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Magnetic kagome metals, in which topologically non-trivial band structures and electronic correlation are intertwined, have recently emerged as an exciting platform to explore exotic correlated topological phases, that are usually not found in weakly interacting materials described within the semi-classical picture of electrons. Here, via a comprehensive single-crystal neutron diffraction and first-principles density functional theory study of the archetypical topological kagome metal Mn$_3$Sn, which is also a magnetic Weyl fermion material and a promising chiral magnet for antiferromagnetic spintronics, we report the realisation of an emergent spin-density wave (SDW) order, a hallmark correlated many-body phenomenon, that is engineered by the Fermi surface nesting of topological flat bands. We further reveal that the phase transition, from the well-known high-temperature coplanar and non-collinear k = 0 inverse triangular antiferromagnetic order to a double-$k$ non-coplanar modulated incommensurate magnetic structure below $T_1$ = 280 K, is primarily driven by the SDW instability. The double-$k$ nature of this complex low-temperature magnetic order, which can be regarded as an intriguing superposition of a longitudinal SDW with a modulation wavevector k$_L$ and a transverse incommensurate helical magnetic order with a modulation wavevector k$_T$, is unambiguously confirmed by our observation of the inter-modulation high-order harmonics of the type of 2k$_L$+k$_T$. This discovery not only solves a long-standing puzzle concerning the nature of the phase transition at $T_1$, but also provides an extraordinary example on the intrinsic engineering of correlated many-body phenomena in topological matter. The identified multi-$k$ magnetic state can be further exploited for the engineering of the new modes of magnetization and chirality switching in antiferromagnetic spintronics.

Anisotropic multiferroic properties of SrMnGe2O6 pyroxene single crystals were systematically investigated by means of magnetization, heat capacity, pyroelectric current measurement and elastic and inelastic neutron scattering experiments. Single crystal neutron diffraction allows us to unambiguously reveal the presence of two incommensurate magnetic orderings: a non-polar amplitude-modulated collinear sinusoidal magnetic structure emerges at TN1=4.36(2)K followed by a polar elliptical cycloidal spin structure below TN2=4.05(2)K. Pyroelectric current measurements on single crystal confirm the appearance of a spontaneous polarization within the (ac) plane below TN2 associated with the latter magnetic symmetry through extended Dzyaloshinsky-Moriya mechanism. The magnetic phase diagram was calculated considering the three isotropic exchange couplings relevant in this system. The magnetic excitations spectra of SrMnGe2O6 measured by inelastic neutron scattering were successfully modeled using a set of exchange interactions consistent with this phase diagram.

Single crystal neutron diffraction, inelastic neutron scattering and electron spin resonance experiments are used to study the magnetic structure and spin waves in Pb$_2$VO(PO$_4$)$_2$, a prototypical layered $S=1/2$ ferromagnet with frustrating next nearest neighbor antiferromagnetic interactions. The observed excitation spectrum is found to be inconsistent with a simple square lattice model previously proposed for this material. At least four distinct exchange coupling constants are required to reproduce the measured spin wave dispersion. The degree of magnetic frustration is correspondingly revised and found to be substantially smaller than in all previous estimates.

International audience; The magnetic structure of the heavy fermion antiferromagnet CePt 2 In 7 is determined using neutron diffraction. We find a magnetic wave vector q M = (1/2,1/2,1/2), which is temperature independent up to T N = 5.5 K. A staggered moment of 0.45(1)μ B at 2 K resides on the Ce ion. The nearest-neighbor moments in the tetragonal basal plane are aligned antiferromagnetically. The moments rotate by 90 • from one CeIn 3 plane to another along the c axis. A much weaker satellite peak with an incommensurate magnetic wave vector q M = (1/2,1/2,0.47) seems to develop at low temperature. However, the experimental data available so far are not sufficient to draw a definitive conclusion about the possible coexistence of commensurate and incommensurate magnetic structures in this material.

