User:Brian P. Josey/Notebook/2009/12/15

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Magnetism

Diamagnetism The orbiting electrons do not act cooperatively when exposed to an external magnetic field, and will have a negative magnetization and negative susceptibility. So as the magnetic field becomes larger, the magnetization becomes more negative, stronger but in the opposite direction. (look up orgo. notes to see if this is how NMR works!)

Paramagnetism Some of the atoms in the paramagnetic material have unpaired electrons, giving rise to a net magnetic moment. When the magnetic field is zero, the magnetization is zero, but as the field increases, the magnetization also increases. The presence of the field creates a partial alignment of the atomic moments in the atoms in the direction of the field. This alignment is opposed by randomizing effects from the temperature of the substance.

Ferromagnetism In a ferromagnetic material the interactions of the atomic magnetic moments are much larger than the ones in paramagnetic materials, as a result the magnetic moments are aligned in parallel, creating a large net magnetization, even without an external magnetic field.

  • Spontaneous Magnetization This is a measure of the net magnetization in a uniformly magnetized volume in the absence of a magnetic field. There is also saturation magnetization, which is the maximum amount of magnetization that can be obtained in a magnetic field. Saturation magnetization is an intrinsic property of a material, where it doesn't depend on the size of a sample of the material, but temperature does affect it. Ferromagnetic materials reach saturation in moderate magnetic fields, and relatively high temperatures, like around room temperature.
  • Curie Temperature If you raise the temperature of a ferromagnetic material, eventually you will reach a point where it will start acting like a paramagnetic material. This temperature is the Curie temperature, and it is where the thermal energy overrides the electronic exchanges, creating a randomizing effect.
  • Hysteresis Hysteresis is the fact that ferromagnetic materials can have a memory of an applied magnetic field after it had been removed. This can occur when a material is saturated in an applied field, and then the field is removed. The magnetization doesn't fall back to its original level, but has some positive value, its saturation remanence. By applying the field in the opposite direction, you can drop it back down to zero at a point called the coercivity. You can then keep applying the field in the opposite direction eventually reaching the saturation point in the opposite direction. Coercivity of remanence is the reverse field that when applied and removed will return the material to a zero saturation remanence.

Ferrimagnetism Ferrimagnetic materials are really similar to ferromagnetic materials in that there is a net magnetic moment, and they share many of the same properties, like the Curie Temperature, but the difference is that there are two sublattices of magnetic moments in a ferrimagnetic material. They point in the opposite direction, and depend on the structure of the material. Like in an oxide, the oxygen atoms separate all the moments pointing in one direction from those pointing in the opposite direction. What is unique about ferrimagnetic materials is that the two different magnetic moments are not equal, so there is a net magnetic moment for the material.

Antiferromagnetism An antiferromagnetic material is a ferrimagnetic material where the two sublattices are equal in magnitude. This creates alternating bands of equal magnetic moments pointing in opposite directions. They have no hysteresis or remanence, but they do have a small susceptibility that varies with the temperature. Above a certain temperature, Neel temperature, the Curie law flips signs from positive to negative. If there are slight tilts in neighboring spins, then a very small net magnetization can be produced. If they are canted, tilted at 90°, then it creates a canted antiferomagnets.