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.
Superparamagnetism is a form of magnetism typical of ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small particles (comparable in size to a magnetic domain of the corresponding massive material), the magnetic moments of individual atoms are aligned. In this situation, the magnetization can randomly change direction due to the effect of temperature.
This phenomenon occurs in materials for which the energy required for the reversal of the magnetic moment of the particles is comparable to the thermal energy of the lattice. The typical time between two magnetic moment reversals is called the Néel relaxation time. In the absence of an external magnetic field, when the time taken to measure the magnetization of nanoparticles is much greater than the Néel relaxation time, their magnetization appears to be zero on average: the nanoparticles are said to be in a superparamagnetic state. In this state, an external magnetic field can magnetize the nanoparticles, similar to a paramagnetic material. However, their magnetic susceptibility is much greater than that of paramagnetic materials.
We speak of paramagnetism, whose theory was developed by Paul Langevin, when we have a set of atoms or molecules each of which possesses a permanent magnetic dipole moment. An aggregate of such objects in general does not hold a stable magnetization because it is continuously disturbed by thermal agitation. By applying a magnetic field H to such a system of particles, they will orient themselves following the applied field H, but once the field is removed they will revert to being a “magnetically disordered” whole.
When we talk about superparamagnetism we refer to a set of objects such that the single element is not a single atom but a compact region formed by an aggregate of 1000÷100.000 atoms, each of which has its own magnetization. However, the set of these objects (e.g., a dispersion of nanoparticles) does not possess a stable magnetization because it is continuously disturbed by thermal agitation. Applying a magnetic field H to this system, the objects will orient themselves following the applied field H, but once the field is removed they will return to be a “magnetically disordered” ensemble, with a behavior analogous to that of a paramagnet.
However, it is possible that going down to low temperatures – a few tens of Kelvin – the superparamagnetic particles remain “magnetically frozen”. If a magnetic field H were applied to the system, ferromagnetic behavior would be observed at these temperatures. The temperature for which this occurs is called the blocking temperature.
Superparamagnetism finds applications in spintronic (such as computer hard drives) ultrathin mixed-magnet/semiconductor devices and ferrofluids.