Demagnetizing Curves
Demagnetizing curves describe the remanent flux density as a function of applied magnetizing field intensity. This is a useful tool for understanding the weakening of a permanent magnet’s field over time. It is a graphic representation of the behavior of a permanent magnet when it is exposed to increasing magnetic fields and decreasing fields.
When a permanent magnet is originally Magnetized, the material forms a series of domains that are aligned in the same direction. This allows the magnet to build up a strong field and achieve a high level of resiliency. This field allows the magnet to remain in a “spelled”, or charged, state and retain its field even when it’s removed from the original source of magnetization.
The forces generated during magnetization depend on the strength and nature of the applied field. When the external field is weak, the domain alignment is only partially satisfied, resulting in a lower flux density. As the field intensity increases, the domains come under greater and greater stress, eventually reaching full alignment. This results in the highest level of magnetization and is often referred to as “saturation”.
When the applied field is reversed, it forces the domains back into alignment in the opposite direction. After dominating, the domains remain partially stuck in the same position and the resultant flux density is dropped down to a lower level, often referred to as “residual magnetization”. This level of residual magnetization is the starting point for measuring the demagnetizing effect and is likely to vary from one material to another.
The main display of a demagnetizing curve is an x-y plot with the magnetization in Tesla (or Gauss) on the y-axis, and the magnetizing field intensity in Oersted (or Ampere-turn) on the x-axis. As the applied field intensity increases, the flux density increases and a line is drawn that exemplifies the behavior. When the field is reversed, the flux density decreases in an opposite direction. The peak remains when the magnetizing field is zero, showing the level of residual magnetization.
The degree of magnetism decreases with aging of the magnet material due to the decrease of coercivity. To illustrate these forces, another x-y plot is used where the reciprocal of the retention ratio is plotted on the y-axis and the temperature is plotted on the x-axis. This plot will show the steam of remanence versus the temperature, as the temperature increase the remanent magnetism decreases, thus resulting in a decrease of the coercivity.
The demagnetizing curves are useful for understanding the attributes of a material’s magnetic field and its ability to remain magnetized over time. It is important for engineers to understand how the material enters and leaves the state of saturation, as this can impact its performance in different applications. Knowing the shape of the curve and its parameters is essential for designing an efficient and durable piece of permanent magnet equipment.
