Magnetic Materials

Properties

  Introduction
Magnetic Properties of Materials
Superconductors
A History of Superconductivity
Molecule-Based Magnets
Spintronics
References

Magnetic Properties of Materials

Magnetic properties other than diamagnetism, which is present in all substances, arise from the interactions of unpaired electrons. These properties are traditionally found in transition metals, lanthanides, and their compounds due to the unpaired d and f electrons on the metal. There are three general types of magnetic behaviors: paramagnetism, in which the unpaired electrons are randomly arranged, ferromagnetism, in which the unpaired electrons are all aligned, and antiferromagnetism, in which the unpaired electrons line up opposite of one another. Ferromagnetic materials have an overall magnetic moment, whereas antiferromagnetic materials have a magnetic moment of zero. A compound is defined as being ferrimagnetic if the electron spins are orientated antiparrallel to one another but, due to an inequality in the number of spins in each orientation, there exists an overall magnetic moment. There are also enforced ferromagnetic substances (called spin-glass-like) in which antiferromagnetic materials have pockets of aligned spins (see Figure 1).

Figure 1. Types of magnetism: (A) paramagnetism (B) ferromagnetism (C) antiferromagnetism (D) ferrimagnetism (E) enforced ferromagnetism

Magnetic character of materials is typically analyzed relative to its magnetic susceptibility (χ). Magnetic susceptibility is the ratio of magnetization (M) to magnetic field (H). The type of magnetic behavior of a compound can be defined by its value of χ (see Table 1 for a comparison of magnetic behavior versus χ and Table 2 for the susceptibilities of some common paramagnetic materials).


Magnetic Behavior Value of χ
Diamagnetic small and negative
Paramagnetic small and positive
Ferromagnetic large and positive
Antiferromagnetic small and positive

Table 1. Magnetic behavior versus values of magnetic susceptibility


Compound/ Element Formula Mass Susceptibility (χm) (m3/kg) Mass Susceptibility (χm) (emu/Oe•g) x 10-3
Cerium Ce 64.84 5.160
Chromium(III) oxide Cr2O3 24.63 1.960
Cobalt(II) oxide CoO 61.57 4.900
Dysprosium Dy 1301 103.500
Dysprosium oxide Dy2O3 1126 89.600
Erbium Er 556.7 44.300
Erbium oxide Er2O3 928.9 73.920
Europium Eu 427.3 34.000
Europium oxide Eu2O3 126.9 10.100
Gadolinium Gd 9488 755.000
Gadolinium oxide Gd2O3 668.5 53.200
Iron(II) oxide FeO 90.48 7.200
Iron(III) oxide Fe2O3 45.06 3.586
Iron(II) sulfide FeS 13.5 1.074
Neodymium Nd 70.72 5.628
Neodymium oxide Nd2O3 128.2 10.200
Potassium superoxide KO2 40.59 3.230
Praseodymium Pr 62.96 5.010
Samarium Sm 28.02 2.230
Samarium oxide Sm2O3 24.98 1.988
Terbium Tb 1822 146.000
Terbium oxide Tb2O3 984.4 78.340
Thulium Tm 320.4 25.500
Thulium oxide Tm2O3 646.5 51.444
Vanadium oxide V2O3 24.83 1.976


Table 2.
Mass susceptibilities of some common paramagnetic materials [emu = electromagnetic unit (10-3amp·m2), Oe = Oersted 103·4 š-1·amp·m-1)]


Antiferromagnetic materials can be distinguished from paramagnetic substances in that the value of χ increases with temperature, whereas χ shows no change or decreases in value as temperature rises for paramagnetic compounds. Ferromagnetic and antiferromagnetic materials will lose magnetic character and become paramagnetic if sufficiently heated. The temperature at which this occurs is defined as the Curie temperature (Tc) for ferromagnetic compounds and the Néel temperature (TN) for antiferromagnetic compounds. Some substances, particularly the later lanthanides, will go from paramagnetic to antiferromagnetic to ferromagnetic as temperature decreases (Table 3).


  Curie Temperature Néel Temperature Curie Temperature Néel Temperature
Metal TC (°C) TN (°C) TC (K) TN (K)
Ce   -260.65   12.5
Pr   -248   25
Nd   -254   19
Sm   -258.35   14.8
Eu   -183   90
Gd 20   293  
Tb -51 -44 222 229
Dy -188 -94 85 179
Ho -253 -142 20 131
Er -253 -189 20 84
Tm -248 -217 25 56

Table 3. Curie and Néel temperatures of some lanthanides.1

There are several unique properties of magnetic materials which are exploited. Changing magnetic fields induce an electrical voltage making magnetic materials a central component of nearly all electrical generators. Magnetic materials are also essential components for information storage in computers, sensors, actuators, and a variety of telecommunications devices ranging from telephones to satellites.

Some materials, known as soft magnetic materials, exhibit magnetic properties only when they are exposed to a magnetizing force such as a changing electric field. Soft ferromagnetic materials are the most common of these as they are widely used in both AC and DC circuits to amplify the electrical flux. Magnetic nanopowders have shown great promise in advanced soft magnetic materials.2 Magnetocaloric materials heat up in the presence of a magnetic field and subsequently cool down when removed from the magnetic field. Pure iron, for example will change temperature by 0.5 – 2.0 °C/Tesla. More recently alloys of the formula Gd5SixGe1-x (where x = 0 – 5) will exhibit a 3 – 4 °C/Tesla change.3,4 Some nanomagnetic materials have shown significant magnetocaloric properties.

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