Weyl semimetals have attracted worldwide attention due to their wide range of exotic properties predicted in theories. The experimental realization had remained elusive for a long time despite much effort. Very recently, the first Weyl semimetal has been discovered in an inversion-breaking, stoichiometric solid TaAs. So far, the TaAs class remains the only Weyl semimetal available in real materials. To facilitate the transition of Weyl semimetals from the realm of purely theoretical interest to the realm of experimental studies and device applications, it is of crucial importance to identify other robust candidates that are experimentally feasible to be realized. In this paper, we propose such a Weyl semimetal candidate in an inversion-breaking, stoichiometric compound strontium silicide, SrSi2, with many new and novel properties that are distinct from TaAs. We show that SrSi2 is a Weyl semimetal even without spin–orbit coupling and that, after the inclusion of spin–orbit coupling, two Weyl fermions stick together forming an exotic double Weyl fermion with quadratic dispersions and a higher chiral charge of ±2. Moreover, we find that the Weyl nodes with opposite charges are located at different energies due to the absence of mirror symmetry in SrSi2, paving the way for the realization of the chiral magnetic effect. Our systematic results not only identify a much-needed robust Weyl semimetal candidate but also open the door to new topological Weyl physics that is not possible in TaAs.

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New type of Weyl semimetal with quadratic double Weyl fermions

There is an increasing understanding of the mechanisms underlying the development of magnetoelectric coupling and multiferroic order in both single-phase and composite materials. The investigations underlying this advance include a range of studies on thin films, which are expected to play an important role in the development of novel magnetoelectric devices. The properties of both single-phase and composite systems are widely studied. While single-phase materials can exhibit rich spin-charge coupling physics, the magnetizations, polarizations, and transition temperatures are often too small to be innately useful for device design. Conversely, a number of ferromagnetic–piezoelectric composites can show strong magnetoelectric coupling at ambient temperatures, which develops as a product-property mediated by elastic deformation, making these systems more directly amenable to fabricating devices. In this review, we provide a short overview of the mechanisms for magnetoelectric coupling in multiferroics, together with a discussion of how this magnetoelectric coupling is relevant for designing new multiferroic devices, including magnetic field sensors, dual electric and magnetic field tunable microwave and millimetre wave devices and miniature antennas. We present a brief summary of some of the significant results in studies on thin-film multiferroics, with an emphasis on single-phase materials, and covering systems where the magnetic and ferroelectric transitions fall at the same temperature as well as systems where they fall at different temperatures.

https://iopscience.iop.org/article/10.1088/0022-3727/44/24/243001/pdf

Introduction to magnetoelectric coupling and multiferroic films

We demonstrate a tunable bandgap from 2.32 eV to 2.09 eV in phase-pure BiFeO3 by controlling the particle size from 65 nm to 5 nm. Defect states due to oxygen and microstrain show a strong dependence on BiFeO3 particle size and have a significant effect on the shape of absorbance curves. Oxygen-defect induced microstrain and undercoordinated oxygen on the surface of BiFeO3 nanoparticles are demonstrated via HRTEM and XPS studies. Microstrain in the lattice leads to the reduction in rhombohedral distortion of BiFeO3 for particle sizes below 30 nm. The decrease in band gap with decreasing particle size is attributed to the competing effects of microstrain, oxygen defects, and Coulombic interactions.

Tunable Bandgap in BiFeO 3 Nanoparticles: The Role of Microstrain and Oxygen Defects

A previous study of the growth conditions has shown that single-phase BiFeO3 thin films can only be obtained in a narrow pressure-temperature window and that these films display a weak magnetic moment. Here we study in more detail the structure and the magnetism of single-phase BiFeO3 films by means of reciprocal space mapping, Raman spectroscopy and neutron diffraction. X-ray and Raman data suggest that the BiFeO3 structure is tetragonal for 70 nm-thick films and changes to monoclinic for 240 nm-thick films, thus remaining different from that of the bulk (rhombohedral) structure. In the 240 nm monoclinically distorted film neutron diffraction experiments allow the observation of a G-type antiferromagnetic order as in bulk single crystals. However, the satellite peaks associated with the long-wavelength cycloid present in bulk BiFeO3 are not observed. The relevance of these findings for the exploitation of the magnetoelectric properties of BiFeO3 is discussed.

