NUM department - Scientific Highlights

Magnetic Proximity Effect in YBa₂Cu₃O₇/La_2/3Ca_1/3MnO₃ and YBa₂Cu₃O₇/LaMnO_3+δ Superlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa₂Cu₃O₇ and ferromagnetic-metallic La_0.67Ca_0.33MnO₃ or ferromagnetic-insulating LaMnO_3+δ. We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La_0.67Ca_0.33MnO₃ and almost absent for ferromagnetic-insulating LaMnO_3+δ. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature..

Direct Observation of the Quantum Critical Point in Heavy Fermion CeRhSi₃

We report on muon spin rotation studies of the noncentrosymmetric heavy fermion antiferromagnet CeRhSi₃. A drastic and monotonic suppression of the internal fields, at the lowest measured temperature, was observed upon an increase of external pressure. Our data suggest that the ordered moments are gradually quenched with increasing pressure, in a manner different from the pressure dependence of the Neel temperature. At 23.6 kbar, the ordered magnetic moments are fully suppressed via a second-order phase transition, and T_N is zero. Thus, we directly observed the quantum critical point at 23.6 kbar hidden inside the superconducting phase of CeRhSi₃.

Spin density wave induced disordering of the vortex lattice in superconducting La_2−xSr_xCuO₄

We use small-angle neutron scattering to study the superconducting vortex lattice in La_2−xSr_xCuO₄ as a function of doping and magnetic field. We show that near optimally doping the vortex lattice coordination and the superconducting coherence length ξ are controlled by a Van Hove singularity crossing the Fermi level near the Brillouin zone boundary. The vortex lattice properties change dramatically as a spin-density-wave instability is approached upon underdoping. The Bragg glass paradigm provides a good description of this regime and suggests that spin-density-wave order acts as a source of disorder on the vortex lattice.

Muonium Emission into Vacuum from Mesoporous Thin Films at Cryogenic Temperatures

We report on muonium (Mu) emission into vacuum following μ⁺ implantation in mesoporous thin SiO₂ films. We obtain a yield of Mu into vacuum of (38±4)% at 250 K and (20±4)% at 100 K for 5 keV μ⁺ implantation energy. From the implantation energy dependence of the Mu vacuum yield we determine the Mu diffusion constants in these films: D_Mu^250K=(1.6±0.1)x10^-4  cm²/s and D_Mu^100K=(4.2±0.5)x10^-5  cm²/s. Describing the diffusion process as quantum mechanical tunneling from pore to pore, we reproduce the measured temperature dependence ∼T^3/2 of the diffusion constant. We extract a potential barrier of (-0.3±0.1) eV which is consistent with our computed Mu work function in SiO₂ of [-0.3,-0.9]  eV. The high Mu vacuum yield, even at low temperatures, represents an important step toward next generation Mu spectroscopy experiments.

Dimensionality Selection in a Molecule-Based Magnet

Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy. Here we demonstrate a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S=1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two- to quasi-one-dimensional while effectively maintaining the nearest-neighbor coupling strength.

Superconducting properties of single-crystalline A_xFe_2-ySe₂ (A=Rb, K) studied using muon spin spectroscopy

We report on the superconducting properties of A_xFe_2-ySe₂ (A=Rb, K) single crystals studied with the muon spin relaxation or rotation (μSR) technique. At low temperatures, close to 90% of the sample volumes exhibit large-moment magnetic order which impedes the investigation of their superconducting properties by μSR. On the other hand, about 10% of the sample volumes remain paramagnetic and clearly show a superconducting response. The temperature dependence of the superconducting carrier density was analyzed within the framework of a single s-wave gap scenario. The zero-temperature values of the in-plane magnetic penetration depths λ_ab(0)=258(2) and 225(2) nm and the superconducting gaps Δ(θ)=7.7(2) and 6.3(2) meV have been determined for A = Rb and K, respectively. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed.

Absolute value and temperature dependence of the magnetic penetration depth in Ba(Co_0.074Fe_0.926)₂As₂

The absolute value and temperature dependence of the in-plane magnetic penetration depth λ have been measured on a single crystal of Ba(Co_0.074Fe_0.926)₂As₂ using low-energy muon-spin rotation and microwave cavity perturbation. The magnetic field profiles in the Meissner state are consistent with a local London model beyond a depth of 15 nm. We determine the gap symmetry through measurements of the temperature dependence of the superfluid density which follows a two-gap s-wave model over the entire temperature range below Tc. While the intermediate to high temperature data is well fit by an energy gap model in the BCS-like (weak-coupling) limit, a second smaller gap becomes apparent at low temperatures.

Vortex Lattice Studies in CeCoIn5 with H⊥c

We present small angle neutron scattering studies of the vortex lattice (VL) in CeCoIn5 with magnetic fields applied parallel (H) to the antinodal [100] and nodal [110] directions. For H || [100], a single VL orientation is observed, while a 90° reorientation transition is found for H || [110]. For both field orientations and VL configurations we find a distorted hexagonal VL with an anisotropy, Γ=2.0±0.05. The VL form factor shows strong Pauli paramagnetic effects similar to what have previously been reported for H || [001]. At high fields, above which the upper critical field (H_c2) becomes a first-order transition, an increased disordering of the VL is observed.

Coupling of Magnetic and Ferroelectric Hysteresis by a Multicomponent Magnetic Structure in Mn₂GeO₄

The olivine compound Mn₂GeO₄ is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity, and a canted commensurate order leads to weak ferromagnetism. Symmetry suggests that the direct coupling between the ferromagnetism and ferroelectricity is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multicomponent magnetic structures found in magnetically frustrated materials like Mn₂GeO₄ provide a new route towards functional materials that exhibit coupled ferromagnetism and ferroelectricity.

First-Order Reorientation Transition of the Flux-Line Lattice in CaAlSi

The flux-line lattice in CaAlSi has been studied by small-angle neutron scattering. A well-defined hexagonal flux-line lattice is seen just above Hc1 in an applied field of only 54 Oe. A 30° reorientation of this vortex lattice has been observed in a very low field of 200 Oe. This reorientation transition appears to be first-order and could be explained by nonlocal effects. The magnetic field dependence of the form factor is well-described by a single penetration depth of λ=1496(1)  Å and a single coherence length of ξ=307(1)  Å at 2 K. At 1.5 K, the penetration depth anisotropy is γλ=2.7(1), with the field applied perpendicular to the c axis, and agrees with the coherence length anisotropy determined from critical field measurements.