ZERO-FIELD MUON SPIN RELAXATION STUDIES OF LOW-TEMPERATURE MAGNETISM IN YBXY1-XBIPT (X 1.0 AND

Abstract Zero-field muon spin relaxation studies are reported for crystalline Yb x Y 1− x BiPt for x = 1.0 and 0.5. YbBiPt exhibits an extremely large C p / T ratio y ≅ 8J/(K 2 mol Yb) below T ≅ 0.2 K and shows evidence for some type of magnetic order below 0.4 K from χ AC . The μSR data are consistent with a highly frustrated spin system segmented into two types of magnetic domains, one of which freezes into a disordered state below T f ≅ 0.4−0.5 K at a temperature relatively independent of the Yb concentration. The frozen spin moment of ∼0.1μ B is highly reduced from the high temperature free-ion value, and is independent of the Yb concentration.

Zero-field muon spin relaxation studies are reported for crystalline YbxYt_~BiPt for x = 1.0 and 0.5. YbBiPt exhibits an extremely large Cp/T ratio 3' = 8J/( K2 mol Yb) below T-~ 0.2 K and shows evidence for some type of magnetic order below 0.4 K from XAc. The ~SR data are consistent with a highly frustrated spin system segmented into two types of magnetic domains, one of which freezes into a disordered state below Tf--~0.4-0.5 K at a temperature relatively independent of the Yb concentration. The frozen spin moment of ~0.1p. B is highly reduced from the high temperature free-ion value, and is independent of the Yb concentration.
The rare-earth bismuth platinum series of compounds (RBiPt) exhibits a variety of magnetic and transport phenomena, ranging from small-gap semiconducting behaviour in NdBiPt to metallic heavy-electron behaviour in YbBiPt [1]. The latter material exhibits an electronic specific heat Cp which is linear in temperature below 0.2K with a slope 7 corresponding to 8 J/(K 2 mol Yb), indicating the presence of extremely massive quasiparticles [2]. An abrupt change of OXAc/OT in the AC susceptibilty at T o = 0.4 K in single crystals is characteristic of some sort of magnetic order below this temperature. The specific heat also peaks near T o = 0.4 K. Muon spin relaxation (IxSR) studies have been reported previously for crushed powders of YbBiPt [3]. The specific heat in powders is qualitatively the same as in single crystals but with a 7-value reduced by about 50% and a maximum at T=0.5-0.6K.
The IxSR experiments in crushed powders could be interpreted [3] in terms of two types of magnetic domains: one type exhibiting disordered static magnetism below about 0.5 K, with a Yb moment much less than the free-ion value, and a second type which Correspondence to: A. Amato, Institut f/Jr Mittelenergiephysik der ETH-Z/irich, CH-5232 Villigen PSI, Switzerland.
remains paramagnetic down to the lowest temperature measured (0.06 K). Rapid muon spin-lattice relaxation rates were also observed, characteristic of anomalously long Yb-spin correlation times. These observations are consistent with a highly frustrated Yb spin system which partially freezes below T= 0.5 K (similar to but not identical with classical spin glasses) and raise the possibility that a high density of low-lying magnetic excitations contribute significantly to the large observed 3' value. It was conjectured [3] that strain introduced by powdering could create the two types of magnetic domains, only one of which gives rise to the large value of 7.
To futher understand these issues, we have performed additional IXSR experiments on crystalline materials. Initial zero-field results from these new experiments in YbxYl_xBiPt for x = 1.0 and 0.5 are reported here.
The ix + SR experiments were carried out at the I~SR facility of the Paul Scherrer Insitute, Villigen, Switzerland. The samples were prepared at Los Alamos by the flux-growth technique and several relatively large (~<30 mm 3) pieces of crystalline material were glued to the cryostat cold finger. The temperature was regulated to better than 0.05 K. The measured zero-field relaxation functions GzF(/) in the crystalline materials were found to be qualitatively similar to those seen 0921-4526/93/$06.00 (~) 1993 -Elsevier Science Publishers B.V. All rights reserved previously in the powdered samples. As before, good fits were obtained for a sum of fast and slow components of the form specific heat and AC susceptibility measurements. Similar behaviour was seen in crushed powders of YbBiPt. Note that the freezing temperature Tf is only slightly lower in the 50% Y sample. The extrapolated zero-temperature values for ~f are 6.7-+ 0.5 ixs ' for x = 1.0 and 5.2 + 0.5 Ixs ' for x = 0.5, corresponding to rms local fields z~H e = ~f/y, of about 77 and 60 Oe, respectively. Here y, is the muon gyromagnetic ratio. The measurement in crushed powders of YbBiPt gave ~f = 5 Ixs 1 at zero temperature. The reduction in ~1 between x = 1.0 and x = 0.5 in the crystalline materials is consistent with that expected [4] for concentrated but uniformly diluted spin systems with no change in moment between x = 1.0 and x = 0.5 (i.e. crf~VY).
Assuming dipolar Yb-muon coupling, these line widths correspond to a static Yb moment -0.1/z B, about 3% of that deduced [2] from the susceptibility below 10 K. The temperature dependences of the fast (A f) and slow (As) component amplitudes are different for the x = 1.0 and x = 0.5 crystalline materials. As the temperature is lowered below 1.1 K, fig. l(c) shows that Af for x = 1.0 grows in magnitude, reaching Af = 2/3 at the lowest temperatures (where Af+ A s=-1). Qualitatively similar temperature dependent amplitudes were also observed for the YbBiPt crushed powders, where Af = As was seen at the lowest temperatures and Af,~ A~ at 1.1 K. For the x = 0.5 crystalline material, however, Af = A~ = 1/2 is observed for all temperatures between 0.08 and 1.1 K. Transverse field measurements indicate that only a single/x* stopping site is probable. Thus the fast and slow components defined by GzF(t ) most likely correspond to different magnetic domains. Therefore appreciable dilution of the Yb spins may produce isolated magnetic domains (of different character) which do not significantly change their size with temperature.
Relatively large zero-field spin lattice relaxation rates In conclusion, p, SR experiments on the crystalline YbBiPt give stong evidence for a highly frustrated spin system with anomalously long correlation times compared to the observed freezing temperatures. The frozen spin moment is significantly reduced from the free-ion value, as seen in other heavy electron systems. Dilution of Yb by 50% Y produces the expected reduction in the zero-temperature frozen spin linewidth gf(0) but with little reduction in the spin-freezing temperature Tf. The existence of two different magnetic domains appears in both crystalline and crushed powder samples.