Spin transport in insulating altermagnets: from hopping conduction to altermagnons
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Abstract
Altermagnetism has recently emerged as a particular symmetry-defined magnetic phase
that combines compensated magnetic order in real space with large spin polarization in
reciprocal space. This thesis establishes consequences of the altermagnetic symmetry
that are experimentally testable and have technologically potentially relevant transport
functionalities, with an emphasis on insulating systems where magnons carry spin. A
unified theoretical background is explored for spin injection, propagation, and detection
in compensated magnets, combined with the symmetry constraints that govern
altermagnetic spin splitting and altermagnon polarization.
Experimentally, the thesis implements from device fabrication to harmonic transport
protocols, synchrotron photoemission electron microscopy, and magnon spin transport.
In Ti-doped hematite, angle-dependent Hall measurements reveal a symmetry controlled
odd transverse response that emerges only in the spin flop phase, accompanied
by crystal orientation-dependent sign inversions. XMLD and XMCD-based
PEEM reconstruct the absolute Néel vector orientation and correlate real-space domain
changes with changes of the transport pseudovector. In insulating orthoferrites,
spin Hall magnetoresistance is established as a local electrical probe of the interfacial
magnetic configuration. Non-local measurements then demonstrate altermagnetic
magnon transport in d-wave orthoferrites, including direction-dependent sign inversions
between the Γ–U and Γ–U′ directions, finite responses at zero magnetic field, and
a non-monotonic distance dependence with sign reversal results from the competition
between different exchange split magnon modes. Simulations reproduce these features
and show that they vanish when the altermagnetic exchange splitting is removed, establishing
a direct link between the observed transport signatures and altermagnetism.
These results identify insulating orthoferrites and hematite as concrete platforms
for field-free, symmetry-defined, low-power magnon spin transport and for combined
transport and microscopy diagnostics of altermagnetic order.