V. Markovich

Correlation between electroresistance and magnetoresistance in La0.82Ca0.18MnO3 single crystal

The resistivity of La0.82Ca0.18MnO3 single crystal has been investigated as a function of external magnetic field and separately under an applied current flow. The measurements were carried out at various temperatures below and above the ferromagnetic transition temperature TC. It has been found that the dynamic electroresistance exhibits stunning similarities to the colossal magnetoresistance at the corresponding temperatures. The correlation observed between the electric- and magnetic-field effects is attributed to electrically induced magnetoresistance.

Preparation of La1−xSrxMnO3 nanoparticles by sonication-assisted coprecipitation

La{sub 1-x}Sr{sub x}MnO{sub 3} (x=0.3) (LSM) nanoparticles were prepared by a sonication-assisted coprecipitation method. The coprecipitation reaction is carried out with ultrasound radiation. Lower sintering temperatures are required for the sonication-assisted product. Fully crystallized LSM with an average particle size 24 nm is obtained after the as-prepared mixture is annealed at 900 deg. C for 2 h. Magnetic properties indicate that the transition temperature from the paramagnetic to ferromagnetic state of the sample is quite sharp and occurs at 366 K for samples annealed for 2 h at 900 and 1100 deg. C.

Magnetic properties of nanocrystalline La1- xMnO3+δ manganites : Size effects

The magnetic properties of nanocrystalline manganites La1?xMnO3+? with particle size of 20 (LMO20), 25 (LMO25), and 30 nm (LMO30), prepared by the citrate method, have been investigated in the temperature range 5?320?K, magnetic field up to 90?kOe and under quasi-hydrostatic pressures up to 14.5?kbar. The studies involve sequential zero-field-cooled magnetization (M) measurements followed by magnetization measurements during cooling in the same magnetic field (H) and complementary measurements of ac susceptibility. Additional measurements of M versus H were carried out at ambient and applied pressures. All nanoparticles exhibit a paramagnetic to ferromagnetic transition (PFT) at a Curie temperature TC>200?K. It was found that the relative volume of the ferromagnetic phase increases for larger particle size and approaches a value of about 93% for LMO30. The real part of the ac susceptibility of sample LMO20 exhibits strong frequency dependence in a wide temperature range below TC, whereas for sample LMO30 only relatively weak frequency dependence was observed. The magnetization of sample LMO30 exhibits a PFT of second order; the type of transition could not be established for the smaller particles. It was found that an applied pressure enhances the TC of La1?xMnO3+? nanoparticles with a pressure coefficient of dTC/dP?1.9?K?kbar?1 for LMO20 and dTC/dP?1.4?K?kbar?1 for LMO25 and LMO30 samples. Peculiar magnetic memory effects observed for sample LMO20 are discussed.

Magnetotransport in granular LaMnO3+δ manganite with nano-sized particles

Transport and magnetic properties of compacted LaMnO3+? manganite nanoparticles of an average size of 18?nm have been investigated in the temperature range 5?300?K. The nanoparticles exhibit a paramagnetic-to-ferromagnetic (FM) transition at the Curie temperature TC ~ 246?K. However, the spontaneous magnetization disappears at a higher temperature of about 270?K. It was found that at low temperatures the FM core occupies about 50% of the particle volume. The temperature dependence of the resistivity shows a metal?insulator transition and a low-temperature upturn below the resistivity minimum at T ~ 50?K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. It has been found that the charging energy decreases monotonically with increasing magnetic field. The low temperature I?V characteristics are well described by an indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates. The experimental features are discussed in the framework of a granular ferromagnet model.

Magnetic, electric and electron magnetic resonance properties of orthorhombic self-doped La1−xMnO3 single crystals

The effect of lanthanum deficiency on structural, magnetic, transport, and electron magnetic resonance (EMR) properties has been studied in a series of La1−xMnO3 (x = 0.01, 0.05, 0.11, 0.13) single crystals. The x-ray diffraction study results for the crystals were found to be compatible with a single phase of orthorhombic symmetry. The magnetization curves exhibit weak ferromagnetism for all samples below 138 K. It was found that both the spontaneous magnetization and the coercive field increase linearly with x. The pressure coefficient dTN/dP decreases linearly with self-doping, from a value of 0.68 K kbar−1 for La0.99MnO3 to 0.33 K kbar−1 for La0.87MnO3. The resistivity of low-doped La0.99MnO3 crystal is of semiconducting character, while that of La0.87MnO3 depends weakly on temperature between 180 and 210 K. It was found that the magnetic and transport properties of the self-doped compounds may be attributed to a phase separation involving an antiferromagnetic matrix and ferromagnetic clusters. The latter phases as well as their paramagnetic precursors have been directly observed by means of EMR.

