J. H. Kim

Magnetic resonant mode in the low-energy spin-excitation spectrum of superconducting Rb2Fe4Se5 single crystals.

We have studied the low-energy spin-excitation spectrum of the single-crystalline Rb(2)Fe(4)Se(5) superconductor (T(c)=32 K) by means of inelastic neutron scattering. In the superconducting state, we observe a magnetic resonant mode centered at an energy of ℏω(res)=14 meV and at the (0.5 0.25 0.5) wave vector (unfolded Fe-sublattice notation), which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors. Our finding suggests that the 245-iron selenides are unconventional superconductors with a sign-changing order parameter, in which bulk superconductivity coexists with the √5×√5 magnetic superstructure. The estimated ratios of ℏω(res)/k(B)T(c)≈5.1±0.4 and ℏω(res)/2Δ≈0.7±0.1, where Δ is the superconducting gap, indicate moderate pairing strength in this compound, similar to that in optimally doped 1111 and 122 pnictides.

Formation and rotation of skyrmion crystal in the chiral-lattice insulator Cu2OSeO3

Small-angle neutron scattering experiments were performed on a bulk single crystal of chiral-lattice multiferroic insulator Cu2OSeO3. In the absence of an external magnetic field, helical spin order with magnetic modulation vector q∥⟨001⟩ was identified. When a magnetic field is applied, a triple-q magnetic structure emerges normal to the field in the A phase just below the magnetic ordering temperature Tc, which suggests the formation of a triangular lattice of skyrmions. Notably, the favorable q direction in the A phase changes from q∥⟨110⟩ to q∥⟨001⟩ upon approaching Tc. Near the phase boundary between these two states, the external magnetic field induces a 30∘ rotation of the skyrmion lattice. This suggests a delicate balance between the magnetic anisotropy and the spin texture near Tc, such that even a small perturbation significantly affects the ordering pattern of the skyrmions.

Magnetic proximity effect in YBa2Cu3O7 / La2/3Ca1/3MnO3 and YBa2Cu3O7 / LaMnO3+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 La0.67Ca0.33MnO3 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 La0.67Ca0.33MnO3 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.

Symmetry of spin excitation spectra in the tetragonal paramagnetic and superconducting phases of 122-ferropnictides

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe1.85Co0.15As2 and BaFe1.91Ni0.09As2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the 42/m screw symmetry around the (1/2 1/2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of-plane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken (nematic) ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode Er, as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship Er=4.3*kB*Tc holds for all studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.

Symmetry and disorder of the vitreous vortex lattice in overdoped BaFe2-xCoxAs2: Indication for strong single-vortex pinning

The disordered flux line lattice in single crystals of the slightly overdoped aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by magnetization measurements, small-angle neutron scattering (SANS), and magnetic force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex pinning precludes the formation of an ordered Abrikosov lattice. Instead, a vitreous vortex phase (vortex glass) with a short-range hexagonal order is observed. Statistical processing of MFM datasets lets us directly measure its radial and angular distribution functions and extract the radial correlation length zeta. In contrast to predictions of the collective pinning model, no increase in the correlated volume with the applied field is observed. Instead, we find that zeta decreases as 1.3*R1 ~ H^(-1/2) over four decades of the applied magnetic field, where R1 is the radius of the first coordination shell of the vortex lattice. Such universal scaling of zeta implies that the vortex pinning in iron arsenides remains strong even in the absence of static magnetism. This result is consistent with all the real- and reciprocal-space vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published to date and is thus sample-independent. The failure of the collective pinning model suggests that the vortices remain in the single-vortex pinning limit even in high magnetic fields up to 9 T.

Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr3Fe2O7.

We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr(3)Fe(2)O(7), which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe(4+) moments adopt incommensurate spiral order below T(N) = 115 K and provide a comprehensive description of the corresponding spin-wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third-nearest-neighbor pairs. The results indicate that the helical magnetism in Sr(3)Fe(2)O(7) results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals.

A model for giant magnetoresistance in magnetic granular solids

Abstract We propose a model for giant magnetoresistance (GMR) in granular magnetic solids where the moments within a grain form a single ferromagnetic domain. The electrical transport properties of such solids due to (1) the scattering from grains that act as centers for the potential barrier and (2) the magnetic scattering due to the dipole-dipole interaction between a ferromagnetic domain and spin of a conduction electron are calculated. The magneto-conductance computed as a function of the grain size and the global magnetization shows that GMR in magnetically inhomogeneous media may arise from the interplay between the potential-barrier and the magnetic scatterings.

On the Feasibility to Study Inverse Proximity Effect in a Single S/F Bilayer by Polarized Neutron Reflectometry ¶

Here we report on a feasibility study aiming to explore the potential of Polarized Neutron Reflectometry (PNR) for detecting the inverse proximity effect in a single superconducting/ferromagnetic bilayer. Experiments, conducted on the V (40 nm)/Fe (1 nm) S/F bilayer, have shown that experimental spin asymmetry measured at T = 0.5TC is shifted towards higher Q values compared to the curve measured at T = 1.5TC. Such a shift can be described by the appearance in superconducting vanadium of magnetic sublayer with a thickness of 7 nm and a magnetization of +0.8 kG.

On the explanation of the paramagnetic Meissner effect in superconductor/ferromagnet heterostructures

An increase of the magnetic moment in superconductor/ferromagnet (S/F) bilayers V(40 nm)/F (, Co(3 nm), Ni(3 nm)) was observed using SQUID magnetometry upon cooling below the superconducting transition temperature T C in magnetic fields of 10 Oe to 50 Oe applied parallel to the sample surface. A similar increase, often called the paramagnetic Meissner effect (PME), was observed before in various superconductors and superconductor/ferromagnet systems. To explain the PME effect in the presented S/F bilayers a model based on a row of vortices located at the S/F interface is proposed. According to the model the magnetic moment induced below T C consists of the paramagnetic contribution of the vortex cores and the diamagnetic contribution of the vortex-free region of the S layer. Since the thickness of the S layer is found to be 3-4 times less than the magnetic-field penetration depth, this latter diamagnetic contribution is negligible. The model correctly accounts for the sign, the approximate magnitude and the field dependence of the paramagnetic and the Meissner contributions of the induced magnetic moment upon passing the superconducting transition of a ferromagnet/superconductor bilayer. (Less)

Structural, Magnetic, and Superconducting Characterization of the CuNi/Nb Bilayers of the S/F Type Using Polarized Neutron Reflectometry and Complementary Techniques

Structural, magnetic, and superconducting properties of S/F bilayers Nb/Cu 40Ni 60 deposited on silicon substrate have been characterized using polarized neutron reflectometry and complementary techniques. The study allowed to determine real thicknesses of the S and F layers as well as the r.m.s. roughness of the S/F interfaces. The latter does not exceed 1 nm, showing the high quality of the S/F interface. Using SQUID and a mutual inductance setup, we determined the superconducting transition temperatures of the samples, which are in agreement with the literature data. Using of polarized neutron reflectometry (PNR) for the single S layer allowed to determine the screening length λ of the superconducting layer, λ = 120 nm. This value is higher than the London penetration depth for pure niobium which may indicate that the superconductor is in the dirty limit. PNR and SQUID studies of magnetic properties of the CuNi layer have shown the presence of ferromagnetism in all investigated samples.