Department of Physics, University of California, Santa Cruz, CA 95064, USA.
J Phys Condens Matter. 2013 Sep 25;25(38):382203. doi: 10.1088/0953-8984/25/38/382203. Epub 2013 Aug 30.
Neutron scattering and magnetometry measurements have been used to study phase transitions in LaCoO3 (LCO). For H ≤ 100 Oe, evidence for a ferromagnetic (FM) transition is observed at Tc ≈ 87 K. For 1 kOe ≤ H ≤ 60 kOe, no transition is apparent. For all H, Curie-Weiss analysis shows predominantly antiferromagnetic (AFM) interactions for T > Tc, but the lack of long-range AFM order indicates magnetic frustration. We argue that the weak ferromagnetism in bulk LCO is induced by lattice strain, as is the case with thin films and nanoparticles. The lattice strain is present at the bulk surfaces and at the interfaces between the LCO and a trace cobalt oxide phase. The ferromagnetic ordering in the LCO bulk is strongly affected by the Co-O-Co angle (γ), in agreement with recent band calculations which predict that ferromagnetic long-range order can only take place above a critical value, γC. Consistent with recent thin film estimations, we find γC = 162.8°. For γ > γC, we observe power-law behavior in the structural parameters. γ decreases with T until the critical temperature, To ≈ 37 K; below To the rate of change becomes very small. For T < To, FM order appears to be confined to regions close to the surfaces, likely due to the lattice strain keeping the local Co-O-Co angle above γC.
中子散射和磁强计测量被用于研究 LaCoO3（LCO）中的相变。在 H ≤ 100 Oe 时，在 Tc ≈ 87 K 处观察到铁磁（FM）转变的证据。在 1 kOe ≤ H ≤ 60 kOe 时，没有明显的转变。对于所有 H，居里-外斯分析表明，在 T > Tc 时主要表现为反铁磁（AFM）相互作用，但缺乏长程 AFM 序表明存在磁阻。我们认为，体相 LCO 中的弱铁磁性是由晶格应变引起的，这与薄膜和纳米颗粒的情况相同。晶格应变存在于体相表面以及 LCO 与痕量氧化钴相之间的界面处。LCO 体相中的铁磁有序强烈受到 Co-O-Co 角（γ）的影响，这与最近的能带计算结果一致，该结果预测只有在临界值 γC 以上才能发生铁磁长程有序。与最近的薄膜估计一致，我们发现 γC = 162.8°。对于 γ > γC，我们观察到结构参数呈幂律行为。γ 随 T 减小直到临界温度 To ≈ 37 K；在 To 以下，变化率变得非常小。对于 T < To，FM 有序似乎仅限于靠近表面的区域，这可能是由于晶格应变使局部 Co-O-Co 角保持在 γC 以上。
