Peculiar magnetism of UAu2Si2

Abstract

Single-crystalline UAu2Si2 has been grown by a floating-zone melting method, and its magnetic, thermal and transport properties have been investigated through measurements of magnetization, specific heat and electrical resistivity to reveal its peculiar magnetism. It is shown that UAu2Si2 undergoes a second-order phase transition at Tm = 19 K, which had been believed to be ferromagnetic ordering in the literature, from a paramagnetic phase to an uncompensated antiferromagnetic phase with spontaneous magnetization along the tetragonal c-axis (the easy magnetization direction). The magnetic entropy analysis points to the itinerant character of 5f electrons in the magnetic ordered state of UAu2Si2 with large enhancement of the electronic specific heat coefficient of γ ∼ 150 mJ/K 2 mol at 2 K. It also reveals the relatively isotropic crystalline electric field effect of this compound, with contrast to the other relative isostructural compounds. The observed magnetization curves strongly suggest that there is a parasitic ferromagnetic component developing below ∼ 50 K in high coercivity with the easy axis along the tetragonal c-axis. The results are discussed in the context of evolution of magnetism within the entire family of isostructural UT2Si2 compounds. arXiv:1608.06035v2 [cond-mat.str-el]

Magnetoelastic phenomena in antiferromagnetic uranium intermetallics: The UAu2Si2 case

“…15 Quite recently we have successfully prepared a UAu 2 Si 2 single crystal and commenced systematic investigations of its intrinsic properties. The results obtained by measurements of magnetization, specific heat and electrical resistivity 27 followed by neutron diffraction 28 and 29 Si-NMR 29 experiments corroborate the conclusion about the ground state of UAu 2 Si 2 as an uncompensated antiferromagnet, contrary to previous reports on polycrystals. 17,[30][31][32][33] UAu 2 Si 2 undergoes a FM-like transition at 50 K (referred to as T 2 ) followed by another magnetic phase transition (T m ) around 20 K. All our previous measurements show a large magnetocrystalline anisotropy with the direction of magnetic moments along the c axis.…”

“…Unlike the low-field transition, the µ 0 H 2 anomaly can be traced to temperatures above T m and is clearly distinguishable even at 40 K. We have discussed this behavior in Ref. 27 , especially in the parts connected with Figures 11,[18][19][20] Two components apparently coexist at temperatures below T m : a) a very weak ferromagnetic one which emerges around 50 K with cooling; its coercive field µ 0 H 2 increases with decreasing temperature and exceeds 4 T at 2 K, b) an uncompensated AFM one (UAFM) with a considerably larger spontaneous magnetization but a much smaller coercive field µ 0 H 1 . The magnetization isotherms in the vicinity of T m show another field-induced step-like transition at higher fields.…”

“…The UAu 2 Si 2 single crystal used in this study was prepared using the floating-zone method in an optical fur-nace (Crystal Systems Co.) in a similar way as in our previous work. 27 Nevertheless, to obtain a higher-quality and larger single crystals we have optimized the entire growth process. The initial polycrystalline rod with diameter of 6 mm and length of ∼ 100 mm was prepared from the starting elements of U (initially 99.9% and consequently purified by the Solid State Electrotransport Method under ultra high vacuum 34 ), Au (99.99%) and Si (99.999%).…”

“…The individual data are vertically shifted to set them equal to 0 at T m = 20.5 K. Below this magnetic phase transition we observe a significant change in the temperature dependence of the thermal expansion in line with our previous reports. 27,28 There obviously is a large anisotropy of the thermal expansion over the entire temperature range. The linear thermal expansion along the a axis shows a continuous decrease from 100 K down to the ordering temperature T m = 20.5 K, where it bends down rapidly pointing at a large contraction of the unit cell along the a axis (−1.5 × 10 −4 between T m and 2 K).…”

“…The results of the present paper, are complementary to our previous neutron-diffraction work, confirming the uncompenasted antiferromagnetic (UAFM) ground state of the compound, 28 and bring evidence for the intrinsic nature of the FM component below ∼ 50 K previously reported as parasitic in our first single-crystal study. 27 Magnetization measurements in pulsed high magnetic fields helped us to complete the phase diagram of UAu 2 Si 2 and revealed signs of the presence of a tricritical point (TCP).…”