Search Results (11)

Heusler alloy research has increased considerably in recent years. This is mostly due to their strong desire to develop future smart device applications. However, many limiting variables remain for researchers to overcome in order to enhance their functional properties. The poor mechanical properties of these alloys restrict their use as solid-state cooling materials in magnetic refrigeration devices. A promising strategy, resulting in novel compounds with better mechanical properties and substantial magnetocaloric effects, is favoring the d–d hybridization with transition-metal elements to replace p–d hybridization. The term given to these materials is “all-d-metal”. In light of recent experimental results of the magnetocaloric effect and the increased mechanical characteristics in these alloys (with complex crystallographic behavior due to off-stoichiometry and disorder), a review of this advanced functional behavior is offered. Moreover, the impact of the substitution of transition metal for the p-group to increase mechanical ductility and considerable magnetocaloric effects has also been addressed. These Heusler alloys are a potential new class of materials for technological applications because of their optimum functional behavior. Finally, we highlighted the potential challenges and unsolved issues in order to guide future studies on this topic. Full article

The magnetic and electrical characteristics of Ni-Mn quinary Heusler alloys are studied in the current work. The results concern the materials’ magnetic and electrical behavior. The physical property measurement system (PPMS) and superconducting quantum interference device (SQUID) were used at various magnetization levels to determine the results. The addition of Fe helps to form the alloy into a smart memory alloy with magnetocrystalline anisotropy, twin border mobility, and varied magnetic and martensite transition temperature characteristics. Character changes in the superparamagnetic (SPM) and paramagnetic (PM) alloys occur between 26 and 34 °C. The curves are supported by the alloy’s martensitic transition temperature change. A large refrigeration capacity is identified in the alloy. These properties are an indication of the alloys’ application prospects. Entropy change helps to detect the inverse magnetocaloric effect in the alloy, whereas adiabatic temperature change helps identify the origin and validity of reverse magnetic properties. The transition temperature changes occur when austenite’s sigma is larger than that of martensite, and as the magnetic field increases, the temperature declines. Isothermal magnetization curves, a large (MR)/B value at low and high magnetic fields, and temperatures near the transformation point suggest that small-crystal Heusler alloys have tremendous promise for low and high magnetic field magnetoresistance applications. Full article

Iron-transition metal-based binary and ternary alloys have attracted great attention due to their relevant mechanical, electrical, and magnetic properties. In this paper, we systematically investigate the structural, magnetic, and magnetocaloric behavior of as-milled Fe₆₅T₃₅ (T = Ni and Mn) alloy. The polycrystalline alloys were produced by the planetary ball milling, using a powder-to-ball ratio of 1:3. A structural study reveals that both Fe₆₅Ni₃₅ and Fe₆₅Mn₃₅ compounds have stabilized in α and γ mixed phase within the cubic crystal structure. The alloyed compounds are further characterized by high-resolution field emission scanning electron microscopy (HR-FESEM), which confirms the mixing of both metals in the alloying process. Temperature-dependent magnetic studies do not show any blocking in zero-field-cooled and field-cooled results; however, the field-dependent magnetization study demonstrates the ferromagnetic nature with small hysteresis in both compounds. Both compounds show a significant magnetocaloric effect over a wide temperature range around room temperature. Fe₆₅Ni₃₅ exhibit a slightly higher value in comparison to Fe₆₅Mn₃₅. In both the alloys, magnetic entropy change follows the power law behavior against the external magnetic field, and the value of exponent ‘m’ explains the presence of magnetic correlation. Our investigation in this study communicates that the phase control or coexistence of both phases may be efficacious in obtaining the desirable characteristic of magnetic and magnetocaloric demeanors in such a binary Fe-T alloy. Full article

