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Magnetocaloric microwires are very promising for energy-efficient magnetic refrigeration in micro electromechanical systems (MEMS) and nano electromechanical systems (NEMS). Creating microwires that exhibit large magnetocaloric effects around room temperature represents an important but challenging task. Here, we report a tunable giant magnetocaloric effect around room temperature in Mn$_x$Fe$_{2-x}$P$_{0.5}$Si$_{0.5}$ ($x$ = 0.7-1.2) microwires by utilizing a melt-extraction technique paired with thermal treatment and chemical engineering. The isothermal magnetic entropy change DeltaSiso and Curie temperature (TC) can be tuned by adjusting the Mn/Fe ratio. The TC varies from 351 to 190 K as x increases from 0.8 to 1.2. Among the compositions investigated, the x = 0.9 sample shows the largest value of DeltaSiso = 18.3 J kg$^{-1}$ K$^{-1}$ for a field change of 5 T around 300 K. After subtracting magnetic hysteresis loss, a large refrigerant capacity of ~284.6 J kg$^{-1}$ is achieved. Our study paves a new pathway for the design of novel magnetocaloric microwires for active magnetic refrigeration at ambient temperatures.

Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) is a promising alternative to conventional vapor compression refrigeration techniques. The cooling efficiency of a magnetic refrigerator depends on its refrigeration capacity and operation frequency. Existing refrigerators possess limited cooling efficiency due to the low operating frequency (around tens of Hz). Theory predicts that reducing geometrical effects can increase the operation frequency by reducing the relaxation time of a magnetic material. As compared to other shapes, magnetocaloric wires transfer heat most effectively to a surrounding environment, due to their enhanced surface area. The wire shape also yields a good mechanical response, reducing the relaxation time and consequently increasing the operation frequency of the cooling device. Experiments have validated the theoretical predictions. By assembling microwires with different magnetocaloric properties and Curie temperatures into a laminate structure, a table-like magnetocaloric bed can be created and used as an active cooling device for micro-electro-mechanical system (MEMS) and nano-electro-mechanical system (NEMS). This paper assesses recent progress in the development of magnetocaloric microwires and sheds light on the important factors affecting the magnetocaloric behavior and cooling efficiency in microwire systems. Challenges, opportunities, and strategies regarding the development of advanced magnetocaloric microwires are also discussed.

Magnetic skyrmions offer promising prospects for constructing future energy-efficient and high-density information technology, leading to extensive explorations of new skyrmionic materials recently. The topological Hall effect has been widely adopted as a distinctive marker of skyrmion emergence. Alternately, here we propose a novel signature of skyrmion state by quantitatively investigating the magnetoresistance (MR) induced by skyrmionic bubbles in CeMn2Ge2. An intriguing finding was revealed: the anomalous MR measured at different temperatures can be normalized into a single curve, regardless of sample thickness. This behavior can be accurately reproduced by the recent chiral spin textures MR model. Further analysis of the MR anomaly allowed us to quantitatively examine the effective magnetic fields of various scattering channels. Remarkably, the analyses, combined with the Lorentz transmission electronic microscopy results, indicate that the in-plane scattering channel with triplet exchange interactions predominantly governs the magnetotransport in the Bloch-type skyrmionic bubble state. Our results not only provide insights into the quantum correction on MR induced by skyrmionic bubble phase, but also present an electrical probing method for studying chiral spin texture formation, evolution and their topological properties, which opens up exciting possibilities for identifying new skyrmionic materials and advancing the methodology for studying chiral spin textures., Comment: 17 pages,5 figures,submitted

Amorphous Co-rich microwires with excellent soft magnetic and mechanical properties produced by melt-extraction technique are emerging as a multifunctional material for a variety of applications ranging from ultrasensitive magnetic field sensors to structural health self-monitoring composites. There is a pressing need for enhancing these properties to make the microwires practical for integration into new technologies. Conventional heat treatments at temperature below crystallization may improve the magnetic softness of an as-quenched amorphous wire, but usually deteriorate the good mechanical characteristic of the wire due to crystallization. To overcome this, we propose a new approach that utilizes the advantages of a multi-step Joule current annealing method to design novel (nanocrystal, amorphous)/amorphous core/shell structures directly from as-quenched amorphous microwires. These results show that the density and size of nanocrystals in the core can be optimized by controlling the Joule current intensity, resulting in the large enhancement of soft magnetic and giant magneto-impedance properties, while the amorphous shell preserves the excellent mechanical strength of the microwire. This study also provides a new pathway for the design of novel core/shell structures directly from rapidly quenched amorphous magnetic materials that are currently exploited in high frequency transformers, sensing and cooling devices.

The polycrystalline Mn1.26Fe0.60P0.48Si0.52 microwires were successfully fabricated for the first time by the melt-extraction technique, and their magnetic and magnetocaloric properties were investigated systematically. The structural analysis shows that the microwires possess a hexagonal phase with Fe2P type, with a homogeneous composition distribution. Magnetometry measurements show that the microwires undergo a weak first-order magnetic phase transition at a temperature of 142 K. The maximum magnetic entropy change of the microwires reaches 4.64 Jkg-1K-1 for a field change of 5 T. These low-cost Mn1.26Fe0.60P0.48Si0.52 microwires are promising for active magnetic refrigeration in the liquid nitrogen temperature range.

We have designed a new magnetic bed structure with desirable table-like magnetocaloric effect (MCE) by using three kinds of soft ferromagnetic Gd-Al-Co microwire arrays with different Curie temperatures ($T_C$). The $T_C$ interval of these three wires is ~10 K and the designed new structure named Sample A. This sample shows a smooth table-like magnetic entropy change ($\Delta S_M$) at high applied field change ($\mu_0 \Delta H=5 T$) ranging from ~92 K to ~107 K. The maximum entropy change ($-\Delta S_M^{\rm max}$) and refrigerant capacity (RC) for Sample A at $\mu_0 \Delta H=5 T$ are calculated to be ~9.42 Jkg$^{-1}$K$^{-1}$ and ~676 Jkg$^{-1}$. The calculated curves of $-\Delta S_M(T)$ and the corresponding experimental data match well with each other, suggesting that the desirable magnetocaloric properties of the microwire arrays can be designed. Simulation shows that the RC values of the designed systems increase when increasing the interval of $T_C$. The table-like MCE and the enhanced heat-transfer efficiency due to the enhanced surface areas of the microwires make this newly designed magnetic bed very promising for use in energy-efficient magnetic refrigerators.