Tricritical point from high-field magnetoelastic and metamagnetic effects in UN

Abstract

Uranium nitride (UN) is one of the most studied actinide materials as it is a promising fuel for the next generation of nuclear reactors. Despite large experimental and theoretical efforts, some of the fundamental questions such as degree of 5 f–electron localization/delocalization and its relationship to magneto-vibrational properties are not resolved yet. Here we show that the magnetostriction of UN measured in pulsed magnetic fields up to 65 T and below the Néel temperature is large and exhibits complex behavior with two transitions. While the high field anomaly is a field-induced metamagnetic-like transition and affects both magnetisation and magnetostriction, the low field anomaly does not contribute to the magnetic susceptibility. Our data suggest a change in the nature of the metamagnetic transition from first to second order-like at a tricritical point at T tri ∼ 24 K and H tri ∼ 52 T. The induced magnetic moment at 60 T might suggest that only one subset of magnetic moments has aligned along the field direction. Using the results obtained here we have constructed a magnetic phase diagram of UN. These studies demonstrate that dilatometry in high fields is an effective method to investigate the magneto-structural coupling in actinide materials.

Synchrotron x-ray scattering of magnetic and electronic structure of UN and U2N3 epitaxial films

“…Our results give a lower value, similar to that derived from a polycrystalline sample as measured by Marples [18]. This is particularly interesting, as the study by Shrestha et al [21] shows that this expansion may be partially suppressed by the application of a modest magnetic…”

“…Relative variation of the lattice parameters in UN below T N as measured in our work and previously reported in Refs [18][19][20][21]…”

“…Apart from Marples [18], all studies were performed on single crystals, although Refs. [20,21] used strain-gauge techniques, not x rays. What is surprising about this figure is that the expansion of the lattice appears to depend on the sample form, and the magnitude is thus not a true bulk property, as it varies by almost a factor of three between different samples.…”

“…field (<30 T), although there seems no obvious explanation for this in the AF state. Magnetic fields of ∼60 T are required [8,21] to disrupt the AF order of UN.…”

Fiber Bragg Grating Dilatometry in Extreme Magnetic Field and Cryogenic Conditions

“…Sensing of magnetostriction with single mode SiO 2 FBGs in pulsed magnetic fields is successfully utilized at the National High Magnetic Field Laboratory (NHMFL) with a resolution as good as a few parts per hundred million ( ΔL/L ≈ 10 −8 ) in the best cases. This capability allows for the study of a variety of insulating and metallic condensed matter systems including geometrically frustrated magnets, quantum magnets, multiferroics, and uranium- and cerium-based antiferromagnets [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]. Figure 1 shows an example of magnetoelastic effects in pulsed magnetic fields to 60 T at cryogenic temperatures on a sample of uranium dioxide (UO 2 ), which is the most commonly used nuclear fuel.…”