An extended investigation of the electronic phase diagram of FeSe1−x up to pressures of p 2.4 GPa by means of ac and dc magnetization, zero field muon spin rotation (ZF µSR), and neutron diffraction is presented. ZF µSR indicates that at pressures p ≥ 0.8 GPa static magnetic order occurs in FeSe1−x and occupies the full sample volume for p 1.2 GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above p ≥ 1.2 GPa both the superconducting transition temperature Tc and the magnetic ordering temperature TN increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe1−x, located on the line connecting the Se atoms along the c-direction. Different magnetic structures are proposed and checked by combining the muon stopping calculations with a symmetry analysis, leading to a similar structure as in the LaFeAsO family of Fe-based superconductors. Furthermore, it is shown that the magnetic moment is pressure dependent and with a rather small value of µ ≈ 0.2 µB at p 2.4 GPa.

Alkali borohydrides MBH 4 and their deuterides have been investigated by X-ray and neutron powder diffraction (M = K, Rb, Cs) and by infrared and Raman spectroscopy (M = Na, K, Rb, Cs). At room temperature the compounds crystallize with a cubic high temperature (HT) structure having Fm3m symmetry in which the [BH 4 ] − complexes are disordered. At low temperature (LT) the potassium compound transforms into a tetragonal low temperature structure having P4 2 /n mc symmetry in which the [BH 4 ] − complexes are ordered such as in the isotypic sodium congener. The B-H distances within the complex as measured on the deuteride at 1.5 K are 1.205(3) Å. Indications for a partial ordering in the rubidium and cesium compounds exist but are not sufficient for a full structural characterization. Infrared and Raman spectra at room temperature are fully assigned for both hydrides and deuterides, including the overtones and combination bands, the Fermi resonance type interactions and the 10 B to 11 B splitting due to the presence of natural boron in the samples.

A detailed investigation of the phase diagram of 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF(6)]) is presented on the basis of a wide set of experimental data accessing thermodynamic, structural, and dynamical properties of this important room temperature ionic liquid (RTIL). The combination of quasi adiabatic, continuous calorimetry, wide angle neutron and X-ray diffraction, and quasi elastic neutron scattering allows the exploration of many novel features of this material. Thermodynamic and microscopic structural information is derived on both glassy and crystalline states and compared with results that recently appeared in the literature allowing direct information to be obtained on the existence of two crystalline phases that were not previously characterized and confirming the view that RTILs show a substantial degree of order (even in their amorphous states), which resembles the crystalline order. We highlight a strong connection between structure and dynamics, showing the existence of three temperature ranges in the glassy state across which both the spatial correlation and the dynamics change. The complex crystalline polymorphism in [bmim][PF(6)] also is investigated; we compare our findings with the corresponding findings for similar RTILs. These results provide a strong experimental basis for the exploration of the features of the phase diagram of RTILs and for the further study of longer alkyl chain salts.

Magnetic topological phases of quantum matter are an emerging frontier in physics and material science [1][2][3][4]. Along these lines, several kagome magnets [5][6][7][8][9] have appeared as the most promising platforms. However, the magnetic nature of these materials in the presence of topological state remains an unsolved issue [5][6][7][8][9]. Here, we explore magnetic correlations in the kagome magnet Co 3 Sn 2 S 2 . Using muon spin-rotation, we present evidence for competing magnetic orders in the kagome lattice of this compound. Our results show that while the sample exhibits an outof-plane ferromagnetic ground state, an in-plane antiferromagnetic state appears at temperatures above 90 K, eventually attaining a volume fraction of 80% around 170 K, before reaching a nonmagnetic state. Strikingly, the reduction of the anomalous Hall conductivity above 90 K linearly follows the disappearance of the volume fraction of the ferromagnetic state. We further show that the competition of these magnetic phases is tunable through applying either an external magnetic field or hydrostatic pressure. Our results taken together suggest the thermal and quantum tuning of Berry curvature field via external tuning of magnetic order. Our study shows that Co 3 Sn 2 S 2 is a rare example where the magnetic competition drives the thermodynamic evolution * Electronic address: zurab.guguchia@psi.ch of the Berry curvature field, thus tuning its topological state.The kagome lattice is a two-dimensional pattern of corner-sharing triangles. With this unusual symmetry and the associated geometrical frustration, the kagome lattice can host peculiar states including flat bands [8], Dirac fermions [5,6] and spin liquid phases [7,10]. In particular, magnetic kagome materials offer a fertile ground to study emergent behaviors resulting from the interplay between unconventional magnetism and band topology. Recently, transition-metal based kagome magnets [5][6][7][8][9][10][11][12][13] are emerging as outstanding candidates for such studies, as they feature both large Berry curvature fields and unusual magnetic tunability. In this family, the kagome magnet Co 3 Sn 2 S 2 is found to exhibit both a large anomalous Hall effect and anomalous Hall angle, and is identified as a promising Weyl semimetal candidate [9,11,14,15]. However, despite knowing the magnetic ground state is ferromagnetic below T C = 177 K [16] with spins aligned along the c-axis [9, 11, 17] (see Figs. 1 a and b) there is no report of its magnetic tunability or phase diagram, and its interplay with the topological band structure. Here we use high-resolution µSR to systematically characterize the phase diagram, uncovering another intriguing in-plane antiferromagnetic phase. The magnetic competition between these two phases is further found to be highly tunable via applying either pressure [18][19][20][21] or magnetic field. Combined with first principles calculations, we discover that the tunable magnetic correlation plays a key role in determining the giant anomalous Hall transp...

