In history, the list of candidate magnetic semiconductors can be grouped into two categories: undoped magnetic semiconductors, such as europium chalcogenides and semiconducting spinels that have a periodic array of magnetic elements on their own; and doped magnetic semiconductors, such as (Cd,Mn)Se and (Ga,Mn)As, which are achieved by doping magnetic elements into conventional nonmagnetic semiconductors to make them ferromagnetic.
Undoped ferromagnetic semiconductors were extensively studied in the late 1960s to early 1970s. EuO, for example, is considered the first magnetic semiconductor, however, with a low Curie temperature (Tc) of only 77 K. Besides the low Tc, crystal growth of these compounds is extremely difficult. To obtain even a small single crystal requires weeks of preparation and growth. What is worse, the crystal structure of such magnetic semiconductors is quite different from that of Si and GaAs, so they cannot be incorporated into complex integrated circuits. On the other hand, doped magnetic semiconductors seemed to be more promising with regard to compatibility with modern semiconductor industry as well as tunability of the resulting magnetic properties.
II–VI magnetic semiconductors
II–VI compounds (such as CdTe, ZnTe, and CdSe) doped with transition metal (TM) elements (such as Mn) have been studied quite extensively in the 1980s[10, 13, 14]. The possible composition range and crystal structure of II–VI ferromagnetic semiconductors are shown in Table 1. Although magnetic ions can be introduced into II–VI semiconductors up to 40%–50%, the II–VI ferromagnetic semiconductors are predominantly antiferromagnetic or paramagnetic. The overwhelming contribution to magnetism is the super-exchange mechanism, which can be viewed as an indirect exchange interaction mediated by the anion. In the presence of an external magnetic field, the Mn2+ ions become magnetized, and consequently the electrons and holes are effected by a large magnetization from the Mn2+ ions. This results in a Zeeman splitting that can be hundreds of times larger than that in non-magnetic semiconductor quantum structures. This, in turn, results in a giant Faraday rotation. The II–VI compounds therefore can be used as optical isolator. However, the interest in II–VI magnetic semiconductors ebbed very quickly because of the lack of ferromagnetism, which was the first depression on this field.
III–V dilute magnetic semiconductors
III–V alloys, such as Ga0.95Mn0.05As, took centre stage in 1996, when Hideo Ohno’s group demonstrated ferromagnetic order in this compound with Tc below 60 K, which, with improvements in the control of the material quality, has subsequently risen to 190 K. This discovery struck an immediate chord with researchers interested in spintronic applications, as the host semiconductor GaAs is a mainstream semiconductor used in lasers, high-mobility transistors and other devices. By introducing Mn into this material it could lead to seamless electrical manipulation of magnetic states and magnetic modification of electrical or optical signals. For example, Hideo Ohno’s group has already shown electrical control of ferromagnetism (Fig. 2) and spin polarized electroluminescence (Fig. 3) based (Ga,Mn)As. Furthermore, (Ga,Mn)As is able to provide interesting insights into a variety of spintronic phenomena, such as domain wall motion[21, 22], spin torque, spin Seebeck effect[24, 25], planar Hall effect, and tunnelling anisotropic magnetoresistance. It is well accepted in the III–V dilute magnetic semiconductors that the long range ferromagnetic order between localized moments provided by substitutional Mn is mediated by the itinerate holes generated also by substitutional Mn through RKKY exchange coupling. Unfortunately, though the Curie temperature of Mn-doped GaAs has been greatly improved compared to previous II–VI group compounds, it is still far below room temperature[28, 29]. Substitution of divalent Mn atoms into trivalent Ga sites leads to severely limited chemical solubility (usually ≤ 5%). The hetero-valent substitution, which simultaneously dopes both hole carriers and magnetic atoms, makes it difficult to individually control the local spin and carrier densities to enhance the effective p-d coupling that is necessary for a higher Tc.
