17 research outputs found
Spin dynamics of FeGaGe studied by Electron Spin Resonance
The intermetallic semiconductor FeGa acquires itinerant ferromagnetism
upon electron doping by a partial replacement of Ga with Ge. We studied the
electron spin resonance (ESR) of high-quality single crystals of
FeGaGe for from 0 up to 0.162 where ferromagnetic order is
observed. For we observed a well-defined ESR signal, indicating the
presence of pre-formed magnetic moments in the semiconducting phase. Upon Ge
doping the occurrence of itinerant magnetism clearly affects the ESR properties
below ~K whereas at higher temperatures an ESR signal as seen in
FeGa prevails independent on the Ge-content. The present results show
that the ESR of FeGaGe is an appropriate and direct tool to
investigate the evolution of 3d-based itinerant magnetism.Comment: 12 pages, 7 figure
Interplay between Co-3d and Ce-4f magnetism in CeCoAsO
We have investigated the ground state properties of polycrystalline CeCoAsO
by means of magnetization, specific heat and solid state NMR. Susceptibility
and specific-heat measurements suggest a ferromagnetic order at about,
=75 K. No further transitions are found down to 2 K. At 6.5 K a
complex Schottky type of anomaly shows up in the specific heat results. The
interplay between Ce-4f and Co-3d magnetism being responsible for that anomaly
is discussed. Furthermore As NMR investigations have been performed to
probe the magnetism on a microscopic scale. As-NMR spectra are analysed in
terms of first and second order quadrupolar interaction. The anisotropic shift
component and could be derived from the
As powder spectra. Towards lower temperature a strong shift anisotropy
was found. Nonetheless tracks the bulk susceptibility down
to 50 K very well. Furthermore the presence of weak correlations among the
Ce ions in the ferromagnetic state is discussed. The observed increase of
towards lower temperatures supports this interpretation.Comment: 6 pages, 4 figures, Accepted in Physical Review
Low field extension for magnetometers (TinyBee) used for investigations on low-dimensional superconductors with Bc1 < 5G
In this article a simple and easy to install low magnetic field extension of
the SQUID magnetometer Quantum Design MPMS-7 is described. This has been
accomplished by complementing the MPMS-7 magnet control system with a
laboratory current supply for the low magnetic field region (B \leq 200G). This
hard- and software upgrade provides a significant gain in the magnetic field
accuracy up to an order of magnitude compared with the standard instrument's
setup and is improving the resolution to better than 0.01G below 40G. The field
control has been integrated into the Quantum Design MultiVu software for a
transparent and user-friendly operation of this extension. The improvements
achieved are especially useful, when low magnetic field strengths (B < 1G) are
required at high precision. The specific advantages of this application are
illustrated by sophisticated magnetic characterisation of lowdimensional
superconductors like Sc3CoC4 and SnSe2{Co({\eta}-C5H5)2}x.Comment: 16 pages, 7 figure
Quantum phase transitions and multicriticality in Ta(Fe1-xVx)2
We present a comprehensive study of synthesis, structure analysis, transport
and thermodynamic properties of the C14 Laves phase Ta(Fe1-xVx)2. Our
measurements confirm the appearance of spin-density wave (SDW) order within a
dome-like region of the x - T phase diagram with vanadium content 0.02 < x <
0.3. Our results indicate that on approaching TaFe2 from the vanadium-rich
side, ferromagnetic (FM) correlations increase faster than the
antiferromagnetic (AFM) ones. This results in an exchange-enhanced
susceptibility and in the suppression of the SDW transition temperature for x <
0.13 forming the dome-like shape of the phase diagram. This effect is strictly
related to a significant lattice distortion of the crystal structure manifested
in the c/a ratio. At x = 0.02 both FM and AFM energy scales have similar
strength and the system remains paramagnetic down to 2 K with an extremely
large Stoner enhancement factor of about 400. Here, spin fluctuations dominate
the temperature dependence of the resistivity \rho ~ T ^ 3/2 and of the
specific heat C/T ~ - log(T) which deviate from their conventional Fermi liquid
forms, inferring the presence of a quantum critical point of dual nature.Comment: 9 pages, 13 figure
Electron Spin Resonance on the spin-1/2 triangular magnet NaYbS2
The delafossite structure of NaYbS2 contains a planar spin-1/2 triangular
lattice of Yb3+ ions and features a possible realisation of a quantum
spin-liquid state. We investigated the Yb3+ spin dynamics by Electron Spin
Resonance (ESR) in single-crystalline samples of NaYbS2. Very clear spectra
with a well-resolved and large anisotropy could be observed down to the lowest
accessible temperature of 2.7 K. In contrast to the ESR properties of other
known spin-liquid candidate systems, the resonance seen in NaYbS2 is accessible
at low fields (< 1T) and is narrow enough for accurate characterisation of the
relaxation rate as well as the g factor of the Yb3+ spins.Comment: 8 page
Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias
The discovery of materials with improved functionality can be accelerated by
rational material design. Heusler compounds with tunable magnetic sublattices
allow to implement this concept to achieve novel magnetic properties. Here, we
have designed a family of Heusler alloys with a compensated ferrimagnetic
state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant
exchange bias (EB) of more than 3 T and a similarly large coercivity are
established. The large exchange anisotropy originates from the exchange
interaction between the compensated host and ferrimagnetic clusters that arise
from intrinsic anti-site disorder. We demonstrate the applicability of our
design concept on a second material, Mn-Fe-Ga, with a magnetic transition above
room temperature, exemplifying the universality of the concept and the
feasibility of room-temperature applications. Our study points to a new
direction for novel magneto-electronic devices. At the same time it suggests a
new route for realizing rare-earth free exchange-biased hard magnets, where the
