arXiv subjects

J. F. Mitchell

Publications and source records attributed to J. F. Mitchell.

At least 127 records · Page 7Linked to original sources

arXiv2003

Low energy spin wave excitations in bilayered magnetic manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ ($0.30\leq{x}\leq{0.50}$)

We have studied the low-temperature behavior of the magnetization and the specific heat of the bilayered perovskite system $% La_{2-2x}Sr_{1+2x}Mn_{2}O_{7}$, for $0.30\leq x\leq 0.50$. Our analysis reveals that below 30 K the temperature dependence of the magnetization of the ferromagnetic samples, $x=0.30$, 0.32, and 0.36, in a field of 1T can be interpreted in terms of the thermal excitations of a two-dimensional gas of ferromagnetic magnons. The specific heat in zero field for these samples as for the $x=0.50$ antiferromagnetic one, is linear with temperature between the range of 1.8K $\leq $ T $\leq $ 10K. This behavior can be also explained by the magnon gas model. By comparing specific heat measurements in zero field with those taken in a field of 9T we are able to extract the lattice and electronic contributions and determine the in-plane exchange interactions. That are found to be in reasonable agreement with the values inferred from the analysis of the magnetization data and also with the values reported by inelastic neutron scattering studies. In addition, we found that the electronic density of states obtained for the $x=0.50$ sample is in agreement with previous band structure calculations.

cond-mat.mtrl-sci↗

arXiv2003

Spin freezing and magnetic inhomogeneities in bilayer manganites

We have performed a muon spin rotation study on polycrystalline samples of electron-doped layered manganites, La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (0.4< x<1), in order to investigate the local magnetic structure and spin dynamics. Our results provide evidence for phase separation into A-type antiferromagnetic and charge-ordered phases for x=0.52 and spin freezing at low temperatures (T<100K) for 0.52<x<0.75. A new phase diagram which includes this spin freezing region is proposed.

cond-mat.str-el↗

arXiv2002

Ultra-sharp magnetization steps in perovskite manganites

We report a detailed study of step-like metamagnetic transitions in the magnetization and resistivity of polycrystalline Pr0.5Ca0.5Mn0.95Co0.05O3. The steps have a sudden onset below a critical temperature, are extremely sharp (width < 0.2 mT), and occur at critical fields which are linearly dependent on the absolute value of the cooling field in which the sample is prepared. Similar transitions are also observed at low temperature in non-Co doped manganites, including single crystal samples. These data show that the steps are an intrinsic property, qualitatively different from either previously observed higher temperature metamagnetic transitions in the manganites or metamagnetic transitions observed in other materials.

cond-mat.mtrl-sci↗

arXiv2002

Tricritical Point and the Doping Dependence of the Order of the Ferromagnetic Phase Transition of La1-xCaxMnO3

We report the doping dependence of the order of the ferromagnetic metal to paramagnetic insulator phase transition in La1-xCaxMnO3. At x = 0.33, magnetization and specific heat data show a first order transition, with an entropy change (2.3 J/molK) accounted for by both volume expansion and the discontinuity of M ~ 1.7 Bohr magnetons via the Clausius-Clapeyron equation. At x = 0.4, the data show a continuous transition with tricritical point exponents alpha = 0.48+/- 0.06, beta = 0.25+/- 0.03, gamma = 1.03+/- 0.05, and delta = 5.0 +/- 0.8. This tricritical point separates first order (x<0.4) from second order (x>0.4) transitions.

cond-mat.str-el↗

arXiv2002

Structural and magnetic ordering in Pr0.65(CaySr1-y)0.35MnO3: "quantum critical point" versus phase segregation scenarios

The phase diagram of Pr0.65(CaySr1-y)0.35MnO3, 0.6 <= y <= 0.8, has been determined by neutron diffraction, magnetization and electrical conductivity measurements in order to investigate the nature of the transition between ferromagnetic metallic and charge-ordered insulating states near y=0.75. Two possible scenarios for this transition have been proposed: a "quantum critical point"-like feature, near which an associated charge-disordered paramagnetic phase is present, or a phase coexistence region. We demonstrate that the latter case is true, phase segregation occurring on a mesoscopic/macroscopic length-scale (several hundred Angstroms to several microns). Our results show that no significant amount of the charge-disordered paramagnetic phase is present at low temperatures. Our data also indicate that the charge-ordered insulator to ferromagnetic metal phase boundary is temperature as well as composition-dependent.

cond-mat.str-el↗

arXiv2002

Glass Transition in the Polaron Dynamics of CMR Manganites

Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polarons correlations are completely dynamic at high T, but then freeze upon cooling to a temperature T* 310 K. This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (Tc=114 K), where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.

