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- ItemSingle Molecule Magnetism with Strong Magnetic Anisotropy and Enhanced Dy∙∙∙Dy Coupling in Three Isomers of Dy-Oxide Clusterfullerene Dy2O@C82(Chichester : John Wiley and Sons Ltd, 2019) Yang, W.; Velkos, G.; Liu, F.; Sudarkova, S.M.; Wang, Y.; Zhuang, J.; Zhang, H.; Li, X.; Zhang, X.; Büchner, B.; Avdoshenko, S.M.; Popov, A.A.; Chen, N.A new class of single-molecule magnets (SMMs) based on Dy-oxide clusterfullerenes is synthesized. Three isomers of Dy2O@C82 with Cs(6), C3v(8), and C2v(9) cage symmetries are characterized by single-crystal X-ray diffraction, which shows that the endohedral Dy−(µ2-O)−Dy cluster has bent shape with very short Dy−O bonds. Dy2O@C82 isomers show SMM behavior with broad magnetic hysteresis, but the temperature and magnetization relaxation depend strongly on the fullerene cage. The short Dy−O distances and the large negative charge of the oxide ion in Dy2O@C82 result in the very strong magnetic anisotropy of Dy ions. Their magnetic moments are aligned along the Dy−O bonds and are antiferromagnetically (AFM) coupled. At low temperatures, relaxation of magnetization in Dy2O@C82 proceeds via the ferromagnetically (FM)-coupled excited state, giving Arrhenius behavior with the effective barriers equal to the AFM-FM energy difference. The AFM-FM energy differences of 5.4–12.9 cm−1 in Dy2O@C82 are considerably larger than in SMMs with {Dy2O2} bridges, and the Dy∙∙∙Dy exchange coupling in Dy2O@C82 is the strongest among all dinuclear Dy SMMs with diamagnetic bridges. Dy-oxide clusterfullerenes provide a playground for the further tuning of molecular magnetism via variation of the size and shape of the fullerene cage.
- ItemShape-adaptive single-molecule magnetism and hysteresis up to 14 K in oxide clusterfullerenes Dy2O@C72 and Dy2O@C74 with fused pentagon pairs and flexible Dy-(μ2-O)-Dy angle(Cambridge : Royal Society of Chemistry, 2020) Velkos, G.; Yang, W.; Yao, Y.-R.; Sudarkova, S.M.; Liu, X.; Büchner, B.; Avdoshenko, S.M.; Chen, N.; Popov, A.A.Dysprosium oxide clusterfullerenes Dy2O@Cs(10528)-C72 and Dy2O@C2(13333)-C74 are synthesized and characterized by single-crystal X-ray diffraction. Carbon cages of both molecules feature two adjacent pentagon pairs. These pentalene units determine positions of endohedral Dy ions hence the shape of the Dy2O cluster, which is bent in Dy2O@C72 but linear in Dy2O@C74. Both compounds show slow relaxation of magnetization and magnetic hysteresis. Nearly complete cancelation of ferromagnetic dipolar and antiferromagnetic exchange Dy⋯Dy interactions leads to unusual magnetic properties. Dy2O@C74 exhibits zero-field quantum tunneling of magnetization and magnetic hysteresis up to 14 K, the highest temperature among Dy-clusterfullerenes.
- ItemAll-on-Chip Concurrent Measurements of the Static Magnetization and of the Electron Spin Resonance with Microcantilevers(Wien [u.a.] : Springer, 2021) Alfonsov, A.; Büchner, B.; Kataev, V.A large variety of the samples of novel magnetic materials, which are of high interest due to their exotic properties, are only available in very small sizes. In some cases, it is not possible to synthesize large single crystals; in other cases, the small size itself is the key prerequisite to manifest a specifically interesting property of the material. The smallness of a sample rises a problem of the detection of the static magnetic response and of the electron spin resonance (ESR) signal. To overcome this problem, we propose to use a cantilever-based (torque-detected) setup with the capability of a simultaneous measurement of ESR and static magnetization. This setup offers a high sensitivity and the ability to acquire along with the ESR signal the components of the magnetization tensor in a single experimental run. Here, we present the working principle of this setup, as well as the estimate of its sensitivity from the measurements on the standard Co Tutton salt sample. © 2021, The Author(s).
