Sawtooth chain magnets have been a subject of historical interest in the field of frustrated magnetism, with classical olivine family M_2TX_4, (M - 3d, T - 4p, X - chalcogen elements) typically realizing simple 𝐤 = (000) states. The magnetism of the Mn_2GeO_4 olivine is surprisingly complex, proceeding from commensurate states to a multiferroic commensurate + incommensurate phase. Here we report inelastic neutron scattering results from a Mn_2GeO_4 single crystal and develop an effective Hamiltonian including long-distance bilinear and dipolar interactions. The magnetic interactions are predominantly antiferromagnetic and span a three-dimensional exchange network consisting of coupled sawtooth chains. Based on the determined strength of the couplings, the dominant sawtooth chains appear at third- and fourth- rather than next-nearest-neighbor. However the next-nearest-neighbor interaction is, along with a modest Dzyaloshinskii-Moriya interaction, important for modeling the observed incommensurability. We use the best-fit Hamiltonian as the basis for Langevin dynamics simulations and Luttinger-Tisza calculations of the high-temperature commensurate transition.

We conducted small-angle neutron scattering (SANS) experiments on the ferromagnetic semimetal EuB6, where we observed first-hand evidence for the presence of magnetic polarons. We carried out SANS experiments over a large range of scattering vectors |q|, ranging from 0.006 to 0.140 angstrom-1, and temperatures of 2 to 60 K. Just above TC our experiments show magnetic scattering intensity, which has a Lorentzian dependence on the wave vector, characteristic of the presence of magnetic polarons. Below TC the polarons merge, and most of the observed intensity is due to scattering from domains. We were able to extract a correlation length xi that ranges from 100 to 300 angstrom for the size of the magnetic polarons. This size is much larger than one would expect for magnetic fluctuations of a three-dimensional Heisenberg ferromagnet, demonstrating the influence of magnetic polarons on the phase transition.

Magnetoelectric multiferroics promise direct cross-control between coexisting ferroelectric and ferromagnetic orders, which is of interest for applications in magnetism and spintronics. A particularly interesting type of cross-control is found in spin-spiral multiferroic Mn_2GeO_4, where a ferroelectric multi-domain distribution can be globally inverted by a single magnetic field sweep. In this work we consider the initial domain evolution from zero-field cooling, imaging the evolution of domains under both magnetic and electric fields via optical second harmonic generation. We find that polarization and magnetization domains form independently when entering the multiferroic phase, and a single deterministic initialisation procedure, spanning three quarters of a field cycle, is required to achieve reliable magnetoelectric cross-coupling. This initialisation behaviour originates from a deterministic pathway from metastable to equilibrium domain patterns, in contrast to more common and less reliable domain "training" procedures that require repeated field cycles. Understanding the initial domain evolution thus enables reliable cross-control in magnetoelectric devices with highly interlinked order parameters.

Encoding information in antiferromagnetic (AFM) domains is a promising solution for the ever growing demand in magnetic storage capacity. The absence of a macroscopic magnetization avoids crosstalk between different domain states, enabling ultrahigh density spintronics1 while being detrimental to the domain detection and manipulation. Disentangling these merits and disadvantages seemed so far unattainable. We report evidence for a new AFM domain selection mechanism based on non-Zeeman susceptibility anisotropy induced by the relative orientation of external magnetic fields to the k-domains. Consequently, the charge transport response is controlled by the rotation of the magnetic field and a pronounced anisotropic magnetoresistance is found in the AFM phase of bulk materials Nd1−xCex CoIn5. Our results and the domain switching theory2 indicate that this constitutes a new effect which might be universal across multiband materials. It provides a novel mechanism to control and detect AFM domains opening new perspectives for AFM sprintronics.

