arXiv subjects

P. Schiffer

Publications and source records attributed to P. Schiffer.

At least 91 records · Page 5Linked to original sources

arXiv2002

Dirty Spin Ice: The Effect of Dilution on Spin Freezing in Dy2Ti2O7

We have studied spin freezing in the diluted spin ice compound Dy2-xYxTi2O7 where the non-magnetic Y ions replace the magnetic Dy ions on the frustrated pyrochlore lattice. Magnetic a.c. and d.c. susceptibility data are presented with an analysis of relaxation times for dilutions of x = 0, 0.1, 0.2, and 0.4. Site dilution apparently decreases the relative number of spins participating in the ice-like freezing near 16 K while leaving the freezing temperature unchanged. Correspondingly the distribution of relaxation times associated with the freezing is broadened only slightly with increasing dilution, suggesting that the freezing process observed near T = 16 K involves local correlations among the spins.

cond-mat.mtrl-sci↗

arXiv2002

Avalanche Dynamics in Wet Granular Materials

We have studied the dynamics of avalanching wet granular media in a rotating drum apparatus. Quantitative measurements of the flow velocity and the granular flux during avalanches allow us to characterize novel avalanche types unique to wet media. We also explore the details of viscoplastic flow (observed at the highest liquid contents) in which there are lasting contacts during flow, leading to coherence across the entire sample. This coherence leads to a velocity independent flow depth at high rotation rates and novel robust pattern formation in the granular surface.

cond-mat.dis-nn↗

arXiv2002

Saturated Ferromagnetism and Magnetization Deficit in Optimally Annealed (Ga,Mn)As Epilayers

We examine the Mn concentration dependence of the electronic and magnetic properties of optimally annealed Ga1-xMnxAs epilayers for 1.35% < x < 8.3%. The Curie temperature (Tc), conductivity, and exchange energy increase with Mn concentration up to x ~ 0.05, but are almost constant for larger x, with Tc ~ 110 K. The ferromagnetic moment per Mn ion decreases monotonically with increasing x, implying that an increasing fraction of the Mn spins do not participate in the ferromagnetism. By contrast, the derived domain wall thickness, an important parameter for device design, remains surprisingly constant.

cond-mat↗

arXiv2002

Above-Room-Temperature Ferromagnetism in GaSb/Mn Digital Alloys

Digital alloys of GaSb/Mn have been fabricated by molecular beam epitaxy. Transmission electron micrographs showed good crystal quality with individual Mn-containing layers well resolved; no evidence of 3D MnSb precipitates was seen in as-grown samples. All samples studied exhibited ferromagnetism with temperature dependent hysteresis loops in the magnetization accompanied by metallic p-type conductivity with a strong anomalous Hall effect (AHE) up to 400 K (limited by the experimental setup). The anomalous Hall effect shows hysteresis loops at low temperatures and above room temperature very similar to those seen in the magnetization. The strong AHE with hysteresis indicates that the holes interact with the Mn spins above room temperature. All samples are metallic, which is important for spintronics applications. * To whom correspondence should be addressed. E-mail: luo@buffalo.edu

cond-mat.mtrl-sci↗

arXiv2002

Magnetic relaxation in La0.250Pr0.375Ca0.375MnO3 with varying phase separation

We have studied the magnetic relaxation properties of the phase-separated manganite compound La0.250Pr0.375Ca0.375MnO3 . A series of polycrystalline samples was prepared with different sintering temperatures, resulting in a continuous variation of phase fraction between metallic (ferromagnetic) and charge-ordered phases at low temperatures. Measurements of the magnetic viscosity show a temperature and field dependence which can be correlated to the static properties. Common to all the samples, there appears to be two types of relaxation processes - at low fields associated with the reorientation of ferromagnetic domains and at higher fields associated with the transformation between ferromagnetic and non-ferromagnetic phases.

cond-mat.str-el↗

arXiv2002

Experimental determination of superconducting parameters for the intermetallic perovskite superconductor ${\text {MgCNi}}_3

