44 | New Scientist | 30 November 2019
Materials like iron, which are capable of
becoming magnetised and retaining their
magnetism even when the external field
is removed, are said to be ferromagnetic.
Ferromagnets are everywhere in our daily
lives. A compass needle is one example, and
your fridge is probably covered with dozens
more, holding up your holiday snaps and
reminders. Less well known to most of us,
but also well-established, are ferroelectrics –
materials that can produce electric fields, used
today to power some types of computer chip.
Their superpower, just like ferromagnetism,
starts with electrons. Briefly put, some
materials have a mix of charged atoms built
into their structure. If an electric field is applied
to the material, these charges can permanently
shift, and the separation of negative and
positive charges generates a tiny electric
field, called a dipole. When these dipoles
line up in the same direction, they form what
is called an electric polarisation. This means
the material produces an electric field.
Materials that can do this are said to be
ferroelectric (see “Fields of dreams”, right).
The first observation of ferroelectric
behaviour came in an unlikely substance:
a laxative called Rochelle salt, developed
by a French pharmacist in the 17th century.Its creator wouldn’t reveal his recipe, but its
ingredients weren’t its only secret. In 1824,
Scottish physicist David Brewster observed
that Rochelle salt is pyroelectric, which
means it produces a small voltage when
heated or cooled. And in 1880, the Curie
brothers – Jacques and Pierre – showed
that it was also piezoelectric, generating
voltage when it was squeezed, stretched
or otherwise physically deformed. In 1899,
Thomas Edison took advantage of Rochelle
salt’s piezoelectricity to build a commercial
version of his phonograph to play back
sound recordings.
Those early findings suggested something
strange was happening in the salt’s atoms.
The situation became even more interesting
in 1921, when a physicist at the University
of Minnesota found that if Rochelle salt
was immersed in an electric field, its electric“ Ferroelectric
behaviour was first
seen in a laxative
called Rochelle salt”
UNPICKING
STRING THEORYString theory is one of the most
popular (and controversial)
candidates for a theory
of everything, a unified
mathematical framework
capable of describing the
entirety of physics. Among its
predictions is that everything
in the universe is made of
unbelievably tiny strings,
whose vibrations correspond
to the subatomic particles we
see in our daily lives.
In the 1970s, physicist Tom
Kibble described how such
strings might arise in the early
universe, in the moments
shortly after the big bang.
Gaining insight into those
conditions appears impossibly
difficult, but Kibble identified
a number of mathematical
symmetries that the early
universe should obey.
If someone could only find
something with those
symmetries, they might
be able to model those
primordial conditions.
Forty years later, Nicola
Spaldin, while at the University
of California, Santa Barbara,
proposed a multiferroic
called yttrium manganite
as the answer. The equations
that describe the material
as it changes polarisation,
wrote Spaldin, match the
conditions Kibble laid out to
such an extent that it is the
“crystallographic equivalent of
cosmic strings”. She suggests
that the material may enable
physicists to simulate the
conditions that prevailed
billions of years in the past.charges line up. Even if the electric field is
taken away, the charges stay put, and the salt
produces its own electric field. By analogy
with ferromagnetism, which had been
known about for millennia, this property
was called ferroelectricity.
Coming just a century after Øersted’s
demonstration, this Rochelle salt experiment
deepened the known connections between
electricity and magnetism. Given this
relationship, you might think that getting
ferroelectricity and ferromagnetism into the
same material would be easy. But no such luck.
“When you have magnetic materials, they’re
almost by definition not ferroelectric,” says
materials scientist Manfred Fiebig at the Swiss
Federal Institute of Technology.Mutually exclusive
The logic is fairly simple: magnetism only
occurs because electrons, in order to align their
spins, must be free to move between atoms.
For a ferroelectric material to create an electric
field, charges must be free to move when an
external field is applied – but then stay in place.
“It is not a trivial thing. You want to relate
two different kinds of physical phenomenon.
One with currents, one with stationary
charges. How do you create materials that have
both of these properties?” asks Ramamoorthy
Ramesh at the University of California,
Berkeley. “These two things are in some
sense pointing in opposite directions.”
But that didn’t stop scientists from looking
for examples. In the 1950s, Soviet physicists
developed a synthetic material that had
flickers of promising properties when cooled
to below 0°C, but these vanished at room
temperature, limiting their usefulness. In 1965,
Swiss physicists overcame some of these
difficulties, but the fragility of their material
meant that industry wouldn’t bite.
The next three decades brought a steady
trickle of experimental attempts to mix
magnetic and ferroelectric ingredients, but
multiferroics remained mostly out of reach,
difficult to make and harder to use.
This is where Spaldin comes in. When she
left Yale for a new position at the University
of California, Santa Barbara, she took a bold
tack. She abandoned her original research
plans and instead dedicated herself to hunting
multiferroics full-time. Then, in 2000, she
published an electrifying paper that changed
everything. It was titled, simply: “Why Are
There So Few Magnetic Ferroelectrics?”
Her short, sharp analysis of the necessary
properties of such materials was inspirational.2