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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.
- 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).
- ItemHyper-domains in exchange bias micro-stripe pattern(Milton Park : Taylor & Francis, 2008) Theis-Bröhl, K.; Westphalen, A.; Zabel, H.; Rücker, U.; McCord, J.; Höink, V.; Schmalhorst, J.; Reiss, G.; Weis, T.; Engel, D.; Ehresmann, A.; Toperverg, B.P.A combination of experimental techniques, e.g. vector-MOKE magnetometry, Kerr microscopy and polarized neutron reflectometry, was applied to study the field induced evolution of the magnetization distribution over a periodic pattern of alternating exchange bias (EB) stripes. The lateral structure is imprinted into a continuous ferromagnetic/antiferromagnetic EB bilayer via laterally selective exposure to He-ion irradiation in an applied field. This creates an alternating frozen-in interfacial EB field competing with the external field in the course of the re-magnetization. It was found that in a magnetic field applied at an angle with respect to the EB axis parallel to the stripes the re-magnetization process proceeds via a variety of different stages. They include coherent rotation of magnetization towards the EB axis, precipitation of small random (ripple) domains, formation of a stripe-like alternation of the magnetization, and development of a state in which the magnetization forms large hyper-domains comprising a number of stripes. Each of those magnetic states is quantitatively characterized via the comprehensive analysis of data on specular and off-specular polarized neutron reflectivity. The results are discussed within a phenomenological model containing a few parameters, which can readily be controlled by designing systems with a desired configuration of magnetic moments of micro- and nano-elements.
- ItemMagnetoelastic coupling and ferromagnetic-type in-gap spin excitations in multiferroic α-Cu2V2O7(Bristol : Institute of Physics Publishing, 2018) Wang, L.; Werner, J.; Ottmann, A.; Weis, R.; Abdel-Hafiez, M.; Sannigrahi, J.; Majumdar, S.; Koo, C.; Klingeler, R.We investigate magnetoelectric coupling and low-energy magnetic excitations in multiferroic α-Cu2V2O7 by detailed thermal expansion, magnetostriction, specific heat and magnetization measurements in magnetic fields up to 15 T and by high-field/high-frequency electron spin resonance studies. Our data show negative thermal expansion in the temperature range ≤200 K under study. Well-developed anomalies associated with the onset of multiferroic order (canted antiferromagnetism with a significant magnetic moment and ferroelectricity) imply pronounced coupling to the structure. We detect anomalous entropy changes in the temperature regime up to ∼80 K which significantly exceed the spin entropy. Failure of Grüneisen scaling further confirms that several dominant ordering phenomena are concomitantly driving the multiferroic order. By applying external magnetic fields, anomalies in the thermal expansion and in the magnetization are separated. Noteworthy, the data clearly imply the development of a canted magnetic moment at temperatures above the structural anomaly. Low-field magnetostriction supports the scenario of exchange-striction driven multiferroicity. We observe low-energy magnetic excitations well below the antiferromagnetic gap, i.e., a ferromagnetic-type resonance branch associated with the canted magnetic moment arising from Dzyaloshinsii-Moriya (DM) interactions. The anisotropy parameter meV indicates a sizeable ratio of DM- and isotropic magnetic exchange.
- ItemEvidence of the Anomalous Fluctuating Magnetic State by Pressure-Driven 4f Valence Change in EuNiGe3(Washington, DC : ACS, 2023) Chen, K.; Luo, C.; Zhao, Y.; Baudelet, F.; Maurya, A.; Thamizhavel, A.; Rößler, U. K.; Makarov, D.; Radu, F.In rare-earth compounds with valence fluctuation, the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration and the magnetic moment. Here, we provide direct experimental evidence for an induced magnetic polarization of the Eu3+ atomic shell with J = 0, due to intra-atomic exchange and spin-orbital coupling interactions with the Eu2+ atomic shell. By applying external pressure, a transition from antiferromagnetic to a fluctuating behavior in EuNiGe3 single crystals is probed. Magnetic polarization is observed for both valence states of Eu2+ and Eu3+ across the entire pressure range. The anomalous magnetism is discussed in terms of a homogeneous intermediate valence state where frustrated Dzyaloshinskii-Moriya couplings are enhanced by the onset of spin-orbital interaction and engender a chiral spin-liquid-like precursor.