UNUSUAL MAGNETIC-BEHAVIOR OF TMIR2 AND YBIR2

Abstract In contrast to the other magnetic Rare Earth (RE) ions for which the cubic Laves REIr 2 compound is ferromagnetic, YbIr 2 and TmIr 2 are antiferromagnetic below 0.40 and 0.05 K, respectively. Both are trivalent above 4 K, with a possible reduced moment for TmIr 2 below 0.3 K. In addition, CeIr 2 and LuIr 2 are superconductive below 0.20 and 0.23 K, respectively.

The predominant behavior of the magnetic Rare Earth (RE)Ir, (Cl& cubic Laves phase) intermetallic compounds is local moment paramagnetism at high temperature and ferromagnetic order at low temperature, as reported by Bozorth et al. [l]. Curie points rc were found to vary roughly as (g -1)2J(J + l), the De Gennes factor [2], and ordered moment values to fall between 2S and gJ. Here S is the spin and J is the total quantum number, and g is the LandC g factor. The data for Eu-, Tm-and YbIr, were questionable as pointed out by the authors of ref. [l] because of unusual magnetic behavior for EuIr, and sample preparation difficulties for the other two compounds. The behavior of EuIr, was resolved when Matthias, Fisk and Smith [3] reported superconductivity at 0.2 K, proving that here Eu is in its purely trivalent, nonmagnetic state. To reexamine the properties of the often mixed-valent elements Yb and Tm in the REIr, system, we have prepared single crystals by the flux growth technique using a Cu solvent. The crystals were all cubic Laves phase; lattice parameters of 0.74638(l) nm for YbIr, and 0.74736(2) nm for TmIr, were measured. Susceptibility measurements were made in a Faraday magnetometer over the range 230-4 K. A Curie-Weiss fit to the susceptibility x = (p&/8)/( T -0) yields perr = 7.57~~ and 0 = -4 K for TmIr, and perr = 4.49~~ and 0 = -4 K for YbIr,, in good agreement with the trivalent free-ion values for the effective moments. The ac susceptibilities were measured from 4 to 0.012 K with arbitrary sensitivity, and therefore no low temperature effective moment values could be obtained. A constant was subtracted from the ac susceptibility to correct for a temperature-independent paramagnetic background. The constant xac (less than about 30% of the peak susceptibility) which gave the best straight line fit to l/x was employed in the analysis. in an applied magnetic field at the rate of -1.5 K/T. The situation for TmIr, is more complex. Below 4 K, a linear fit to l/xac extrapolates to 0 = -1.4 K, compared to 0 = -4 K from the high temperature data. In addition, at 0.3 K, xac has a discontinuity in the slope (steeper at lower temperature) followed by a cusp at 0.05 K, seen in fig. 2. The cusp moves to lower temperatures in a field at the rate of -2.5 K/T. We have also grown single-crystal CeIr, and LuIr,; these compounds are superconductive below 0.20 and 0.23 K with upper critical fields of 0.11 and 0.10 T, respectively.
The cubic Laves Cl5 structure with REIr, appears EuIr, is superconducting at 0.2 K and thus must be in its trivalent J = 0 configuration.
In contrast, Et&h, is not trivalent; rather it is mixed valent as determined from "'Eu Mossbauer effect measurements [5] and is strongly paramagnetic at low temperatures, indicating a probable divalent state. Generally then, the nonmagnetic RE ions form superconductors in the REIr, series; the magnetic RE ions order magnetically.
Ferromagnetic order is the rule except for Yb-and TmIr, which are antiferromagnetic with NeCl temperatures much lower than the 0 values; this may be due to crystal field effects. TmIr, has an additional susceptibility feature below 0.3 K which may be interpreted as a moment reduction. This may be due to crystal-field effects, the onset of mixed valency or quadrupolar ordering as seen in TmZn slightly above a magnetic ordering temperature [6]. The very low temperatures at which these effects occur make further investigation most difficult; they also indicate that extremely small energies are responsible for this unusual behavior.
As a further note on the unusual properties of these materials, we comment on arc melted Tm,,,Ir,,s. This composition forms a eutectic of Ir and TmIr, as determined by X-ray diffraction.
Susceptibility measurements on as-cast material yield prff = 7.44~~ and 0 = -2.6 K, consistent with a full, trivalent moment on the Tm ion. Superconductivity is observed by ac susceptibility techniques at 1.5-1.8 K. Annealing this sample (16 days, 1OOO'C) destroys superconductivity above 0.1 K, the transition temperature of pure Ir. We speculate that one of two phenomena may be occurring: enhanced superconductivity [7] in Ir due to lattice mismatch with TmIr, giving rise to a lattice expansion and softening in Ir, which has been shown to lack credibility [El; or the first case of superconductivity in a binary compound of undetermined crystal structure containing a rare earth element carrying its full local moment at low temperatures. At present, more experimental work needs to be done to confirm either of these hypotheses.