A vibrating-sample magnetometer (VSM) (also referred to as a Foner magnetometer) is a scientific instrument that measures magnetic properties based on Faraday’s Law of Induction. Simon Foner at
MIT Lincoln Laboratory
The MIT Lincoln Laboratory, located in Lexington, Massachusetts, is a United States Department of Defense federally funded research and development center chartered to apply advanced technology to problems of national security. Research and dev ...
invented VSM in 1955 and reported it in 1959. Also it was mentioned by G.W. Van Oosterhout and by P.J Flanders in 1956. A sample is first placed in a constant magnetic field and if the sample is magnetic it will align its magnetization with the external field. The
magnetic dipole moment of the sample creates a magnetic field that changes as a function of time as the sample is moved up and down. This is typically done through the use of a
piezoelectric
Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied Stress (mechanics), mechanical s ...
material. The alternating magnetic field induces an electric field in the pickup coils of the VSM.
The current is proportional to the magnetization of the sample - the greater the induced current, the greater the magnetization. As a result, typically a
hysteresis curve will be recorded
and from there the magnetic properties of the sample can be deduced.
The idea of vibrating sample came from D. O. Smith's
vibrating-coil
magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
.
Typical VSM overview
Parts of a typical VSM setup
![VSM-schematic-highres](https://upload.wikimedia.org/wikipedia/commons/8/8b/VSM-schematic-highres.jpg)
* Actively cooled electromagnet/power supply
* Amplifier
* Control Chassis
* Meter
* Computer interface
* Sensor coils
* Vibration exciter with sample holder
* Hall probe (optional)
Sample Operating Procedure:
# Mounting the sample in the sample holder
# Turning on the VSM system
# Run the computer software to initialize the system
# Calibrate the system
# Optimize the system for
''M''
# Calibrate the Lock-In Amplifier
# Take measurements and record data
Conditions for VSM to be effective
# Magnetic field must be strong enough to fully saturate the samples (or else inaccurate measurements will be taken)
# Magnetic field must be uniform across the sample space (otherwise the addition of field gradients
will induce force that alter the vibration once again leading to inaccurate results
Importance of pick-up coils
These allow the VSM to maximize the induced signal, reduce the noise, give a wide saddle point, minimize the volume in between the sample and electromagnet to achieve a more uniform magnetic field at the sample space.
The configuration of the coils can vary depending on the type of material being studied.
Relation to Physics
The VSM relies on
Faraday's law of induction
Faraday's law of induction (briefly, Faraday's law) is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (emf)—a phenomenon known as electromagnetic inducti ...
, with the detection of the emf given by
,
where ''N'' is the number of wire turns, ''A'' is the area, and
the angle between the normal of the coil and the B field. However, ''N'' and ''A'' are often unnecessary if the VSM is properly calibrated.
By varying the strength of the electromagnet through computer software, the external field is sweeped from high to low and back to high.
Typically this is automated through a computer process and a cycle of data is printed out. The electromagnet is typically attached to a rotating base
so as to allow the measurements be taken as a function of angle. The external field is applied parallel to the sample length
and the aforementioned cycle prints out a
hysteresis loop
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
. Then using known magnetization of the calibration material and wire volume the high field voltage signal can be converted into
emu
The emu () (''Dromaius novaehollandiae'') is the second-tallest living bird after its ratite relative the ostrich. It is endemic to Australia where it is the largest native bird and the only extant member of the genus '' Dromaius''. The emu ...
units - useful for analysis.
Advantages and Disadvantages
The precision and accuracy of VSM's are quite high even among other
magnetometers
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...
and can be on the order of ~
emu.
VSM's further allow for a sample to be tested at varying angles with respect to its magnetization letting researchers minimize the effects of external influences.
However, VSM's are not well suited for determining the magnetization loop due to the demagnetizing effects incurred by the sample.
VSM's further suffer from temperature dependence and cannot be used on fragile samples that cannot undergo acceleration (from the vibration).
References
{{reflist
See also
*
Magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
*
Alternating (Field) Gradient Magnetometer (AFGM or AGM)
*
SQUID Magnetometer
True squid are molluscs with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not st ...
American inventions
Magnetic devices
Measuring instruments
Magnetometers