Modified form of graphene could be used to make an energy-efficient data storage device

In the ongoing quest to store more data in smaller packages, a Stanford engineer has simulated how to use electric fields to control spin, a quantum property of the electrons, in a modified form of graphene.

Graphene is made from sheets of carbon atoms arranged in a lattice. Each sheet is just one atom thick. A recent paper by Elton Santos, a Stanford research fellow in chemical engineering, shows how modifying this material could create small, energy efficient storage devices with high capacities.

His recent paper in the online edition of ACS Nano delves into the science of spintronics - an emerging field in which researchers exploit the spin of the electron to develop novel electronic devices.

To understand spin, imagine electrons as planets.

Planets revolve around the sun, just like electrons orbit the nucleus of an atom. Planets also rotate, creating day and night. The rotation of electrons is called spin. Rotating electrons spin up or spin down. These spin values can be used to represent digital zeroes and ones.

When several neighboring electrons have the same spin value, they act together to create a magnetic field. These collections of electrons are called domains. Domains occur naturally in the cobalt alloys used to store data in the current generation of hard drives.

The magnetic field created by each domain is like a switch. Disk drive manufacturers can flip the switch from zero to one using a large external magnetic field.

As storage devices get smaller, domains get packed together more densely. Because of this increasing density, stronger magnetic fields are needed to flip the domains back and forth from zero to one. Creating stronger magnetic fields requires more energy.

Santos' work demonstrates how to use electric fields to flip the switch on data storage devices. It takes less energy to generate an electric field than it does to generate a magnetic field.

The problem has been that researchers cannot switch spins in conventional storage materials like cobalt.

So scientists are investigating new materials called magnetolectric materials that couple electrical and magnetic properties. Very few materials are magnetoelectric and, of these, most are expensive and difficult to handle, Santos said.

His paper looks at how to turn graphene into a magnetoelectric material. Graphene is easy to make, but it is not naturally magnetoelectric.

''You have to break the symmetry between the lattices, otherwise the graphene is inert,'' Santos said.

He describes how to make graphene magnetoelectric by adding naturally-occurring organic molecules called nitrophenyl diazonium (NPD). He said researchers routinely perform NPD modifications of graphene for other purposes, such as building graphene-based chemical sensors, so the process he describes is familiar to researchers.

His simulations show how modifying graphene with NPD gives the material magnetoelectric properties.

The NPD molecule forms a special chemical bond with the graphene. One carbon in the NPD molecule bonds to a single carbon atom in the graphene. Without this bond, the spin of electrons arranged within the carbon lattices of graphene are balanced, creating symmetry.

But when this single NDP bond is present, it upsets the balanced electron spins and breaks the symmetry of the lattices. Due to the broken symmetry, the unbalanced spin of electrons in the carbon can be controlled with an electric field.

''Our research suggests that graphene modified in this way could be a member of one the most exclusive clubs in materials science: the magnetoelectric materials,'' Santos said.

In his simulations, devices made with just two or three layers of NPD-coated graphene already exhibited magnetoelectric behavior similar to the other bulkier and more expensive materials being researched.

Santos also showed that graphene made from multiple sheets of carbon atoms has better magnetoelectric properties than graphene made from fewer sheets. It is easier to produce graphene with several sheets, meaning it would be easier to make better magnetoelectric switches.

A group of researchers have independently measured such effects, confirming the predictions Santos made in his simulations. The experiments were announced in the same issue of ACS Nano.

''Elton's work could soon transform the building blocks of modern electronics,'' said Jeongmin Hong, a postdoctoral researcher at University of California, Berkeley and the lead author of the experimental report. ''This is a breakthrough in the push for extremely energy-efficient computing.''

In addition to data storage, magnetoelectronic graphene could be used to make spintronic chemical and biological sensors and energy-harvesting devices, Santos said.

''Spintronics is a wide open field with so much potential,'' Santos said. ''We need materials that are light, cheap and easy to manipulate in looking for exciting spintronics applications.''

Santos performed this research jointly with Stanford Engineering's Department of Chemical Engineering and the School of Engineering and Applied Sciences at Harvard University.