252 research outputs found
Specific heat in magnetic field and magnetocaloric effects of α-R2S3 (R = Tb, Dy) single crystals
The magnetocaloric effects (MCE) of α-Tb2S3 and α-Dy2S3 single crystals exhibiting successive antiferromagnetic (AFM) transitions have been investigated by analyzing specific heat measured in magnetic field. The temperature dependence of specific heat in the vicinity of the successive transitions shows obvious distinction depending on the orientations of the applied magnetic field for both α-Tb2S3 and α-Dy2S3 that having orthorhombic crystal structures. When the magnetic field is increased, the specific heat is as follows: For α-Tb2S3 in H‖b, the peak around TN2 shifts to lower temperature but the other one peak around TN1 barely moves; In H⊥b, the peak around TN2 has no shift almost within 3 T but suddenly moves to lower temperature in 4 T and the other one peak around TN1 shifts to lower temperature in specific heat versus temperature. In the case of α-Dy2S3, the two peaks around TN2 and TN1 shift to lower temperatures in H‖b but move to higher temperatures when the magnetic field is increased up to 5 T by H⊥b in spite of antiferromagnetic transitions. Therefore, the maximum value and corresponding temperature of both isothermal magnetic entropy change (ΔSm) and adiabatic temperature change (ΔTad) in the magnetic field H⊥b are extremely different in low temperature range from that in the field of H‖b. The results propone that the MCE of α-Tb2S3 and α-Dy2S3 could be controlled at low temperature by the magnitude and orientation of magnetic field. It also indicates that the refrigerating capacity and thermal absorption capacity will be controlled by changing magnitude and orientation of magnetic field on the α-Tb2S3 and α-Dy2S3 single crystals
Structure, magnetism, and magnetocaloric properties of MnFeP1−xSix compounds
MnFeP1-xSix compounds with x=0.10,0.20,0.24,0.28,...,0.80,1 were prepared by high-energy ball milling and solid-state reaction. The structural, magnetic, and magnetocaloric properties are investigated as a function of temperature and magnetic field. X-ray diffraction studies show that the samples in the range from x=0.28 to 0.64 adopt the hexagonal Fe2P-type structure with a small amount of second phase which increases with increasing Si content. The samples with lower Si content show the orthorhombic Co2P-type structure. Magnetic measurements show that the paramagnetic-ferromagnetic transition temperatures range from 214 to 377 K. Of much importance is the fact that these compounds do not contain any toxic components and exhibit excellent magnetocaloric properties
Kondo hybridisation and the origin of metallic states at the (001) surface of SmB6
SmB6, a well-known Kondo insulator, has been proposed to be an ideal
topological insulator with states of topological character located in a clean,
bulk electronic gap, namely the Kondo hybridisation gap. Seeing as the Kondo
gap arises from many body electronic correlations, this would place SmB6 at the
head of a new material class: topological Kondo insulators. Here, for the first
time, we show that the k-space characteristics of the Kondo hybridisation
process is the key to unravelling the origin of the two types of metallic
states observed directly by ARPES in the electronic band structure of
SmB6(001). One group of these states is essentially of bulk origin, and cuts
the Fermi level due to the position of the chemical potential 20 meV above the
lowest lying 5d-4f hybridisation zone. The other metallic state is more
enigmatic, being weak in intensity, but represents a good candidate for a
topological surface state. However, before this claim can be substantiated by
an unequivocal measurement of its massless dispersion relation, our data raises
the bar in terms of the ARPES resolution required, as we show there to be a
strong renormalisation of the hybridisation gaps by a factor 2-3 compared to
theory, following from the knowledge of the true position of the chemical
potential and a careful comparison with the predictions from recent
LDA+Gutzwiler calculations. All in all, these key pieces of evidence act as
triangulation markers, providing a detailed description of the electronic
landscape in SmB6, pointing the way for future, ultrahigh resolution ARPES
experiments to achieve a direct measurement of the Dirac cones in the first
topological Kondo insulator.Comment: 9 pages, 4 Figures and supplementary material (including Movies and
CORPES13 "best prize" poster
Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys
The magnetocaloric effect (MCE) in paramagnetic materials has been widely
used for attaining very low temperatures by applying a magnetic field
isothermally and removing it adiabatically. The effect can be exploited also
for room temperature refrigeration by using recently discovered giant MCE
materials. In this letter, we report on an inverse situation in Ni-Mn-Sn
alloys, whereby applying a magnetic field adiabatically, rather than removing
it, causes the sample to cool. This has been known to occur in some
intermetallic compounds, for which a moderate entropy increase can be induced
when a field is applied, thus giving rise to an inverse magnetocaloric effect.
However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is
just as large as that reported for giant MCE materials, but with opposite sign.
The giant inverse MCE has its origin in a martensitic phase transformation that
modifies the magnetic exchange interactions due to the change in the lattice
parameters.Comment: 12 pages, 4 figures, to appear in Nature Materials (online published,
15 May 2005
Exotic (anti)ferromagnetism in single crystals of Pr6Ni2Si3
The ternary intermetallic compound Pr6Ni2Si3, is a member of a structure
series of compounds based on a triangular structure where the number of Pr
atoms in the prism cross section can be systematically varied. Pr6Ni2Si3
contains two distinct Pr lattice sites which result in complex interactions
between the magnetic ions. Extensive measurements of specific heat and
magnetization on single crystal samples indicate that Pr6Ni2Si3 orders with
both a ferromagnet and an antiferromagnet component, with ordering temperatures
of 39.6 K and ~ 32 K, respectively. The ferromagnetic component // c-axis is
accompanied by a large hysteresis, and the antiferromagnetic component,_|_
c-axis is accompanied by a spin-flop-type transition. More detailed
measurements, of the vector magnetization, indicate that the ferromagnetic and
the antiferromagnetic order appear independent of each other. These results not
only clarify the behavior of Pr6Ni2Si3 itself, but also of the other members of
the structure series, Pr5Ni2Si3 and Pr15Ni7Si10.Comment: 9 pages, 13 figures, submitted to PR
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