Supervisor Letter:

Dear Prospective Graduate Student,

Our research is in the area of strongly correlated electronic systems. Much of our work is on the theory of frustrated magnets, high temperature superconductors and other novel superconductors. We are also interested in other systems where electron-electron interactions and quantum effects are important, such as spin chains and ladders, carbon nanotubes, and quantum Hall systems. Other faculty members at McMaster who work in the area of strongly correlated electronic systems include Professor Erik Sorensen and, on the experimental side, Professors Bruce Gaulin, Takashi Imai, Graeme Luke and Tom Timusk.

Currently, we have two PhD students, Philip Ashby and Saeed Zelli, and one postdoctoral fellow, Rahul Roy, in our group. Some of our recent graduates include Michael Young (now pursuing his PhD with Sung-Sik Lee), Eric Mills, Kiri Nichol, Fei Lin, Rastko Skepnik, Denis Dalidovich, Laura Filion, and Bo Zhou. We are interested in taking on new graduate students this year.

Here is a quick summary of some of the projects our group is currently working on. Philip Ashby is currently studying chiral p-wave superconductivity, which has been proposed as the state for strontium ruthenate. This is a novel state with a “topological order”, which under certain conditions can support Majorana fermions and non-abelian statistics. Saeed Zelli is working on understanding the mysterious pseudogap phase of the high temperature superconducting cuprates. His work involves fairly large scale numerical calculations which include the effect of Gutzwiller projection (no double occupancy) which is believed to be important for this phase which arises from doping a Mott insulator. Rahul Roy works on topological insulators, systems which exhibit neither conventional spontaneous symmetry breaking or Fermi liquid behavior at low temperatures, but which have a type of subtle order and can be described by topological field theories. A chiral p-wave superconductor is one example, but other examples include quantum Hall states and novel states arising from a strong spin-orbit coupling in some semiconductor systems.

In collaboration with Xiaoliang Qi, a postdoctoral fellow at Stanford, one of us (Catherine Kallin) is also studying other problems related to chiral p-wave superconductivity and the puzzle of the “missing edge currents” predicted by theory by not observed in experiments. We are also working on a problem in frustrated quantum magnetism, stimulated by recent experiments on CsCuCl.

We are both members of the Canadian Institute for Advanced Research (CIAR) program on Superconductivity. This program runs an annual summer school which gives our graduate students and postdocs the opportunity to meet other students and researchers from across Canada who are working on research problems related to their own.

Both of us are also Affiliates of the Perimeter Institute in Waterloo, and John Berlinsky is the Academic Program Director for Perimeter. Perimeter is launching a new graduate program, Perimeter Scholars International, and we expect some lively and interesting interactions to result from our involvement.

You can learn about where our former students and postdocs are now from our home webpage. You can also learn a bit about work from some of the general or review papers we have listed and/or linked to on our webpage. Please contact us at kallin@mcmaster.caand berlinsk@mcmaster.caif you think you might be interested in condensed matter theory or if you have any other questions.

Professor Catherine Kallin
Professor John Berlinsky

Research Interests: Identifying chemical bond with electric flux linking two nuclei to the bonding charge leads to a theory that extends the rules of covalent, ionic and VSEPR models.
David Brown came to McMaster in 1959 after completing a Ph.D in crystallography at the University of London (UK). He has spent sabbaticals in Oxford working in biocrystallography and in Cambridge at the Cambridge Crystallographic Data Centre. In 1994 he was a visiting professor at the University of Amsterdam. He formally retired from McMaster University in 1996 and is emeritus professor of Physics and Astronomy. He is no longer taking on students.
 
