Paramagnetism

Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. Paramagnetic materials are characterized at the atomic level by magnetic dipoles that align with the applied magnetic field, being weakly attracted to it.

In particular paramagnetism is observed in those materials whose molecules have a magnetic dipole moment of their own, so there is an effect mainly due to magnetic polarization by orientation and negligible Larmor precession, such as air and aluminum. In the case of air, the paramagnetic effect is at the expense of the oxygen molecule that possesses electronic doublets split in the external orbitals responsible for the effect. It is for this reason that it is possible to find oxygen even in the depths of the earth (vertically very long caves) and dissolved in sea water even beyond 5000 meters.

Contrary to ferromagnetic materials (which are also attracted by magnetic fields), paramagnetic materials do not retain magnetization in the absence of an applied external field.

Examples of paramagnetic compounds

Many metal ions are endowed with paramagnetic properties, even some not belonging to the corresponding metal with the same property. These include the bi- and tetra-valent manganese ions (Mn2+/ Mn4+), the trivalent chromium ion (Cr3+), the bivalent nickel ion (Ni2+), and all trivalent rare earth ions (such as neodymium, gadolinium, terbium, dysprosium, holmium, europium, and thulium).

Many coordination complexes are also endowed with this property. Some examples are:

  • nickel hexafluoride (NiF6)2-
  • chromium hexafluoride (CrF6)4-
  • iron tetrachloride (FeCl4)2-
  • cobalt hexafluoride (CoF6)3-
  • tantalum octafluoride (TaF8)3-
  • molybdenum octacyanide (Mo(CN)8)4-

Metal sulfides such as vanadium monosulfide and octasulfide (VS and V7S8), titanium trisulfide (Ti2S3), and molybdenum disulfide (MoS2) also possess metal-like properties and are paramagnetic. Among metal alloys, excluding those containing iron, nickel and cobalt, there are tantalum nitride (TaN), vanadium telluride (V4Te5) and nickel arsenide coordinated with cadmium iodide (NiAs/CdI2) which are paramagnetic. Finally, austenitic stainless steel also exhibits paramagnetic properties.

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