Superconductivity and magnetic ordering in the pseudoternary Ho(IrxRh1−x)4B4 system

Volume 76A, number 5,6

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14 April 1980

SUPERCONDUCTIVITY AND MAGNETIC ORDERING IN THE PSEUDOTERNARY Ho(Ir~Rhi_~)4B4SYSTEM 1 H.C.KUandF.ACKER Inst itute for Pure and Applied Physical Sciences2 University of California, San Diego, La Jolla, CA 92093, USA

:

and B.T. MATTHIAS institute for Pure and Applied Physical Sciences 2, University of California, San Diego, La Jolla, CA 92093, USA and Bell Laboratories, Murray Hill, NJ 07974, USA Received 10 January 1980

The boundaries between the paramagnetic, superconducting and magnetically ordered phases in the tetragonal pseudoternary system Ho(IrxRhl_x) 4B4 have been established by means of magnetic susceptibility measurements down to 1.2 K. Antiferromagnetic ordering occurs for x ~ 0.60 with NéeI temperatures higher than the superconducting transition temperatures, showing a new aspect of the coexistence of magnetic order and superconductivity.

The compound HoRh4B4, which crystallizes with the primitive tetragonal CeCo4B4-type structure [1], is a member of the MRh4B4 system with a = 5.29 A, c = 7.38 A and orders magnetically at 6.6 K [2]. Specific heat measurements are consistent with a doublet3 state due crystal field of the Ho~ Jground = 8 Hund’s rule to multiplet. The splitting specific heat anomaly associated with the magnetic ordering exhibits almost ideal mean field behavior [3,4]. In the neutron diffraction study, the magnetic moments were found to be parallel to the c axis, with value 3 atunique 2.8 K,tetragonal which extrapolates to a8.7 TMB of 7.8 MB/Ho at OK [5]. In recent measurements, we observed marked anisotropy in the magnetic properties and obtained cvidence that the magnetic order is not simple ferromagnetism [6]. The (true) zero field susceptibility of HoRh4B4 takes on a finite and constant value below Tm, down to 1.68 K, even in the easy direction. ConPartially supported on a grant from the Swiss National 2

Science Foundation. Research in La Jolla supported by National Science Foundation Grant No. DMR77-08469.

sequently, there is no spontaneous magnetization in this temperature range, in contrast to the behavior of GdRh4B4 and DyRh4B4 [61.Below I K, an upturn was detected in the specific heat of HoRh4B4 which has with nuclear Schottky anomaly [3,4]been butassociated may also be duea to a magnetic transition. However, superconductivity is not observed in HoRh 4B4, although the extrapolation from the superconducting transition temperatures of the Lu, Tm and Er compounds [2] and the study of the pseudoternary (Ho~Eri_~)Rh4B4 system [7] and the (Ho~Lui_~) Rh 4B4 system [3] suggest that this compound should have a Tc around 5.5—6.5 K. The magnetic order below Tm 6.6 K whatever its nature may be does prevent the occurrence of superconductivity. Recently, the compound Holr4B4 was found to become superconducting at 2.1 K. The superconducting phase is metastable and has the same CeCo4B4-type structure as the HoRh4B4 compound [81.More surprisingly, the pseudoternary compound Ho(1r05 Rh05)4 B4 was found in the same study to become superconducting at 6.4 K, a value nearly as high as the magnetic ordering temperature of HoRh4B4. In an attempt to gain more information about the —

