Ferromagnetic Superconductor Research Articles

Inverse proximity effect at superconductor-ferromagnet interfaces: Evidence for induced triplet pairing in the superconductor

Considerable evidence for proximity-induced triplet superconductivity on the ferromagnetic side of a superconductor-ferromagnet (S-F) interface now exists; however, the corresponding effect on the superconductor side has hardly been addressed. We have performed scanning tunneling spectroscopy measurements on NbN superconducting thin films proximity coupled to the half-metallic ferromagnet La2/3Ca1/3MnO3 (LCMO) as a function of magnetic field. We have found that at zero and low applied magnetic fields the tunneling spectra on NbN typically show an anomalous gap structure with suppressed coherence peaks and, in some cases, a zero-bias conductance peak. As the field increases to the magnetic saturation of LCMO where the magnetization is homogeneous, the spectra become more BCS-like and the critical temperature of the NbN increases, implying a reduced proximity effect. Our results therefore suggest that triplet-pairing correlations are also induced in the S side of an S-F bilayer.

Physical realization of the generalized fully frustrated XY model in an array of SFS junctions

We show that a physical realization of the phase diagram proposed by Minnhagen et al. [Phys. Rev. B 78, 184432 (2008)] for so-called generalized fully frustrated XY model on square lattice can be achieved in arrays of SFS (superconductor-ferromagnet-superconductor) junctions near the transition of the junctions to $\pi$-state. Moreover, the phase diagram with such a topology has to be reproduced twice, on both sides of the $0-\pi$ transition.

Spatially resolved penetration depth measurements and vortex manipulation in the ferromagnetic superconductorErNi2B2C

We present a local probe study of the magnetic superconductor, ErNi$_2$B$_2$C, using magnetic force microscopy at sub-Kelvin temperatures. ErNi$_2$B$_2$C is an ideal system to explore the effects of concomitant superconductivity and ferromagnetism. At 500 mK, far below the transition to a weakly ferromagnetic state, we directly observe a structured magnetic background on the micrometer scale. We determine spatially resolved absolute values of the magnetic penetration depth $\lambda$ and study its temperature dependence as the system undergoes magnetic phase transitions from paramagnetic to antiferromagnetic, and to weak ferromagnetic, all within the superconducting regime. In addition, we estimate the absolute pinning force of Abrikosov vortices, which shows a position- and temperature dependence as well, and discuss the possibility of the purported spontaneous vortex formation.

Evolution of ferromagnetic and non-Fermi-liquid states with doping: The case of Ru-doped UCoGe

We have investigated the impact of Ru substitution for Co on the behavior of the ferromagnetic superconductor UCoGe by performing x-ray diffraction, magnetization, specific heat and electrical resistivity measurements on polycrystalline samples of the $\mathrm{UCo}_{1-x}\mathrm{Ru}_{x}\mathrm{Ge}$ series ($0\geq x\leq0.9$). The initial Ru substitution up to $x\approx0.1$ leads to a simultaneous sharp increase of the Curie temperature and spontaneous magnetization up to maximum values of $T_{\mathrm{C}}=8.6 K$ and $M_{\mathrm{S}}=0.1 \mu_{\mathrm{B}}$ per formula unit, respectively, whereas superconductivity vanishes already for $x\approx0.03$. Further increase of the Ru content beyond $x\approx0.1$ leads to a precipitous decrease of both, $T_{\mathrm{C}}$ and $M_{\mathrm{S}}$ towards a ferromagnetic quantum critical point (QCP) at $x_{\mathrm{cr}}=0.31$. Consequently the $T-x$ magnetic phase diagram consists of a well-developed ferromagnetic dome. We discuss the evolution of ferromagnetism with $x$ on the basis of band structure changes due to varying 5$f$-ligand hybridization. This scenario is supported by the results of electronic structure calculations and consideration of the simplified periodic Anderson model. The analysis of the temperature dependencies of the electrical resistivity and heat capacity at low temperatures of the samples in the vicinity of the QCP reveals a non-Fermi liquid behavior and assigns the ferromagnetic quantum phase transition to be most likely of a continuous Hertz-Millis type.

