Nanotec 2005

Magnetization Curve

• Human Anatomy And Physiology Study Course
• High Blood Pressure Remedy
• How to Fight and Overcome Cancer

As indicated in Section 2.2, the magnetic flux B and field H are scaled by the permeability /; similarly the magnetization M and field H are scaled by the susceptibility x. For ferro- and ferrimagnetic materials, / and x are not constants; instead they are nonlinear functions that depend on the previous state of magnetization (hysteresis). Figure 16 is a plot of B and /0M versus H, known as the BH or MH loop, where / and x are the tangents of the curve at different operating H. Note that from Figure 16, the BH and

Domain Wall

Domain 1

Domain 2

Figure 14. A hypothetical infinitely thin domain wall separating two domains with a 180° difference in direction.

Easy axis

Figure 15. To minimize the exchange energy associated with the infinitely thin domain wall of Figure 35, the directions of the moments (represented by the arrows) change gradually so the angle between two consecutive moments is less than 180°.

MH curves are offset by the vacuum induction /0H. For most ferro- and ferrimagnetic materials, the susceptibility is much larger than 1, so the BH and MH loops are commonly considered the same.

The BH loop begins with an initial curve, which is obtained by applying a field on a demagnetized sample. As the field increases, the sample magnetizes through reversible domain wall movement including irreversible Barkhausen jumps until reaching the saturation magnetization BS. As the H field reduces to zero, the magnetization does not reduce to zero but remains at the remanence Br due to the irreversible rotation of the magnetization vectors within the material. The magnetization becomes zero only when H reaches to the coercive field H = -Hc .As the H field continues to decrease, the sample will saturate at -BS. When the H field increases again, the MH curve reaches Br, Hc, and BS sequentially. Note that the BH loop will not go back to the origin. To reset the BH loop to the origin the sample needs to be demagnetized by either annealing the sample over Tc or by applying a time-varying amplitude-decaying ac field. Annealing over Tc is the best way to erase all magnetic memory of the sample; however, the process may also alter the mechanical properties of the material. Applying a decaying ac field is the most common way because it is easy to implement and imposes no permanent physical damage, but it yields no random distribution of the domain magnetization

Domain 1

Domain 2

aPPl

0 0