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Bipolar Magnetic Semiconductor (BMS)

Bipolar magnetic semiconductors (BMSs) are a special class of magnetic semiconductors characterized by a unique electronic structure, where valence band maximum (VBM) and conduction band minimum (CBM) are fully spin polarized in the opposite spin direction. BMSs can be described by three energy gaps, the spin-ﬂip gap Δ1 in valence band (VB), band gap Δ2 and spin-flip gap Δ3 in conduction band (CB). The concept of bipolar magnetic semiconductors was first proposed by Jinlong Yang and his colleagues [1]. Up to now, bipolar magnetic semiconductors, together with half metals and spin gapless semiconductors, have been viewed as three important classes of spintronic materials [2,3].

Properties and Potential Applications

The proposal of bipolar magnetic semiconductor (BMS) is aimed to realize electrical control of carriers’ spin orientation, which is a key scientific problem in developing high performance spintronic devices, since electric ﬁeld can be easily applied locally, in contrast to magnetic ﬁeld. In BMS, completely spin-polarized currents with reversible spin polarization can be created and controlled simply by applying a gate voltage. Under zero gate voltage (VG = 0), BMS exhibits intrinsic semiconducting behavior. Under an appropriate negative gate voltage (VG < 0), when the Fermi level (EF) is shifted down into the spin-flip gap Δ1, BMS presents half metallic conduction with the carriers fully spin polarized in the spin up direction. While the Fermi level (EF) shifts up into the spin-flip gap Δ3 under an appropriate positive gate voltage (VG > 0), BMS also possesses half metallic conduction but with the carriers fully spin polarized in the spin down direction. Through this way, the carrier’s spin orientation in BMS can be easily reversed just by altering the sign of the applied gate voltage. Based on BMS, several electrically controlled spintronics devices have been developed, such as bipolar field eﬀect spin filter and field eﬀect spin valve. BMS can also be used to detect and separate entangled electrons from superconductors, which is very attractive in quantum information processing [4,5].

Materials Developments

A number of BMS materials have been theoretically predicted, such as MnPSe3 nanosheets, Heusler alloys FeVXSi (X = Ti, Zr), double perovskites A2CrOsO6 (A=Ca, Sr, Ba) and DPP-based organometallic sheet [6-9]. However, the experimental realization of electrical control of spin orientation in these materials still keeps a challenge and needs further experimental efforts.

References

1. Xingxing Li, Xiaojun Wu, Zhenyu Li, Jinlong Yang and J. G. Hou, Bipolar magnetic semiconductors: a new class of spintronics materials, Nanoscale 2012, 4 (18), 5680-5685, DOI:10.1039/C2NR31743E.

2. Hongzhe Pan, Yuanyuan Sun, Yongping Zheng, Nujiang Tang and Youwei Du, B4CN3 and B3CN4 monolayers as the promising candidates for metal-free spintronic materials, New Journal of Physics 2016, 18, 093021, DOI:10.1088/1367-2630/18/9/093021.

3. Jiří Tuček, Piotr Błoński, Juri Ugolotti, Akshaya Kumar Swain, Toshiaki Enoki and Radek Zbořil, Emerging chemical strategies for imprinting magnetism in graphene and related 2D materials for spintronic and biomedical applications, Chemical Society Reviews 2018, 47 (11), 3899-3990, DOI:10.1039/C7CS00288B.

4. Xingxing Li and Jinlong Yang, Bipolar magnetic materials for electrical manipulation of spin-polarization orientation, Physical Chemistry Chemical Physics 2013, 15 (38), 15793-15801, DOI:10.1039/C3CP52623B.

5. Xingxing Li and Jinlong Yang, First-principles design of spintronics materials, National Science Review 2016, 3 (3), 365-381, DOI:10.1093/nsr/nww026.

6. Xingxing Li, Xiaojun Wu and Jinlong Yang, Half-metallicity in MnPSe3 exfoliated nanosheet with carrier doping, Journal of The American Chemical Society, 2014, 136 (31), 11065-11069, DOI:10.1021/ja505097m.

7. Jiahui Zhang, Xingxing Li and Jinlong Yang, Electrical control of carriers' spin orientation in the FeVTiSi Heusler alloy, Journal of Materials Chemistry C 2015, 3 (11), 2563-2567, DOI:10.1039/C4TC02587C.

8. Xingxing Li, Xiaojun Wu, Zhenyu Li and Jinlong Yang, Proposal of a general scheme to obtain room-temperature spin polarization in asymmetric antiferromagnetic semiconductors, Physical Review B 2015, 92 (12), 125202, DOI: 10.1103/PhysRevB.92.125202.

9. Xingxing Li and Jinlong Yang, Toward Room-Temperature Magnetic Semiconductors in TwoDimensional Ferrimagnetic Organometallic Lattices, The Journal of Physical Chemistry Letters 2019, 10 (10), 2439-2444, DOI:10.1021/acs.jpclett.9b00769.