Antiferro–ferromagnetic transformation in LaMnO₃ under neutron irradiation

Abstract

It is known (see, e.g., the review [1]), that the dielectric LaMnO 3 of stoichiometric composition has an O′-type orthorhombic structure formed with the participation of both the tilting modes (ϕ, ψ ) and the Jahn-Teller (JT) mode Q 2 [2]. Measurements of the magnetic susceptibility of LaMnO 3 show preferably positive exchange interaction between Mn 3+ ions, but at low temperatures this oxide behaves as an A-type antiferromagnet [1]. The main role in formation of this type of antiferromagnetism is played by JT distortions causing orbital ordering. As a result, the magnetic moments of Mn 3+ ions are found to be ferromagnetically ordered in the orthorhombic cell a -c planes (space group Pnma) and directed along axis a, with the magnetic moments of the adjacent planes being ordered antiferromagnetically. For quite a long time there has been going a discussion in the literature of an idea that if a stoichiometric LaMnO 3 without the cooperative JT distortions were obtained, its magnetic state would rather be ferromagnetic than antiferromagnetic [1]. An attempt might be made to obtain such a structural state by irradiating the sample with fast neutrons, which would not affect the sample chemical composition. It may be supposed that the most significant effect of irradiation would be mutual disordering of the "lanthanum" and "manganese" sublattices. Radiation-induced disordering of cations with significantly differing ion radii causes significant noncorrelated displacements of neighbouring ions, the displacement values being comparable with the tilting and JT displacements of anions. At high degree of structural disordering this process results in radiation-induced amorphization of perovskites [3]. As it may be seen on an example of La 0.8 Ba 0.2 MnO 3 [4], at partial (~10%) radiation disordering the crystalline structure is preserved, but under irradiation the tilting modes are suppressed, and the initial rhombohedral structure gets transformed to cubic.A stoichiometric LaMnO 3 sample obtained by way of high-temperature annealing under argon, which had been earlier investigated in detail in paper [5], was selected for the study. A piece to be irradiated, ~0.3 g by weight, was cut from the initial solid sample. All subsequent diffraction and magnetic measurements were carried out on this particular sample. The sample under study was irradiated to a fluence of 5.5 × 10 23 m -2 of fast (E n ≥ 1 MeV) neutrons (~0.2 displacement per atom) at a temperature not over 340 K in the water cavity of the IVV-2M research nuclear reactor. Structural investigations before and after irradiation were carried out on the neutron diffractometer D2 (wavelength λ = 1.805 Å) at room temperature (RT) and at 4.2 K, as well as on the DRON-UM1 X-ray diffractometer (CuK α radiation) at RT. Structural parameters were refined by the Rietveld full-profile analysis method with the application of the FULLPROF software package. The magnetic Using the methods of neutron and X-ray diffraction analysis and magnetic measurements, polycrystalline samples o...

Enhanced magnetization in proton irradiated Mn3Si2Te6 van der Waals crystals

“…The radiation induced defects and vacancies were believed to generate ferromagnetism in the above materials. Interestingly, it is also shown that neutron and electron irradiation can be employed in manipulating the magnetic properties of magnetically active materials such as LaMnO3 (antiferromagnetism to ferromagnetism) and La0.9Ca0.1CoO3 (increase in magnetization) 7,21 . These changes were attributed to ion displacements and lattice expansion.…”