Our measurements probe the magnetic anisotropy in UTe2 with fine control over the magnetic field orientation in the sample. While measuring UTe2 in short (<100 ms) 60 tesla pulses, we discovered a large change in the magnetic susceptibility transverse to the external magnetic field. The large change in the transverse magnetic susceptibility is observed for magnetic field orientations near its superconducting phases, suggesting a clear connection to high-field superconductivity. Explanations of unconventional superconductivity often rely on magnetic fluctuations as the mediating source, and evidence for magnetic fluctuations exists in related uranium-based compounds. In these related compounds, reentrant superconductivity exists when magnetic field is applied perpendicular to the direction of their ferromagnetically ordered moments. UTe2 is unique in that it does not exhibit long-range magnetic order; there was no prior evidence for magnetic order, or a reason that moments would be fluctuating. Previous measurements report only a first-order metamagnetic transition. Our high field studies probing the magnetic susceptibility transverse to the magnetic field suggest that large transverse magnetic fluctuations are likely responsible for its remarkable high-field superconducting phase.
Prior measurements suggested that several critical endpoints exist at high magnetic fields in UTe2. Our measurement technique is a second-derivative of the free energy (it’s the angular derivative of magnetic torque), which is ideal for identifying 1st and 2nd order phase transitions. We already have evidence for a couple of the critical endpoints that exist in the vicinity of the magnetic fluctuations, but we want to confirm their presence and fully map out the phase diagram using our measurements of the magnetotropic susceptibility at high magnetic fields. We will also explore the evolution (both in field-orientation and temperature) of observed second-order phase transitions to determine whether their influence stems from a quantum critical point. To extend our measurements on UTe2, we were recently awarded additional magnet time for experiments up to 65 tesla at the National High Magnetic Field Laboratory in the US.
We completed a full angle dependence of the magnetotropic susceptibility in RuCl3, which requires rotation of the crystal in 30 degree increments between each measurement. Our measurements on RuCl3 confirm a TN = 14 K, typical for the monoclinic structure at low temperatures. We mapped out the boundaries of the AFM phase, as well as the boundary of ZZ2 phase for field applied near the a-axis. Our results are consistent with other measurements showing these phases, however, they are much more comprehensive. We show that the Neel temperature is dependent upon in-plane magnetic field angle. We also confirm that there is only one transition (or two for field near the a-axis and symmetric directions due to the ZZ2 phase) when magnetic field is used to suppress the AFM phase for nearly all field orientations. This is not in accordance with longitudinal thermal conductivity measurements that displayed oscillatory behavior under similar conditions. Upon rotating a fixed magnetic field of 14 tesla, oscillatory behavior is observed in our measurements upon crossing the AFM phase boundary, but only when detecting ac-plane anisotropy. We are still working to understand the origin of this behavior, but we believe that the oscillatory behavior may be related to rotation of the ZZ2 structure under a magnetic field.