High temperature superconductors start out as two-dimensional antiferromagnetic Mott insulators. These are then lightly doped, where the motion of these free charges distorts the surrounding Néel order and forms a string of distorted spins in the form of a magnetic polaron. Thus, magnetic polarons are thought to be central to the high-$T_c$ problem. The shape and size of such dynamic magnetic domains can be determined from small angle neutron scattering (SANS). In order to show that magnetic polarons can be detected by SANS we studied the three-dimensional ferromagnetic model system \eu, where the results of other measurements suggest the presence of magnetic polarons. In \eu, just above $T_{\mathrm{C}}$, our experiments show magnetic scattering intensity, which has a Lorentzian dependence on the wave vector, characteristic for the presence of magnetic polarons. Below $T_{\mathrm{C}}$ the polarons merge and most of the observed intensity is due to scattering from domain walls. We were able to extract a correlation length $\xi$ which ranges from 100 to 300~\AA\ for the size of the magnetic polarons. This size is much larger than one would expect for magnetic fluctuations of a 3D Heisenberg ferromagnet, demonstrating the usefulness of SANS for detecting magnetic polarons.

We present high-quality angle-resolved photoemission (ARPES) and density functional theory calculations (DFT+U) of SmCoIn5. We find broad agreement with previously published studies of LaCoIn5 and CeCoIn51,2, confirming that the Sm 4f electrons are mostly localized. Nevertheless, our model is consistent with an additional delocalized Sm component, stemming from hybridization between the 4f electrons and the metallic bands at “hot spot” positions in the Brillouin zone. The dominant hot spot, called γZ, is similar to a source of delocalized f states found in previous experimental and theoretical studies of CeCoIn51,3. In this work, we identify and focus on the role of the Co d states in exploring the relationship between heavy quasiparticles and the magnetic interactions in SmCoIn5, which lead to a magnetically ordered ground state from within an intermediate valence scenario4–6. Specifically, we find a globally flat band consisting of Co d states near E = − 0.7 eV, indicating the possibility of enhanced electronic and magnetic interactions in the “115” family of materials through localization in the Co layer, and we discuss a possible origin in geometric frustration. We also show that the delocalized Sm 4f states can hybridize directly with the Co 3dxz/3dyz orbitals, which occurs in our model at the Brillouin zone boundary point R in a band that is locally flat and touches the Fermi level from above. Our work identifies microscopic ingredients for additional magnetic interactions in the “115” materials beyond the RKKY mechanism, and strongly suggests that the Co d bands are an important ingredient in the formation of both magnetic and superconducting ground states.

Understanding the Kondo lattice system is challenging because of the complexity of the Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions and the interplay of the Kondo effect with the crystal electric field (CEF). To understand the interplay between the competing interactions, high-quality single crystals of La-substituted CeIn 3 were synthesized by the flux method, and their single-impurity Kondo scattering effects were studied in comparison with the Kondo lattice behavior of CeIn 3 . The magnetic susceptibility reveals a scaling behavior above T N , indicating that local Kondo impurity scattering is important for the entire La 1− x Ce x In 3 series. The CEF gap estimated from the specific heat and neutron scattering experiments did not vary significantly with La substitution. Although the localized f -electron behavior observed in the La 1− x Ce x In 3 series is consistent with the de Haas-van Alphen (dHvA) and angle-resolved photoemission spectroscopy (ARPES) results in previous reports, the appearance of a coherent peak in the resistivity at ~ 47 K suggests that a mild coherence effect coexists in the prototypical antiferromagnetic (AFM) compound CeIn 3 .

