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Crystal Growth Laboratory

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A single crystal grown from solution.

The growth of large and pristine single crystals of new materials is central to our research program. Our research projects really begin with materials preparation and crystal growth, and only if that is successful do we undertake the scattering studies. I prefer to have all of my graduate students have experience with crystal growth, as it is an important and under-appreciated technical competence which will serve them well in their future careers.

We produce most of the single and polycrystalline materials that we study with scattering techniques. We sometimes are given crystals by other crystal growers (for example, we’ve had a very successful collaboration with Prof. F. C. Chou of the National Taiwan University on unconventional spin-Peierls systems TiOBr and TiOCl), but mostly we make our own materials. This allows us to set up long term and systematic studies of entire families of materials, and to explore certain themes in condensed matter physics.

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Hanna Dabkowska, Senior Research Scientist in the BIMR's Center for Crystal Growth, with one of our two Optical Floating Zone Furnaces (four mirror type).

The cornerstones of our crystal growth effort are two Floating Zone Image Furnaces. Polycrystalline starting material of the desired composition, or material close to the desired composition, is placed near one of the common focii in a 2 or 4 elliptical mirror cavity. Halogen light bulbs placed at the other elliptical mirror focii ensure that light is focused at the sample position, producing a “hot spot” which is only a few mm across. Depending on how well the material absorbs the light, the “hot spot” may be locally heated up to as high as 2200 C, which is enough to melt most materials. A molten zone is then formed at the common focus, and it is held in place by the surface tension of the molten material. Then, either the polycrystalline sample, or the mirror assembly, or both, can be translated such that the molten zone passes along the polycrystalline rod. The polycrystalline rods are placed in a sealed quartz tube, so that the crystal growth can be carried out in specialized oxidizing or reducing, or inert atmospheres – up to 10 atmospheres. If everything works well, a zone refined single crystal is left behind the passing molten zone, and a high quality single crystal of approximate dimensions 5mm diameter by 50 mm long is produced.

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One of our two Optical Floating Zone Furnaces (two mirror type), with a ceramic that has been melted at the "hot spot".

These furnaces are particularly well suited to growing large single crystals of transition metal oxides. Our recent successes have been the cubic pyrochlores Yb2Ti2O7, Er2Ti2O7, Tb2Ti2O7, Ho2Ti2O7 ; kagome staircase materials Co3V2O8; high temperature superconductors La(2-x)Ba(x)CuO4, Bi2Sr2CaCu2O8; and quantum magnets SrCu2(BO3)2 and CuGeO3.

A very important prerequisite for all single crystal growth is the capacity to make the polycrystalline materials which are the starting point. We have extensive facilities for the accurate weighing, mixing, and annealing of polycrystalline materials, as well as infrastructure, such as x-ray diffraction capabilities, which allow us to assess whether or not the starting polycrysyalline material is what we think it is.

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An example of a finished floating zone growth. The ceramic starting material (the "seed rod") is still attached to the single crystal growth.

We also perform some Flux Growth which involves slow cooling of a melt, and single crystals are then cut out of the resulting boule. We have the capacity for performing Growths from supersaturated solution, Bridgeman growths, Czochralski growths, as well as Tri-Arc growths for intermetallic materials.

Taken together, the infrastructure for both crystal growth and for the assessment of crystalline materials in the Centre for Crystal Growth is the most sophisticated and extensive of its type in Canada, and one of the most sophisticated in North America. This infrastructure underpins our entire program in the advanced characterization of materials with exotic ground states.

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X-ray Scattering

We perform x-ray scattering measurements on all new materials which we are interested in. Some of these measurements are connected with the growth and preparation of the materials themselves, and allow us to verify that the material we are growing has the correct structure and is free of impurity phases.

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Bruce with the four circle x-ray diffractometer at McMaster University.

We also perform full fledged x-ray scattering studies of the new materials, using either the rotating anode laboratory at McMaster, or at synchrotron x-ray sources. These x-ray scattering experiments inform on structural phase transitions, charge ordering, and magnetoelastic effects that may be relevant to the crystals of interest. Although most of the problems we are currently interested in are related to exotic magnetism, the lattice on which the magnetism resides, and the charge correlations which may underlie the magnetism, are not simply bystanders to the interesting physics in these materials. It is therefore important to be able to probe the charge correlations in our materials with a powerful diffraction probe complementary to neutron scattering; that is x-ray scattering.

X-ray Sources: We use a variety of x-ray sources, appropriate to the nature of the problem we are interested in. Tube sources are relatively low power (~ 2 kW) x-ray sources which are based on a stationary metal target, typically Cu or Mo. Rotating anode sources are similar to tube sources, but the metal target rotates, as the name implies, allowing their use at higher powers (~ 18 kW) and hence higher incident fluxes. The spectrum of x-ray radiation coming from either rotating anode or tube sources is peaked at the elemental K-alpha, K-beta, etc., energies typical of the target, and is unpolarized. In contrast, synchrotron sources produce x-ray radiation by the acceleration of charged particles. This occurs using either the bending magnets in the synchrotron storage ring, or using insertion devices which cause the charged particles to “wiggle” or “undulate” within a straight section of the storage ring. Synchrotron radiation is intense, brilliant, continuously tunable in energy, and polarized.

