Ultrafast helicity-dependent all-optical switching in hybrid magnetic nanomaterials
Lead Research Organisation:
UNIVERSITY OF EXETER
Department Name: Physics and Astronomy
Abstract
Information technology has transformed our lives, connecting us and providing access to knowledge that was previously the preserve of experts and scholars. And yet today's technology is only a beginning. Embedding information gathering and remote control of objects that we either use or wear will lead to an Internet of Things that greatly extends the amount of information that we need to process and store. For this vision to be realised, the capabilities of the underlying hardware must continue to advance at breakneck speed. The principal focus of this project is on how we might store information in future.
Today we are increasingly dependent upon cloud computing that stores and retrieves information from data centres that contain enormous arrays of magnetic hard disk drives (HDDs). We have become accustomed to the idea that the capacity of each HDD will increase every year so that increased demand for storage capacity can be met by regularly replacing each HDD. However, the success of this strategy is now uncertain because the continued miniaturisation of technology needed to increase capacity has reached physical limits that are not easily overcome. Specifically, the size of the region on the surface of a disk that is used to store one "bit" of information (1 or 0), has become so small as to be unstable on the timescale of 10 years that is the industrial standard for date retention. Materials with enhanced stability have been developed, but it is not possible to switch their magnetization, i.e. write data, with the magnetic field available from a conventional magnetic recording head. This has generated intense interest in new mechanisms for magnetic switching that can bypass this seemingly unavoidable bottleneck.
This project will explore how light may be used to switch the magnetization of new magnetic materials in which the electronic and magnetic properties can be tailored to optical control. In 2004 it was shown that atomic monolayers of graphene can be peeled from a crystal of graphite, a form of carbon, by a technique known as mechanical exfoliation. The technique is effective because graphene layers are only weakly bonded to their neighbours by what are known as van der Waals forces. In fact, there are many other crystals with similar bonding from which few and monolayer films may be exfoliated. By exfoliating layers from different crystals and stacking them to form a multilayer, it is possible to create hitherto unknown hybrid materials that can combine the favourable properties of their parent crystals. Furthermore, the interface between successive layers can be extremely clean and well ordered. Here the aim is to combine 2 dimensional ferromagnetic (2dFM) layers that have permanent magnetic order with semiconducting transition metal dichalcogenide (TMDC) layers in which electrons can be optically excited with very high efficiency. Furthermore, it is possible to excite electrons that have a magnetic moment, which is associated with their quantum mechanical "spin", with direction determined by the polarization of the incident light.
Experiments will be performed in which an ultrafast laser pulse with duration less than 1 trillionth of a second is used to excite electrons in the TMDC layer so that their magnetic moments can interact with the magnetic moments in the 2dFM layer. By controlling the direction of the excited magnetic moments through the polarization of the light, the aim is to switch the magnetization of the 2dFM backwards and forwards at will. Furthermore, the manner and timescales on which the magnetization changes will be determined by using a second laser pulse to interrogate the instantaneous magnetic state at a time of our choosing. While the initial goal is to observe and understand the mechanism of all-optical switching, the ability to combine many different materials will facilitate the search for the combinations that are best suited to data storage applications.
Today we are increasingly dependent upon cloud computing that stores and retrieves information from data centres that contain enormous arrays of magnetic hard disk drives (HDDs). We have become accustomed to the idea that the capacity of each HDD will increase every year so that increased demand for storage capacity can be met by regularly replacing each HDD. However, the success of this strategy is now uncertain because the continued miniaturisation of technology needed to increase capacity has reached physical limits that are not easily overcome. Specifically, the size of the region on the surface of a disk that is used to store one "bit" of information (1 or 0), has become so small as to be unstable on the timescale of 10 years that is the industrial standard for date retention. Materials with enhanced stability have been developed, but it is not possible to switch their magnetization, i.e. write data, with the magnetic field available from a conventional magnetic recording head. This has generated intense interest in new mechanisms for magnetic switching that can bypass this seemingly unavoidable bottleneck.
