Field-Induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

Abstract

We review our recent studies on ferromagnetic superconductors, UGe 2 , URhGe and UCoGe, where the spin-triplet state with the so-called equal spin pairing is realized. We focus on experimental results of URhGe and UCoGe in which the superconductivity occurs already at ambient pressure. The huge upper critical field H c2 on UCoGe for the field along the hard magnetization axis (b-axis) is confirmed by the AC susceptibility measurements by the fine tuning of field angle. Contrary to the huge H c2 along the hard-magnetization axis, H c2 along the easy-magnetization axis (c-axis) is relatively small in value. However, the initial slope of H c2 , namely dH c2 /dT (H → 0) both in UCoGe and in URhGe indicates the large value, which can be explained by the magnetic domain effect detected in the magnetization measurements. The specific heat measurements using a high quality single crystal of UCoGe demonstrate the bulk superconductivity, which is extended under magnetic field for the field along c-axis. KEYWORDS: superconductivity, ferromagnetism, uranium compound, superconducting upper critical field, AC susceptibility, specific heat, resistivity, magnetization Ferromagnetism and superconductivity had been thought to be competitive phenomena, since the large internal field due to the ferromagnetism easily destroys the superconducting Cooper pairs. In 1980s', some exceptional cases were found for example in ErRh 4 B 4 and HoMo 6 Se 8 [1]. In these materials, the superconductivity is observed in the intermediate temperature range. Further decreasing temperature, the ferromagnetism appears and the superconducting phase collapses at low temperature, meaning that the Curie temperature (T Curie ) is lower than the superconducting transition temperature (T sc ), and there is no microscopic coexistence of superconductivity and ferromagnetism with large ordered moment. The microscopic coexistence was theoretically proposed in the weak ferromagnet ZrZn 2 [2], where the spin-triplet state is formed near the ferromagnetic quantum critical point. There are, however, no experimental evidences.The first microscopic coexistence of ferromagnetism and superconductivity was found in UGe 2 under pressure [3], then in URhGe at ambient pressure [4], and more recently in UCoGe at ambient pressure [5]. All the known ferromagnetic superconductors are uranium compounds, and T sc is lower than T Curie [6,7]. The ordered moments of U atom are substantially reduced, compared to 1 J. Phys. Soc. Jpn.

Pairing mechanism in the ferromagnetic superconductor UCoGe

“…At first, according to previous resistivity measurements, H c2 is strongly anisotropic: it is >20 times higher along the two transverse directions ( a , b ) than along the easy magnetization axis 18 . In addition to this strong anisotropy, the angular dependence of H c2 (0) in the ( a , c ) plane is extremely sharp near the a -axis 20 21 , at odds with the usual elliptical behaviour. Strong anisotropy in H c2 is commonly observed in low dimensional superconductors, like organics 22 .…”

Ferromagnetism and superconductivity in CeFeAs1−xPxO (Jesche1, Förster2, Spehling3 et al. 2012Phys. Rev. B

“…A comparison with UCoGe, one of the few established FM superconductors, supports this possibility since the diamagnetic signal at T SC can be small compared to the anomaly at T C (Ref. 21) and a Meissner phase might be even completely absent when the internal FM field is larger than H c1 . 22 x > 30% and the absence of SC in these samples, we conclude a competing character of both effects (see the phase diagram in Fig.…”

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

“…The superconductivity extends over a wide range of pressure and it survives in the paramagnetic phase [3,8] experimentally. Although the Mineev's mechanism of suppressing the Pauli depairing effect does not work in the paramagnetic phase, observed H c2 for in-plane magnetic fields are large, H c2 7-8 T, and they well exceed the Pauli limiting field ∼ 1 T, which is naively expected for the equal-spin triplet pairing with T sc 1 K [9]. H c2 near the magnetic phase transition point p c 1.3 GPa is especially nontrivial, since there is a competition between enhanced spin fluctuations and Pauli depairing effect.…”

“…Among these uranium compounds, UCoGe has the lowest Curie temperature T C ∼ 2.7 K and the superconducting transition temperature T sc ∼ 0.6 K at ambient pressure [3,[5][6][7][8][9][10]. The ferromagnetism is suppressed by applying pressure and T C seems to approach zero at a critical pressure p c ∼ 1.3 GPa.…”

“…In these compounds, 5f -electrons are responsible both for the magnetism and the superconductivity, in sharp contrast to the previously found ferromagnetic superconductors such as ErRh 4 B 4 and HoMo 6 S 8 where the magnetism and superconductivity have distinct origins 4 . Among these uranium compounds, UCoGe has the lowest Curie temperature T C ∼ 2.7K and the superconducting transition temperature T sc ∼ 0.6K at ambient pressure 3,[5][6][7][8][9][10] . The ferromagnetism is suppressed by applying pressure and T C seems to approach zero at a critical pressure p c ∼ 1.3GPa.…”