Charge ordered ferromagnetic phase in La0.5Ca0.5MnO3

Abstract

Mixed valent manganites are noted for their unusual magnetic,electronic and structural phase transitions. The La1-xCaxMnO3 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 states are thought to coexist. Uehara et al. have shown that charge order and charge disorder can coexist in the related compound La0.25Pr0.375Ca0.375MnO3. Here, we present electron microscopy data for La0.5Ca0.5MnO3 that sheds light on the distribution of coexisting phases and uncovers a novel and unexpected phase. Using electron holography and Fresnel imaging, we find micron sized ferromagnetic regions spanning several grains coexisting with similar sized regions with no local magnetisation. Holography shows that the ferromagnetic regions have a local magnetisation of 3.4 +- 0.2 mB/Mn (the spin aligned value is 3.5 mB/Mn). We use electron diffraction and dark field imaging to show that charge order exists in regions with no net magnetisation and, surprisingly, can also occur in ferromagnetic regions.