Magnetic structure and phase diagram of TmB4

Magnetic structure of single crystalline TmB4 has been studied by magnetization, magnetoresistivity and specific heat measurements. A complex phase diagram with different antiferromagnetic (AF) phases was observed below TN1 = 11.7 K. Besides the plateau at half-saturated magnetization (1/2 MS), also plateaus at 1/9, 1/8 and 1/7 of MS were observed as function of applied magnetic field B//c. From additional neutron scattering experiments on TmB4, we suppose that those plateaus arise from a stripe structure which appears to be coherent domain boundaries between AF ordered blocks of 7 or 9 lattice constants. The received results suggest that the frustration among the Tm3+ magnetic ions, which maps to a geometrically frustrated Shastry-Sutherland lattice lead to strong competition between AF and ferromagnetic (FM) order. Thus, stripe structures in intermediate field appear to be the best way to minimize the magnetostatic energy against other magnetic interactions between the Tm ions combined with very strong Ising anisotropy.Comment: 4 pages, 4 figures, conference contribution - CSMAG 0

Surface and bulk components of electrical conductivity in the (presumably special topological) Kondo insulator SmB6 at lowest temperatures

Samarium hexaboride (SmB6) has recently been considered to be a topological Kondo insulator (TKI), the first strongly correlated electron system to exhibit topological surface conduction states. In this contribution, results of electrical resistivity measurements between 80 K and 0.08 K of various SmB6 single crystalline samples are presented, analyzed and discussed. The received results imply that the residual conductivity of SmB6 below about 4 K is of non-activated (metallic-like) nature. It is shown that this metallic-like behavior can be attributed both to surface (2D) conduction states, as may be expected in case of a topological insulator, as well as to the highly correlated many-body (3D) bulk ground state which is formed within the gap of this compound. From this it follows that in SmB6, where surface conductivity states are clearly present, there is in parallel also a bulk contribution to residual electrical conductivity originating from the strongly correlated electron system with valence fluctuations. This raises the question whether SmB6 does not form a new / special type of topological insulator in which in the energy gap besides the surface conduction states, there is also a conducting narrow in-gap band originating from the bulk strongly correlated electron system.Comment: 9 pages, 5 figure

Investigation of the magnetic structure and crystal field states of pyrochlore antiferromagnet Nd2Zr2O7

We present synchrotron x-ray diffraction, neutron powder diffraction and time-of-flight inelastic neutron scattering measurements on the rare earth pyrochlore oxide Nd2Zr2O7 to study the ordered state magnetic structure and cystal field states. The structural characterization by high-resolution synchrotron x-ray diffraction confirms that the pyrochlore structure has no detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals long range all-in/all-out antiferromagnetic order below T_N ~ 0.4 K with propagation vector k = (0 0 0) and an ordered moment of 1.26(2) \mu_B/Nd at 0.1 K. The ordered moment is much smaller than the estimated moment of 2.65 \mu_B/Nd for the local Ising ground state of Nd3+ (J=9/2) suggesting that the ordering is partially suppressed by quantum fluctuations. The strong Ising anisotropy is further confirmed by the inelastic neutron scattering data which reveals a well-isolated dipolar-octupolar type Kramers doublet ground state. The crystal field level scheme and ground state wavefunction have been determined.Comment: 12 pages, 15 figures, 2 table

Magnetic study of M type doped barium hexaferrite nanocrystalline particles

Co Ti and Ru Ti substituted barium ferrite nanocrystalline particles BaFe12 2xCoxTixO19 with 0 lt;x lt;1 and BaFe12 2xRuxTixO19 with 0 lt;x lt;0.6 were prepared by ball milling method, and their magnetic properties and their temperature dependencies were studied. The zero field cooled ZFC and field cooled FC processes were recorded at low magnetic fields and the ZFC curves displayed a broad peak at a temperature TM. In all samples under investigation, a clear irreversibility between the ZFC and FC curves was observed below room temperature, and this irreversibility disappeared above room temperature. These results were discussed within the framework of random particle assembly model and associated with the magnetic domain wall motion. The resistivity data show some kind of a transition from insulator to perfect insulator around . At 2 K, the saturation magnetization slightly decreased and the coercivity dropped dramatically with increasing the Co Ti concentration x. With Ru Ti substitution, the saturation magnetization showed small variations, while the coercivity decreased monotonically, recording a reduction of about 73 at x 0.6. These results were discussed in light of the single ion anisotropy model and the cationic distributions based on previously reported neutron diffraction data for the CoTi substituted system, and the results of our Mössbauer spectroscopy data for the RuTi substituted system

Phosphonate Metal–Organic Frameworks: A Novel Family of Semiconductors

Herein, the first semiconducting and magnetic phosphonate metal–organic framework (MOF), TUB75, is reported, which contains a 1D inorganic building unit composed of a zigzag chain of corner‐sharing copper dimers. The solid‐state UV–vis spectrum of TUB75 reveals the existence of a narrow bandgap of 1.4 eV, which agrees well with the density functional theory (DFT)‐calculated bandgap of 1.77 eV. Single‐crystal conductivity measurements for different orientations of the individual crystals yield a range of conductances from 10−3 to 103 S m−1 at room temperature, pointing to the directional nature of the electrical conductivity in TUB75. Magnetization measurements show that TUB75 is composed of antiferromagnetically coupled copper dimer chains. Due to their rich structural chemistry and exceptionally high thermal/chemical stabilities, phosphonate MOFs like TUB75 may open new vistas in engineerable electrodes for supercapacitors.TU Berlin, Open-Access-Mittel - 202