The effects of element substitution and element doping on the magnetic properties and magnetocaloric effect of the LaFe_11.5Si_1.5 compound were investigated. The crystals of the LaFe_11.5Si_1.5, La_0.8Nd_0.2Fe_11.5Si_1.5, and LaFe_11.5Si_1.5C_0.2 compounds all showed cubic NaZn₁₃-type structures, but the lattice of the La_0.8Nd_0.2Fe_11.5Si_1.5 shrank and the lattice of the LaFe_11.5Si_1.5C_0.2 expanded. All three compounds had the characteristic of first-order magnetic transition due to the obvious itinerant-electron metamagnetic (IEM) transition occurring above Curie temperature (T_C). For the LaFe_11.5Si_1.5, La_0.8Nd_0.2Fe_11.5Si_1.5, and LaFe_11.5Si_1.5C_0.2 compounds, the T_C were approximately 194 K, 188 K, and 232 K, respectively. Meanwhile, the maximum magnetic entropy changes (−ΔS_M) under a magnetic field change of 0–3 T were approximately 18.7 J/kg·K, 22.8 J/kg·K, and 16.4 J/kg·K, respectively. The T_C was mainly affected by the lattice constant. Furthermore, the −ΔS_M was mainly affected by the latent heat of the first-order magnetic transition. Full article

In this paper, we report on a successful synthesis of dysprosium iron garnet Dy₃Fe₅O₁₂ (DyIG) by a reactive synthesis method involving dysprosium iron perovskite and hematite. Phase formation was traced using dilatometry, and XRD measurements attested to the formation of the desired structure. Samples with relative density close to 97% were fabricated. The samples were characterized using vibrating sample magnetometry, dielectric spectroscopy, and UV-Vis-NIR spectroscopy. Magnetic properties were probed in temperatures between 80 and 700 K with a maximum applied field of 1 kOe. The measurements revealed several effects: the compensation of magnetic moments at a certain temperature, the inversion of the magnetocaloric effect, and the ability to measure the Curie temperature of the material. Activation energy was determined from UV-Vis-NIR and dielectric spectroscopy measurements. Characteristic magnetic temperatures and activation energy values of the samples were similar to bulk DyIG obtained using other methods. Full article

It is well recognized that intermetallics based on rare-earth (R) and transition metals (T) display numerous interesting magnetic properties, leading to potential applications in different fields. The latest progress regarding magnetic properties and the magnetocaloric effect (MCE) in the nanostructured Pr${}_2$Co${}_7$ compound, as well as its carbides and hydrides, is reviewed in this paper. Some of this progress reveals remarkable MCE performance, which makes it attractive in the field of magnetic refrigeration at high temperatures. With the purpose of understanding the magnetic and magnetocaloric characteristics of these compounds, the crystal structure, microstructure, and magnetism are also brought into focus. The Pr${}_2$Co${}_7$ compound has interesting magnetic properties, such as a high Curie temperature T${}_C$ and uniaxial magnetocrystalline anisotropy. It crystallizes in a hexagonal structure (2:7 H) of the Ce${}_2$Ni${}_7$ type and is stable at relatively low temperatures (T${}_a$ ≤ 1023 K), or it has a rhombohedral structure (2:7 R) of the Gd${}_2$Co${}_7$ type and is stable at high temperatures (T${}_a$ ≥ 1223 K). Studies of the magnetocaloric properties of the nanocrystalline Pr${}_2$Co${}_7$ compound have shown the existence of a large reversible magnetic entropy change ($\Delta$S${}_M$) with a second-order magnetic transition. After its substitution, we showed that nanocrystalline Pr${}_2$Co${}_{7-x}$Fe${}_x$ compounds that were annealed at T${}_a$ = 973 K crystallized in the 2:7 H structure similarly to the parent compound. The extended X-ray absorption fine-structure (EXAFS) spectra adjustments showed that Fe atoms preferably occupy the 12k site for x ≤ 1. The study of the magnetic properties of nanocrystalline Pr${}_2$Co${}_{7-x}$Fe${}_x$ compounds revealed an increase in T${}_C$ of about 26% for x = 0.5, as well as an improvement in the coercivity, H${}_c$ (12 kOe), and the (BH)${}_{\max }$ (9 MGOe) product. On the other hand, the insertion of C atoms into the Pr${}_2$Co${}_7$ cell led to a marked improvement in the T${}_C$ value of 21.6%. The best magnetic properties were found for the Pr${}_2$Co${}_7$C${}_{0.25}$ compound annealed at 973 K, H${}_c$ = 10.3 kOe, and (BH)${}_{\max }$ = 11.5 MGOe. We studied the microstructure, hydrogenation, and magnetic properties of nanocrystalline Pr${}_2$Co${}_7$H${}_x$ hydrides. The crystal structure of the Pr${}_2$Co${}_7$ compound transformed from a hexagonal (P6₃/mmc) into an orthorhombic (Pbcn) and monoclinic (C2/c) structure during hydrogenation. The absorption of H by the Pr${}_2$Co${}_7$ compound led to an increase in the T${}_C$ value from 600 K at x = 0 to 691 K at x = 3.75. The Pr${}_2$Co${}_7$H${}_{0.25}$ hydride had optimal magnetic properties: H${}_c$ = 6.1 KOe, (BH)${}_{\max }$ = 5.8 MGOe, and T${}_C$ = 607 K. We tailored the mean field theory (MFT) and random magnetic anisotropy (RMA) methods to investigate the magnetic moments, exchange interactions, and magnetic anisotropy properties. The relationship between the microstructure and magnetic properties is discussed. The obtained results provide a fundamental reference for adapting the magnetic properties of the Pr${}_2$Co${}_7$, Pr${}_2$Co${}_{6.5}$Fe${}_{0.5}$, Pr${}_2$Co${}_7$C${}_{0.25}$, and Pr${}_2$Co${}_7$H${}_{0.25}$ compounds for potential permanent nanomagnets, high-density magnetic recording, and magnetic refrigeration applications. Full article