The focus of Dr. Brown’s career has been understanding the physics that lies behind the empirical bond model used to describe the structures of organic molecules. He has developed the bond valence model which provides a similar description for inorganic compounds, and he has shown that the bond valence, i.e., the amount of charge an atom uses to form a bond, is the same as the electrostatic flux that links the negative bonding charge to its positive atomic core. The flux is proportional to the number of Faraday lines of field linking these two charges, and since the flux does not depend on where the charge is located, the theory independent of the charge distribution described by quantum mechanics. This allows classical electrostatic theory to be used to derived a set of simple theorems for modelling chemical structures, answering questions such as: which atoms will bond to each other, how many bonds does each atom form, and what is the length of the resulting bonds? The flux theory describes all localize bonds, both covalent and ionic. It extends the scope of the VSEPR model and explains why hydrogen bonds are asymmetric. It can be used to predict and analyse complex inorganic structures, for example by quantifying the spatial stresses responsible for the unusual electrical properties of perovskites, or tracing the paths of the mobile ions in an ionic conductor. Unlike the Lewis and orbital models, the flux theory gives a correct physical description of the chemical bond, leading, for example, to a simple prediction of solubility, and modelling the structures of aqueous solutions and surfaces.
 
Developing the bond valence model started with deriving empirical rules from the large numbers of reported inorganic crystal structures, work that required crystallographic databases and a common crystallographic file structure. To meet these needs Dr. Brown pioneered both the Inorganic Crystal Structure Database and the CIF crystallographic file structure. He was an editor of Acta Crystallographica and served on several commissions of the International Union of Crystallography and committees of the American Crystallographic Association.

Research Interests: Non invasive elemental analysis of the human body; x-ray fluorescence; neutron activation analysis

Measuring the Elemental Composition of the Human Body

We develop and use techniques based in atomic and nuclear physics for non destructive elemental analysis of bulk samples. Specifically, we use x-ray fluorescence and neutron activation analysis with the aim of finding out the elemental content of living human subjects. I work closely with Fiona McNeill on many of these projects and we both collaborate with Soo-Hyun Byun, particularly drawing on his expertise in nuclear instrumentation and pulse processing. These measurements involve ionising radiation (photons or neutrons) so a key preliminary to any proposed measurement is to ensure that the dose is acceptably low, typically well within the range employed by established medical diagnostic procedures.

The most widely applied of our techniques is the analysis of lead in bone using x-ray fluorescence. In this we collaborate with industry, with academic colleagues in the US and, in a study headed by Fiona McNeill, with Health Canada. Our work has contributed to a change in thinking about long term lead exposure, particularly highlighting the importance of endogenous exposure, that is what happens when lead stored over time in bone is released back into the body via the blood.

In another example, we recently completed a pilot study, using neutron activation to measure aluminum in bone of a group of people suffering from Alzheimer’s Disease and a comparison group with no signs of dementia.

Strontium seems to be ambiguous with regards to bone health. In excess it causes a rickets-like condition. However, people suffering from osteoporosis administered a strontium supplement had fewer fractures than a comparison group. We have developed an x-ray fluorescence method to measure strontium in bone and we are seeking to find ways (and funding!) to use this further in exploring the interaction of strontium with bone health.

I have a strong interest in the interaction between science and faith from the perspective of an activate participant in both communities. With colleagues I help to run the Hamilton Science and Faith Forum, the local chapter of the Canadian Scientific and Christian Affiliation.

Research Interests: Growth of structure in the universe via gravitational N-body simulations

Computer simulated galaxies

The universe took billions of years to evolve, but computers can simulate the process in a matter of hours or days. As part of the McMaster Unbiased Galaxy Survey (MUGS), astrophysics professor Hugh Couchman is using computational modelling to create a sample of 25 galaxies. “I try to model how these galaxies formed and why they have the structure they do,” he explained. The model galaxies are being compared to what galaxies actually look like today.

The 25 galaxies in the survey are “unique members of the population of galaxies, which we’ve chosen according to unbiased criteria,” said Couchman. In order to qualify for the survey, the galaxies only needed to have a mass close to that of the Milky Way galaxy and avoid the chaotic environments of dense clusters of galaxies.

Three hundred thousand years after the Big Bang, the universe had cooled and was very “smooth” (the density of matter was evenly distributed). If the universe had remained completely smooth, nothing would have changed, but very small fluctuations in the density allowed matter to group together. “Those tiny little ripples in the matter density slowly grow,” said Couchman. “When you have a slightly over-dense region, it has a slightly stronger gravitational attraction, so it tends to pull matter towards it and gets even more over-dense.”

