Scientific Highlights

The compound Sr_0.5Ce_0.5FBiS₂ belongs to the intensively studied family of layered BiS₂ superconductors. It attracts special attention because superconductivity at T_sc = 2.8 K was found to coexist with local-moment ferromagnetic order with a Curie temperature T_C = 7.5 K. Recently it was reported that upon replacing S by Se T_C drops and ferromagnetism becomes of an itinerant nature. At the same time T_sc increases and it was argued superconductivity coexists with itinerant ferromagnetism. Here we report a muon spin rotation and relaxation study (μSR) conducted to investigate the coexistence of superconductivity and ferromagnetic order in Sr_0.5Ce_0.5FBiS_2-xSe_x with x = 0.5 and 1.0. By inspecting the muon asymmetry function we find that both phases do not coexist on the microscopic scale, but occupy different sample volumes. For x = 0.5 and x = 1.0 we find a ferromagnetic volume fraction of ≈8 % and ≈30 % at T = 0.25 K, well below T_C = 3.4 K and T_C = 3.3 K, respectively. For x = 1.0 (T_sc = 2.9 K) the superconducting phase occupies most (≈64 %) of the remaining sample volume, as shown by transverse field experiments that probe the Gaussian damping due to the vortex lattice. We conclude ferromagnetism and superconductivity are macroscopically phase separated.

In its orthorhombic T_d polymorph, MoTe₂ is a type-II Weyl semimetal, where the Weyl fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance and superconductivity were also observed in T_d-MoTe₂. Understanding the superconductivity in T_d-MoTe₂, which was proposed to be topologically non-trivial, is of eminent interest. Here, we report high-pressure muon-spin rotation experiments probing the temperature-dependent magnetic penetration depth in T_d-MoTe₂. A substantial increase of the superfluid density and a linear scaling with the superconducting critical temperature T_c is observed under pressure. Moreover, the superconducting order parameter in T_d-MoTe₂ is determined to have 2-gap s-wave symmetry. We also exclude time-reversal symmetry breaking in the superconducting state with zero-field μSR experiments. Considering the strong suppression of T_c in MoTe₂ by disorder, we suggest that topologically non-trivial s⁺⁻ state is more likely to be realized in MoTe₂ than the topologically trivial s⁺⁺ state.

The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. Here we demonstrate in the pyrochlore Tb₂Hf₂O₇, that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.

We report muon-spin rotation and neutron-scattering experiments on nonmagnetic Zn impurity effects on the static spin-stripe order and superconductivity of the La214 cuprates. Remarkably, it was found that, for samples with hole doping x ≈ 1/8, the spin-stripe ordering temperature T_so decreases linearly with Zn doping y and disappears at y ≈ 4%, demonstrating a high sensitivity of static spin-stripe order to impurities within a CuO₂ plane. Moreover, T_so is suppressed by Zn in the same manner as the superconducting transition temperature T_c for samples near optimal hole doping. This surprisingly similar sensitivity suggests that the spin-stripe order is dependent on intertwining with superconducting correlations.

Unlike the widely studied ReFeAsO series, the newly discovered iron-based superconductor ThFeAsN exhibits a remarkably high critical temperature of 30 K, without chemical doping or external pressure. Here we investigate in detail its magnetic and superconducting properties via muon-spin rotation/relaxation and nuclear magnetic resonance techniques and show that ThFeAsN exhibits strong magnetic fluctuations, suppressed below ≈35 K, but no magnetic order. This contrasts strongly with the ReFeAsO series, where stoichiometric parent materials order antiferromagnetically and superconductivity appears only upon doping. The ThFeAsN case indicates that Fermi-surface modifications due to structural distortions and correlation effects are as important as doping in inducing superconductivity. The direct competition between antiferromagnetism and superconductivity, which in ThFeAsN (as in LiFeAs) occurs at already zero doping, may indicate a significant deviation of the s-wave superconducting gap in this compound from the standard s^± scenario.

We study by means of bulk and local probes the d-metal alloy Ni_1-xV_x close to the quantum critical concentration, x_c ≈ 11.6%, where the ferromagnetic transition temperature vanishes. The magnetization-field curve in the ferromagnetic phase takes an anomalous power-law form with a nonuniversal exponent that is strongly x dependent and mirrors the behavior in the paramagnetic phase. Muon spin rotation experiments demonstrate inhomogeneous magnetic order and indicate the presence of dynamic fluctuating magnetic clusters. These results provide strong evidence for a quantum Griffiths phase on the ferromagnetic side of the quantum phase transition.

The magnetic order induced by the pressure was studied in FeSe by means of muon spin rotation (μSR) technique. By following the evolution of the oscillatory part of the μSR signal as a function of angle between the initial muon spin polarization and 101 axis of the studied FeSe sample, it was found that the pressure-induced magnetic order in FeSe corresponds either to the collinear (single-stripe) antiferromagnetic order as observed in parent compounds of various FeAs-based superconductors or to the bi-collinear order as obtained in the FeTe system, but with the Fe spins turned by 45° within the ab plane. The value of the magnetic moment per Fe atom was estimated to be ≅0.13–0.14 μ_B at p ≅ 1.9 GPa.

