Hard magnetic films
Permanent magnets made of hard magnetic materials are used for applications that require efficient transformation of kinetic energy in electric energy, and vice versa. Examples for such devices are generators and electric motors. The importance of this application will increase in the future since a growing share of private transportation will be based on electric power. Today’s best hard magnetic materials contain rare earth elements like Neodymium, Samarium or Dysprosium. It became apparent in the last years that the availability of these elements may be restricted at times. Therefore, alternative alloys have gained much interest.
Fe-Co, basically a soft magnetic material, has a great potential to become a good permanent magnet. Theoretical calculations showed that the crystal, once it is brought into a tetragonally distorted state, will have a great magnetic anisotropy, which is necessary for hard magnetic materials. In order to strain the lattice, very thin films of Fe-Co can be deposited epitaxially. In this way, they adapt their in-plane lattice constants to the substrate on which they are deposited and can be strained along the perpendicular direction.
The drawback of this approach is that the induced strains are only stable up to a critical film thickness. For thicker films, a relaxation back into the cubic state with a very low magnetic anisotropy takes place. Our group succeeded to stabilize lattice strains up to very high critical film thicknesses of over 100 nm by alloying of only 2 at% of a third element like C or B. These Fe-Co-X alloys have a uniaxial magnetic anisotropy of 0.5 MJ/m³. Unfortunately, this magnetocrystalline anisotropy is still smaller than the shape anisotropy of a thin film, hence the easy direction of magnetization is still lying in the film plane. One possibility to further increase the magnetic anisotropy could be to increase the X-content, but we could show that this reduces the crystallinity of the layers and therefore also the crystalline anisotropy.
Adding 2 at.% C to Fe-Co leads to a tetragonal distortion and to an increased magnetocrystalline anisotropy. This is promising to produce rare-earth-free permant magnets.