Superposed symmetry-equivalent magnetic ordering wave vectors can lead to topologically non-trivial spin textures, such as magnetic skyrmions and hedgehogs, and give rise to novel quantum phenomena due to fictitious magnetic fields associated with a non-zero Berry curvature of these spin textures. To date, all known spin textures are constructed through the superposition of multiple spiral orders, where spins vary in directions with constant amplitude. Recent theoretical studies have suggested that multiple sinusoidal orders, where collinear spins vary in amplitude, can construct distinct topological spin textures regarding chirality properties. However, such textures have yet to be experimentally realised. In this work, we report the observation of a zero-field magnetic hedgehog lattice from a superposition of triple sinusoidal wave vectors in the magnetically frustrated Kondo lattice CePtAl4Ge2. Notably, we also observe the emergence of anomalous electrical and thermodynamic behaviours near the field-induced transition from the zero-field topological hedgehog lattice to a non-topological sinusoidal state. These observations highlight the role of Kondo coupling in stabilising the zero-field hedgehog state in the Kondo lattice and warrant an expedited search for other topological magnetic structures coupled with Kondo coupling.

The microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn 5 and other Ce-based heavy fermion materials, depends strongly on the efficiency with which f electrons are delocalized from the rare earth sites and participate in a Kondo lattice. Replacing Ce 3+ (4 f 1 , J = 5/2) with Sm 3+ (4 f 5 , J = 5/2), we show that a combination of the crystal electric field and on-site Coulomb repulsion causes SmCoIn 5 to exhibit a Γ 7 ground state similar to CeCoIn 5 with multiple f electrons. We show that with this single-ion ground state, SmCoIn 5 exhibits a temperature-induced valence crossover consistent with a Kondo scenario, leading to increased delocalization of f holes below a temperature scale set by the crystal field, T v ≈ 60 K. Our result provides evidence that in the case of many f electrons, the crystal field remains the dominant tuning knob in controlling the efficiency of delocalization near a heavy fermion quantum critical point, and additionally clarifies that charge fluctuations play a general role in the ground state of “115” materials.

Van der Waals (vdW) magnets are an ideal platform for tailoring 2D magnetism with immense potential for spintronics applications and are intensively investigated. However, little is known about the microscopic origin of magnetic order in these antiferromagnetic systems. X‐ray photoemission electron microscopy is used to address the electronic and magnetic properties of the vdW antiferromagnet FePS 3 down to the monolayer. The experiments reveal a giant out‐of‐plane magnetic anisotropy of 22 meV per Fe ion, accompanied by unquenched magnetic orbital moments. Moreover, the calculations suggest that the Ising magnetism in FePS 3 is a visible manifestation of spin–orbit entanglement of the Fe 3 d electron system.

We conducted a small-angle neutron scattering experiments (SANS) on the ferromagnetic semi-metal EuB$_6$, where we observed direct evidence for the presence of magnetic polarons. We carried out SANS experiments over a large range of scattering vectors $|\vec{q}|$ from 0.006 to 0.140~\AA$^{-1}$ and 2 to 60~K. Just above $T_{\mathrm{C}}$ our experiments show magnetic scattering intensity, which has a Lorentzian dependence on the wave vector, which is characteristic for the presence of magnetic polarons. Below $T_{\mathrm{C}}$ the polarons merge and most of the observed intensity is due to scattering from domain walls.We were able to extract a correlation length $\xi$ which ranges from $100$ to $300$~\AA\ for the size of the magnetic polarons. This size is much larger than one would expect for magnetic fluctuations, demonstrating the influence the magnetic polarons on the phase transition. We were able to extract a correlation length $\xi$ which ranges from $100$ to $300$~\AA\ for the size of the magnetic polarons. This size is much larger than one would expect for magnetic fluctuations, demonstrating the influence the magnetic polarons on the phase transition.

