The phase‐down scenario of conventional refrigerants used in gas–vapor compressors and the demand for environmentally friendly and efficient cooling make the search for alternative technologies more ...important than ever. Magnetic refrigeration utilizing the magnetocaloric effect of magnetic materials could be that alternative. However, there are still several challenges to be overcome before having devices that are competitive with those based on the conventional gas–vapor technology. In this paper a rigorous assessment of the most relevant examples of 14 different magnetocaloric material families is presented and those are compared in terms of their adiabatic temperature and isothermal entropy change under cycling in magnetic‐field changes of 1 and 2 T, criticality aspects, and the amount of heat that they can transfer per cycle. The work is based on magnetic, direct thermometric, and calorimetric measurements made under similar conditions and in the same devices. Such a wide‐ranging study has not been carried out before. This data sets the basis for more advanced modeling and machine learning approaches in the near future.
Magnetic refrigeration utilizing the magnetocaloric effect of magnetic materials is a promising alternative to conventional gas–vapor technology. In this paper a rigorous assessment of the most relevant magnetocaloric material families is presented and compared in terms of their adiabatic temperature and isothermal entropy change under cycling, criticality aspects, and the amount of heat that they can transfer per cycle.
The ideal magnetocaloric material would lay at the borderline of a first-order and a second-order phase transition. Hence, it is crucial to unambiguously determine the order of phase transitions for ...both applied magnetocaloric research as well as the characterization of other phase change materials. Although Ehrenfest provided a conceptually simple definition of the order of a phase transition, the known techniques for its determination based on magnetic measurements either provide erroneous results for specific cases or require extensive data analysis that depends on subjective appreciations of qualitative features of the data. Here we report a quantitative fingerprint of first-order thermomagnetic phase transitions: the exponent n from field dependence of magnetic entropy change presents a maximum of n > 2 only for first-order thermomagnetic phase transitions. This model-independent parameter allows evaluating the order of phase transition without any subjective interpretations, as we show for different types of materials and for the Bean-Rodbell model.
Solid‐state magnetic refrigeration is a high‐potential, resource‐efficient cooling technology. However, many challenges involving materials science and engineering need to be overcome to achieve an ...industry‐ready technology. Caloric materials with a first‐order transition—associated with a large volume expansion or contraction—appear to be the most promising because of their large adiabatic temperature and isothermal entropy changes. In this study, using experiment and simulation, it is demonstrated with the most promising magnetocaloric candidate materials, La–Fe–Si, Mn–Fe–P–Si, and Ni–Mn–In–Co, that the characteristics of the first‐order transition are fundamentally determined by the evolution of mechanical stresses. This phenomenon is referred to as the stress‐coupling mechanism. Furthermore, its applicability goes beyond magnetocaloric materials, since it describes the first‐order transitions in multicaloric materials as well.
Solid‐state magnetic refrigeration is a high‐potential cooling technology. Materials with a first‐order transition are most promising because of their large adiabatic temperature and isothermal entropy changes. In this study, it is demonstrated with the magnetocaloric materials La–Fe–Si, Mn–Fe–P–Si, and Ni–Mn–In–Co, using experiment and simulation, that the characteristics of the first‐order transition are fundamentally determined by the evolution of mechanical stresses.
The effect of electric current assisted annealing was investigated in amorphous Fe40Ni40B20 melt spun ribbons in terms of manipulating the crystallization temperature and phase formation. A specific ...setup was developed, which allows to measure the sample temperature and the electrical resistance in-situ during the annealing process. By employing these simultaneous measurements, the shift of the crystallization temperature was evaluated by employing different electric current modes and current densities. It was found that the onset temperature for crystallization is shifted to lower temperatures when a certain threshold of electric current density is surpassed. Maximum temperature shifts up to 130°C were achieved using direct and alternate current modes, while in pulsed current mode the maximum shift was 100°C. Even though a noticeable change was observed in the crystallization temperature when an electric current is applied during annealing, the nucleated phases were found to be the same as in the samples annealed without current. The reasons for the observed behavior are discussed and compared to propositions found in the literature. It is concluded that the ‘‘hot spots’’ mechanism is in good agreement with the results obtained in this work.
•In-house built set up was developed to detect crystallization temperature.•Electric current annealing decreases the crystallization temperature of Fe–Ni–B.•Different current density and mode affects the onset crystallization temperature.•The mechanism of the observed behavior is related to creation of “hot spots".
