Elastic and antiferromagnetic anomalies in Pr 0.48 Ca 0.52 MnO 3 as determined by resonant ultrasonic spectroscopy (original) (raw)
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Physical Review B, 2010
Elastic and anelastic properties of single crystal samples of EuTiO 3 have been measured between 10 and 300 K by resonant ultrasound spectroscopy at frequencies in the vicinity of 1 MHz. Softening of the shear elastic constants C 44 and 1 2 (C 11 − C 12 ) by ∼20-30% occurs with falling temperature in a narrow interval through the transition point, T c = 284 K, for the cubic-tetragonal transition. This is accounted for by classical coupling of macroscopic spontaneous strains with the tilt order parameter in the same manner as occurs in SrTiO 3 . A peak in the acoustic loss occurs a few degrees below T c and is interpreted in terms of initially mobile ferroelastic twin walls, which rapidly become pinned with further lowering of temperature. This contrasts with the properties of twin walls in SrTiO 3 , which remain mobile down to at least 15 K. No further anomalies were observed that might be indicative of strain coupling to any additional phase transitions above 10 K. A slight anomaly in the shear elastic constants, independent of frequency and without any associated acoustic loss, was found at ∼140 K. It marks a change from elastic stiffening to softening with falling temperature and perhaps provides evidence for coupling between strain and local fluctuations of dipoles related to the incipient ferroelectric transition. An increase in acoustic loss below ∼80 K is attributed to the development of dynamical magnetic clustering ahead of the known antiferromagnetic ordering transition at ∼5.5 K. Detection of these elastic anomalies serves to emphasize that coupling of strain with tilting, ferroelectric, and magnetic order parameters is likely to be a permeating influence in determining the structure, stability, properties, and behavior of EuTiO 3 .
Symmetry and strain analysis of structural phase transitions inPr0.48Ca0.52MnO3
Physical Review B, 2010
Structural evolution as a function of temperature through the Pnma ↔ incommensurate ͑IC͒ phase transition in Pr 0.48 Ca 0.52 MnO 3 perovskite has been analyzed from the perspectives of symmetry and strain. The structure and stability of both phases are shown to depend on combinations of order parameters which have symmetries associated with irreducible representations M
Strain mechanism for order-parameter coupling through successive phase transitions inPrAlO3
Physical Review B, 2005
High-resolution powder neutron diffraction data have been collected at ϳ50 temperatures between 4.2 and 300 K for perovskites Pr 1−x La x AlO 3 with x = 0, 0.05, 0.1, and 0.25. Structural and lattice parameter data extracted by Rietveld refinement have then been used to follow details of octahedral tilting and spontaneous strains associated with the sequence of phase transitions Pm3m ↔ R3c ↔ Imma ↔ C2/m. These are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a Pm3m reference structure with two instabilities ͑R 4 + and ⌫ 3 + active͒. Data from the literature relating to the Pm3m ↔ R3c transition are consistent with second-order character and an extrapolated transition temperature of 1864± 31 K. The R3c ↔ Imma transition is first order in character and can be understood as occurring because coupling takes place between tilting and electronic order-parameter components via a common tetragonal strain. Strains for the Imma ↔ C2/m transition conform closely to the Landau solution for a proper ferroelastic transition with second-order character and low-temperature saturation. The acoustic anomaly reported at 118 K appears to be associated with a metrically tetragonal structure which develops, as an accidental strain degeneracy, at a temperature between ϳ110 and ϳ120 K. Differences in saturation temperatures for the order parameters associated with tilting and electronic ordering are probably responsible for small additional anomalies in the evolution of strains below ϳ110 K.
Physical Review B, 2006
Magnetic properties of polycrystalline Pr 0.5 Ca 0.5 Mn 1−x Cr x O 3 ͑x = 0.015, 0.03͒ samples have been investigated in the temperature range 1.5-300 K, magnetic field up to 16 kOe and under hydrostatic pressures up to 11 kbar. The studies involved sequential zero field cooled ͑ZFC͒ magnetization ͑M͒ measurements followed by measurements during cooling in the same magnetic field ͑H͒. Additional measurements of magnetization vs H were carried out at ambient and applied pressure. The volume of the ferromagnetic phase for x = 0.015 sample exhibits a considerable decrease upon thermal cycling even after many cycles. On the other hand, both the Curie temperature ͑T C ͒ and the width of thermal hysteresis remain practically unchanged during the thermal cycling. In contrary to the effect of thermal cycling, applied pressure enhances T C and the volume of the ferromagnetic phase below T C and reduces considerably the width of thermal hysteresis. A ferromagnetic contribution to magnetization due to short range ferromagnetic coupling was observed in the vicinity of the charge order transition temperature T CO Ϸ 220 K; it appears to enhance under pressure. In the case of Pr 0.5 Ca 0.5 Mn 0.97 Cr 0.03 O 3 an applied pressure also increases the magnetization and T C with a pressure coefficient of dT C / dP Ϸ 1.8 K / kbar, at the same time the hysteresis in the vicinity of T C narrows. The results observed are related to the interfacial elastic energy at the phase boundaries of the phase separated Cr doped manganites. The effect of the suppression of the Jahn-Teller distortions and of the electron-phonon coupling in interfacial regions and in ferromagnetic domains under an applied hydrostatic pressure is also discussed.
