500 research outputs found
Pressure-induced ferromagnetism with strong Ising-type anisotropy in YbCuSi
We report dc magnetic measurements on YbCuSi at pressures above 10
GPa using a miniature ceramic anvil cell. YbCuSi shows a
pressure-induced transition from the non-magnetic to a magnetic phase at 8 GPa.
We find a spontaneous dc magnetization in the pressure-induced phase above 9.4
GPa. The pressure dependence of the ferromagnetic transition temperature T_C
and the spontaneous magnetic moment m_0 at 2.0 K have been determined. The
value of m_0 in the present macroscopic measurement is less than half of that
determined via Mossbauer experiment. The difference may be attributed to
spatial phase separation between the ferromagnetic and paramagnetic phases.
This separation suggests that the pressure-induced phase boundary between the
paramagnetic and ferromagnetic states is of first order. Further, we have
studied the magnetic anisotropy in the pressure-induced ferromagnetic state.
The effect of pressure on the magnetization with magnetic field along the
magnetic easy -axis is much larger than for field along the hard -axis in
the tetragonal structure. The pressure-induced phase has strong Ising-type
uniaxial anisotropy, consistent with the two crystal electric field (CEF)
models proposed for YbCuSi.Comment: 5 pages, 3 figure
Unconventional critical scaling of magnetization in uranium ferromagnetic superconductors UGe and URhGe
We report a dc magnetization study of the critical phenomenon around the
ferromagnetic transition temperature T_C in high-quality single crystals of
uranium ferromagnetic superconductors UGe2 and URhGe. The critical exponents,
beta for the temperature dependence of the magnetization below T_C, gamma for
the magnetic susceptibility, and delta for the magnetic isothermal at T_C have
been determined with a modified Arrott plot, a Kouvel-Fisher plot, and the
scaling analysis. Magnetization in the ferromagnetic state has strong uniaxial
magnetic anisotropy in the two compounds. However, the universality class of
the critical phenomena do not belong to the three dimensional (3D) Ising
system. Although the values of beta in UGe2 and URhGe are close to those in the
3D magnets, the values of gamma are close to unity, that expected from the mean
field theory. Similar critical exponents have been reported previously for the
3D Ising ferromagnet UIr where superconductivity appears under high pressure.
The critical behavior may be limited to a very narrow Ginzburg critical region
of 1 mK because of the strong itinerant character of the 5f electrons in the
ferromagnetic superconductor UCoGe where the mean field behavior of the
magnetization has been reported. The unconventional critical scaling of
magnetization in UGe2, URhGe and UIr cannot be explained via previous
approaches to critical phenomena. The ferromagnetic correlation between the 5f
electrons differs from that in the 3D Ising system and this difference may be a
key point for the understanding of the ferromagnetism where superconductivity
emerges.Comment: 8 pages, 5 figure
Strong Correlation between Anomalous Quasiparticle Scattering and Unconventional Superconductivity in Hidden Order Phase of URuSi
The pressure dependent electrical resistivity of URuSi has been
studied at high pressure across the first order phase boundary of where
the ground state switches under pressure from "hidden order" (HO) to large
moment antiferromagnetic (LAFM) states. The electrical transport in
URuSi at low temperatures shows a strong sample dependence. We have
measured an ultra-clean single crystal whose quality is the highest among those
used in previous studies. The generalized power law
analysis finds that the electric
transport property deviates from Fermi liquid theory in the HO phase but obeys
the theory well above . The analysis using the polynomial in
expression
reveals the relation in the HO
phase. While the pressure dependence of is very weak,
is roughly proportional to . This suggests a strong
correlation between the anomalous quasiparticle scattering and the
superconductivity and that both have a common origin. The present study
clarifies a universality of the HO phase inherent in strongly correlated
electron superconductors near quantum criticality
Magnetic measurements at pressures above 10 GPa in a miniature ceramic anvil cell for a superconducting quantum interference device magnetometer
A miniature ceramic anvil high pressure cell (mCAC) was earlier designed by
us for magnetic measurements at pressures up to 7.6 GPa in a commercial
superconducting quantum interference (SQUID) magnetometer [N. Tateiwa et al.,
Rev. Sci. Instrum. 82, 053906 (2011)]. Here, we describe methods to generate
pressures above 10 GPa in the mCAC. The efficiency of the pressure generation
is sharply improved when the Cu-Be gasket is sufficiently preindented. The
maximum pressure for the 0.6 mm culet anvils is 12.6 GPa when the Cu-Be gasket
is preindented from the initial thickness of 0.30 to 0.06 mm. The 0.5 mm culet
anvils were also tested with a rhenium gasket. The maximum pressure attainable
in the mCAC is about 13 GPa. The present cell was used to study YbCu2Si2 which
shows a pressure induced transition from the non-magnetic to magnetic phases at
8 GPa. We confirm a ferromagnetic transition from the dc magnetization
measurement at high pressure. The mCAC can detect the ferromagnetic ordered
state whose spontaneous magnetic moment is smaller than 1 mB per unit cell. The
high sensitivity for magnetic measurements in the mCAC may result from the the
simplicity of cell structure. The present study shows the availability of the
mCAC for precise magnetic measurements at pressures above 10 GPa
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