$Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$马氏体相的相分离和超顺磁性
Phase Separation and Superparamagnetism in the Martensitic Phase of $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$
摘要：
$5 \le x \le 10$范围内的$Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$形状记忆合金显示出应用所需的性能以及迷人的磁性。这些非化学计量的Heusler合金在300 - 400 K的温度$T_{M}$下经历从铁磁性（F）到非铁磁性的马氏体相变，具有异常低的热滞后和大的磁化变化。这种合金的马氏体相的低温磁结构是明显不均匀的，引起了人们极大的兴趣，但还没有很好地理解。我们目前使用的自旋回波核磁共振，在大超精细领域在$^{55}Mn$网站，提供了令人信服的证据表明，纳米级的磁性相分离到F和反铁磁（AF）地区发生以下$T_{M}$在合金与x在0至7。在有限的Co替代的F区域被发现是两种不同的类型，对应于高和低的本地浓度的Co Ni网站。F和AF nanoregions的尺寸分布的估计已经作出。在x = 7时，AF成分不是长程有序的，甚至低于4 K，并且与x = 0时的AF成分完全不同;到x = 14时，F相完全占主导地位。特别感兴趣的是，我们发现，对于x = 7，场冷却导致AF区域的急剧变化。这些发现提供了对这些复杂合金中磁相分离和超顺磁性起源的深入了解，特别是它们的固有交换偏置，这是当前相当感兴趣的。
英文摘要：
$Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ shape memory alloys in the approximate range $5 \le x \le 10$ display desirable properties for applications as well as intriguing magnetism. These off-stoichiometric Heusler alloys undergo a martensitic phase transformation at a temperature $T_{M}$ of 300 - 400 K, from ferromagnetic (F) to nonferromagnetic, with unusually low thermal hysteresis and a large change in magnetization. The low temperature magnetic structures in the martensitic phase of such alloys, which are distinctly inhomogeneous, are of great interest but are not well understood. Our present use of spin echo NMR, in the large hyperfine fields at $^{55}Mn$ sites, provides compelling evidence that nanoscale magnetic phase separation into F and antiferromagnetic (AF) regions occurs below $T_{M}$ in alloys with x in the range 0 to 7. At finite Co substitution the F regions are found to be of two distinct types, corresponding to high and low local concentrations of Co on Ni sites. Estimates of the size distributions of both the F and AF nanoregions have been made. At x = 7 the AF component is not long-range ordered, even below 4 K, and is quite different to the AF component found at x = 0; by x = 14 the F phase is completely dominant. Of particular interest, we find, for x = 7, that field cooling leads to dramatic changes in the AF regions. These findings provide insight into the origins of magnetic phase separation and superparamagnetism in these complex alloys, particularly their intrinsic exchange bias, which is of considerable current interest.
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