Influence of magnetic order on superconducting properties of Ho_xY_1-xNi₂¹¹B₂C

M. Loewenhaupt, A. Kreyssig, C. Sierks (TU Dresden), K.-H. Müller, J. Freudenberger (IFW Dresden), C. Ritter, H. Schober (ILL Grenoble).

The discovery of superconductivity in quaternary intermetallic RNi₂B₂C compounds (R = Sc, Y, Th or lanthanides) has attracted growing interest since the superconductivity is observed not only for the RNi₂B₂C compounds with nonmagnetic R, but also for the ones with magnetic rare-earths Tm, Er, Ho, and Dy which order magnetically at low temperatures. This makes these systems suitable for a study of the interplay between superconductivity and magnetism. The systems where the Néel-temperature T_N and the superconducting transition-temperature T_c are of the same order of magnitude seem to be the most interesting since they show effects which are not yet completely understood.

A system with this property is HoNi₂B₂C with T_N ~ T_c ~ 8 K. Furthermore, it exhibits a reentrant behaviour, this means that the superconducting properties do not strengthen with decreasing temperature but weaken in a narrow temperature range. We investigated the origin of the reentrant behaviour by comparing the temperature dependence of the magnetic ordering, of the low energy magnetic excitations, and of the superconducting properties. To study in a systematic way the correlation between the different properties, we prepared a series of samples where we diluted magnetic Ho with nonmagnetic Y.

HoNi₂B₂C crystallises in the tetragonal ThCr₂Si ₂ structure with the space group I4/mmm. The structure can be visualised as a framework of alternating R-C and
Ni₂ B ₂ layers as shown in Fig. 1a. Three types of magnetic order are observed in HoNi₂B₂C and the diluted samples: a commensurate antiferromagnetic structure with wave vector ₁ = (0 0 1), an incommensurate structure with ₂ = (0 0 0.915), and an incommensurate structure with ₃ = (0.585 0 0). In certain temperature regions two or three of these structures seem to coexist. The commensurate antiferromagnetic c-axis modulated structure with ₁ consists of Ho-moments ferromagnetically ordered in the (a, b)-planes pointing along (1 1 0) and stacked antiferromagnetically in the c-direction. In Fig. 1a the incommensurate c-axis modulated structure with ₂ is illustrated. It consists of ferromagnetically ordered (a, b)-planes stacked along c-direction with a turning angle of approximately 165°. The incommensurate a-axis modulated structure with ₃ is still under discussion. A proposal for it is shown in Fig. 1b.

To determine the behaviour of these three types of magnetic order, temperature-dependent neutron powder-diffraction experiments were carried out at the multidetector instrument D1B.

The powder samples of Ho_xY_1-xNi₂¹¹B₂C were first cooled down to the lowest temperature of 1.5 K and then heated in narrow temperature steps of 0.15 K up to 10 K. In Fig. 2 the low-angle parts of the diffraction patterns are shown for a temperature of 3 K.

In Fig. 3a-c the temperature dependence of peak intensities corresponding to the three types of magnetic order is presented. Peak intensities connected to the commensurate c-axis modulated structure decrease with decreasing Ho content and are below the detection limit for x = 0.75. The incommensurate a-axis modulated structure with ₃ exists for all investigated samples only in a narrow temperature range. For x = 1 the temperature dependence of the incommensurate c-axis modulated structure with ₂ is similar to that for ₃, but for x < 1 the peak intensities increase monotonically with decreasing temperature down to the lowest measured temperatures.

To characterise the superconducting properties, we determined the upper critical field H_c2 of the type-II-superconductor by measuring the electrical resistivity as a function of magnetic field and temperature in the range between 1.7 K and the critical temperature. The upper critical field H _c2 of the samples increases with decreasing temperature from zero at T_c up to a local maximum (see Fig. 3d). As a direct consequence of the reentrant behaviour H _c2 decreases in a narrow temperature range and increases again monotonically to the lowest measured temperature for all investigated samples.

We observe the peculiar behaviour that although T_c increases with increasing dilution the temperature range where the reentrant behaviour can be observed is shifted to lower temperatures. This correlates for all three samples only with the temperature dependence of the incommensurate a-axis modulated structure which is indicated by the arrows in Fig. 3d.

Inelastic neutron-scattering experiments were carried out at the time-of-flight spectrometer IN6. No Q-dependence of the spectra could be detected for separate groupings of detectors. In the magnetic response function shown in Fig. 4 a sharp peak at 1.8 meV and a very broad one at 0.6 meV can be found for T = 1.6 K. Up to 5 K there is no significant change of the excitation spectrum. For higher temperatures the peak at 1.8 meV vanishes and a strong quasielastic scattering develops (see Fig. 3e). This is also the temperature region where the magnetic Bragg peaks of the commensurate c-axis structure decrease drastically (Fig. 3a). For x = 0.85 we could barely observe the 1.8 meV peak and for x = 0.75 we did not find it at all. It seems that the low-energy excitation at 1.8 meV is correlated to the existence of the commensurate c-axis modulated structure.

In summary we can state the following: In the system Ho_xY_1-xNi₂¹¹B₂C the correlation between the reentrant behaviour and the temperature dependence of the incommensurate magnetic structure along the a-axis strongly suggests that the a-axis structure (₃ ) is responsible for the suppression of superconductivity.

Both magnetic structures with c-axis wave vectors (₁ , ₂ ) do not interfere with superconductivity, as their temperature dependencies and the reentrant behaviour are uncorrelated for x = 0.85 and x = 0.75. For the pure Ho-compound, however, ₂ and ₃ coexist in nearly the same temperature region. Similar investigations which have been undertaken by us in the system Ho_xLu_1-xNi₂¹¹B₂C yield qualitatively the same results but with more pronounced effects for the dilution.