H. Szymczak

Magnetic and magnetotransport properties of Co-doped manganites with perovskite structure

Abstract A systematic study of the doping of the Mn-sites by cobalt in three series of manganites — La 0.76 Ba 0.24 (Mn 1− x Co x )O 3 single crystals, La 2/3 Ba 1/3 (Mn 1− x Co x )O 3 and La(Mn 1− x Co x )O 3 ceramics has been performed. It was found that La(Mn 1− x Co x )O 3 annealed at 800°C in the range 0.4⩽ x ⩽0.9 is a mixture of ferromagnetic domains with ordered Mn and Co ions and ionically disordered spin-glass domains. In the quenched samples the fraction of spin-glass-type component increases strongly. The La 2/3 Ba 1/3 (Mn 1− x Co x )O 3 solid solutions exhibit also an evidence for phase separation in the range 0.5⩽ x ⩽0.8. All the La(Mn 1− x Co x )O 3 samples show an insulating behavior, however, magnetoresistance reduces strongly when the cobalt content rises to x =0.5. The La 0.76 Ba 0.24 (Mn 1− x Co x )O 3 single crystals show first-order phase transition below their Curie points associated with a change of ground state of the Co 2+ ions. The magnetic phase diagrams are depicted. The results are discussed in terms of positive Mn 3+ –O–Mn 4+ , Mn 3+ –O–Mn 3+ , Mn 4+ –O–Co 2+ and negative Mn 4+ –O–Mn 4+ , Co 2+ –O–Co 2+ , Co 2+ –O–Mn 3+ superexchange interactions as well as Co 2+ and Mn 4+ ionic ordering.

Magnetic phase diagrams of the manganites Ln1-xBaxMnO3 (Ln = Nd, Sm)

The magnetization and magnetoresistance of Ln1-xBaxMnO3 have been studied for a single crystal (Ln = Nd, x = 0.23) and for ceramics (Ln = Nd, Sm; 0 x0.44). It was shown that Nd1-xBaxMnO3 compositions in the range 0 x0.05 are inhomogeneous antiferromagnets; a mixed antiferro-ferromagnet state appears in the range 0.05 0.12. Both Nd and Sm sublattices are coupled antiferromagnetically with manganese ions. All the ferromagnetic Nd1-xBaxMnO3 samples show similar magnetotransport properties despite of a strong difference in the resistivity behaviour. Our results could be understood in terms of Mn-O-Mn superexchange and 3d orbital ordering as well as variation of band structure due to the ionic size effect, Mn4+ ion appearance and magnetic ordering. It is supposed that the gap between the narrow 3d manganese band and wide 2p oxygen band decreases strongly below TC.

Magnetic state of the structural separated anion-deficient La0.70Sr0.30MnO2.85 manganite

The results of neutron diffraction studies of the La0.70Sr0.30MnO2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups

Effect of the size factor on the magnetic properties of manganite La 0.50 Ba 0.50 MnO 3

R\bar 3c

Magnetic phase transitions in the system La1−xBixMnO3+λ

and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La0.70Sr0.30MnO2.85 is a cluster spin glass, which is the result of frustration of Mn3+-O-Mn3+ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.

Spin-reorientational transitions in low-doped Nd1?xCaxMnO3 manganites: the evidence of an inhomogeneous magnetic state

The pinning induced magnetostriction in an isotropic superconductor was calculated for different sample shapes. We analyzed some special shapes for which the solution can be found analytically. The magnetostriction of a finite slab was considered. In order to determine the influence of demagnetization effects, the pinning induced magnetostriction of an infinitely long and thin strip was calculated. A simple, approximate formula can be used in particular cases to analyze the magnetostriction induced by pinning forces. In this formula the magnetostriction of a sample is connected directly with its magnetization. We also present some experimental results on high temperature superconductors which are analyzed in frames of the developed theory.

Evolution of magnetic state in the La1-xCaxMnO3-γ (x=0.30, 0.50) manganites depending on the oxygen content

Nanocrystalline manganite La0.50Ba0.50MnO3 was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size D increases from ∼30 nm to ∼7 μm with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously from ∼3.787 to ∼3.904 Å with the average crystallite size. The most significant lattice compression (≈3%) occurs in the sample with an average crystallite size of ∼30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curic temperature TC from ∼264 to ∼331 K and in the spontaneous magnetic moment σS from ∼1.52 to ∼3.31 μB/f.u. The anomalous behavior of the magnetic properties of the manganite La0.50Ba0.50MnO3 obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+-O-Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

Crystal structure and magnetic ordering of the LaCo1?xFexO3 system

For La0.8253+Sr0.1752+Mn3+O2.9122− anion-deficient manganite, the specific magnetization, the dynamic magnetic susceptibility, and the heat capacity are investigated. This material is found to be an inhomogeneous ferromagnet below the Curie point TC ≈ 122 K, which is much lower than the Curie point determined for the stoichiometric composition (TC ≈ 268 K). An increase in magnetic field by two orders of magnitude leads to an increase in the Curie temperature by ΔT ≈ 12 K. The presence of oxygen vacancies leads to the frustration of a part, namely, Vfr ≈ 22%, of the indirect Mn3+-O-Mn3+ exchange interactions, but the spin glass state is not realized. The ferromagnetic matrix of the material under study is characterized by a scatter in the exchange interaction intensities. The heat capacity is found to exhibit an anomalous behavior. Based on the Banerjee magnetic criterion, it is established that the ferromagnet-paramagnet transition observed for La0.8253+Sr0.1752+Mn3+O2.9122− anion-deficient manganite is a second-order thermodynamic phase transition. The mechanism and origin of the critical behavior of the system under investigation are discussed.