Propagating Low-Energy 4f Paramagnons

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The concept of spin waves and their quanta magnons was introduced by F. Bloch in 1930. Today, the field of science investigating spin waves and magnons and utilizing them for data processing is known as magnonics. The typical materials used to investigate magnons are the magnetically ordered ferro- and anti-ferromagnets (FMs and AFMs) at temperatures below the critical (Curie or Neel) temperature. Above these temperatures, the magnetic order is lost, but the exchange stiffness, responsible for the spontaneous magnetic ordering, although decreases, remains finite. This phenomenon allowed for the observation of high-energy (> 10 meV) magnons also in the paramagnets (PMs) by inelastic neutron scattering. These magnons are called paramagnons. In the project "ParaMagnonics", we will apply a strong external magnetic field and will decrease the temperature of the PMs, to achieve the essential ordering of the magnetic moments. These quasi-independent moments will still be coupled by the remaining exchange-stiffness and/or by the dipole-dipolar interaction. Exploiting the variation of the materials, temperature and applied magnetic fields, we will prove the existence of propagating low-energy (< 0.5 meV) dipolar and exchange paramagnons in PMs and in FMs and AFMs above the critical temperatures. Consequently, we will investigate their dispersions, damping, and propagation characteristics. Our hypothesis is that paramagnons carried by 4f ions should have the fundamentally lowest damping values due to their isolation from the crystal lattice. Finally, we will exploit spin-orbit torque (SOT) phenomena to excite, amplify, and detect ballistic and diffusive paramagnons. We will use different PM and the family of Eu-chalcogenides of FM and AFM nature in particular. The paramagnons will be detected and characterized by low-temperature microwave and Brillouin Light Scattering spectroscopy. Electrical measurements and SOT phenomena will be utilized to understand the nature of the diffusive paramagnons spin transport as well as to excite and amplify them. ParaMagnonics has a very high degree of originality, as propagating low-energy paramagnons have never been observed experimentally and they have a whole range of advantages over ordinary magnons: a very broad class of PM materials for magnonics, low damping, sensitive control of paramagnon properties and new degrees of freedom for this control, and a novel toolbox for quantum magnonics.