Electronic structure and spin-dependent transport of altermagnets and odd-parity-wave magnets

Abstract

Spintronics is a field of research and technology employing electron spin in electronic devices. Conventionally, generating a spin degree of freedom has been achieved using ferromagnets and systems with large spin-orbit coupling. More recently, antiferromagnetic spintronics has explored the principle ability of compensated magnets to be highly densified in devices, their ultrafast dynamics, and the possibility of high switching energy efficiency. However, functionalizing conventional collinear antiferromagnets for spin-dependent effects is nontrivial. Their antiferromagnetic electronic band structure is either spin degenerate, or it is spin-split due to relativistic spin-orbit coupling, which is weak for common elements and more significant only in heavy, rare elements.

This thesis concerns a different emerging mechanism for generating a spin degree of freedom based on unconventional, magnetically compensated magnets. Among these are the recently identified even-parity (e.g., d- or g-wave) altermagnets and odd-parity (e.g., p- or f-wave) magnets—the latter we theoretically predict in this thesis [publication1, publication2]. Generating a spin polarization in the electronic structure in these unconventional magnets does not require net magnetization or heavy relativistic elements but instead forms due to distributions of nonrelativistic exchange fields in the crystal. We discuss and formulate the spin group symmetry criteria for these unconventional magnets. Unlike the commonly studied magnetic symmetries, which consider a single operation acting on the spin and lattice degree of freedom, spin symmetries consider pairs of operations that can act differently on these two degrees of freedom, opening new possibilities for studying and identifying magnetic phases.

We also contribute to the first experimental confirmations of the altermagnetic electronic structure with our ab initio calculations and symmetry analysis [publication3, publication4, publication5].

Moreover, we propose concepts for spintronics effects based on altermagnets and p-wave magnets. We develop new concepts for giant and tunneling magnetoresistance effects in altermagnets which originate from their spin-polarized, exchange-split band structure in the nonrelativistic limit, which is unprecedented for collinear compensated magnets [publication6]. We have also proposed spin-dependent effects in our p-wave magnets with a special focus on the coplanar class, including nonrelativistic transport anisotropy [publication1] and Edelstein effects [publication2], which is however beyond the scope of this thesis.

Our findings can have broad implications, ranging from fundamental research, such as topology and charge-to-spin conversion with lighter-element materials, to the more applied, such as memory and sensing applications.