The term spinelectronics or spintronics refers to a field, which aims to exploit the electrons' spin degree of freedom within electronic applications. Thanks to the well-known Hall effect and the anisotropic magnetoresistance (AMR), magnetic fields can be sensed electronically. Both effects are, however, limited when it comes to extreme miniaturization of the sensors. Such miniaturization is required, e.g., for modern high density hard disk drives. Here, one uses multilayer systems which exploit the giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR). Devices built from such multilayers of just a few nanometers thickness allow in principle for the required downsizing.
Based on the TMR in magnetic tunnel junctions (MTJs) it is also possible to think of non-volatile magnetic random access memory (MRAM), which has some similarity to the ancient magnetic-core memory. An MTJ basically consists of two ferromagnetic films, separated by a thin insulating film. By pinning one of the magnetic electrodes with an antiferromagnet, a so-called "spin valve" is created, which usually has low resistance in the parallel magnetic configuration and high resistance in the antiparallel configuration. The magnetic configuration can be switched by external fields (or can be used for sensing them) or by strong current pulses, which is called spin transfer torque switching (STT). Making the magnetization perpendicular to the film plane allows for further reduction in size without compromising on the thermal stability of the junctions.
Being both fast and non-volatile, the MRAM technology holds the promise of merging today's RAM and hard disk or solid state drive technologies. This will make data access faster and allow for considerably lower power consumption.
The demands for many applications of spinelectronic devices pose a serious challenge for materials science. An ideal STT-MTJ requires an antiferromagnet as well as ferromagnets with tailored properties.
The antiferromagnet is required to have a large Neel temperature, a large interfacial exchange coupling energy and high anisotropy. A thin ferromagnet deposited directly on top of the antiferromagnet (or vice versa) will show the exchange bias effect, a shift of the magnetic hysteresis away from zero field. This allows us to construct MTJs that have a well-defined anti-parallel magnetic state, which is necessary to construct magnetic sensors or to build magnetic memories. Nowadays, materials containing Pt or Ir are used as antiferromagnets, which are costly and their production causes significant pollution. Alternatives without noble metals or rare earth element are necessary for a large-scale usage of spintronics.
The ferromagnets (or ferrimagnets) form the magnetic electrodes in a tunnel junction or a GMR stack. Ideally, it will have low magnetization, high Curie temperature, high spin polarization, and low magnetic damping. While the high critical temperatures ensure a good thermal stability, the low magnetization and low magnetic damping allow for small spin transfer torque switching currents. A high spin polarization reduces the switching current and increases the TMR effect amplitude, which in turn is necessary for a useful signal-to-noise ratio in practical applications. Meeting all these demands in a single, affordable material is difficult and requires careful tuning. Such tuning can be done based on rational design principles, to which a detailed understanding of the band structure of a material is necessary.
To investigate the properties of new materials, I deposit thin films of promising intermetallic compounds by magnetron sputtering. Our co-sputtering tools allow us to create virtually any quaternary intermetallic compound. By carefully choosing the substrates and seed layers, the crystal structure, strain, and crystal orientation can be manipulated.
The films are characterized with various techniques, such as x-ray diffraction, magnetization measurements, optical absorption (from IR to UV), and electrical measurements. With synchrotron techniques such as soft x-ray absorption it is possible to investigate the magnetic properties of a film for each element individually.
Based on the known crystal structure of a material, we can compute its band structure and many more properties with density functional theory (DFT) and many-body perturbation theory. Comparison of measured data with calculations allows us to obtain a deeper understanding of the materials and the measurements, particularly of spectroscopic data. Computational materials science using DFT helps to find new useful materials and predict their phase diagrams. The stability of new compounds can be predicted and the search can be narrowed to materials which are expectedly stable, thus saving resources in the laboratory.
Research is moving on every day, just visit me in my office and ask for the latest opportunities for bachelor's and master's theses. I will be happy to give you an introduction into my field and answer your firstname.lastname@example.org