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of about 50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions., 26 pages, 5 figures

International audience; We present macroscopic and neutron diffraction data on multiferroic lightly Co doped Ni3V2O8. The magnetic H-T phase diagrams have been derived from magnetization and electric polarization measurements with field directions parallel to the principal crystallographic axes. While the phase diagram for H parallel to b is very similar to that of the parent compound Ni3V2O8 for the commonly involved phases, the zero-field phases in (Ni0.9Co0.1)(3)V2O8 show a stronger instability for applied magnetic fields along the a or c axis. Neutron single-crystal diffraction revealed the magnetic structure of the field-induced phase for H parallel to c with a collinear spin alignment along the a and b axes for the two magnetically inequivalent sites. A pronounced irreversibility has been observed for the transition between the zero-field spin cycloid and the field-induced phase, which is manifested in a propagation vector change from q = (0.322 0 0) to q = (0.306 0 0), with slight modifications of the magnetic structure after reentering the zero-field phase. The reentrant phase is characterized by a significantly larger b component of the cross-tie site spin, therefore showing remanent features of the high-field phase. For H parallel to a the magnetization data reveal anomalies, one of which was proved to reflect a field-induced transition from the cycloidally to the sinusoidally modulated magnetic structure.

International audience; Chemical and structural phase compositions of two single-crystalline samples prepared with different cooling rates from stoichiometric FeTe0.6Se0.4 melts were studied. Both types of samples were investigated in a very comprehensive way using magnetic and electrical transport measurements combined with X-ray, neutron and electron backscatter diffraction. We show that slowly cooled samples are homogeneous on a microscopic scale with only a small excess of iron. Those slowly cooled samples do not exhibit bulk superconductivity down to 1.8 K. In contrast, fast-cooled samples are superconducting below about 14 K but are composed of several chemical phases: they consist of a matrix preserving the crystal structure of slow-cooled samples, and of core-shell structured dendritic inclusions (about 2030 vol%). These have different crystal structures and chemical compositions and order magnetically at temperatures far above the superconducting transition temperature of the inhomogeneous samples. These structural and chemical inhomogeneities seem to play a vital role in the superconducting properties of this and similar iron-based systems as they lead to internal stress and act in a similar way as the application of the external pressure that reportedly increase the superconducting transition temperature in many iron pnictides and chalcogenides. We argue that a phase pure, homogeneous and stress-free FeTe0.6Se0.4 is non-superconducting. (C) 2015 Elsevier B.V. All rights reserved.

The interplay of magnetic and charge fluctuations can lead to quantum phases with exceptional electronic properties. A case in point is magnetically-driven superconductivity, where magnetic correlations fundamentally affect the underlying symmetry and generate new physical properties. The superconducting wave-function in most known magnetic superconductors does not break translational symmetry. However, it has been predicted that modulated triplet p-wave superconductivity occurs in singlet d-wave superconductors with spin-density wave (SDW) order. Here we report evidence for the presence of a spatially inhomogeneous p-wave Cooper pair-density wave (PDW) in CeCoIn5. We show that the SDW domains can be switched completely by a tiny change of the magnetic field direction, which is naturally explained by the presence of triplet superconductivity. Further, the Q-phase emerges in a common magneto-superconducting quantum critical point. The Q-phase of CeCoIn5 thus represents an example where spatially modulated superconductivity is associated with SDW order.