The application of anisotropic parameters in NMR‐spectroscopy enables the acquisition of spatial and angular information, complementary to those from conventional isotropic NMR‐measurements. The use ...of alignment media is a well‐established method for inducing anisotropy. PBPMLG is a recently discovered polyglutamate‐based alignment medium, exhibiting thermoresponsive behavior in the lyotropic liquid crystalline (LLC) phase, thus offering potential for deeper understanding of the alignment process. We present one approach for investigating the thermoresponsive behavior by synthesizing specifically deuterated PBPMLG‐isotopologues and their subsequent analyses using 2H NMR‐spectroscopy. It was possible to relate the observed thermoresponsive behavior to a flip of the polymer with respect to the external magnetic field—an effect never observed before in glutamate‐based polymeric alignment media. Furthermore, a solvent‐induced temperature dependent gelation was verified in THF, which might provide yet another opportunity to manipulate the properties of this alignment medium in the future.
Deuterated isotopologues of a thermoresponsive, glutamate‐based alignment medium have been synthesized and investigated by 2H NMR‐spectroscopy, vibrating sample magnetometry and polarized optical microscopy. The measurements show a flip of the polymeric α‐helix in lyotropic liquid crystalline phase, providing further insights into the alignment process of the polymer inside the magnetic field.
The application of anisotropic parameters in NMR-spectroscopy enables the acquisition of spatial and angular information, complementary to those from conventional isotropic NMR-measurements. The use ...of alignment media is a well-established method for inducing anisotropy. PBPMLG is a recently discovered polyglutamate-based alignment medium, exhibiting thermoresponsive behavior in the lyotropic liquid crystalline (LLC) phase, thus offering potential for deeper understanding of the alignment process. We present one approach for investigating the thermoresponsive behavior by synthesizing specifically deuterated PBPMLG-isotopologues and their subsequent analyses using
H NMR-spectroscopy. It was possible to relate the observed thermoresponsive behavior to a flip of the polymer with respect to the external magnetic field-an effect never observed before in glutamate-based polymeric alignment media. Furthermore, a solvent-induced temperature dependent gelation was verified in THF, which might provide yet another opportunity to manipulate the properties of this alignment medium in the future.
Bulk Mn
52
Al
46
C
2
in
τ
-phase was prepared by vacuum induction melting and used as precursor for the production bulk permanent magnets by suction casting and hot-extrusion. Part of the precursor ...alloy was mechanically milled into a
τ
-phase powder and used as precursor for production of samples by electron beam melting, hot-compaction and high pressure torsion processes. The microstructure and magnetic properties of all samples were investigated and correlated. It was found that the mechanical deformation enhances coercivity, up to 0.58 T, while the absence of this strain is beneficial for magnetization. Among the observed techniques, hot extrusion and high pressure torsion have shown promising possibilities to further develop Mn-Al-C as permanent magnets. However, it should be taken into account the challenges related to design a proper processing window for hot extrusion and the limitation of HPT regarding the absence of texture.
Strong unidirectional anisotropy in bulk polycrystalline B20 FeGe has been measured by ferromagnetic resonance spectroscopy. Such anisotropy is not present in static magnetometry measurements. B20 ...FeGe exhibits inherent Dzyaloshinskii-Moriya interaction, resulting in a nonreciprocal spin-wave dispersion. Bulk and micron sized samples were produced and characterized. By X-band ferromagnetic resonance spectroscopy at 276 K ± 1 K, near the Curie temperature, a distribution of resonance modes was observed in accordance with the cubic anisotropy of FeGe. This distribution exhibits a unidirectional anisotropy, i.e. shift of the resonance field under field inversion, of K
= 960 J/m
± 10 J/m
, previously unknown in bulk ferromagnets. Additionally, more than 25 small amplitude standing spin wave modes were observed inside a micron sized FeGe wedge, measured at 293 K ± 2 K. These modes also exhibit unidirectional anisotropy. This effect, only dynamically measurable and not detectable in static magnetometry measurements, may open new possibilities for directed spin transport in chiral magnetic systems.
The application of anisotropic parameters in NMR‐spectroscopy enables the acquisition of spatial and angular information, complementary to those from conventional isotropic NMR‐measurements. The use ...of alignment media is a well‐established method for inducing anisotropy. PBPMLG is a recently discovered polyglutamate‐based alignment medium, exhibiting thermoresponsive behavior in the lyotropic liquid crystalline (LLC) phase, thus offering potential for deeper understanding of the alignment process. We present one approach for investigating the thermoresponsive behavior by synthesizing specifically deuterated PBPMLG‐isotopologues and their subsequent analyses using 2H NMR‐spectroscopy. It was possible to relate the observed thermoresponsive behavior to a flip of the polymer with respect to the external magnetic field—an effect never observed before in glutamate‐based polymeric alignment media. Furthermore, a solvent‐induced temperature dependent gelation was verified in THF, which might provide yet another opportunity to manipulate the properties of this alignment medium in the future.
Deuterated isotopologues of a thermoresponsive, glutamate‐based alignment medium have been synthesized and investigated by 2H NMR‐spectroscopy, vibrating sample magnetometry and polarized optical microscopy. The measurements show a flip of the polymeric α‐helix in lyotropic liquid crystalline phase, providing further insights into the alignment process of the polymer inside the magnetic field.
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