Interdependence of Magnetism and Superconductivity in the BorocarbideTmNi2B2C

Abstract

We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron diffraction. The ordering vector is QA = (0.48, 0, 0) and the phase appears above a critical in-plane magnetic field of 0.9 T. The field was applied in order to test the assumption that the zero-field magnetic structure at QF = (0.094, 0.094, 0) would change into a c-axis ferromagnet if superconductivity were destroyed. We present theoretical calculations which show that two effects are important: A suppression of the ferromagnetic component of the RKKY exchange interaction in the superconducting phase, and a reduction of the superconducting condensation energy due to the periodic modulation of the moments at the wave vector QA.PACS numbers: 74.70. Dd, 75.25.+z, 74.20.Fg The interplay between magnetism and superconductivity is inherently of great interest since the two phenomena represent ordered states which are mutually exclusive in most systems. Therefore, the borocarbide intermetallic quaternaries with stoichiometry (RE)Ni 2 B 2 C have attracted great attention since the publication of their discovery in 1994 [1,2], as they exhibit coexistence of magnetism and superconductivity if the rare earth (RE) is either Dy, Ho, Er or Tm. The magnetic moments in these compounds are due to the localized 4f electrons of the rare-earth ions. The 4f and the itinerant electrons are coupled weakly by the exchange interaction, resulting in the indirect Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction between the 4f -moments. Thus the RKKY interaction, which is decisive for the cooperative behavior of the magnetic electrons, depends on the state of the metallic ones.The borocarbides have a tetragonal crystal structure with space group (I4/mmm) [3], and TmNi 2 B 2 C has a superconducting critical temperature T c = 11 K and a Néel temperature T N = 1.5 K [4]. The crystalline electric field aligns the thulium moments along the c axis, and the magnetic structure has a short fundamental ordering vector Q F = (0.094, 0.094, 0) with several higher-order odd harmonics [5,6]. In the magnetic structures detected in any of the other systems, the moments of the rare-earth ions are confined to the basal plane, and they have short wavelength antiferromagnetically ordered states. For example, they are commensurate with a propagation vector Q = (0, 0, 1) for RE = Ho and Dy, or incommensurate with Q ≈ (0.55, 0, 0) for RE = Gd, Tb, Ho and Er [5]. An especially tight coupling between magnetism and superconductivity has been clearly demonstrated in TmNi 2 B 2 C, where a magnetic field applied along the c axis induced concurrent changes of the magnetic structure and the symmetry of the flux line lattice [7]. Similar effects have not yet been observed in any of the other borocarbides.One question that immediately arises, and which we believe is important for the general understanding of the interaction between superconductivity and magnetism, is why the long-period magnetic ordering found in TmNi 2 B 2 C is stable. Bandstructure calculations on the normal state of non-magne...

Magnetic and quadrupolar ordering in TmNi2B2C

“…The neutron diffraction studies of the magnetic ordering were performed at the BER-II reactor at BENSC, Hahn-Meitner Institut, and at Risø's DR3 reactor (see Refs. [3,4] for more details). The magnetoelastic strain field was studied at the BW5 highenergy beam line at HASYLAB using 100 keV photons and Si/Ge gradient crystals as monochromator and analyzer (see Ref.…”

“…[7]. They have irregular shapes with dimensions 2 Â 2 Â 0:2 mm 3 and 4 Â 4 Â 0:8 mm 3 for the neutron and X-ray experiments, respectively. The neutron diffraction experiments were carried out in the a-b scattering plane, the synchrotron experiment in the a-c plane.…”

“…Results obtained in the lowand zero-field Q F phase and for more field values may be found in Refs. [3,4,8]. Fig.…”

“…In TmNi 2 B 2 C, superconductivity is found below T c ¼ 11 K, and neutron diffraction studies have shown that it orders below T N ¼ 1.8 K in zero field with an incommensurable antiferromagnetic (AF) structure having modulation vector Q F ¼ (0.094, 0.094, 0) and Ising-like magnetic moments oriented along the c-axis [2]. Another incommensurable AF structure with modulation vector Q A ¼ (0.483, 0, 0) is observed when a magnetic field larger than approximately 10 kOe is applied along the a-axis [3,4]. This AF phase is also found in other magnetic RNi 2 B 2 C compounds, but in TmNi 2 B 2 C the magnetic field dependence is peculiar because the transition temperature T N increases significantly with field, indicating that additional forces than the conventional crystal field (CF) and magnetic interactions act on the magnetic moments.…”

On The Influence of a Non-Local Electrodynamics in the Irreversible Magnetization of Non-Magnetic Borocarbides

“…The most remarkable feature is the coexistence of superconductivity and antiferromagnetism for R = Tm, Er, Ho and Dy. The high superconducting transition temperatures T c and the broad variation of the ratio T N /T c (where T N is the Neel temperature, ranging from 1.5 K to 10 K) make this family particularly appropriate to explore that coexistence [5][6][7][8][9] .…”