Asked by Cheesecake
What happens to the magnetic domains inside iron when it becomes magnetized?
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Iron is a ferromagnet, so its atoms’ electron spins tend to align with their neighbors. However, in an unmagnetized piece of iron those aligned regions (magnetic domains) point in many different directions so their fields cancel and the net magnetization is near zero.
When you magnetize iron the following happens:
- Domain wall motion and domain growth: domains whose magnetization is already aligned with the applied field grow at the expense of domains that are not aligned. Domain walls move so favorably oriented domains become larger. This is the dominant process at low and moderate fields.
- Rotation of magnetization: as the field increases further, the magnetization vectors inside domains rotate toward the field direction. At high fields most domains are aligned with the field.
- Saturation: when almost all domains are aligned with the applied field further increases in field produce little additional magnetization (magnetic saturation).
- Remanence and coercivity: when the applied field is removed some domains stay aligned because of energy barriers and defects that pin domain walls — this produces a remanent (residual) magnetization. To reverse the magnetization you must apply a reverse field large enough to overcome those barriers (coercivity). Soft iron has easily moving domain walls (low coercivity), hard magnetic materials have strongly pinned walls (high coercivity).
- Thermal demagnetization: heating above the Curie temperature destroys the exchange alignment and the domains disappear; the material becomes paramagnetic.
Microscopically the behavior is governed by competition among exchange energy (favors parallel spins), magnetocrystalline anisotropy (favored directions), magnetostatic (demagnetizing) energy, and energy associated with domain walls and defects. In short: magnetization occurs mainly by domain walls moving so domains aligned with the applied field grow, and eventually domains rotate and align producing net magnetization.
When you magnetize iron the following happens:
- Domain wall motion and domain growth: domains whose magnetization is already aligned with the applied field grow at the expense of domains that are not aligned. Domain walls move so favorably oriented domains become larger. This is the dominant process at low and moderate fields.
- Rotation of magnetization: as the field increases further, the magnetization vectors inside domains rotate toward the field direction. At high fields most domains are aligned with the field.
- Saturation: when almost all domains are aligned with the applied field further increases in field produce little additional magnetization (magnetic saturation).
- Remanence and coercivity: when the applied field is removed some domains stay aligned because of energy barriers and defects that pin domain walls — this produces a remanent (residual) magnetization. To reverse the magnetization you must apply a reverse field large enough to overcome those barriers (coercivity). Soft iron has easily moving domain walls (low coercivity), hard magnetic materials have strongly pinned walls (high coercivity).
- Thermal demagnetization: heating above the Curie temperature destroys the exchange alignment and the domains disappear; the material becomes paramagnetic.
Microscopically the behavior is governed by competition among exchange energy (favors parallel spins), magnetocrystalline anisotropy (favored directions), magnetostatic (demagnetizing) energy, and energy associated with domain walls and defects. In short: magnetization occurs mainly by domain walls moving so domains aligned with the applied field grow, and eventually domains rotate and align producing net magnetization.
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