Microscopic magnetic properties of the ferromagnetic superconductor UCoGe reviewed by x-ray magnetic circular dichroism

The ferromagnetic superconductor UCoGe has been investigated by high field x-ray magnetic circular dichroism (XMCD) at the U ${M}_{4,5}$ and Co/Ge $K$ edges. The analysis of the branching ratio and XMCD at the U ${M}_{4,5}$ edges reveals that the U $5f$ electrons count is close to 3. The orbital $(\ensuremath{\sim}0.70{\ensuremath{\mu}}_{B})$ and spin $(\ensuremath{\sim}\ensuremath{-}0.30{\ensuremath{\mu}}_{B})$ moments of U at 2.1 K and 17 T (H$\ensuremath{\parallel}$c) have been determined. Their ratio $(\ensuremath{\sim}\ensuremath{-}2.3)$ suggests a significant delocalization of the $5f$ electron states. The similar field dependences of the local U/Co and the macroscopic magnetization indicate that the Co moment is induced by the U moment. The XMCD at the Co/Ge $K$ edges reveal the presence of small Co $4p$ and Ge $4p$ orbital moments parallel to the macroscopic magnetization. In addition, the Co $3d$ moment is estimated to be at most of the order of $0.1{\ensuremath{\mu}}_{B}$ at 17 T. Our results rule out the possibility of an unusual polarizability of the U and Co moments as well as their antiparallel coupling. We conclude that the magnetism which mediates the superconductivity in UCoGe is driven by U.

D-band metalY9Co7revisited: Evidence for local coexistence of superconductivity and itinerant ferromagnetism

The heat capacity, magnetic, and electrical transport properties of the binary intermetallic compound ${\mathrm{Y}}_{9}{\mathrm{Co}}_{7}$ are re-investigated through careful low-temperature measurements performed on one and the same high-quality polycrystalline sample. Our results indicate a local coexistence of itinerant ferromagnetism and bulk superconductivity in ${\mathrm{Y}}_{9}{\mathrm{Co}}_{7}$ below ${T}_{\mathrm{sc}}=2.95$ K, as opposite to a previous conviction that superconductivity occurs in the paramagnetic phase embedded in a basically normal magnetic environment. Since the clean-limit condition is satisfied for a pure sample of ${\mathrm{Y}}_{9}{\mathrm{Co}}_{7}$, the question whether magnetic fluctuations contribute in the formation of Cooper pairs in this sole $d$-band ferromagnetic superconductor remains open.

Magnetic, electrical, and thermodynamic properties of NpIr: Ambient and high-pressure measurements, and electronic structure calculations

We present bulk property measurements of NpIr, a newly synthesized member of the Np-Ir binary phase diagram, which is isostructural to the noncentrosymmetric pressure-induced ferromagnetic superconductor UIr. Magnetic susceptibility, electronic transport properties at ambient and high pressure, and heat capacity measurements have been performed for temperatures $T=0.55\text{--}300$ K in a range of magnetic fields up to 14 T and under pressure up to $17.3$ GPa. These reveal that NpIr is a moderately heavy fermion Kondo system with strong antiferromagnetic interactions, but there is no evidence of any phase transition down to $0.55$ K or at the highest pressure achieved. Experimental results are compared with ab initio calculations of the electronic band structure and lattice heat capacity. An extremely low lattice thermal conductivity is predicted for NpIr at temperatures above 300 K.

73Ge-Nuclear Magnetic Resonance/Nuclear Quadrupole Resonance Investigation of Magnetic Properties of URhGe

We report on the 73Ge-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) results for the ferromagnetic (FM) superconductor URhGe. The magnitude and direction of the internal field, Hint, and the parameters of the electric field gradient at the Ge site were determined experimentally. By using powdered polycrystalline samples oriented by different methods, the field dependences of NMR shift and nuclear spin relaxation rates for H0 || c (easy axis) and H0 || b were obtained. From the NMR shifts for H0 || b, we confirmed a gradual suppression of the Curie temperature and observed a phase separation near the spin reorientation. The observation of the phase separation gives microscopic evidence that the spin reorientation under H0 || b is of first order at low temperatures. The nuclear spin–lattice relaxation rate 1/T1 indicates that the magnetic fluctuations are suppressed for H0 || c, whereas the fluctuations remain strongly for H0 || b. The enhancements of both 1/T1T and the nuclear spin–spi...

