Antiferromagnetism is magnetic order, yet without a macroscopic magnetization as in iron. The absence of the field has interesting technological potential, but it also makes the antiferromagnetic state harder to control and set in a certain direction. Light, however, can do this if antiferromagnets from a special class of materials called multiferroics are selected.
By Sebastian Manz and Manfred Fiebig, ETH Zürich
Based on article published in Nature Photonics
Magnetic materials are used in manifold ways: power generators, sensors and computer data storage are just a few examples. Controlling the magnetic state of a device is relatively laborious. An electric current has to flow through or at least close to it and generate a magnetic field that acts on the magnetization of ferromagnets like iron or nickel. This costs energy and poses a technological challenge. Current-less remote-control of magnetism with light is therefore explored as an alternative.
But how about magnets not carrying a macroscopic magnetization? These materials are called antiferromagnets. They represent the dominating form of magnetic order in nature and play an important role in devices like hard disk read/write heads. They cannot be controlled by magnetic fields, however, and optical remote-control seems even less likely.
Our team has been able to show that such optical control is nevertheless possible. This discovery rests on three ingredients:
First, the ability to literally see the antiferromagnetic state by a special, laser-frequency-doubling optical process called second harmonic generation. This allows us to image regions of different orientation and spatially resolve the antiferromagnetic domain state.
The second key ingredient is the coupling of the antiferromagnetic state to an electric polarization which is found in a class of materials called multiferroics. Here, antiferromagnetism and electric order possess a one-to-one coupling, since one order is responsible for the other to occur. If the system is now excited by a laser pulse it is locally heated above the transition temperature TC. During subsequent cooling below TC local electric stray fields exerted by the non-heated environment direct the cooling region into the opposite domain state, i.e. reversal occurs. Due to the intrinsic coupling, reversing the electric state simultaneously reverses the antiferromagnetic state. The difference to mere ferroelectric switching is hence that we are switching a truly magnetic state, yet with a mechanism acting outside spin space.
The third ingredient is attributed to the reversibility of this process: In order to switch the antiferromagnetic order back and forth, differently colored light beams are necessary. Here, we exploit the different optical and thus thermal penetration depth of different wavelength. This realizes fully-reversible switching of an antiferromagnetic structure.
Applying the above mentioned procedure, we can locally switch the antiferromagnetic order (±C) at least 200 times. Furthermore, even arbitrarily shaped domain patterns can be written by sweeping a laser across the sample without any spatial crosstalk, i.e. the reversal occurs only where the laser hits the sample.
The effect is described in a recent article in Nature Photonics . The work is a good example for the interesting new route the field of multiferroics, compounds uniting magnetic and electric order, is taking. The focus on the multiferroic order as such is replaced by an interest in functionalities resulting from it, like reversible optical control of antiferromagnetism in the present case. This shift of focus is reviewed in another recent work of our team that was published in Nature Review Materials .
 S. Manz, M. Matsubara, Th. Lottermoser, J. Büchi, A. Iyama, T. Kimura, D. Meier, M. Fiebig, Reversible optical switching of antiferromagnetism in TbMnO3, Nature Photonics 10, 653 (2016).
 M. Fiebig, Th. Lottermoser, M. Trassin, D. Meier, The evolution of multiferroics, Nature Reviews Materials 1, 16046 (2016).
Contact: Manfred Fiebig, (Tel. +41 44 633 26 90)
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