Miniaturized Tunnel Magnetoresistance Sensors for Novel Applications of Atomic Force Microscopy
Magnetostrictive tunnel magnetoresistance (TMR) sensors pose a bright perspective in micro- and nano-scale strain sensing technology due to their high strain sensitivity and compatibility with mass-production techniques. They can be exploited in the development of the self-sensing mechanism for atomic force microscopy (AFM) as they offer a simple and cost-effective detection system compared to the conventional optical read-out, especially in vacuum and in liquid. The primary focus of this study is to understand TMR sensor responses to mechanical stress. Therefore, micro-sized junctions based on the CoFeB/MgO/CoFeB structure in squared, round, elliptical shapes are investigated using a four-point bending apparatus. Measured strain loops at different bias fields reveal dependency of strain sensitivity, gauge factor, on the bias field. Tensile and compressive strain loops for a round junction with a nominal diameter of 30.5 µm exhibit large gauge factors of 2150 ± 30 and 1750 ± 35 at -3.2 kA/m and +4 kA/m magnetic bias fields, respectively. Implemented shape anisotropy in the elliptical junctions is not suitable to replace the need of the bias field. Aiming for the same purpose, gauge factors of 400 are measured by round junctions at zero field. This can be beneficial for the development of a simple and compact detection system. Such junctions with high strain sensitivity are successfully integrated to AFM cantilevers. The deflection can be easily detected as resistance changes in a TMR sensor. Self-sensing TMR cantilevers exhibit deflection sensitivity of about 3.9 x 10-6 1/Å. These cantilevers can be an alternative for replacing the conventional optical read-out as they are successfully used to record typical force-distance curves and topography imaging in contact and dynamic modes. Minimum detectable deflection is determined by imaging atomic-step edges of Au (111) terraces and self-assembled monolayers of FDTS in amplitude- and frequency-modulated AFM modes.