Relationship between Ferromagnetic Criticality and the Enhancement of Superconductivity Induced by Transverse Magnetic Fields in UCoGe

Abstract

We have performed 59 Co NMR experiments on the ferromagnetic superconductor UCoGe under magnetic fields (H) along the a-and b-axes to investigate the relationship between ferromagnetic properties and superconductivity. The ferromagnetic ordering temperature T Curie is suppressed and the nuclear spin-lattice relaxation rate 1/T 1 at 2 K is enhanced in H || b, although T Curie and 1/T 1 are unchanged in H || a, indicating that the ferromagnetic criticality is induced only when H is applied along the b axis. We show the close relationship between the magnetic anisotropies and the superconducting ones reported by Aoki et al.: the superconductivity is gradually suppressed in H || a, but enhanced in H || b above 5 T. We strongly suggest that the enhancement of the superconductivity observed in H || b originates from the field induced ferromagnetic criticality, as pointed out by Aoki et al and Mineev.UCoGe exhibits the ferromagnetic (FM) ordering at a low Curie temperature T Curie of ∼ 3 K and a superconducting (SC) transition temperature T SC of ∼ 0.8 K at ambient pressure, 1 which is the highest among the FM superconductors discovered so far. Although UCoGe possesses a three-dimensional crystal structure, its magnetic property is the Ising anisotropy with the c-axis being the easy axis.2, 3 In addition, its SC upper critical fields (H c2 ) also have strong anisotropy; superconductivity survives far beyond the Pauli-limiting field along the aand b-axes, whereas H c2 along the c-axis is as small as 0.5 T. 2, 3Since UCoGe includes the familiar NMR-active nucleus 59 Co, it is a suitable compound for NMR measurements within the FM superconductors. We have shown that superconductivity occurs in the FM region from the 59 Co nuclear quadrupole resonance (NQR), resulting in the microscopic coexistence of ferromagnetism and superconductivity. This is consistent with the µSR result.4 From the precise angleresolved NMR at 1.7 K and Meissner measurements at 85 mK, we show that the magnetic field along the c-axis strongly suppresses the Ising FM fluctuations along the c-axis and that the superconductivity is observed only in the limited magnetic field region where the FM spin fluctuations are active.5 These results, combined with model calculations, strongly suggest that the Ising FM spin fluctuations tuned by H || c induce the unique spin-triplet superconductivity by resolving the above puzzling H c2 behavior.Although the external magnetic field along the c-axis is the tuning parameter of the Ising FM fluctuations and the key to understanding the small H c2 along the c-axis, the magnetic field along the b-axis, which is the second magnetic easy axis and perpendicular to the U-U zigzag chain, also * E-mail: t.hattori@scphys.kyoto-u.ac.jp † E-mail: kishida@scphys.kyoto-u.ac.jp tunes the superconductivity. 3 The superconductivity becomes robust against the external field along the b axis when µ 0 H greater than 5 T is applied, which is reminiscent of the reentrant superconductivity in the sister compound URhGe, 6,7 as discussed l...

Magnetism and superconductivity in ferromagnetic heavy-fermion system UCoGe under in-plane magnetic fields

“…As mentioned in the previous section, the Curie temperature is decreased by magnetic field along the b axis, while it is unchanged by a magnetic field applied parallel to the a axis within the experimental range [8][9][10]22]. If the magnetic fields are further increased, T C will be suppressed for a-axis magnetic fields as well.…”

“…The action includes spin-dependent terms described byh = h + h ex , where h = μ B H is the applied Zeeman field and g factor is simply taken to be g = 2, and h ex is the exchange splitting energy of the Fermi surface in the ferromagnetic state. From the experiments [8,22] and the previous sections, it is reasonable to assume that the exchange splitting at zero temperature h ex c axis depends only on b-axis applied fields. It is phenomenologically approximated as…”

Superconductivity in uranium compounds

“…Similar mechanism also stimulates the arising of the reentrant superconducting state in URhGe in fields along b-axis about 12 Tesla discovered about decade ago 13 . It should be noted that the presented ideas about the relationship between ferromagnetic properties and superconductivity quite recently have been essentially supported experimentally 14 . The paper is organized as follows.…”

Reentrant superconductivity in URhGe

“…We have operated with frequency independent magnetic susceptibilities found in thermodynamically equilibrium state. Certainly, there are many properties of these materials like field dependence of electron effective masses 1,2 , NMR and NQR relaxation anisotropy 11,12 , temperature dependence of the upper critical fields 7 which demand development of theory of frequency dependent electron -magnon interaction determining superconducting pairing in magnetically strongly anisotropic media.…”