Oxide magnetic semiconductors
In April 2000, Tomasz Dietl et al. published a theoretical paper in Science that provided a model to explain the origin of ferromagnetism in (Ga,Mn)As, and used the same model to predict ferromagnetism in wide bandgap materials ZnO and GaN. They predicted that the ferromagnetism of ZnO-based magnetic semiconductors can persist above room temperature. This marked the starting point of an exciting race for room temperature ferromagnetic semiconductors and pushed ZnO into the focus of materials research. Indeed, room temperature ferromagnetism has commonly been obtained in ZnO systems doped with transition metal elements such as V[31–33], Cr[34–36], Mn[37–42], Fe, Co[44–46], Ni[47, 48], and Cu[49, 50], and co-doping such as CoFe[51–53] and MnCo[54, 55]. Similar to (Ga,Mn)As, oxide magnetic semiconductor research was also confined to dilute magnetic compounds containing minute amounts of magnetic ions, owing to the very low thermodynamic miscibility (typically ≤ 10%) of transition metals. These are the so-called dilute magnetic oxides. In order to explain why dilute magnetic oxides without itinerary carriers become ferromagnetic at room temperature, the concept of bound magnetic polarons was introduced. In the bound magnetic polaron model, oxygen vacancies act as both electron donors and electron traps, which can bind the electrons and maintain insulating behavior. As shown in Fig. 4, each trapped electron couples the local moments of doped magnetic ions that lie within its orbit ferromagnetically, leading to a bound polaron with a large net magnetic moment. If neighboring polarons do not interact strongly, a paramagnetic, insulating phase will be resulted in. However, for certain polaron–polaron distances and combinations of electron–electron and electron–local-moment exchange constants, the polarons couple ferromagnetically.
Although a considerable amount of experimental data and corresponding mechanisms have been accumulated, the origin and control of ferromagnetism in dilute magnetic oxides are the most controversial research topic in materials science and condensed-matter physics. The data are notoriously plagued by instability and a lack of reproducibility[57, 58]. The weak magnetization displayed by thin-film specimens at room temperature is typically only 10 emu/cm3. In particular, spin-polarized electrical transport is barely noticeable in such materials, because the coupling between the mobile carriers and the localized 3d electrons is very weak[59–61]. Many scientists are now convinced that Dietl’s predictions on room temperature ferromagnetism are not realizable, due to unrealistic assumption of Mn0.05Zn0.95O with hole concentration up to 3.5 × 1020 cm–3[62, 63].
Two dimentional magnetic semiconductors
Since graphene was successfully prepared from graphite in 2004, two dimentional (2D) layered materials have received extensive attentions, which provided new opportunities to make 2D magnetic semiconductors. The first attempt was to add ferromagnetism to the long list of graphene’s capabilities. One can imagine that the ferromagnetic graphene could lead to novel transport phenomena such as the quantized anomalous Hall effect, which is very promising for dissipationless transport. From a theoretical point of view, magnetic moments in graphene can be induced from dopants, defects, or edges[66–68]. The possibility of long-range magnetic ordering has been predicted for randomly distributed point defects and grain boundaries[69, 70], and bilayer graphene was suggested to exhibit spontaneous many-body ferromagnetism. However, experimental studies yielded contradictory results. Some observed room temperature ferromagnetic order[72–75], while other reported the absence of any sign of ferromagnetism in graphene from room temperature down to low temperature[76, 77]. Other 2D nanomaterials, such as layered transition metal dichalcogenides (TMDs), are promising for flexible and transparent electronics applications due to their sizeable band gaps within the 1–2 eV range[78–80]. Significant efforts have been used to prepare 2D dilute magnetic semiconductors by introducing magnetic atoms (e.g., Mn, Fe, Co, and Ni) into 2D TMD[81–85], and ferromagnetic ordering has been reported recently in TM-doped TMDs. Nevertheless, lack of clear experimental evidence for ferromagnetic order formation in 2D layer hinders development of this nascent field.
The breakthrough of 2D magnetism came in 2017, ferromagnetism was demonstrated in van der Waals (vdW) crystals in the monolayer limit[86, 87]. In order to detect the ferromagnetic ordering of spins in a single atomic layer, a technique of much greater sensitivity than that provided by conventional magnetometers is required. To achieve the necessary level of sensitivity, a method called polar magneto-optical Kerr effect microscopy was used to determine the spatial extent of ferromagnetic order in their materials. With this powerful weapon, ferromagnetic order was demonstrated to remain intact in CrI3 even in a single layer of the material (albeit with a suppressed Curie temperature of 45 K). A single layer of CrI3 had a substantial remnant magnetization in the absence of a magnetic field, directed perpendicular to the plane of the lattice (Fig. 5). The magnetic system is therefore well described by the 2D Ising model. In strong contrast to CrI3, ultrathin layers of Cr2Ge2Te6 had a highly suppressed Curie temperature in the 2D limit. Consistent with the Mermin–Wagner theorem, the ferromagnetic order was not present in a single layer of Cr2Ge2Te6 even at 4.7 K, which was well described by the Heisenberg model. Despite low Curie temperature, the progresses opened a door to explore low-dimentional magnetism and related phenomena, such as highly tunable s, p-d coupling via electrostatic gating, strain, and proximity effects in 2D system[90–93]. When it comes to practicality, however, achieving robust magnetic ordering at room temperature is still prerequisite.