Xiaobing Chen

Unconventional magnets combining antiferromagnetic structures with ferromagnetic-like responses are essential for the development of next-generation spintronics. Their emergent properties are fundamentally dictated by the interplay between exchange-driven magnetic geometry and spin-orbit coupling, described by spin space groups and magnetic space groups, respectively. However, the lack of direct correspondence between these frameworks hinders the systematic tracking of symmetry evolution of these intertwined physical contributions. Here we introduce FINDSPINGROUP, an online platform based on the oriented spin space group framework to unify spin space group and magnetic space group descriptions. By automating the tracking of symmetry-breaking pathways from the absence to the presence of spin-orbit coupling effects, FINDSPINGROUP enables the classification of magnetic phases and the derivation of symmetry-constrained tensors for phenomena such as momentum-dependent spin splitting and the anomalous Hall effect. By standardizing data exchange through the spin crystallographic information file, FINDSPINGROUP establishes a computational infrastructure for the high-throughput discovery and design of unconventional magnets.

Quantum transport phenomena in two‐dimensional electron gases (2DEGs) at oxide interfaces have garnered significant interest owing to their potential in spintronic and quantum information technologies. Here, we systematically investigate the quantum conductance corrections of spin‐polarized 2DEGs formed at the interfaces between two insulating oxides, ferromagnetic EuTiO ₃ (ETO) films and (111)‐oriented KTaO ₃ (KTO) substrates. The anomalous Hall effect and hysteretic magnetoresistance provide evidence for long‐range ferromagnetic order in the 2DEGs, which could be attributed to interfacial Eu doping in combination with the magnetic proximity effect of the ETO layer. The breaking of time‐reversal symmetry by ferromagnetism in the 2DEGs, and with the assistance of the spin‐orbit coupling effect, leads to a non‐trivial Berry phase. These results are explained by the competition between weak localization (WL) and weak antilocalization (WAL) in quantum transport due to Dirac‐like fermions at the KTO (111) interfaces, strikingly similar to that observed in spin‐polarized 2DEGs in LAO/ETO/STO (111) heterostructures and magnetically doped topological insulators. Moreover, we find that this competition can be tuned by optical stimuli via a photo‐excitation‐induced shift of the Fermi level. Our findings demonstrate a controllable platform based on spin‐polarized oxide 2DEGs for quantum transport, opening new avenues for spin‐orbitronic and topological electronic applications.

Magnetism has seen substantial progress in recent decades, driven largely by its potential for next-generation storage devices. However, the classification of magnetic orders, even for fundamental concepts such as ferromagnetism (FM) and antiferromagnetism (AFM), remains a topic of active evolution, particularly with the discovery of unconventional magnetic materials and advances in antiferromagnetic spintronics^1,2,3,4. Here we present a classification of magnetic order using the state-of-the-art spin space group (SSG) theory^{5,6,7,8,9,10,11}. On the basis of whether the net spin magnetization is constrained to zero by the SSG framework, we systematically categorize magnetic orders into FM (including ferrimagnetism) and AFM. We further introduce an ‘oriented spin space group’ (OSSG) description, that is, a SSG with a fixed magnetic orientation, thereby unifying the SSG and magnetic space group (MSG)^12,13,14 frameworks. This approach clearly reveals the symmetry-breaking pathway induced by spin–orbit coupling (SOC). On the basis of the proposed group framework, we identify a distinct magnetic phase, termed spin–orbit magnetism (SOM), in which the net spin magnetization is induced by SOC. Our work provides a comprehensive symmetry-based perspective for classifying magnetic order, offering fresh insights into unconventional magnets and broad applicability in spintronics and quantum materials design.

The combination of quantum geometry and magnetic geometry in magnets excites diverse phenomena, some critical for antiferromagnetic spintronics. However, very few material platforms have been predicted and experimentally verified to date, with the material pool restricted by the assumed need for strong spin-orbit coupling (SOC). Here, we bypass the need for SOC by considering magnetic order induced quantum geometry and corresponding nonlinear transports (NLTs) in antiferromagnets (AFMs). By integrating spin space group theory into the symmetry analysis, we find that collinear and coplanar magnetic geometries can only induce NLT driven by Berry curvature dipole, and noncoplanar ones may trigger NLT driven by dipoles of Berry curvature, inverse mass, and quantum metric. Using this approach, we establish a materials database of 260 AFMs with SOC-free NLT effects, and complement this with first-principles calculations on several prototypical material candidates. Our work not only provides a universal theoretical framework for studying various magnetism-driven transport effects, but also predicts broad, experimentally accessible material platforms for antiferromagnetic spintronics.

