Epitaxial growth of magnetic thin films and heterostructures for data storage and energy applications

The more and more challenging requests of the data storage industry and new findings in the field of spintronics have driven the interest of magnetism research community towards the design and growth of thin films with high structural quality. Since the Nineties several research groups have demonstrated the possibility to employ a sputtering apparatus to epitaxially grow thin films and heterostructures of metallic alloys with different functional magnetic properties, e.g. huge magnetocrystalline anisotropy, ferromagnetic shape-memory, giant magnetoresistance. This demonstration has paved the way to the development of new devices, leading in some cases to important technological breakthroughs, as in the case of hard disk's reading heads based on the tunnel magnetoresistance effect. We have used a RF sputtering apparatus to grow thin films and heterostructures of different magnetic metallic materials. Using single-crystalline substrates with different lattice parameters, i.e., MgO, SrTiO3, and LSAT, we have obtained epitaxial thin layers (thickness from 3.5 to 200 nm) of L10-FePt, Ni-Mn-Ga, and Mn-Ga; the films show a variety of different morphologies, depending on substrate, film thickness, growth temperature and annealing temperature. We have exploited the alternate layer deposition from three different targets to obtain specific and variable compositions of the metallic alloys; this technique has shown a high reliability in spanning a wide compositional range and, in the case of the FePt alloy, allowed us to reduce the growth temperature for chemical ordering. A couple of remarkable examples will be presented, which demonstrate that understanding and controlling structure is crucial for tailoring magnetism. We have proposed and realized different exchange-coupled composite systems, i.e., epitaxial heterostructures where a magnetically soft phase is grown onto a magnetically hard phase (L10-FePt, 10 nm thickness), to improve the performances of next generation hard disks. We have obtained a giant magnetically induced reorientation of martensitic variants by microstructure engineering in magnetic shape-memory Ni-Mn-Ga films, which have a great potential for the fabrication of new-concept actuators, sensors and energy harvesters. We have been able to epitaxially grow thin films of the metastable tetragonal phase of Mn-Ga alloy (close to Mn3Ga composition) (Fig. 1); these films possess exceptional magnetic and electronic properties, which make them promising as ferromagnetic electrodes in Spin-Transfer-Torque Magnetic RAMs.