When doping a
disordered magnetic insulator material with atoms of a nonmagnetic material,
the conventional wisdom is that the magnetic interactions between the magnetic
ions in the material will be weakened.
However, when
the antiferromagnetic insulator barium manganate was doped—a process in which
atoms of nonmagnetic vanadium were substituted for the manganese—the barium
manganate’s magnetic excitations (i.e., its magnons) were surprisingly unreduced
in strength and energy.
Matthew Stone of
the Neutron Scattering Science Division and collaborators from Stanford University conducted this research at
the SNS Cold Neutron Chopper Spectrometer (CNCS) and Powder Diffractometer
(POWGEN). The work will contribute to fundamental understanding of magnetic
interactions in insulator materials. “But from what we’re learning one can
also apply that to magnets that are used in applications, such as magnets in
motors, for example: figuring out how to improve magnets used in devices,”
Stone explains.
“We have a
nice model system, the barium manganate (they call it ‘Bamno’), that we
understand. We’re putting dopants in that system, and we want to see what
changes occur. We thought that if we put enough vanadium in there, we would no
longer see the known excitations. But it turned out that we could put up to 30%
vanadium in there and we still saw those original excitations.”
“That was
the main, interesting part of the paper. The extra was that in addition to this
excitation known in the original material, and now seen by neutron scattering
in the nonmagnetic substituted material, we also see low-energy excitations in
these doped materials. And that
was unexpected,” Stone says.
Stone and his
collaborators used Ba3Mn2O8 powder samples
made by Ian Fisher’s research group at Stanford University.
For the neutron scattering, they needed the CNCS and POWGEN instruments at SNS.
At POWGEN, they were able to “see” the material’s crystal structure.
At CNCS, they were able to see the excitations.
“It is very
important to know where these vanadium atoms (the dopant) are going. Just
because the chemist or the crystal grower mixes two compounds together and puts
in a pinch of vanadium, doesn’t mean we know that it’s going into the crystal
at the right spot,” Stone says. “So POWGEN can show us, as we are
putting in more vanadium, that it is it going to the right spot, the right
atomic site.”
Then the CNCS
instrument measured the magnetic excitations in various dopings of the samples.
“What is powerful about the CNCS is that we had five different dopings of
material, ” Stone explains, “and because of the higher flux—the
number of neutrons coming in from the neutron source—and the good detector
coverage, we were able to churn through those samples very quickly and get
reasonable data.”
The magnetic
excitations in the pure compound are called “spin dimers”—pairs of
interacting spins, strongly coupled together. There are two, hence they are
called dimers. The dimers have an energy level, and neutrons then excite the
dimers to a higher energy state. In so doing, the neutrons give up energy and
slow down. The CNCS instrument measures the neutrons’ energy loss by neutron
scattering.
Here is how it
works in a model system such as Ba3Mn2O8.
“If you put those dimers in a lattice,” Stone explains, “and you
have them talking to each other, then you have a dimer excitation that can
propagate through the lattice; it is an energy propagation that’s called a
magnon. You have a magnetic wave that can move through the material from these
dimers that are weakly coupled together in the lattice. The whole crystal is
filled with them, and we measure how these excitations propagate. This is called
their dispersion relation.”
Enter the
doping. The researchers break up some of the dimers by inserting nonmagnetic
vanadium into them. Theoretically, the broken dimers can no longer support the
magnetic excitations. “Only we found that the propagating magnetic
excitation still exists. Even though a third of those dimers may be
broken,” Stone says.
The researchers
believe their data indicate that something previously unknown is occurring in
this doped insulator. “We believe that in this material there are strong
bonds, strong dimers, and that these are actually coupled together in a network
and that network has many different linkages.
“So one of
these linkages in the material, one dimer, actually can see the influence of up
to six or eight neighbors. It can see neighbors that are different distances
away. And because it can see them, it is still able to propagate its magnetic
information along the network, even around one of its neighbors that may have a
broken dimer,” Stone explains. “That is why we believe that we still
see that magnetic excitation and why we still see that it is disordered.”
Their next step
is to get a crystal. The Stanford group will grow the crystal and will align a
number of them together to have as large a sample mass in the neutron beam as
possible. The measurements will then be taken at CNCS or at the Cold
Triple-Axis Spectrometer (CTAX) at HFIR.
“With the
powder measurement we are able to see the existence of this magnon and its
lower energy and its upper energy. With a crystal we can actually see how it is
propagating through the lattice—and with much better detail,” Stone says.
Does this work
belong to the superconductivity family of research? It is actually parallel,
Stone says. “Superconductor excitation measurements are very similar to
the ones we’ve examined. It too is about a magnetic moment that is propagating
magnetic information through the lattice. What we call a dispersing magnetic
excitation. People measure these in superconductors to figure out what the
exchange energy is between magnetic ions in a superconductor.
“We are doing
the same thing. Our material is an insulator, but we are doing identical
measurements, trying to find out this information about propagating magnetic
waves—the same type of excitation in fact that people see in superconductors—in
our material.”
