Long-scale phase separation versus homogeneous magnetic state in (La_ {1-y} Pr_ {y}) _ {0.7} Ca_ {0.3} MnO_ {3}: A neutron diffraction study (original) (raw)
Physical Review B, 2012
Bulk manganites of the form La 5/8−y Pr y Ca 3/8 MnO 3 (LPCMO) exhibit a complex phase diagram due to coexisting charge-ordered antiferromagnetic (CO/AFM), charge-disordered paramagnetic (PM), and ferromagnetic (FM) phases. Because phase separation in LPCMO occurs on the microscale, reducing particle size to below this characteristic length is expected to have a strong impact on the magnetic properties of the system. Through a comparative study of the magnetic and magnetocaloric properties of single-crystalline (bulk) and nanocrystalline LPCMO (y = 3/8) we show that the AFM, CO, and FM transitions seen in the single crystal can also be observed in the large particle sizes (400 and 150 nm), while only a single PM to FM transition is found for the small particles (55 nm). Magnetic and magnetocaloric measurements reveal that decreasing particle size affects the balance of competing phases in LPCMO and narrows the range of fields over which PM, FM, and CO phases coexist. The FM volume fraction increases with size reduction, until CO is suppressed below some critical size, ∼100 nm. With size reduction, the saturation magnetization and field sensitivity first increase as long-range CO is inhibited, then decrease as surface effects become increasingly important. The trend that the FM phase is stabilized on the nanoscale is contrasted with the stabilization of the charge-disordered PM phase occurring on the microscale, demonstrating that in terms of the characteristic phase separation length, a few microns and several hundred nanometers represent very different regimes in LPCMO.
The European Physical Journal B, 2001
The atomic structure of (La1−yPry)0.7Ca0.3MnO3 compound with 0.5 ≤ y ≤ 1 has been systematically studied by neutron powder diffraction in the temperature range from 15 to 293 K. For composition with y = 0.75, the structural analysis was performed on two samples, one containing the natural mixture of oxygen isotopes and the other one 75% enriched by 18 O. The room temperature structural characteristics of the series, including cell volume, average Mn-O bond distance, and average Mn-O-Mn bond angle, are the linear functions of the rA . Temperature dependencies of these parameters are quite smooth, except for the point T = TFM, where a jump like changes occur. The isotope enriched samples have been found identical in crystal and magnetic structure down to the temperature of transition of the sample with 16 O into the metallic ferromagnetic phase. It confirms that different transport and magnetic properties of the samples with 16 O and 18 O at low temperature are driven by the different oxygen atoms dynamics solely. Temperature dependencies of the CO and AFM diffraction peak intensities and of the peak widths for compositions close to the metal-insulator boundary (y ≈ 0.75) indicate the macroscopically phase separated AFM-dielectric + FM-metallic state below TFM. PACS. 75.30.Vn Colossal magnetoresistance -61.12.Ld Single-crystal and powder diffraction
Unconventional Ferromagnetic Transition in La_ {1-x} Ca_ {x} MnO_ {3}
Physical review letters, 1996
Neutron scattering has been used to study the magnetic correlations and long wavelength spin dynamics of La 1-x Ca x MnO 3 in the ferromagnetic regime (0≤x<1 / 2). For x = 1 / 3 (T C = 250 K) where the magnetoresistance effects are largest the system behaves as an ideal ...
Imaging the First-Order Magnetic Transition in La_{0.35}Pr_{0.275}Ca_{0.375}MnO_{3}
Physical Review Letters, 2012
The nature of the ferromagnetic, charge, orbital, and antiferromagnetic order in La 0:35 Pr 0:275 Ca 0:375 MnO 3 on the nano-and microscale was investigated by photoemission electron microscopy (PEEM) and resonant elastic soft x-ray scattering (RSXS). The structure of the ferromagnetic domains around the Curie temperature T C indicates that they nucleate under a high degree of lattice strain, which is brought about by the charge, orbital, and antiferromagnetic order. The combined temperaturedependent PEEM and RSXS measurements suggest that the lattice distortions associated with charge and orbital order are glassy in nature and that phase separation is driven by the interplay between it and the more itinerant charge carriers associated with ferromagnetic metallic order, even well below T C .
