D. D. Khalyavin

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 properties of anion deficit manganites Ln0.55Ba0.45MnO3−γ (Ln=La, Nd, Sm, Gd, γ⩽0.37)

Abstract A detailed study of oxygen deficit effect on magnetic properties was carried out for Ln 0.55 Ba 0.45 MnO 3− γ (Ln=La,xa0Nd,xa0Sm,xa0Gd). It was shown that Ln 0.55 Ba 0.45 MnO 3− γ undergoes a concentrational phase transition from ferromagnetic to spin glass state at γ =0.15. The freezing temperature T f decreases with decreasing oxygen content down to 80xa0K for γ =0.37. Another behavior has been revealed for Ln=Nd,xa0Sm,xa0Gd series; these compounds undergo a transition from ferromagnetic state (Ln=Nd) or spin-glass one (Ln=Sm,xa0Gd) into weak ferromagnetic state ( γ =0.35) through intermediate antiferromagnetic state ( γ =0.2). In contrast to stoichiometric manganites the Neel point increases from 136xa0K (Ln=Nd) to 160xa0K (Ln=Gd) with decreasing rare-earth ionic radii. It is supposed that both lanthanide and barium ions as well as oxygen vacancies are ordered in the strongly reduced rare-earth manganites.

Low-temperature structural and dielectric phenomena in La1/3NbO3 and La1/3TaO3: Comparative study

The crystal structures of the perovskites La1/3NbO3 and La1/3TaO3 were studied between 10 and 350 K using high-resolution neutron powder diffraction and compared with their radio-frequency dielectric response over the same temperature range. The structure of La1/3NbO3 remains orthorhombic Cmmm, while La1/3TaO3 undergoes continuous transition from the high-temperature tetragonal P4/mmm to Cmmm phase at about 220 K. This transition is tricritical in nature and accompanied by no dielectric anomaly. In La1/3NbO3, the frequency-dependent peak of the dielectric permittivity is associated with an atypical increase in the lattice parameters below about 80 K.

Singularities of magnetic and elastic characteristics of La2/3Ba1/3MnO3: Analysis of martensitic kinetics

Abstract A coordinated temperature behavior of magnetic susceptibility and internal friction has been observed in the La 2/3 Ba 1/3 MnO 3 manganite in the temperature region of the crystal phase separation 5–340xa0K. Stepwise temperature behavior of the susceptibility of the single crystal sample and corresponding singular behavior of the internal friction in the polycrystalline manganite have been found. These small-scale features of the temperature dependences of the susceptibility and the internal friction are considered to be a reflection of martensitic kinetics of the structural phase transformation R3¯c↔Imma in the 200xa0K temperature region.

The nonlinear magnetic properties of the pseudocubic Nd0.77Ba0.23MnO3 single crystal in the critical paramagnetic region and phase separation

The second magnetization harmonic was studied for a moderately doped Nd0.77Ba0.23MnO3 neodymium manganite single crystal in parallel constant and harmonic magnetic fields in the critical paramagnetic region. According to the neutron and X-ray diffraction data, the crystal was crystallographically single-phase and had a pseudocubic structure both at room temperature and below the Curie point TC=124.1 K. Although the specific resistance of this compound had a singularity near TC and exhibited giant magnetoresistance, it remained an insulator in the ferromagnetic state. Nonlinear response measurements in the TC<T<T*≈146.7 K paramagnetic region were indicative of the existence of two magnetic phases. Above T*, the crystal was magnetically single-phase, and its critical behavior was well described by dynamical similarity theory for isotropic 3D ferromagnets. The unexpected appearance of a new magnetic phase in the structurally homogeneous crystal was discussed based on phase separation ideas; such a phase separation could occur in moderately doped cubic manganites experiencing orbital ordering.

