2023, Tschirner, Teresa, Keßler, Philipp, Gonzalez Betancourt, Ruben Dario, Kotte, Tommy, Kriegner, Dominik, Büchner, Bernd, Dufouleur, Joseph, Kamp, Martin, Jovic, Vedran, Smejkal, Libor, Sinova, Jairo, Claessen, Ralph, Jungwirth, Tomas, Moser, Simon, Reichlova, Helena, Veyrat, Louis

Observations of the anomalous Hall effect in RuO2 and MnTe have demonstrated unconventional time-reversal symmetry breaking in the electronic structure of a recently identified new class of compensated collinear magnets, dubbed altermagnets. While in MnTe, the unconventional anomalous Hall signal accompanied by a vanishing magnetization is observable at remanence, the anomalous Hall effect in RuO2 is excluded by symmetry for the Néel vector pointing along the zero-field [001] easy-axis. Guided by a symmetry analysis and ab initio calculations, a field-induced reorientation of the Néel vector from the easy-axis toward the [110] hard-axis was used to demonstrate the anomalous Hall signal in this altermagnet. We confirm the existence of an anomalous Hall effect in our RuO2 thin-film samples, whose set of magnetic and magneto-transport characteristics is consistent with the earlier report. By performing our measurements at extreme magnetic fields up to 68 T, we reach saturation of the anomalous Hall signal at a field Hc ≃ 55 T that was inaccessible in earlier studies but is consistent with the expected Néel-vector reorientation field.

2019, Souchay, Daniel, Nentwig, Markus, Günther, Daniel, Keilholz, Simon, de Boor, Johannes, Zeugner, Alexander, Isaeva, Anna, Ruck, Michael, Wolter, Anja U.B., Büchnerde, Bernd, Oeckler, Oliver

The crystal structures of new layered manganese bismuth tellurides with the compositions Mn0.85(3)Bi4.10(2)Te7 and Mn0.73(4)Bi6.18(2)Te10 were determined by single-crystal X-ray diffraction, including the use of microfocused synchrotron radiation. These analyses reveal that the layered structures deviate from the idealized stoichiometry of the 12P-GeBi4Te7 (space group P3m1) and 51R-GeBi6Te10 (space group R3m) structure types they adopt. Modified compositions Mn1-xBi4+2x/3Te7 (x = 0.15-0.2) and Mn1-xBi6+2x/3Te10 (x = 0.19-0.26) assume cation vacancies and lead to homogenous bulk samples as confirmed by Rietveld refinements. Electron diffraction patterns exhibit no diffuse streaks that would indicate stacking disorder. The alternating quintuple-layer [M2Te3] and septuple-layer [M3Te4] slabs (M = mixed occupied by Bi and Mn) with 1 : 1 sequence (12P stacking) in Mn0.85Bi4.10Te7 and 2 : 1 sequence (51R stacking) in Mn0.81Bi6.13Te10 were also observed in HRTEM images. Temperature-dependent powder diffraction and differential scanning calorimetry show that the compounds are high-temperature phases, which are metastable at ambient temperature. Magnetization measurements are in accordance with a MnII oxidation state and point at predominantly ferromagnetic coupling in both compounds. The thermoelectric figures of merit of n-type conducting Mn0.85Bi4.10Te7 and Mn0.81Bi6.13Te10 reach zT = 0.25 at 375 °C and zT = 0.28 at 325 °C, respectively. Although the compounds are metastable, compact ingots exhibit still up to 80% of the main phases after thermoelectric measurements up to 400 °C. © The Royal Society of Chemistry 2019.

2022, Naselli, Gabriele, Moghaddam, Ali G., Di Napoli, Solange, Vildosola, Verónica, Fulga, Ion Cosma, van den Brink, Jeroen, Facio, Jorge I.

We analyze the electronic structure of topological surface states in the family of magnetic topological insulators MnBi2nTe3n+1. We show that, at natural-cleavage surfaces, the Dirac cone warping changes its symmetry from hexagonal to trigonal at the magnetic ordering temperature. In particular, an energy splitting develops between the surface states of the same band index but opposite surface momenta upon formation of the long-range magnetic order. As a consequence, measurements of such energy splittings constitute a simple protocol to detect the magnetic ordering via the surface electronic structure, alternative to the detection of the surface magnetic gap. Interestingly, while the latter signals a nonzero surface magnetization, the trigonal warping predicted here is, in addition, sensitive to the direction of the surface magnetic flux. Our results may be particularly useful when the Dirac point is buried in the projection of the bulk states, caused by certain terminations of the crystal or in hole-doped systems, since in both situations the surface magnetic gap itself is not accessible in photoemission experiments.

2008, Scheerbaum, N., Heczko, O., Liu, J., Hinz, D., Schultz, L., Gutfleisch, O.

Magnetic field-induced twin boundary motion leading to large magnetic field-induced strain of ~1.0% was established in polycrystalline Ni50.9Mn27.1Ga22.0 (at.%) fibres at room temperature (~60–100 μm in diameter and ~3 mm in length). The fibres' grains are as large as the fibre diameter and of random orientation. At room temperature, a ferromagnetic 5M martensite is found. Magnetic field-induced twin boundary motion was indicated by magnetic measurements and validated by electron backscatter diffraction (EBSD). The application of a magnetic field shifts the equilibrium temperature of martensite and austenite by ~0.4 K T−1, which agrees with calculations using the Clapeyron–Clausius approach.