The x-ray scattering facilities we use extensively are:

The Rotating Anode Lab at McMaster University: We have one 18 kW Cu – rotating anode x-ray source, which feeds a very versatile 4-circle diffractometer. We can mount both a low temperature displex for sample temperatures between 7 K and 325 K, and a 3He fridge which allows us access to sample temperatures as low as 0.3 K. This latter capability, to perform x-ray measurements at temperatures as low as 0.3 K, is very unusual worldwide. We are presently building a second 4-circle diffractometer which uses an 18 kW Mo rotating anode source and focusing optics. This new facility should be entering commissioning in the summer of 2010.

The Advanced Photon Source (APS) at Argonne National Lab: We have made extensive use of two insertion device beamlines at the APS to carry out very high resolution scattering studies on a number of topical problems in new materials. Presently, we are most involved in time-resolved x-ray scattering measurements on exotic magnets in pulsed magnetic fields. We have developed a very exciting capability to make x-ray measurements on exotic new magnetic materials as they experience an ultra-high magnetic field, pulsed from 0 to ~ 30 or 40 T over a few milliseconds. We are just now exploring this new capability, in collaboration with Dr. Zahir Islam at the APS and Prof. Hiroyuki Nojiri at Tohoku University in Japan, and have used it to study giant magnetoelastic effects in geometrically frustrated magnets. The APS, just outside of Chicago, is about a 9 hour drive from McMaster. (http://www.aps.anl.gov/)

The Brockhouse Sector at the Canadian Light Source (CLS): In collaboration with Prof. Stefan Kycia at the University of Guelph, and other Canadian x-ray scientists, we are building a state-of-the-art sector of beamlines devoted to scattering studies at the CLS in Saskatoon. This sector is based at a straight section of the CLS with two insertion devices (one undulator, one wiggler) which will feed three or more experimental stations for simultaneous scattering experiments. When complete in ~ 2013, we will have the capacity for very sophisticated resonant and non-resonant x-ray scattering studies of new materials under a host of extreme environments, including low temperatures and high magnetic fields. (http://www.lightsource.ca/)

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Neutron Scattering

We carry out neutron scattering studies on the materials we are interested in, as neutrons can provide unique and often definitive information regarding the structure, dynamics, and phase behaviour of materials. The neutron is uncharged and carries a spin ½ magnetic moment, so it is a very powerful probe of magnetism in solids, and most of the problems we are currently studying involve the magnetic properties of new materials.

The neutron is a rather weak probe of matter, which has both positive and negative dimensions to it. The negative is that, as a weak probe, we typically need to study samples which are relatively large – volumes of ~ 1-5 cc are typical. The positive side of this is that, as a weak probe, the neutron scattering cross section is well understood theoretically and theoretical calculations can be very easily compared to experiment.

Our neutron experiments are carried out at a variety of pulsed and steady state neutron sources, mostly in North America, but also around the world. These are:

  1. The NRU reactor at Chalk River. Here we primarily use the C5 triple axis spectrometer and the C2 powder spectrometer. Chalk River is in the Ottawa Valley, about a 6 ½ hour drive from McMaster. (http://www.aecl.ca/Science/CRL.htm)
  2. The NIST Center for Neutron Research. We have been major users of the Disk Chopper Spectrometer, the SPINS and MACS cold triple axis spectrometers, as well as SANS diffraction instruments. NIST is located just northwest of Washington, DC, about a 9 hour drive from McMaster. (http://www.ncnr.nist.gov/)
  3. The Spallation Neutron Souce at Oak Ridge National Lab. This is the world’s next generation neutron source, designed to operate at 1.4 MW. We have played a major leadership role in developing two state-of-the-art neutron instruments at SNS – SEQUOIA, a high resolution chopper instrument, and VULCAN, a powder diffractometer optimized for the study of engineering materials. Our work to date has focused on the inelastic chopper spectrometers SEQUOIA, ARCS, and CNCS. With figures-of-merit for instruments exceeding those at other sources by factors of 10 – 100, we will continue to pursue our most challenging experiments at SNS. Oak Ridge National Lab is in East Tennessee, and is about a 12 hour drive from McMaster. (http://neutrons.ornl.gov/)
  4. The Lujan Neutron Scattering Center at Los Alamos National Lab. Here we have carried out very interesting neutron pair distribution function (NPDF) measurements. Los Alamos Laboratory is in New Mexico, near Santa Fe. (http://lansce.lanl.gov/lujan/)
  5. The ISIS pulsed neutron facility at the Rutherford Appleton Laboratory in the UK. This is a very well developed spallation neutron source with advanced and sophisticated instrumentation. ISIS has recently completed the installation and commissioning of their Second Target Station, which significantly expands their capability for cold neutron research. Our group has been a significant user of the OSIRIS and MAPS chopper spectrometers. The Rutherford Appleton Lab is in Oxfordshire, UK, about a 90 minute drive west of Heathrow airport. (http://www.isis.rl.ac.uk/)
  6. The reactor at the Helmholtz Zentrum Berlin (HZB). This center is known particularly for its very sophisticated low temperature and high magnetic field sample environments, which complement excellent cold neutron instrumentation very well. We have been significant users of the FLEX cold triple axis instrument. The HZB is on the western edge of Berlin, Germany near Potsdam. (http://www.helmholtz-berlin.de/index_en.html)
  7. MAD, the McMaster Alignment Diffractometer at the McMaster Nuclear Reactor. McMaster has a 3 MW research reactor on campus, and we are just about to complete construction of a modern and robust triple axis spectrometer with fixed momonchromator take-off angle (2thm). This instrument, MAD, will be used for a variety of purposes, including aligning and co-aligning single crystals in advance of experiments at other neutron sources around the world. It will also be used for real science experiments, and for graduate and undergraduate education. Most of my research group has been involved in the design, and commissioning of MAD at some level, and we are very excited that we will soon have access to this powerful research and education tool.

