- Magnetic Susceptibility Definition: Magnetic susceptibility is defined as the extent to which a substance can be magnetised by an external magnetic field.
- Types of Magnetic Materials: Diamagnetic substances have negative susceptibility, paramagnetic substances have positive but small susceptibility, and ferromagnetic substances have large susceptibility.
- Ferromagnetic Susceptibility: Ferromagnetic susceptibility refers to the strong magnetization observed in materials like iron.
- Curie Temperature: The temperature above which paramagnetic materials lose their magnetism is called the Curie temperature.
- Measurement Methods: Magnetic susceptibility is measured using methods like Faraday’s scale, Guoy’s scale, inductive method, and MR method.
Before defining the term magnetic susceptibility, we need to know some terms such as magnetic field (H), magnetic flux density (B) and magnetic permeability (µ)
Magnetic field : Lines of force will be produced which diffuses through the medium where the field is applied.
Magnetic flux density : The amount of lines of force per unit area.
Magnetic permeability : It denotes the relation between the magnetic field and magnetic flux density. (B = µH)
Now, Magnetic susceptibility can be simply defined as the measurement of the extent to which a substance can be magnetised by applying a peripheral magnetic field. It is denoted by 
(from the Greek letter chi). It can be measured as
M – magnetisation of the material
H – Applied peripheral magnetic field strength
Magnetic susceptibility is caused by the interaction of electrons and nuclei when a magnetic field is applied. This interaction either opposes or enhances the magnetic field. If it opposes the field, it results in diamagnetism.
If the interaction enhances the magnetic field (electrons and nuclei align with the field), it results in paramagnetism, ferromagnetism, or super magnetism, depending on the degree of enhancement.
The range of the value of magnetic susceptibility () of different magnetic substances is
- Diamagnetic substances : < 0
- Paramagnetic substances : 1 > > 0
- Super paramagnetic and ferromagnetic substances : >> 1
Consider a material with magnetization M placed in an external magnetic field B0, where the magnetic permeability of free space is µ0. The total magnetic field in the material after applying the external field is then given by:
When the internal magnetic field of the material is considered, µ0 can be substituted by µ which is given by µ = Km µ0. Where, Km is relative permeability. If the material does not produce any internal magnetism with applied magnetic field, then the value of Km will be equal to 1. When Km>1, due to the application of a peripheral magnetic field the material is magnetised. The can be related with Km, i.e is defined as how much value the relative permeability differs from one.
m and Km will give same information. Both quantities are dimensionless.
Magnetic Susceptibility and Temperature
The paramagnetic material will be in its magnetic state only when it is above a specific temperature. This particular temperature is called Curie temperature.
The relation of same with magnetic susceptibility is explained by Curie law which is given by
C – Curie constant.
T – Temperature in Kelvin.
The paramagnetic material with weak interactive electrons has to obey the Curie-Weiss law which is given by
m – Molar magnetic susceptibility
θ – Weiss constant
The Curie-Weiss plot can be obtained by plotting a graph of 1/ m verses temperature. From that graph, we will get the curie constant by the inverting the slope and Weiss constant can be obtained from the y-intercept (Figure 1). If the Curie-Weiss law is satisfied then the plot should be linear.
Magnetic Susceptibility Measurement
A sensor in an oscillator circuit generates a low-energy alternating magnetic field. When a material with magnetic susceptibility is near the sensor, it changes the oscillator frequency. This frequency change is converted into magnetic susceptibility by electronics. Measurement methods include Faraday’s scale, Guoy’s scale, the inductive method using a SQUID magnetometer, and the MR method.
