154 research outputs found
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
Damagnetization cooling of a gas
We demonstrate demagnetization cooling of a gas of ultracold Cr atoms.
Demagnetization is driven by inelastic dipolar collisions which couple the
motional degrees of freedom to the spin degree. By that kinetic energy is
converted into magnetic work with a consequent temperature reduction of the
gas. Optical pumping is used to magnetize the system and drive continuous
demagnetization cooling. Applying this technique, we can increase the phase
space density of our sample by one order of magnitude, with nearly no atom
loss. This method can be in principle extended to every dipolar system and
could be used to achieve quantum degeneracy via optical means.Comment: 10 pages, 5 figure
Physical Foundations of Landauer's Principle
We review the physical foundations of Landauer's Principle, which relates the
loss of information from a computational process to an increase in
thermodynamic entropy. Despite the long history of the Principle, its
fundamental rationale and proper interpretation remain frequently
misunderstood. Contrary to some misinterpretations of the Principle, the mere
transfer of entropy between computational and non-computational subsystems can
occur in a thermodynamically reversible way without increasing total entropy.
However, Landauer's Principle is not about general entropy transfers; rather,
it more specifically concerns the ejection of (all or part of) some correlated
information from a controlled, digital form (e.g., a computed bit) to an
uncontrolled, non-computational form, i.e., as part of a thermal environment.
Any uncontrolled thermal system will, by definition, continually re-randomize
the physical information in its thermal state, from our perspective as
observers who cannot predict the exact dynamical evolution of the microstates
of such environments. Thus, any correlations involving information that is
ejected into and subsequently thermalized by the environment will be lost from
our perspective, resulting directly in an irreversible increase in total
entropy. Avoiding the ejection and thermalization of correlated computational
information motivates the reversible computing paradigm, although the
requirements for computations to be thermodynamically reversible are less
restrictive than frequently described, particularly in the case of stochastic
computational operations. There are interesting possibilities for the design of
computational processes that utilize stochastic, many-to-one computational
operations while nevertheless avoiding net entropy increase that remain to be
fully explored.Comment: 42 pages, 15 figures, extended postprint of a paper published in the
10th Conf. on Reversible Computation (RC18), Leicester, UK, Sep. 201
Influence of Pr on the magnetic structure of Er
Time-of-flight neutron diffraction has been used to determine the temperature dependence of the magnetic structure of three binary hexagonal Er-Pr alloys, Er90Pr10, Er80Pr20, and Er60Pr40. In overall agreement with magneto-thermal measurements performed on these compounds, the addition of Pr initially increases the Curie temperature and decreases the Neel temperature, observed at 20 K and 86 K, respectively, for pure Er. The neutron diffraction data for Er90Pr10, however, also clearly show that a ferromagnetic phase, with moments parallel to the c-axis, coexists with modulated structure components, with increasing temperature up to 35 K, above which a pure sine modulated structure sets in up to the Neel temperature at approximately 65 K. A similar behavior is observed for Er80Pr20, where the sine modulated phase is observed to disappear at 48 K. In sharp contrast, only one magnetic phase, identified as ferromagnetic with moments parallel to the c-axis, is observed for Er60Pr40 from low temperatures up to the Curie point at 35 K. The propagation vectors of the sine modulated phases are found to be temperature dependent
The nature of the first order isostructural transition in GdRhSn
We present structural, magnetic, thermal, magnetocaloric, and electrical transport properties of polycrystalline GdRhSn. Magnetization data show that it orders antiferromagnetically at T-N = 16.2 K. The compound has the ZrNiAl type hexagonal crystal structure at room temperature and undergoes a first order iso-structural transition in the paramagnetic state at 245 K. The unit cell volume change at the transition is small (-0.07%) but discontinuous, in agreement with the first-order nature of the transition observed by magnetic, transport, and heat capacity measurements. The anisotropic changes of the lattice parameters are Delta alpha/alpha = 0.28% and Delta c/c = 0.64% on cooling. A substantial change in the 4f and conduction electron hybridization, giving rise to an increased integrated DOS, occurs when the high temperature phase transforms to the low temperature phase. A moderate magnetocaloric effect at T-N (Delta S-M = 6.5 J/kg K and Delta T-ad = 4.5 K for Delta H = 50 kOe) has been measured using both magnetization and heat capacity data. (C) 2014 Elsevier B.V. All rights reserved
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