Beyond the Three Traditional Assumptions of Spintronics

Spintronics has transformed our ability to generate, transfer, and manipulate angular momentum in solids, enabling powerful strategies for current-driven control of magnetic states and devices. Despite its remarkable success, the field has traditionally been built upon three simplifying assumptions: a classical description of localized spins (magnetic moments), exchange interactions that act purely between spin degrees of freedom, and the predominance of spin dipole moments as the primary carriers of angular momentum. This talk explores what new physics emerges when these assumptions are relaxed and when additional internal degrees of freedom are taken seriously.
In particular, I will discuss the role of longitudinal quantum fluctuations in spin transfer processes [1], revealing regimes where angular momentum transport cannot be captured by conventional transverse spin dynamics. I will then introduce current-driven control of magnetism mediated by orbital exchange interactions [2], highlighting how orbital degrees of freedom provide a qualitatively new channel for coupling electric currents to magnetic order. Finally, I will present multipolar pathways for angular-momentum transport that go beyond conventional dipolar mechanisms [3], opening a broader framework for spintronics and orbitronics in which higher-rank moments play an active and controllable role.

[1] T. Lee et al., “Signatures of longitudinal spin pumping in a magnetic phase transition,” Nature, vol. 638, pp. 106–111, Jan. 2025, doi: 10.1038/s41586-024-08367-z. 
[2] G.-H. Lee, K.-W. Kim, and K.-J. Lee, “Orbital exchange-mediated current control of magnetism,” unpublished, 2025. 
[3] H.-W. Ko and K.-J. Lee, “Magnetic octupole Hall effect in d-wave altermagnets,” unpublished, Aug. 2025, arXiv:2508.00794.