“The idea of creating or destroying magnetization with only voltage was first put forward back in the 1960s in studies on magnetoelectrics and multiferroics,” says team leader Manuel Bibes of the University Paris-Sud and Thales. These materials have been enjoying a renaissance since the beginning of this century, so such experiments are in the news again.

Bibes and colleagues studied films of FeRh around 20 nm thick. FeRh is a magnetic material that boasts a peculiar transition from antiferromagnetic to ferromagnetic as the temperature is increased from room temperature to about 100 °C. The researchers deposited these films on ferroelectric barium titanate (BaTiO3 single crystals and looked at how the magnetism of the FeRh changed as a voltage was applied.

The team found that electric fields as low as 0.4 kVcm–1 increased the magnetic transition temperature of the FeRh by 25 °C, which was enough to convert the FeRh from an antiferromagnetic state (with a very small magnetization) to a ferromagnetic one (with a large magnetization) just above room temperature. This giant magnetoelectric response comes primarily thanks to voltage-induced strain, says the team.

Two types of domains

Between 7 °C and 130 °C, BaTiO3 adopts a tetragonal structure with its unit cell elongated in one direction (called c) along which the ferroelectric polarization is oriented. This polarization is shortened in the two equivalent perpendicular directions (called a). Looking down at the surface of a (001)-oriented BaTiO3 crystal, we typically see two types of domains, explains Bibes: those where c points out of the surface plane (c domains) and those where it is in the plane (a domains).

“When we apply a voltage between the top and bottom surfaces, c domains grow in size at the expense of a domains, until the whole crystal becomes one giant c domain,” he told nanotechweb.org. “The in-plane lattice constant of the c domains is shorter, so the FeRh film grown on top is mechanically ‘squeezed’. This compression favours the antiferromagnetic state, which is stable over a wide range of temperatures. Just above room temperature, the FeRh is antiferromagnetic at high voltage but becomes ferromagnetic when the voltage is turned off.”

Voltage-induced strain

So why does this happen? The interatomic distances in FeRh are naturally smaller in the antiferromagnetic state than in the ferromagnetic one thanks to quantum mechanical interactions between the electron spins in the material and the bonding electrons, he continued. “Therefore, when we bring atoms closer together through the voltage-induced strain exerted by the BaTiO3, we change the energy balance between the two states – and favour antiferromagnetism,” he explained.

According to the researchers, the result could bode well for hybrid (or composite) multiferroics in which a ferroelectric (or piezoelectric) material is combined, not with a standard ferromagnetic, but with a material that transitions between different magnetic states. “In terms of applications, we might envisage voltage-driven solid-state magnetic memories, for example,” said Bibes.

“As well as the strain-mediated effect we have observed here, some of the voltage-induced magnetization change might be coming from accumulated and depleted charges in the BaTiO3,” he added. “We now plan to study this effect further – in the system we looked at in this work as well as in other material combinations.”

The current work is detailed in Nature Materials doi:10.1038/nmat3870.

Further reading

Electric fields control spin currents (Jan 2010)
Multiferroics feel the strain (May 2013)