Structural distortion and magnetism of BiFeO3 epitaxial thin films: a Raman spectroscopy and neutron diffraction study

Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or may produce emergent spin–orbit effects that enable efficient spin–charge interconversion. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen–vacancy defect in diamond, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen–vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

Real-space imaging of non-collinear antiferromagnetic order with a single spin magnetometer

We have synthesized a range of transition-metal-doped BiFeO3 thin films on conducting silicon substrates using a spin-coating technique from metal–organic precursor solutions. Bismuth, iron and transition-metal–organic solutions were mixed in the appropriate ratios to produce 3% transition-metal-doped samples. X-ray diffraction studies show that the samples annealed in a nitrogen atmosphere crystallize in a rhombohedrally distorted BiFeO3 structure with no evidence for any ferromagnetic secondary phase formation. We find evidence for the disappearance of the 404 cm−1 Raman mode for certain dopants indicative of structural distortions. The saturation magnetization of these BiFeO3 films has been found to increase on doping with transition metal ions, reaching a maximum value of 8.5 emu cm−3 for the Cr-doped samples. However, leakage current measurements find that the resistivity of the films typically decreases with transition metal doping. We find no evidence for any systematic variation of the electric or magnetic properties of BiFeO3 depending on the transition metal dopant, suggesting that these properties are determined mainly by extrinsic effects arising from defects or grain boundaries.

http://iopscience.iop.org/article/10.1088/0953-8984/21/3/036001/pdf

Structural, magnetic, and electrical studies on polycrystalline transition-metal-doped BiFeO3 thin films

We report on the electronic energy-level diagram of polycrystalline ${\text{BiFeO}}_{3}$ using the elemental, optical, and current-density-electric field $(J\text{\ensuremath{-}}E)$ characteristics. The elemental, electronic composition, and valence-band structure of ${\text{BiFeO}}_{3}$ ceramics were studied using x-ray photoelectron spectroscopy. The diffuse reflectance spectrum of a mixture of ${\text{BiFeO}}_{3}$ and ${\text{BaSO}}_{4}$, used as a standard, was recorded to test the Kubelka-Munk model. From the graph of the Kubelka-Munk function versus wavelength, two charge-transfer bands and two doubly degenerated $d\text{\ensuremath{-}}d$ transitions (${^{6}\text{A}}_{1\text{g}}\ensuremath{\rightarrow}{^{4}\text{T}}_{2\text{g}}$ and ${^{6}\text{A}}_{1\text{g}}\ensuremath{\rightarrow}{^{4}\text{T}}_{1\text{g}}$) were observed in polycrystalline ${\text{BiFeO}}_{3}$. The $J\text{\ensuremath{-}}E$ curves measured on the ${\text{BiFeO}}_{3}$ ceramics showed space-charge-limited conduction mechanism.

Charge transfer and electronic transitions in polycrystalline BiFeO 3

We report on the structural, thermal, and magnetic properties of polycrystalline BiFeO3 synthesized by sol-gel route. Powder x-ray diffraction data of the BiFeO3 sample was refined with rhombohedral structure with space group R3c. Magnetization and coercive field measurements showed weak ferromagnetic nature below 10 K. We also investigated the low temperature magnetocaloric effect in polycrystalline BiFeO3 ceramics. The anomalies observed in the magnetic entropy change of the BiFeO3 ceramics were close to that of low temperature phase transitions. The combined relative errors in the magnetic entropy change are found to vary from 4% to 15% with increase in temperature.

Low temperature magnetocaloric effect in polycrystalline BiFeO3 ceramics

Magnetic and optical properties of Mn-doped BaSnO3

Structural and magnetic properties of polycrystalline BiFeO3, Bi0.9Ca0.1FeO2.95, Bi0.9Ba0.05Ca0.05 FeO2.95, and Bi0.9Ba0.1FeO2.95 ceramic samples were studied to establish the effects of doping in BiFeO3 on the magnetic property. X-ray diffraction data of the undoped and doped BiFeO3 samples were refined to a rhombohedral structure with space group R3c. X-ray photoelectron spectroscopy study showed the formation of a single-phase in both the undoped and doped BiFeO3 ceramics with Fe in the 3+ valence state. Ca doped and Ba-Ca co-doped BiFeO3 ceramic samples show weak ferromagnetic ordering at room temperature. This observation makes Ca doped and Ba-Ca co-doped BiFeO3 samples an interesting material system for magnetoelectric coupling studies.

B. Ramachandran

Papers

Structural, magnetic, and electrical studies on polycrystalline transition-metal-doped BiFeO3 thin films

Charge transfer and electronic transitions in polycrystalline BiFeO 3

Low temperature magnetocaloric effect in polycrystalline BiFeO3 ceramics

Magnetic and optical properties of Mn-doped BaSnO3

Weak ferromagnetic ordering in Ca doped polycrystalline BiFeO3