Mechanisms of anisotropy control by strain in FePt/BaTiO3

L.B. STEREN,; M RODRIGUEZ; M. CABABIE; A. LOPEZ PEDROSO ; L. PIETRASANTA; D. RUBI; M. SIRENA; J. GOMEZ; A. BUTERA

Glasgow

Conferencia; 8th Joint European Magnetic Symposia; 2016

Nowadays, the design and fabrication of multiferroics (MF) is one of the main challenges for the development ofoxide spintronics. Tuning the magnetic state of nanostructures by electric field or strainsappears to be the key for low-energy devices. Magneto-electric andmagneto-elastic coupling at interfaces between ferromagnetic (FM) andferroelectric (FE) layers are at the origin of these phenomena. Both, themagnetic order and anisotropy of the FM component can be affected by thestrains induced by the FM/FE lattice mismatch while charge transfer betweenboth compounds can depend on the ferroelectric polarization of the structure. Avariety of FM and FE have been combined into artificial MF, being the magneticcompounds generally oxides or transition metal alloys.                In particular, FePt/BaTiO3seems to be a very promising system. First-principle calculations revealedrecently that important changes of magnetic anisotropy should be observed onthe FePt overlayer when the BaTiO3 is poled.  The FePtdisordered phase is a relatively soft magnetic material with an out-of-planestrain-induced anisotropy. Above a critical thickness, tc,the orientation of the magnetization in the magnetic domains changes fromin-plane alignment to a system of stripes in which a component perpendicular tothe film plane points alternately in opposite directions. Based on theseproperties we decided to investigate the structure FePt/BaTiO3 inorder to develop a new multifunctional material.  We do expect strain-inducedeffects originated at the BaTiO3 underlayer onto the magneticproperties of the FePt. On the one hand, there is a large lattice mismatch, ç~8%,  betweenFePt and BaTiO3 lattices and it has been shown in previous worksthat FePt magnetic properties are very sensitive to substrate-induced strains. On the other hand, we intend to observe more subtle effects in the FePt magnetizationas the BaTiO3 overcomes different structural phase transitions belowroom temperature.  To achieve these goals,   FePt(tFePt)/BaTiO3bilayers with 10nm≤tFePt≤60nm were fabricated by sputteringand pulsed-laser deposition techniques. X-ray diffraction patterns show thatboth components grow textured. The magnetization of FePt films is in the planeof the layers for thicknesses lower than 40nm. For thicker films, themagnetization is oriented at an oblique direction with respect to the filmplane. The magnetic domain patterns change drastically with the film thicknessand Magnetis Force Microscopy shows that an array of out of plane stripesappears in the thicker films at room temperature (Fig. 1).  Thecompetition between shape and strain-induced anisotropies leds to an anomaloustemperature dependence of the in-plane coercivity above tc   (Fig. 2).  A first comparison of thisresult with those of FePt single thin films indicates that the criticalthickness has shifted to higher values. Moreover, a careful look of the Hc(T)curve for the thickest film (arrows in Fig. 2) reveals small but measurable effectsof the BaTiO3 structural transitions on the coercivity of FePtfilms.