Maximal magnetism: Single-atom limit

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  • Published: May 15, 2014
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Maximal magnetism: Single-atom limit

To the max

Reaching the magnetic anisotropy limit of a 3d metal atom. Harald Brune EPFL press photo

Scientists have shown for the first time the maximum theoretical limit on the energy required to control the magnetization of a single atom. The fundamental work could have implications for the future of research into magnetism and devices that utilize the phenomenon in technology.

Magnetism all around, sometimes up, sometimes down. Essential for so many technologies including those of the computer age, magnetic hard drives, magnetic random access memories (MRAMs), molecular magnets, and quantum computers. From the atomic perspective, the phenomenon arises due to the spin and orbital momentum of the electrons. 'Magnetic anisotropy', of course, arising in the bulk because of discrepancies between those electron alignments and the structure of the material itself. It would be so much easier if there were a way to look at a single atom in terms of its magnetic properties.

Multi-centred

Now, writing in the journal Science, researchers led by researchers at the École polytechnique fédérale de Lausanne (EPFL) describe how they have combined various experimental and computational methods to measure for the first time the energy needed to change the magnetic anisotropy of a single cobalt atom. Their methodology and findings could affect many disparate fields of fundamental research to the advancement of technologies that revolve around magnetism and spintronic architectures, for instance.

The team led by EPFL's Harald Brune (pictured) explains that the total energy corresponding to a material's magnetic anisotropy is a fundamental constraint to the downscaling of magnetic devices such as MRAM devices and quantum computers that exploit electron spin states as distinct information units, in 'qubits'. Brune and colleagues have worked with colleagues at ETH Zurich, the Paul Scherrer Institute, the IBM Almaden Research Center, Georgetown University's Department of Physics and the Swiss Light Source, to develop a method for determining the maximum possible magnetic anisotropy for a single atom of the transition metal cobalt, a familiar component of many magnetic materials used in technology.

The researchers used a technique called inelastic electron tunnelling spectroscopy to probe the quantum spin states of a single cobalt atom bound within a layer of magnesium oxide, MgO. The technique uses an atom-sized scanning tip that allows electrons to tunnel through to the bound cobalt atom, transferring their energy and their spin properties as they go.

Qubits

The experiments revealed the maximum magnetic anisotropy energy of a single atom (about 60 milli-electronvolts) and the longest spin life-time for a single transition metal atom. This large anisotropy leads to a remarkable magnetic moment, which has been determined with synchrotron-based measurements at the X-Treme beamline at the Swiss Light Source, the team reports. Though a piece of fundamental science , these findings do open the way for a better understanding of magnetic anisotropy and present a single-atom model system that could itself be used as a qubit in a future device.

"Quantum computing uses quantum states of matter, and magnetic properties are such a quantum state", explains Brune. "They have a life-time, and you can use the individual surface adsorbed atoms to make qubits. Our system is a model for such a state. It allows us to optimize the quantum properties, and it is easier than previous ones, because we know exactly where the cobalt atom is in relation to the MgO layer."

"The next step will be to engineer longer spin-relaxation times. This is the lifetime of a magnetic state. If we can increase that time, we can preserve a given magnetic state for longer time, which is of interest for magnetic information storage and for quantum computation," Brune told SpectroscopyNOW.

Related Links

Science, 2014, online: "Reaching the Magnetic Anisotropy  Limit of a 3d Metal Atom"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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