Magnetic refrigeration is a fascinating superior choice technology as compared with traditional refrigeration that relies on a unique property of particular materials, known as the magnetocaloric effect (MCE). This paper provides a thorough understanding of different magnetic refrigeration technologies using a variety of models to evaluate the coefficient of performance (COP) and specific cooling capacity outputs. Accordingly, magnetic refrigeration models are divided into four categories: rotating, reciprocating, C-shaped magnetic refrigeration, and active magnetic regenerator. The working principles of these models were described, and their outputs were extracted and compared. Furthermore, the influence of the magnetocaloric effect, the magnetization area, and the thermodynamic processes and cycles on the efficiency of magnetic refrigeration was investigated and discussed to achieve a maximum cooling capacity. The classes of magnetocaloric magnetic materials were summarized from previous studies and their potential magnetic characteristics are emphasized. The essential characteristics of magnetic refrigeration systems are highlighted to determine the significant advantages, difficulties, drawbacks, and feasibility analyses of these systems. Moreover, a cost analysis was provided in order to judge the feasibility of these systems for commercial use. Full article

In the present work, the microstructure and its effect on the magnetic, magnetocaloric, and magnetoelastic properties of the Tb₅₅Co₃₀Fe₁₅ melt-spun ribbon were investigated. The ribbon exhibits typical amorphous characteristics in its X-ray diffraction examination and differential scanning calorimetry measurement. However, the magnetic properties of the ribbon indicate that the ribbon is inhomogeneous in the nanoscale, as ascertained by a high-resolution electron microscope. Compared to the Tb₅₅Co₄₅ amorphous alloy, the Tb₅₅Co₃₀Fe₁₅ ribbon shows poor magnetocaloric properties but outstanding magnetostriction. A rather high value of reversible magnetostriction up to 788 ppm under 5 T was obtained. The mechanism for the formation of nanoparticles and its effect on the magnetocaloric and magnetostrictive properties were investigated. Full article

With the purpose to optimize the functional properties of Heusler alloys for their use in solid-state refrigeration, the characteristics of the martensitic and magnetic transitions undergone by Ni₅₀Mn_25−xGa₂₅Cu_x (x = 3–11) alloys have been studied. The results reveal that, for a Cu content of x = 5.5–7.5, a magnetostructural transition between paramagnetic austenite and ferromagnetic martensite takes place. In such a case, magnetic field and stress act in the same sense, lowering the critical combined fields to induce the transformation; moreover, magnetocaloric and elastocaloric effects are both direct, suggesting the use of combined fields to improve the overall refrigeration capacity of the alloy. Within this range of compositions, the measured transformation entropy is increased owing to the magnetic contribution to entropy, showing a maximum at composition x = 6, in which the magnetization jump at the transformation is the largest of the set. At the same time, the temperature hysteresis of the transformation displays a minimum at x = 6, attributed to the optimal lattice compatibility between austenite and martensite. We show that, among this system, the optimal caloric performance is found for the x = 6 composition, which displays high isothermal entropy changes (−36 J·kg⁻¹·K⁻¹ under 5 T and −8.5 J·kg⁻¹·K⁻¹ under 50 MPa), suitable working temperature (300 K), and low thermal hysteresis (3 K). Full article

