Defect induced magnetism in SiC

Defect induced magnetism, which can be controllably generated by ion or neutron irradiation, is attracting intensive research interest. It not only challenges the traditional opinions about magnetism, but also has some potential applications in spin-electronics. SiC is a new candidate for the investigation of defect-induced ferromagnetism after graphitic materials and oxides due to its high material purity and crystalline quality [1, 2]. In this contribution, we present a comprehensive investigation on the structural and magnetic properties of ion implanted and neutron irradiated SiC samples. In combination with X-ray absorption spectroscopy, high-resolution transmission electron microscopy and first-principles calculations, we try to understand the mechanism in a microscopic picture.
For neon or xenon ion implanted SiC, we identify a multi-magnetic-phase nature [3]. The magnetization of SiC can be decomposed into paramagnetic, superparamagnetic and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. By combining X-ray magnetic circular dichroism and first-principles calculations, we clarify that p-electrons of the nearest-neighbor carbon atoms around divacancies are mainly responsible for the long-range ferromagnetic coupling [4]. Thus, we provide a correlation between the collective magnetic phenomena and the specific electrons/orbitals. Moreover, a negative magnetoresistance has been observed in ferromagnetic an conducting SiC, indicating the interplay between magnetism and free carriers [5].
With the aim to verify if a sample containing defects through its bulk volume can persist ferromagnetic coupling, we applied neutron irradiation to introduce defects into SiC [6]. Besides a weak ferromagnetic contribution, we observe a strong paramagnetism, scaling up with the neutron fluence. The ferromagnetic contribution only occurs in a narrow fluence window or after annealing. First-principles calculations hint towards a mutually exclusive role of the concentration of defects: Defects favor spin polarization at the expense of magnetic interaction. Moreover, the interaction between the nuclear spin and the paramagnetic defect can effectively tune the spin-lattice relaxation time (T1) as well as the nuclear spin coherent time (T2) [7].

[1] L. Li, et al., Appl. Phys. Lett. 98, 222508 (2011).
[2] Y. Wang, et al., Phys. Rev. B 90, 214435 (2014).
[3] Y. Wang, et al., Phys. Rev. B 89, 014417 (2014).
[4] Y. Wang, et al., Scientific Reports, 5, 8999 (2015).
[5] Y. Liu, et al., Phys. Rev. B 95, 195309 (2017).
[6] Y. Wang, et al., Phys. Rev. B 92, 174409 (2015).
[7] Z. Zhang, et al., Phys. Rev. B 95, 085203 (2017).