The low energy dynamical properties of the multiferroic hexagonal perovskite ErMnO3 have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon only excitable by the electric field of an electromagnetic wave. Using comparison with the isostructural YMnO3 compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electro-active crystal field transition., 6 pages

The N$\acute{\rm e}$el temperature of the new frustrated family of Sr\emph{RE}$_2$O$_4$ (\emph{RE} = rare earth) compounds is yet limited to $\sim$ 0.9 K, which more or less hampers a complete understanding of the relevant magnetic frustrations and spin interactions. Here we report on a new frustrated member to the family, SrTb$_2$O$_4$ with a record $T_{\rm N}$ = 4.28(2) K, and an experimental study of the magnetic interacting and frustrating mechanisms by polarized and unpolarized neutron scattering. The compound SrTb$_2$O$_4$ displays an incommensurate antiferromagnetic (AFM) order with a transverse wave vector \textbf{Q}$^{\rm 0.5 K}_{\rm AFM}$ = (0.5924(1), 0.0059(1), 0) albeit with partially-ordered moments, 1.92(6) $\mu_{\rm B}$ at 0.5 K, stemming from only one of the two inequivalent Tb sites mainly by virtue of their different octahedral distortions. The localized moments are confined to the \emph{bc} plane, 11.9(66)$^\circ$ away from the \emph{b} axis probably by single-ion anisotropy. We reveal that this AFM order is dominated mainly by dipole-dipole interactions and disclose that the octahedral distortion, nearest-neighbour (NN) ferromagnetic (FM) arrangement, different next NN FM and AFM configurations, and in-plane anisotropic spin correlations are vital to the magnetic structure and associated multiple frustrations. The discovery of the thus far highest AFM transition temperature renders SrTb$_2$O$_4$ a new friendly frustrated platform in the family for exploring the nature of magnetic interactions and frustrations., Comment: 19 pages, 8 Figures, 1 Table

The phonon structure of antiferromagnetic Bi2Fe4O9 (space group Pbnm No. 55, TN≈240 K) was studied theoretically by calculations of lattice dynamics and experimentally between 10 and 300 K by polarized Raman spectroscopy. Most of the 12Ag+12B1g+9B2g+9B3g Raman modes were unambiguously identified. Strong second-order scattering was observed for ab-plane-confined incident and scattered light polarizations. In addition to the phonon-scattering, broad Raman bands with typical characteristics of magnon scattering appear below TN. The magnon bands are analyzed on the basis of magnetic structure of Bi2Fe4O9 and attributed to two- magnon excitations.

International audience; We have revisited the magnetic structure of manganese phosphorus trisulfide MnPS3 using neutron diffrac- tion and polarimetry. MnPS3 undergoes a transition toward a collinear antiferromagnetic order at 78 K. The resulting magnetic point-group breaks both the time reversal and the space inversion thus allowing a linear magnetoelectric coupling. Neutron polarimetry was subsequently used to prove that this coupling provides a way to manipulate the antiferromagnetic domains simply by cooling the sample under crossed magnetic and electrical fields, in agreement with the nondiagonal form of the magnetoelectric tensor. In addition, this tensor has, in principle, an antisymmetric part that results in a toroidic moment and provides with a pure ferrotoroidic compound.

The ${\mathrm{Fe}}^{3+}$ lattice in the ${\mathrm{Bi}}_{2}{\mathrm{Fe}}_{4}{\mathrm{O}}_{9}$ compound is found to materialize the first analogue of a magnetic pentagonal lattice. Because of its odd number of bonds per elemental brick, this lattice, subject to first neighbor antiferromagnetic interactions, is prone to geometric frustration. The ${\mathrm{Bi}}_{2}{\mathrm{Fe}}_{4}{\mathrm{O}}_{9}$ magnetic properties have been investigated by macroscopic magnetic measurements and neutron diffraction. The observed noncollinear magnetic arrangement is related to the one stabilized on a perfect tiling as obtained from a mean field analysis with direct space magnetic configuration calculations. The peculiarity of this structure arises from the complex connectivity of the pentagonal lattice, a novel feature compared to the well-known case of triangle-based lattices.

