The local atomic structure of La 12x Ca x MnO 3 (x 0.12, 0.21, and 0.25) has been studied using pairdistribution-function analysis of neutron powder-diffraction data. A change is seen in the local structure which can be correlated with the metal-insulator transition in the x 0.21 and 0.25 samples. This local structural change is modeled as an isotropic collapse of oxygen towards Mn of magnitude d 0.12 Å occurring on one in four Mn sites. We argue that this is the direct observation of lattice polaron formation, associated with the metal-insulator transition in these materials. [S0031-9007(96)00719-3] PACS numbers: 71.30.+h, 71.38.+i, 72.80.Ga The discovery in the 1950s of ferromagnetism coupled with a metal-insulator (MI) transition in the class of materials La 12x A x MnO 3 , where A is a divalent ion [1], led to a theoretical explanation which involved a new type of exchange interaction called double exchange (DE) [2]. There has recently been a resurgence of interest in these materials for two reasons. First, a very large magnetoresistance, first observed by Volger [3], was rediscovered in thin films of La 0.67 Ba 0.33 MnO 3 [4], and also in a variety of thin films and bulk samples with similar chemical compositions [5,6]. This raises the possibility of using these materials in magnetic field sensors and magnetic read-write heads. Second, a question has been raised as to whether double exchange is the only physics governing the properties of these materials [7]. Millis et al. [7] argue that, to explain the experimental observations, an additional effect must be present to reduce the electron kinetic energy at the MI transition. A number of authors have proposed that it is a polaronic mechanism, either magnetic [6,8] or lattice [7,9,10] which is important. The presence of Jahn-Teller effects in these materials also is expected to enhance polaron formation [7,11]. It is clearly necessary to establish the presence of polarons experimentally and to characterize them. The presence of lattice polarons can be inferred indirectly from changes in refined thermal parameters [7]. However, except in the special cases where they are ordered over long range, a local structural probe is required to analyze them directly.We have used the atomic-pair distribution function (PDF) analysis of neutron powder-diffraction data to study the local structure of La 12x Ca x MnO 3 for x 0.12, 0.21, and 0.25 as a function of temperature. This enables us to look for a local structural response to the metal-insulator transition indicating the formation of randomly distributed lattice polarons. We recover experimentally the real-space pair correlation function which shows the relative positions of atoms in the structure as a function of distance. This approach has been shown to yield quantitative local-structural information as a function of distance in well-ordered materials [12,13] as well as in its traditional application to glasses and amorphous materials [14]. The approach has been described in detail elsewhere [12,15]. The temperatur...

Measured distortions of the Mn-O bond length distribution from x-ray-absorption fine-structure measurements are found to relate linearly to the doped hole concentration x at room temperature in La 12x Ca x MnO 3. Comparison of the distortions above and below T c for colossal magnetoresistor (CMR) samples gives an estimate of the number of delocalized holes n dh , and we find that ln͑n dh ͒~M (magnetization). These results are complementary to resistance measurements that show that ln͑r͒~2M. We have thus established the functional relationship between the electronic, spin, and lattice degrees of freedom in the CMR perovskites. [S0031-9007(97)05093-X]

No abstract

No abstract

The inhuence of hydrostatic pressure on the electrical resistivity p and ferromagnetic transition temperature T, of La& Ca"Mn03 is presented for x=0.21, 0.33, and 0.40. Pressure strongly decreases p and drives T, up at rates as large as +37 K/GPa for the x=0.21 specimen; this is significantly larger than dT, /dP values observed in conventional ferromagnets and about eight times larger than d Tz /dP observed in Ca& Sr Mn03 antiferromagnets. The transport mechanism in these oxides is influenced in a qualitatively similar way by both pressure and magnetic field. The results are interpreted in the framework of the doubleexchange interaction involving conduction via magnetic polarons.