Astronomers can measure the cosmic microwave background emission that was produced 400,000 years after the Big Bang to determine the spectrum of fluctuations, which is used as an input in computational modelling to see if these fluctuations produce the same types of galaxies that exist today.

“The biggest challenge at the moment is getting the physics right,” said Couchman. “A galaxy is a complicated ecosystem. It’s a balance between gravity and other significant processes.” In addition to taking gas dynamics and dark matter into account, astrophysical processes such as star formation greatly influence the development of galaxies.

Massive stars use up their energy and burn out more quickly than smaller stars. When a massive star evolves, it releases huge amounts of radiation; when it dies, it explodes as a supernova, sending an enormous blast wave into the interstellar medium. These processes regulate how gas cycles through the star formation process. “There’s a complex interaction between stars and the gas in galaxies,” said Couchman. “That’s turned out to be incredibly challenging to model.” The range of scales involved makes it impossible to incorporate stars into the model directly because they are so much smaller than galaxies.

The collaboration between astronomers and physicists in the Department of Physics and Astronomy is one of the features that drew Couchman to McMaster. “One of the reasons I wanted to come here is because astronomy is part of physics,” he said. “In many places, the astronomers are either in their own separate department, or quite separated from the physicists. Here, we share offices on the same floor and talk to each other. I really like that about this department.”

Supervisor Letter:

Hugh Couchman
Department of Physics & Astronomy
McMaster University

Dear Prospective Graduate Student,

This letter is to introduce myself and to describe my group and the kind of research that we do.

I am interested in post recombination cosmology which is the study of the universe and its contents after the universe cooled and first became neutral about 100,000 years after the Big Bang. In particular I investigate how structure – galaxies, clusters of galaxies and large-scale structure – grows from small density ripples present at recombination. Much of this work involves computer simulation and is a field which has been named as one of the “Grand Challenges” of the physical sciences!

A Master’s student, Sam Bromley, got his degree in September and Todd Fuller, a student of mine at the University of Western Ontario, successfully defended his Ph.D. in May. I expect to have openings for two and possibly three new students this coming year. There are currently two postdoctoral fellows working with me: Rob Thacker – who is investigating the formation of spiral galaxies, a much sought-after goal in cosmology – and James Wadsley – who works with me and Ralph Pudritz on areas of common interest between star formation theorists and cosmologists. Ralph Pudritz and I are hoping to hire another postdoc this coming year.

The group is very informal and interacts closely. The postdocs, in particular, are a great asset and frequently collaborate with and help students. I usually suggest several projects and encourage students to pick one which excites them. My group forms part of the wider theoretical astrophysics group with Alison Sills‘ and Ralph Pudritz’ groups and I have close ties with the other theorists in Physics and with those in other departments, often because we all use computers to do science (it’s amazing the range of science you come into contact with this way). Student have their own workstations and access to a wide range of other computers.

I am part of several external collaborations, including “Virgo” which is based in Durham, UK and with connections in Munich, Germany and with “C4” which is a collaboration between Victoria and McMaster, as well as ties with the Astronomy groups in Sussex, UK, the University of Washington in Seattle and the Canadian Institute for Theoretical Astrophysics at the University of Toronto. My students can expect to be involved with this joint work and will have opportunities for travel to meetings and conferences.

Please do not hesitate to contact me at couchman@mcmaster.ca if you think this is a research area in which you might be interested or simply if you have any questions.

Best wishes,
Hugh Couchman

Research Interests: Nonlinear dynamics and chaos theory
David Goodings was educated at the University of Toronto and Cambridge University. Following postdoctoral positions at the Atomic Energy Research Establishment, Harwell, U.K. and the University of Pittsburgh, he taught for several years at the University of Sussex and at the American University of Beirut, Lebanon. In 1969 he joined the Physics Department at McMaster. For many years he was a condensed matter theorist, working on the electronic structure of metals and alloys, the magnetic properties of rare earth metals and compounds, and molecular crystals.

