Dhiman Indu, Das A, Nigam A K
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India.
J Phys Condens Matter. 2009 Sep 23;21(38):386002. doi: 10.1088/0953-8984/21/38/386002. Epub 2009 Aug 27.
We report a structural, magnetic and transport study in La(0.5)Ca(0.5-x)Ba(x)MnO(3) (0<x≤0.5) using neutron diffraction, magnetization and resistivity measurements. The samples crystallize in an orthorhombic structure with Pnma space group and no structural transition is observed with Ba doping. Substituting Ca with Ba leads to an increase in average A-site ionic radii, ⟨r(A)⟩ and cation size disorder (σ(2)). Consequently, the nature of magnetic ordering undergoes changes as a function of x. At low temperature for x≤0.1, the CE-type antiferromagnetic spin structure is stabilized, similar to the parent compound. At x = 0.15, the magnetic phase is a mixture of CE-type and A-type spin structure. For x≥0.2, the ferromagnetic phase is stabilized. The signature of orbital ordering in addition to spin and charge ordering is observed in all samples with x≤0.15. T(C), T(CO) and T(N) remain nearly constant for x≤0.2. The transport studies reveal that for samples x≤0.2, adiabatic hopping of small polarons governs the transport behaviour above T(CO) (∼160 K). A parallel combination of small polaron hopping and variable range hopping mechanisms is found to describe resistivity data below T(CO). At x = 0.2, an insulator to metal transition is observed as temperature is lowered, and at x = 0.5 the sample becomes metallic below 280 K. In the metallic regime, resistivity data follows the Bloch-Grüneisen model together with contributions from high frequency optical phonons and magnetic scattering. The experimentally established phase diagram from the present study is compared with theoretical studies in the intermediate electron-phonon interaction regime.
我们通过中子衍射、磁化强度和电阻率测量，对La(0.5)Ca(0.5 - x)Ba(x)MnO(3)（0 < x ≤ 0.5）进行了结构、磁性和输运性质研究。样品结晶为具有Pnma空间群的正交结构，且未观察到Ba掺杂引起的结构转变。用Ba替代Ca会导致平均A位离子半径⟨r(A)⟩和阳离子尺寸无序度(σ(2))增加。因此，磁有序的性质随x变化。在x ≤ 0.1的低温下，CE型反铁磁自旋结构稳定，与母体化合物相似。在x = 0.15时，磁相是CE型和A型自旋结构的混合。对于x ≥ 0.2，铁磁相稳定。在所有x ≤ 0.15的样品中，除了自旋和电荷有序外，还观察到轨道有序的特征。对于x ≤ 0.2，T(C)、T(CO)和T(N)几乎保持不变。输运研究表明，对于x ≤ 0.2的样品，小极化子的绝热跳跃在T(CO)（~160 K）以上主导输运行为。发现小极化子跳跃和变程跳跃机制的并联组合可描述T(CO)以下的电阻率数据。在x = 0.2时，随着温度降低观察到绝缘体到金属的转变，在x = 0.5时，样品在280 K以下变为金属。在金属区域，电阻率数据遵循布洛赫 - 格律恩森模型，并伴有高频光学声子和磁散射的贡献。将本研究通过实验建立的相图与中间电子 - 声子相互作用区域的理论研究进行了比较。
