New Scientist - USA (2019-11-30)

30 November 2019 | New Scientist | 45

One researcher she inspired was Ramesh,
then working on the other side of the country.
He had been conducting experiments on a
synthetic compound called bismuth ferrite,
and the weird results he was seeing seemed to
match the signature of Spaldin’s multiferroics.
So he picked up the phone.
“I remember it very clearly,” says Spaldin.
“He was very Californian. He didn’t know me
very well, but he just asked, ‘What do you think
is the electric polarisation of bismuth ferrite?’ ”
That unconventional opening line launched
a collaboration: Spaldin with the theory and
big vision, Ramesh with the materials-making
background. As it turned out, bismuth ferrite
was the perfect candidate. On a microscopic
level, it consists of a lattice of bismuth atoms
interspersed with charged ions of iron and
oxygen. The structure of the bismuth atoms
provides the ferroelectricity, and the

Fields of dreams

A material’s electric and magnetic properties depend on the behaviour of its

electrons. Individual electrons can generate electric or magnetic fields that

cluster together in small regions called domains. In ferroelectric and

ferromagnetic materials, these domains line up in the presence of external fields.

In multiferroic materials, both sets of domains line up at the same time

FERROMAGNETIC MATERIAL

FERROELECTRIC MATERIAL

MULTIFERROIC MATERIAL

Apply a

magnetic field

Apply an

electric field

“ Spaldin changed

her plans and

started to hunt

multiferroics

full-time”

FINDING

DARK MATTER

Around 85 per cent of the

matter in the universe is

invisible. We know this

so-called dark matter is

out there because of its

gravitational effects, but

nobody has yet spotted it

directly. Sinéad Griffin at the

Lawrence Berkeley National

Lab in California is one of the

many physicists looking to

change that.

Her idea is simple. As the

Earth whooshes through

a big cloud of dark matter,

that directional motion might

give rise to a kind of invisible

wind. Such a wind would impart

energy that ordinary matter

could pick up, providing

evidence of dark matter’s

existence and possibly its

composition.

“A smoking gun for a dark

matter experiment would be

getting this directionality,”

says Griffin. “If you have a

target that can pick this up,

it’s enough.”

Conveniently, the energy

range that multiferroics are

sensitive to is exactly right

for picking up the likely

constituents of dark matter.

Most dark matter detectors

are huge vats of inert liquids,

tucked away deep underground,

that are looking for high-energy

particles. But they have largely

seen a lot of nothing. The

detectors that Griffin envisions

would pick up even fainter

signals, making them an

exciting option.

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