A.N. Ulyanov

Giant magnetic entropy change in La0.7Ca0.3MnO3 in low magnetic field

A giant magnetocaloric effect was found in La0.7Ca0.3MnO3 manganite at low magnetic fields. The magnetic entropy changes caused by a magnetic field change of 0.5 and 1.0 T were as large as 5.04 and 6.25 J kg−1 K−1, respectively, which exceed those of perovskite-like magnetic refrigerants or elemental Gd by a factor of 2–3. The extremely high values of the magnetocaloric effect are attributed to a sharp drop in the magnetization at a first-order ferromagnetic–paramagnetic phase transition and S-shaped magnetization versus field dependence.

Ferromagnetic ordering temperature and internal pressure in manganites La0.7Ca0.3-xSrxMnO3

Abstract Lanthanum manganites La0.7Ca0.3−xSrxMnO3 were synthesized and investigated. The jump of Tc at the magnetic ordering temperature dependence on concentration at x=0.15 was revealed. The structural phase transition from the orthorhombic phase (x

Metamagnetic transition and extremely large low field magnetocaloric effect in La0.7Ca0.3mnO3 manganite

Metamagnetic transition was observed in low magnetic field for La0.7Ca0.3MnO3−δ lanthanum manganite prepared by solid-state technology. Magnetization shape causes giant low field magnetocaloric effect at Curie temperature TC=242K. Magnetic entropy changes ΔSm, deduced with isothermal magnetization curves, are equal to 7.0 and 8.0J∕kgK in the field change from 0 to 1.0 and 2.0T, respectively. The obtained ΔSm are essentially higher than that reported before for lanthanum manganites with similar TC. The observation shows the way for the construction the magnetic refrigerants, working in the field of permanent magnets, which is beneficial for household and automotive applications.

Oxygen deficiency as a driving force for metamagnetism and large low field magnetocaloric effect in La0.7Ca0.3−xSrxMnO3−δ manganites

We report the importance of metamagnetism for the giant magnetocaloric effect in lanthanum manganites and the key role of oxygen deficiency for this phenomenon. It is found that La0.7Ca0.3−xSrxMnO3−δ samples with x=0.0, 0.025, 0.05, 0.075, 0.1, and 0.125 are orthorhombic (O) phase, while those with x=0.15, 0.175, 0.2, and 0.3 are rhombohedral (R) phase at room temperature. The samples possess relatively high oxygen deficiency δ. With increasing x, the oxygen content increases from about 2.92 to 2.94 in the O phase and from 2.93 to 2.97 in the R phase, while it decreases in the vicinity of the O-R phase transition. The Curie temperature increases with increasing x in both the structural phases and shows a positive jump around the O-R phase transition. The x=0 sample shows S-shape magnetization, which is typical for metamagnetic materials. The S shape is continuously suppressed with increase in Sr and disappears for x≥0.075. Interestingly, the magnetic entropy change, adiabatic temperature change, and refri...

Phase diagram and anomalous properties of La0.7M0.3MnO3 lanthanum manganites near the structural phase transition

The La0.7Ca0.3−xBaxMnO3 (x=0; 0.03; 0.06…0.3) and La0.7Ca0.3−xSrxMnO3 (x=0.105, 0.12, 0.135, 0.15, 0.165, and 0.18) lanthanum manganites were synthesized and studied. The Curie temperature TC of Ba range showed a sharp change near the concentration structural orthorhombic–rhombohedral phase transition. The TC values for that range are under the “classical” ferromagnetic–paramagnetic line. The TC temperatures of Sr range of samples are independent on x value at the vicinity of structural transition in the rhombohedral phase. Temperature dependence of magnetization of the La0.7Ca0.165Sr0.135MnO3 composition looks like a two-step curve which is caused by the closeness of ferromagnetic ordering and structural phase transition temperatures.

