Modern Magnetism and Spintronics

Fundamental Physics in Antiferromagnets

Transport of spin angular momenta has been a central topic in the field of magnetism and spintronics. Recently, it has been identified that quantum spin-wave excitations (dubbed "magnons") can transmit spins without physical movements of electrons. As a result, magnetic insulators can function as "metals" concerning pure spin transport (i.e., conducting spins while blocking charges), introducing a new paradigm of pure spin current.

Physical behaviors of magnons are fundamentally different from their electronic counterparts. Magnons are charge neutral quasiparticles that obey the Bose-Einstein statistics with zero chemical potential. Therefore, being able to transport spins does not automatically grant magnons a similar level of physical significance as electrons. In particular, the electron spin serves as an internal degree of freedom controllable via magnetic fields or spin-orbit interactions. By contrast, the magnon spin polarization in a ferromagnetic material (FM) is always opposite to the local magnetization, which is essentially a fixed quantity.

The situation, however, is fundamentally different when turning to antiferromagnetic materials (AFMs) in which neighboring magnetic moments compensate at the atomic length scale. In a collinear AFM with uniaxial anisotropy, symmetry guarantees that spin-up and spin-down magnons coexist; they form an intrinsic degree of freedom capable of encoding information. This unique feature opens an intriguing playground to explore the magnonic counterparts of pure spin phenomena usually associated with electrons and photons. For example, the magnon spin Nernst effect refers to the generation of a pure spin current transverse to a temperature gradient drive; it can intuitively be regarded as the magnonic analog of the spin Hall effect. What plays the role of spin-orbit coupling here is the Dzyaloshinskii-Moriya interaction (DMI) enabled by the symmetry breaking of a magnet.

The transport properties of magnons have profound physical implications embedded in the geometric phase (a.k.a. Berry phase), which arises from the non-trivial phase relation of the spin waves that reflects the hidden topology of the magnonic band structures not perceptible in traditional perspectives.

Being one of the pioneers in the emerging frontier of antiferromagnetic spintronics, our research group strives to unravel the fundamental mechanisms behind a wide variety of transport phenomena in magnetic materials. We are especially interested in the interplay among spin, charge, thermal, and mechanical transport taking place on the nanometer scale where quantum behaviors become prominent. We utilize theoretical tools including, but not limited to, the Boltzmann equation, spin diffusion equation, effective Lagrangian, lattice Green's function, and micromagnetics.

• Topological magnonics: Similar to electrons, the Berry phase associated with magnons gives rise to topologically protected edge states and chiral spin currents confined on the boundaries. In particular, when the magnon edge states appear near zero energy, they can be easily populated by thermal agitations (remember: magnons are bosons), which leads to exotic thermomagnetic behavior not available in electrons. Meanwhile, the magnonic Berry phase in AFMs exhibits a hyperbolic geometry in which the radius of Bloch sphere is "-1" (c.f., normal unit sphere has radius +1), entailing a distinct mathematical structure. Candidate materials for exploring topological magnons include, but not limited to, MnPS₃, CrI₃, alpha-Fe₂O₃ and their variances.
• Quantum Behaviors: Magnons are bosons that exhibit zero-point quantum fluctuations. At ultra-low temperatures, quantum effects become dominant as thermal magnons are suppressed. For example, the quantum energy of antiferromagnetic magnons confined by two adjacent magnets depends on the relative orientation of the two magnets, which induces a magnonic Casimir effect acting on the spin degree of freedom. This effect contributes to the magnon-mediated interlayer coupling and alters the magnetization dynamics of the whole system. The quantum behavior depends on the magnon spectrum of AFM, which can be classified into two categories: uniaxial AFMs such as Cr₂O₃, MnF₂, and FeF₂; bi-axial AFMs such as NiO and MnO.
• Interfacial Spin Interconversion: Magnons can deliver spin angular momenta to an adjacent conductor via spin-dependent scattering at the interface, which creates a pure spin current to the outside. Reciprocally, a spin current flowing towards a magnetic insulator can generate Terahertz spin dynamics through spin-transfer torques. Theses effects manifest the interconversion of spin angular momenta between different physical systems that usually involve intricate spin-charge dynamics. What is unique to AFMs is that the sublattice degree of freedom (or "pseudo-spin") always entangles with the spin. Consequently, electrons transferring into and out of an AFM will mingle spins with pseudo-spins, invoking fundamentally different physics compared to FMs.

Physics-Enabled Novel Applications

It is an appealing development in spintronics to recognize AFMs as promising candidate materials for next-generation nanodevices. AFMs can potentially lead to tranformative technological advances because of the following salient features:

• Terahertz: AFMs operate in the Terahertz (THz) regime, which is 2~3 orders of magnitude faster than ferromagnets that have been widely utilized in electronics. Ultrafast operations of AFMs arise from unique physics rather than smart designs, holding huge potential to break the theoretical limit of ferromagnet-based devices. My recent predictions of THz spin pumping, ultrafast switching (10's picosecond), THz nano-oscillator (THz "clock" generator), high-speed domain wall motions, etc. based on AFMs, have triggered an intense experimental search for novel and multi-functional AFMs.

• Spin: As mentioned above, AFMs allow for magnon excitations with both spin polarizations. This unique degree of freedom - ONLY available in AFMs - enables magnons to be active information carriers. Since magnons are charge neutral, they do not incur Joule heating when propagating in space. Consequently, magnons can preserve and deliver spin information over long distances (many orders of magnitude longer than electrons!) with rather low dissipation. Moreover, it is theoretically possible to coherently manipulate the magnon spin and process information beyond binary, which ushers a new arena for quantum computing. At present, quantum computing is being widely investigated using electrons and photons, but a magnon-based quantum computing has not been clearly visioned. With the recent progress in AFM spintronics, however, the inquiry into the magnon chirality will eventually establish a new thrust in quantum information science.

• Robustness: Macroscopic magnetization vanishes in most AFMs because neighboring atomic spins compensate. For this reason, AFM-based devices are free from stray fields and the concominant crosstalk between adjacent computing or memory units. Utilizing AFMs as active spintronic materials will significantly improve the scalability and immunity of nanodevices. In the absence of magnetization, however, the read-out mechanism should also change radically, which calls for additional (and presently unclear) physical effects including magneostriction, multiferroic responses, etc. Addressing those challenges are at the heart of current research and will shape the forthcoming landscape of spintronics and magnetism.

Funding Sources

• NSF CAREER award (2024 - Present)
• DoD MURI award (2019 - Present)
• Regents' Faculty Fellowships (Intramural, 2021 - 2022)
• Omnibus Research and Travel Award (Intramural, 2020 - 2021)
• SHINES center Seed fund (Intramural, 2019 - 2020)