Frequency Mixing Magnetic Detection Setup Employing Permanent Ring Magnets as a Static Offset Field Source

Abstract

Frequency mixing magnetic detection (FMMD) has been explored for its applications in fields of magnetic biosensing, multiplex detection of magnetic nanoparticles (MNP) and the determination of core size distribution of MNP samples. Such applications rely on the application of a static offset magnetic field, which is generated traditionally with an electromagnet. Such a setup requires a current source, as well as passive or active cooling strategies, which directly sets a limitation based on the portability aspect that is desired for point of care (POC) monitoring applications. In this work, a measurement head is introduced that involves the utilization of two ring-shaped permanent magnets to generate a static offset magnetic field. A steel cylinder in the ring bores homogenizes the field. By variation of the distance between the ring magnets and of the thickness of the steel cylinder, the magnitude of the magnetic field at the sample position can be adjusted. Furthermore, the measurement setup is compared to the electromagnet offset module based on measured signals and temperature behavior.

Keywords: biosensors; frequency mixing magnetic detection; magnetic nanoparticles; magnetic sensors.

Figure 1

(Left) Schematic block diagram of the excitation and readout circuitry connected to the cross-sectional view of the measurement head assembly. (Right) Photo of the internal configuration of the measurement head and the high- and low-frequency excitation coils around the detection coils. The sample is inserted into the measurement head from above, and a light sensor is used to monitor the insertion and removal of the sample.

Figure 2

(Left) Schematic representation of the PMOM measurement head, the individual parts are labelled. (Right) The picture of the PMOM measurement head, which is held inside a place holder and fastened using three hexagonal screws.

Figure 3

Simulated magnetic field as a function of the distance between the two magnets with steel cylinders of different thicknesses (color-coded). The magnets are moved symmetrically away from each other, as shown in (a–c).

Figure 4

The dynamic applicable static magnetic field through PMOM. The magnetic fields were measured in 28 steps by moving the two permanent ring magnets towards each other (black squares following the black arrow), thus increasing the magnetic field, and then pulling them away from each other to decrease the field (red circles following red arrow).

Figure 5

Nonlinear magnetic moment trace of sample Syn70 measured with the PMOM setup (blue squares), and with the EMOM setup in pulsed (red squares) and continuous (black squares) mode over a field range of 0 to 24 mT, for mixing harmonic f₁ + f₂. The temperature development in the measurement head is plotted for each case as faded solid lines with matching colors. The signal values of the different setups for four different regions (a–d, marked in green) are given in Table 1.

Figure 6

Nonlinear magnetic moment trace of sample Syn70 measured with EMOM−pulsed setup (black squares) over a field range of 0 to 24 mT and the PMOM setup (red circles) over a field range of 2 to 26.5 mT, for mixing harmonics f₁ + f₂, f₁ + 2·f₂, f₁ + 3·f₂ and f₁ + 4·f₂.