亚铁磁Heusler合金Mn2CoGa和Mn2CoAl掺杂Cr,Fe和Co的局域铁磁结构

通过实验和计算的方法研究了Mn₂CoM_xGa_1-x 和Mn₂CoM_xAl_1-x (M=Cr, Fe, Co)掺杂系列合金样品. 研究发现, 在共价作用的影响下, Fe和Co原子占A位, 使被取代的MnA (-2.1 μ_B)变成MnD (3.2 μ_B), 在最近邻的强交换作用下亚铁磁基体中形成了MnB-CoC-MnD局域铁磁性结构, 使分子磁矩的增量最高可达6.18 μ_B. Fe, Co 掺杂后建立同样的局域铁磁结构, 居里温度的变化趋势却不同. 实验观察到Mn₂Co_1+xAl_1-x中掺杂容忍度高达x=0.64, 远高于在Mn₂CoGa中(x=0.36)的结果; 以及随着Al的减少, 合金由B2有序向A2混乱转变等现象, 为共价作用对合金结构稳定的影响提供了证据. 磁测量中发现Cr掺杂后磁矩增量高达3.65 μ_B以及居里温度快速上升的反常现象, 意味着对占位规则的违背.

Local ferromagnetic structure in Heusler alloy Mn2CoGa and Mn2CoAl doped by Cr, Fe and Co

The crystal structures and magnetic properties of Mn₂CoM_xGa_1-x and Mn₂CoM_xAl_1-x (M=Cr, Fe, Co) alloys are investigated through experiment and calculation. Due to the covalent effect, the doped Fe and Co atoms preferentially occupy the A sites. It causes that some MnA (-2.1 μ_B) atoms become MnD (3.2 μ_B) and a local ferromagnetic structure of MnB-CoC-MnD is generated in the ferrimagnetic matrix, showing that an increment of molecular moment is as high as 6.18 μ_B. The achievement of the ferromagnetic structure consumes the exchange interaction energy, consequently, reducing the T_C in Fe doping alloys. It is found that the toleration for doping Co in Mn₂CoAl reaches up to x=0.64, much more than that in Mn₂CoGa (x=0.36), and the change from ordered B2 to A2 structures along with the decrease of Al content. These observations reveal the importance of the covalent effect in these intermetallic compounds. The Cr doping shows an abnormal increment of molecular moment of 3.65 μ_B and increases the T_C rapidly, which implies that Cr atoms may take an atomic configuration thereby disobeying to the occupation rule.