Paramagnetism
Paramagnetism is one of the manifestations of magnetism in matter: paramagnet have just as long a nonzero magnetization, as they are in an external magnetic field. The randomly arranged magnetic moments of an atom or a molecule arrange themselves partly in an external magnetic field so that the magnetic field inside the paramagnetic substance amplified, but only as long as the external magnetic field exists (as opposed to stable magnetization in the ferro- magnetism ). Paramagnetic materials have a tendency to be drawn into a magnetic field.
The proportionality of the field gain is determined by the magnetic permeability ĩr (or magnetic susceptibility ĩr -1) and is at paramagnet > 1 (see diamagnetism ).
In physics all materials with positive magnetic susceptibility and without magnetic ordering are classified as paramagnetic.
- 5.1 Alkali metals
- 5.2 alkaline earth metals
- 5.3 Rare Earth
- 5.4 molecules
- 5.5 magnetite
- 6.1 See also
- 6.2 Literature
- 6.3 External links
Origin
Paramagnetism occurs only in the fabrics, the unpaired electrons have (radicals, transition metal cations, lanthanide cations ) and their atoms or molecules possess a magnetic moment. Reasons for this are, among others the intrinsic angular momentum (spin ) and orbital angular momentum of the electrons as they move around the nucleus.
A model, one can get a paramagnetic sample from nothing but small bar magnets imagine constructed, the wheels spin, but do not move. Brings you the sample in a magnetic field, then the bar magnets are preferably aligned in the direction of the magnetic field lines. An important feature is that the bar magnets do not affect each other - they address all independently from. The thermal fluctuations cause a constant, random reorientation of the bar magnets. In this case, random rod magnet orientations, configurations of the bar magnet in the central magnetic orientations with vanishing moment much more likely as an ordered distribution in which the magnetic moments are aligned in the same direction and lead to a non-zero total magnetic moment. So therefore you need more stronger magnetic fields, the more you want to align the magnets.
In terms of physics: the cause of a paramagnetic behavior lies in the alignment of the microscopic magnetic moments of a substance in a magnetic field. The individual magnetic moments are mutually independent. In contrast to ferromagnets such an alignment after switching off the magnetic field by thermal fluctuations is destroyed immediately. The magnetization of the material is proportional to the applied magnetic field
The greater is the magnetic susceptibility of the material, the easier it is these magnetize accordingly. The susceptibility is a measure of the strength of paramagnetism. Because of the simple relationship of the susceptibility of the relative magnetic permeability is often taken as a measure of the latter.
Frequently, one can read that a very large susceptibility means a sample is ferromagnetic. This statement is not entirely correct. Although the susceptibility of ferromagnets is very large in many cases, however, the cause lies in the said coupling. Ferromagnet point even after switching off the magnetic field or magnetization, the so-called remanence, while paramagnet, as already stated, after switching off the field, the magnetization disappears.
Species
A classic observation does not explain the presence of the above discussed magnetic moments. This, however, can understand quantum mechanics. The important for magnetism statement is that the total angular momentum of an atomic state is always associated with a magnetic moment
Here is the Landé factor and the Bohr magneton. The total angular momentum results from three components:
The core spin associated magnetic moment is - because of the much larger mass of the nucleons - all too weak to provide a significant contribution to the susceptibility can. Therefore, this will be ignored in the following. It should be noted, however, that the magnetic moment of the nucleus is quite measurable, which is used in medicine in magnetic resonance imaging (MRI) (hence the procedure is also called nuclear magnetic resonance imaging ).
The main contributions to the susceptibility stir from various sources, which are listed below. However, as it always is also diamagnetic contributions to the susceptibility, decided only a summation of all contributions, whether a substance is ultimately paramagnetic. Whose contribution, however, occurs Langevin paramagnetism (see below ), so is usually dominant.
Magnetic moments of atoms in the ground state ( Langevin paramagnetism )
The total angular momentum of an atom in the ground state can be theoretically determine rules on the so-called Hund's. Most important line is the fact that the total angular momentum
Always adds to zero. In all other cases, ie has a nuclear magnetic moment.
The temperature dependence of this contribution is determined by the law Curiesche
Described, while the Curie constant ( a material constant ).
A more detailed analysis of the Langevin paramagnetism is done using the Langevin and the Brillouin function.
Magnetic moments of the conduction electrons (Pauli paramagnetism )
Electrons can move virtually free in metals. Each electron has a magnetic moment - so you'd expect a Curie -like contribution to the susceptibility. However, only the excited conduction electrons due to the Pauli principle, the freedom to align their spin in the magnetic field. Their number is proportional to (the Fermitemperatur, another material constant ):
A detailed analysis shows, however, that there is a dependence on the strength of the external magnetic field.
Magnetic moments of atoms in excited states ( Van Vleck paramagnetism )
Even if the total angular momentum of an atom in its ground state is zero, so does not apply to the excited states. At a finite temperature are always some atoms in an excited state, so this contribution occurs in all materials. Of appreciable size, it is only in molecular crystals; there he may even exceed in strength the Langevin paramagnetism. But to calculate the size of this contribution is rather complex, especially for molecules.
Comparison of the orders of magnitude
Superparamagnetism
The magnetic properties of granular ferromagnetic solids are dependent on the grain size. For reduction of the grain size, the number of magnetic domains (Weiss - district) from per grain. Below a critical size, it is energetically unfavorable to form more of these areas. There is therefore only a white district per grain, that is, all the atomic magnetic moments of a grain are arranged parallel to each other. Below a critical size, a more stable orientation of the total magnetic moment is not possible at finite temperature, because the energy required for the magnetization reversal is smaller than the thermal energy. The solid body as a whole now behaves paramagnetic with the particularity that the magnetic moments, not individually respond in blocks to external magnetic fields. This particular shape of the paramagnetism is called superparamagnetism.
Application
The paramagnetism of oxygen is utilized in the physical vapor analysis.
Examples
Alkali metals
The electron shell of the alkali metals consists of a noble gas configuration and an additional s- electron. According to Hund's rules, the atoms in the ground state therefore have a magnetic moment. This is the first case (see above), which provides a strong contribution to the susceptibility. The alkali metals are therefore paramagnetic.
Alkaline earth metals
As opposed to the alkali metals, the alkaline earth metals have two s- electrons, and thus a closed shell. However, they are among the metals and thus affect the second case. With the exception of beryllium outweighs this contribution the diamagnetic, so that the alkaline earth metals are weakly paramagnetic.
Rare earths
The rare earths are among the most technically important materials for alloys in permanent magnets. The reason is that the crucial not fully occupied shell inside the electron shell (f- electrons) and thus no impact is on the chemical properties of atoms. Almost all the rare earths are thus paramagnetic ( for the first case ), but its thickness varies. This makes them ideal candidates in alloys with ferromagnetic materials, making very strong permanent magnets can be made.
Molecules
Since molecules often have a closed electronic configuration and no metals, they only show a contribution by the third case. Some examples of paramagnetic substances are:
- Nitrogen dioxide
- Oxygen
Lodestone
Magnetite ( Fe3O4) usually shows ferrimagnetic behavior ( ferrimagnetism ). For particle sizes that are smaller than 20 to 30 nm, is shown at room temperature superparamagnetic behavior. In the presence of an external magnetic field, all particles align in the direction of this field. After removal of the external field, the thermal energy is high enough, so that the mutual orientation of the particles and the magnetization relaxes again goes to zero.