I. Fita

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...

Irreversibility, remanence, and Griffiths phase in Sm0.1Ca0.9MnO3 nanoparticles

Magnetic properties of compacted Sm0.1Ca0.9MnO3 particles with 25 and 60 nm average sizes have been investigated. Particular attention has been paid to Griffiths-like features at temperatures above magnetic transition temperature TC and to the system glassiness at low temperatures. Griffiths-like features in inverse magnetic susceptibility of Sm0.1Ca0.9MnO3 nanoparticles have been linked to the presence of short range ferromagnetically correlated spin clusters above TC. Glassy behavior has been revealed in temperature and frequency dependence of ac-susceptibility, temperature and field dependence of thermoremanent and isothermoremanent magnetization, and time decay of the remanent magnetization. Experiments revealed the major impact of the glassy component on magnetic properties of investigated nanoparticles. The magnetic relaxation associated with glassy features was found to be much more pronounced in smaller particles, where a formation of collective state in an ensemble of phase separated nanoparticle...

Vacancies at Mn-sites in LaMn1−xO3 manganites: Interplay between ferromagnetic interactions and hydrostatic pressure

Magnetic properties of self-doped LaMn1−xO3 manganites (0⩽x⩽0.06) under hydrostatic pressure up to 12 kbar have been investigated. Samples with x=0 and 0.02 are antiferromagnetic insulators with a Neel temperature TN=138 K and 124 K, respectively. Materials with x=0.04 and 0.06 are ferromagnetic with a Curie temperature TC close to 110 K. With increasing self-doping, the ferromagnetic moment at T=5 K increases significantly while the pressure coefficient of TN decreases significantly. Pressure dependences of TC and of spontaneous magnetization are markedly different. For x=0.04, both quantities increase under increasing pressure. However, for x=0.06 transition temperature becomes pressure independent while magnetization markedly decreases with increasing pressure. Suppression of the ferromagnetism under pressure is tentatively attributed to the development of antiferromagnetic domains capable of competing with the ferromagnetic order under pressure.

Interplay between itinerant and localized states in CaMn1- xRuxO3 (x≤0.5) manganites

Magnetic properties of polycrystalline CaMn1-xRuxO3 (x = 0 - 0.5) samples were investigated in the temperature range 4.2 - 250 K, under external magnetic field up to 15 kOe and under hydrostatic pressure up to 12 kbar. Transport properties of the samples with x = 0.1, 0.2, 0.4, 0.5 were also investigated under pressure up to 10 kbar. For x up to 0.4, the pressure was found to suppress ferromagnetic correlations and to increase the resistivity, while for x = 0.5 to act in the opposite way. While long ferromagnetic order is completely suppressed, in small clusters ferromagnetic correlations probably survive under pressure, as was revealed for CaMn0.9Ru0.1O3. The pressure effect on the magnetic interactions and on the volume of ferromagnetic phase was found to depend strongly on the Ru-content, and absolute value of the pressure coefficient of spontaneous magnetization was found to decrease practically linearly with increasing x in the range 0.1 < x < 0.5. The experimental data are discussed in the frame of proposed energy-level diagram, which includes magneto-impurity states at low and moderate Ru-doping and mixed-valence states of Ru presented by a strongly-correlated t2g-like band at heavy Ru-doping. An impact of disorder introduced by Ru-doping on the energy diagram and on derived magnetic interactions is discussed. Predictions of the model regarding the pressure effects on conductivity and temperature scales characteristic for magnetic interactions are in reasonable agreement with experiment.

Doping dependent magnetism and exchange bias in CaMn1−xWxO3 manganites

Magnetic properties of CaMn1−xWxO3 (0 ≤ x ≤ 0.1) have been investigated, and the research was focused on the exchange bias (EB) phenomenon in CaMn0.93W0.07O3. Magnetic ground state was found to be dependent on tungsten doping level and the following states were distinguished: (i) G-type antiferromagnetic (AFM) state with a weak ferromagnetic (FM) component at x = 0 and 0.04; (ii) mostly orbitally ordered C-type AFM at x = 0.07 and 0.1. For the studied manganites, spontaneous magnetization increases sharply with increasing doping level reaching M0 ≈ 9.5 emu/g at T = 10 K for x = 0.04, and then decreases rapidly reaching zero for x = 0.1. Exchange bias effect, manifested by vertical and horizontal shifts in the hysteresis loop for field cooled sample, has been observed in CaMn0.93W0.07O3. Exchange bias field, coercivity, remanence asymmetry, and magnetic coercivity depend strongly on temperature, cooling field, and maximal measuring field. Horizontal and vertical shifts of magnetization loop sharply decreas...