International audience; A novel doubly chiral magnetic order is found out in the structurally chiral langasite compound Ba$_3$NbFe$_3$Si$_2$O$_{14}$. The magnetic moments are distributed over planar frustrated triangular lattices of triangle units. On each of these they form the same triangular configuration. This ferro-chiral arrangement is helically modulated from plane to plane. Unpolarized neutron scattering on a single crystal associated with spherical neutron polarimetry proved that a single triangular chirality together with a single helicity is stabilized in an enantiopure crystal. A mean field analysis allows discerning the relevance on this selection of a twist in the plane to plane supersuperexchange paths.

We report neutron-diffraction experiments to determine the low-temperature magnetic structure of NpPd2Al3, the isostructural Np analog of the heavy-fermion superconductor UPd2Al3. The systems orders initially at about 40 K with an incommensurate magnetic propagation vector q=(,,0.36). Below 25nK the intensity of this phase is reduced and a new commensurate magnetic phase with q=(,,) is observed as the dominant phase. The fact that we observe two magnetic phases in this system may have relevance to understanding the ground state in UPd2Al3.

El presente manuscrito resume el trabajo realizado durante los últimos tres años en el Institut Laue Langevin (Grenoble) en colaboración con el Instituto de Ciencia de Materiales de Barcelona, la Universitat Autònoma de Barcelona y el Commissariat à l’énergie atomique et aux énergies alternatives (Grenoble). Cuando se descubrió que la ferroelectricidad estaba estrechamente ligada al orden magnético en TbMnO3, el interés en los materiales multiferroicos resurgió, tanto por razones fundamentales como por las posibles aplicaciones tecnológicas de dichos materiales. Hace cinco años, aproximadamente, se encontró experimentalmente que la polarización espontánea en MnWO4 está acoplada una estructura magnética de tipo cicloidal. Este material presenta tres estados magnéticos: AF3 (13.5 K < T < 12.5 K), los momentos magnéticos se ordenan colinealmente a lo largo de la dirección u en el plano ac y con sus amplitudes moduladas sinusoidalmente, con un vector de modulación k = (-0.214, 1 , 0.457); AF2 (12.5 K < T < 6.8 K), la estructura magnética tiene una componente adicional a lo largo del eje b, y en ella permanence inalterado el vector de propagación. De esta manera, la fase AF2 es una estructura cicloidal donde los momentos están contenidos en el plano ub. En esta fase aparece la polarización eléctrica a lo largo del eje b y por lo tanto, es ésta la fase multiferroica de este material. AF1 (T < 6.8 K), la estructura magnética es colineal (a lo largo de u) con k = (¼, ½, ½). La secuencia de estructuras magnéticas está inducida por la fuerte competición entre interacciones magnéticas que da lugar a la frustración magnética. La aplicación de campos externos o la sustitución química puede modificar el sutil equilibrio entre las interacciones magnéticas. En un trabajo experimental se demostró que sustituyendo manganeso por cobalto la fase multiferroica se estabiliza a bajas temperaturas; éste fue el germen del presente trabajo de tesis doctoral. En esta tesis se han estudiado las estructuras cristalinas y magnéticas, y la polarización eléctrica de Mn1-xCoxWO4 (x = 0, 0.05, 0.10, 0.15 and 0.20) sin campo magnético y bajo campo hasta 12 T. Se ha estudiado la relación entre las estructuras magnéticas y la polarización eléctrica. El análisis mediante el formalismo superespacial de las estructuras magnéticas incommensurables de x = 0 nos ha ayudado a ver la relación entre los átomos magnéticos en cada una de las fases, de una forma muy sencilla. Se ha observado que este dopaje da lugar a nuevas estructuras magnéticas, algunas de ellas multiferroicas. En cuanto a la polarización asociada a dichas estructuras hemos observado que en todos los casos no se encuentra a lo largo del eje b, obteniendo así un control químico de la dirección de la polarización. Además, se han encontrado evidencias de acoplamiento espín-red a pesar de que éstas sean muy débiles. Respecto a las medidas bajo campo, hemos visto distintos tipos de transiciones: rotaciones continuas del plano que contiene a los momentos (rotación de la polarización), transiciones