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occurrence of superconductivity and its relation to the magnetic order, we investigated the composition dependence of the upper (Tci) and lower (Tc2) superconducting critical temperature as well as the magnetic ordering temperatures Tm of a series of pseudoternary Ho(Ir~ Rhi~)4B4compounds. The samples were synthesized by arc-melting the high purity elements under Ar. Following the measurements of their critical temperatures, they were wrapped an Ta foils, sealed in quartz tubes under Ar for annealing (5 days at 1150°Cfollowed by two weeks at 900°C) and finally powdered. Superconducting and magnetic transition temperatures were determined from ac magnetic susceptibility measurements (20 Hz) down to 1.2 K, except for the Néel temperatures, which were obtained by means of static susceptibility measurements, using a vibrating sample magnetometer. The superconducting critical temperatures T~iand Tc2 for x > x~(x~ 0.075 is the composition at which Tci, Tc2 and Tm coincide) and the magnetic ordering temperature Tm for x 0.50 are plotted versus composition in fig. 1 for the arc-melted samples. The main features of this diagram are the occurrence of a superconducting phase between two magnetically ordered phases of different kinds, and a region where superconductivity and antiferromagnetism appear to coexist. The values of Tci, Tc2 and Tm are slightly increased by annealing For x > 0.80, the concentration of the CeC04B4-type phase decreases sharply in the annealed samples and finally disappears. In the annealed samples at x = 0.9 and x = 1.0, no superconducting or magnetic transition can be detected above 1.2 K. Powdering the samples did not affect the results, there were thus no filaments of superconducting impurity phases in our specimen. There are several features to be noticed in fig. I: (1) Tm shows a nearly linear decrease withx, up to x~,at a rate of dTm/dx —0.22 K/at% Jr. (2) The Tc2 versus x phase boundary for x > xcr is depressed relative to the extrapolation of Tm versus x for x 0.60 (possibly x > 0.50), Tm reappears and increases with x, saturating to a value of 2.8 K (no ~.

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A similar diagram was established for the annealed samples; we found little difference with fig. 1, up to x 0.7, where the X-ray patterns still show a 95% concentration of the CeC04B4-type phase.

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magnetic transition could be detected above 1.6 K for x 0.55, even in a 7 kOe field). Forx >0.80, the concentration of the phase was too low for the magnetic order to be detected. (4) Tci increases with x, reaches a maximum value of 6.4 K, decreases abruptly between x = 0.50 and x = 0.60, and then flattens out. (5) There is a minimum in Tci at x 0.80. The variation of Tci with composition, apart from the noticeable depression on the Rh-rich side, is a cornmon feature of the pseudoternary M(Ir~Rh1 —x )~ B4 systems, as can be seen from the comparison between the present results and those for the Er(lr~Rhi x)4 B4 system in fig. 2 [81. From the Tc variation of nonmagnetic YRh4B4 and LuRh4B4 [2] and from the band calculations for ErITh4B4 and HoRh4B4 [9,10] which show a strong contribution of the 4d states of Rh to the peak in the total density of states at the Fermi energy, it appears that the Rh4 tetrahedra are responsible for the high Tc’S of the MRh4 B4 compounds [111. The lower value of Tc for the MIr4B4 compounds [81 are in all likelihood due to the smaller density of states at EF in the case of Sd electrons. The occurrence of minima in Tc around x 0.3 and x 0.8 for the Er(Ir~ Rh1 —x )4B4 system as well as around x 0.8 for the Ho(Ir~Rhi~)4B4 system, together with the spectacular drop of Tc observed for both systems around x 0.5 indicate that the Rh and Ir atoms are not completely =

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14 April 1980

If ~ = Tm, this depression would be the result of the weakening of the magnetic interaction in the superconducting state. The possibility that superconductivity and magnetic order might coexist in the temperature

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randomly distributed in the 8(g) sites of the space group P42/nmc. Moreover, since there are two formula units per unit cell in this tetragonal structure [1], the assumption that the existence of at least one pure Rh4 tetrahedron for x ~ 0.50 can explain not only the sudden drop of T~around x 0.50 where Ir atoms occupy partially both tetrahedra but also the minima in both systems which are due to the disorder scattering by the randomly distributed Rh and Ir atoms in one tetrahedron instead of two. For Er(Ir~Rhi~)4B4, there is a change around x 0.50 in the slope of c/a versus composition [8], which may be connected with a modification of the electronic structure. However, more experiments are necessary to clarify these arguments. The reason for the variation of Tm (or Tc2) withx in Ho(Ir~Rhi~)4B4 is different from that for the (Ho~Eri_~)Rh4B4 [7] and (Ho~Lui_~)Rh4B4 [3] systems, where the decrease (orions Tc2)orresults 3ionsofbyTm Er~3 by thefrom the dilution ofLu~3ions. the Ho~ In the present system, the Ho nonmagnetic sublattice is kept intact and the gradual substitution of Ir on the Rh sites alters the environment of the Ho+3 ions. Modifications are to be expected in the crystal field as well as in the exchange interaction which determine the magnetic behavior of these ions. =