Dr. Goodings research interests are now in the area of nonlinear dynamics and chaos theory, particularly in the active field known as “quantum chaos”. What is of central interest is to study the quantum analogs of classically chaotic systems. Using as an example a model of a billiard bouncing elastically in a wedge while subject to a constant downward force, he and graduate student Tom Szeredi showed how all the dynamical features of this classically chaotic system can be extracted from a knowledge of the quantum energy eigenvalues. They have also investigated the more difficult problem of going in the other direction, that is, of calculating approximate quantum energy eigenvalues from a knowledge of the (chaotic) classical dynamics. Current research is focussed on a scheme for incorporating the Heisenberg uncertainty principle into the classical-quantum correspondence.

In collaboration with people in cardiology at the McMaster Medical Centre, Dr. Goodings and graduate student Julie Lefebvre have used chaos theory to study normal heart rhythms. Their research has shown that there is an element of deterministic chaos in normal heart rhythms, although the evidence is not strong or persistent.

Research Interests: Globular clusters, stellar populations, evolution of galaxies

William Harris and The Big Picture

My main research interests are in the earliest stages of galaxy evolution — the first few Gigayears of a galaxy’s history during which structures like its halo stars and globular clusters emerged.¬† Their properties yield unique clues to the most active part of galaxy formation.¬† See my webpage for more, but here’s a shortlist of projects I’m currently involved with:

• Hubble Space Telescope imaging of a series of supergiant elliptical galaxies in the cosmologically “nearby” universe at distances from 40 to 200 Megaparsecs.¬† These giants have the largest globular cluster populations known (tens of thousands of clusters per galaxy) and with this data, our team is building up the biggest photometric database for globular clusters in existence.¬† With this material we are exploring patterns in the distributions of globular cluster luminosity, heavy-element enrichment, and spatial distributions in their galaxy halos — all of which are tracers of their formation epoch.
• Correlations between globular cluster populations and other large-scale properties of their host galaxies, including dynamical mass, galaxy type, and (very puzzlingly) their central supermassive black holes.
• Dynamics and assembly history of groups of galaxies over the redshift range z=0 to 1.
• Photometry and modelling of the halo-star populations in nearby galaxies such as M33, NGC 5128, and M87.
• Developing hydrodynamic modelling for the formation of massive star clusters.¬† The formation stage of “true” globular clusters (in the mass range of 0.1 to 10 million Solar masses) from their parent molecular clouds is the least well understood, but also most crucial, stage in their histories, and is likely to have produced important feedback on the larger-scale star formation history of the entire galaxy.

Supervisor Letter: Dear Student,

I work in observational astronomy, particularly on nearby galaxies and star clusters. I especially like investigating the very oldest types of stars in galaxies, especially ones that are found in globular clusters and galactic halos. These objects give us a unique way of tracing what happened to the galaxies during their first, crucial formation period 10 to 12 billion years ago. In astrophysical terms, this is a route to connecting the detailed predictions from cosmological model theory to the real universe that we cannot do in any other way.

My students and colleagues get involved, like me, in analyzing and interpreting the raw data that we obtain from observatories like the Hubble Space Telescope, the Canada-France-Hawaii Telescope (CFHT), and the Gemini telescopes in Chile and Hawaii.¬† Travel to take part in astronomical society meetings and professional conferences is also an expected part of your educational experience here. You’ll find that modern astronomy is a thoroughly international subject right from the start — and one that Canada is an integral part of.

Our astrophysics group at McMaster is a very close-knit one. We work together. I welcome shared supervisions, such as with one of our theorists so that both observation and modelling can be carried out effectively. See my webpage for more, and please contact me at harris@physics.mcmaster.ca if you think this is a research area you might be interested in. This is a great group to be part of!

