Abstract
1. Introduction
Magnetic semiconductors[
Figure 1.(Color online) Publications per year on magnetic semiconductors according to the Web of Science. The term “Magnetic semiconductors” was selected as research topic.
Despite considerable research efforts, a magnetic semiconductor that exhibits usefully large, gateable spin polarizations at room temperature is still missing[
Unfortunately, neither of them has been demonstrated as a room temperature intrinsic magnetic semiconductor so far. Due to small feasibilities of application, the development of magnetic semiconductors slowed down. In recent years, publications on this subject declined rapidly, as shown in Fig. 1. When a scientific gold-rush excitement passed, it is a good time to think calmly about the ways for seeking room temperature magnetic semiconductors. Here, we would like to quote from Churchill: “This is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning.” Indeed, this is not the first time that the study on magnetic semiconductors sank into a low valley. The question is still open, and a lot of rooms remain for people to exploit. In this review, we will start with a simple historical review to draw inspirations from the past. Then we discuss recent experimental progresses to pursue strong s, p–d interaction to realize room temperature magnetic semiconductors, which are achieved by introducing a very high concentration of magnetic atoms by means of low-temperature nonequilibrium growth.
2. A brief historical perspective on magnetic semiconductors
In history, the list of candidate magnetic semiconductors can be grouped into two categories: undoped magnetic semiconductors, such as europium chalcogenides and semiconducting spinels[
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[
2.1. 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[
2.2. III–V dilute magnetic semiconductors
III–V alloys, such as Ga0.95Mn0.05As, took centre stage in 1996[
Figure 2.(Color online) (a) Electric field control of the hole-induced ferromagnetism in magnetic semiconductor (In,Mn)As field-effect transistors. (b) Hall resistance versus field curves under three different gate biases. Inset, the same curves shown at higher magnetic fields. Reprinted with permission from Ref. [
Figure 3.(Color online) Electrical spin injection in an epitaxially grown ferromagnetic semiconductor heterostructure based on GaAs. (a) Spontaneous magnitization develops below the Curie temperature
2.3. 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[
Figure 4.(Color online) Representation of magnetic polarons. A donor electron in its hydrogenic orbit couples with its spin antiparallel to impurities with a 3
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[
2.4. 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[
The breakthrough of 2D magnetism came in 2017, ferromagnetism was demonstrated in van der Waals (vdW) crystals in the monolayer limit[
Figure 5.(Color online) (a) Out-of-plane view of the CrI3 structure depicting the Ising spin orientation. (b) Polar MOKE signal for a CrI3 monolayer at a temperature of 15 K. The inset shows an optical image of an isolated monolayer (the scale bar is 2
3. Room temperature magnetic semiconductors with high transitional metal concentration
As a model system, (Ga,Mn)As had provided a good test bed to explore new physics and to design proof-of-concept spintronics devices. So people strived for a new material and transferred the experience with (Ga,Mn)As to it. It is noteworthy that it may be unrealistic if we are committed to find a room temperature dilute magnetic semiconductor with carrier-mediated ferromagnetism just like (Ga,Mn)As. To the best of our knowledge, nobody has demonstrated carrier-mediated ferromagnetism in a dilute magnetic semiconductor at room temperature. The problem is that the s, p–d exchange interaction mediated by free carriers is not large enough in such materials to align local spins ferromagnetically at room temperature, which precludes carrier-mediated ferromagnetism at room temperature in dilute system.