We have measured upper-critical-field $H_{\text c2}$, specific heat C, and tunneling spectra of the intermetallic perovskite superconductor MgCNi${}_3$ with a superconducting transition temperature $T_{\text c}\approx 7.6$ K. Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field $H_{\text c}$(0), coherence length $ξ$(0), penetration depth $λ$(0), lower-critical-field $H_{\text c1}$(0), and Ginsberg-Landau parameter $κ$(0). From the specific heat, we obtain the Debye temperature $\it Θ_{\text D} \approx$ 280 K. We find a jump of $ΔC/γT_{\text c}$=2.3 at $T_{\text c}$ (where $\it γ$ is the normal state electronic specific coefficient), which is much larger than the weak coupling BCS value of 1.43. Our tunneling measurements revealed a gap feature in the tunneling spectra at $\it Δ$ with $2\it Δ/{\text k}_{\text B}T_{\text c}\approx$ 4.6, again larger than the weak-coupling value of 3.53. Both findings indicate that MgCNi$_3$ is a strong-coupling superconductor. In addition, we observed a pronounced zero-bias conductance peak (ZBCP) in the tunneling spectra. We discuss the possible physical origins of the observed ZBCP, especially in the context of the pairing symmetry of the material.

cond-mat.supr-con↗

arXiv2001

Granular Drag on a Discrete Object: Shape Effects on Jamming

We study the drag force on discrete objects with circular cross section moving slowly through a spherical granular medium. Variations in the geometry of the dragged object change the drag force only by a small fraction relative to shape effects in fluid drag. The drag force depends quadratically on the object's diameter as expected. We do observe, however, a deviation above the expected linear depth dependence, and the magnitude of the deviation is apparently controlled by geometrical factors.

cond-mat.dis-nn↗

arXiv2001

Effects of annealing time on defect-controlled ferromagnetism in Ga1-xMnxAs

We have studied the evolution of the magnetic, electronic, and structural properties of annealed epilayers of Ga1-xMnxAs grown by low temperature molecular beam epitaxy. Annealing at the optimal temperature of 250 C for less than 2 hours significantly enhances the conductivity and ferromagnetism, but continuing the annealing for longer times suppresses both. These data indicate that such annealing induces the defects in Ga1-xMnxAs to evolve through at least two different processes, and they point to a complex interplay between the different defects and ferromagnetism in this material.

cond-mat.mtrl-sci↗

arXiv2001

Two-Carrier Transport in Epitaxially Grown MnAs

Magneto-transport measurements of ferromagnetic MnAs epilayers grown by molecular beam epitaxy reveal the presence of both positive and negative charge carriers. Electrical transport at high temperatures is dominated by holes, and at low temperatures by electrons. We also observe distinct changes in the magnetoresistance associated with the transition between the electron- and hole-dominated transport regimes. These results are of direct relevance to MnAs/semiconductor hybrid heterostructures and their exploitation in electronic and optical spin injection experiments.

cond-mat.mtrl-sci↗

arXiv2001

Stick-Slip Fluctuations in Granular Drag

We study fluctuations in the drag force experienced by an object moving through a granular medium. The successive formation and collapse of jammed states give a stick-slip nature to the fluctuations which are periodic at small depths but become "stepped" at large depths, a transition which we interpret as a consequence of the long-range nature of the force chains and the finite size of our experiment. Another important finding is that the mean force and the fluctuations appear to be independent of the properties of the contact surface between the grains and the dragged object. These results imply that the drag force originates in the bulk properties of the granular sample.

cond-mat.dis-nn↗

arXiv2001

Thermodynamic Study of Excitations in a 3D Spin Liquid

In order to characterize thermal excitations in a frustrated spin liquid, we have examined the magnetothermodynamics of a model geometrically frustrated magnet. Our data demonstrate a crossover in the nature of the spin excitations between the spin liquid phase and the high-temperature paramagnetic state. The temperature dependence of both the specific heat and magnetization in the spin liquid phase can be fit within a simple model which assumes that the spin excitations have a gapped quadratic dispersion relation.

cond-mat↗

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

Modeling relaxation and jamming in granular media

We introduce a stochastic microscopic model to investigate the jamming and reorganization of grains induced by an object moving through a granular medium. The model reproduces the experimentally observed periodic sawtooth fluctuations in the jamming force and predicts the period and the power spectrum in terms of the controllable physical parameters. It also predicts that the avalanche sizes, defined as the number of displaced grains during a single advance of the object, follow a power-law, $P(s)\sim s^{-τ}$, where the exponent is independent of the physical parameters.

cond-mat.soft↗

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

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

Jamming and Fluctuations in Granular Drag

We investigate the dynamic evolution of jamming in granular media through fluctuations in the granular drag force. The successive collapse and formation of jammed states give a stick-slip nature to the fluctuations which is independent of the contact surface between the grains and the dragged object -- thus implying that the stress-induced collapse is nucleated in the bulk of the granular sample. We also find that while the fluctuations are periodic at small depths, they become "stepped" at large depths, a transition which we interpret as a consequence of the long-range nature of the force chains.

cond-mat.dis-nn↗