Ferromagnetism

Cornell University researchers have demonstrated a way to generate strong, localized static magnetic fields using only light, without external magnets or magnetic materials, an approach they say could advance spintronics, magnetic data storage, and photonic and quantum computing.

Illustration of the metasurface designed to control infrared light. Laser pulses can selectively activate regions of the material, allowing researchers to dynamically tune its optical properties. Image credit: Cornell University 
 

The technique relies on what the team calls a "time interface." Just as a light wave partially reflects and partially transmits when it crosses a spatial boundary, such as the surface between air and water, it does something analogous when it encounters a sudden change in the optical properties of the medium it's traveling through over time, rather than in space. A sufficiently abrupt time interface can also excite a static, zero-frequency mode of the system, effectively pulling part of the light's rapidly oscillating magnetic field out of circulation and freezing it into a stationary magnetic field pattern. Unlike more established light-driven magnetization methods such as the inverse Faraday effect, which tend to rely on the specific nonlinear optical properties of the material used, the team's approach works with the geometry of an engineered surface rather than the material itself. "We used an approach known as localized free carrier generation, which has advantages over other methods of nanoscale magnetization," author Shivaksh Rawat said. "One of the important contributions of our work is that our approach is material agnostic. Any non-metallic surface will work."

Researchers at Helmholtz-Zentrum Berlin (HZB), KAIST, the Technical University of Munich, the Max Planck Institute for Chemical Physics of Solids, the Technical University of Dresden, the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, the Paul Scherrer Institut's Swiss Light Source, and RIKEN's Center for Emergent Matter Science have shown that magnetic fields of just a few millitesla, applied in-plane rather than perpendicular to a sample, can continuously rotate and reshape the chiral magnetic stripe domains of a room-temperature noncentrosymmetric magnet, offering a new vector-field-based way to control magnetic domain orientation.

Micromagnetic simulations show how a small magnetic field in plane with the magnetic stripes can change the magnetic texture from a stripe pattern to a fan pattern. Image credit: HZB

Noncentrosymmetric magnets, which lack inversion symmetry, host a competition between conventional exchange interaction and the antisymmetric Dzyaloshinskii-Moriya interaction (DMI), the effect responsible for the chirality of textures such as helices, cycloids, and skyrmions. How that competition plays out depends heavily on a material's crystal symmetry, and most experimental work on it has relied on Lorentz transmission electron microscopy (LTEM), a technique generally limited to magnetic field tilt angles of 45 degrees or less. That has left the response of these materials to genuinely in-plane fields, and the role of a material's full three-dimensional field environment, largely unexplored.

Researchers at Wenzhou University, MIT, the Shanghai Institute of Technical Physics (Chinese Academy of Sciences), the University of Chinese Academy of Sciences, Caltech, the Hong Kong University of Science and Technology, and Fuzhou University have developed a general method for growing large-scale, single-crystalline films of two-dimensional metal oxides that are directly compatible with silicon technology - a longstanding barrier to building spintronic, ferroelectric, and neuromorphic components directly onto conventional chips.

Hybrid integration of 2D single-crystal metal oxides with silicon circuits is central to the "More-than-Moore" roadmap for extending chip functionality beyond simple transistor scaling, but growing such oxides as true single crystals has proven difficult. Conventional epitaxy relies on strong chemical bonding to lock a growing film into registry with its substrate, which causes defects when integrating chemically dissimilar materials like oxides and silicon. Van der Waals epitaxy on layered substrates such as mica avoids that problem by using weak, non-directional interactions instead, but the resulting surfaces are typically too chemically inert to guide film growth in a single orientation, so films tend to form as a mosaic of randomly oriented crystallites rather than a true single crystal. The team's solution was to reconstruct the oxygen atomic plane exposed at the surface of cleaved mica. Quantum mechanics calculations showed that this reconstruction creates a periodic potential landscape that energetically favors one crystallographic orientation for incoming oxide atoms, without requiring strong chemical bonds - effectively letting correctly oriented crystal nuclei grow faster and dominate while misoriented ones are starved of material.

