The objective of the DFG funded project is the Tunnel Magnetoresistance (TMR) effect for Atomic Force Microscopy (AFM) applications. The TMR effect is observed in thin film systems that are composed of two ferromagnetic electrodes and a thin insulating barrier. Due to the spin-dependent tunneling probability, the tunnel resistance across such a thin film junction depends on the relative orientation of the magnetization of the electrodes. TMR junctions with high sensitivity to mechanical strain can be fabricated if a magnetostrictive sense layer is implemented. Figure 1a shows a resistance versus strain curve for a 10µm x 10µm sized junction. According to the positive magnetostriction of the sense layer the easy axis begins to rotate into the perpendicular direction with respect to the applied compressive strain leading to a decrease of the resistance. This decrease saturates at about 0.1% strain when the perpendicular magnetization configuration is reached. The gauge factor equals 840 and is several times larger than the maximum gauge factors for conventional metallic strain sensors.

Fig. 1: (a) Tunnel resistance versus strain and gauge factor (compressive stress). (b) AFM operation with magnetostrictive TMR sensor.

If a magnetostrictive TMR junction is placed on an AFM cantilever the change of its tunnel resistance can be used to detect a cantilever bending (Fig. 1b). Accordingly, AFM tip to sample interactions can be monitored by a simple resistance measurement during scanning. An optical alignment as in conventional atomic force microscopes is obsolete when using this new detection method. It clearly simplifies the operation and also widens the applicability to problems where an alignment is difficult, i.e. automated cantilever exchange in vacuum systems or operation in liquids, for instance. Moreover, new cantilever geometries can be used, since the TMR sensors can be miniaturized by standard lithography.

The TMR stack development and the cantilever fabrication is performed in the nanolab. The characterization of the cantilevers equipped with TMR sensors is done by our project partner at the Institute for Microstructure Technology in Karlsruhe.