates that these materials are magnetoresistant. The resistivity curves, registered in a magnetic field of 7T confirm this viewpoint as shown in Fig. 4 for the A₁ and A₂ samples. One indeed observes resistance ratios close to 6 at 250-260 K, in a magnetic field of 7T, which are the highest values that have been obtained to date, near room temperature in the La-Mn-O system. On the opposite, no CMR effect was observed for the orthorhombic samples.
    In conclusion, this study shows the important role of the lanthanum deficiency in the La_1-xMnO_3- system to optimize, the CMR properties of those perovskites, taking into consideration the oxygen stoichiometry. It appears clearly that T_c, but also the resistance ratio are very sensitive to the hole carrier density so that the TC of 260 K and the resistance ratio of six, may be still improved by varying the experimental parameters. In any case, the non stoichiometric La_0.9MnO_3- is more promising that the LaMnO_3+x phase, for which a maximum T_c of 200 K could be achieved [8]. There is no doubt that the orthorhombic forms do not exhibit CMR properties, in agreement with the previous studies [8]. The issue of the rhombohedrai or monoclfnic symmetry in all the compounds of the system La-Mn-O has to be studied carefully. A neutron diffraction study coupled with electron microscopy investigation will be necessary.

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