In a previous article, we introduced the basics of magnetometers and some of their main applications. Today we will go a step further and take a look at the most common types of magnetometers.
Scalar magnetometers perform an accurate measurement of the numeric value of the magnetic field. Each type is based on different physical phenomena:
- Hall effect: Sense the voltage induced across an electrical conductor when applying a magnetic field can be perfectly used to measure magnetic fields
- Proton precession (PPM): Make use of nuclear magnetic resonance to measure the resonance of protons in the magnetic field, measuring the voltage induced in a coil due to their reorientation
- Overhauser: Similar to the Hall effect and proton precession magnetometers, but use radio-frequency signals to polarize the electron spins
An Overhauser magnetometer for geophysical applications. Image used courtesy of Gem System
- Inductive: Measure the dipole moment of some particles by measuring the current induced in some detection coils after having subjected the sample to a variating magnetic field
- Fluxgate: Composed of a magnetic ring core with at least two coil windings: the drive winding and the sense winding
Fluxgate magnetometers windings. Image courtesy of the Imperial College London
- Hall effect: Generate a voltage proportional to the magnetic field and provide information about its module and direction; widely used for sensing applications rather than for characterizing magnetic materials
- Microelectromechanical system (MEMS): Detect the motion of a resonant structure using optical means at the microscopic scale
MEMS magnetometers are cheap and accessible. Image used courtesy of Sparkfun Electronics
Although each gradient magnetometer is a little different, each roughly has the same elements. First, they require a device to generate a known magnetic field, which can be alternating or constant. Second, gradient magnetometers require a source for an alternating gradient field. Finally, they also require an electronic or optical means to detect and measure the resultant force.
They also all have resonant operation, so the magnetic samples move around their resonant frequency when the maximum amplitude is achieved.
Another relevant aspect of magnetometers is the orientation of the magnetic field. In some magnetometers, such as the Zijlstra’s, the alternating and DC field were both aligned and oriented vertically. In contrast, in Foner’s magnetometer, the sample vibrates perpendicularly to the magnetic field, which reduces the complexity of the necessary set-up.
Vibrating Reed Magnetometer
Zijlstra introduced one of the first alternating gradient magnetometers in 1970. It was intended to overcome the limitation of previous magnetometers and measure the complete hysteresis curve of magnetic materials.
The reed magnetometer consists of a thin wire with a quite small sample to be characterized attached at its end. There are two coils connected in series opposition, or differentially coupled, to create a field gradient. This field creates a force on the sample, and consequently a vibration of the reed. Since the movement is very subtle, the frequency is set equal to the mechanical resonance of the reed, so the movement is amplified and easier to detect. The movement of the reed is observed using a microscope and a stroboscope lamp. When the current through the coils is constant, so is the magnetic field; the movement we measure is proportional to the magnetic moment of the sample.
The most prominent difference between Zijlstra’s magnetometers and the previous ones is the sensitivity and also the capability to completely characterize magnetic materials. To have full magnetic characterization, samples need to be very small to avoid imperfections, the problem is that magnetometers able to characterize samples with the size of microns can only characterize some magnetic properties such as the remanence or the susceptibility, but not the complete hysteresis cycle.
Vibrating Sample Magnetometers (VSM)
Most devices that measure the magnetic moment have a detection coil horizontally aligned with the coils generating an alternating magnetic field.
Vibrating sample magnetometers (VSM), invented by Foner in 1959, introduced the novelty that the sample motion is perpendicular to the applied magnetic field. Foner reduced the complexity of the set-up, avoiding hard modifications of the magnets.
VSMs are present in many laboratories and commercially available.
A commercial vibrating sample magnetometer (VSM). Image courtesy of Microsense
Combined Alternating Field Magnetometers
There is a third category of magnetometers that combines characteristics of the previous ones; they are so-called combined magnetometers. They still use two magnetic fields; however, instead of applying only one alternating field and another constant one, they apply two alternating fields. The greatest advantage is the characterization of samples in AC, as well as in DC, compared to VSMs or other magnetometers that are limited to DC fields.
Other magnetometers generate a magnetic field of a frequency equal to the mechanical resonance frequency of the sample. Combined magnetometers generate two magnetic fields whose difference is equal to the resonance frequency. Since one of the magnetic fields can be set to 0 Hz, it can perfectly work as a traditional gradient magnetometer. When varying both frequencies, the device works as a susceptometer, measuring high-order harmonics of the magnetic moment. This type of magnetometer was invented in 2015 by researchers at the Technical University of Madrid.