Alternating Gradient Magnetometer

For the manipulation of magnetic particles, it is very important to know their magnetic moment. This was measured with the Alternating Gradient Magnetometer (AGM) MICROMAG 2900 from the PRINCETON MEASUREMENT CORPORATION. The maximum magnetic field, generated by an electromagnet, is 14kOe at an air gap of 12mm. The sensitivity goes down to 10pAm$^2$ with an accuracy of 2%.

Figure 2.4: Example for an AGM measurement of magnetic markers.

Although the AGM is very sensitive, single magnetic markers cannot be measured directly. Instead, several millions of markers are measured in the AGM and the average magnetic moment for a single bead can be calculated then. Additionally, the number of the measured magnetic markers cannot be counted exactly, but only estimated by the given dilution. Figure 2.4 shows exemplarily the calculated magnetic moment per bead for CHEMAGEN beads with a concentration of 50$\mu$g/ml. For the bond-force measurements, the magnetic moment at a small outer field ($\approx$100Oe) is interesting and not the moment for saturated magnetic beads.

Beside the fact that only the mean magnetic moment of the beads is known, more issues were found during the measurements. Although the beads are superparamagnetic, some of them show a remanent magnetisation. The reasons can be the clustering of the beads, not fully oxidised Magnetite (Fe$_3$O$_4$) particles inside the beads or a few very big beads. To avoid clustering, the magnetic markers are pipette spotted onto a heated Si-wafer ($\approx$100). Because the effect remained, the clustering cannot be the only reason for the ferromagnetic behaviour. Additionally, the magnetic moment shows a dependency on the concentration of the beads that cannot be neglected (6 different concentrations are tested for every bead type). All of this has to be taken into account, to calculate the magnetic moment at a small outer magnetic field.