Hard magnetic films
Permanent magnetic films are required for applications in magnetic microelectromechanical systems (MEMS) and as magnetic data storage media. Both applications require better materials. For MEMS, the aim is to provide high power in reduced dimensions and thus a high energy product BHmax is required. In recording media the challenge is to avoid losing information due to the superparamagnetic limit, where the magnetisation switches by thermal activation when the grain size drops below a critical diameter Dp. Therefore, materials with a higher magnetocrystalline anisotropy are required for a further increase of bit density.
The basis for high performance magnetic films is the choice of material. The intrinsic properties and the resulting extrinsic magnetic properties of current hard magnetic materials are summarized in the table below. We use pulsed laser deposition for the growth of thin Nd-Fe-B, Sm-Co, Pr-Co and FePt films and electrodeposition for thick FePt and CoPt films. Additionally we examine exchange coupled Sm-Co/Fe and FePt/Fe films and magnetic nanowires. We study the growth conditions required for an appropriate microstructure and texture, since these are the key issue to transform the good intrinsic properties given by the material to good extrinsic properties like a high coercivity. Each of these systems have certain advantages and require quite different optimum preparation conditions. Follow the links to these compounds for details.
Materials currently under investigation in department 26:
Related hard magnetic films are also examined in department 21:Table: Intrinsic and derived properties of hard magnetic materials under investigation: TC Curie temperature, KU uniaxial magnetocrystalline anisotropy constant, JS saturation polarization, µ0HA anisotropy field, BHth,max theoretical maximum energy product, DP stable grain size against superparamagnetism at RT. References to these values can be found here. In addition, the best coercivity values at RT obtained in our group are given.
| Material | TC (K) | KU (MJ/m3) | JS (T) | µ0HA (T) | BHth,max (kJ/m3) | DP (nm) | Our best coercivity µ0HC (T) |
|---|---|---|---|---|---|---|---|
| Nd2Fe14B | 585 | 4.6 | 1.60 | 7.3 | 512 | 3.7 | 2.0 |
| SmCo5 | 1000 | 11-20 | 1.14 | 24-44 | 258 | 2.7-2.2 | 3.4 |
| PrCo5 | 905 | 8 | 1.24 | 14.5 | 310 | 3.0 | 2.24 |
| FePt | 750 | 6.6 | 1.43 | 11.6 | 407 | 2.8 | 5.6 (PLD) 1.1 (Electrodep.) |
| CoPt | 840 | 4.9 | 1.0 | 12.3 | 199 | 3.6 | 1.1 (Electrodep.) |
Further information can be found in our publications and in this introductional paper.