Chapter 17
Magnetic Semiconductors
Abstract Materials and properties of two types of semiconductors with spontaneous magnetization or
ferromagnetic and paramagnetic properties are explained: Compound materials and diluted magnetic
semiconductors. A short introduction to semiconductor spintronics covers the concepts of spin transistor
and spin LED.
17.1 Introduction
Magnetic semiconductors exhibit spontaneous magnetic order. Even ferromagnetism, important for
spin polarization, as needed in spinelectronics (also called spintronics), can occur below the Curie
temperature that is characteristic of the material. Magnetic semiconductors can be binary compounds
such as EuTe (antiferromagnetic) or EuS (ferromagnetic). Another class of magnetic semiconductors
contains paramagnetic ions in doping concentration (typically < 10
21 cm
−1 ) or alloy concentration
x (typically x ≥ 0.1%). Such materials are termed diluted magnetic semiconductors (DMS). The
incorporation of the magnetic atoms leads first to conventional alloy effects, such as the modification
of the lattice constant, the carrier concentration or the band gap. The status of the field up to the mid1980s can be found in [1516, 1517], mostly focused on II–VI DMS. A review of work on III–V based
materials for spintronics, mostly GaAs:Mn, can be found in [1518]. A 2003 review of wide band gap
ferromagnetic semiconductors is given in [1519], a 2014 review of Mn-containing DMS in [1520].
17.2 Magnetic Semiconductors
In a magnetic semiconductor, one sublattice is populated with paramagnetic ions. The first two ferromagnetic semiconductors discovered were CrBr 3 [1521] in 1960 and EuO [1522] one year later.
Europium monoxide has an ionic Eu
2+ O
2− character, such that the electronic configuration of europium
is [Xe]4f
7 5d
0 6s
0 and that of oxygen is 1s
2 2s
2 2p
6 . Some properties of europium chalcogenides [1523]
are summarized in Table 17.1.
EuO can be modeled as a Heisenberg ferromagnet with dominant nearest- and next-nearest Eu–
Eu interactions [1524]. The Heisenberg exchange parameters J 1 and J 2 for these four compounds
are shown in Fig. 17.1. In the nearest-neighbor interaction J 1 a 4f electron is excited to the 5d band,
experiences an exchange interaction with the 4f spin on a nearest neighbor and returns to the initial state.
This mechanism generally leads to ferromagnetic exchange. The next-nearest-neighbor interaction J 2
is weakly ferromagnetic (EuO) or antiferromagnetic (EuS, EuSe, EuTe). In the superexchange process,
© Springer Nature Switzerland AG 2021
M. Grundmann, The Physics of Semiconductors, Graduate Texts in Physics,
https://doi.org/10.1007/978-3-030-51569-0_17
465
Magnetic Semiconductors
Abstract Materials and properties of two types of semiconductors with spontaneous magnetization or
ferromagnetic and paramagnetic properties are explained: Compound materials and diluted magnetic
semiconductors. A short introduction to semiconductor spintronics covers the concepts of spin transistor
and spin LED.
17.1 Introduction
Magnetic semiconductors exhibit spontaneous magnetic order. Even ferromagnetism, important for
spin polarization, as needed in spinelectronics (also called spintronics), can occur below the Curie
temperature that is characteristic of the material. Magnetic semiconductors can be binary compounds
such as EuTe (antiferromagnetic) or EuS (ferromagnetic). Another class of magnetic semiconductors
contains paramagnetic ions in doping concentration (typically < 10
21 cm
−1 ) or alloy concentration
x (typically x ≥ 0.1%). Such materials are termed diluted magnetic semiconductors (DMS). The
incorporation of the magnetic atoms leads first to conventional alloy effects, such as the modification
of the lattice constant, the carrier concentration or the band gap. The status of the field up to the mid1980s can be found in [1516, 1517], mostly focused on II–VI DMS. A review of work on III–V based
materials for spintronics, mostly GaAs:Mn, can be found in [1518]. A 2003 review of wide band gap
ferromagnetic semiconductors is given in [1519], a 2014 review of Mn-containing DMS in [1520].
17.2 Magnetic Semiconductors
In a magnetic semiconductor, one sublattice is populated with paramagnetic ions. The first two ferromagnetic semiconductors discovered were CrBr 3 [1521] in 1960 and EuO [1522] one year later.
Europium monoxide has an ionic Eu
2+ O
2− character, such that the electronic configuration of europium
is [Xe]4f
7 5d
0 6s
0 and that of oxygen is 1s
2 2s
2 2p
6 . Some properties of europium chalcogenides [1523]
are summarized in Table 17.1.
EuO can be modeled as a Heisenberg ferromagnet with dominant nearest- and next-nearest Eu–
Eu interactions [1524]. The Heisenberg exchange parameters J 1 and J 2 for these four compounds
are shown in Fig. 17.1. In the nearest-neighbor interaction J 1 a 4f electron is excited to the 5d band,
experiences an exchange interaction with the 4f spin on a nearest neighbor and returns to the initial state.
This mechanism generally leads to ferromagnetic exchange. The next-nearest-neighbor interaction J 2
is weakly ferromagnetic (EuO) or antiferromagnetic (EuS, EuSe, EuTe). In the superexchange process,
© Springer Nature Switzerland AG 2021
M. Grundmann, The Physics of Semiconductors, Graduate Texts in Physics,
https://doi.org/10.1007/978-3-030-51569-0_17
465