Interfaces are critical in quantum physics, and therefore we must explore the potential for designer hybrid materials that profit from promising combinatory effects. In particular, the fine-tuning of spin polarization at metallo–organic interfaces opens a realm of possibilities, from the direct applications in molecular spintronics and thin-film magnetism to biomedical imaging or quantum computing. This interaction at the surface can control the spin polarization in magnetic field sensors, generate magnetization spin-filtering effects in nonmagnetic electrodes, or even give rise to a spontaneous spin ordering in nonmagnetic elements such as diamagnetic copper and paramagnetic manganese.

In the temperature-magnetic field phase diagram, the binary metallic compound MnSi exhibits three magnetic phases below T_c ≈ 29K.An unconventional helicoidal phase is observed in zero field. At moderate field intensity a conical phase sets in. Near T_c, in an intermediate field range, a skyrmion lattice phase appears. Here we show the magnetic structure in the conical phase to strongly depend on the field direction and to deviate substantially from a conventional conical structure.

Muon-spin rotation data collected at ambient pressure (p) and at p = 2.42 GPa in MnP were analyzed to check their consistency with various low- and high-pressure magnetic structures reported in the literature. Our analysis con rms that in MnP the low-temperature and low-pressure helimagnetic phase is characterised by an increased value of the average magnetic moment compared to the high-temperature ferromagnetic phase. An elliptical double-helical structure with a propagation vector Q=(0,0,0.117), an a-axis moment elongated by approximately 18% and an additional tilt of the rotation plane towards c-direction by ∼ 4-8° leads to a good agreement between the theory and the experiment. The analysis of the high-pressure μSR data reveals that the new magnetic order appearing for pressures exceeding 1.5 GPa can not be described by keeping the propagation vector Q||c. Even the extreme case—decoupling the double-helical structure into four individual helices— remains inconsistent with the experiment. It is shown that the high-pressure magnetic phase which is a precursor of superconductivity is an incommensurate helical state with Q||b.

A quantum spin liquid is a state of matter characterized by quantum entanglement and the absence of any broken symmetry. In condensed matter, the frustrated rare-earth pyrochlore magnets Ho₂Ti₂O₇ and Dy₂Ti₂O₇, so-called spin ices, exhibit a classical spin liquid state with fractionalized thermal excitations (magnetic monopoles). Evidence for a quantum spin ice, in which the magnetic monopoles become long range entangled and an emergent quantum electrodynamics arises, seems within reach. The magnetic properties of the quantum spin ice candidate Yb₂Ti₂O₇ have eluded a global understanding and even the presence or absence of static magnetic order at low temperatures is controversial. Here we show that sensitivity to pressure is the missing key to the low temperature behaviour of Yb₂Ti₂O₇. By combining neutron diffraction and muon spin relaxation on a stoichiometric sample under pressure, we evidence a magnetic transition from a disordered, non-magnetic, ground state to a splayed ferromagnetic ground state.

Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain α_intr = 0.0036±0.0003 at 310K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.

The RNiO₃ perovskites are known to order antiferromagnetically below a material-dependent Néel temperature TN. We report experimental evidence indicating the existence of a second magnetically ordered phase in TlNiO₃ above T_N = 104K, obtained using nuclear magnetic resonance and muon spin rotation spectroscopy. The new phase, which persists up to a temperature T_N^* = 202K, is suppressed by the application of an external magnetic field of approximately 1T. It is not yet known if such a phase also exists in other perovskite nickelates.

We investigated the depth dependence of current-induced magnetic fields in a bilayer of a normal metal (Au) and a ferrimagnetic insulator (Yttrium Iron Garnet—YIG) by using low energy muon spin spectroscopy (LE-μSR). This allows us to explore how these fields vary from the Au surface down to the buried Au|YIG interface, which is relevant to study physics like the spin-Hall effect. We observed a maximum shift of 0.4 G in the internal field of muons at the surface of Au film which is in close agreement with the value expected for Oersted fields. As muons are implanted closer to the Au|YIG interface, the shift is strongly suppressed, which we attribute to the dipolar fields present at the Au|YIG interface. Combining our measurements with modeling, we show that dipolar fields caused by the finite roughness of the Au|YIG interface consistently explain our observations. Our results, therefore, gauge the limits on the spa|ial resolution and the sensitivity of LE-μSR to the roughness of the buried magnetic interfaces, a prerequisite for future studies addressing current induced fields caused by the spin-accumulations due to the spin-Hall effect.

We present a detailed investigation of the temperature and depth dependence of the magnetic properties of the three-dimensional topological Kondo insulator SmB₆, in particular, near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB₆, which produce local magnetic fields of about ∼1.8 mT fluctuating on a time scale of ∼60 ns. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of ∼40-90 nm, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state.