Percolative transition on ferromagnetic insulator manganites: Uncorrelated to correlated polaron clusters

Abstract

We report an atomic-scale study on the ferromagnetic insulator manganite LaMnO 3.12 using ␥-␥ perturbed angular correlation spectroscopy. Data analysis reveals a nanoscopic transition from an undistorted to a JahnTeller ͑JT͒ distorted local environment upon cooling. The percolation thresholds of the two local environments enclose a macroscopic structural transition ͑rhombohedral-orthorhombic͒. Two distinct regimes of JT distortions were found: a high-temperature regime where uncorrelated polaron clusters with severe distortions of the Mn 3+ O 6 octahedra survive up to T Ϸ 800 K and a low-temperature regime where correlated regions have a weaker JT-distorted symmetry. DOI: 10.1103/PhysRevB.73.100408 PACS number͑s͒: 75.47.Lx, 31.30.Gs, 64.60.Ak, 76.80.ϩy Intense experimental and theoretical work has been devoted to manganite systems due to their colossal magnetoresistance ͑CMR͒, polaron dynamics, and charge-orbital ordering phenomena. The undoped manganites ͑AMnO 3 where A is a trivalent ion of La, Pr, . . .͒ typically show antiferromagnetic insulator behavior and cooperative Jahn-Teller ͑JT͒ distortion of MnO 6 octahedra. Oxygen excess or the presence of divalent ions at A sites reduce the static JT distortion by the creation of Mn 4+ ions. This effect favors the ferromagnetic interaction via dynamic electron transfer between Mn 3+ and Mn 4+ , the so-called double-exchange ͑DE͒ interaction. 1 Although DE interaction explains qualitatively the CMR, it does not fully account for the large resistivity of the paramagnetic and ferromagnetic insulator phases. Polaron formation must certainly play an important role in this respect. [2][3][4][5] Polarons are formed due to the strong electron-lattice coupling that leads to charge localization via JT distortions. Recently, the nature of such local distortions, their dynamics and correlations have been addressed by several authors. [6][7][8][9][10][11] In spite of such an effort, several issues as the detailed structure of polarons, the temperature evolution of polaron clusters, or the effect of such evolution on the average macroscopic lattice structure still remain as open questions.Local distortions and their dynamics can be studied by using ␥-␥ perturbed angular correlation spectroscopy ͑PAC͒, a nuclear hyperfine method specially effective to sample atomic-scale environments. PAC efficiency is T independent, allowing us to explore a wide range of temperatures. To gain further insight on the microscopic nature of polaronic distortions, their spatial correlations, and the role of polarons in ferromagnetic insulator manganites ͑FMI͒, we have studied in detail the compound LaMnO 3.12 using the PAC technique. This compound is a prototypical FMI manganite that undergoes a rhombohedral ͑R͒-orthorhombic ͑O͒ structural transition around room temperature, which provides us with an ideal scenario to probe the evolution of local lattice distortions through different average lattice symmetries. In particular, we show that random distributed polaron clusters survive in the undist...

New Phase Transition in thePr1−xCaxMnO3System: Evidence for Electrical Polarization in Charge Ordered Manganites

“…For all compositions the system presents an orthorhombic distorted perovskite structure and CO is stabilized over a broad range of compositions, with x extending from 0.30 to 0.90, in good agreement with the literature [16 -18]. Using the 111m Cd -perturbed angular correlation technique (PAC) [19] the measurement of the EFG tensor at the Ca= Pr site was performed in a series of samples ranging from x 0-1, in the 10 -1000 K temperature range (for typical experimental details see [15]). The samples under study were implanted at ISOLDE/CERN with 111m Cd to a dose lower than 1 ppm of the Pr =Ca concentration.…”

“…This is connected to the relatively high conductivity of these materials, and to the possibility that the suspected electric dipole order may only occur within nanoscopic regions. However, a very recent work of Jooss et al [13] provides, by refinements of electron diffraction microscopy data, indirect evidence for canted antiferroelectricity in Pr 0:68 Ca 0:32 MnO 3 .The measurement of the electric field gradient tensor (EFG) via hyperfine techniques offers a very sensitive tool to locally study phase transitions and probe electric ordering [14,15]. Aiming to get further insight on the microscopic nature of CO we performed a detailed study on the EFG temperature trends across the Pr 1ÿx Ca x MnO 3 phase diagram.…”

“…The measurement of the electric field gradient tensor (EFG) via hyperfine techniques offers a very sensitive tool to locally study phase transitions and probe electric ordering [14,15]. Aiming to get further insight on the microscopic nature of CO we performed a detailed study on the EFG temperature trends across the Pr 1ÿx Ca x MnO 3 phase diagram.…”

Perturbed angular correlations investigations on YMnO3 multiferroic manganite

“…Based on XRD and magnetic measurements, also performed after the PAC experiments, the high quality of the samples show that the origin of this EFG b also cannot be attributed to impurities. Thus the origin of the higher V zzb local environment is not clear and do not seem related to the quality of the samples nor defects created by the implantation process since previous studies showed that the present annealing was sufficient to recover from implantation defects [15,16]. The existence of this second EFG distribution seems to be intrinsic to this system and has to be found in more subtle grounds.…”

“…The EFG principal component, V zz , shows the typical decrease on its magnitude with increasing temperature for both EFG a and EFG b . This trend was found in perovskite systems [15,16] and is frequently associated to an increase of the atomic vibrations due to the increase of temperature. The asymmetry parameter, η, is also constant with temperature, with small variations within the error bars for both EFG distributions.…”