Non-trivial spin arrangements in magnetic materials give rise to the topological Hall effect observed in compounds with a non-centrosymmetric cubic structure hosting a skyrmion lattice, in double-exchange ferromagnets and magnetically frustrated systems. The topological Hall effect has been proposed to appear also in presence of non-coplanar spin configurations and thus might occur in an antiferromagnetic material with a highly noncollinear and non-coplanar spin structure. Particularly interesting is a material where the non-collinearity develops not immediately at the onset of antiferromagnetic order but deep in the antiferromagnetic phase. This unusual situation arises in non-cubic antiferromagnetic Mn 5 Si 3 . Here we show that a large topological Hall effect develops well below the Néel temperature as soon as the spin arrangement changes from collinear to non-collinear with decreasing temperature. We further demonstrate that the effect is not observed when the material is turned ferromagnetic by carbon doping without changing its crystal structure.

Interfacing superconducting quantum processors, working in the GHz frequency range, with optical quantum networks and atomic qubits is a challenging task for the implementation of distributed quantum information processing as well as for quantum communication. Using spin ensembles of rare-earth ions provides an excellent opportunity to bridge microwave and optical domains at the quantum level. In this Rapid Communication, we demonstrate the ultralow-power, on-chip, electron-spin-resonance spectroscopy of Er 3+ spins doped in a Y 2 SiO 5 crystal using a high-Q, coplanar, superconducting resonator.Quantum communication is a rapidly developing field of science and technology, which allows the transmission of information in an intrinsically secure way. 1 As well as its classical counterpart, a quantum communication network can combine various types of systems which transmit, receive, and process information using quantum algorithms. 2 For example, the nodes of such a network can be implemented by superconducting (SC) quantum circuits operated in the GHz frequency range, 3 whereas fiber optics operated at near infrared can be used to link them over long distances. For the reversible transfer of quantum states between systems operating at GHz and optical frequency ranges, one must use a hybrid system. 4 Spin ensembles coupled to a microwave resonator or to a SC qubit represent one of the possible implementations of such a system. 5-8 The collective coupling strength of a spin ensemble is increased with respect to a single spin by the square root of the number of spins. Transparent crystals doped with paramagnetic ions often possess long coherence times, 9,10 and the collective coupling has been recently demonstrated with nitrogen-vacancy centers in diamond, 11-13 organic molecules, 14 and (Cr 3+ ) ions in ruby. 12 In this Rapid Communication, we report on the ultralowpower electron-spin-resonance (ESR) spectroscopy of an erbium-ion spin ensemble at sub-Kelvin temperatures using a high-Q, coplanar, SC resonator. The Er 3+ ions are distinct from other spin ensembles due to their optical transition at the telecom C band, i.e., inside the so-called erbium window at 1.54 μm wavelength, and their long measured optical coherence time. 15 The energy-level diagram of erbium ions embedded inside a crystal is shown in Fig. 1(a). The electronic configuration of a free Er 3+ ion is 4f 11 , with a 4 I term. The spin-orbit coupling splits it into several fine structure levels. An optical transition at the telecom wavelength occurs between the ground state 2S+1 L J = 4 I 15/2 and the first excited state 4 I 13/2 , where S, L, and J are the respective spin, orbital, and total magnetic momenta of the ion. The weak crystal field splits the ground state into eight (J + 1/2) Kramers doublets. 16 At cryogenic temperature, only the lowest doublet Z 1 is populated, therefore the system can be described as an effective electronic spin with S = 1/2. However, erbium has five even isotopes, 162 Er, 164 Er, 166 Er, 168 Er, and 170 Er, and one odd ...

We report on large thermoelectric effects in superconductor-ferromagnet tunnel junctions in proximity contact with the ferromagnetic insulator europium sulfide. The combination of a spin-splitting field and spin-polarized tunnel conductance in these systems breaks the electron-hole symmetry and leads to spin-dependent thermoelectric currents. We show that the exchange splitting induced by the europium sulfide boosts the thermoelectric effect in small applied fields and can therefore eliminate the need to apply large magnetic fields, which might otherwise impede applications in thermometry or cooling.

We report on nonlocal spin transport in mesoscopic superconducting aluminum wires in contact with the ferromagnetic insulator europium sulfide. We find spin injection and long-range spin transport in the regime of the exchange splitting induced by europium sulfide. Our results demonstrate that spin transport in superconductors can be manipulated by ferromagnetic insulators, and opens a new path to control spin currents in superconductors.

The anomalous Hall effect (AHE), which in long-range ordered ferromagnets appears as a voltage transverse to the current and usually is proportional to the magnetization, often is believed to be of negligible size in antiferromagnets due to their low uniform magnetization. However, recent experiments and theory have demonstrated that certain antiferromagnets with a non-collinear arrangement of magnetic moments exhibit a sizeable spontaneous AHE at zero field due to a non-vanishing Berry curvature arising from the quantum mechanical phase of the electron’s wave functions. Here we show that antiferromagnetic Mn5Si3 single crystals exibit a large AHE which is strongly anisotropic and shows multiple transitions with sign changes at different magnetic fields due to field-induced rearrangements of the magnetic structure despite only tiny variations of the total magnetization. The presence of multiple non-collinear magnetic phases offers the unique possiblity to explore the details of the AHE and the sensitivity of the Hall effect on the details of the magnetic texture.

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Ferromagnetic Mn5Si3C0.8 and Mn5Ge3C0.8 films with Curie temperatures TC well above room temperature are obtained by C+12-ion implantation in antiferromagnetic Mn5Si3 or ferromagnetic Mn5Ge3. Patterning of the films with a gold mesh serving as a stencil mask during implantation allows a lateral modification of magnetic order creating ferromagnetic regions of Mn5Si3C0.8 which are embedded in antiferromagnetic Mn5Si3. This provides a procedure for the fabrication of magnetoelectronic hybrid devices comprised of different magnetic phases.

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