In order to investigate the effects leading to the strong magnetoelectric coupling in the type II multiferroic TbMnO3 we have studied the thermal properties and temperature dependence of the lattice vibrations of (TbMnO3)-O-16 and its isotopically substituted counterpart (TbMnO3)-O-18. Heat capacity measurements on powder samples revealed no significant change in the Mn3+ and Tb3+ magnetic phase transition temperatures, as well as the multiferroic ordering temperature upon isotope substitution, indicating that a change in the dynamical modulation of the MnO6 octahedral distortions and rotations altering the Mn-O-Mn bond angles has no influence on the magnetic properties of TbMnO3. Raman light scattering experiments have been performed on isotopically substituted single crystals to determine the temperature induced changes in phonon energies and linewidths at the sinusoidal and multiferroic phase transitions. A detailed modeling indicates that the spin-phonon coupling can be accounted for the pronounced anomalies in the temperature dependence of the phonon behaviors at the transition to the sinusoidal spin phase at T-N(Mn) = 41 K and to the multiferroic spin-spiral phase at T-FE = 28 K. No further effects such as the appearance of the electric polarization or the electromagnon were required to explain the data, especially below the multiferroic phase transition.

Topological magnon insulators constitute a growing field of research for their potential use as information carriers without heat dissipation. We report an experimental and theoretical study of the magnetic ground-state and excitations in the van der Waals two-dimensional honeycomb magnet ErBr 3 . We show that the magnetic properties of this compound are entirely governed by the dipolar interactions which generate a continuously degenerate non-collinear ground-state on the honeycomb lattice with spins confined in the plane. We find that the magnon dispersion exhibits Dirac-like cones when the magnetic moments in the ground-state are related by time-reversal and inversion symmetries associated with a Berry phase π as in single-layer graphene. A magnon band gap opens when the dipoles are rotated away from this state, entailing a finite Berry curvature in the vicinity of the K and K’ Dirac points. Our results illustrate that the spin-wave dispersion of dipoles on the honeycomb lattice can be reversibly controlled from a magnetic phase with Dirac cones to a topological antiferromagnetic insulator with non-trivial valley Chern number.

EuB6 has for a long time captured the attention of the physics community, as it shows a ferromagnetic phase transition leading to a insulator the metal transition together with colossal magnetoresistance (CMR). EuB6 has a very low carrier density, which is known to drastically change the interaction between the localized Eu moments and the conduction electrons. One of early triumphs of the quantum theory in condensed matter was the presence of Fermi surface, which is intimately linked to the symmetry of the underlying crystal lattice. This symmetry can be probed by angle resolved magnetoresistance (AMRO) measurements. Here, we present angle resolved magnetoresistance (AMRO) measurements that show a that in EuB6 this symmetry is broken, possibly indicating the presence of a quantum nematic phase. We identify the region in the temperature-magnetic field phase diagram where the magnetoresistance shows two-fold oscillations instead of the expected fourfold pattern. Quantum nematic phases are analogous to classical liquid crystals. Like liquid crystals, which break the rotational symmetry of space, their quantum analogs break the point-group symmetry of the crystal due to strong electron-electron interactions, as in quantum Hall states, Sr3Ru2O7, and high temperature superconductors. This is the same region where magnetic polarons were previously observed, suggesting that they drive the nematicity in EuB6. This is also the region of the phase diagram where EuB6 shows a colossal magnetoresistance (CMR). This novel interplay between magnetic and electronic properties could thus be harnessed for spintronic applications.

The Swiss spallation neutron source SINQ at Paul Scherrer Institute (PSI) has been in operation since 1996 using an optical guide system to transport low-energy neutrons from the source to the neutron beam lines. Clear signs of degradation in the guide system motivated us to redesign all neutron guides, which were then replaced in 2019–2020. All instruments received new tailored guides and SINQ operation was resumed in July 2020. Neutron flux measurements before and after the upgrade show that the expected instrument-dependent flux gain of a factor 2–10 was achieved. Furthermore, the fast-neutron background in the guide hall could be suppressed with an improved shielding in the guide bunker. The upgrade was completed with a slight delay due to the corona pandemic and in full compliance with the financial budget.