second quadrant magnetization can be stabilized by the exchange bias.Comment: Four figure
Observation of the anomalous Hall effect in a layered polar semiconductor
Progress in magnetoelectric materials is hindered by apparently contradictory
requirements for time-reversal symmetry broken and polar ferroelectric
electronic structure in common ferromagnets and antiferromagnets. Alternative
routes could be provided by recent discoveries of a time-reversal symmetry
breaking anomalous Hall effect in noncollinear magnets and altermagnets, but
hitherto reported bulk materials are not polar. Here, we report the observation
of a spontaneous anomalous Hall effect in doped AgCrSe, a layered polar
semiconductor with an antiferromagnetic coupling between Cr spins in adjacent
layers. The anomalous Hall resistivity 3 cm is comparable to the
largest observed in compensated magnetic systems to date, and is rapidly
switched off when the angle of an applied magnetic field is rotated to from the crystalline -axis. Our ionic gating experiments show
that the anomalous Hall conductivity magnitude can be enhanced by modulating
the -type carrier density. We also present theoretical results that suggest
the anomalous Hall effect is driven by Berry curvature due to noncollinear
antiferromagnetic correlations among Cr spins, which are consistent with the
previously suggested magnetic ordering in AgCrSe. Our results open the
possibility to study the interplay of magnetic and ferroelectric-like responses
in this fascinating class of materials.Comment: 8 pages, 5 figure
Observation of the anomalous Hall effect in a layered polar semiconductor
Funding: S.-J.K. acknowledged support from the International Max Planck Research School for Chemistry and Physics of Quantum Materials (IMPRS-CPQM). L.Ć . acknowledged support from Johannes Gutenberg University Grant TopDyn, and support by the Deutsche Forschungsgemein- schaft (DFG, German Research Foundation) for funding through TRR 288 â 422213477 (projects A09 and B05).Progress in magnetoelectric materials is hindered by apparently contradictory requirements for timeâreversal symmetry broken and polar ferroelectric electronic structure in common ferromagnets and antiferromagnets. Alternative routes can be provided by recent discoveries of a timeâreversal symmetry breaking anomalous Hall effect (AHE) in noncollinear magnets and altermagnets, but hitherto reported bulk materials are not polar. Here, the authors report the observation of a spontaneous AHE in doped AgCrSe2, a layered polar semiconductor with an antiferromagnetic coupling between Cr spins in adjacent layers. The anomalous Hall resistivity 3 ΌΩcm is comparable to the largest observed in compensated magnetic systems to date, and is rapidly switched off when the angle of an applied magnetic field is rotated to â80° from the crystalline câaxis. The ionic gating experiments show that the anomalous Hall conductivity magnitude can be enhanced by modulating the pâtype carrier density. They also present theoretical results that suggest the AHE is driven by Berry curvature due to noncollinear antiferromagnetic correlations among Cr spins, which are consistent with the previously suggested magnetic ordering in AgCrSe2. The results open the possibility to study the interplay of magnetic and ferroelectricâlike responses in this fascinating class of materials.Publisher PDFPeer reviewe
Anisotropic superconductivity and quantum oscillations in the layered dichalcogenide TaSnS2
TaSnS2 single crystal and polycrystalline samples are investigated in detail by magnetization, electrical resistivity, and specific heat as well as Raman spectroscopy and nuclear magnetic resonance (NMR). Studies are focused on the temperature and magnetic field dependence of the superconducting state. We determine the critical fields for both directions Bâ„c and Bâ„c. Additionally, we investigate the dependence of the resistivity, the critical temperature, and the structure through Raman spectroscopy under high pressure up to 10 GPa. At a pressure of â3GPa the superconductivity is suppressed below our minimum temperature. The Sn NMR powder spectrum shows a single line which is expected for the TaSnS2 phase and confirms the high sample quality. Pronounced de Haas-van Alphen oscillations in the ac susceptibility of polycrystalline sample reveal two pairs of frequencies indicating coexisting small and large Fermi surfaces. The effective mass of the smaller Fermi surface is â0.5me. We compare these results with the band structures from DFT calculations. Our findings on TaSnS2 are discussed in terms of a quasi-two-dimensional BCS superconductivity
Resonant torsion magnetometry in anisotropic quantum materials
Unusual behavior of quantum materials commonly arises from their effective
low-dimensional physics, which reflects the underlying anisotropy in the spin
and charge degrees of freedom. Torque magnetometry is a highly sensitive
technique to directly quantify the anisotropy in quantum materials, such as the
layered high-T superconductors, anisotropic quantum spin-liquids, and the
surface states of topological insulators. Here we introduce the magnetotropic
coefficient , the second derivative of the
free energy F with respect to the angle between the sample and the
applied magnetic field, and report a simple and effective method to
experimentally detect it. A sub-g crystallite is placed at the tip of a
commercially available atomic force microscopy cantilever, and we show that
can be quantitatively inferred from a shift in the resonant frequency under
magnetic field. While related to the magnetic torque , takes the role of torque susceptibility, and thus provides
distinct insights into anisotropic materials akin to the difference between
magnetization and magnetic susceptibility. The thermodynamic coefficient is
discontinuous at second-order phase transitions and subject to Ehrenfest
relations with the specific heat and magnetic susceptibility. We apply this
simple yet quantitative method on the exemplary cases of the Weyl-semimetal NbP
and the spin-liquid candidate RuCl, yet it is broadly applicable in quantum
materials research.Comment: 7 pages including 6 figures and methods sectio