Magnetoresistance and Phase Diagram of Thin-Film UNi₂Al₃

Abstract

We study the dc resistivity of UNi 2 Al 3 thin films as a function of temperature and magnetic field. We focus on the temperature range around the antiferromagnetic transition (T N ≈ 4 K in zero applied field). From a clear signature of T N in the dc resistance along the crystallographic a-direction, we extract the shape of the magnetic phase diagram. Here we find quantitative differences in comparison to previous studies on bulk crystals.

Terahertz conductivity of the heavy-fermion compound UNi2Al3

“…The absence of any field dependence is a further indication that the observed feature is not connected to the antiferromagnetically ordered phase. In the dc resistivity measurements on the same sample, we observed a magnetoresistace of only 4% for an applied field of 7 T. 30 From the accuracy of our transmission measurements, we cannot exclude that the THz conductivity in the measured frequency range changes by a similar, small amount. Thus also the field-dependent behavior of the THz conductivity could match that of the dc conductivity.…”

“…The sample shows a superconducting transition at T c = 0.46 K and an antiferromagnetically ordered phase below T N = 4.2 K, as determined from the dc resistivity. 30 The residual resistivity ratio (RRR) is 5.5 along the a axis. The substrate YAlO 3 (112) is also anisotropic, 35 and the main optical axes of the YAlO 3 substrate and the UNi 2 Al 3 film, which are equal to the crystallographic axes, are tilted with respect to each other by 45 • .…”

“…3. According to the phase diagram determined on this particular sample, 30 we did not induce the transition to the paramagnetic phase with the highest field we applied in the optical experiments. But increasing the field up to 7 T at a temperature of 2 K, we considerably approached the phase boundary.…”

“…Following the phase diagrams of UNi 2 Al 3 determined for single crystals by Süllow et al 28 and Tateiwa et al, 29 at 2 K the antiferromagnetically ordered state can be suppressed with a magnetic field of about 6 T aligned along the a axis of UNi 2 Al 3 , whereas our own magnetoresistance studies on UNi 2 Al 3 thin films (including the present film) revealed a somewhat higher critical field. 30 For UPd 2 Al 3 much higher fields of the order of 20 T are necessary to reach the phase boundary. 31 One further advantage of UNi 2 Al 3 for our optical experiments is that the a and c axes lie within the plane of the thin-film sample 24,32 and can be probed with incident light of different linear polarization; this allows us to conveniently study whether the anisotropy that is observed in the dc properties 24 of UNi 2 Al 3 is also present in the optical conductivity at frequencies of a few cm −1 .…”

Observing the anisotropic optical response of the heavy‐fermion compound UNi2Al3

“…This analysis requires knowledge of the optical properties of the substrate material; we determined these from measurements on an empty YAlO 3 (112) substrate at the same frequencies and temperatures as the UNi 2 Al 3 -film. In the present case, to obtain the optical conductivity of the film quantitatively, we have to modify the conventional analysis procedure due to substrate birefringence [18]. From the transmission and phase data, the anisotropy of the optical response is evident.…”