Tuning in magnetic modes in Tb(CoxNi1−x)2B

Abstract

Neutron diffraction and thermodynamics techniques were used to probe the evolution of the magnetic properties of Tb(CoxNi1−x)2B2C. A succession of magnetic modes was observed as x is varied: the longitudinal modulated k = (0.55, 0, 0) state at x = 0 is transformed into a collinear k = ( 1 /2, 0, 1 /2) antiferromagnetic state at x = 0.2, 0.4; then into a transverse c-axis modulated k = (0, 0, 1 /3) mode at x = 0.6, and finally into a simple ferromagnetic structure at x = 0.8 and 1. Concomitantly, the low-temperature orthorhombic distortion of the tetragonal unit cell at x = 0 is reduced smoothly such that for x ≥ 0.4 only a tetragonal unit cell is manifested. Though predicted theoretically earlier, this is the first observation of the k = (0, 0, 1 /3) mode in borocarbides; our findings of a succession of magnetic modes upon increasing x also find support from a recently proposed theoretical model. The implication of these findings and their interpretation on the magnetic structure of the RM2B2C series are also discussed.PACS numbers: 71.27.+a,75.25.+z, 75.50.Cc,75.30.Fv Rare-earth 4f moments at regular crystalline sites of intermetallic matrices are subjected to a variety of competing interactions, such as the Ruderman-KittelKasuya-Yosida (RKKY), crystalline electric field, magnetoelastic, and dipolar interactions.1,2 A particular class of such 4f intermetallics is the quaternary isomorphous borocarbides RM 2 B 2 C (R is a rare earth or Y, and M is a transition metal), which have been found to exhibit coexistence between superconductivity and magnetism for a judicious choice of R and M .3-8 Apart from the interesting issue of coexistence (which highlights the importance of electron interactions, with themselves as well as with the 4f moments), the magnetic properties of these materials pose a challenging problem in their own right, especially when R is magnetic and M is nonmagnetic. For the interesting case of M = Co or Ni, Table I indicates that, for fixed R, the Co-based members exhibit collinear and equal-amplitude ferromagnetic (FM)/antiferromagnetic (AFM) structures;9-11 by contrast, the Ni-based members exhibit a variety of modulated structures, few equal-amplitude and commensurate AFM structures, and an absence of FM modes. 12Rhee et al.13 have calculated the generalized susceptibility for borocarbides from band structures obtained through the local-density approximation (LDA): for a fixed pair R and M , three incommensurate peaks were predicted near k 1 = (0.6, 0, 0), k 2 = (0, 0, 0.9) and k 3 = (0, 0, 0.3). Though calculated for the nonmagnetic LuNi 2 B 2 C, the results were expected to be valid for all R, with the precise position and sharpness of each peak depending on R and M . Experimentally, both k 1 and k 2 modes show up in HoNi 2 B 2 C and, moreover, k 1 is evident in, for example, R = Er, Tb, Gd (Ref.12) but, so far, k 3 has not been observed in borocarbides. It is recalled that this model does not account for many modes [e.g. Ref. 13, the authors attributed this to the fact that their theory ...

A mean-field approach to Kondo-attractive-Hubbard model

“…Interestingly, sequences of magnetic modes have been observed in some quaternary borocarbides (QBC) with the general formula RT 2 B 2 C, in which R is a rare earth element and T is a transition metal [10][11][12]36]. In particular, alloying the transition metal, as in, e.g., Tb(Co x Ni 1−x ) 2 B 2 C or Ho(Co x Ni 1−x ) 2 B 2 C, yields sequences of magnetic modes, and may even lead to suppression of superconductivity, as x is varied [37][38][39][40].…”

Evolution of the magnetism of Tb(Co_{x}Ni_{1-x})_{2}B_{2}C

“…The polycrystalline samples used in this work were the same as the ones used previously 5 for collecting neutron diffractograms. The only difference is that the diffractograms and ac susceptibility have been collected on powdered samples, while the magnetization and specific heat had been measured on the same small solid piece.…”

“…This tuning in of magnetic modes by a simple variation of x highlights the importance of electronic band structures (which are dominated by the spacer 3d orbitals): [6][7][8][9][10] indeed, the generalized susceptibility calculations of Rhee et al 11 predict a variety of magnetic modes, one of which was verified only recently. 5 An alternative theoretical analysis was proposed by Bertussi et al, 12 based on an effective microscopic model in which the interplay between local moments ordering and superconductivity is explicitly taken into account. This interplay, as well as the specific layering character of borocarbides 13 have been tested in one spatial dimension (1D), which is amenable to unbiased calculations, and, as it turned out, the key element consists of conduction electrons being mediators of both pairing and magnetic coupling.…”

“…In this way, a phase diagram relating the magnetic coupling to a succession of magnetic modes is established, 12 and this phase diagram is found to be in fair agreement with the experimental observations. 5 The success of these theoretical approaches is impressive, considering that the calculations of Rhee et al do not admit pairing interactions, while those for the effective microscopic model have so far been carried out only in 1D. In both cases, however, theoretical results are only available for the ground state (T = 0), thus not covering the rich features of thermodynamic properties which, for R(T x M 1−x ) 2 B 2 C (T, M = 3d atoms), have been extensively studied (see, e.g., Refs.…”

“…1,2, and references therein). Evidently, an investigation of the above mentioned influence of the spacer on both the thermodynamic properties and magnetic structures would be a welcome contribution for the understanding of the mechanisms behind 5). µND (µM) is the zero-field (90 kOe) value obtained from neutron diffraction (magnetization isotherm).…”