The depression of Tc2 relative to the linear extrapolation of Tm from x > xn was also observed for the (Ho~Eri_~)Rh 4B4 and (Ho~Lui~)Rh4B4systems.

ordering was observed. Fig. 3 shows the inverse magnetic susceptibility for Ho(1r0 7Rh03)4B4 compound as measured in two directions that are approximately parallel and perpendicular to the cooling axis of the specimen in the arc furnace (i.e., possibly the c-axis of 3ions is also The shown for comparison. is afree the crystallites). inverse susceptibilityThere for the Ho~ marked anisotropy in the susceptibility of the opposite sign to that found in HoRh 4B4 [6]. The magnetic transition temperature is well defined and lies 1.1 K above the superconducting transition temperature, which shows in a transparent way that magnetic order and i.s

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superconductivity coexist below T~.This fact would confirm the antiferromagnetic nature of the order t2, since antiferromagnetism is expected and was found in REMo6S8 and REMo6Se8 systems [121, to coexist

14 April 1980

tion and resistivity measurements are needed in order to find the systematics of these interesting compounds.

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with superconductivity. However, the Ho(Ir~Rhi~)4 B4 system brings forth the first evidence of a Née! ternperature that is higher than the superconducting transition temperature. Since the drop of T~inearx 0.55 was also observed in the nonmagnetic Lu(1r~Rhi~)4B4 system [13], it is not expected to be the result of the magnetic ordering. On the other hand, the cause of the drop of in the Ho(Ir~Rh1 ~)4B4 system may also be responsible for the onset of magnetic order near this composition. In order to further study the interplay of superconductivity and magnetic order we are investigating the Dy(Ir~Rhix)4B4 system, where we found Née! temperatures below T~as well as above in prelimmary measurements. In conclusion, the discovery of superconductivity in the Ho(IrxRhix)4B4 system, together with the occurrence of antiferromagnetism and its coexistence with superconductivity, are significant for the behavior of the rare earth ternary and pseudoternary borides. The superconducting transition temperatures and the magnetic ordering temperatures in Ho(Ir~Rhi_~)4B4 are strongly influenced by crystal field effects and changes in the electronic structure. More experimental data, in particular from specific heat, neutron diffracF2

In our range of measurements (0—15 kOe),M varies linearly

We wish to thank L.D. Woolf, H,B. MacKay, H.F. Braun and 0. Fischer for informative discussions and M.B. Maple for his careful reading of the manuscript. References

[1] J.M.

Vandenberg and B.T. Matthias, Proc. Nail. Acad. Sci. USA 74 (1977) 1336. [2] B.T. Matthias, E. Corenzwit, J.M. Vandenberg and H. Barz, Proc. Nati. Acad. Sci. USA 74 (1977) 1334. 13] MB. Maple, H.C. Hamaker, D.C. Johnston, H.B. MacKay and L.D. Woolf, J. Less Common Metals 62 (1978) 251. [41 H.R. Ott, L.D. Woolf, M.B. Maple and D.C. Johnston, J. Low Temp. Phys., to be published. [51 G.H. Lander, 5K. Sinha and I- .Y. Fradin, J. App!. Phys. 50 (1979) 1990. [61 F. Acker and H.C. Ku, to be published. 171 D.C. Johnston, W.A. Fertig, M.B. Maple and B.T. Matthias, Solid State Coinmun. 26 (1978) 144. [8] H.C. Ku, B.T. Matthias and H. Barz, Solid State Commun. 32 (1979) 937.

[91 T. Jarlborg, A.J. Freeman and T.J. Watson-Yang, Phys. Rev. Lett. 39 (1977) 1032. [10] A.J. Freeman and T. Jarlborg, J. App!. Phys. 50(1979) 1876.

[111J.M. Vandenberg and B.T.

Matthias, Science 198 (1977) 194. [12] For a review, see: M.B. Maple, J. Phys. C6 (1979) 1374; M. Ishikawa, ~.Fischer and J. Muller, J. Phys. C6 (1979) 1379. [13] H.C. Ku, unpublished results.

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