2017, Papa, Veronica, Cabrero-Antonino, Jose R., Alberico, Elisabetta, Spanneberg, Anke, Junge, Kathrin, Junge, Henrik, Beller, Matthias

Novel well-defined NNP and PNP manganese pincer complexes have been synthetized and fully characterized. The catalyst Mn-2 containing an imidazolyaminolphosphino ligand shows high activity and selectivity in the hydrogenation of a wide range of secondary and tertiary amides to the corresponding alcohols and amines, under relatively mild conditions. For the first time, more challenging substrates like primary aromatic amides including an actual herbicide can also be hydrogenated using this earth-abundant metal-based pincer catalyst.

2023, Sahoo, M., Salman, Z., Allodi, G., Isaeva, A., Folkers, L., Wolter, A.U.B., Büchner, B., De Renzi, R.

We investigated the magnetic properties of polycrystalline samples of the intrinsic magnetic topological insulators MnPn2Te4, with pnictogen Pn = Sb, Bi, by bulk magnetization and μSR. DC susceptibility detects the onset of magnetic ordering at TN = 27 K and 24 K and a field dependence of the macroscopic magnetization compatible with ferri- (or ferro-) and atiferro- magnetic ordering, respectively. Weak transverse field (wTF) Muon Spin Rotation (μSR) confirms the homogeneous bulk nature of magnetic ordering at the same two distinct transition temperatures. Zero Field (ZF) μSR shows that the Sb based material displays a broader distribution of internal field at the muon, in accordance with a larger deviation from the stoichiomectric composition and a higher degree of positional disorder (Mn at the Pn(6c) site), which however does not affect significantly the sharpness of the thermodynamic transition, as detected by the muon magnetic volume fraction and the observability of a critical divergence in the longitudinal and transverse muon relaxation rates.

2020, Wilhelm, Marek, Giesen, Margret, Duchoň, Tomáš, Moors, Marco, Mueller, David N., Hackl, Johanna, Baeumer, Christoph, Hamed, Mai Hussein, Cao, Lei, Zhang, Hengbo, Petracic, Oleg, Glöß, Maria, Cramm, Stefan, Nemšák, Slavomír, Wiemann, Carsten, Dittmann, Regina, Schneider, Claus M., Müller, Martina

Magneto-ionic control of magnetism is a promising route toward the realization of non-volatile memory and memristive devices. Magneto-ionic oxides are particularly interesting for this purpose, exhibiting magnetic switching coupled to resistive switching, with the latter emerging as a perturbation of the oxygen vacancy concentration. Here, we report on electric-field-induced magnetic switching in a La0.7Sr0.3MnO3 (LSMO) thin film. Correlating magnetic and chemical information via photoemission electron microscopy, we show that applying a positive voltage perpendicular to the film surface of LSMO results in the change in the valence of the Mn ions accompanied by a metal-to-insulator transition and a loss of magnetic ordering. Importantly, we demonstrate that the voltage amplitude provides granular control of the phenomena, enabling fine-tuning of the surface electronic structure. Our study provides valuable insight into the switching capabilities of LSMO that can be utilized in magneto-ionic devices. © 2020 Author(s).

2009, Rößler, S., Harikrishnan, S., Naveen Kumar, C.M., Bhat, H.L., Elizabeth, S., Rößler, U.K., Steglich, F., Wirth, S.

We report on the magnetic and electrical properties of Pr 0.6Sr 0.4MnO 3 single crystals. This compound undergoes a continuous paramagnetic-ferromagnetic transition with a Curie temperature T C301 K and a first-order structural transition at T S64 K. At T S, the magnetic susceptibility exhibits an abrupt jump, and a corresponding small hump is seen in the resistivity. The critical behavior of the static magnetization and the temperature dependence of the resistivity are consistent with the behavior expected for a nearly isotropic ferromagnet with short-range exchange belonging to the Heisenberg universality class. The magnetization (M-H) curves below T S are anomalous in that the virgin curve lies outside the subsequent M-H loops. The hysteretic structural transition at T S as well as the irreversible magnetization processes below T S can be explained by phase separation between a high-temperature orthorhombic and a low-temperature monoclinic ferromagnetic phase.

2021, Liu, Xin, Werner, Thomas

The reduction of polar bonds, in particular carbonyl groups, is of fundamental importance in organic chemistry and biology. Herein, we report a manganese pincer complex as a versatile catalyst for the transfer hydrogenation of amides, carbamates, urea derivatives, and even polyurethanes leading to the corresponding alcohols, amines, and methanol as products. Since these compound classes can be prepared using CO2as a C1 building block the reported reaction represents an approach to the indirect reduction of CO2. Notably, these are the first examples on the reduction of carbamates and urea derivatives as well as on the C-N bond cleavage in amides by transfer hydrogenation. The general applicability of this methodology is highlighted by the successful reduction of 12 urea derivatives, 26 carbamates and 11 amides. The corresponding amines, alcohols and methanol were obtained in good to excellent yields up to 97%. Furthermore, polyurethanes were successfully converted which represents a viable strategy towards a circular economy. Based on control experiments and the observed intermediates a feasible mechanism is proposed. © The Royal Society of Chemistry 2021.

2008, Thomas, M., Heczko, O., Buschbeck, J., Rößler, U.K., McCord, J., Scheerbaum, N., Schultz, L., Fähler, S.

Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni-Mn-Ga films. Ni-Mn-Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45° relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing micro structural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.