First-order magnetostructural phase transitions are inevitably accompanied by large hysteresis, which evokes non-ignorable energy losses a main challenge for the utilization of giant magnetocaloric effect (MCE) materials in the emerging magnetic cooling technology. In this work, we present a novel approach to reduce the hysteresis and simultaneously to remain the giant entropy change in La-Fe-Si-based MCE alloys by microstructural manipulation. The microstructure evolution is comprehensively studied by high angle annular dark field-scanning transmission electron microscope, three-dimensional atom probe and geometric phase analysis. For the LaFe_13-xSi_x system via co-doping of Ce and H atoms, we have observed the appearance of nanograins in size range of 5 – 50 nm that is totally different from the widely reported compositions. Such refinement can be ascribed to the release of internal stress caused by the inhomogeneous distribution of hydrogen atoms. With the formation of the nanocrystals in (La_1-xCe_x)₂Fe₁₁Si₂H_y alloys, the value of hysteresis loss can be monotonously reduced from 48.3 to 0.6 J kg⁻¹. More importantly, the magnetostructural transition keeps an obvious first-order type, which leads to a large adiabatic temperature change of 2.03 K in 1.3 T upon 10⁵ magnetic cycles, as well as a high reversible refrigeration capacity of 89.4 J kg⁻¹ for (La_0.6Ce_0.4)₂Fe₁₁Si₂H_y.

一阶磁结构相变不可避免地伴随着较大的磁滞现象，这会引起不可忽略的能量损失，这是在新兴的磁冷却技术中利用巨磁热效应（MCE）材料的主要挑战。在这项工作中，我们提出了一种新颖的方法，可通过微结构操纵来减少磁滞并同时保持La-Fe-Si基MCE合金的巨大熵变化。通过高角度环形暗场扫描透射电子显微镜，三维原子探针和几何相分析对微观结构的演变进行了全面的研究。对于LaFe _{13- x} Si _x通过Ce和H原子的共掺杂，我们观察到纳米颗粒的出现在5至50 nm的尺寸范围内，这与广泛报道的成分完全不同。这种改进可以归因于由氢原子的不均匀分布引起的内部应力的释放。随着在（La _{1- x} Ce _x）₂ Fe ₁₁ Si ₂ H _y合金中形成纳米晶体，磁滞损耗值可以从48.3单调降低至0.6 J kg ^-1。更重要的是，磁结构转变保持明显的一阶类型，这导致在10 ^5时1.3 T中绝热温度变化为2.03 K磁循环，以及（La _0.6 Ce _0.4）₂ Fe ₁₁ Si ₂ H _y的89.4 J kg ^-1的高可逆制冷能力。