- ItemHolographic vector field electron tomography of three-dimensional nanomagnets(London : Nature Publishing Group, 2019) Wolf, D.; Biziere, N.; Sturm, S.; Reyes, D.; Wade, T.; Niermann, T.; Krehl, J.; Warot-Fonrose, B.; Büchner, B.; Snoeck, E.; Gatel, C.; Lubk, A.Complex 3D magnetic textures in nanomagnets exhibit rich physical properties, e.g., in their dynamic interaction with external fields and currents, and play an increasing role for current technological challenges such as energy-efficient memory devices. To study these magnetic nanostructures including their dependency on geometry, composition, and crystallinity, a 3D characterization of the magnetic field with nanometer spatial resolution is indispensable. Here we show how holographic vector field electron tomography can reconstruct all three components of magnetic induction as well as the electrostatic potential of a Co/Cu nanowire with sub 10 nm spatial resolution. We address the workflow from acquisition, via image alignment to holographic and tomographic reconstruction. Combining the obtained tomographic data with micromagnetic considerations, we derive local key magnetic characteristics, such as magnetization current or exchange stiffness, and demonstrate how magnetization configurations, such as vortex states in the Co-disks, depend on small structural variations of the as-grown nanowire.
- ItemSuperconducting switching due to a triplet component in the Pb/Cu/Ni/Cu/Co2Cr1-xFexAly spin-valve structure(Frankfurt am Main : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2019) Kamashev, A.A.; Garif'yanov, N.N.; Validov, A.A.; Schumann, J.; Kataev, V.; Büchner, B.; Fominov, Y.V.; Garifullin, I.A.We report the superconducting properties of the Co2Cr1-xFexAly/Cu/Ni/Cu/Pb spin-valve structure the magnetic part of which comprises the Heusler alloy layer HA = Co2Cr1-xFexAly with a high degree of spin polarization (DSP) of the conduction band and a Ni layer of variable thickness. The separation between the superconducting transition curves measured for the parallel (α = 0°) and perpendicular (α = 90°) orientation of the magnetization of the HA and the Ni layers reaches up to 0.5 K (α is the angle between the magnetization of two ferromagnetic layers). For all studied samples the dependence of the superconducting transition temperature Tc on α demonstrates a deep minimum in the vicinity of the perpendicular configuration of the magnetizations. This suggests that the observed minimum and the corresponding full switching effect of the spin valve is caused by the long-range triplet component of the superconducting condensate in the multilayer. Such a large effect can be attributed to a half-metallic nature of the HA layer, which in the orthogonal configuration efficiently draws off the spin-polarized Cooper pairs from the space between the HA and Ni layers. Our results indicate a significant potential of the concept of a superconducting spin-valve multilayer comprising a half-metallic ferromagnet, recently proposed by A. Singh et al., Phys. Rev. X 2015, 5, 021019, in achieving large values of the switching effect.
- ItemTopological Electronic Structure and Intrinsic Magnetization in MnBi4Te7: A Bi2Te3 Derivative with a Periodic Mn Sublattice(College Park, MD : American Physical Society, 2019) Vidal, R.C.; Zeugner, A.; Facio, J.I.; Ray, R.; Haghighi, M.H.; Wolter, A.U.B.; Corredor, Bohorquez, L.T.; Caglieris, F.; Moser, S.; Figgemeier, T.; Peixoto, T.R.F.; Vasili, H.B.; Valvidares, M.; Jung, S.; Cacho, C.; Alfonsov, A.; Mehlawat, K.; Kataev, V.; Hess, C.; Richter, M.; Büchner, B.; Van Den Brink, J.; Ruck, M.; Reinert, F.; Bentmann, H.; Isaeva, A.Combinations of nontrivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances, material candidates are emerging. Yet, so far a compound that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized. MnBi2Te4 has been established as the first antiferromagnetic topological insulator and constitutes the progenitor of a modular (Bi2Te3)n(MnBi2Te4) series. Here, for n=1, we confirm a nonstoichiometric composition proximate to MnBi4Te7. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topologically nontrivial surface state on the MnBi4Te7(0001) surface, analogous to the nonmagnetic parent compound Bi2Te3. Our results establish MnBi4Te7 as the first band-inverted compound with intrinsic net magnetization providing a versatile platform for the realization of magnetic topological states of matter.