Vanadium‐substituted LiMnPO4 has been synthesized by the solid state reaction route. To obtain tetravalent state of vanadium, the samples have been additionally annealed under hydrogen atmosphere. Comparative magnetic and neutron powder diffraction investigations of the two solid‐solution lines were performed as a function of V content. The divergence between static zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetizations in the fields of H < 1 T and the magnetization hysteresis indicating the presence of a ferromagnetic component were observed for all vanadium concentrations. The ferromagnetic component in the reduced V‐doped LiMnPO4 is more intensive than in the as‐prepared samples. To gain insight into the reason for this effect, the formation energies of antisite defects in V‐doped and reduced LiMnPO4 were calculated within the density functional theory. The appearance of the ferromagnetic clusters above the Néel temperature is discussed.
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...
The magnetic state of mixed-valence oxide manganites (e.g., La 1-x Me x MnO 3 , where Me is Sr, Ca, Ba), being the object of intensive study in the last 10 years, is formed at the expense of competing ferro-and antiferromagnetic interactions. Antiferromagnetism is determined by superexchange in which localized electrons are involved, while ferromagnetism is governed by kinetic exchange (Mn 4+ -O-Mn 3+ ) of delocalized e g -electrons [1]. A tendency towards electron phase separation, characteristic of systems with strongly correlated electrons, results in the features of the magnetic state of perovskite-like manganites. At low level of doping x (low concentration of holes), ferromagnetic polarons of small radius are formed in the dielectric antiferromagnetic matrix; with a rise of x, polarons merge in metallic ferromagnetic drops, and at x ≥ 0.16, a ferromagnetic order and a metallic state below the Curie temperature (T C ) occur within the whole crystal [2].In our view, another separation scenario, which, in a sense, is opposite to the one above, is of interest, when dielectric antiferromagnetic microregions emerge in a conducting ferromagnetic matrix. Forced realization of the situation of this kind, as distinct from the case of thermodynamic equilibrium phase separation, may be attempted by irradiation of manganites with fast neutrons. It is known [3] that at irradiation of oxides bearing cations of 3d-and 4f-transition metals, through origination and further evolution of atom-atom collision cascades, structurally disordered microregions predominantly of ~12 Å effective sizes are formed. Here, a disordering is understood to be statistical mutual redistribution of cations localized in crystallographically nonequivalent positions. In fact, irradiation with fast neutrons may be regarded as a method for obtaining new nanosize structural formations, without affecting the continuity and chemical composition of the sample.In case of perovskite-like manganites, irradiation with fast neutrons leads to partial disordering only. Investigation of irradiated La-Sr and La-Ba oxide manganites has shown [4,5] that the dopant (Sr, Ba) and manganese cations mainly get redistributed over the nonequivalent sites. Analysis of the results of [5,6] on irradiation of La-Ba and La-Sr manganites shows that structural disordering must lead to electron localization.Polycrystalline samples of manganite of nominal composition La 0.825 Ba 0.175 MnO 3 studied in the initial state earlier in [7] were chosen for investigation. The structurally disordered state of the samples was obtained by their irradiation with fast neutrons (E eff ≥ 1 MeV) at a temperature not higher than 340 K in the water cavity of the IVV-2M research nuclear reactor. The samples were exposed to fast neutron fluences Φ (× 10 19 cm -2 ): 1.6, 2.2, 4.9, 5.5 and 12.The methods of neutron diffraction, X-ray and magnetic measurements were used to study the structural and magnetic states of disordered samples of manganite La 0.825 Ba 0.175 MnO 3 . A disordered state was attaine...
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