By the fabrication of periodically arranged nanomagnetic systems it is possible to engineer novel physical properties by realizing artificial lattice geometries that are not accessible via natural crystallization or chemical synthesis. This has been accomplished with great success in two dimensions in the fields of artificial spin ice and magnetic logic devices, to name just two. Although first proposals have been made to advance into three dimensions (3D), established nanofabrication pathways based on electron beam lithography have not been adapted to obtain free-form 3D nanostructures. Here we demonstrate the direct-write fabrication of freestanding ferromagnetic 3D nano-architectures. By employing micro-Hall sensing, we have determined the magnetic stray field generated by our free-form structures in an externally applied magnetic field and we have performed micromagnetic and macro-spin simulations to deduce the spatial magnetization profiles in the structures and analyze their switching behavior. Furthermore we show that the magnetic 3D elements can be combined with other 3D elements of different chemical composition and intrinsic material properties.

Motivated by the recent observations of incommensurate magnetic order and electric polarization in YBaCuFeO 5 up to temperatures T N2 as high as 230 K [B. Kundys et al., Appl. Phys. Lett. 94, 072506 (2009); Y. Kawamura et al., J. Phys. Soc. Jpn 79, 073705 (2010)], we report here for the first time a model for the incommensurate magnetic structure of this material, which we complement with ab initio calculations of the magnetic exchange parameters. Using neutron powder diffraction, we show that the appearance of polarization below T N2 is accompanied by the replacement of the high-temperature collinear magnetic order by a circular inclined spiral with propagation vector k i = (1/2,1/2,1/2 ± q). Moreover, we find that the polarization approximately scales with the modulus of the magnetic modulation vector q down to the lowest temperature investigated (∼3 K). Further, we observe occupational Fe/Cu disorder in the FeO 5 -CuO 5 bipyramids, although a preferential occupation of such units by Fe-Cu pairs is supported by the observed magnetic order and by density functional calculations. We calculate exchange coupling constants for different Fe/Cu distributions and show that, for those containing Fe-Cu dimers, the resulting magnetic order is compatible with the experimentally observed collinear magnetic structure [k c = (1/2,1/2,1/2), T N2 > T > T N1 = 440 K]. Based on these results, we discuss possible origins for the incommensurate modulation and its coupling with ferroelectricity. M. MORIN et al. PHYSICAL REVIEW B 91, 064408 (2015) J ⊥ (meV) J (meV) J O (meV) J NNN (meV) (a) J 1,2 = 134.5 J 1,3 = 10.6 J 5,3 = −1.6 J 1,5 = −0.05 J 7,8 = 8.7 J 5,7 = 2.8 J 1+c,5 = −0.01 (b) J 1,2 = 129.9

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We report a study of the geometrically frustrated magnetic material Tb 2 Sn 2 O 7 by the positive muonspin relaxation technique. No signature of a static magnetically ordered state is detected while neutron magnetic reflections are observed in agreement with a published report. This is explained by the dynamical nature of the ground state of Tb 2 Sn 2 O 7 : the Tb 3 magnetic moment characteristic fluctuation time is ' 10 ÿ10 s. The strong effect of the magnetic field on the muon-spin-lattice relaxation rate at low fields indicates a large field-induced increase of the magnetic density of states of the collective excitations at low energy. DOI: 10.1103/PhysRevLett.96.127202 PACS numbers: 75.40.ÿs, 75.25.+z, 76.75.+i Magnetic materials with antiferromagnetically coupled spins located on triangular motifs exhibit geometrical magnetic frustration because their spatial arrangement is such that it prevents the simultaneous minimization of all the interaction energies [1]. The frustration, which leads to a highly degenerate ground state, forbids magnetic order to occur. Perturbations to the nearest-neighbor exchange interaction, such as exchange interactions extending beyond nearest-neighbor magnetic atoms, dipole coupling, or magnetic anisotropy, are believed to be responsible for the magnetic order observed in some compounds [2]. Typical examples are given by the spinel structure oxide [6]. A prerequisite for understanding the unanticipated behavior of these latter systems is a careful characterization of their dynamical properties.Here we show that positive muon-spin relaxation ( SR) and ND results in the ordered phase of Tb 2 Sn 2 O 7 can be simultaneously accounted for only if the Tb 3 moments are strongly dynamical. An independent and consistent time scale is obtained from a careful analysis of the neutron data. In addition, the initial strong and counterintuitive increase of the muon relaxation rate when a magnetic field is applied indicates an increase of the density of magnetic excitations at very low energy.Tb 2 Sn 2 O 7 crystallizes with the cubic space group Fd 3m. Rietveld refinements of powder x-ray and ND patterns yield the lattice constant a 10:426 A and the free position parameter allowed by the space group for the 48f site occupied by oxygen, x 0:336 [6]. Magnetic measurements point to a magnetic transition at 0.87 K and to strong antiferromagnetic interactions as deduced from the large and negative Curie-Weiss constant CW ÿ12 K [13]. Powder ND indicates a structure with both ferromagnetic and antiferromagnetic components below T sr 1:3 1 K where short-range magnetic correlations which are not liquidlike appear [6]. A steep increase of the Tb 3 magnetic moment Tb and correlation length L c is observed around T lr 0:87 K, where a peak is seen in the temperature dependence of the specific heat C p T . We present below (i) C p T data recorded using a dynamic adiabatic technique, (ii) ND measurements carried out at the cold neutron powder diffractometer DMC of the SINQ facility at the Paul Scherrer I...