Magnetic materials are important for various aspects of our daily lives, for example in power generation, magnetic resonance imaging and in electronic-devices. Amongst the various types of magnets, ferromagnetic materials like the common elements Fe, Ni and Co have a spontaneous magnetisation, responsible for the intuitive and simplistic understanding of magnetism in permanent magnets encountered in everyday life, and are also the most commonly integrated into electronic devices.
With the ever-increasing demand for faster, smaller and more energy-efficient electronic devices, researchers all over the world are focusing on novel ways to manipulate and control the property of these magnetic materials. Conveniently, a new class of magnetic materials was recently discovered which can be mechanically peeled off layer by layer using tape forming an atomically thick, and therefore a two-dimensional (2D), sheet. Because this sheet is so extremely thin, it can be bent and strained with very little force. Layers of these magnetic 2D materials, like the ferromagnet Cr2Ge2Te6 (CGT) that was used in our work, can also be stacked onto each other forming hybrid materials with new functionalities.
How to efficiently tune the magnetic state of a ferromagnet?
In commercial magnetic-based devices, for example in the magnetic random access memory (MRAM), the magnetic state is switched using a current, which inevitably causes heating. The use of an electric field instead reduces the energy dissipation allowing for the design of more energy-efficient devices, while making them more comparable with conventional Si-based devices. This is why the use of an electric field for manipulating the magnetic state of materials is highly desirable.
In our work, we thought of an indirect and simple way to utilize the electric field for tuning the magnetic state of 2D magnets. The magnetic state of a magnetic material is set by the exchange interaction, a quantum mechanical interaction, of neighbouring magnetic atoms in a crystal lattice. In the macro-world, one can imagine this interaction as analogous to holding two tiny permanent magnets (with both the same North to South pole orientation) close to each other and feeling the interaction of the two magnets. But what would be the simplest way to weaken or strengthen this interaction? That would be by moving the atoms farther apart or closer together, in other words by straining these magnetic layers! Going back to our tiny magnet analogy, this is similar to moving the tiny magnets farther away or closer together.
You may be wondering how we are able to strain these 2D magnetic materials. Well, we simply deform them by pulling them down into cavities using the electrostatic force which is controlled by an electric field. The 2D nature of these materials is important as it allows for high precision strain manipulation in these ultrathin layers, in which tension can be tuned over a large range. This way electric-field induced strain has the potential for becoming an efficient fine-control tuning knob of the magnetic state of 2D magnetic materials.
What do our devices look like and how do they work?
We suspend layers of CGT over micron-sized cavities in Si-based substrates, forming ultrathin nanodrums (Fig. 1a). We fabricate these drums using both layers of only bare CGT and heterostructures made of stacks of CGT with other 2D materials. We ensure that the suspended layers are placed on top of electrodes for electric field application.
Then, we manipulate the magnetic properties of the CGT layers by pulling them into the cavity using an electric field, while reading out their magnetic state using the strain-sensitive laser interferometry method (Fig. 1b). This way both the read-out and tunability of the magnetic state are realised through nanomechanical strain.
While straining our 2D magnetic materials, we observed pronounced signatures of coupling between magnetism and the elastic properties through the resonance frequency of the membranes made of CGT and its heterostructures near the magnetic transition temperature. We also controllably induced a strain of only 0.026% in a suspended CGT-based heterostructure via electrostatic force and showed a substantial enhancement of the magnetic transition temperature by roughly 2.5 K in the absence of an external magnetic field or electric currents.
What is the big picture?
Our findings follow from our previous study, where we demonstrated that by analysing the nanomechanical motion of such 2D material membranes as a function of temperature, one can probe the changes induced by the phase transition from one magnetic state to another. In this work, we followed up to extend the methodology to 2D-layered ferromagnet devices and their heterostructures. These findings together indicate how strongly nanomechanical strain and the magnetic state in various 2D materials are coupled.
This work was done in worldwide collaboration. The Peking University in China and the Universitat de Valencia in Spain provided very high-quality van der Waals crystals. With funding from the EU Graphene Flagship project, NWO and the Kavli Institute of Nanoscience, we fabricated and tested these devices in the Netherlands at the Delft University of Technology using cross-departmental expertise in 2D materials, nanomechanics and magnetism. This work is a great demonstration of cross-cultural synergy.
The presented results, achieved via our international collaboration, open-up various possibilities for further studies. One way to go would be to investigate the magnetic properties which result at interfaces of stacked 2D magnetic materials, also allowing novel magnetic properties to be discovered near the monolayer limit while excluding any substrate interactions. We also anticipate that these studies in future could lead to the development of gate-controlled membrane devices for low-power spintronic and memory applications.
(Images: Samer Kurdi, Irek Rosłoń and Makars Šiškins, Delft University of Technology)