2000
The coexistence of ferromagnetic metallic and antiferromagnetic charge-ordered ͑CO͒ states in Pr 2/3 Ca 1/3 MnO 3 has been investigated using zero-field muon spin relaxation, neutron diffraction, calorimetric, and magnetic measurements. Calorimetric data evidence a high degree of disorder below the CO transition. The data are consistent with large antiferromagnetic ͑AFM͒ CO regions containing structural and magnetic inhomogeneities densely scattered. Below the charge order temperature (T CO Ϸ220 K) the dominant relaxation mechanism of the muon polarization is based on ferromagnetic Mn-Mn correlations and the spin-lattice relaxation rate is peaked at T C instead of T N. The results agree with a spatial distribution of ferromagnetic and AFM regions strongly interpenetrated. The presence of local magnetic order in the whole sample is only achieved just below T C Ϸ120 K.
Journal of Magnetism and Magnetic Materials, 2009
Nd 1Àx Sr x MnO 3 perovskite manganite material with different compositions (x ¼ 0.31, 0.35, 0.37, 0.39 and 0.41) have been prepared employing solid-state reaction technique. The ultrasonic velocities and attenuation of the above samples have been measured employing through transmission method operated at a fundamental frequency of 5 MHz over wide range of temperatures. The temperature dependence of ultrasonic velocities, attenuation, relative percentage variation in velocities and elastic constants show an interesting anomaly in all compositions. The observed anomalies in ultrasonic parameters at T c in all compositions have been revealed in terms of existence of ferromagnetic (FM) state. Similarly, the anomalies at T co show the transition from FM to charge-ordered antiferromagnetic (AFM) state. The observed results have been used to explore the competitions between FM and AFM.
Proceedings of the National Academy of Sciences, 2007
We use neutron scattering to study the lattice and magnetic structure of the layered half-doped manganite Pr0.5Ca1.5MnO4. On cooling from high temperature, the system first becomes chargeand orbital-ordered (CO/OO) near TCO = 300 K and then develops checkerboard-like antiferromagnetic (AF) order below TN = 130 K. At temperatures above TN but below TCO (TN < T < TCO), the appearance of short-range AF spin correlations suppresses the CO/OO induced orthorhombic strain, contrasting with other half-doped manganites, where AF order has no observable effect on the lattice distortion. These results suggest that a strong spin-lattice coupling and the competition between AF exchange and CO/OO ordering ultimately determines the low-temperature properties of the system.
Structure and magnetism in the Pr1−xNaxMnO3 perovskites (0 ⩽ x ⩽ 0.2)
Journal of Magnetism and Magnetic Materials, 2002
The mixed-valence manganites Pr 1Àx Na x MnO 3 have been investigated by neutron diffraction, electric transport and magnetic measurements. Similarly to related systems with divalent alkali earths, the increasing monovalent sodium substitution decreases the Jahn-Teller deformation of the MnO 6 octahedra, lowers the resistivity and changes gradually the magnetic ordering from the layered type antiferromagnetism (x ¼ 0) through spin-canted arrangements (xB0:05) to the pure ferromagnetism (0:10pxp0:15) with T C B125 K. The samples with ferromagnetic ground state are not metallic below T C but show appreciable magnetoresistive effects in a broad temperature region. The electronic localization at low temperatures is further enhanced in the sample with the maximum sodium content xB0:2: Electron and neutron diffraction evidences that Pr 0.8 Na 0.2 MnO 3 exhibits a commensurate charge and orbital ordering of the Mn 3+ /Mn 4+ (1:1) kind below T co ¼ 215 K, followed with a transition to the antiferromagnetic arrangement of pseudo-CE type at T N ¼ 175 K, analogous to that of previously studied Pr 0.65 Ca 0.35 MnO 3 . In addition, Pr 0.8 Na 0.2 MnO 3 undergoes below B50 K a spin reorientation and, simultaneously, ferromagnetic clusters in the charge-ordered matrix are formed. By application of external field of 2-5 T below T co ; the insulating charge-ordered antiferromagnet is transformed to a metallic ferromagnetic state which is persistent below B60 K, i.e. temperature close to the spin reorientation transition. r
Physical Review B, 2015
A Raman spectroscopy study on the half-doped single-layer manganite Pr 0.5 Ca 1.5 MnO 4 has been performed in combination with x-ray diffraction, resistivity, magnetization, and specific heat measurements. The results provide insight into the underlying mechanisms of phenomena that arise from correlations between lattice, charge, orbital, and spin degrees of freedom. The appearance of a new Raman mode at 366 cm −1 , a visible jump in the resistivity, and a doubling of the unit cell signify the onset of charge/orbital ordering at 320 K. This transition is also marked by a sharp peak in the magnetic susceptibility and specific heat, suggesting strong spin-charge coupling. Our structural analysis suggests that the charge disproportionation below 320 K is small. Orbital fluctuations below 320 K are evidenced by the evolution with temperature of the Jahn-Teller Raman mode (situated at 533 cm −1 at 320 K). A coincidence between the onset of two-dimensional short-range antiferromagnetic order at 215 K and anomalies in both the temperature dependence of the Jahn-Teller mode and the Mn-O bonding pattern in the ab plane indicate that the short-range magnetic order plays a role in stabilizing the orbital fluctuations. Below the Néel temperature of 127 K, the softening of both the 366 cm −1 mode and an octahedral tilting mode at 214 cm −1 mark the onset of three-dimensional antiferromagnetic ordering. The estimated spin-phonon coupling constants for these two modes are 2.6 and 6.8 cm −1 , respectively. This study highlights the remarkable coupling of charge, orbital, and spin degrees of freedom to the lattice in single-layer Pr 0.5 Ca 1.5 MnO 4 .