arXiv subjects

L. Ghivelder

Publications and source records attributed to L. Ghivelder.

43 records · Page 3Linked to original sources

arXiv2001

Magnetization studies of phase separation in La$_{0.5}$Ca$_{0.5}$MnO$_{3}$

We present magnetization studies in a series of phase separated La$_{0.5}$Ca$%_{0.5}$MnO$_{3}$ samples, with different low temperature fractions of the ferromagnetic (FM) and charge ordered-antiferromagnetic (CO-AFM) phases. A particular experimental procedure probes the effect of the magnetic field applied while cooling the samples, which promotes FM fraction enlargement and enhances the melting of the CO phase. The response of the system depending on its magnetic field history indicates the existence of three different regimes in the phase separated state which develops below T$_{C}$. Our data allows us to identify the onset temperature below which the system becomes magnetic and structurally phase separated, and an onset field above which FM fraction enlargement occurs.

cond-mat.str-el↗

arXiv2001

Magnetic relaxation phenomena and cluster glass properties of La{0.7-x}Y{x}Ca{0.3}MnO{3} manganites

The dynamic magnetic properties of the distorted perovskite system La{0.7-x}Y{x}Ca{0.3}MnO{3} (0 <= x <= 0.15) have been investigated by ac-susceptibility and dc magnetization measurements, with emphasis on relaxation and aging studies. They evidence for x >= 0.10 the appearance of a metallic cluster glass phase, that develops just below the ferromagnetic transition temperature. The clusters grow with decreasing temperature down to a temperature T(f0) at which they freeze due to severe intercluster frustration. The formation of these clusters is explained by the presence of yttrium induced local structural distortions that create localized spin disorder in a magnetic lattice where double-exchange ferromagnetism is dominant.

cond-mat.str-el↗

arXiv2001

Magnetoresistance induced by low-field control of phase separation in La0.5Ca0.5MnO3

The effect of low magnetic fields on the transport properties of a manganite compound with magnetic phase separation is studied. The different behavior obtained in samples of La0.5Ca0.5MnO3 related to the way in which the low field is applied is consistent with a picture of changes in the metallic fraction induced by the magnetic field in a field-cooled cycle. Using a simple model of conduction through a binary mixture, the interrelation between magnetoresistance, metallic fraction and percolation temperature is accounted for. A new physical coefficient relating magnetic field and metallic fraction emerges as the relevant parameter in the description of phase separated manganites.

cond-mat.mtrl-sci↗

arXiv2000

Controlled Phase Separation in La_0.5 Ca_0.5 Mn O_3

A systematic study of phase separation effects in polycrystalline La_0.5 Ca_0.5 Mn O_3 obtained under different thermal treatments is reported. Samples with average grain size ranging from 200 to 1300 nm were studied. Magnetic and electrical measurements show quantitative differences among samples in their low temperature behavior. indicating that the fraction of the ferromagnetic (FM) phase gradually decreases as the grain size increase. Percolation of the FM phase in samples with a small fraction of this phase suggests that grain boundaries play a distinctive role in the spatial distribution of coexisting phases. The defective structure at the grain surface could explain the local inhibition of the antiferromagnetic charge ordered phase, an effect that is gradually removed with increasing grain size. Qualitative agreement of the data with this description is found. This effect is also found to be highly dependent on the oxygen content of the samples and its spatial distribution.

cond-mat.mtrl-sci↗

arXiv2000

Static critical behavior of the ferromagnetic transition in LaMnO3.14 manganite

The ferromagnetic phase transition in LaMnO3.14 is investigated by measuring the dc magnetization as a function of magnetic field and temperature. Modified Arrott plot and Kouvel Fisher analysis yield estimates for the critical exponents beta, and gama, with values between that predicted for the Heisenberg model and mean field theory. At low fields we found an anomalous small value of beta, indicating that the critical behavior is influenced by the range of magnetic fields used.

cond-mat.stat-mech↗

arXiv2000

Low-temperature specific heat of La0.78Pb0.22MnO3-d manganite

The specific heat of the perovskite manganite La0.78Pb0.22MnO3-d was measured from 0.5 to 25 K. The results yield values of the electronic density of states, the Debye temperature, and the spin-wave stiffness constant of the compound. The magnetic contribution was found to be consistent with the existence of a gap in the spin-wave spectrum, and a phonon term in T^5 was needed to fit the data.

cond-mat.str-el↗

arXiv1999

Specific heat and magnetic order in LaMnO_{3+δ}

Magnetic and specific-heat measurements are performed in three different samples of LaMnO_{3+δ}, with δ=0.11, 0.15 and 0.26, presenting important disorder effects, such as carrier localization, due to high amounts of La and Mn vacancies. For the samples with δ=0.11 and 0.15, magnetic measurements show signatures of a two-step transition: as the temperature is lowered, the system enters a ferromagnetic phase followed by a disorder-induced cluster-glass state. Spin-wave-like contributions and an unexpected large linear term are observed in the specific heat as a function of temperature. In the sample with the highest vacancy content, δ=0.26, the disorder is sufficient to suppress even short-range ferromagnetic order and yield a spin-glass-like state.

cond-mat↗