So we discuss a proposal to achieve thermally robust s, p–d interaction in semiconductors. According to the Anderson model[
3.1. Single crystal magnetic semiconductors with high transitional metal concentration
Over the years, the endeavor for improving quality of materials has led to high control of the growth processes. In many cases, low solubility of magnetic elements can be overcame by low-temperature nonequilibrium molecular beam epitaxial (LT-MBE) growth. Recently, (Ga1–xFex) Sb (x = 3.9%−20%) thin films were successfully grown by LT-MBE[
Figure 6.(Color online) (a) Schematic fabrication of (Ga1–
Another breakthrough was made in Co doped ZnO[
Figure 7.(Color online) (a) XRD and corresponding RHEED patterns of the Co
Figure 8.(Color online) (a) Normalized MR and (b) anomalous Hall resistivity and corresponding M-H loops for the Ga(Co0.4Zn0.6)0.98O film from room temperature down to 5 K. Reprinted with permission from Ref. [
The concentration of Co in the films is high enough to exceed the threshold to percolate together and couple close-neighbor local spins to a parallel ground state. Therefore, the ferromagnetism is originated from ferromagnetic p–d coupling between O (2p) and Co (3d) orbitals in the presence of oxygen vacancies. In this regard, the films can be classified as a ferromagnetic insulator, owing to the localized 2p and 3d electrons lying at a deep level within the large band gap of ZnO. By contrast, electronic transport is dominated by s electrons. The polarization of conducting s electron can be very different from the local polarization determined by d electrons. If the s, p–d exchange coupling is very weak, then no spin polarized transport behavior can be observed, as shown in Fig. 9(a). In order to enhance the s, p–d exchange coupling, Ga was introduced to increase the conducting carrier density, because the s, p–d exchange interaction energy depends on the concentration of magnetic ions as well as the density of states of the conducting carrier. By increasing the conducting carrier concentration, the density of states near the Fermi level can be significantly extended into the gap region, which gives rise to more overlapping between the delocalized electronic states (4s) and the localized impurity band (2p and 3d), as shown in Fig. 9(b). As a result, spin-polarized conducting carriers are created, and their density increases with carrier density, owing to the enhancement of s, p–d coupling between Ga (4s), O (2p), and Co (3d) orbitals.
Figure 9.(Color online) The schematic band diagrams as
3.2. Amorphous and/or nanocrystalline oxide magnetic semiconductors
However, the above dilute magnetic semiconductors and single crystal magnetic semiconductors with relatively high transitional metal concentration are largely limited by their stoichiometry, crystal structure and homogeneity. Beyond these limits, we propose that room temperature semiconductors with high spin-polarization may be realized by preparing amorphous, inhomogeneous and nonstoichiometric materials.
Nanocrystalline Zn1–xCoxO and amorphous Ti1−xCoxO2 magnetic semiconductor films with inhomogeneous composition on the subnanometer scale were prepared by sputtering under thermal nonequilibrium condition[
Figure 10.(a) A low magnification micrograph of Ti0.24Co0.76O2 films and the corresponding electron diffraction pattern in the inset. (b) The high resolution TEM image, and (c) the corresponding elemental mapping of Co. (d) XPS of Co 2p3/2 and Co 2p1/2 peaks. (e) Electron energy-loss spectroscopy of Ti0.24Co0.76O2 films. Reprinted with permission from Ref. [
One of the most important characters of magnetic semiconductors is the spin polarization. Therefore, a series of wide-band-gap ternary oxide ferromagnetic semiconductor films with high transition metal concentration were prepared to study the spin-polarized transport[
Figure 11.(Color online) Resistivity in logarithmic scale versus
4. Summary and outlook
It was long held that magnetic semiconductor research was confined to dilute magnetic compounds containing minute amounts of magnetic ions, which leads to a severe limit on its development. However, useful magnetic semiconductors, such as Cd0.55Mn0.45Te[16], may require high concentration of magnetic atoms. Fortunately, a great progress in the epitaxy of semiconductor compounds has made it possible to introduce such a high concentration of dopants without ruining the crystal structure of semiconductor host. A tip of the iceberg for magnetic semiconductors with high magnetic dopants concentration has been emerged. We have a good reason to believe that the magnetic semiconductors with high concentration of magnetic elements will show us new surprise in the years to come.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 11434006, and 51871112), the National Basic Research Program of China (Grant No. 2015CB921502), the 111 Project (Grant No. B13029), Shandong Provincial Natural Science Foundation (Grant No. ZR2018MA035).
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