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Giant negative thermal expansion in magnetic nanocrystals


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 ZrW2O8 (ref. 1) and the framework material ZnxCd1−x(CN)2 (refs 2,3). This unusual behaviour can be understood in terms of low-energy phonons1,2,3,4,5,6, while the colossal values of both positive and negative thermal expansion recently observed in another framework material, Ag3[Co(CN)6], have been explained in terms of the geometric flexibility of its metal–cyanide–metal linkages7. Thermal expansion can also be stopped in some magnetic transition metal alloys below their magnetic ordering temperature, a phenomenon known as the Invar effect8,9, 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 ZrW2O8) and MnF2 is a general property of nanoparticles in which there is strong coupling between magnetism and the crystal lattice.

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Figure 1: Evidence for the crystalline quality of the nanoparticles.
Figure 2: X-ray diffraction data for nanocrystals of CuO.
Figure 3: NTE in nanocrystals of CuO and MnF2, but not NiO.


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The authors would like to thank W. J. Moon and E. Tanaka at the High Voltage Electron Microscopy Laboratory, Kyushu University, for taking the electron micrographs of the nanoparticles.

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X.G.Z. conceived and designed the experiments. H.K., H.Y., X.G.Z. and K.K. performed the experiments. H.K. and X.G.Z. analysed the data. H.Y., C.N.X. and Y.I. contributed NiO and analysis tools. X.G.Z. wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to X. G. Zheng.

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Zheng, X., Kubozono, H., Yamada, H. et al. Giant negative thermal expansion in magnetic nanocrystals. Nature Nanotech 3, 724–726 (2008).

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