Perpendicular Magnetic Anisotropy

Magnetic anisotropy is the tendency for the magnetization to point along certain preferential axes in a magnetic material. It is an important property for spintronic devices as the magnetic data can be encoded in the magnetization orientation, taking two equally stable values (e.g., up -> 1, down -> 0). There is an energy barrier associated with reversing the magnetization between these two stable states.

A large barrier makes the anisotropy strong and the memory device robust against external disturbances (temperature and magnetic field); however, more energy is required to change its state (write the data). With the same reasoning, low anisotropy devices would be more “energy-efficient” but may be prone to inadvertent memory loss. Therefore, anisotropy engineering is a crucial task for spintronic devices to fulfill the required operational conditions. Magnetic anisotropy pointing perpendicular to the film surface (perpendicular magnetic anisotropy, PMA) is the most suitable option for downsizing magnetic memory bits while retaining large (and tunable) energy barriers.

In MAGNEPIC, we develop ferrimagnetic garnets with PMA. Thanks to the compositional tunability, ferrimagnetic garnets offer an excellent platform for anisotropy engineering for future spintronic applications. The mechanism to obtain PMA in our garnet films relies on the mismatch of their lattice parameters relative to the substrate. We grow our films on deliberately chosen substrates in optimized conditions (temperature, pressure, etc.) to induce tensile strain. As a consequence, the PMA emerges in order to minimize the overall magnetic energy. Since the PMA in our ferrimagnetic garnet films is a bulk property, we can change the thickness of our films to a great extent from a few to tens of nanometers while maintaining the PMA. This aspect offers great flexibility for experiments and future device applications.