Paramagnetism
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Paramagnetism is the tendency of the atomic magnetic dipoles to align with an external magnetic field. This effect occurs due to quantum-mechanical spin as well as electron orbital angular momentum.
Introduction
Paramagnetic materials are attracted when subjected to an applied magnetic field. The alignment of the atomic dipoles with the magnetic field tends to strengthen it and is described by a relative magnetic permeability greater than unity (or, equivalently, a small positive magnetic susceptibility).
Paramagnetism requires that the atoms individually have permanent dipole moments even without an applied field, which typically implies an unpaired electron in the atomic or molecular orbitals. In pure paramagnetism, these atomic dipoles do not interact with one another and are randomly oriented in the absence of an external field, resulting in zero net moment. If they do interact, they can spontaneously align or anti-align, resulting in ferromagnetism (permanent magnets) or antiferromagnetism, respectively. Paramagnetic behaviour can also be observed in ferromagnetic materials that are above their Curie temperature, and in antiferromagnets above their Néel temperature.
In atoms with no permanent dipole moment, e.g. for filled electron shells, a weak dipole moment can be induced in a direction anti-parallel to an applied field, an effect called diamagnetism. Paramagnetic materials also exhibit diamagnetism, but the latter effect is typically orders of magnitude weaker. Paramagnetic materials in magnetic fields will act like magnets but when the field is removed, thermal motion will quickly disrupt the magnetic alignment. In general paramagnetic effects are small (magnetic susceptibility of the order of 10−3 to 10−5).
Curie's Law
Under relatively low magnetic field saturation when the majority of the atomic dipoles are not aligned with the field, paramagnetic materials exhibit magnetisation according to Curie's Law:
[\mathbf = C \cdot \frac}]
where
- M is the resulting magnetisation
- B is the magnetic flux density of the applied field, measured in teslas
- T is absolute temperature, measured in kelvins
- C is a material-specific Curie constant
Paramagnetic Materials
- Aluminium Al [13] (metal) // Al is the preferred paramagnetic material for lunar mass driver applications using regolith as an ore.
- Barium Ba [56] (metal)
- Calcium Ca [20] (metal)
- Oxygen, its liquid form. O [8] (non-metal)
- Platinum Pt [78] (metal)
- Sodium Na [11] (metal)
- Strontium Sr [38] (metal)
- Uranium U [92] (metal)
- Magnesium Mg [12] (metal)
- Technetium Tc [43] (artificial)
- Dysprosium
Types of Paramagnetism
Speromagnetism - Local moments in random orientation, no net magnetisation
Asperomagnetism - Frozen spins
Spin Glass - Dilute magnetic ions
Helimagnetism - Crystalline aspero
Mictomagnetism - Cluster glass
See also
| Magnetic states |
| diamagnetism – superdiamagnetism – paramagnetism – superparamagnetism – ferromagnetism – antiferromagnetism – ferrimagnetism – metamagnetism – spin glass |
References
- Charles Kittel, Introduction to Solid State Physics (Wiley: New York, 1996).
- Neil W. Ashcroft and N. David Mermin, Solid State Physics (Harcourt: Orlando, 1976).
- John David Jackson, Classical Electrodynamics (Wiley: New York, 1999).
External links
- [Classification of Magnetic Materials] by Applied Alloy Chemistry Group at University of Birmingham.
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