Ultrafast Laser Induced Conductive and Resistive Transients inLa0.7Ca0.3MnO3

Abstract

Pulsed laser excitation induced conductance changes in colossal magnetoresistance material La 0.7 Ca 0.3 MnO 3 were studied on the picosecond time scale. A two-component signal was seen consisting of a fast positive transient associated with the paramagnetic insulating state and a slower negative signal associated with the ferromagnetic metallic state. The fast component corresponds to the photoionization of the Jahn-Teller small polaron. The slow component is explained in terms of the reduced carrier mobility due to photogenerated magnetic excitations. [S0031-9007(98)06798-2] PACS numbers: 75.70. -i, 78.47. + pThe observation of colossal magnetoresistance (CMR) in the doped rare earth manganites has stimulated research on these strongly coupled metallic oxide materials [1][2][3]. It is now accepted that both double exchange [5] and Jahn-Teller lattice distortions [6] play an important role in the magnetic and transport properties of the manganites. However, the phase diagram of the manganites has a number of intriguing features [4]. The ferromagnetic order in the manganites does not have a simple dependence on the Mn 31 ͞Mn 41 ratio. The ferromagnetic Curie temperature ͑T C ͒ in La 12x Ca x MnO 3 , for example, peaks close to 30% Ca 21 (corresponding to 30% Mn 41 ) concentration rather than 50% Ca 21 concentration. Further, as one enhances the Ca 21 concentration in the system, disruption of the ferromagnetic order is observed. In fact, for .50% of the Ca 21 concentration no ferromagnetic phase is seen. Also the propensity of the system to form charge ordered insulator also increases with x. These features show that spin, charge, and lattice are strongly coupled in the CMR materials [5][6][7][8].In this paper, we describe the transient transport response of La 0.7 Ca 0.3 MnO 3 upon excitation with a 100 fs laser pulse. If charge or spin excitations are introduced via pulsed photoexcitation, and the change in conductance of the material is monitored, important information about the interactions among spin, charge, and lattice, and also the relaxation processes of the nonequilibrium state may be obtained on these CMR materials. There have been some studies in the past on the picosecond spin dynamics in magnetic systems [9][10][11][12]. For example, the study by Awschalom and Halbout [9] on magnetic polaron dynamics in a dilute magnetic semiconductor Cd 12x Mn x Te showed that the spin organization and relaxation times are hundreds of picoseconds. To our knowledge, no picosecond dynamical study on CMR materials has yet been reported in the literature.The epitaxial La 0.7 Ca 0.3 MnO 3 (LCMO) films used in the study were prepared by the pulsed laser deposition process on LaAlO 3 substrates. The details of the deposition and annealing process are described elsewhere [13] and the temperature dependence of the electrical re

Electron–Phonon Coupling Assisted Emission in Single Magnetic La<SUB>0.67</SUB>Ca<SUB>0.33</SUB>MnO<SUB>3</SUB> Domains of Thin Nanoplates

“…19 The higher energy band at 652.6 nm is attributed to the transition, determined by the crystal field energy E C , between a t 2g core electron of Mn 3+ to the spin-up e g bands of Mn 4+ by a dipole allowed charge transfer process, the 700.5 nm band is assigned to the transition between the spin-up and spin-down e g bands separated by the Hund's coupling energy E J , while the 789.8 nm band is identified as the charge transfer excitation band of an electron from the lower J-T split e g level of a Mn 3+ ion to the e g level of an adjacent Mn 4+ ion. A basic electronic structure of the e g and t 2g levels of Mn 3+ and Mn 4+ as proposed by Zhao et al 24 from a pulsed laser excitation induced conductance in bulk La 0 7 Ca 0 3 MnO 3 is followed. Evidently, a markedly modified emission arises with e-p coupling in single domain La 0 67 Ca 0 33 MnO 3 nanoplates as follows.…”

Influence of film thickness on the physical properties of manganite heterojunctions

“…Take the kinetic/ dynamic behavior of the charge carriers as an example. 12,13 When the diffusion distance of the photocarriers is comparable or smaller than film thickness, size effect may occur, resulting in dramatic changes in the photovoltaic properties of the junctions. As we know, the injection, drift and diffusion of charge carriers are important topics of electronics.…”

Photoinduced coherent magnetization precession in epitaxialLa0.67Ca0.33MnO3films

“…The study of the picosecond magnetization dynamics of manganites was reported by Zhao et al, 13 who monitored the optically induced conductance changes in LCMO films with 20-ps time resolution. Long-lived spin excitations were found responsible for a resistivity increase in the ferromagnetic phase.…”