The series of Mn_2−xFe_xP_1−ySi_y types of compounds form one of the most promising families of magnetocaloric materials in term of performances and availability of the elemental components. Potential for large scale application needs to optimize the synthesis process, and an easy and rather fast process here described is based on the use of two main type of precursors, providing the Fe-P and Mn-Si proportions. The series of Mn_2−xFe_xP_1−ySi_y compounds were synthesized and carefully investigated for their crystal structure versus temperature and compared interestingly with earlier results. A strong magnetoelastic effect accompanying the 1st order magnetic transition—as well as the parent phosphide–arsenides—was related to the relative stability of both the Fe magnetic polarization and the Fe–Fe exchange couplings. In order to better understand this effect, we propose a local distortion index of the non-metal tetrahedron hosting Fe atoms. Besides, from Mn-rich (Si-rich) to Fe-rich (P-rich) compositions, it is shown that the magnetocaloric phenomenon can be established on demand below and above room temperature. Excellent performance compounds were realized in terms of magnetic entropy ΔS_m and adiabatic temperature ΔT_ad variations. Since from literature it was seen that the magnetic performances are very sensitive to the synthesis process, correspondingly here a new effective process is proposed. Mössbauer spectroscopy analysis was performed on Mn-rich, equi-atomic Mn-Fe, and Fe-rich compounds, allowing determination of the distribution of hyperfine fields setting on Fe in the tetrahedral and pyramidal sites, respectively. Electronic structure calculations confirmed the scheme of metal and non-metal preferential ordering, respectively. Moreover, the local magnetic moments were derived, in fair agreement with both the experimental magnetization and the Fe contributions, as determined by Mössbauer spectroscopy. Full article

Octacyanometallate-based compounds displaying a rich pallet of interesting physical and chemical properties, are key materials in the field of molecular magnetism. The [M(CN)₈]ⁿ⁻ complexes, (M = W^V, Mo^V, Nb^IV), are universal building blocks as they lead to various spatial structures, depending on the surrounding ligands and the choice of the metal ion. One of the functionalities of the octacyanometallate-based coordination polymers or clusters is the magnetocaloric effect (MCE), consisting in a change of the material temperature upon the application of a magnetic field. In this review, we focus on different approaches to MCE investigation. We present examples of magnetic entropy change ΔS_m and adiabatic temperature change ΔT_ad, determined using calorimetric measurements supplemented with the algebraic extrapolation of the data down to 0 K. At the field change of 5T, the compound built of high spin clusters Ni₉[W(CN)₈]₆ showed a maximum value of −ΔS_m equal to 18.38 J·K⁻¹ mol⁻¹ at 4.3 K, while the corresponding maximum ΔT_ad = 4.6 K was attained at 2.2 K. These values revealed that this molecular material may be treated as a possible candidate for cryogenic magnetic cooling. Values obtained for ferrimagnetic polymers at temperatures close to their magnetic ordering temperatures, T_c, were lower, i.e., −ΔS_m = 6.83 J·K⁻¹ mol⁻¹ (ΔT_ad = 1.42 K) and −ΔS_m = 4.9 J·K⁻¹ mol⁻¹ (ΔT_ad = 2 K) for {[Mn^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n and{[Fe^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n, respectively. MCE results have been obtained also for other -[Nb(CN)₈]-based manganese polymers, showing significant T_c dependence on pressure or the remarkable magnetic sponge behaviour. Using the data obtained for compounds with different T_c, due to dissimilar ligands or other phase of the material, the ΔS_m ~ T_c^−2/3 relation stemming from the molecular field theory was confirmed. The characteristic index n in the ΔS_m ~ ΔHⁿ dependence, and the critical exponents, related to n, were determined, pointing to the 3D Heisenberg model as the most adequate for the description of these particular compounds. At last, results of the rotating magnetocaloric effect (RMCE), which is a new technique efficient in the case of layered magnetic systems, are presented. Data have been obtained and discussed for single crystals of two 2D molecular magnets: ferrimagnetic {Mn^II(R-mpm)₂]₂[Nb^IV(CN)₈]}∙4H₂O (mpm = α-methyl-2-pyridinemethanol) and a strongly anisotropic (tetren)Cu₄[W(CN)₈]₄ bilayered magnet showing the topological Berezinskii-Kosterlitz-Thouless transition. Full article