Bill Harris

Research Interests: Electron systems, including high temperature superconductors, frustrated antiferromagnets, and quantum Hall systems

Catherine works in the general area of Quantum Condensed Matter Theory.  Her current research is primarily on quantum magnets and novel superconductors, including high temperature superconductors and chiral superconductors that may exhibit topological order.  More information about her research group and activities can be found on her website.

Art is by Pamela Davis Kivelson.

Supervisor Letter:

Catherine Kallin
John Berlinsky
Department of Physics & Astronomy
McMaster University

Dear Prospective Graduate Student,

Our research is in the area of strongly correlated electronic systems. Much of our work is on the theory of frustrated magnets, high temperature superconductors and other novel superconductors. We are also interested in other systems where electron-electron interactions and quantum effects are important, such as spin chains and ladders, carbon nanotubes, and quantum Hall systems. Other faculty members at McMaster who work in the area of strongly correlated electronic systems include Professors Sung-Sik Lee and Erik Sorensen and, on the experimental side, Professors Bruce Gaulin, Takashi Imai, Graeme Luke and Tom Timusk.

Currently, we have four PhD students, Philip Ashby, Sedigh Ghamari, Wen Huang and Saeed Zelli, and one postdoctoral fellow, Ed Taylor, in our group. Some of our recent graduates include Eric Mills, Kiri Nichol, Fei Lin, Rastko Skepnik, Denis Dalidovich, and Rahul Roy.

Here is a quick summary of some of the projects our group is currently working on. Philip Ashby is currently studying chiral p-wave superconductivity, which has been proposed as the state for strontium ruthenate. This is a novel state with a “topological order”, which under certain conditions can support Majorana fermions and non-abelian statistics. Saeed Zelli is working on understanding the mysterious pseudogap phase of the high temperature superconducting cuprates. His work involves fairly large scale numerical calculations which include the effect of Gutzwiller projection (no double occupancy) which is believed to be important for this phase which arises from doping a Mott insulator. Rahul Roy works on topological insulators, systems which exhibit neither conventional spontaneous symmetry breaking or Fermi liquid behavior at low temperatures, but which have a type of subtle order and can be described by topological field theories. A chiral p-wave superconductor is one example, but other examples include quantum Hall states and novel states arising from a strong spin-orbit coupling in some semiconductor systems.

We are both members of the Canadian Institute for Advanced Research (CIAR) program on Superconductivity. This program runs an annual summer school which gives our graduate students and postdocs the opportunity to meet other students and researchers from across Canada who are working on research problems related to their own.

Both of us are also Affiliates of the Perimeter Institute in Waterloo, and John Berlinsky is the Academic Program Director for Perimeter. Perimeter has launched recently a new graduate program, Perimeter Scholars International, and we expect some lively and interesting interactions to result from our involvement.

You can learn about where our former students and postdocs are now from our home webpage. You can also learn a bit about work from some of the general or review papers we have listed and/or linked to on our webpage. Please contact us at kallin@mcmaster.ca and berlinsk@mcmaster.ca if you think you might be interested in condensed matter theory or if you have any other questions.

Professor Catherine Kallin
Professor John Berlinsky

Research Interests: Lattice vibrations, subsitutionally disordered crystals, embedded atom model for anharmonic vibrational properties of crystals
David W. Taylor did his undergraduate and graduate studies at Oxford University, obtaining his D.Phil. in 1965. He spent 1965-67 as a Member of the Technical Staff at Bell Telephone Laboratories, Murray Hill, N.J. and then joined the Physics Department at McMaster. He has been Professor of Physics since 1977 and retired in 2004 . He spent 1974-75 on a Senior Research Fellowship at the Department of Theoretical Physics, Oxford.

Dr. Taylor’s research was mainly in the area of lattice vibrations, with the emphasis being on the vibrational properties of substitutionally disordered crystals. Of particular interest was the use of the embedded atom model both for disordered crystals and also to calculate the anharmonic vibrational properties of crystals.

Dr. Taylor is now involved with scuba diving and is the financial director for Save Ontario Shipwrecks and is also financial director for the Ontario Underwater Council.

?A collection of his point and shoot pictures of reefs and wrecks is available at http://www.physics.mcmaster.ca/~taylordw/photos/?

