The Theoretical And Experimental Investigation Of Hysteresis Loops And Anisotropy In The Ferromagnetic Films

Magnetoelectrical anisotropies of monolayer ferromagnetic films grown on different substrates along different directions and magnetic reversal processes of a FePt/a-Fe/FePt ferromagnetic trilayer system have been carefully investigated within experimental and theoretical approach, respectively.1.(110)-oriented manganite thin films are crucial for studies on interface effect concerning the injection of spin polarized electrons, especially the proximity effect with high temperature cuprate superconductors. However, due to the peculiarity of the (110) plane, atomically smooth surface generally can only be obtained at very low film thickness. Therefore anisotropy along the two crystallographically asymmetric [001] and [110] in-plane directions in ultra thin films are worthy of careful investigation. Here we report our study on the magnetoelectrical anisotropy of (110)-oriented epitaxial La2/3Ca1/3MnO3(LCMO) thin films of20nm thickness grown by pulsed laser depostition. Both SrTiO3(STO) and LaAlO3(LAO) substrates were adopted to highlight the lattice misfit strain e(?)ects. While in LCMO/STO films the magnetic easy axis is along [110] direction, in LCMO/LAO films [001] direction is easier, dominated by the misfit strain induced magnetoelastic anisotropy. Accordingly, the resistivity and magnetoresistance measured along the easy axes are lower than that along the hard axes. The correlation between the magnetic and transport anisotropy has been undoubtedly demonstrated. As a comparison, properties for the LCMO films on STO are free from small abnormal features, and thus are clear and easy understood, making STO a more appropriate substrate for interface studies in (110)-oriented heterostructures.2. Magnetic reversal processes of a FePt/a-Fe/FePt trilayer system with in-plane easy axes have been investigated within a micromagnetic approach. It is found that the magnetic reversal process consists of three steps:nucleation of a prototype of domain wall in the soft phase, the evolution as well as the motion of the domain wall from the soft to the hard phase and finally, the magnetic reversal of the hard phase. For small soft layer thickness Ls,the three steps are reduced to one single step, where the magnetizations in the two phases reverses simultaneously and the hysteresis loops are square with nucleation as the coercivity mechanism. As Ls increases, both nucleation and pinning fields decrease. In the meantime, the single-step reversal expands to a standard three-step one and the coercivity mechanism changes from nucleation to pinning. The critical thickness where the coercivity mechanism alters, could be derived analytically, which is found to be inversely proportional to the square root of the crystalline anisotropy of the hard phase. Such a scaling law might provide an easy way to test the present theory. Further increase of Ls leads to the change of the coercivity mechanism from pinning to nucleation.