Magnetic Sensors

The practical application of magnetic field sensors is continuously increasing. The range of application extends from the primary measurement of magnetic fields, such as in biomagnetism (e.g., in magnetocardiography and magnetoencephalography), in geophysics (e.g., in magnetotellurics), over the measurement of geometric quantities, such as length, position, angle, and revolution, to the readout of magnetic storage media. Advantages of magnetic techniques are that they are contactless and that magnetic field lines unobstructedly penetrate nonmagnetic materials, such as dirt. The choice of the proper sensor for a given application depends on a number of factors, such as signal amplitude, measurement bandwidth, dynamic range, size and price.

In our group we deal with the design, the fabrication and the application of highly sensitive magnetic field sensors, such as superconducting quantum interference devices (SQUIDs) and fluxgate sensors. In addition, we also work on the development of our own AMR sensors. A central topic in the sensor optimization is the understanding of the noise mechanisms which limit the ultimate sensor resolution.

One of the most widely deployed magnetic field sensor is the AMR sensor. AMR stands for anisotropic magnetoresistance. In contrast to GMR sensors („giant magnetoresistance“), which require complex multilayer systems, the AMR sensor is characterized by its simplicity. It consists of a thin permalloy layer and metal stripes (so-called barber poles) which cause a linearization of the sensor characteristics. The spontaneous magnetization lies in the easy axis direction which is fixed by shape anisotropy. A magnetic field along the heavy axis (perpendicularly to the easy axis) provides a rotation of the magnetization in the permalloy strip and thus a change of its resistance.

AMR sensors are nowadays commercially available from various manufacturers, either as primary magnetic field sensors or – in combination with integrated readout electronics – as e.g. rotation speed sensors. In collaboration with different manufacturers we characterize AMR sensors, especially with regard to the noise performance. In addition, we fabricate our own AMR sensors with the goal to tailor magnetometers for various applications where the very low noise of SQUIDs or fluxgate magnetometers is not needed. Furthermore, we want to investigate the influence of fabrication and geometrical parameters on the sensor noise. The permalloy films are deposited by rf sputtering, the metal layers for the barber poles and contact pads are deposited either by sputtering of thermal evaporation. Patterning is performed with conventional photolithography in combination with wet-chemistry of Ar plasma etching.

In order to characterize the AMR sensors an automated measurement setup was built. A typical sensor characteristics of an AMR sensor without barber poles fabricated without an additional annealing treatment is depicted in the figure below.

Messkurve — Ideal characteristics of an AMR sensor with (red line) and without (dotted line) barber poles

AMR — Magnetoresistance curve of a 50 µm-wide permalloy stripe

Fluxgate Sensor — Complete fluxgate sensor in racetrack geometry made at the emg

One very important low-noise sensor for magnetic fields is the fluxgate. It is used today in many applications, especially where robust and reliable sensors of very high sensitivity are needed. Fluxgates are used in space missions to detect and analyze the magnetic fields of the planets and moons in our solar system.

A fluxgate consists of a soft magnetic core, which is periodically driven into saturation with an ac-current of frequency f. A detection coil around the core measures the induction voltage and detects the signal at the doubled frequency 2f.

By demodulation of this harmonic signal the external magnetic field around the fluxgate can be measured as induction voltage. With special low noise core material very sensitive fluxgates with low noise can be realized.

Rauschen — Magnetic noise spectra of a fluxgate in shielded and unshielded environment

Typical noise spectra of one of our fluxgate sensors are shown in Fig. 3. We find at 1 kHz a magnetic flux density noise of 1 pT/Hz^1/2 and at 1 Hz the noise level is around 10 pT/Hz^1/2. This is measured in shielded as well as in unshielded environment. Fluxgates can measure in a very large dynamic range of magnetic fields between several pT and up to 0.1 mT. In contrast to SQUID magnetometers they are not sensitive to high frequency interference and operate at room temperature. The are very well suited for applications in nondestructive evaluation and testing and even allow simple biomagnetic measurements.

Dünnschicht Fluxgate — Photo of upper part of racetrack thin-film fluxgate magnetometer

In addition to the wire-wound, bulk fluxgate sensors, we also work on the design and fabrication of thin-film fluxgate magnetometers. To wind the coils around the permalloy core, a multilayer technology including two metallic layers to form the coils, one insulating layer and the magnetic layer are needed. In our process, Ti-Au-Ti trilayers are used to fabricate the coils, SiO₂ is used for the insulation, and the core is fabricated from permalloy. Fig. 4 shows a photo of a part of one of our thin-film fluxgate magnetometers. The sensitivity using a standard excitation frequency of 15 kHz ranges around 1 V/T.

To further improve the performance, especially of our thin-film fluxgate sensors, we also work on the development of a readout electronics with considerably increased excitation frequency (typically 1 MHz). An increased excitation frequency allows one to considerably increase the measurement bandwidth which in conventional systems lies in the range of a few kHz and to considerably increase the sensitivity.

Superconducting Quantum Interference Devices (SQUIDs) are highly sensitive magnetic field sensors that offer a large bandwidth in conjunction with a good noise performance. Hence, they can be employed in a wide range of applications. The sensors are based on the quantum-interference effect that occurs in a superconducting ring which is interrupted by weak links. By connecting the SQUID to dedicated SQUID electronics, changes in the magnetic flux can be directly translated into a change of the output voltage.

There are several kinds of SQUIDs whose properties are tailored to a specific measurement task. In the institute, directly-coupled SQUID magnetometers are fabricated based on the high-temperature superconductor yttrium-barium-copper-oxide. After the characterization, they are employed for biomagnetic applications, such as magnetrelaxometrie (MRX) on magnetic nanoparticles.

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