Who
Zheyuan Liu (University of Tokyo)
Abstract
Recently, spin-symmetry classifications have identified a third type of collinear magnetic order—altermagnetism—named after its alternating spin polarizations in both real and reciprocal spaces [1]. Materials belonging to this class, termed altermagnets, combine
characteristics of conventional antiferromagnets and ferromagnets: they exhibit zero net
magnetization within a unit cell, yet possess spin-split electronic bands originating from a
non-relativistic exchange origin [2]. This unique combination enables efficient spin current
generation while excluding stray magnetic fields, offering great potential for the
miniaturization of spintronic devices. In this talk, I will discuss how neutron scattering
contributes to the discovery of altermagnetism, highlighting two recent studies on MnTe as
examples: the observation of chiral-split magnon via inelastic neutron scattering (INS) [3],
and the detection of altermagnetic order switching via polarized neutron diffraction (PoND).
In the INS experiment (performed at HRC, J-PARC), an altermagnetic magnon splitting
was clearly identified in MnTe. The measured magnon spectra were well reproduced by the
linear spin-wave theory based on a Heisenberg spin Hamiltonian with a pair of alternating
exchange interactions, highlighting the non-relativistic origin of altermagnetism.
Furthermore, the calculated neutron chiral factor revealed the magnon dispersions are
chiral-split, thereby confirming altermagnetism from the perspective of spin excitation.
In the PoND experiment (performed at PONTA, JRR-3), a pronounced nuclear-magnetic
interference was observed in MnTe bulk crystals. The time-reversal symmetry breaking in
altermagnets results in a non-zero NMI term [4], which directly manifests the altermagnetic
order, which is characterized by the Néel vector in altermagnet. The sign of NMI term was
reversed by field cooling with opposite magnetic field in the milli-Tesla-scale, suggesting
the switching of altermagnetic order. Complementary remanent magnetization
measurements confirmed that the weak ferromagnetism (WFM) in MnTe is coupled to the
altermagnetic order, establishing a general route for the magnetic control of altermagnetic
order via flipping the WFM. Moreover, we demonstrate PoND as a powerful bulk-sensitive
probe of altermagnetic order, complementing the local detection techniques like X-ray
dichroism [5], which are limited to the surfaces or thin films.
[1] L. Šmejkal et al., Phys. Rev. X 12, 031042 (2022). Phys. Rev. X 12, 040501 (2022).
[2] J. Krempaský et al., Nature 626, 517 (2024). S. Lee et al., Phys. Rev. Lett. 132, 036702
(2024). T. Osumi et al., Phys. Rev. B 109, 115102 (2024).
[3] Z. Liu et al., Phys. Rev. Lett. 133, 156702 (2024).
[4] P. A. McClarty et al., Phys. Rev. B 111, L060405 (2025).
[5] O. J. Amin et al., Nature 636, 348 (2024).
characteristics of conventional antiferromagnets and ferromagnets: they exhibit zero net
magnetization within a unit cell, yet possess spin-split electronic bands originating from a
non-relativistic exchange origin [2]. This unique combination enables efficient spin current
generation while excluding stray magnetic fields, offering great potential for the
miniaturization of spintronic devices. In this talk, I will discuss how neutron scattering
contributes to the discovery of altermagnetism, highlighting two recent studies on MnTe as
examples: the observation of chiral-split magnon via inelastic neutron scattering (INS) [3],
and the detection of altermagnetic order switching via polarized neutron diffraction (PoND).
In the INS experiment (performed at HRC, J-PARC), an altermagnetic magnon splitting
was clearly identified in MnTe. The measured magnon spectra were well reproduced by the
linear spin-wave theory based on a Heisenberg spin Hamiltonian with a pair of alternating
exchange interactions, highlighting the non-relativistic origin of altermagnetism.
Furthermore, the calculated neutron chiral factor revealed the magnon dispersions are
chiral-split, thereby confirming altermagnetism from the perspective of spin excitation.
In the PoND experiment (performed at PONTA, JRR-3), a pronounced nuclear-magnetic
interference was observed in MnTe bulk crystals. The time-reversal symmetry breaking in
altermagnets results in a non-zero NMI term [4], which directly manifests the altermagnetic
order, which is characterized by the Néel vector in altermagnet. The sign of NMI term was
reversed by field cooling with opposite magnetic field in the milli-Tesla-scale, suggesting
the switching of altermagnetic order. Complementary remanent magnetization
measurements confirmed that the weak ferromagnetism (WFM) in MnTe is coupled to the
altermagnetic order, establishing a general route for the magnetic control of altermagnetic
order via flipping the WFM. Moreover, we demonstrate PoND as a powerful bulk-sensitive
probe of altermagnetic order, complementing the local detection techniques like X-ray
dichroism [5], which are limited to the surfaces or thin films.
[1] L. Šmejkal et al., Phys. Rev. X 12, 031042 (2022). Phys. Rev. X 12, 040501 (2022).
[2] J. Krempaský et al., Nature 626, 517 (2024). S. Lee et al., Phys. Rev. Lett. 132, 036702
(2024). T. Osumi et al., Phys. Rev. B 109, 115102 (2024).
[3] Z. Liu et al., Phys. Rev. Lett. 133, 156702 (2024).
[4] P. A. McClarty et al., Phys. Rev. B 111, L060405 (2025).
[5] O. J. Amin et al., Nature 636, 348 (2024).