Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

Above 40 T, URu 2Si 2 appears to be a polarized heavy-fermion metal with a large density of carriers, whose
effective mass rapidly decreases with increasing magnetic polarization.
Fig 6 : Magnetoresistance of CeRh2Si2 in pulsed
magnetic fields up to 50 T at different
temperatures.
18
CeRh 2Si 2 is a heavy-fermion compound
characterized by two successive transitions, at
T N,1 = 36 K and T N,2 = 26 K, towards
antiferromagnetic order [15]. Application of
hydrostatic pressure induces a quantum phase
transition to a paramagnetic Fermi liquid regime,
in the vicinity of which an unconventional
superconducting pocket develops [16]. Recently,
we have studied the effects of high magnetic
fields on this system (at ambient pressure) at the
LNCMI-Toulouse [17]. Transport (see Fig. 6)
and torque measurements have been performed
in pulsed magnetic fields up to 60 Tesla, and
thermal expansion measurements were
performed in continuous fields up to 13 Tesla.
Fig. 6 shows measurements of the
magnetoresistance up to 50 Tesla and
temperatures between 1.5 K and 80 K. At low
temperatures, a step-like anomaly is observed at
a critical field H * of around 26 T. This
corresponds to a field-induced first-order
transition to a magnetically polarized regime,
where all the microscopic magnetic moments (on the Ce 3+ sites) are aligned ferromagnetically along the field
direction. Additional torque measurements (not shown here) permitted to see in fact two transitions to the
polarized state, at H * 1 ≈25.6 and H * 2 ≈26.0 Tesla (see also [18]). The strong change of resistance between the
antiferromagnetic and the polarized phases may indicate an important modification of the electronic properties,
which may be related to a reconstruction of the Fermi surface of the system. At higher temperatures, a magnetic
anomaly characteristic of the field-induced transition between the antiferromagnetic and polarized states can be
followed from our measurements: its characteristic field decreases with increasing temperature, and vanishes at
TN,1 = 36 K. A change from a first order to a second order transition is observed at about 20 Kelvin. At
temperatures higher than T N,1, a broad anomaly (at H pol) is associated to the crossover between the low-field nonpolarized
paramagnetic regime and the high-field polarized paramagnetic regime.
Fig 6 : Magnetic field-temperature phase diagram of
CeRh2Si2 obtained by transport, torque and thermal
expansion.
This study permitted to determine carefully
the magnetic field-temperature phase
diagram of CeRh 2Si 2, when a magnetic field
is applied along the easy-axis c. The phase
diagram shown in Fig. 6 was obtained from
the combination of our transport, torque, and
thermal expansion measurements. The
transition temperatures T N,1 and T N,2 are
found to decrease with increasing magnetic
fields, before merging at about 24 T and 20 K.
This point may correspond to a tetra-critical
point, where two first-order transition lines
reported below 20 K (and above 24 T) also
end. Changes of the physical properties close
to this point are not trivial since, from our
resistance measurements, a reminiscence of
the first order transition can be followed up
to 25 Tesla, while the second order line can
also be defined down to 15 Tesla. At higher
temperatures, the characteristic field Hpol of
the crossover between the paramagnetic nonpolarized
and polarized regimes is found to
vary almost linearly with T, which confirms that its nature is simply related to the Zeeman effect.