Size-dependent spin state and ferromagnetism in La0.8Ca0.2CoO3 nanoparticles

Magnetic and structural properties of nanocrystalline low-doped La0.8Ca0.2CoO3 cobaltites with particle size of 8, 13, 23, and 50 nm, prepared by the glycine-nitrate method, were investigated in temperature range 5–320 K, magnetic field up to 50 kOe and under hydrostatic pressure up to 10 kbar. With particle downsizing, a noticeable expansion of unit cell, with concomitant changes in the rhombohedral structure toward the cubic one was observed. It was found that the increased surface-disorder effect strongly suppresses the ferromagnetic state in La0.8Ca0.2CoO3 nanoparticles leading to a decrease, by factor of about 2, both in spontaneous magnetization, MS, and Curie temperature, TC, when particle’s size decreases from 23 to 8 nm. The effective magnetic moment μeff was found also to decrease distinctly due to the strong interdependence between Co–O–Co interactions and Co spin state. The size-induced magnetic disorder drives the La0.8Ca0.2CoO3 nanoparticles to a dominant glassy behavior for 8 nm particles. This is evidenced by the fact that the freezing temperature varies with magnetic field in a strict conformity with the de Almeida–Thouless law for spin glasses and also by the observation of characteristic slowing down in the spin dynamics. The applied pressure suppresses TC, MS, and coercive field HC, like it is observed for bulk La0.8Ca0.2CoO3. Nevertheless, in nanoparticles the pressure effect on TC is noticeably stronger, while HC diminishes with pressure much slower then in bulk material.

Chapter One - Magnetic Properties of Perovskite Manganites and Their Modifications

Abstract This chapter attempts to systematically outline some fundamentals and key experimental results concerning magnetic properties of perovskite manganites, focusing on (i) magnetocaloric properties, (ii) pressure effect on magnetic properties, and (iii) magnetism of manganite nanoparticles. Each family of manganites has unique properties that can be used as a way of tuning the optimum magnetocaloric response. The relatively easy possibility of tuning the Curie temperature of manganites is a key point in developing efficient magnetocaloric materials. The most interesting effects of applied external pressure observed for various classes of manganite systems, such as hole-doped manganites; parent, single-valent, and self-doped manganites; hexagonal manganites, near-half-doped manganites, electron-doped manganites, and manganite nanoparticles are reviewed. Some of the most relevant finite-size and surface effects on the magnetic properties of ferromagnetic and antiferromagnetic manganite nanoparticles are also discussed. New phenomena such as a suppression of charge/orbital ordering with decreasing particle size, collective states, and nonequilibrium dynamics in ensembles of antiferromagnetic manganite nanoparticles are presented.

Ferromagnetic domain structure of La0.78Ca0.22MnO3 single crystals

The magneto-optical technique has been employed to observe spontaneous ferromagnetic domain structures in La0.78Ca0.22MnO3 single crystals. The magnetic domain topology was found to be correlated with the intrinsic twin structure of the investigated crystals. With decreasing temperature the regular network of ferromagnetic domains undergoes significant changes resulting in apparent rotation of the domain walls in the temperature range of 70–150 K. The apparent rotation of the domain walls can be understood in terms of the Jahn-Teller deformation of the orthorhombic unit cell, accompanied by additional twinning.

FMR probing of ‘spontaneous’ and Ru-doping induced ferromagnetism in Sm0.2Ca0.8Mn1−xRuxO3 (x≤0.08) manganites

Abstract The FMR spectra of electron-doped Sm0.2Ca0.8Mn1−xRuxO3 (0≤x≤0.08) system are measured and analyzed in the temperature interval 100≤T≤320 K , including all critical points of magnetic transitions. It is shown that the origin of macroscopic magnetization in the parent Sm0.2Ca0.8MnO3 and Ru-doped compounds is essentially different. The bound magnetic polarons formation at temperatures below the Neel point is suggested to be responsible for the appearance of a minute ‘spontaneous’ magnetic moment in Sm0.2Ca0.8MnO3. Doping with Ru induces formation of ‘conventional’ ferromagnetic phase in antiferromagnetic matrix. Such a phase is characterized by a Curie temperature, which is higher than the Neel temperature of the matrix. Magnetic phase diagrams of Sm0.2Ca0.8Mn1−xRuxO3 determined by ferromagnetic resonance and dc magnetic measurements are discussed.

Magnetic properties of Sm0.1Ca0.9MnO3 nanoparticles

Magnetic properties of compacted Sm0.1Ca0.9MnO3 nanoparticles with average particle size of 25 and 60 nm have been investigated. It was found that the relative volume of the ferromagnetic phase decreases with decreasing particle size. Magnetization curves measured in field cooled and zero field cooled mode separate near the transition temperature TC and remain different even in magnetic field of 15 kOe. AC-susceptibility is strongly frequency dependent below TC, although the temperature of the maximum depends on frequency only slightly. Magnetization hysteresis loops exhibit horizontal and vertical shifts, relatively small in 60 nm and much larger in 25 nm particles, due to size-dependent exchange bias effect. The exchange bias field and the coercive field depend in a non-monotonic way on cooling magnetic field, while the asymmetry of remanence magnetization and magnetic coercivity increase monotonously with the increase of cooling field. Applied pressure enhances Curie temperature TC of nanoparticles wit...