Electronic structures of ferromagnetic superconductorsUGe2and UCoGe studied by angle-resolved photoelectron spectroscopy

The electronic structures of the ferromagnetic superconductors ${\mathrm{UGe}}_{2}$ and $\mathrm{UCoGe}$ in the paramagnetic phase were studied by angle-resolved photoelectron spectroscopy using soft x rays $(h\ensuremath{\nu}=400\text{--}500\phantom{\rule{0.28em}{0ex}}\mathrm{eV})$. The quasiparticle bands with large contributions from $\mathrm{U}\phantom{\rule{0.28em}{0ex}}5f$ states were observed in the vicinity of ${E}_{\mathrm{F}}$, suggesting that the $\mathrm{U}\phantom{\rule{0.28em}{0ex}}5f$ electrons of these compounds have an itinerant character. Their overall band structures were explained by the band-structure calculations treating all the $\mathrm{U}\phantom{\rule{0.28em}{0ex}}5f$ electrons as being itinerant. Meanwhile, the states in the vicinity of ${E}_{\mathrm{F}}$ show considerable deviations from the results of band-structure calculations, suggesting that the shapes of Fermi surface of these compounds are qualitatively different from the calculations, possibly caused by electron correlation effect in the complicated band structures of the low-symmetry crystals. Strong hybridization between $\mathrm{U}\phantom{\rule{0.28em}{0ex}}5f$ and $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}3d$ states in $\mathrm{UCoGe}$ were found by the $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}2p\ensuremath{-}3d$ resonant photoemission experiment, suggesting that $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}3d$ states have finite contributions to the magnetic, transport, and superconducting properties.

Phenomenological description of anisotropy effects in some ferromagnetic superconductors

We study phenomenologically the role of anisotropy in ferromagnetic superconductors UGe2, URhGe, and UCoGe for the description of their phase diagrams. We use the Ginzburg–Landau free energy in its uniform form as we will consider only spatially independent solutions. This is an expansion of previously derived results where the effect of Cooper-pair and crystal anisotropies is not taken into account. The three compounds are separately discussed with the special stress on UGe2. The main effect comes from the strong uniaxial anisotropy of magnetization while the anisotropy of Cooper pairs and crystal anisotropy only slightly change the phase diagram in the vicinity of Curie temperature. The limitations of this approach are also discussed.

Upper critical field under hydrostatic pressure in UCoGe

We report on the pressure dependence of the upper superconductivity critical field Hc2 of the ferromagnetic superconductor UCoGe for field along the c axis, which is the easy magnetization axis. The surprising result is the upward curvature and strong enhancement of the upper critical field on cooling. This enhancement is even more pronounced above the critical pressure pc in the paramagnetic phase than in the ferromagnetic phase below pc. This effect cannot be explained by models based on the suppression of pair building magnetic fluctuations under magnetic field. That points out either decrease of the pair breaking mechanism under field or fluctuations favorable for the Cooper pairing linked to the wing structure of ferromagnetism near the critical pressure, where ferromagnetism is suppressed.

The critical current reaction on hydrostatic pressure of a superconductor–semimetal composite

The transport characteristics of a composite consisting of microparticles of a superconductor MgB2 and nanopowder of a ferromagnetic half-metal La0.7Ca0.3MnO3 (LCMO) were investigated under hydrostatic compression. It was observed that as the concentration of the LCMO manganite is increased above the percolation threshold of MgB2, the superconducting current flows through the superconductor–ferromagnet–superconductor chains with the LCMO thickness exceeding the magnetic coherence length ξM in the half-metal (ξM ≤ 10 Å) by an order of magnitude. This behavior can be attributed to the presence of triplet superconducting correlations in the half-metal, which arise due to the spin-active surface of LCMO nanograins. An increase in pressure led to an increase in the critical current of the MgB2–LCMO composite, while the critical current and the transition temperature of pure MgB2 decreased under pressure.