cond-mat.str-el↗

arXiv2001

The Structure of Nanoscale Polaron Correlations in La1.2Sr1.8Mn2O7

A system of strongly-interacting electron-lattice polarons can exhibit charge and orbital order at sufficiently high polaron concentrations. In this study, the structure of short-range polaron correlations in the layered colossal magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by a crystallographic analysis of broad satellite maxima observed in diffuse X-ray and neutron scattering data. The resulting q=(0.3,0,1) modulation is a longitudinal octahedral-stretch mode, consistent with an incommensurate Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual orbital-stripe pattern parallel to the <100> directions.

cond-mat.mtrl-sci↗

arXiv2001

Low Temperature Magnetothermodynamics of Pr{0.7}Ca{0.3}MnO{3}

We present a detailed magnetothermal study of Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$, a perovskite manganite in which an insulator-metal transition can be driven by magnetic field, but also by pressure, visible light, x-rays, or high currents. We find that the field-induced transition is associated with a large release of energy which accounts for its strong irreversibility. In the ferromagnetic metallic state, specific heat and magnetization measurements indicate a much smaller spin wave stiffness than that seen in any other ferromagnetic manganite, which we explain in terms of ferromagnetism among the Pr moments. The Pr ferromagnetism also appears to influence the low temperature thermodynamic phase diagram of this material and the uniquely sensitive metastability of the insulating state.

cond-mat↗

arXiv2000

Phase Separation and the Low-Field Bulk Magnetic Properties of Pr0.7Ca0.3MnO3

We present a detailed magnetic study of the perovskite manganite Pr0.7Ca0.3MnO3 at low temperatures including magnetization and a.c. susceptibility measurements. The data appear to exclude a conventional spin glass phase at low fields, suggesting instead the presence of correlated ferromagnetic clusters embedded in a charge-ordered matrix. We examine the growth of the ferromagnetic clusters with increasing magnetic field as they expand to occupy almost the entire sample at H ~ 0.5 T. Since this is well below the field required to induce a metallic state, our results point to the existence of a field-induced ferromagnetic insulating state in this material.

cond-mat.str-el↗

arXiv2000

Soft spin waves in the low temperature thermodynamics of Pr_{0.7}Ca_{0.3}MnO_{3}

We present a detailed magnetothermal study of Pr(0.7)Ca(0.3)MnO(3), a perovskite manganite in which an insulator-metal transition can be driven by magnetic field, but also by pressure, visible light, x-rays, or high currents. We find that the field-induced transition is associated with an enormous release of energy which accounts for its strong irreversibility. In the ferromagnetic metallic state, specific heat and magnetization measurements indicate a much smaller spin wave stiffness than that seen in any other manganite, which we attribute to spin waves among the ferromagnetically ordered Pr moments. The coupling between the Pr and Mn spins may also provide a basis for understanding the low temperature phase diagram of this most unusual manganite.

cond-mat↗

arXiv2000

Interplay of spin and orbital ordering in the layered colossal magnetoresistance manganite La2-2xSr1+2xMn2O7 (0.5<=x<=1.0)

The crystallographic and magnetic phase diagram of the n=2 layered manganite La2-2xSr1+2xMn2O7 in the region x=>0.5 has been studied using temperature dependent neutron powder diffraction. The magnetic phase diagram reveals a progression of ordered magnetic structures generally paralleling that of 3-D perovskites with similar electronic doping: A (0.5<=x<=0.66) --> C (0.75<=x<=0.90) --> G (0.90<=x<=1.0). However, the quasi-2-D structure amplifies this progression to expose features of manganite physics uniquely accessible in the layered systems: (a) a "frustrated" region between the A and C regimes where no long-range magnetic order is observed; (b) magnetic polytypism arising from weak inter-bilayer magnetic exchange in the Type-C regime; and (c) a tetragonal to orthorhombic phase transition whose temperature evolution directly measures ordering of d3y2-r2 orbitals in the a-b plane. This orbital-ordering transition is precursory to Type-C magnetic ordering, where ferromagnetic rods lie parallel to the b-axis. These observations support the notion that eg orbital polarisation is the driving force behind magnetic spin ordering. Finally, in the crossover region between Type-C and Type-G states, we see some evidence for the development of local Type-C clusters embedded in a Type-G framework, directly addressing proposals of similar short-range magnetic ordering in highly-doped La1-xCaxMnO3 perovskites.

cond-mat.mtrl-sci↗

arXiv2000

Competition of charge, orbital, and ferromagnetic correlations in layered manganites

The competition of charge, orbital, and ferromagnetic interactions in layered manganites is investigated by magneto-Raman scattering spectroscopy. We find that the colossal magnetoresistance effect in the layered compounds results from the interplay of the orbital and ferromagnetic double-exchange correlations. Inelastic scattering by charge-order fluctuations dominates the quasiparticle dynamics in the ferromagnetic-metal state. The scattering is suppressed at low frequencies, consistent with the opening of a charge-density wave pseudogap.