The magnetetoelectric CaBaCo 4 O 7 compound offers an interesting scenario to study frustrated magnetic configurations. The Co 2+ and Co 3+ ions in tetrahedral oxygen coordination form a three-dimensional framework of interconnected triangular and kagome layered arrangements [1]. The compound becomes ferrimagnetic below 60 K, and displays a strong increase of electric polarization of 17 000 μC /cm 2 , driven by exchange-striction. In this work, we present our results on the thermal evolution of magnetic and crystallographic properties of powder samples of Ca 1- x Sr x BaCo 4 O 7 ( x = 0, 0.02, 0.05, 0.07) to study the effect of substitution at the Ca site. We will show that low doping levels (<10 at.%) change quite dramatically the magnetic behavior of the compound, as observed in magnetization vs. temperature measurements. Combined with extensive use of Neutron Power Diffraction we analysed the evolution of the magnetic order as a function of temperature and composition of the samples. The reported non collinear ferrimagnetic order of the parent compound is only retained for the lowest doping level x = 0.02 and is accompanied by a strong unit cell distortion. In turn, further Sr doping blurs this distortion and favors other magnetic arrangements. In the temperature range 62 K < T < 82 K, samples with x ≥ 0.02 show a plateau in the magnetization. By using the superspace group theory and its implementation in the Rietveld refinement of neutron diffraction data, we have solved the incommensurate magnetic structure that appears at these intermediate temperatures. The magnetic order has a propagation vector k = (1/2, 1/2, g ) with g ≈ 0.02 and it belongs to the sup erspace group Pna 2 1 1’(1/2, 1/2, g ) qq 0 s . This phase corresponds to a modulated spin structure with distinct behaviors of the triangular and kagome cobalt sites and could explain previous findings reported in the literature for other substitution sites in the CaBaCo 4 O 7 family.

PbMO3 (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO3 remain unclear. Herein, we reveal that PbFeO3 crystallizes into an unusual 2ap × 6ap × 2ap orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb2+0.5Pb4+0.5FeO3 lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb2+ and Pb4+) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe3+ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

Kondo metal CePtAl4Ge2 exhibits long-range antiferromagnetic order below 2.3 K. Neutron powder-diffraction experiments reveal that Ce moments order antiferromagnetically with an incommensurate ordering wave vector k = (1.39, 0, 0.09). Inelastic neutron powder scattering experiments show a magnetic excitation at 14.5 meV, which corresponds to the first excited state due to the crystalline electric field splitting of the ground-state multiplet of Ce3+. The temperature and field dependence of the magnetization of CePtAl4Ge2 is consistent with a doublet ground state with a dominant vertical bar j(z) = 1/2 > character and a first excited doublet vertical bar j(z) = 3/2 > at 14.5 meV.

Quantum spin liquids are materials that feature quantum entangled spin correlations and avoid magnetic long-range order at T = 0 K. Particularly interesting are two-dimensional honeycomb spin lattices where a plethora of exotic quantum spin liquids have been predicted. Here, we experimentally study an effective S = 1/2 Heisenberg honeycomb lattice with competing nearest and next-nearest-neighbour interactions. We demonstrate that YbBr3 avoids order down to at least T = 100 mK and features a dynamic spin–spin correlation function with broad continuum scattering typical of quantum spin liquids near a quantum critical point. The continuum in the spin spectrum is consistent with plaquette type fluctuations predicted by theory. Our study is the experimental demonstration that strong quantum fluctuations can exist on the honeycomb lattice even in the absence of Kitaev-type interactions, and opens a new perspective on quantum spin liquids.

Numerous materials feature unexplained phases with invisible or hidden order of electronic origin. A particularly mysterious case is that of Tb2Ti2O7, which avoids magnetic order to the lowest temperatures, but nevertheless has an unexplained second-order phase transition near T = 0.5 K. Our ultrasound measurements of Tb2Ti2O7 provide direct evidence of a huge softening followed by strong hardening of the structural lattice below T = 0.5 K. In the absence of magnetic order at this temperature, our results provide conclusive evidence for the proposed quadrupolar order and emphasize the importance of higher-order multipolar interactions in rare-earth frustrated magnets.