Abstract:
Multifunctional material systems unite different material properties in a single functional unit. This makes them interesting for both device applications and basic research. One prominent example of multifunctional systems are the so-called magnetoelectric multiferroic materials, which simultaneously exhibit coupled ferroelectric and ferromagnetic properties. Thus, an in-situ electric-field control of magnetization is enabled - a long-standing experimental challenge offering entirely novel device paradigms. A promising approach to magnetoelectric materials are composite-type multifunctional structures made from ferromagnetic and ferroelectric constituents, which are referred to as ferromagnetic/ferroelectric hybrid systems. Such artificial compounds enable an elastic strain-mediated, indirect magnetoelectric coupling across the interface of the constituents by exploiting piezoelectricity in one phase and magnetostriction in the other.
Within the framework of this thesis, we study the spin-mechanics approach. This novel concept is based on the strain-mediated electric-field control of magnetization in ferromagnetic thin film/piezoelectric actuator hybrid systems. More precisely, we investigate magnetization manipulation concepts for both polycrystalline and single-crystalline ferromagnetic thin films. We particularly focus on a control of the magnetization orientation, i.e., we study the feasibility and limitations of a piezo-voltage generated manipulation of the macrospin magnetization. We address the reversibility and nonvolatility of all-electric-field controlled magnetization reorientation processes in polycrystalline nickel, single-crystalline dilute magnetic semiconductor (Ga,Mn)As, and single-crystalline magnetite ferromagnetic thin films. The magnetic anisotropy of these multifunctional hybrids is quantified as a function of the voltage Vp applied to the piezoelectric actuator both using ferromagnetic resonance spectroscopy and anisotropic magnetoresistance techniques. The evolution of the magnetization orientation as a function of Vp is determined via superconducting quantum interference device magnetometry and magneto-optical Kerr effect spectroscopy. In short, we show that the magnetization orientation can be rotated continuously and reversibly within up to 90° at zero external magnetic field at room temperature solely by changing Vp. We also demonstrate irreversible and nonvolatile voltage-controlled magnetization reorientations of up to 180° upon an appropriate magnetic field preparation of the magnetization. We furthermore realize a proof-of-principle multifunctional memory device based on a nonvolatile and reversible all-electric-field control of remanent magnetization orientation. All data can be quantitatively understood within a single-domain (macrospin) Stoner-Wohlfarth type of approach. Furthermore, we investigate different concepts towards a nonvolatile, all-voltage-controlled magnetization switching in single-crystalline ferromagnets at room temperature.