Determination of the magnetic network of the double perovskite Ca2MnReO6 through neutron diffraction.

Name: Arthur SantAna Cavichini
Type: PhD thesis
Publication date: 23/08/2018
Advisor:

Namesort descending Role
Marcos Tadeu DAzeredo Orlando Advisor *

Examining board:

Namesort descending Role
Antonio Eduardo Martinelli External Examiner *
Eliana Navarro dos Santos Muccillo External Examiner *
Haimon Diniz Lopes Alves External Examiner *
Márcia Carvalho de Abreu Fantini External Examiner *
Marcos Tadeu DAzeredo Orlando Advisor *
Wanderlã Luis Scopel Internal Examiner *

Summary: This work investigated the magnetic structure of the double perovskite Ca2MnReO6 by powder neutron diffraction. Up to the present work, there were no conclusive studies about its magnetic structure. In order to clarify this structure, polycrystalline samples of this compound were
produced in sufficient quantities to perform neutron diffraction studies. About 10 g of the compound were synthesized in batches by solid state reaction in a vacuum-sealed quartz tube. X-ray diffraction measurements showed crystallinity and quality of the batches. The dc magnetization with 𝐻 = 0.1 T showed a paramagnetic transition at 𝑇 = 121 K, a magnetic frustration on the ZFC magnetization curve and a significant magnetization of 𝑀 s 0.5𝜇𝐵/f.u in FC. The fit of the Curie-Weiss Law in the paramagnetic region in 𝜒−1 resulted in a 𝜃𝐶𝑊 = 70 K, indicating a predominance of ferromagnetic interactions in the lattice. ZFC magnetic hysteresis at 𝑇 = 10 K showed that the compound exhibits a hard magnetic behavior and an 𝐻𝑐 = 4 T, indicating a magneto-crystalline coupling. Rietveld refinement of the cold neutron diffraction spectra as a function of temperature revealed that between 75 K-120 K there is an anomalous expansion of the 𝑏 axis and a large contraction of the 𝑐 parameter with the cooling, which may be related to the magneto-crystalline coupling. The evolution of the total moments as a function of the temperature show that their values arrive at zero near 𝑇𝑐, and that a above 𝑇 = 80 K there is an increase of the ferromagnetic component of Mn. Rietveld refinement of the thermal neutron diffraction spectra showed no structural phase transition of the compound as a function of temperature. As a conclusion of this work, it has been determined of unpublished form that the magnetic group 𝑃21/𝑛 describes the magnetic structure of the compound Ca2MnReO6. Such a conclusion was based on a detailed crystallographic analysis of the purely magnetic peaks of the cold neutron diffraction spectra at 𝑇 = 2 K and using the propagation vector ⃗𝑘 = (0,0,0). Moreover, it was found that the Mn and Re magnetic sub-lattice present an A-AFM type ordering, with magnetic vectors [2.41(3),±0.8(3), 3.53(3)] and [−0.16(4), 0.15(4)], respectively. It was also found that the magnetic vector of Re makes an angle of 100° with respect to the magnetic vector of Mn in even 𝑧, revealing an orthogonality relationship between them. The orthogonal magnetic
coupling between the moments of Mn and Re indicates the predominance of an antisymmetric exchange interaction (Dzyaloshinskii-Moriya) in comparison to a symmetric exchange interactions (Heisenberg). This coupling characterizes the compound as a spin-orbit-assisted Mott
insulator, similar to that observed in Iridates.

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