44 research outputs found
Lockin to Weak Ferromagnetism in TbNi2B2C and ErNi2B2C
This article describes a model in which ferromagnetism necessarily
accompanies a spin-density-wave lockin transition in the borocarbide structure
provided the commensurate phase wave vector satisfies Q = (m/n)a* with m even
and n odd. The results account for the magnetic properties of TbNi2B2C, and are
also possibly relevant also for those of ErNi2B2C.Comment: 4 page
Magnetic pair breaking in HoNi2B2C
Neutron-diffraction techniques have been used to study the interplay between superconductivity and magnetism in HoNi2B2C (Tc=8 K). The experimental results, obtained on single crystals, show that below approximately 4.7 K, this compound is in a simple antiferromagnetic state that coexists with superconductivity. Between approximately 4.7 and 6 K, an incommensurate modulated magnetic structure has been found. This observation strongly suggests that pair breaking associated with this incommensurate magnetic structure is responsible for the deep minimum in Hc2 and the near-reentrant behavior observed in this compound at approximately 5 K
Magnetic Phase Diagram of GdNi2B2C: Two-ion Magnetoelasticity and Anisotropic Exchange Couplings
Extensive magnetization and magnetostriction measurements were carried out on
a single crystal of GdNi2B2C along the main tetragonal axes. Within the
paramagnetic phase, the magnetic and strain susceptibilities revealed a weak
anisotropy in the exchange couplings and two-ion tetragonal-preserving
alpha-strain modes. Within the ordered phase, magnetization and
magnetostriction revealed a relatively strong orthorhombic distortion mode and
rich field-temperature phase diagrams. For H//(100) phase diagram, three
field-induced transformations were observed, namely, at: Hd(T), related to the
domain alignment; Hr(T), associated with reorientation of the moment towards
the c-axis; and Hs(T), defining the saturation process wherein the exchange
field is completely counterbalanced. On the other hand, For H//(001) phase
diagram, only two field-induced transformations were observed, namely at: Hr(T)
and Hs(T). For both phase diagrams, Hs(T) follows the relation
Hs[1-(T/Tn)^2]^(1/2)kOe with Hs(T-->0)=128.5(5) kOe and Tn(H=0)=19.5 K. In
contrast, the thermal evolution of Hr(T) along the c-axis (much simpler than
along the a-axis) follows the relation Hr[1-T/Tr]^(1/3) kOe where
Hr(T-->0)=33.5(5) kOe and Tr(H=0)=13.5 K. It is emphasized that the
magnetoelastic interaction and the anisotropic exchange coupling are important
perturbations and therefore should be explicitly considered if a complete
analysis of the magnetic properties of the borocarbides is desired
Phonon spectrum and soft-mode behavior of MgCNi_3
Temperature dependent inelastic neutron-scattering measurements of the
generalized phonon density-of-states for superconducting MgCNi_3, T_c=8 K, give
evidence for a soft-mode behavior of low-frequency Ni phonon modes. Results are
compared with ab initio density functional calculations which suggest an
incipient lattice instability of the stoichiometric compound with respect to Ni
vibrations orthogonal to the Ni-C bond direction.Comment: 4 pages, 5 figure
Nesting properties and anisotropy of the Fermi surface of LuNiBC
The rare earth nickel borocarbides, with the generic formula
NiBC, have recently been shown to display a rich variety of
phenomena. Most striking has been the competition between, and even coexistence
of, antiferromagnetism and superconductivity. We have measured the Fermi
surface (FS) of LuNiBC, and shown that it possesses nesting
features capable of explaining some of the phenomena experimentally observed.
In particular, it had previously been conjectured that a particular sheet of FS
is responsible for the modulated magnetic structures manifest in some of the
series. We report the first direct experimental observation of this sheet.Comment: 4 pages, 4 PS figure
A study of the superconducting gap in RNiBC (R = Y, Lu) single crystals by inelastic light scattering
Superconductivity-induced changes in the electronic Raman scattering response
were observed for the RNiBC (R = Y, Lu) system in different scattering
geometries. In the superconducting state, 2-like peaks were observed in
A, B, and B spectra from single crystals. The peaks in
A and B symmetries are significantly sharper and stronger than
the peak in B symmetry. The temperature dependence of the frequencies of
the 2-like peaks shows typical BCS-type behavior, but the apparent
values of the gap are strongly anisotropic for both systems. In
addition, for both YNiBC and LuNiBC systems, there exists
reproducible scattering strength below the gap which is roughly
linear to the frequency in B and B symmetries. This discovery of
scattering below the gap in non-magnetic borocarbide superconductors, which are
thought to be conventional BCS-type superconductors, is a challenge for current
understanding of superconductivity in this system.Comment: Added text, changed a figure, and added references. Will appear in
Phys. Rev.
Magneto-electrodynamics at high frequencies in the antiferromagnetic and superconducting states of DyNi_2B_2C
We report the observation of novel behaviour in the radio frequency (rf) and
microwave response of DyNi_2B_2C over a wide range of temperature (T) and
magnetic field (H) in the antiferromagnetic (AFM) and superconducting (SC)
states. At microwave frequencies of 10 GHz, the T dependence of the surface
impedance Z_s=R_s+iX_s was measured which yields the T dependence of the
complex conductivity \sigma_1-i\sigma_2 in the SC and AFM states. At radio
frequencies (4 MHz), the H and T dependence of the penetration depth
\lambda(T,H) were measured. The establishment of antiferromagnetic order at
T_N=10.3 K results in a marked decrease in the scattering of charge carriers,
leading to sharp decreases in R_s and X_s. However, R_s and X_s differ from
each other in the AFM state. We show that the results are consistent with
conductivity relaxation whence the scattering rate becomes comparable to the
microwave frequency. The rf measurements yield a rich dependence of the
scattering on the magnetic field near and below T_N. Anomalous decrease of
scattering at moderate applied fields is observed at temperatures near and
above T_N, and arises due to a crossover from a negative magnetoresistance
state, possibly associated with a loss of spin disorder scattering at low
fields, to a positive magnetoresistance state associated with the metallic
nature. The normal state magnetoresistance is positive at all temperatures for
\mu_0H>2T and at all fields for T>15K. Several characteristic field and
temperature scales associated with metamagnetic transitions (H_M1(T), H_M2(T))
and onset of spin disorder H_D(T), in addition to T_c, T_N and H_c2(T) are
observed in the rf measurements.Comment: 9 pages, Latex, Uses REVTeX, This and related publications also
available at http://sagar.physics.neu.edu/ Submitted to Phys. Rev.
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Neutron scattering studies of RENi{sub 2}B{sub 2}C: Magnetic structures and lattice dynamics
Neutron scattering techniques have been used to study the magnetic structure and lattice dynamical properties of various members of the recently discovered RENi{sub 2}B{sub 2}C family, were RE stands for a rare-earth element. The magnetic structures of superconducting DyNi{sub 2}B{sub 2}C, ErNi{sub 2}B{sub 2}C, HoNi{sub 2}B{sub 2}C, and non superconducting TbNi{sub 2}B{sub 2}C have been determined as a function of temperature, in the 2-300 K temperature range
Microscopic Study of the Low Temperature Metamagnetic States of HoNi₂B₂C.
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