Research Interests: Optical properties of novel quantum materials (e.g., new H3S 203 K superconductor), strange metals and terahertz optical structures.
I am working on the optical properties of novel quantum materials. The recently discovered  superconductor, hydrogen sulfide with a transition temperature of 203 K at a pressure of 150 Gpa is one example. Here the goal is to establish the mechanism of superconductivity using optical spectroscopy. Another interest is the strange behavior of many metals. Their optical properties are anomalous and seem to contradict accepted theories. The optical spectrum of the mysterious hidden order state in uranium ruthenium silicon is another quantum phenomenon that my group has been working since its discovery.  Recently I have returned to study the strange resonance at terahertz frequencies  in arrays of holes  in metal plates. These devices, discovered by Paul Richards and I years ago are used in millimeter wave astronomy as filters but the mechanism of their high transmission has not been explained.

Research Interests: Numerical solutions of quantum wave equations, time-dependent phenomena, quantum search algorithms, statistical mechanics of ultracold gases

Wytse van Dijk completed his undergraduate and graduate studies at McMaster University, receiving in 1968 the Ph.D in nuclear theory. He spent two years as a postdoctoral fellow at the Theoretical Physics Department of the University of Oxford, England. After a one-year faculty appointment at Mount Allison University, New Brunswick, he joined the faculty of Dordt College, Sioux Center, Iowa. During the 1977-78 academic year he was on leave at the Theoretical Physics Institute of the University of Alberta, Edmonton, Alberta. In 1982 he was appointed Professor of Mathematics and Physics at Redeemer University College, Ancaster, Ontario and Adjunct Professor in the Department of Physics and Astronomy at McMaster University; both positions he holds at the present.

Dr. van Dijk’s earlier research focused on the nuclear force problem and its application to low-energy nuclear systems. This research was motivated by the desire to understand the effect of quark or elementary-particle degrees of freedom in nuclear structure or scattering. The most recent work has dealt with sensitivity of the mixing parameter associated with the ³S₁ – ³D₁ coupling in the nuclear force to the one-pion exchange.

Recently van Dijk has become interested in decaying quantum systems which simulate the nuclear alpha decay. In particular the theoretical determination of the ionization and bremsstrahlung probabilities due to an alpha particle being emitted from its nucleus is found to shed light on fundamental issues in quantum mechanics as well as mechanisms responsible for nuclear structure. The Bohm trajectory method is also being studied in connection with decaying and scattering systems.

Dr. van Dijk is also interested in theoretical and numerical analyses of quantum mechanical scattering phenomena such as time delay and advance in (un)coupled one-dimensional systems. Aspects of this study have been applied to semiconductor superlattices and quantum computing. This is part of a program of studying time-dependent quantum mechanical systems.

Research Interests: Surfaces, films of a few atomic layers in thickness, and the artificially structured materials that can be made by combinations of these.

Thin films

We live in a three-dimensional world where most objects have a length, width and depth. Since the number of atoms inside a three-dimensional material far exceeds the number of atoms found on its surface, interior atoms determine the material’s structure and properties. By contrast, two-dimensional films consist entirely of surface atoms and can be as thin as one atom.

“Atoms on the surface can behave very differently from atoms deep inside a material because they have fewer neighbouring atoms to constrain them,” said David Venus, professor of condensed matter physics and Chair of the Department of Physics and Astronomy at McMaster University. “As a result, ultrathin films can act as two-dimensioanl systems and may have very different structural, electronic and magnetic properties. One of our aims is to characterize and understand these differences.”

Magnetism is very sensitive to the environment of the atoms in a material. In a three-dimensional magnetic material, the atoms form bonds with six or eight other atoms, forming a cooperative system in which the individual atomic magnetic moments (or internal “bar magnets”) all point in the same direction. The resulting ferromagnetic state can be very stable, even above 1000 Kelvin.

