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."
Surfaces, films of a few atomic layers in thickness, and the artificially structured materials that can be made by combinations of these.
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, firstname.lastname@example.org. 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.