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A single crystal of YFE2Zn20
shown next to a mm scale. It grows in this shape naturally and has
mirrored facets.
Image © US Department of Energy's
Ames Laboratory |
Their versatility makes the new zinc compounds
ideal for basic research efforts to observe and learn more about the
origins of phenomena such as magnetism. Basic research is the building
block. Once scientists understand how these materials work, products
and/or processes can follow.
In addition, zinc is very cheap. In 1982, the
U.S.Mint switched the composition of the penny to 97.5 percent zinc
and only 2.5 percent copper. In a similar manner, this class of
compounds is over 85 percent zinc. If technological applications can
be found, these compounds will literally only cost pennies to make.
The unique aspect of the RT2Zn20 (R=rare earth, T=transition
metal, Zn=zinc) compounds' properties that Canfield, Bud'ko, and Jia
discovered lies in the fact that they display extraordinary tunability,
even though they are over 85 percent zinc. Indeed, these researchers
have been able to make scores of different compounds with this "one
rare earth-two transition metals-twenty zincs" formula.
"We can make compounds for up to 10 transition
metals, and for each of those we can include between seven and 14 rare
earths," said Canfield. "So that's between 70 and 140 compounds."
One of the compounds the researchers made, YFe2Zn20
(Y=yttrium, Fe=iron, Zn=zinc), turned out to be even closer to being
ferromagnetic than palladium, a nearly ferromagnetic material that
scientists have traditionally studied to better understand magnetism.
Canfield describes palladium as a "runner-up" in
terms of band magnetismthe magnetism of the common metals like iron,
cobalt or nickel. These metals become ferromagnetic at such high
temperatures that it's difficult to study them in detail, so palladium
is the next-best option. In addition, palladium acts as a "before"
picture to their "after" in terms of ferromagnetism.
"The problem is that as an element, palladium is a
little hard to tune," said Canfield. "There is one palladium site, and
it's not that versatile. For basic research as well as possible
applied materials, you want compounds that allow for the manipulation
of their properties. We can tune the rare earth-iron(2)-zinc(20) so
we're able to push these compounds even closer to ferromagnetism and
try to understand the consequences of this," he explained.
Canfield, Bud'ko, and Jia have also tuned the
zinc(20) compounds by substituting on the rare earth side, for example,
by exchanging yttrium for gadolinium. Canfield explained, "It's like
having a panicky crowd and someone yelling, 'Quick, run this way!' All
of a sudden, everyone runs that way. That's what adding the gadolinium
does – the compound just suddenly goes ferromagnetic at an
unexpectedly high temperature."
The researchers can also tune the zinc(20)
compounds by "playing" with the transition metal site. "By
substituting cobalt for iron, we can back this material off," said
Canfield. The yttrium-cobalt-zinc(20) is about as ferromagnetic as
copper, which means it's not. So we can calm the crowd down a little
and see what happens."
The remarkable tunability of the new family of
zinc(20) compounds is allowing Canfield, Bud'ko and Jia to approach
the ferromagnetic transition point from where they hope to achieve
another ambition – pushing the material to become ferromagnetic at
very low temperatures by tweaking and tuning. "If we could do that,"
said Canfield, "then we could actually witness the birth of this type
of small moment ferromagnetism – instead of just before and after
pictures, we could watch the whole film."
As they continue to work toward that goal, Canfield
and Bud'ko stress the importance of being able to do materials
research at a DOE lab. "There are many different skills and resources
available to draw on," said Canfield. "Experimentally, it's very
important to have design, synthesis and characterization very tightly
linked. "You need to have your intrepid band of explorers able to
investigate and contribute. Let me give you two extreme examples.
First, being in Ames gives us access to the world's highest purity
rare earth elements. We need these to explore the effects of
substitution on the rare earth site. On the other extreme, in these
nearly ferromagnetic materials, band structure calculations have been
very important, and being able to tie into the Ames Lab band structure
expertise of German Samolyuk has been incredibly useful in helping us
understand it and trying to figure out where the next moves are." |
