On the magnetocaloric properties of Heusler compounds: Reversible, time- and size-dependent effects of the martensitic phase transition

Large magnetocaloric effects can be obtained in the Heusler alloy systems Ni-Mn-In and Ni-Mn-In-Co during the magnetostructural phase transformation between the low temperature paramagnetic martensite and the high temperature ferromagnetic austenite phase. The martensitic transition takes place by a nucleation and growth process and can be tuned in a wide temperature window by varying the chemical composition. It is furthermore sensitive to a magnetic field but also to hydrostatic pressure. The phase transformation can therefore be induced by those external stimuli, which is investigated in this thesis by means of a phenomenological model. The martensitic transition is related to a significant thermal hysteresis, which limits the reversible adiabatic temperature and isothermal entropy change of the material. However, the magnetocaloric effect under cycling can be enhanced when the material remains all the time in a mixed-phase state, in so-called minor loops of hysteresis. On the contrary, in very high magnetic-field rates as well as in micrometer-sized single particles, the thermal hysteresis increases significantly, which needs to be considered in terms of application. In order to understand the contrasting behavior of small fragments in comparison to their bulk representatives, a finite element model is introduced, from which the importance of mechanical stress during the first-order transition becomes apparent. Furthermore, an attempt is made to improve the sustainability of magnetocaloric Heusler alloys by the substitution of critical elements to move this interesting material class further towards application

Multicaloric effects in metamagnetic Heusler Ni-Mn-In under uniaxial stress and magnetic field

The world's growing hunger for artificial cold, on the one hand, and the ever more stringent climate targets, on the other, pose an enormouschallenge to mankind. Novel, efficient, and environmentally friendly refrigeration technologies based on solid-state refrigerants can offer away out of the problems arising from climate-damaging substances used in conventional vapor-compressors. Multicaloric materials standout because of their large temperature changes, which can be induced by the application of different external stimuli such as a magnetic, elec-tric, or a mechanical field. Despite the high potential for applications and the interesting physics of this group of materials, few studies focuson their investigation by direct methods. In this paper, we report on the advanced characterization of all relevant physical quantities thatdetermine the multicaloric effect of a Ni-Mn-In Heusler compound. We have used a purpose-designed calorimeter to determine the isother-mal entropy and adiabatic temperature changes resulting from the combined action of magnetic field and uniaxial stress on this metamag-netic shape-memory alloy. From these results, we can conclude that the multicaloric response of this alloy by appropriate changes of uniaxialstress and magnetic field largely outperforms the caloric response of the alloy when subjected to only a single stimulus. We anticipate thatour findings can be applied to other multicaloric materials, thus inspiring the development of refrigeration devices based on the multicaloriceffect

Reversibility of minor hysteresis loops in magnetocaloric Heusler alloys

The unavoidable existence of thermal hysteresis in magnetocaloric materials with a first-order phase transition is one of the central problems limiting their implementation in cooling devices. Using minor loops, however, allows achieving significant cyclic effects even in materials with relatively large hysteresis. Here, we compare thermometric measurements of the adiabatic temperature change Delta T-ad and calorimetric measurements of the isothermal entropy change Delta S-T when moving in minor hysteresis loops driven by magnetic fields. Under cycling in 2 T, the Ni-Mn-In-Co Heusler material provides a reversible magnetocaloric effect of Delta S-T(rev) = 10.5 J kg(-1) K-1 and Delta T-ad(rev) = 3.0 K. Even though the thermodynamic conditions and time scales are very different in adiabatic and isothermal minor loops, it turns out that after a suitable scaling, a self-consistent reversibility region in the entropy diagram is found. This region is larger than expected from basic thermodynamic considerations based on isofield measurements alone, which opens new opportunities in application. Published by AIP Publishing

The role of Debye temperature in achieving large adiabatic temperature changes at cryogenic temperatures: a case study on Pr2InPr_2In

The excellent magnetic entropy change ($\Delta S_T$) in the temperature range of 20 $\sim$ 77 K due to the first-order phase transition makes $Pr_2In$ an intriguing candidate for magnetocaloric hydrogen liquefaction. As an equally important magnetocaloric parameter, the adiabatic temperature change ($\Delta T_{ad}$) of $Pr_2In$ associated with the first-order phase transition has not yet been reported. In this work, the $\Delta T_{ad}$ of $Pr_2In$ is obtained from heat capacity measurements: 2 K in fields of 2 T and 4.3 K in fields of 5 T. While demonstrating a $\Delta T_{ad}$ that is not as impressive as its remarkable $\Delta S_T$, $Pr_2In$ exhibits an unusual low Debye temperature ($T_D$) of around 110 K. Based on these two observations, an approach that combines the mean-field and Debye models is developed to study the correlation between $\Delta T_{ad}$ and $T_D$. The role of $T_D$ in achieving large $\Delta T_{ad}$ is revealed: materials with higher $T_D$ tend to exhibit larger $\Delta T_{ad}$, particularly in the cryogenic temperature range. This discovery explains the absence of an outstanding $\Delta T_{ad}$ in $Pr_2In$ and can serve as a tool for designing or searching materials with both a large $\Delta S_T$ and a $\Delta T_{ad}$

A unified approach to describe the thermal and magnetic hysteresis in Heusler alloys

