(Nanowerk News) A research team, led by University of Texas at Dallas scientists, has developed a new technique for growing very large, high-quality crystals that could help make advanced electronics, such as spintronic and magnetic optoelectronic devices, a reality (ACS nano, “Three-Phase Magnetic Phase Transitions Revealed in High-Quality CrSBr van der Waals Bulk Magnets”).
Very thin layers – only one or two atoms thick – can be easily peeled off from bulk crystals. These two-dimensional layers exhibit surprising magnetic properties and are highly stable in air at room temperature, making them attractive for use in devices combining stacked layers of different materials.
Dr. Wenhao Liu, postdoctoral research fellow in the Department of Physics in the School of Natural Sciences and Mathematics, developed a vapor solid synthesis technique to grow chromium sulfide bromide crystals, which are typically prepared via the chemical vapor transport (CVT) method. .
“While this material itself is not new, our method for making it stands out from other methods,” said Liu, who works in Dr. Bing Lv, professor of physics and co-author of the report on this new technique. . “Our method is simpler, more direct, and produces much larger, higher quality crystals than conventional CVT methods.”
The synthesis technique, which involves using a simple box furnace, produces crystals measuring 1 to 2 centimeters, which are 10 times larger than those produced by other methods, says Lv (pronounced “love”).
The magnetic properties of the exfoliated layers surprised the research team.
Every electron in a material has a property called spin, which can go up or down, and the orientation of the spin determines the material’s magnetic properties. For example, when all the spins are aligned in the same direction, the material is ferromagnetic, whereas when the spins are aligned in parallel but in opposite directions—antiparallel—the material is antiferromagnetic.
“Usually, this magnetic ordering – spin alignment – occurs instantaneously within a material,” says Lv. “The uniqueness of our 2D material is that the antiferromagnetic order increases and spreads throughout the crystal in three stages as we decrease the temperature.”
The magnetic anomaly—an increase in the ferromagnetic order—occurs in the material at 185 kelvin (minus 126.7 degrees Fahrenheit), 156 kelvin (minus 178.9 degrees Fahrenheit) and 132 kelvin (minus 222 degrees Fahrenheit).
“This is the first time this phenomenon has been observed in chromium sulfide bromide crystals. I couldn’t believe it when I first saw this because it went against our intuitive thinking,” said Lv. “This is an interesting observation, as these kinds of ‘soft’ magnets may be useful in certain magnetic memory devices.”
The crystals are stable in air, a convenient and important property for future research and possible use in high-performance devices, Liu said.
“Ordinary crystals can react with water or oxygen in the air, which changes the chemical composition or structure of the crystals,” said Liu, one of the report’s lead authors. “That is why in most cases, crystals must be covered with an inert material. Our crystals remove that barrier, which makes them easier to use for the creation and development of new devices.”
Nikhil Dhale, a physics doctoral student, and Aswin Kondusamy, a materials science and engineering doctoral student, also contributed to the findings, along with researchers from the University of Michigan, University of North Texas and University of Houston.