Researchers at Shenzhen University, the Japan Atomic Energy Agency, the National Institutes for Quantum Science and Technology (QST), the University of Tokyo, the University of Science and Technology of China, Xi'an Jiaotong University, Shenzhen Technology University (SZTU), and The Hong Kong Polytechnic University, have provided the first element-resolved confirmation of intrinsic carbon ferromagnetism at room temperature in a bulk, wafer-scale material. The result, obtained in a hyper-porous graphene network (HGN) grown across a 2-inch silicon wafer, adds carbon to the small list of room-temperature magnetic materials and offers a new, structurally tunable platform for exploring spintronic and magnetic applications of carbon allotropes.

Structural features of the hyper-porous graphene network (HGN). Structural characterization of the (HGN). (a) Optical image of HGN on a 2-inch Si wafer. (b) TEM image showing nanoscale pores. (c) Converted TEM sketch with pores (<2 nm) as hollow islands. (d-f), Plan-view HAADF TEM images: overview with pores (d), zoomed-in cross-linked edges (e), and atomic detail with 0.35 nm interlayer and 0.21 nm zigzag edge spacing (f). (g-i) Side-view TEM images: vertical layers (g), crosslinking features (h), and 0.12 nm armchair spacing (i). Image from: Advanced Materials
 

Claims of intrinsic magnetism in carbon have long been difficult to establish, since even a trace of ordinary magnetic contamination, iron, cobalt, or nickel, can produce a magnetic signal comparable to the faint ferromagnetism reported in some carbon systems, which typically sits near 10⁻³ emu/g or below in bulk graphite. Prior demonstrations of edge-localized magnetism in graphene nanoribbons and multi-spin nanographene molecules showed the underlying physics could work at the nanoscale, but establishing the same effect reliably in bulk carbon had remained an open problem, requiring evidence that carbon itself, not a contaminant, carries the magnetism, and a structural handle whose removal reversibly weakens the effect.

Researchers from the University of Maryland, University of California, South Dakota School of Mines and Technology, East China Normal University, KAUST and other institutes recently reported all‑van der Waals multiferroic tunnel junctions (MFTJs) that combine ferromagnetism and ferroelectricity in a single nanoscale spintronic device, enabling four non‑volatile resistance states for multibit memory operation. 

These multistate junctions are realized by vertically stacking three atomically thin crystals: two ferromagnetic electrodes and a ferroelectric tunnelling barrier, all obtained by mechanical exfoliation and then assembled into a clean, defect‑sparse heterostructure. In their prototypical structure, Fe₃GeTe₂/CuInP₂S₆/Fe₃GeTe₂, multilayer Fe₃GeTe₂ serves as the ferromagnetic electrodes, while CuInP₂S₆ (CIPS) provides a ferroelectric spacer with switchable polarization. Because the layers are coupled by van der Waals forces rather than epitaxial bonding, the stack avoids stringent lattice‑matching and chemical‑compatibility constraints that hinder oxide‑based MFTJs and is far less susceptible to interfacial defects and interdiffusion.

Researchers from ETH Zürich, University of Washington, University of Basel and National Institute for Materials Science have demonstrated all-optical control over the spin-valley polarization in twisted molybdenum ditelluride (t‑MoTe₂) homobilayers - a step toward dynamically reconfigurable quantum materials and optically defined topological circuits. The work shows how circularly polarized light can reversibly switch the magnetic orientation of a strongly correlated ferromagnetic state, all without changing the sample temperature.

The experiments, led by Prof. Ataç Imamoğlu (ETH Zürich), Prof. Tomasz Smoleński (University of Basel), and colleagues, exploit a system where two atomically thin MoTe₂ layers are stacked with a small twist angle. This twist creates a moiré superlattice with flat, valley‑contrasting Chern bands, giving rise to highly correlated quantum phases - including Chern insulators and ferromagnetic metals - depending on the electron filling. Because the electronic bands are nearly dispersionless, electron-electron interactions dominate, resulting in spontaneous spin alignment even at cryogenic but steady temperatures.

