Our research has been funded from a number of different sources including:
Geometrically
Frustrated Magnetism :
Spin Liquid and Spin Ice
The large degeneracy of states resulting from the geometrical frustration of competing interactions is an essential ingredient of important problems in fields as diverse as magnetism, protein folding, and neural networks. Geometrically frustrated magnetic materials are those in which the geometry of the magnetic sublattice prevents the simultaneous minimization of the energies of different spin-spin interactions. Such materials can have a spin liquid ground state in which frustration prevents the onset of long range order, and the spins thus continue to fluctuate down to the lowest temperatures measured. We have studied the spin liquid ground state of gadolinium gallium garnet, in which the application of a magnetic field results in antiferromagnetic long range order. We have demonstrated that the spin liquid state is distinct from a spin glass, and that the phase boundary with the ordered state is directly analogous to the melting curve of 4He.
Our recent studies, in collaboration with Prof. Robert Cava from Princeton, have been of a "spin ice" material, dysprosium titanate, in which the low temperature statistical properties of the spins are analogous to those of protons in frozen water. We have found a cooperative spin-freezing transition leading to the spin ice ground state in this material. This transition is associated with a very narrow range of relaxation times and represents a new modality for spin-freezing. The dynamics are analogous to those associated with the freezing of protons in ice, and they provide a new window through which to study glass-like behavior and the consequences of frustration in the limit of low disorder.
Schematic of frustration in water ice and spin ice. a. In water ice, each hydrogren ion is close to one or the other of its two oxygen neighbors, and each oxygen must have two hydrogen ions closer to it. b. In spin ice, the spins point either directly toward or away from the centers of the tetrahedra, and each tetrahedron is constrained to have two spins pointing in and two pointing out.
Spintronic Materials
The possibility of devices based on an electron's spin rather than its charge has lead to the growing field of "spintronics." In this field the injection of a spin-polarized current into a semiconductor could lead to novel electronic devices such as spin polarized LEDs, spin transistors, and integration of memory and computation on the same chip. Ferromagnetic semiconductors are currently the most viable method of injecting polarized carriers into semiconductors, and (Ga,Mn)As is of particular interest because it has the highest established Curie temperature, a small coercive field, and can be grown on a number of semiconductors with molecular beam epitaxy. In collaboration with Dr. Samarth's group, our recent work at Penn State has focused on characterizing the magnetic and transport properties of (Ga,Mn)As in the hope of improving current theoretical understanding of this material.
We have examined the magnetic, electronic, and structural properties of molecular beam epitaxially grown (Ga,Mn)As epilayers as a function of annealing time. An optimal annealing time was established where the ferromagnetism and conductivity were significantly enhanced, and the maximum reported value of the Curie temperature was achieved. These data indicate that such annealing induces the defects in (Ga,Mn)As to evolve through at least two different processes, and they point to a complex interplay between the different defects and ferromagnetism in this material.
We have also made a comprehensive study of (Ga,Mn)As as a function of Mn concentration at the previously established optimal annealing time. The Curie temperature, lattice constant, and conductivity all increase with Mn concentration up to ~5% and then saturate (reproducibly achieving a Curie temperature of 110K). The moment per Mn atom is seen to decrease with increasing Mn concentration throughout the range - indicating that adding more Mn to the system becomes less effective and, from the saturation in the other properties, seems to be ineffective above ~5%. This work will hopefully help improve current theoretical understanding of ferromagnetic semiconductors pointing the way to materials with a large percentage of the Mn contributing to the ferromagnetism and Curie temperatures above room temperature. Indeed, our most recent work has shown that with appropriate annealing and control of the growth process, the transition temperature can be pushed up to 150 K.
Colossal Magnetoresistance Materials
Rare earth perovskites based on manganese display an extraordinarily strong coupling between their electronic, magnetic, and lattice degrees of freedom. In addition to extremely large magnetoresistance (for which these compounds have been called "colossal magnetoresistance materials") this coupling results in a wide range of novel behavior including real-space charge ordering, both ferromagnetic and antiferromagnetic spin ordering, and macroscopic phase separation between different magnetoelectronic ground states. Our recent research, in collaboration with Dr. John Mitchell from Argonne and Prof. Sang-Wook Cheong from Rutgers, has probed the phase separated ground states through a range of thermodynamic and bulk magnetic measurements. This work has elucidated the nature of unusual low energy excitations in these materials and also has shown that the phase separated ground states, display unusual glassy behavior associated with the competition between different magnetic and electronic interactions. Additionally, in collaboration with Prof. Bruce Bunker at Notre Dame, we have demonstrated that the colossal magnetoresistance manganites can have intrinsic chemical inhomogeneities at the atomic scale, which may nucleate the magnetoelectronic phase separation. Most recently, in collaboration with Prof. Bernard Raveau from Caen, we have found and characterized extraordinarily sharp metamagnetic transitions which appear suddenly at very low temperatures (T < 5K).
Physics of Wetting and Drag Force in Granular Media
A separate research program in our lab is in the study of granular media. In particular we have investigated two different physical phenomena in granular media:
Dynamics of Wet Granular Materials The dynamics of granular materials have been the subject of considerable attention from the non-linear physics community over the past decade, but almost all of the attention has focused on dry materials in which there is no significant attractive force between the grains. In collaboration with Prof. Albert-László Barabási at Notre Dame and Prof. Tamás Vicsek at the Eötvös University, we have explored the physics of wet granular materials, in which small amounts of liquid added to grains lead to intergrain cohesion and lubrication. Through quantitative studies of both the increased stability of wet granular media and the dynamics of avalanches among wet grains, we have demonstrated that microscopic amounts of interstitial liquid can result in macroscopic changes in the granular properties. We observe a wide range of novel behavior with increasing liquid content, including the development of clumps of correlated grains and a viscoplastic regime of behavior in which the entire granular sample behaves coherently and the surface spontaneously forms regular patterns.Drag Force and Local Jamming When a stress is applied to a granular material, the grains "jam" to oppose the stress by forming a rigid structure. In collaboration with Prof. Albert-László Barabási at Notre Dame and Prof. Byungnam Kahng at Seoul, we have explored local jamming resulting from stress applied by a discrete solid object moving slowly through a dense granular medium. We have measured the low velocity drag force associated with such motion, a relatively simple property which had not been the subject of previous systematic study. We have characterized the average drag force on various objects to study geometrical and frictional effects, and we have also studied large stick-slip fluctuations in the drag which reflect the long range nature of force propagation in dense granular materials. Aside from providing a detailed understanding of the drag process in granular materials, these studies lend insight into the process of how locally jammed grains collapse, and we are continuing with studies of the effects of finite sample size on the strength of locally jammed granular materials.
Wet Sand Slide Show (.pdf format)
Granular Drag Slide Show (.pdf format)
New experiments on dynamics of surface flow in wet granular materials
Peter Schiffer ©2005