discontinuas... Para todo ello, se han utilizado las siguientes técnicas experimentales: magnetometría, medidas de piroelectricidad, y difracción de neutrones y de sincrotrón. En la mayoría de los experimentos se han utilizado monocristales. El manuscrito está organizado de la siguiente manera: (i) Parte I, Introducción: se presentan, en general, los materiales multiferroicos, las estructuras magnéticas, los materiales estudiados y las técnicas que se han utilizado. Al final, se enumeran los objetivos de este trabajo. (ii) Parte II, Propiedades magnéticas, ferroeléctricas y estrucurales de Mn1-xCoxWO4 sin campo magnético aplicado. (iii) Parte III, Transiciones inducidas por el campo mangético aplicado. (iv) Parte IV, Resumen y Conclusiones. (v) Parte V, Apéndices., The manuscript summarizes the work done during the last three years in the Institut Laue Langevin (Grenoble) in collaboration with the Instituto de Ciencia de Materiales de Barcelona, the Universitat Autònoma de Barcelona and the Commissariat à l’énergie atomique et aux énergies alternatives (Grenoble). Since ferroelectricity was found to be coupled to the magnetic order in TbMnO3, there has been a renewed interest in multiferroics, both for fundamental reasons and for possible technological applications. About five year ago, it was discovered that the electric polarization that emerges in MnWO4 is coupled to a cycloidal magnetic structure. This material has three magnetic states: AF3 (13.5 K < T < 12.5 K), moments order collinearly along a direction u within the ac plane with a sinusoidally modulated amplitude and an incommensurate propagation vector k = (-0.214, 1 , 0.457); AF2 (12.5 K < T < 6.8 K), it presents an additional magnetic component along b, the propagation vector be- ing the same. Thus, the magnetic ordering is an elliptical cycloidal spin structure within the ub plane. In this phase the electric polarization arises along the b axis and therefore, this is the multiferroic phase. AF1 (T < 6.8 K), the magnetic structure is collinear (along u) with k = (¼, ½, ½). The succession of magnetic structures is induced by a strong magnetic competition which turns into magnetic frustration. The application of external fields or chemical substitution could unbalance the subtle equilibrium between the magnetic interactions. The work on the thesis was initiated by a publication in the literature (on polycrystalline samples) which showed that a subtle substitution of manganese by cobalt induces changes and stabilizes the multiferroic structure at low temperatures. The thesis presents a study of the crystal and magnetic structures, and electric polarization of Mn1-xCoxWO4 (x =0, 0.05, 0.10, 0.15 and 0.20) at zero and non-zero magnetic field (up to 12 T). We have studied the interplay between the magnetic structure and the electric polarization. The superspace formalism, used to analyze the incommensurate mangetic structures of the pure MnWO4 compound, showed the relation between the mangetic atoms in each phase in a very simple way. The Co-doping gives rise to new magnetic structures and some of them are multiferroics. Regarding the electric polarization associated to those new magnetic structures, depending on the structure the polarization is oriented along a different direction. Therefore, we have obtained a chemical control of the orientation of the electric polarization. Moreover, we have found weak but clear spin-lattice coupling in some compositions. Concerning the measurements under field, several types of transitions have been observed: continuous rotations of the rotation plane of the spins (and continuous rotation of the associated polarization), discontinuous transitions... To achieve our purposes bulk magnetometry, pyroelectric measurements, neutron- and synchrotron-diffraction experiments have been carried out on single crystals. The manuscript is organized in the following way: (i) Part I, Introduction: multiferroics, magnetic structures, the studied materials and the experimental techniques used are introduced. At the end the interest and objectives of the thesis are enumerated. (ii) Part II, Magnetic, ferroelectric and structural properties of Mn1-xCoxWO4 under zero magnetic field. (iii) Part III, Magnetic-field-induced transitions. (iv) Part IV, Summary and Conclusions. (v) Part V, Appendices.