Surface Ferromagnetism in Pr0.5Ca0.5MnO3 Nanoparticles as a Consequence of Local Imbalance in Mn3+:Mn4+ Ratio,Chemistry of Materials

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Surface Ferromagnetism in Pr0.5Ca0.5MnO3 Nanoparticles as a Consequence of Local Imbalance in Mn3+:Mn4+ Ratio
Chemistry of Materials ( IF 7.2 ) Pub Date : 2018-09-24 00:00:00 , DOI: 10.1021/acs.chemmater.8b03070
Haruna D. Aliyu _{1,

2} , Jose M. Alonso _{1,

3} , Patricia de la Presa _{1,

4} , Walmir E. Pottker ₁ , Benedict Ita ₂ , Mar Garcia-Hernández ₃ , Antonio Hernando _{1,

4}

Affiliation

Half-doped praseodymium manganites, Pr_0.5Ca_0.5MnO₃, synthesized by the sol–gel method produce highly crystalline nanoparticles with no structural disorder at the surface. As for most half-doped (Mn³⁺:Mn⁴⁺ = 1) antiferromagnetic (AFM) charge-ordered manganites, ferromagnetism (FM) appears as particle size decreases. A possible origin of FM phase development is the lacking oxygen ligands at the surface that increases Mn³⁺ at the expense of Mn⁴⁺ in order to maintain electronic neutrality. In this work, we show that Mn³⁺:Mn⁴⁺ = 1 ratio is preserved in the whole crystalline particle with the exception of the surface, where this ratio changes to Mn³⁺:Mn⁴⁺ > 1. This Mn³⁺ excess makes a double exchange Mn³⁺ → O^2– → Mn⁴⁺ prevail over the superexchange, giving rise to the FM interactions. We show that a shell thickness of only one unit cell, with a_cell = 0.38 nm, is enough to explain the onset of FM at the surface, whereas the volume remains AFM. Furthermore, the FM to AFM ratio fits to the increase of surface-to-volume ratio with decreasing particle size.

中文翻译：

Pr _0.5 Ca _0.5 MnO ₃纳米颗粒中的表面铁磁作为Mn ³⁺：Mn ⁴⁺比例局部失衡的结果

通过溶胶-凝胶法合成的半掺杂锰Pr _0.5 Ca _0.5 MnO ₃产生了高度结晶的纳米颗粒，表面没有结构紊乱。对于大多数半掺杂（Mn ³⁺：Mn ⁴⁺ = 1）反铁磁（AFM）电荷排序的锰铁矿，随着颗粒尺寸的减小，铁磁性（FM）出现。FM相发展的可能原因是表面缺乏氧配体，该配体会增加Mn ³⁺，而以Mn ⁴⁺为代价以保持电子中性。在这项工作中，我们表明Mn ³⁺：Mn ⁴⁺= 1比率保留在整个晶体颗粒中，但表面除外，该比率变为Mn ³⁺：Mn ⁴⁺ >1。这种Mn ³⁺过量使Mn ³⁺ →O ^2– →Mn发生两次交换^超交换占主导地位的是⁴⁺，从而引起了FM互动。我们显示，只有一个单位晶胞的壳厚度，一个_晶胞= 0.38 nm，足以说明表面上FM的发生，而体积仍然是AFM。此外，FM与AFM之比适合于随着颗粒尺寸减小而增加的表面体积比。

更新日期：2018-09-24

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