Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases, but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.

Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases

MoS2 decorated on one-dimensional MgFe2O4/MgO/C composites for high-performance microwave absorption.

Fabrication of one-dimensional ZnFe2O4@carbon@MoS2/FeS2 composites as electromagnetic wave absorber.

Hysteresis is more than just an interesting oddity, which occurs in materials with a first-order transition. It is a real obstacle on the path from existing lab-scale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and therefore it is necessary to understand the mechanisms causing hysteresis and to find solutions how to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss fundamental aspects, which can contribute to thermal hysteresis and we are developing strategies for at least partially overcoming the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. Doing so, we refer to the most relevant classes of magnetic refrigerants La-Fe-Si-, Heusler- and Fe2P-type compounds.

Mastering hysteresis in magnetocaloric materials

Large and sensitive magnetostriction (large strain induced by small magnetic fields) is highly desired for applications of magnetostrictive materials. However, it is difficult to simultaneously improve magnetostriction and reduce the switching field because magnetostriction and the switching field are both proportional to the magnetocrystalline anisotropy. To solve this fundamental challenge, we report that introducing tetragonal nanoprecipitates into a cubic matrix can facilitate large and sensitive magnetostriction even in random polycrystals. As exhibited in a proof-of-principle reference, Fe–Ga alloys, the figure of merit—defined by the saturation magnetostriction over the magnetocrystalline anisotropy constant—can be enhanced by over 5-fold through optimum aging of the solution-treated precursor. On the one hand, the aging-induced nanodispersive face-centered tetragonal (FCT) precipitates create local tetragonal distortion of the body-centered cubic (BCC) matrix, substantially enhancing the saturation magnetostriction to be comparable to that of single crystal materials. On the other hand, these precipitates randomly couple with the matrix at the nanoscale, resulting in the collapse of net magnetocrystalline anisotropy. Our findings not only provide a simple and feasible approach to enhance the magnetostriction performance of random polycrystalline ferromagnets but also provide important insights toward understanding the mechanism of heterogeneous magnetostriction. The performance of materials that convert magnetic to mechanical energy and vice versa has been improved by introducing nanoprecipitates into their structure. Magnetostrictive materials can expand or contract under the influence of an external magnetic field. This property makes them useful for low-power sensors and energy harvesting. Simultaneously optimizing both the size of the magnetostrictive effect and the material’s sensitivity to small magnetic fields is difficult because increasing one effect tends to decrease the other. Tianyu Ma, Xi’an Jiaotong University, China, and Xiaobing Ren, National Institute for Materials Science, Tsukuba, Japan, and co-workers developed an approach to solve this problem by introducing tiny imperfections into a ferromagnetic alloy. By changing the atomic structure of small regions within the alloy iron−gallium, the researchers were able to achieve a five-fold improvement in the material’s magnetostrictive performance. a Aging-time dependent saturation magnetostriction for Fe73Ga27 random polycrystals. Bright-field images for the b 1373 K-quenched, c 1 h-aged and d 12 h-aged Fe73Ga27 random polycrystals. e Figure of merit as a function of aging time for Fe81Ga19 random polycrystals. f Comparison of saturation magnetostriction among our optimally aged Fe–Ga random polycrystals, the quenched random polycrystals doped with a third element and the single crystals.

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Large and sensitive magnetostriction in ferromagnetic composites with nanodispersive precipitates

Formation mechanism of tetragonal nanoprecipitates in Fe–Ga alloys that dominate the material's large magnetostriction

Cell-boundary-structure controlled magnetic-domain-wall-pinning in 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

Enhancing reversible entropy change of all-d-metal Ni37.5Co12.5Mn35Ti15 alloy by multiple external fields

Revisiting the pinning sites in 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

The caloric effects under combined applications of magnetic field and hydrostatic pressure to a MnCoSi meta-magnet were investigated. Under a magnetic field change of 0–5 T, the maximum magnetic entropy change was enhanced by 35.7% when a 3.2kbar hydrostatic pressure was applied, and the cooling temperature span was extended by 60 K when a hydrostatic pressure of 9.7 kbar was applied. The coupled caloric entropy change, which originates from the coupling between the magnetism and volume, was calculated and accounted for the enhanced entropy change of MnCoSi. The present work facilitates the use of MnCoSi as a solid-state refrigerant and also enriches the investigation of the multicaloric effect under multiple external fields.

Yao Liu

Papers

Formation mechanism of tetragonal nanoprecipitates in Fe–Ga alloys that dominate the material's large magnetostriction

Cell-boundary-structure controlled magnetic-domain-wall-pinning in 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

Enhancing reversible entropy change of all-d-metal Ni37.5Co12.5Mn35Ti15 alloy by multiple external fields

Revisiting the pinning sites in 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

Strengthened caloric effect in MnCoSi under combined applications of magnetic field and hydrostatic pressure