San-Dong Guo

Zero-net-magnetization magnets possess ultradense and ultrafast application potential, benefiting from their intrinsic zero stray field and terahertz dynamics characteristics. Herein, we propose the concept of zero-net-magnetization hybrid magnet, in which magnetic atoms with opposite spin polarization are partially coupled via spatial inversion ( P ) symmetry, partially via rotation/mirror ( C / M ) symmetry or partially without any symmetry correlation. From a local perspective and neglecting the interactions between local regions, hybrid magnet can be regarded as being composed of P T -antiferromagnet (possessing the combined symmetry ( P T ) of P and time-reversal ( T )), altermagnet, or fully compensated ferrimagnet. To realize hybrid magnet, we propose that such system can be constructed by forming heterojunction with three types of zero-net-magnetization magnetic monolayers. We mainly investigate the heterojunction composed of two kinds of zero-net-magnetization magnets, among which one type corresponds to fully compensated ferrimagnet. When heterojunction hybrid magnet exhibits a type-II band alignment, only one of electron doping and hole doping can induce a net magnetic moment, while the other hardly generates any net magnetization. Taking the heterojunction constructed by P T -antiferromagnet and fully compensated ferrimagnet as an example, we verify our proposal by means of the tight-binding (TB) model. Finally, taking the Cr 2 C 2 S 6 / CrMoC 2 S 6 heterojunction as an example, we perform first-principles calculations combined with electric field variation to validate our TB model and theoretical proposal. Lastly, we also note that both hidden altermagnets and surface altermagnets qualify as special zero-net-magnetization hybrid magnets.

Fully-compensated (FC) ferrimagnet has garnered widespread attention due to its zero-net total magnetic moment and non-relativistic global spin splitting. In general, for a FC ferrimagnet, at least one spin channel should be gaped to ensure a zero-net total magnetic moment, which would lead to a FC ferrimagnetic (FC-FIM) semiconductor or half-metal, and appears to limit the existence of an FC-FIM metal. Here we propose that an FC-FIM metal can be achieved by electrically closing the gap of a bilayer system. Using two-dimensional (2D) ferromagnetic (FM) semiconductor as building block, we examine both FM and antiferromagnetic (AFM) interlayer couplings and distinguish unipolar magnetic semiconductor (UMS) and bipolar magnetic semiconductor monolayers. It is concluded that an electric field can annihilate the bilayer gap and realize the FC-FIM metal only when the interlayer coupling is AFM and the building block is a UMS. Our scheme for realizing an FC-FIM metal can be generalized to electrically tuned 2D spin-degenerate metal with spin-layer locking. Using first-principles calculations, we have validated our proposal by taking bilayer MnOF, bilayer ScI 2 and monolayer Hf 2 S as examples. Our work offers an alternative route to realize the originally forbidden FC-FIM metal, paving the way for further exploration of FC-FIM metal.

Using high-throughput GGA+U first-principles calculations we survey the formation energy, magnetic ground state and spin-valley-coupled electronic structure of 87 monolayer 2H-phase MX₂ (M = Groups-IIIB–IIB, X = S, Se, Te). A systematic group-by-group evolution is uncovered: 3d-based TMDs favor antiferromagnetic (AFM) semiconductors, 4d congeners stabilize ferromagnetic (FM) metals or bipolar magnetic semiconductors (BMSs), whereas 5d members are overwhelmingly non-magnetic metals. Thermodynamic stability (ΔH_f ≤ 0) is fulfilled for all sulfides/selenides/tellurides of Groups-IIIB–VIB and most of Groups-VIIB–VIII, whereas post-transition-metal TMDs (Groups IB–IIB) are unstable. Valley polarization is dictated by the interplay between hexagonal lattice symmetry and magnetic order: FM Group-VB VX₂ and BMS VSe₂/VTe₂ exhibit 100% spin-polarized K/K′ band edges ideal for the anomalous valley Hall effect, while AFM Group-VIB CrX₂ and Mo/WX₂ retain spin-degenerate K-point valleys suitable for reversible valleytronics. The resulting atlas provides an experimentally verifiable blueprint for wafer-scale synthesis of high-temperature FM, half-metallic or valley-polarized 2D crystals and accelerates the materialization of next-generation spin-valleytronic devices.

Hidden spin polarization (HSP) with zero-net spin polarization in total but non-zero local spin polarization has been proposed in certain nonmagnetic centrosymmetric compounds, where the individual sectors forming the inversion partners are all inversion asymmetry. Here, we extend this idea to antiferromagnetic materials with PT symmetry (the joint symmetry of space inversion symmetry (P) and time-reversal symmetry (T)), producing zero-net spin polarization in total, but either of the two inversion-partner sectors possesses altermagnetism, giving rise to non-zero local spin polarization in real space. This phenomenon can also be termed as hidden altermagnetism. By the first-principle calculations, we predict that PT-symmetric bilayer Cr₂SO can serve as a possible candidate showing altermagnetic HSP. By applying an external electric field to break the global P lattice symmetry, the altermagnetic HSP can be separated and observed experimentally. Our works extend the hidden physics, and will also advance the theoretical and experimental search for new type of spin-polarized materials.

In addition to altermagnets, fully compensated ferrimagnets are another category of collinear magnetic materials that possess zero-net total magnetic moment and exhibit spin-splitting, making them promising for low-energy spintronics, high-density data storage, and high-sensitivity sensors. Although many methods, such as alloying, external electric field, Janus engineering, ferroelectric field, and spin ordering, have been proposed to achieve fully compensated ferrimagnetism, these approaches either face experimental difficulties or produce a small spin-splitting or are volatile. Here, we propose to form vertical heterostructures by stacking two different but equally magnetized two-dimensional ferromagnetic materials. If an A-type antiferromagnetic ordering is satisfied, a fully compensated ferrimagnet can be formed. This vertical heterostructure approach is insensitive to lattice matching and stacking manner, thus being more conducive to experimental realization. Through first-principles calculations, we verify our proposal with several examples, focusing in particular on CrI3/CrGeTe3 heterojunction composed of experimentally synthesized CrI3 and CrGeTe3 monolayers. The calculations show that CrI3/CrGeTe3 is a fully compensated ferrimagnet, with pronounced spin-splitting, and that tensile strain is more favorable for achieving fully compensated ferrimagnetism. Our work provides an experimentally feasible strategy for realizing fully compensated ferrimagnetism, thereby further advancing the development of this field.

The total spin polarization of hidden fully-compensated ferrimagnet is zero, but either of the two inversion-partner sectors possesses fully-compensated ferrimagnetism with non-zero local spin polarization in real space.