Phase separations in La0.7−xDyxCa0.3Mn(Fe)O3
Mössbauer spectroscopy of La 0.7−x Dy x Ca 0.3 Mn͑Fe͒O 3 shows phase separation, both above and below T C . The behavior in the two regions are independent and related to different phenomena. Above T C , the phase splitting is due to different lattice distortions. In one of the phases, Jahn-Teller distortion is significant. Below T C , the spin orderings in the two phases are ferromagnetic ͑FM͒ and spin glass ͑SG͒ orderings, respectively. At low temperatures, SG phase converts into FM phase when the applied magnetic field increases, but only partially. We find a drastic increase in magnetoresistivity ͑MR͒ as x is increased beyond 0.07. It attains a maximum value for x Ϸ 0.12 ͑greater than 750 000%͒ and decreases again rapidly as x increases. We have provided an explanation for the high MR in x = 0.12 phase. Other differences between the compositions studied here can also be identified. First, the phase with Jahn-Teller distortion disappears when the temperature is lowered to T C in a composition with x ജ 0.12. This is not the case when x = 0.07. Second, we observe anomalous hysteresis loop when x = 0.12. The anomaly is less pronounced in other compositions. Third, in composition x = 0.07, the hyperfine magnetic fields ͑H int ͒ in FM and SG components are distinctly different. This is not so when x = 0.12.
Possible magnetic phase separation in Ru-doped La 0. 67 Ca 0. 33 MnO 3.
Journal of Magnetism …, 2003
X-ray diffraction, resistivity, ac susceptibility and magnetization studies on La 0.67 Ca 0.33 Mn 1-x Ru x O 3 (0 ≤ x < 0.1) were carried out. A significant increase in the lattice parameters indicated the presence of mixed valance state of Ru: Ru 3+ and Ru 4+ . The resistivity of the doped compounds exhibited two features: a broad maximum and a relatively sharp peak. While a para to ferromagnetic transition could be observed for the latter peak, no magnetic signal either in ac susceptibility or in magnetization measurements could be observed for the broad maximum. The magnetic moment decreases non linearly from 3.55 to 3 µ B over the Ru composition from 0 to 8.5 at.%. Based on the results of the present studies and on existing literature on the Mn-site substituted systems, we argue that a magnetic phase separation occurs in the Ru doped system. While the sharp peak in the resistivity corresponds to Ru 4+ enriched region with a ferromagnetic coupling with neighboring Mn ions, the broad peak corresponds to a Ru 3+ rich regions, with an antiferromagnetic coupling with neighboring Mn ions.
Physical Review B, 2007
Effect of oxygen isotope substitution and crystal micro-structure on magnetic ordering and phase separation in (La 1−y Pr y ) 0.7 Ca 0.3 MnO 3 Abstract The crystal and magnetic structures of the specified CMR manganite system have been studied as a function of y = (0.2 − 1) across the metal-insulator (MI) transition, and of the oxygen mass ( 16 O, 18 O). We quantitatively show how the polaronic narrowing of the carrier bandwidth and the crystal lattice micro-strains control the volume fractions of the mesoscopic ferro-and antiferromagnetic clusters. A well-defined dip in the transition temperatures and the suppression of all the types of long range ordering seen near the MI-transition at y ≃ 0.9 indicate a key role of the quenched disorder for the formation of the long-scale phase separated state. PACS numbers: 75.47.Gk, 61.12.Ld, 75.30.-m,
Journal of the Physical Society of Japan, 2004
We investigated the structural phase transition occurring around 760 K in a colossal magnetoresistance material, La 0:625 Ca 0:375 MnO 3. In order to uncover detailed features of this transition between the high temperature rhombohedral (R 3 3c) and the low temperature orthorhombic phase (Pnma), high resolution neutron powder diffraction, resistivity measurements, and elastic constant measurements were carried out up to 1000 K. From the Rietveld refinement analysis of the powder diffraction data, it is shown that the transition is of first order as expected from symmetry and that there exists a temperature region of $40 K around the transition temperature where the two phases coexist. This phase coexistence in the transition region accounts for the results of the elastic constant and resistivity measurements. Neutron powder diffraction data also allowed us to determine the tilt angles of the soft lattice modes, R 25 and M 3 , relevant to the phase transition.
Neutron scattering study of magnetic phase separation in nanocrystalline La5/8Ca3/8MnO3
Physical Review B, 2011
We demonstrate that magnetic phase separation and competing spin order in the colossal magnetoresistive (CMR) manganites can be directly explored via tuning strain in bulk samples of nanocrystalline La1-xCaxMnO3. Our results show that strain can be reversibly frozen into the lattice in order to stabilize coexisting antiferromagnetic domains within the nominally ferromagnetic metallic state of La5/8Ca3/8MnO3. The measurement of tunable phase separation via magnetic neutron powder diffraction presents a direct route of exploring the correlated spin properties of phase separated charge/magnetic order in highly strained CMR materials and opens a potential avenue for realizing intergrain spin tunnel junction networks with enhanced CMR behavior in a chemically homogeneous material.