In a thin film consisting of only one layer of atoms, the number of neighbouring magnetic atoms that can form bonds is much smaller. Since there are fewer atoms cooperating with each other, the magnetic state is much weaker, and a modified, or completely different magnetic state might be preferred. Furthermore, the weaker ferromagnetic state is more easily disrupted by thermal energy. As the temperature increases, “the aligned magnetic moments start to jostle around so much, they become disorganized,” Venus explained. “They no longer point in the same direction, and the film is no longer a ferromagnet.” This abrupt change from an ordered to a disordered state is called a phase transition. The study of phase transitions in two-dimensional magnetic films is a major focus of Venus’s research.

The use of thin films is an effective method for creating novel, artificial materials. “You can make completely new materials using these very thin films,” said Venus. “Just by choosing a different substrate, you can grow a different atomic structure that may not occur naturally.” The new structure will have a different arrangement of bonds and could have different magnetic properties.

There are challenges to overcome when exploring these new possibilities. Surface atoms are difficult to study because they are easily contaminated by chemical reactions with molecules in the air. To prevent contamination, experiments must be conducted in an ultra high vacuum chamber, and the films cannot be removed from the vacuum chamber to measure their magnetic properties using standard techniques. Instead, a laser beam is directed through a window in the vacuum chamber, reflected off the magnetic film, and exits through another window. During the reflection from a magnetic film only one atom thick, the polarization of the laser light is changed a tiny amount. “We are able to detect those changes in polarization and work back to determine what the magnetic properties of the film must be,” said Venus. “The polarization change is larger or smaller, depending on how strong the magnetization is.”

Thin magnetic films are often used in computer hard drives to store information. The memory disk inside a hard drive, is coated with a thin layer of magnetic material onto which data is recorded. The read-and-write head passes over the disk as it spins, detecting (reading) and modifying (writing) the magnetic state of tiny regions, or bits, on the hard drive. Information encoded as the binary states “0” and “1” are stored as magnetic north-pointing and south-pointing bits.

Advances in magnetic data storage have enabled the modern information age, much like the transistor enabled the earlier computer revolution, but it is always difficult to predict which discovery will have an impact on society. “We are not designing hard drives in our laboratory”, said Venus. “Our role is to create new magnetic materials and understand their novel properties. They might improve hard drives or magnetic sensors, or have uses that have not yet even been thought of.”

Supervisor Letter:

Dear Prospective Graduate Student:

Thank you for your interest in research in thin magnetic films at McMaster University. I have openings for one or two graduate students beginning in September 2012, working on projects related to the topics described below. At present, I do not foresee a position for a postdoctoral researcher. If you are interested in this type of graduate research, please contact me by email, venus@physics.mcmaster.ca. Also visit the “Graduate” section of the Physics and Astronomy website for the procedures and policies for graduate application and support.

Our work concentrates on the novel magnetic phenomena which arise when the magnetic response of a system is driven by surface and interface atoms. Due to their low-symmetry environments, these atoms have a disproportionately large influence on magnetic properties of thin films, and often dominate the magnetic response in the ultrathin limit (1-10 ML, monolayers or atomic layers).

In addition to studying thin magnetic films, students in my group obtain practical skills in film deposition and characterization, surface science techniques and procedures and in apparatus design. All past graduates are presently employed in the wider physics community in positions such as development research in magnetic sensor heads, or in flat panel displays, research and managerial positions in the semiconductor industry and national facilities, and in scientific publishing. Others have moved to postdoctoral or other academic positions.

We are pursuing three main areas of research at the present time, but there are other directions that we are also interested in. These three areas are:

1. Domain pattern dynamics in perpendicularly-magnetized films, measured using the magnetic susceptibility.

Films may be magnetized perpendicularly to their surface because of the large magnetic anisotropy associated with surface and interface atoms. The perpendicular magnetic anisotropy and large aspect ratio of these ultrathin films changes the character of domain formation when compared to bulk material or films magnetized in-plane. In particular, an equilibrium density of magnetic domains arises spontaneously, and long-range dipole interactions between the domains can organize them in to ordered patterns.

