Επίδραση των ελαστικών τάσεων επιταξίας στο μηχανισμό της μαγνητικής πόλωσης ανταλλαγής της πολυστρωματικής δομής [La2/3Ca1/3MnO3/La1/3Ca2/3MnO3]15

Abstract

The exchange bias (EB) phenomenon has received considerable attention because of its important applications in magnetic storage devices. Of particular interest are the EB properties of colossal magnetoresistance (CMR) compositionally modulated structures consisting of antiferromagnetic (AF) and ferromagnetic (FM) (La,Ca)MnO3 layers because manganites are strongly correlated electron systems, in which the magnetic, electronic and crystal structures interact strongly with each other. Complementary x-ray synchrotron radiation diffraction (XRD) and resonant scattering (RXS) measurements were performed at the Mn K-edge between 10 and 300 K in order to analyze the effect of epitaxial strain and tetragonal lattice distortions on the exchange bias (EB) mechanism observed in [La2/3Ca1/3MnO3(FM)/La1/3Ca2/3MnO3(AF)]15 multilayers below a blocking temperature, TB, of 80 K. XRD measurements showed that the c/a axial ratio, an indication of the tetragonal lattice distortion in pseudocubic lattice settings, reaches its maximum at the onset of the EB effect and the corresponding structural correlation length varies substantially at the onset of TB. The in-plane lattice parameter a at TB is close to the bulk lattice parameters of the AF layers, thus indicating that the EB effect is related with the accommodation of strain inside the FM and AF layers. RXS measurements revealed that such anisotropic lattice strains force the MnO6 octahedral sites into a ferrodistortive (FD) alignment, where the FD supercell coincides with the unit cell of the crystalline lattice. The RXS intensity difference signal exhibits a main-edge feature and a post-edge feature at 6.57 keV that scales linearly with temperature and the c/a ratio up to 80 K. The disappearance of the post-edge feature above 80 K(=TB) may signify a rearrangement of Mn-O-Mn bonding angles due to strain-driven effects at the FM/AF interfaces, inducing disorder in FD octahedral tilt ordering which may pin the local distortions below the TB.