Maria Krautz

Element-Resolved Thermodynamics of Magnetocaloric LaFe 13−x Si x

By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe(13-x)Si(x). These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties.

Effect of carbon on magnetocaloric effect of LaFe11.6Si1.4 compounds and on the thermal stability of its hydrides

La(Fe,Si)13 alloys display a giant magnetocaloric effect when a magnetic field is applied near the Curie temperatureT C. However, to use these alloys for domestic refrigeration based on magnetic cooling, it is vital to increase T C near to the room-temperature range while simultaneously maintaining a large magnetocaloric effect. With this aim, we studied the effect of interstitialcarbon on the microstructure and magnetocaloric effect in LaFe11.6Si1.4C x (x = 0–0.4). The investigation was carried out in cast samples annealed for seven days at 1323 K. The study of microstructure shows that annealing led to about 90 wt. % of 1:13 magnetocaloric phase. Magnetization data revealed that the addition of carbon leads to an increase in T C and a decrease of the thermal hysteresis width. For x > 0.2, the magnetic transition changes from first-order to second-order, with a corresponding reduction in magnetocaloric effect. A small amount of C (x up to 0.2) improves the magnetocaloric properties of the parent alloy La(Fe,Si)13, and, furthermore, the carbon addition leads to an increase in the thermal stability of hydrided LaFe11.6Si1.4C x . The onset of hydrogen desorption increases from 460 K for the x = 0 (carbon-free alloy) to 500 K and 540 K, respectively, for x = 0.1 and x = 0.2.

Reversible solid-state hydrogen-pump driven by magnetostructural transformation in the prototype system La(Fe,Si)13Hy

In magnetocaloric La(Fe,Si)13 alloys, it is known that hydrogen addition shifts the magnetic transition temperature TC near room temperature. By partial hydrogen desorption, TC can be adjusted precisely towards the working temperature of a magnetic cooling device. In this work, we studied the stability of partially desorbed LaFe11.6Si1.4Hy and show that the large volume difference of ferro- and para-magnetic phases drives the system from a single phase to a stable two-phase configuration. The hydrogen concentration separates on a macroscopic scale. The phase-coexistence is visualized by Kerr Microscopy. Differential scanning calorimetry measurements show that hydrogen can be reversibly recombined and separated again. We explain the separation by the coexistence of a high-volume ferromagnetic and low-volume paramagnetic phase that can be induced either by temperature or other external stimuli. This phenomenon can be applied to material systems that show a coupling of physical and structural transitions.

Reentrant spin-glass behavior and bipolar exchange-bias effect in “Sn” substituted cobalt-orthotitanate

We report the co-existence of longitudinal ferrimagnetic behavior with Neel temperature TN ∼ 46.1 K and reentrant transverse spin-glass state at 44.05 K in Tin (Sn) doped cobalt-orthotitanate (Co2TiO4). The ferrimagnetic ordering is resulting from different magnetic moments of Co2+ on the A-sites (3.87 μB) and B-sites (5.069 μB). The magnetic compensation temperature (TCOMP) shifts from 31.74 K to 27.1 K when 40 at. % of “Sn4+” substitutes “Ti4+” at B-sites where the bulk-magnetization of two-sublattices balance each other. For T > TN, the dc-magnetic susceptibility (χdc = M/Hdc) fits well with the Neels expression for the two-sublattice model with antiferromagnetic molecular field constants NBB ∼ 15.44, NAB ∼ 32.01, and NAA ∼ 20.88. The frequency dependence of ac-magnetic susceptibility χac data follows the Vogel-Fulcher law, and the power-law of critical slowing-down with “zν” = 6.01 suggests the existence of spin-clusters (where “z” and “ν” being dynamic critical-exponent and correlation length of cri...

Moment-Volume Coupling in La(Fe1−x Si x )13

We investigate the origin of the volume change and magnetoelastic interaction observed at the magnetic first-order transition in the magnetocaloric system La(Fe1-xSix)(13) by means of first-principles calculations combined with the fixed-spin moment approach. We find that the volume of the system varies with the square of the average local Fe moment, which is significantly smaller in the spin disordered configurations compared to the ferromagnetic ground state. The vibrational density of states obtained for a hypothetical ferromagnetic state with artificially reduced spin-moments compared to a nuclear inelastic X-ray scattering measurement directly above the phase transition reveals that the anomalous softening at the transition essentially depends on the same moment-volume coupling mechanism. In the same spirit, the dependence of the average local Fe moment on the Si content can account for the occurence of first- and second-order transitions in the system.

The dynamics of spontaneous hydrogen segregation in LaFe13−xSixHy

By means of time- and temperature-dependent magnetization measurements, we demonstrate that the timescale of hydrogen diffusion in partially-hydrogenated LaFe

Anisotropic thermal conductivity in epoxy-bonded magnetocaloric composites

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The dynamics of spontaneous hydrogen segregation in LaFe

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