反磁性

因反磁性產生磁浮的熱解碳(pyrolytic carbon)。

反磁性是一些類別的物質，當處在外加磁場中，會對磁場產生的微弱斥力的一種磁性現象。

起因

反磁性的成因，是當物質處在外加磁場中，外加磁場使得物質電子軌道（更精確的講法：軌域）運動產生改變的連帶效應。當施加一外源磁場B時，會對運動中的電子（電荷q）產生了磁力F：F = qv × B。此力改變了電子所受的向心力，使得電子軌道運動或是加速，或是減慢。電子速度因此受到改變，而連帶改變了其與外加磁場相反方向上的軌道磁矩。

考慮兩個電子軌域：一個順時針運動，一個逆時針運動。一進入頁面方向的外加磁場會使順時針轉動電子的向心力增加，而使其自頁面出來方向上的磁矩增加。同樣的外加磁場則會使逆時針轉動電子的向心力減少，而使其進入頁面方向上的磁矩減少。兩者的改變都與進入頁面方向的外加磁場相抗衡。然而，外加磁場對於多數日常物質所感生的磁矩卻非常小，因此淨效應會是一種斥力。

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All materials show a diamagnetic response in an applied magnetic field; however for materials which show some other form of magnetism (such as ferromagnetism or paramagnetism), the diamagnetism is completely overpowered. Substances which only, or mostly, display diamagnetic behaviour are termed diamagnetic materials, or diamagnets. Materials that are said to be diamagnetic are those which are usually considered by non-physicists as "non magnetic", and include water, DNA, most organic compounds such as petroleum and some plastics, and many metals such as mercury, gold and bismuth.

Diamagnetic materials have a relative magnetic permeability that is less than 1, thus a magnetic susceptibility which is less than 0, and are therefore repelled by magnetic fields. However, since diamagnetism is such a weak property its effects are not observable in every-day life. For example, the magnetic susceptibility of diamagnets such as water is $\ \chi _{v}$ = −9.05×10⁻⁶. The most strongly diamagnetic material is bismuth, $\ \chi _{v}$ = −166×10⁻⁶, although pyrolytic graphite may have a susceptibility of $\ \chi _{v}$ = −400×10⁻⁶ in one plane. Nevertheless these values are orders of magnitudes smaller than the magnetism exhibited by paramagnets and ferromagnets. Superconductors may be considered to be perfect diamagnets ($\ \chi _{v}$ = −1), since they expel all field from their interior due to the Meissner effect.

歷史

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In 1778 S. J. Brugmans was the first person to observe that bismuth and antimony were repelled by magnetic fields. However, the term "diamagnetism" was coined by Michael Faraday in September 1845, when he realized that all materials in nature possessed some form of diamagnetic response to an applied magnetic field.

反磁性磁浮

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A live frog levitates inside a 32 mm diameter vertical bore of a Bitter solenoid in a magnetic field of about 16 teslas at the Nijmegen High Field Magnet Laboratory. Movie

A particularly fascinating phenomenon involving diamagnets is that they may be levitated in stable equilibrium in a magnetic field, with no power consumption. Earnshaw's theorem seems to preclude the possibility of static magnetic levitation. However, Earnshaw's theory only applies to objects with permanent moments m, such as ferromagnets, whose magnetic energy is given by m·B. Ferromagnets are attracted to field maxima, which do not exist in free space. Diamagnetism is an induced form of magnetism, thus the magnetic moment is proportional to the applied field B. This means that the magnetic energy of diamagnets is proportional to B², the intensity of the magnetic field. Diamagnets are also attracted to field minima, and there can be a minimum in B² in free space (in fact $\nabla ^{2}\mathbf {B} ^{2}\geq 0$).

A thin slice of pyrolytic graphite, which is an unusually strong diamagnetic material, can be stably floated in a magnetic field, such as that from rare earth permanent magnets. This can be done with all components at room temperature, making a visually effective demonstration of diamagnetism.

The Radboud University Nijmegen has conducted experiments where they have successfully levitated water and a live frog, amongst other things.

Recent experiments with studying the growth of protein crystals has led to a technique that utilizes powerful magnets to allow growth in ways that counteract Earth's gravity.

參看

• 反铁磁性

外部連結

• Videos of frogs and other diamagnets levitated in a strong magnetic field
• Video of levitating pyrolytic graphite
• Video of a piece of neodymium magnet levitating between blocks of bismuth.
  • Website about this device, with images (in Finnish).