SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023
Sayantika Bhowal
Recently, a new class of antiferromagnets, also known as altermagnets, has been proposed that exhibit non-relativistic spin-splitting in contrast to the conventional antiferromagnets with degenerate up and down spin-polarized bands. Taking the example of prototypical non-relativistic spin-split centrosymmetric antiferromagnet rutile MnF2, in my talk, I will show that the non-relativistic spin-splitting is conveniently described in terms of the ferroic ordering of magnetic octupoles. Using density-functional and model calculations, we show that the magnetic octupole is the lowest-order ferroically ordered magnetic quantity in this case, and so is the natural order parameter for the transition into the magnetically ordered state. They provide a unified description of the broken time-reversal symmetry and the non-relativistic spin splitting as well as a platform for manipulating the latter and account for other phenomena, such as piezomagnetism, characteristic of this class of antiferromagnets. Unusually for antiferromagnets, we show that the magnetic octupoles cause a non-zero magnetic Compton scattering, providing also a route for their direct experimental detection.
Reference:
Bhowal and N. A. Spaldin, arXiv:2212.03756 (2022).
SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023
Andrew Mackenzie
I will describe an instrument that is under construction in our department that will enable the imaging of a number of optical response functions in magnets and correlated electron systems. The design philosophy is for a moving-sample, fixed-optics setup, to maximise the flexibility. Spatial resolution will be of order microns, over a temperature range of approximately 15 - 330 K, and the instrument includes a uniaxial pressure cell with which the samples of interest can be tuned.
SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023
Angel Rubio
The experimental realization of atomically thin layers of magnetic van der Waals (vdW) materials has sparked a new interest in two-dimensional magnetism. In particular, and due to the presence of strong anisotropy, quantum fluctuations, and spin-orbit e ects, these materials are prime candidates to host exotic and topological phenomena of interest for future spintronics and quantum computation devices. They are also sensitive to a wide range of material engineering techniques, allowing their electronic and magnetic state to be tuned with high precision. Although recent progress has demonstrated that optical engineering techniques can be used realize exotic non-equilibrium states, driving a system with lasers is often associated with excessive heating. A path to circumvent this problem is to instead embed the system in an optical cavity, where the effective light-matter coupling can be enhanced via photon mode volume compression. Here, we identify a nearly ideal material platform to explore cavity quantum electrodynamics (c-QED) engineering by extending c-QED into the magnetic regime of two-dimensional heterostructures. Speci cally, we demonstrate how an optical cavity can be used to control the magnetic ground state of the proximate quantum spin liquid -RuCl3. Depending on the cavity frequency, photon occupation, and the strength of the e ective light-matter coupling, we nd that it is possible to transform the equilibrium zigzag antiferromagnetic order into any of the magnetic phases supported by the extended Kitaev model. As our key result we nd that for frequencies of a few THz and for moderate light-matter couplings, the interaction between the magnetic system and the vacuum uctuations of the cavity is su fficient to transform -RuCl3 from a zigzag antiferromagnet to a ferromagnet. This constitute a realistic proposal of where the magnetic order of a material system is controlled solely by vacuum fluctuations. In addition, we nd that by pumping the cavity to the few photon regime, it is possible to push the system into the antiferromagnetic Kitaev quantum spin liquid state.
SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023
Edgar Felipe Galindez Ruales
Altermagnets, i.e., materials with alternating order both in real as well as in reciprocal space are of major interest for fundamental physics as well as possible applications. This novel class of magnetically ordered materials features a large crystal electric field driven splitting of the valence states associated with the antiferromagnetically ordered sublattices. As a consequence, both spin polarized currents as well as pure spin currents are possible without any net magnetization in altermagnets. We can now utilize this type of magnetic order as well a spin source based on altermagnets. Based on the theoretical prediction of several altermagnetic materials in oxides such as RuO2 [1], we explore high transport spin-polarization and novel effects such as the spin-splitter torque and associated large magnetoresistance effects. In particular by doping typically insulating altermagnetic oxides, we can open a new class of altermagnets to transport investigations [2].
[1] A. Bose et al., Nature Electron.
5, 267 (2022)
[2] E. Galindez-Ruales et al., (in preparation)
SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023
Paul McClarty
The group theory of magnetic materials has a long history dating back to the early days of quantum mechanics and culminating in the relatively recently completed representation theory for the 1651 magnetic space groups. But we know from old work by Bill Brinkman and Roger Elliott that the magnetic space groups do not complete the set of symmetry groups relevant to magnetism. Instead we must consider the larger set of spin space groups. In this talk I discuss magnon band structures, how symmetries constrain and enrich their features, how the magnetic interactions feed into the relevant symmetries and I give examples of natural models with magnetically enhanced degeneracies, gapped topology and magnonic alternmagnetism.
SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023
Sebastian Goennenwein
In conventional antiferromagnets, transverse transport phenomena such as the anomalous (spontaneous) Hall or Nernst effects must vanish owing to symmetry reasons. In contrast, altermagnetic materials – a new class of magnetic materials with zero net magnetic moment, however featuring broken time reversal symmetry and momentum-dependent spin splitting – do allow for finite spontaneous transverse transport response [1].
In the presentation, I will discuss our recent experiments with Mn5Si3 thin films [2]. Owing to a dominant antiferromagnetic exchange interaction, the net magnetic moment of the films is vanishingly small. Nevertheless, we observe a large and strongly hysteretic anomalous Hall effect in our samples, which characteristically depends on film crystallinity. In addition, the samples also show a non-trivial Nernst response. All these experimental findings can be straightforwardly rationalised in terms of the altermagnetic properties of Mn5Si3 proposed by theory [1].
[1] L. Smejkal, J. Sinova, and T. Jungwirth, Phys. Rev. X
12, 040501 (2022)
[2] H. Reichlova
et al., arXiv:2012.15651
SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023
Nirmal J. Ghimire
The new concept of altermagnetism brings a new excitement in the materials landscape of antiferromagnetic spintronics. An unambiguous experimental realization remains challenging not just due to the limited number of the candidate materials, but also due to strict requirement for its manifestation in magnetotransport measurements. In this talk we will present our recent effort in the synthesis and magnetotransport studies of new altermagnetic materials. We will primarily focus on the study of MnTe and CrSb.
SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023
Helena Reichlova
Spintronics with antiferromagnets is a quickly developing research direction and in the last decade many spintronics phenomena were demonstrated in magnetically ordered systems with vanishing magnetization [1,2]. Very recently also the collinear magnets – altermagnets - were predicted to exhibit the anomalous Hall effect [3] arising from their spin and crystal symmetry. I will discuss magneto-transport experiments in altermagnets MnTe [4] and RuO2, and compare the results to the measurements of the anomalous Hall effect in Mn5Si3 thin films [5].
References
[1] T. Jungwirth et al., Nat. Nano. 11, 231–241 (2016)
[2] V. Baltz et al., Rev. Mod. Phys. 90, 015005 (2018)
[3] L. Smejkal et al., Sci. Adv. 6, aaz8809 (2020)
[4] R. Gonzalez-Betancourt et al., Phys. Rev.Lett. (2023)
[5] H. Reichlova et al., arXiv:2012.15651 (2021)