Pressure-induced exchange bias effect in phase-separated CaMn0.9Ru0.1O3

Strong enhancement of exchange bias (EB) effect in phase-separated CaMn0.9Ru0.1O3 manganite with ferromagnetic phase heavily suppressed under pressure was observed in the measurements of field-cooled magnetization hysteresis loop. It was found that both EB field HE and normalized remanence asymmetry pronouncedly increase with increasing pressure P, enlarging by one order of magnitude at P = 10 kbar. In contrast, the coercive field HC shows a non-monotonic variation with pressure, reaching a maximum value at around 5 kbar. Such unusual HC(P) dependence may be explained by transformation from multi-domain to single-domain state in a system of ferromagnetic (FM) size-variable particles. Comparison of HE(P) with HC(P) data indicates remarkable enhancement of EB with decrease in HC in the pressure range between 5 and 10 kbar. It is argued that a strong increase of HE under pressure is attributed to the suppression of FM phase, i.e., to the decrease in size of FM clusters embedded in antiferromagnetic matrix. P...

The effect of Ni doping on the magnetic and transport properties in Pr0.5Ca0.5Mn1−xNixO3 manganites

Structural, magnetic, and transport properties in Pr0.5Ca0.5Mn1−xNixO3 (x=0, 0.04, 0.07, 0.1) were investigated. It is remarkable that low Ni-doping levels at Mn sites induce drastic changes in the physical properties of Pr0.5Ca0.5MnO3 due to melting of the charge ordered state and the consequent capability of Ni ions to create ferromagnetic (FM) clusters. It was found that oxygen deficient samples (3−δ=2.84±0.03) exhibit resistivities higher by four to five orders than that of their stoichiometric counterparts and do not exhibit metal-insulator transition. Only a stoichiometric x=0.04 sample with higher content of the FM phase shows metal-insulator transition at T≈80 K. A change in slope in the zero field cooling magnetization curve observed for x=0.04 and 0.07 (may be slightly oxygen deficient samples) are indicative of spin-glass-like state. Applied hydrostatic pressure of about 10 kbars reduces the temperature of charge ordering in x=0 sample by about 10 K indicating on pressure induced suppression of...

Pressure-induced oxygen-ordering processes in GdBa1.5Sr0.5Cu3O6+x

Abstract The influence of pressure on the superconducting properties of GdBa1.5Sr0.5Cu3O6+x (x = 0.63−0.93) ceramics has been studied. The pressure effect on Tc strongly depends on the oxygen content. Unlike YBa2Cu3O6+x the d T c d P value increases monotonically with decreasing x without the peak for x ≈ 0.7. The value d T c d P = 13.7 K (for x = 0.63) is the highest value for HTSCs of the 123 type. Under a prolonged action of pressure at room temperature the relaxation of the critical parameters (Tc, Hc1) as well as hysteretic phenomena in the case of pressure decrease have been observed. These effects are interpreted as being due to oxygen ordering in the CuO chains. The effect resulting from the oxygen ordering under pressure depends stronger on the oxygen index x than the intrinsic pressure effect connected only with the compression of the unit cell.

Pressure effect on Bi0.4Ca0.6Mn1−xRuxO3 manganite: Enhanced ferromagnetism and collapsed exchange bias

Pressure effect on magnetic state of Ru-doped charge-ordered manganite Bi0.4Ca0.6Mn1 − xRuxO3 (x = 0.1, 0.2) was investigated by magnetization measurements in the temperature range of 5−315 K and under pressure up to ∼10 kbar. It was found that the x = 0.2 composition is basically ferromagnetic (FM) while the low-doped (x = 0.1) one exhibits a FM cluster glass behavior and exchange bias (EB) effect at low temperatures. Bi0.4Ca0.6Mn0.9Ru0.1O3 demonstrates a substantial pressure-enhanced ferromagnetism, evidenced by ∼70% increase in spontaneous magnetization at 10 K under 10.3 kbar. It appears that an applied pressure strongly increases the FM to antiferromagnetic (AFM) phase ratio in the sample, leading to a notable decrease in both coercive field HC and EB field HE (HC and HE at 10 K decrease under pressure of ∼10 kbar by about 40% and 50%, respectively). The pressure-induced changes are qualitatively described within the simple model for FM size-variable clusters embedded in an AFM matrix. It is argued t...

Different path of pressure-induced oxygen ordering and disordering processes in YBa2Cu3O6+x near a metal-insulator transition

The relaxation of the superconducting transition temperature Tc in YBa2Cu3O6.38 is investigated with increasing oxygen order in the CuOx plane under 1 GPa pressure and with decreasing oxygen order after the pressure is relieved. It is established that the oxygen disordering process is more rapid than the pressure-induced ordering process: The ratio of the relaxation times of Tc in these processes τord/τdisord≈5. This behavior could be caused by different mechanisms of the pressure-induced increase in the Cu-O chain length and decrease of this length after pressure relief.