Research Group for Actinide Materials ScienceGroup Leader : Zachary FiskMembers : Yoshinori Haga, Etsuji Yamamoto, Naoyuki Tateiwa, Pospíšil Jiří, Akio Nakamura

5f electrons in actinide compounds play essential roles invarious phase transitions in actinide compounds. The electronicstates can be modified by the chemical or physical environment
around these electrons. It is therefore interesting to investigate
new materials under various conditions such as pressure,magnetic field or low temperature to find new phenomena. The
purification of a known compound also provides new insightparticularly for phenomena occurring at low temperatures,
where impurity disorder can deeply disturb and hence hide theintrinsic behavior. Here we report recent results on a new
uranium compound and anomalous ferromagnetic criticalbehavior in the uranium compounds.
New uranium compounds grown from metal flux
A series of actinide compounds with the tetragonal
HoCoGa5-type structure have been extensively studied becauseof the variety of physical phenomena, including unconventional
superconductivity and magnetic/multipole orderings. Amongthem URhGa5 is known as a nonmagnetic metal, in contrast tothe unconventional superconductor PuRhGa5 and NpRhGa5
showing multiple magnetic orderings. We have successfullygrown a new uranium compound URhIn5, which is isostructuraland isoelectronic to URhGa5 but has a significantly larger unit
cell volume [1]. Physical properties measured on a bulk single
crystal, as well as microscopic investigation using 115In-NMR[2], demonstrated an antiferromagnetic ground state. This result
directly shows that a modification of 5f states from itinerant tomagnetic characteristics takes place in URhIn5. It is naturally
expected that the application of pressure would drive URhIn5
back to a nonmagnetic state as in URhGa5. However, evenunder the hydrostatic pressure of 4 GPa, the antiferromagnetism
is stabilized as demonstrated by the increase of the transition
temperature under pressure.

Fig.1 A single crystal of URhIn5 grown from the indium flux, and its
crystal structure determined from the single-crystal X-ray
diffraction.

Fig.2 Examples of experimental data used to determine criticalexponents. From the magnetization as a function of appliedmagnetic field, one of the critical exponents  has been
determined.

Unconventional critical scaling in uranium ferromagnet
Ferromagnetism is observed in several uranium compounds.
Among them UGe2, URhGe and UCoGe are known as
ferromagnets showing superconductivity at lower temperatures.
Superconductivity carried by the ferromagnetic electrons is onlyobserved in uranium compounds. This fact strongly suggeststhat ferromagnetism in uranium compounds has a peculiarfeature which could induce superconductivity. We haveinvestigated the ferromagnetic ordering in these compounds bycharacterizing the critical phenomena around the ferromagnetictransition temperature. Note that the critical phenomena arethermodynamic properties characterizing the global features ofphase transition, and therefore are insensitive to the details ofmicroscopic difference of the material in question. For example,
the critical exponents which characterize the critical phenomena
are exactly the same for the liquid-gas transition and uniaxial
ferromagnetism. For ferromagnetic transition in general, theory
successfully predicts critical exponents, in agreement withexperiments. For UGe2 and URhGe, however, we found that thecritical exponents determined experimentally differ significantly
from the theoretical predictions [3]. We looked for the reasonsof the difference by considering the characteristic feature for
uranium compounds, including the itinerant 5f states, peculiarityof the crystal structure in these compounds, possible long-range
interaction between magnetic moments, classical dipole-dipoleinteractions, etc. Although the satisfactory explanation has notbeen made, the present work would stimulates the further
theoretical investigation on ferromagnetism and its coexistencewith superconductivity.
References[1] Y. Matsumoto et al., Phys. Rev. B 88, 045120 (2013).
[2] H. Sakai et al., Phys. Rev. B 88, 045123 (2013).
[3] N. Tateiwa et al., Phys. Rev. B 89, 064420 (2014).