  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.