The formation and motion of the domain walls (domain “lines” in an ultrathin film) influences the magnetic properties of the films, and dominates the magnetic susceptibility. Measurements of the complex susceptibility (using the magneto-optical Kerr effect) have allowed us to make a quantitative study of domain dynamics and dissipation as a function of temperature, and to test models of domain wall pinning and the distribution of pinning energies which act upon the domain walls in a quantitative manner.

More recently, we have been studying the mechanisms and dynamics by which the domain patterns change as the temperature is changed. This includes the creation or annihilation of domains, and the role played by topological defects (or dislocations) in the evoltuion of the domain pattern. At higher temperatures a phase transition between two domain patterns is predicted. One of the patterns has long range positional order and the other does not. This is related to a 2-dimensional “melting” of the ordered domain, and can be observed indirectly through a signature in the magnetic susceptibility.

“Dynamics of topological defects in a two-dimensional magnetic stripe pattern”, N. Abu-Libdeh and D. Venus, Phys. Rev. B 84 (2011), 094428.

“Dynamics of domain growth driven by dipolar interactions in a perpendicularly magnetized ultrathin film”, N. Abu-Libdeh and D. Venus, Phys. Rev. B 81 (2010), 195416. “Dynamical signature of a domain phase transition in a perpendicularly magnetized ultrathin film”, N. Abu-Libdeh and D. Venus, Phys. Rev. B 80 (2009) 184412.

2. Magnetic phase transitions in ultrathin films.

Since ultrathin films are not nearly thick enough to support a magnetic domain wall parallel to their surface, the entire film thickness contributes to a correlated moment. Thus, it is possible to study magnetic transitions in 2-dimensional and quasi-2-dimensional systems using ferromagnetic films. We have studied the critical exponents of the complex magnetic susceptibility for ultrathin iron films. From the real part of the susceptibility we have measured the critical exponent “gamma”, and demonstrated its systematic sensitivity on film roughness. Using the imaginary part of the susceptibility, we have made the first measurements of critical slowing down in an ultrathin film, 2D Ising system. Our most research investigated the departures from 2D Ising behaviour as the film thickness is changed by only fractions of an atomic layer from a complete monolayer. These departures are reflected in both the critical exponents and the magnetic susceptibility measured along the “hard” magnetic axis. They are related to the presence of step atoms at the edges of the incomplete monolayer, and can be understood by applying the Harris criterion to this inhomogeneous system.

“Observation of mixed anisotropy in the critical susceptibility of an ultrathin magnetic film”, K. Fritsh, R. D’Ortenzio and D. Venus, Phys. Rev. B 83, (2011), 075421.

“Measurements of critical slowing down in the 2D Ising model using ferromagnetic ultrathin films”, M.J. Dunlavy and D, Venus, Phys. Rev. B 71, (2005), 144406.

“Critical susceptibility exponent measured from ultrathin Fe/W(110) films”, M.J. Dunlavy and D. Venus, Phys. Rev. B 69, (2004), 094411.

3. Growth and characterization of ordered antiferromagnetic films.

Surface and interface atoms are expected to play an important role in antiferromagnetic films as well (although, of course, the magnetic dipole interaction will not). Of particular interest is the magnetic coupling between antiferromagnetic and ferromagnetic films along a shared interface. This is related to the “exchange bias” which shifts the hysteresis loop of the ferromagnetic films so that it is not symmetric about H=0, and is important in the fabrication of commercial spin valve structures. We are currently investigating the growth of antiferromagnetic alloys in the ultrathin film limit, to determine suitable magnetic systems to study. Once again, the magnetic susceptibility will be a versatile method to characterize the pinning of domain walls in the ferromagnetic layer by the antiferromagnetic film.

“Structural and magnetic properties of a chemically ordered fcc (111) Mn alloy film”, Z. Zhou, Q. Li and D. Venus, J. Appl. Phys. 99, 08N504 (2006).

“Competition between magnetic relaxation mechanisms in exchange coupled CoO/Co bilayers”, Phys. Rev. B 72, (2005), 024404.

Yours truly,
David Venus