cond-mat.str-el↗

arXiv2000

Mesoscopic and Microscopic Phase Segregation in Manganese Perovskites

Mesoscopic (500-2000 Angstrom) and microscopic (5-20 Angstrom) phase segregation with temperature and magnetic field was studied in the model manganite Pr0.7Ca0.3MnO3 by high-resolution neutron diffraction and inelastic neutron scattering. Intra-granular strain-driven mesoscopic segregation between two insulating phases, one of which is charge ordered (CO), sets in below the CO temperature in zero field. The CO phase orders antiferromagnetically, while the other insulating phase shows spin-glass behavior. After field-induced metallization, the CO phase coexists with a ferromagnetic metallic phase.

cond-mat.mtrl-sci↗

arXiv2000

Field Dependent Specific-Heat of Rare Earth Manganites

The low temperature specific heat C(H) of several rare-earth manganites (La_(0.7)Sr_(0.3)MnO_(3), Nd_(0.5)Sr_(0.5)MnO_(3), Pr_(0.5)Sr_(0.5)MnO_(3), La_(0.67)Ca_(0.33)MnO$_(3), La_(0.5)Ca_(0.5)MnO_(3), La_(0.45)Ca_(0.55)MnO_(3) and La_(0.33)Ca_(0.67)MnO_(3)) was measured as a function of magnetic field. We observed behaviour consistent with thermodynamic expectations, i.e., C(H) decreases with field for ferromagnetic metallic compounds by an amount which is in quantitative agreement with spin wave theory. We also find that C(H) increases with field in most compounds with a charge-ordered antiferromagnetic ground state. In compounds which show evidence of a coexistence of ferromagnetic metallic and antiferromagnetic charge-ordered states, C(H) displays some unusual non-equilibrium effects presumably associated with the phase-separation of the two states. We also observe a large anomalous low temperature specific heat at the doping induced metal-insulator transition (at x = 0.50) in La_(1-x)Ca_(x)MnO_(3).

cond-mat↗

arXiv2000

Time Dependent Effects and Transport Evidence for Phase Separation in La_{0.5}Ca_{0.5}MnO_{3}

The ground state of La_{1-x}Ca_{x}MnO_{3} changes from a ferromagnetic metallic to an antiferromagnetic charge-ordered state as a function of Ca concentration at x ~ 0.50. We present evidence from transport measurements on a sample with x = 0.50 that the two phases can coexist, in agreement with other observations of phase separation in these materials. We also observe that, by applying and then removing a magnetic field to the mainly charge-ordered state at some temperatures, we can "magnetically anneal" the charge order, resulting in a higher zero-field resistivity. We also observe logarithmic time dependence in both resistivity and magnetization after a field sweep at low temperatures.

cond-mat↗

arXiv1999

Observation of Kosterlitz-Thouless spin correlations in the colossally magnetoresistive layered manganite $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$

The spin correlations of the bilayer manganite $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$ have been studied using neutron scattering. On cooling within the paramagnetic state, we observe purely two-dimensional behavior with a crossover to three-dimensional scaling close to the ferromagnetic transition. Below $T_C$, an effective finite size behavior is observed. The quantitative agreement of these observations with the conventional quasi two-dimensional Kosterlitz-Thouless model indicates that the phase transition is driven by the growth of magnetic correlations, which are only weakly coupled to polarons above $T_C$.

cond-mat.str-el↗

arXiv1999

Charge melting and polaron collapse in $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$

X-ray and neutron scattering measurements directly demonstrate the existence of polarons in the paramagnetic phase of optimally-doped colossal magnetoresistive oxides. The polarons exhibit short-range correlations that grow with decreasing temperature, but disappear abruptly at the ferromagnetic transition because of the sudden charge delocalization. The "melting" of the charge ordering as we cool through $T_C$ occurs with the collapse of the quasi-static polaron scattering, and provides important new insights into the relation of polarons to colossal magnetoresistance.

cond-mat.str-el↗

arXiv1999

Lattice Displacements Above Tc in the Layered Manganite La1.2Sr1.8Mn2O7

Neutron diffraction data presented in this paper demonstrates the relevance of lattice displacement above TC, in our understanding of the evolution of the crystal structure with temperature in the layered CMR manganite La1.2Sr1.8Mn2O7. The anomalous temperature behavior of thermal diffuse scattering (TDS) in La1.2Sr1.8Mn2O7 strongly suggests that it arises from lattice displacements and correlates directly with anomalies in the displacement parameters of the O- and Mn-atoms and Mn-O bond lengths. From our measurements, the insulator - metal transition can be described as a transition from a high temperature state with disordered Mn-O bond lengths to a low temperature state with a more uniform distribution on Mn-O bonds. These observations are in agreement with polaronic charge transport above TC in the perovskite manganites; as electron hopping is responsible for bond disorder above TC, below the transition where eg carries are delocalized, any lattice displacements are uniformly averaged.

cond-mat.mtrl-sci↗