Giant negative thermal expansion in magnetic nanocrystals
Giant negative thermal expansion in magnetic nanocrystals
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DOI：
10.1038/nnano.2008.309
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发表时间：
2008-12-01
影响因子：
38.3
通讯作者：
Xu, C. N.
中科院分区：
文献类型：
--
作者：
Zheng, X. G.;Kubozono, H.;Xu, C. N.
大多数固体在受热时会膨胀，但在一些材料中观察到了一种被称为负热膨胀的性质，包括氧化物ZrW2O-8(参考文献1)。1)和骨架材料ZnxCd1-x(CN)(2)(参考文献2，3)。这种不寻常的行为可以用低能声子(1-6)来解释，而最近在另一种骨架材料Ag-3[Co(CN)(6)]中观察到的正和负热膨胀的巨大值，则是用其金属-氰化物-金属键(7)的几何柔性来解释的。一些磁性过渡金属合金的热膨胀也可以在其磁序温度以下停止，这种现象被称为因瓦效应(8，9)，在工业应用中开发具有可调正或负热膨胀的材料的可能性导致了对因瓦效应和负热膨胀的强烈兴趣。本文报道了三种磁性纳米晶CuO、MnF2和NiO的热膨胀实验结果，发现CuO和MnF2的热膨胀都低于其磁序温度，但NiO没有。较大的CuO和MnF2颗粒也表现出显著的磁致伸缩(即它们随着外加磁场的变化而改变形状)，这导致热膨胀显著降低到它们的磁有序温度以下；这种行为在NiO中没有观察到。我们认为，CuO和MnF_2中的负热膨胀效应是磁性与晶格强耦合的纳米粒子的普遍性质。
Most solids expand when they are heated, but a property known as negative thermal expansion has been observed in a number of materials, including the oxide ZrW2 O-8 (ref. 1) and the framework material ZnxCd1-x(CN)(2) (refs 2,3). This unusual behaviour can be understood in terms of low-energy phonons(1-6), while the colossal values of both positive and negative thermal expansion recently observed in another framework material, Ag-3[Co(CN)(6)], have been explained in terms of the geometric flexibility of its metal-cyanide-metal linkages(7). Thermal expansion can also be stopped in some magnetic transition metal alloys below their magnetic ordering temperature, a phenomenon known as the Invar effect(8,9), and the possibility of exploiting materials with tuneable positive or negative thermal expansion in industrial applications has led to intense interest in both the Invar effect and negative thermal expansion. Here we report the results of thermal expansion experiments on three magnetic nanocrystals-CuO, MnF2 and NiO-and find evidence for negative thermal expansion in both CuO and MnF2 below their magnetic ordering temperatures, but not in NiO. Larger particles of CuO and MnF2 also show prominent magnetostriction ( that is, they change shape in response to an applied magnetic field), which results in significantly reduced thermal expansion below their magnetic ordering temperatures; this behaviour is not observed in NiO. We propose that the negative thermal expansion effect in CuO ( which is four times larger than that observed in ZrW2 O-8) and MnF2 is a general property of nanoparticles in which there is strong coupling between magnetism and the crystal lattice.