Weak itinerant ferromagnetism and clean superconductivity in Y9Co7

We present low-temperature magnetic properties for a high-quality polycrystalline sample of the first ferromagnetic superconductor Y9Co7. The results of susceptibility and magnetization measurements show the coexistence of a weak itinerant ferromagnetic order with a Curie temperature of TC ≅ 4.5 K and superconductivity below TS = 3 K. Several electronic quantities and parameters characterizing the superconducting and normal state are calculated within the Ginzburg–Landau–Abrikosov–Gorkov theory. The data reveal clean limit superconductivity in Y9Co7 due to the good chemical purity and structural order of the specimen.

Reentrant superconductivity in URhGe

There is presented a phenomenological description of phase diagram of ferromagnet superconductor URhGe. In frame of the Landau phenomenological theory it was found that phase transition between anisotropic ferromagnetic and paramagnetic states under strong enough magnetic field perpendicular to direction of easy magnetization changes from the second to the first order type. It is shown that magnetic susceptibility corresponding to longitudinal magnetic fluctuations strongly increases in vicinity of the first order transition stimulating reentrance of superconducting state. The reentrant superconductivity observed near the first order transition line at temperatures about twice lower than the tricritical point temperature exists both in ferromagnet and paramagnet state. The critical temperature of transition to superconducting state falls down at intersection with line of ferromagnet-paramagnet phase transition.

Evolution of magnetism in UCoGe and UCoAl with Ru doping**Invited talk at the 7th International Workshop on Advanced Materials Science and Nanotechnology IWAMSN2014, 2–6 November, 2014, Ha Long, Vietnam.

Intriguing magnetic properties of the UCo1−xRuxGe and UCo1−xRuxAl pseudoternary systems studied on single crystals are presented and discussed in terms of anisotropic 5f-ligand hybridization as the principal driving mechanism. Ferromagnetism of the ferromagnetic superconductor UCoGe is initially stabilized by Ru substitutions for Co (superconductivity suppressed already for x > 0.01) providing maximum Curie temperature (TC), spontaneous magnetic moment (μs) and magnetic entropy (Smag) for 10% Ru. Further increasing Ru concentration leads to gradual suppression of these parameters towards a ferromagnetic quantum critical point (FM QCP) at ≈0.31. Non-Fermi liquid scaling of specific heat and electrical resistivity is observed in the vicinity of FM QCP. The itinerant electron metamagnet UCoAl is transformed to a ferromagnet by only 0.5% Ru substituted for Co. The ferromagnetism is suppressed for x > 0.7. On the descending branch of the dome-like T − x magnetic phase diagram two distinctly different ferromagnetic phases are observed. The strong magnetocrystalline anisotropy leaving the hard magnetization directions Pauli paramagnetic like irrespective of magnetic ground states projected exclusively in the easy magnetization directions is accounted to the anisotropic 5f-5f wave-function overlap and 5f-ligand hybridization, and 5f-electron orbital moment.

Fabrication and measurements of hybrid Nb/Al Josephson junctions and flux qubits with π-shifters

We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing π-phase shifters fabricated using well-established Nb-based technology of superconductor–ferromagnet–superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in situ Ar etching using e-beam gun. Shadow-evaporated Al/AlOx/Al Josephson junctions with Nb bias pads show the expected current–voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb π-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without π-junction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.

