Loudon James C, Mathur Neil D, Midgley Paul A
Department of Materials Science and Metallurgy, University of Cambridge, UK.
Nature. 2002;420(6917):797-800. doi: 10.1038/nature01299.
Mixed-valent manganites are noted for their unusual magnetic, electronic and structural phase transitions. For example, the La(1-x)Ca(x)MnO(3) phase diagram shows that below transition temperatures in the range 100-260 K, compounds with 0.2 < x < 0.5 are ferromagnetic and metallic, whereas those with 0.5 < x < 0.9 are antiferromagnetic and charge ordered. In a narrow region around x = 0.5, these totally dissimilar ground states are thought to coexist. It has been shown that charge order and charge disorder can coexist in the related compound, La(0.25)Pr(0.375)Ca(0.375)MnO(3). Here we present electron microscopy data for La(0.5)Ca(0.5)MnO(3) that shed light on the distribution of these coexisting phases, and uncover an additional, unexpected phase. Using electron holography and Fresnel imaging, we find micrometre-sized ferromagnetic regions spanning several grains coexisting with similar-sized regions with no local magnetization. Holography shows that the ferromagnetic regions have a local magnetization of 3.4 +/- 0.2 Bohr magnetons per Mn atom (the spin-aligned value is 3.5 micro (B) per Mn). We use electron diffraction and dark-field imaging to show that charge order exists in regions with no net magnetization and, surprisingly, can also occur in ferromagnetic regions.
混合价态的锰氧化物因其异常的磁、电子和结构相变而受到关注。例如，La(1-x)Ca(x)MnO(3)相图表明，在100 - 260 K的转变温度以下，0.2 < x < 0.5的化合物是铁磁性和金属性的，而0.5 < x < 0.9的化合物是反铁磁性和电荷有序的。在x = 0.5附近的一个狭窄区域内，这些完全不同的基态被认为是共存的。已经表明，在相关化合物La(0.25)Pr(0.375)Ca(0.375)MnO(3)中电荷有序和电荷无序可以共存。在此，我们展示了La(0.5)Ca(0.5)MnO(3)的电子显微镜数据，这些数据揭示了这些共存相的分布，并发现了一个额外的、意想不到的相。使用电子全息术和菲涅耳成像，我们发现跨越几个晶粒的微米级铁磁区域与尺寸相似但无局部磁化的区域共存。全息术表明，铁磁区域每个锰原子的局部磁化为3.4 +/- 0.2玻尔磁子（自旋排列值为每个锰3.5 μB）。我们使用电子衍射和暗场成像表明，电荷有序存在于没有净磁化的区域，并且令人惊讶的是，也可以出现在铁磁区域。