Uniaxial pressure effect on magnetic susceptibility of perovskite manganite La0.66Ba0.34MnO3

Abstract Magnetization of La 0.66 Ba 0.34 MnO 3 and its temperature behavior under a uniaxial pressure of 0.1xa0kbar are measured between 5 and 270xa0K in magnetic fields 0 H

Neutron diffraction, magnetization, and ESR studies of pseudocubic Nd0.75Ba0.25MnO3 and its critical behavior above TC

Results of structural neutron diffraction study, magnetization, and electron spin resonance (ESR) measurements are presented for insulating Nd{sub 1-x}Ba{sub x}MnO{sub 3}(x=0.25) with the Curie temperature T{sub C}{approx_equal}129 K. Detailed analysis of the data is performed by using Pbnm space group in a temperature range 4.2-300 K. The compound is found to exhibit the Jahn-Teller (JT) transition at T{sub JT}{approx}250 K. The character of the coherent JT distortions and their temperature evolution differ from those of the x=0.23 manganite. The field cooled magnetization data are in reasonable agreement with the predictions for a three-dimensional (3D) isotropic ferromagnet above T{sub C}. These measurements, however, reveal a difference between the field cooled and zero field cooled data in the paramagnetic region. The ESR results also correspond with behavior of a 3D isotropic ferromagnet above T*{approx_equal}143 K[{tau}*{approx_equal}0.12{<=}{tau}<1,{tau}=(T-T{sub C})/T{sub C}]. The T-dependence of the ESR linewidth is found to be proportional to [T{chi}(T)]{sup -1}, where {chi}(T) is the susceptibility. This uncritical behavior results from the anisotropic spin interactions that can be attributed to the Dzyaloshinsky-Moria (DM) coupling. The critical enhancement is not observed. It can be explained by the strong uncritical contribution to the linewidth, and suppression of the critical enhancement by a magnetic field.morexa0» The different temperature treatments (slow/fast cooling/heating, with/without external magnetic field) of the sample reveal a temperature hysteresis of the ESR spectra below T* indicating an anomalous response in the paramagnetic region. The study of the magnetic phase transition in the x=0.23 and 0.25 manganites suggests change in its character from second to first order at T*. The conventional free energy including the magnetization and magnetic field is not found to describe this first order transition. This suggests that the charge, orbital, and JT phonon degrees of freedom, in addition to magnetization, may be the critical variables, the unusual character of the transition being determined by their coupling. The unconventional critical behavior is attributed to an orbital liquid metallic phase that begins to coexist with the initial orbital ordered phase below T*.«xa0less

Neutron diffraction, magnetization and ESR studies of pseudocubic Nd(0.75)Ba(0.25)MnO3 and its unusual critical behavior above Tc

Results of structural neutron diffraction study, magnetization, and electron spin resonance (ESR) measurements are presented for insulating Nd{sub 1-x}Ba{sub x}MnO{sub 3}(x=0.25) with the Curie temperature T{sub C}{approx_equal}129 K. Detailed analysis of the data is performed by using Pbnm space group in a temperature range 4.2-300 K. The compound is found to exhibit the Jahn-Teller (JT) transition at T{sub JT}{approx}250 K. The character of the coherent JT distortions and their temperature evolution differ from those of the x=0.23 manganite. The field cooled magnetization data are in reasonable agreement with the predictions for a three-dimensional (3D) isotropic ferromagnet above T{sub C}. These measurements, however, reveal a difference between the field cooled and zero field cooled data in the paramagnetic region. The ESR results also correspond with behavior of a 3D isotropic ferromagnet above T*{approx_equal}143 K[{tau}*{approx_equal}0.12{<=}{tau}<1,{tau}=(T-T{sub C})/T{sub C}]. The T-dependence of the ESR linewidth is found to be proportional to [T{chi}(T)]{sup -1}, where {chi}(T) is the susceptibility. This uncritical behavior results from the anisotropic spin interactions that can be attributed to the Dzyaloshinsky-Moria (DM) coupling. The critical enhancement is not observed. It can be explained by the strong uncritical contribution to the linewidth, and suppression of the critical enhancement by a magnetic field.morexa0» The different temperature treatments (slow/fast cooling/heating, with/without external magnetic field) of the sample reveal a temperature hysteresis of the ESR spectra below T* indicating an anomalous response in the paramagnetic region. The study of the magnetic phase transition in the x=0.23 and 0.25 manganites suggests change in its character from second to first order at T*. The conventional free energy including the magnetization and magnetic field is not found to describe this first order transition. This suggests that the charge, orbital, and JT phonon degrees of freedom, in addition to magnetization, may be the critical variables, the unusual character of the transition being determined by their coupling. The unconventional critical behavior is attributed to an orbital liquid metallic phase that begins to coexist with the initial orbital ordered phase below T*.«xa0less