Different excitations, like temperature, magnetic field, or pressure, can drive a martensitic transition in Heusler alloys. Coupled phenomena in these materials lead to interesting magnetocaloric and barocaloric effects ascribed to this transition. In this work, we demonstrate that isothermal transformations induced by a magnetic field and isofield transformations induced by the temperature can be described using the same framework. By defining an effective temperature that relates field and temperature through the properties of the system (magnetic moment and entropy of the transition), both kinds of loops can be transformed into the other kind, therefore providing a more effective way of characterizing hysteretic samples. The validity of this effective temperature approach to describe the transition holds for martensite to austenite transformations as well as reversal ones, and thus, the hysteresis phenomena can be described using this single general excitatio

Designing magnetocaloric materials for hydrogen liquefaction with light rare-earth Laves phases

Magnetocaloric hydrogen liquefaction could be a "game-changer" for liquid hydrogen industry. Although heavy rare-earth-based magnetocaloric materials show strong magnetocaloric effects in the temperature range required by hydrogen liquefaction (77 ~ 20 K), the high resource criticality of the heavy rare-earth elements is a major obstacle for upscaling this emerging liquefaction technology. In contrast, the higher abundances of the light rare-earth elements make their alloys highly appealing for magnetocaloric hydrogen liquefaction. Via a mean-field approach, it is demonstrated that tuning the Curie temperature ($T_C$) of an idealized light rare-earth-based magnetocaloric material towards lower cryogenic temperatures leads to larger maximum magnetic and adiabatic temperature changes ($\Delta S_T$ and $\Delta T_{ad}$). Especially in the vicinity of the condensation point of hydrogen (20 K), $\Delta S_T$ and $\Delta T_{ad}$ of the optimized light rare-earth-based material are predicted to show significantly large values. Following the mean-field approach and taking the chemical and physical similarities of the light rare-earth elements into consideration, a method of designing light rare-earth intermetallic compounds for hydrogen liquefaction is proposed: tunning $T_C$ of a rare-earth alloy to approach 20 K by mixing light rare-earth elements with different de Gennes factors. By mixing Nd and Pr in Laves phase $(Nd,Pr)Al_2$, and Pr and Ce in Laves phase $(Pr,Ce)Al_2$, a fully light rare-earth intermetallic series with large magnetocaloric effects covering the temperature range required by hydrogen liquefaction is developed, demonstrating a competitive maximum effect compared to the heavy rare-earth compound $DyAl_2$

A multicaloric cooling cycle that exploits thermal hysteresis

The giant magnetocaloric effect, in which large thermal changes are induced in a material on the application of a magnetic field, can be used for refrigeration applications, such as the cooling of systems from a small to a relatively large scale. However, commercial uptake is limited. We propose an approach to magnetic cooling that rejects the conventional idea that the hysteresis inherent in magnetostructural phase-change materials must be minimized to maximize the reversible magnetocaloric effect. Instead, we introduce a second stimulus, uniaxial stress, so that we can exploit the hysteresis. This allows us to lock-in the ferromagnetic phase as the magnetizing field is removed, which drastically removes the volume of the magnetic field source and so reduces the amount of expensive Nd-Fe-B permanent magnets needed for a magnetic refrigerator. In addition, the mass ratio between the magnetocaloric material and the permanent magnet can be increased, which allows scaling of the cooling power of a device simply by increasing the refrigerant body. The technical feasibility of this hysteresis-positive approach is demonstrated using Ni-Mn-In Heusler alloys. Our study could l

Tailoring magnetocaloric effect in all-d-metal Ni-Co-Mn-Ti Heusler alloys: a combined experimental and theoretical study

Novel Ni-Co-Mn-Ti all-d-metal Heusler alloys are exciting due to large multicaloric effects combined with enhanced mechanical properties. An optimized heat treatment for a series of these compounds leads to very sharp phase transitions in bulk alloys with isothermal entropy changes of up to 38 J kg$^{-1}$ K$^{-1}$ for a magnetic field change of 2 T. The differences of as-cast and annealed samples are analyzed by investigating microstructure and phase transitions in detail by optical microscopy. We identify different grain structures as well as stoichiometric (in)homogenieties as reasons for differently sharp martensitic transitions after ideal and non-ideal annealing. We develop alloy design rules for tuning the magnetostructural phase transition and evaluate specifically the sensitivity of the transition temperature towards the externally applied magnetic fields ($\frac{dT_t}{\mu_0dH}$) by analyzing the different stoichiometries. We then set up a phase diagram including martensitic transition temperatures and austenite Curie temperatures depending on the e/a ratio for varying Co and Ti content. The evolution of the Curie temperature with changing stoichiometry is compared to other Heusler systems. Density Functional Theory calculations reveal a correlation of T$_C$ with the stoichiometry as well as with the order state of the austenite. This combined approach of experiment and theory allows for an efficient development of new systems towards promising magnetocaloric properties. Direct adiabatic temperature change measurements show here the largest change of -4 K in a magnetic field change of 1.93 T for Ni$_{35}$Co$_{15}$Mn$_{37}$Ti$_{13}$

High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 10$^{11}$ photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH$_3$I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses

Energetic sub-2-cycle laser with 216 W average power

Few-cycle lasers are essential for many research areas such as attosecond physics that promise to address fundamental questions in science and technology. Therefore, further advancements are connected to significant progress in the underlying laser technology. Here, two-stage nonlinear compression of a 660 W femtosecond fiber laser system is utilized to achieve unprecedented average power levels of energetic ultrashort or even few-cycle laser pulses. In a first compression step, 408 W, 320 mu J, 30 fs pulses are achieved, which can be further compressed to 216 W, 170 mu J, 6.3 fs pulses in a second compression stage. To the best of our knowledge, this is the highest average power few-cycle laser system presented so far. It is expected to significantly advance the fields of high harmonic generation and attosecond science. (C) 2016 Optical Society of Americ