Periodic Reporting for period 2 - EXAFONIS (Exploring antiferromagnetic order at the nanoscale with a single spin microscope)
Periodo di rendicontazione: 2022-03-01 al 2023-08-31
Antiferromagnetic materials (AFs) are emerging as a new paradigm for the development of innovative spintronic devices combining (i) non-volatile and high-density data storage capabilities, (ii) highspeed logic operations, and (iii) minimal energy consumption. Despite such appealing prospects, most of conventional real-space magnetic microscopy techniques cannot probe the AF order at the nanoscale because magnetic moments are mostly compensated, resulting in very weak magnetic signals. This is a major obstacle to the fundamental understanding of nanoscale AF order and its response to external stimuli. To harness the unique features of AFs for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems.
In this context, the project EXAFONIS proposes to exploit the unique performances offered by a new generation of quantum magnetometers based on a single Nitrogen-Vacancy (NV) defect in diamond to investigate the AF order at the nanoscale. Two main objectives are pursued. The first objective is to provide a deep understanding of the microscopic mechanisms at the origin of AF manipulation by external stimuli, such as strain, electric fields or spin-polarized currents. The second objective is to demonstrate the detection and manipulation of magnetic skyrmions in AF materials, thus integrating topology in the vibrant area of AF spintronics.
In this context, the project EXAFONIS proposes to exploit the unique performances offered by a new generation of quantum magnetometers based on a single Nitrogen-Vacancy (NV) defect in diamond to investigate the AF order at the nanoscale. Two main objectives are pursued. The first objective is to provide a deep understanding of the microscopic mechanisms at the origin of AF manipulation by external stimuli, such as strain, electric fields or spin-polarized currents. The second objective is to demonstrate the detection and manipulation of magnetic skyrmions in AF materials, thus integrating topology in the vibrant area of AF spintronics.
The first material studied in EXAFONIS is BiFeO3 (BFO), an insulating multiferroic compound which has emerged as a promising platform for spintronic applications because its multiferroic order is preserved well above room temperature. In addition to a strong ferroelectric polarization, BFO exhibits a cycloidal AF order that can be deterministically controlled by electric fields through magnetoelectric coupling, thus opening perspectives for low-power reconfigurable AF spintronics. During the first reporting period, we have imaged topological defects emerging from the cycloidal AF order at the surface of bulk BFO crystals. Combining reciprocal and real-space magnetic imaging techniques, we first observed, in a single ferroelectric domain, the coexistence of AF domains in which the cycloidal AF order propagates along different wavevectors. At the junctions between these magnetic domains, we observed the formation of topological line defects identical to those found in a broad variety of lamellar physical systems with rotational symmetries. This work established, for the first time, the presence of these magnetic objects in a multiferroic AF material, offering additional topological AF textures for future use in spintronics. We then studied how the cycloidal AF order evolves in BFO thin films grown on different substrates allowing to tune epitaxial strain. Using scanning-NV magnetometry, we were able to identify (i) two different types of spin cycloids stabilized in strain-engineered BFO epitaxial thin films and (ii) a critical epitaxial strain at which the cycloidal AF state is on the brink of destabilization with a period starting to diverge. This study has consolidated the design of optimized piezoelectric-BFO heterostructures enabling to manipulate the AF order through operando strain actuation.
Besides the study of BFO, we have also shown that non-collinear AF spin textures can be imaged with nanoscale spatial resolution by probing the magnetic noise they locally produce via thermal populations of magnons. This was achieved by adding a relaxometry-based imaging mode to the scanning-NV magnetometry toolbox, which relies on measurements of variations in the photoluminescence signal of the NV defect induced by magnetic noise. As a proof-of-concept, the efficiency of this novel method was first demonstrated by imaging domain walls and spin spirals in synthetic antiferromagnets (SAF). We then showed that NV-based relaxometry enables to image isolated skyrmions in SAF, which was an important goal of EXAFONIS.
Besides the study of BFO, we have also shown that non-collinear AF spin textures can be imaged with nanoscale spatial resolution by probing the magnetic noise they locally produce via thermal populations of magnons. This was achieved by adding a relaxometry-based imaging mode to the scanning-NV magnetometry toolbox, which relies on measurements of variations in the photoluminescence signal of the NV defect induced by magnetic noise. As a proof-of-concept, the efficiency of this novel method was first demonstrated by imaging domain walls and spin spirals in synthetic antiferromagnets (SAF). We then showed that NV-based relaxometry enables to image isolated skyrmions in SAF, which was an important goal of EXAFONIS.
In the next phase of the EXAFONIS project, we will mainly focus on three different tasks. Capitalizing on the study of strain-engineered BFO epitaxial thin films, we will first grow BFO thin films on a piezoelectric substrate (PMN-PT) enabling to manipulate the AF order through operando strain actuation. This heterostructure will be used to study the variations of the AF order while tuning the voltage applied to the piezoelectric material in order to assess the efficiency of in situ AF actuation by local strain. We will also investigate if BFO thin films could be driven through a topological transition when increasing uniaxial anisotropy via strain engineering. In addition, we will study the dynamics of skyrmions in synthetic AF under spin polarized currents, as planned.