Spin Polarization and Suppression of Superconductivity at Nanoscale Ferromagnet–Superconductor Interfaces

We have investigated point contacts between ferromagnetic Fe and different BCS-type superconductors using Andreev reflection spectroscopy. With the standard BTK-type analysis, the extracted spin polarization of Fe does not depend on the superconductor and agrees well with literature data. However, the characteristics of the Andreev reflection spectra vary with the superconductor. While most of the contacts with the strong superconductors Nb and Ta appear to be barely affected by the presence of Fe, the contacts with weaker superconductors Sn and Al have a reduced critical temperature $$T_\mathrm{c}$$ and occasionally a second superconducting transition at still lower temperature, and contacts with the weakest superconductor Zn have strongly reduced superconducting anomalies. We propose that this variation is related to the magnetic field generated by the ferromagnetic electrode, which can also affect the extracted spin polarization.

(Li(0.8)Fe(0.2))OH]FeS and the ferromagnetic superconductors [(Li(0.8)Fe(0.2))OH]Fe(S(1-x)Se(x)) (0 < x ≤ 1).

Superconductivity up to 43 K and magnetic ordering coexist in the iron chalcogenides [(Li(0.8)Fe(0.2))OH]Fe(S(1-x)Se(x)) (0 < x ≤ 1). Substitution of sulphur for selenium gradually suppresses superconductivity while the ferromagnetic signature persists up to non-superconducting [(Li(0.8)Fe(0.2))OH]FeS.

Topological superconductivity induced by ferromagnetic metal chains

Recent experiments have provided evidence that one-dimensional (1D) topological superconductivity can be realized experimentally by placing transition metal atoms that form a ferromagnetic chain on a superconducting substrate. We address some properties of this type of systems by using a Slater-Koster tight-binding model. We predict that topological superconductivity is nearly universal when ferromagnetic transition metal chains form straight lines on superconducting substrates and that it is possible for more complex chain structures. The proximity induced superconducting gap is $\sim \Delta E_{so} / J$ where $\Delta$ is the $s$-wave pair-potential on the chain, $E_{so}$ is the spin-orbit splitting energy induced in the normal chain state bands by hybridization with the superconducting substrate, and $J$ is the exchange-splitting of the ferromagnetic chain $d$-bands. Because of the topological character of the 1D superconducting state, Majorana end modes appear within the gaps of finite length chains. We find, in agreement with experiment, that when the chain and substrate orbitals are strongly hybridized, Majorana end modes are substantially reduced in amplitude when separated from the chain end by less than the coherence length defined by the $p$-wave superconducting gap. We conclude that Pb is a particularly favorable substrate material for ferromagnetic chain topological superconductivity because it provides both strong $s-$wave pairing and strong Rashba spin-orbit coupling, but that there is an opportunity to optimize properties by varying the atomic composition and structure of the chain. Finally, we note that in the absence of disorder a new chain magnetic symmetry, one that is also present in the crystalline topological insulators, can stabilize multiple Majorana modes at the end of a single chain.

Superconductivity tuned through magnetic irreversibility in two-dimensional Co/Nb/Co trilayers under a parallel magnetic field

In ferromagnetic/superconducting (FM/SC) planar hybrids, a reentrance of the upper-critical field line Hc2(T) is observed close to Tc. The effect is generally ascribed to the so-called domain-wall superconductivity and has been theoretically explored in great detail. Experimental investigations are limited mostly to FM/SC bilayers in which the FM layers host out-of-plane magnetic domains of strong anisotropy and large width (&amp;gt;300 nm), the SC layer is quite thick (&amp;gt;30 nm), and the external magnetic field was applied normal. To expand our knowledge to until now unexplored conditions, we study a series of Co(dCo)/Nb(dNb)/Co(dCo) trilayers under a parallel magnetic field; Co outer layers exhibit out-of-plane magnetic domains of weak anisotropy and small width (&amp;lt;150 nm) and the Nb interlayer is very thin (&amp;lt;20 nm). We demonstrate a strong reentrance of Hc2(T) that can be tuned through the irreversible magnetization processes of the FM outer layers, the two-dimensional character of the SC interlayer, and the matching between the interfering SC and FM length scales. These results refine the nature of the reentrance of Hc2(T) in FM/SC hybrids and can motivate new theoretical and experimental investigations.