Magnetic Properties of Materials

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 (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 (Table 1 for a comparison of magnetic behavior versus χ and Table 2 for the susceptibilities of some common paramagnetic materials).

Table 1. Magnetic behavior versus values of magnetic susceptibility

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

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

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

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).

Table 3. Curie and Néel temperatures of some lanthanides.

	Curie Temperature	Néel Temperature	urie 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

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.