Researchers from the University of Maryland, King Abdullah University of Science and Technology (KAUST), Nankai University, Cornell University, University of Wisconsin–Madison, Oak Ridge National Laboratory, University of California, University of Tennessee, Air Force Research Laboratory and Rice University recently reported the first experimental realization of non-volatile, electrical control of magnetism in a two-dimensional (2D) material system. The collaborative work demonstrates a robust interferroic magnetoelectric coupling in a van der Waals heterostructure made of atomic layers of ferroelectric CuCrP₂S₆ and ferromagnetic Fe₃GeTe₂ - marking a milestone for 2D multiferroic research and energy-efficient spintronic applications.

At the heart of this work lies the long-standing challenge of stabilizing ferroic order in truly two-dimensional materials. While ferroelectric and ferromagnetic phenomena are both well-established in bulk materials, their coexistence in 2D is difficult to maintain due to depolarization fields and thermal fluctuations that destabilize long-range order. The team overcame these limitations by stacking exfoliated layers of the ferroelectric CuCrP₂S₆ and ferromagnetic Fe₃GeTe₂ with atomically clean interfaces, enabling short-range, interfacial coupling between their ferroic orders.

Researchers from Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Donostia International Physics Center, Leibniz Institute for Solid State and Materials Research Dresden and IMDEA Nanoscience have identified ferromagnetic hexagonal close-packed (hcp) cobalt as a prototypical magnetic nodal-line semimetal that remains robust at room temperature, turning a classic ferromagnet into a highly tunable topological platform for spintronics.

Cobalt has long been regarded as a textbook elemental ferromagnet with a supposedly well-understood band structure, examined in detail for over 40 years. Using spin- and angle-resolved photoemission spectroscopy (spin-ARPES) at the BESSY II synchrotron, the team now observes entangled, spin-polarized bands that cross along extended paths in momentum space without opening an energy gap, even at room temperature. These measurements reveal a dense network of magnetic nodal lines - topological band crossings between two spin-polarized states - that give rise to fast, robust charge carriers central to future information and spin-based technologies.

Researchers at the University of Waterloo recently demonstrated fully electrical, field‑free control of perpendicular magnetization using spin‑orbit torque (SOT) in a low‑symmetry 2D magnet/topological‑insulator heterostructure, paving the way for scalable, energy‑efficient spintronic memory and logic devices.

Stacking the three-fold symmetry of BiSbTe on top of the two-fold symmetry of intercalated-CrTe, the interface only permits a unidirectional symmetry which produces an extremely strong out-of-plane spin torque and can deterministically switch a very hard, perpendicular magnet with ease. Image credit: University of Waterloo  

Modern MRAM and related spintronic memories need dense, robust perpendicular magnetic anisotropy (PMA) bits that can be switched deterministically with low energy consumption, but conventional SOT easily switches only in‑plane moments and typically requires an external bias field to tilt perpendicular spins “up” or “down”. In perpendicular configurations, bits point out of the film plane, which boosts storage density but makes the energy‑efficient, fully electrical control of their state difficult. Standard heavy‑metal/ferromagnet stacks already break out‑of‑plane symmetry and can support in‑plane switching, yet deterministic out‑of‑plane reversal demands breaking additional in‑plane symmetries - usually via an applied magnetic field, which adds circuit complexity, power overhead, and risks cross‑talk between neighboring bits.

Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have partnered with NTNU, the Norwegian University of Science and Technology in Trondheim, and the Institute of Nuclear Physics in the Polish Academy of Sciences to develop a method that facilitates the manufacture of particularly efficient magnetic nanomaterials in a relatively simple process based on inexpensive raw materials. 

Using a highly focused ion beam, they imprint magnetic nanostrips consisting of tiny, vertically aligned nanomagnets onto the materials. This geometry makes the material highly sensitive to external magnetic fields and current pulses.