Search

Magnetic properties of polycrystalline Sm0.1Ca0.9−ySryMnO3 (y = 0–0.3) samples have been investigated in the temperature range 5–250 K, magnetic fields up to 16 kOe and under hydrostatic pressures up to 11 kbar. The studies involved sequential measurements of zero field cooled (ZFC) magnetization and measurements of magnetization upon cooling in the same magnetic field (FCC). The low-doped group (y = 0–0.1) exhibits magnetic phase separation below TN≈TC≈100–110 K, consisting of ferromagnetic (FM) clusters embedded in an antiferromagnetic (AFM) G-type matrix. Magnetization and ac-susceptibility measurements of this group of materials indicate features reminiscent of a cluster glass-like state, below TC. It was found that the volume fraction of the FM phase at 10 K decreases with increasing y from 28% for Sm0.1Ca0.9MnO3 to 18% for Sm0.1Ca0.8Sr0.1MnO3. It was found that an applied pressure enhances TC with a pressure coefficient of dTC/dP≈0.4–0.5 K kbar−1. The low-temperature magnetization at this doping range (y = 0–0.1) depends on pressure only slightly, except for the case of Sm0.1Ca0.8Sr0.1MnO3, where an applied pressure enhances the FM phase volume considerably. Samples with y = 0.2 and 0.3 exhibit a heterogeneous spin configuration in their ground state, consisting of a C-AFM phase and a G-AFM phase with a very weak FM moment. The temperature of transition from the paramagnetic to the C-AFM state is almost insensitive to applied pressure, whereas the lower transition temperature to the G-AFM state increases slightly under pressure. It was found also that an applied pressure considerably reduces the FM correlations in the paramagnetic phase as well as the FM component of the G-AFM state.

We present the study of magnetic and transport properties of Pb(1-x-y-z)Mn(x)Sn(y)Eu(z)Te. AC magnetic susceptibility measurements as well as transport characterization were performed. The obtained results indicate that the presence of two types of magnetic ions influences magnetic properties of investigated IV-VI semimagnetic semiconductor. A qualitative analysis and possible mechanisms of the substantial dependence of the Curie temperature on the Eu content are presented. The most likely reason of the observed Curie temperature behaviour is a strong dependence of the location of a heavy mass Sigma band upon the alloy composition. The theoretical calculations in frame of simple models confirm this point.

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.

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.

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.