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Register a new userHigh Curie Temperature Ferromagnetic Semiconductor: Bimetal Transition Iodide V$_2$Cr$_2$I$_9$
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Bimetal transition iodides in two-dimensional scale provide an interesting idea to combine a set of single-transition-metal ferromagnetic semiconductors together. Motivated by structural engineering on bilayer CrI$_3$ to tune its magnetism and works that realize ideal properties by stacking van der Waals transitional metal dichalcogenides in a certain order. Here we stack monolayer VI$_3$ onto monolayer CrI$_3$ with a middle-layer I atoms discarded to construct monolayer V$_2$Cr$_2$I$_9$. Based on this crystal model, the stable and metastable phases are determined among 7 possible phases by first-principles calculations. It is illustrated that both the two phases have Curie temperature $sim$ 6 (4) times higher than monolayer CrI$_3$ and VI$_3$. The reason can be partly attributed to their large magnetic anisotropy energy (the maximum value reaches 412.9 $mu$eV/atom). More importantly, the Curie temperature shows an electric field and strain dependent character and can even surpass room temperature under a moderate strain range. At last, we believe that the bimetal transition iodide V$_2$Cr$_2$I$_9$ monolayer would support potential opportunities for spintronic devices.
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Exploring two-dimensional (2D) magnetic semiconductors with room temperature magnetic ordering and electrically controllable spin polarization is a highly desirable but challenging task for nanospintronics. Here, through first principles calculations, we propose to realize such a material by exfoliating the recently synthesized organometallic layered crystal Li$_{0.7}$[Cr(pyz)$_2$]Cl$_{0.7}$0.25$cdot$(THF) (pyz = pyrazine, THF = tetrahydrofuran) [Science 370, 587 (2020)]. The feasibility of exfoliation is confirmed by the rather low exfoliation energy of 0.27 J/m$^2$, even smaller than that of graphite. In exfoliated Cr(pyz)$_2$ monolayer, each pyrazine ring grabs one electron from the Cr atom to become a radical anion, then a strong $d$-$p$ direct exchange magnetic interaction emerges between Cr cations and pyrazine radicals, resulting in room temperature ferrimagnetism with a Curie temperature of 342 K. Moreover, Cr(pyz)$_2$ monolayer is revealed to be an intrinsic bipolar magnetic semiconductor where electrical doping can induce half-metallic conduction with controllable spin-polarization direction.
We demonstrate the manipulation of the Curie temperature of buried layers of the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the effect of annealing. Patterning the GaAs-capped ferromagnetic layers into nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As layers and thus allows the removal of Mn interstitials using annealing. This leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples. For a fixed annealing time, the enhancement of the Curie temperature is larger for narrower nanowires.
We investigate the magnetic and magneto-optic properties of epitaxial GaN:Gd layers as a function of the external magnetic field and temperature. An unprecedented magnetic moment is observed in this diluted magnetic semiconductor. The average value of the moment per Gd atom is found to be as high as 4000 mub as compared to its atomic moment of 8 mub. The long-range spin-polarization of the GaN matrix by Gd is also reflected in the circular polarization of magneto-photoluminescence measurements. Moreover, the materials system is found to be ferromagnetic above room temperature in the entire concentration range under investigation (7$times10^{15}$ to 2$times10^{19}$ cm$^{-3}$). We propose a phenomenological model to understand the macroscopic magnetic behavior of the system. Our study reveals a close connection between the observed ferromagnetism and the colossal magnetic moment of Gd.
We report on a theoretical and experimental study of CuMn-V antiferromagnets. Previous works showed low-temperature antiferomagnetism and semimetal electronic structure of the semi-Heusler CuMnSb. In this paper we present theoretical predictions of high-temperature antiferromagnetism in the stable orthorhombic phases of CuMnAs and CuMnP. The electronic structure of CuMnAs is at the transition from a semimetal to a semiconductor and we predict that CuMnP is a semiconductor. We show that the transition to a semiconductor-like band structure upon introducing the lighter group-V elements is present in both the metastable semi-Heusler and the stable orthorhombic crystal structures. On the other hand, the orthorhombic phase is crucial for the high Neel temperature. Results of X-ray diffraction, magnetization, transport, and neutron diffraction measurements we performed on chemically synthesized CuMnAs are consistent with the theory predictions.
Experimentally Cr doping in the rutile phase of VO$_2$ is found to stabilize a charge ordered ferromagnetic insulating state in the doping range of 10% to 20%. In this work, we investigated its origin at 12.5% Cr doping using a combination of ab-initio electronic structure calculations as well as microscopic modeling. Our calculations are found to reproduce the ferromagnetic insulating state as well as a charge ordering at the V and Cr sites. The mapping of the ab-initio band structure onto a tight-binding Hamiltonian allows one to calculate the energy gain from different exchange pathways. This gain is quantified in this work for the first time and the role of charge ordering in stabilizing a ferromagnetic insulating state is understood.
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