This project will explore how light may be used to switch the magnetization of new magnetic materials in which the electronic and magnetic properties can be tailored to optical control. In 2004 it was shown that atomic monolayers of graphene can be peeled from a crystal of graphite, a form of carbon, by a technique known as mechanical exfoliation. The technique is effective because graphene layers are only weakly bonded to their neighbours by what are known as van der Waals forces. In fact, there are many other crystals with similar bonding from which few and monolayer films may be exfoliated. By exfoliating layers from different crystals and stacking them to form a multilayer, it is possible to create hitherto unknown hybrid materials that can combine the favourable properties of their parent crystals. Furthermore, the interface between successive layers can be extremely clean and well ordered. Here the aim is to combine 2 dimensional ferromagnetic (2dFM) layers that have permanent magnetic order with semiconducting transition metal dichalcogenide (TMDC) layers in which electrons can be optically excited with very high efficiency. Furthermore, it is possible to excite electrons that have a magnetic moment, which is associated with their quantum mechanical "spin", with direction determined by the polarization of the incident light.
Experiments will be performed in which an ultrafast laser pulse with duration less than 1 trillionth of a second is used to excite electrons in the TMDC layer so that their magnetic moments can interact with the magnetic moments in the 2dFM layer. By controlling the direction of the excited magnetic moments through the polarization of the light, the aim is to switch the magnetization of the 2dFM backwards and forwards at will. Furthermore, the manner and timescales on which the magnetization changes will be determined by using a second laser pulse to interrogate the instantaneous magnetic state at a time of our choosing. While the initial goal is to observe and understand the mechanism of all-optical switching, the ability to combine many different materials will facilitate the search for the combinations that are best suited to data storage applications.
Organisations
Publications
Da Browski M
(2022)
All-optical control of spin in a 2D van der Waals magnet.
in Nature communications
Khela M
(2023)
Laser-induced topological spin switching in a 2D van der Waals magnet
in Nature Communications
| Description | 1.Bilayers composed of a monolayer transition metal dichalcogenide (TMDC) and a 2 dimensional ferromagnetic (2dFM) layer were fabricated by mechanical exfoliation and stacking, and their structural and properties confirmed by atomic force microscopy and photoluminescence measurements performed within the glove box. Capping with hexagonal boron nitride allowed the samples to be studied by other measurement techniques in air. 2. Triage of samples for ultrafast laser excitation was carried out by wide field Kerr microscopy, revealing the magnetic domain structure, how over and underlayers affect the magnetic properties of the 2dFM, and hysteresis loop behaviour from selected regions. 3. Ultrafast excitation of the WSe2/CrI3 system, which has perpendicular magnetic anisotropy, was studied in detail, and both helicity dependent (HD) and helicity independent (HI) all optical switching (AOS) were observed. It was confirmed that only regions of the CrI3 overlapped with WSe2 exhibit AOS and then the response of these regions is spatially inhomogeneous, presumably due to the effect of edges and trapped bubbles and impurities. The HD-AOS was explained in terms of transfer of optically excited spin-polarised electrons from the WSe2 to the CrI3. Surprisingly, one monolayer of WSe2 is able to induce switching in up to 10 nm of CrI3, pointing to the importance of simultaneous demagnetisation of the CrI3 and a long spin lifetime in the WSe2. The interplay of these different characterisatic times can explain the HI-AOS, with transfer of minority spins being initially blocked before causing switching on longer timescales. 4. A study of the 2dFM CrGeTe3 revealed that ultrafast excitation can induce transitions between different topological states (bubble/skyrmion, stripe), with the final state being determined by the laser fluence used. 5. A novel time resolved scanning Kerr microscope was constructed to enable measurements to be made at variable temperature within a 5 Tesla superconducting magnet system. Galvanometer mirrors were used to raster the tightly focused probe beam, while a weakly focused pump beam was introduced after the scanning mirrors. Time resolved measurements were made on 2dFM samples of different thickness. The results have been submitted for publication. 6. All magneto-optical measurements were performed within the Exeter time resolved magnetism (EXTREMAG) facility and have been used to publicise the capabilities of EXTREMAG to potential users. |
| Exploitation Route | The results are very interesting of the field of all optical switching and demonstrate that transition metal dichalcogenide layers and interfacial spin transfer are an important new ingredient that others may also wish to exploit. |
| Sectors | Digital/Communication/Information Technologies (including Software) Education Electronics |