Critical state in the Y-Ba-Cu-O ceramics

The temperature and field dependences of the Y-Ba-Cu-O ceramics susceptibility have been studied both experimentally and theoretically. These dependence have been used to reconstruct the temperature dependence of the critical current (jc(T) = jc(0)(1 − T/Tc1)3/2, jc(0) = 340 A/cm2, Tc1 = 94.6 K) and the distribution of the granules with respect to the superconducting transition temperature (with the maximum at Tc2 = 95.3 K and ∆T = 1.1 K). Within the critical state concept diamagnetic response of the granular superconductors at fundamental and multiple frequencies has been calculated. The calculation results are in good agreement with the experimentally obtained susceptibility dependences and explain the different dependences of the diamagnetic response on the magnitude of the d.c. field and the amplitude of a.c. field.

Electron paramagnetic resonance study of La0.7Ca0.3−xBaxMnO3 lanthanum manganites

Electron paramagnetic resonance (ESR) study of La0.7Ca0.3−xBaxMnO3 manganites (x=0; 0.15; 0.3) is presented. Experimentally observed exponential decreasing of line intensity on temperature is in agreement with the deduced one and indicates the spin-lattice relaxation in manganites near the Curie temperature Tc. The obtained dependence of ESR linewidth on the x value shows the decreasing of strength of spin-lattice interaction with the increasing of Tc (caused by the increasing of the x value).

Local structure and destruction of electron pathway effects on Curie temperature of B-site-substituted lanthanum manganites

A study of B-site-substituted La0.7Ca0.3Mn1−xScxO3 lanthanum manganites is presented. Substitution of Sc3+ ion for Mn3+ ion damages a path of motion of itinerant eg electrons, thus causing a weakening of double-exchange interaction and a decrease of Curie temperature, TC (about 35K for each 1% Sc doping from x=0 to 0.03). We suggest that the observed strong decrease of the TC with x is mostly caused by a difference in an electron configuration of the Mn3+ and Sc3+ ions and partially due to a change in the local parameters of the MnO6 octahedra because of the larger Sc3+ ion than the Mn3+ one. A method for distinguishing the effect of the above two factors on Curie temperature is supposed.

EPR line intensity in La0.7Ca0.3−xBaxMnO3 manganites

Abstract The La0.7Ca0.3−xBaxMnO3 (x=0, 0.15, 0.3) manganites were studied by electron paramagnetic resonance (EPR). Temperature dependencies of EPR line intensity can be fitted by the deduced exponential decay I(T)=I 0 exp (E a /k B T) at more wide temperature range as compared with the Curie–Weiss law (Ea and kB are the activation energy and Boltzmann constant, respectively). A deviation of the EPR line intensity from the Curie–Weiss law near the Curie point indicates for the spin–lattice interaction in lanthanum manganites near that temperature. This interaction decreases at high temperatures. The exponential decay is valid in the conditions when the temperature dependence of EPR line width is proportional to the conductivity one.

Local structure and magnetic inhomogeneity of nano-sized La0.7Sr0.3MnO3 manganites

We present the study of the local structure and magnetic inhomogeneity of nano-sized La0.7Sr0.3MnO3 manganites performed by x-ray absorption fine structure (XAFS) spectroscopy and nuclear magnetic resonance (NMR). Nano-powders with particle size of 50–200 nm (NP1) and 20 nm (NP2), and bulk samples (BS) were studied. EXAFS data show the difference between the local structure of core and outer shell of the nano-particle. The data evidenced that the shell of particles presents the disordered crystalline array. According to the NMR, at low temperature the magnetic state of NP1 sample is homogenous. However, with increase of temperature the two ferromagnetic phases, with strong and weakened double exchange, coexist. The NP2 powder is magnetically homogeneous and characterized by weakened double exchange at all temperatures of the existence of the magneto-ordered state. The results specify on possible existence of characteristic spatial scale below which the phase stratification becomes energetically unfavorable.