Our general philosophy is that we carry out our neutron experiments at the facility which is the best match to our particular science interests and needs. Working at these large national and international facilities we develop excellent collaborations. Grad students and postdocs benefit from their own supervisor (Bruce), but also from close interaction with the world’s best neutron scientists with whom we collaborate.

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Extreme Sample Environments

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The 30T pulsed magnet used at the Advanced Photon Source.

The exploration of exotic magnetism in new materials often calls for combinations of extremely low temperatures and high magnetic fields. Low temperatures are necessary to overcome thermal fluctuations, which can prevent the development of interesting magnetic phenomena, and can obscure fascinating quantum mechanical effects that underly the behaviour. Likewise, the ability to apply a magnetic field is important, as the field acts as an external “knob” for tuning magnetic interactions in the system. For example, the quantum magnet SrCu2(BO3)2, which develops a singlet ground state with triplet excitations at T<10K, can be brought to a triplet-condensed state through a magnetic-field-tuned crossing of the triplet and singlet energy levels (20T at 500mK).

Parametric studies that consist of varying temperature and magnetic field independently lead to the development of a phase diagram for the material; this is often a vital “map” that reveals the balance of interactions in the microscopic system.

Low Temperatures: The ability to achieve very low temperatures is especially important for frustrated materials, which by their nature resist forming ordered states until they are extremely cold, or sometimes not at all! As just one example, the frustrated pyrochlore material Tb2Ti2O7 resists long range magnetic order down to the lowest measurable temperatures (30mK), as revealed by neutron scattering experiments. This material is referred to as a “spin liquid” because of the lack of spin freezing that these measurements show.

Our group actively seeks out low temperature sample environments in which to perform neutron and x-ray scattering experiments. On the neutron scattering side of things, several of the instruments that we frequently use (such as DCS, SPINS and MACS at the NIST Center for Neutron Research, or CNCS at the Spallation Neutron Source) are capable of accommodating specially designed dilution refrigerators, which can reach ~30mK. If these extremely low temperatures are not required to access the interesting physics in the system, other more common types of refrigerators can be used at many beamlines, such as 3He fridges (T>300mK), 4He fridges (T>1K), or displexes (T>4K).

On the x-ray scattering side of things, our own x-ray lab at McMaster University features a 3He fridge (T>300mK), that allows us to perform truly unique low-temperature structural studies. At the Advanced Photon Source (APS), high brilliance, high resolution synchrotron x-ray experiments can be carried out a temperatures as low as 4K.

High Magnetic Fields: An invaluable combination for the study of frustrated magnetism is the dilution refrigerator insert with a superconducting magnet; a combination that is available at several neutron beam lines. The superconducting magnets can typically reach a maximum of between 10 and 15T. In frustrated materials this level of magnetic field is often sufficient to see phase transitions, since the energy scales required to tip the delicate balance of microscopic interactions are usually quite low. However, because neutron and x-ray studies have until very recently been restricted to such moderate fields, the high-field region of many materials has been inaccessible to these scattering techniques (for example, the level crossing associated with Bose-condensation of triplets in SrCu2(BO3)2 has not yet been observed with neutron scattering, which is the most direct probe of such effects).

To push the limits of the highest magnetic fields achievable, our group is part of a collaboration with researchers at Tohoku University in Japan and at the APS in Chicago. This collaboration is developing pulsed magnet neutron and x-ray scattering techniques. These pulsed magnets can reach higher fields (30 - 50T), because the power is supplied to the magnet for only milliseconds, preventing excessive heating of the coil. In an x-ray or neutron scattering experiment, the pulsed magnet technique requires either time-resolved measurements (achievable at the high intensity synchrotron source, APS), or the statistical combination of many successive pulses (a technique being considered at some neutron sources).

At the APS, we have been particularly active in developing the pulsed magnet dual-cryostat setup, with which we have successfully explored the low temperature and high-field dependence of the lattice structure of the strongly magneto-elastic spin liquid, Tb2Ti2O7.

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