We propose a novel ferroelectric switchable altermagnetism effect: the reversal of ferroelectric polarization is coupled to the switching of altermagnetic spin splitting. We demonstrate the design principles for the ferroelectric altermagnets and the additional symmetry constraints necessary for switching the spin splitting through flipping the electric polarization based on the state-of-the-art spin-group symmetry techniques. We find 22 ferroelectric altermagnets by screening through the 2001 experimental reported magnetic structures in the MAGNDATA database and identify two of them as ferroelectric switchable altermagnets. Using the hybrid improper ferroelectric material [C(NH_{2})_{3}] Cr(HCOO)_{3} as an example, we show how the altermagnetic spin splitting is tightly coupled to the ferroelectric polarization, providing an ideal platform for designing electric-field-controllable multiferroic devices. Finally, we find that such manipulation of altermagnetism can be detected by monitoring the physical quantities that are related to the nonvanishing Berry curvature dipole, such as the linearly polarized photogalvanic spin current.

Magnonic systems provide a fertile playground for bosonic topology¹, for example, Dirac^2,3,4,5,6 and Weyl^7,8 magnons, leading to a variety of exotic phenomena such as charge-free topologically protected boundary modes^6,7, the magnon thermal Hall effect⁹ and the magnon spin Nernst effect¹⁰. However, their understanding has been hindered by the absence of fundamental symmetry descriptions of magnetic geometries and spin Hamiltonians primarily governed by isotropic Heisenberg interactions. The ensuing magnon dispersions enable gapless magnon band nodes that go beyond the scenario of representation theory of the magnetic space groups^11,12, thus referred to as unconventional magnons. Here we developed spin space group^{13,14,15,16,17} theory to elucidate collinear magnetic configurations, classifying the 1,421 collinear spin space groups into 4 types, constructing their band representations and providing a comprehensive tabulation of unconventional magnons, such as duodecuple points, octuple nodal lines and charge-4 octuple points. On the basis of the MAGNDATA database¹⁸, we identified 498 collinear magnets with unconventional magnons, among which more than 200 magnon band structures were obtained by using first-principles calculations and linear spin wave theory. In addition, we evaluated the influence of the spin–orbit-coupling-induced exchange interaction in these magnets and found that more than 80 per cent are predominantly governed by the Heisenberg interactions, indicating that the spin space group serves as an ideal framework for describing magnon band nodes in most 3d, 4d and half-filled 4f collinear magnets.

Spatial, momentum and energy separation of electronic spins in condensed-matter systems guides the development of new devices in which spin-polarized current is generated and manipulated^1,2,3. Recent attention on a set of previously overlooked symmetry operations in magnetic materials⁴ leads to the emergence of a new type of spin splitting, enabling giant and momentum-dependent spin polarization of energy bands on selected antiferromagnets^5,6,7,8,9,10. Despite the ever-growing theoretical predictions, the direct spectroscopic proof of such spin splitting is still lacking. Here we provide solid spectroscopic and computational evidence for the existence of such materials. In the noncoplanar antiferromagnet manganese ditelluride (MnTe₂), the in-plane components of spin are found to be antisymmetric about the high-symmetry planes of the Brillouin zone, comprising a plaid-like spin texture in the antiferromagnetic (AFM) ground state. Such an unconventional spin pattern, further found to diminish at the high-temperature paramagnetic state, originates from the intrinsic AFM order instead of spin–orbit coupling (SOC). Our finding demonstrates a new type of quadratic spin texture induced by time-reversal breaking, placing AFM spintronics on a firm basis and paving the way for studying exotic quantum phenomena in related materials.

Despite intensive research, the mechanism determining the terahertz (THz) emission of the ferromagnetic (FM) metallic monolayers remains elusive. Here, we report on the results of a systematic investigation on the THz emission generated by pumping Ni80Fe20 monolayers on Al2O3 substrates with a femtosecond laser. We found solid evidence that the THz emission is dominated by the anomalous Nernst effect (ANE), in which a transient spin-polarized charge current can be induced by an ultrafast electron temperature gradient on the picosecond timescale, outputting THz emission. We found a polarity reversal of the THz waveform after the introduction of a SiO2 buffer layer to the sample and found that, based on ultrafast temperature simulation, it was a consequence of direction reversal of temperature gradient. Comparing the THz emission of different FM monolayers further confirms that the THz polarity also strongly depends on the sign of the ANE coefficient. These phenomena unambiguously indicate that the ANE plays a decisive role in the process of THz emission. The present work shows the importance of ultrafast spin caloritronics for a spintronic THz emitter. The principle demonstrated here can be applied to other FM metallic materials.