Using a Highly Sensitive TMR Sensor Array for the Detection of Superparamagnetic Beads

Camelia Albon, Karsten Rott, Michael Schilling, Alexander Weddemann, and Andreas Huetten

Department of Physics, Bielefeld University, P.O. Box 100131, 33501 Bielefeld, Germany




The use of magnetoresistive biosensors for the detection of biomolecules attached to magnetic nanoparticles is already a well-known technique. For the detection of magnetic nanoparticles attached only by one biomolecule the improvement of the magnetoresistive sensor's sensitivity is mandatory. In this way the application of new detection schemes in order to measure the biochemical processes that take place at single biomolecular level can be done more accurately.
TMR sensors with MgO as barrier material represent promising candidates to achieve highly improved sensitivity. In order to acquire a spatial resolution for the detection of magnetic nanoparticles a highly integrated sensor array is created. In our approach a sensor array is patterned by e-beam lithography with 20 elliptical sensors having the dimensions of 100400 nm on an area of 18.2µm2. The area of one sensor must be in the range of bead size for good data achievement.
There are many possible modalities to magnetize the superparamagnetic particles and then to detect their magnetic responses through the changes that are made in the TMR's elements resistance.
In this particular case, the superparamagnetic beads are magnetized with an in-plane, 45 angle and out-of-plane DC magnetic field.
In order to achieve a good detection it is highly important that the sensor output to be completely linear, without any hysteretic behavior.
This linearity can be obtained by inducing a transverse magnetization direction in the pinned layer, while the free layer is in the longitudinal direction. Also, by using a synthetic antiferromagnet, the demagnetizing and coupling fields acting on the free layer are much weak, so the free layer will be more sensitive to the changes induced by the straight field of the magnetized superparamagnetic bead.
This TMR array will be used for detection of single molecule processes with the help of magnetic nanoparticles.