How does magnetic hysteresis work? Physical principles
16. January 2012
Magnetic hysteresis is a phenomenon that occurs in ferromagnetic materials exposed to a changing magnetic field. From the perspective of materials engineering and materials testing, hysteresis in ferromagnetic materials is of interest in permanent magnet production, magnetic particle testing and eddy current testing. Ferromagnetic materials include cobalt and its alloys, nickel and its alloys, and iron and its alloys, such as steels.
What does “hysteresis” actually mean?
The term hysteresis comes from the Greek word hysteros and means “delay” or “lag”. This simple translation is already an important key to understanding the phenomenon: in magnetic hysteresis, a material does not respond to the external magnetic field immediately, but with a certain delay.
Describing magnetic hysteresis
An unmagnetised ferromagnetic material has the potential to become a permanent magnet, but is not yet one. If we place such a material in a magnetic field, for example inside a current-carrying coil, an external magnetic field H acts on it. This field produces a magnetic flux density B within the material.
Put simply, H is the external “command” and B is the material’s response to that command. Magnetic hysteresis thus describes the relationship between magnetic field strength H and magnetic flux density.
The hysteresis curve
To determine a hysteresis curve experimentally, an initially unmagnetised ferromagnetic material is placed inside a coil through which no current initially flows. Thus, H = 0 and B = 0. We are at the origin of the hysteresis curve. If the current in the coil is increased, the field strength H rises. The magnetic flux density B initially responds only weakly, but then increases very rapidly and eventually reaches saturation at a particular field strength +HS.
From this point onwards, B changes very little, even if H is increased further. This curve is called the initial magnetisation curve and occurs only in initially unmagnetised ferromagnetic materials.
Remanence and coercive field strength
If the external magnetic field is then reduced again, the material does not return along the initial magnetisation curve, but along the hysteresis curve itself. At H = 0, a magnetic flux density +BR remains: the remanence.
The term remanence comes from the Latin “remanere” and means “that which remains”. It is precisely this residual magnetisation that allows a permanent magnet to remain magnetic.
We now reverse the direction of the external field H, for example by reversing the current in the coil, and increase H again. We then observe B. At a particular field strength, known as the coercive field strength (HC), we measure B = 0. The Latin word coercere means “to restrain”.
HC is therefore the field strength needed to “restrain” B, forcing B = 0. If we increase H further, we reach another saturation point (-HS). By reversing the direction of H and varying its magnitude, we can now travel around the hysteresis curve through the points +HS, +BR, -HC, -HS, -BR, +HC and +HS.
Soft and hard magnetic materials
Ferromagnetic materials exhibit different hysteresis curves. Soft magnetic materials have narrow hysteresis curves with low remanence and low coercive field strength. Hard magnetic materials have broad hysteresis curves with high remanence and high coercive field strength. These differences are closely related to the microscopic structure of the materials.
The physical basis: Weiss domains
Ferromagnetic materials exhibit collective behaviour involving many atoms. The magnetic moments of numerous atoms are aligned in the same direction within particular regions, forming small magnetic domains. These regions are called Weiss domains.
In an unmagnetised ferromagnetic material, these domains are oriented in different directions, so their magnetic effects cancel one another out. When an external magnetic field is applied, however, the magnetic moments begin to align with it. The stronger the field, the greater this alignment, until magnetic saturation is eventually reached.
Why does hysteresis occur?
The magnetic moments do not follow the external field immediately, but with a time delay. This is caused by internal resistance within the material, for example crystal defects, dislocations, precipitates and foreign atoms.
For the magnetic moments to rotate or move, they must first overcome this resistance. This requires energy, which must initially accumulate in the system. This delay between cause (H) and response (B) is precisely what we observe as magnetic hysteresis.
The significance of the area enclosed by the hysteresis curve
The energy required to reorient the magnetic moments is reflected in the area enclosed by the hysteresis curve:
- Large area → high energy requirement → broad hysteresis curve
- Small area → low energy requirement → narrow hysteresis curve
This also explains the difference between hard and soft magnetic materials.
Hard magnetic materials have a broad hysteresis curve because the processes that make the material hard hinder the rotation of its magnetic moments. Energy must therefore first accumulate for the rotation process, which takes time and causes a delay. The area enclosed by the hysteresis curve is a measure of the energy required for this reorientation. Soft magnetic materials, such as low-carbon steels, offer little resistance to reorientation and consequently have a narrow hysteresis curve.
The width of the hysteresis curve and the magnitude of the delay between field strength H and flux density B therefore depend on how much energy must be supplied to overcome the internal resistance. Once sufficient energy is available, the Weiss domains rotate, and the magnetic flux density B changes, very rapidly. This rapid change also explains the somewhat “rectangular” shape of the hysteresis curve.
An illustrative comparison
The process can be compared to a simple example: place a weight on a carpet, attach an elastic cord to it and start pulling. Initially, nothing happens, even though the cord is already under tension. Only when sufficient energy has been stored in the cord is static friction overcome, causing the weight to move suddenly. The rougher the carpet, the more energy is needed and the greater the delay.
A ferromagnetic material behaves similarly when its Weiss domains rearrange.
Technical applications
Materials with narrow hysteresis curves, known as soft magnetic materials, are used wherever energy losses during remagnetisation should be kept as low as possible. A classic example is the core of a transformer, which “connects” the primary and secondary coils.
Materials with broad hysteresis curves, known as hard magnetic materials, are used in applications where the material should resist remagnetisation as strongly as possible. One example is a permanent magnet, which should offer high resistance to remagnetisation or demagnetisation: after all, it is intended to remain a permanent magnet.
