Near Field Microscopies platform

Coordinators: K. Bouzehouane, A. Vecchiola

Near-field microscopy was introduced to the laboratory in 1998 to support the emergence of nanoscience in its research topics. Since then, this activity has continued to develop and grow in response to the constant changes in the laboratory’s research themes; firstly, by developing state-of-the-art expertise in specific modes of local probe microscopy; secondly, through instrumental developments, both in-house and in collaboration with academic laboratories, major industrial companies, start-ups and French SMEs.

Today, the Laboratoire Albert Fert’s ‘Near Field Microscopies’ platform relies on the skills of two CNRS engineers and a pool of 12 microscopes, each with its own specificities for magnetic imaging, piezoresponse, electronic transport properties, RAMAN spectroscopy and photoluminescence… All of these modes can be operated in a controlled atmosphere, in a vacuum, in a magnetic field, at low temperature, in operando, in a glove box, etc. A veritable Swiss army knife of the near field, the platform is used for most of the laboratory’s research topics (spintronics, magnonics, functional oxides, multiferroics, 2D-VdW, molecular electronics, superconductors, etc.).

The CT-AFM mode involves measuring local conduction between a conductive AFM tip and a sample. It can be used to map the transport properties of thin films and heterostructures, to measure local current-voltage characteristics, or to make electrical contact with a nanometric component to analyse its electronic transport properties.

This mode has been used, for example, to validate the tunnel nature of electronic transport through various ultrathin insulating layers, for the optimization of tunnel junction devices based on h-BN or ferroelectric oxides such as BiFeO3 or BaTiO3 (PhD theses H. Béa, M. Piquemal).

The PFM mode enables the polar textures of ferroelectric materials to be imaged via their piezoelectric response. By combining the out-of-plane response and the response in the plane of the sample surface (vectorial PFM), the polarisation vector in the material being probed can be determined at any point. PFM can also be used to measure hysteretic cycles locally, and to determine the coercive fields of the ferroelectric polarization. This mode can be used to write remanent domain configurations in devices (“ferroelectric lithography”).

State-of-the-art expertise of this type of imaging has enabled, for example, the study of multiferroic tunnel barriers (M. Gajek PhD thesis) or the manipulation of superconductivity by field effect in Ferroelectric/Superconductor hybrid heterostructures (A. Crassous PhD thesis).

Interfacing the PFM microscope with transport measurements also makes it possible to observe the in-operando properties of functional devices such as ferroelectric memristors. The tip of the microscope is used as a mobile electrode to measure the transport properties of these components, to modify their resistance state by applying voltage pulses (ns) and also to image the configuration of the ferroelectric domains corresponding to this state within the device (A. Chanthbouala PhD Thesis).

MFM mode is used to image the magnetic textures of a sample, via the interaction of a magnetically coated AFM tip with the leakage fields radiated by the magnetic domains. The properties of the magnetic coating on the tip are the result of a compromise between having sufficient interaction to generate a measurable signal but limited interaction so as not to disturb the magnetic textures observed. This is why the near-field platform is developing its own magnetic coatings for tips (see growth platform) in order to adapt them as closely as possible to the characteristics of each system (magnetization, coercivity, anisotropy, etc.).

By interfacing the MFM microscope with transport measurements, it is also possible to combine imaging under a magnetic field, electrical measurement of devices and their manipulation by current pulses. For example, the dynamics of skyrmion motion in submicron metal tracks (W. Legrand PhD thesis) could be studied by combining imaging sequences of the device and current pulses.

For some low-magnetization systems (nanostructures, ferrimagnetic materials, ultra-thin ferromagnetic layers, etc.), the sensitivity of standard MFM becomes insufficient, even with optimized tips. The near-field platform has developed a new approach to vacuum MFM (in collaboration with Park Instrument) that enables us to gain more than an order of magnitude in sensitivity while remaining within a conventional amplitude modulation scheme. To enable the manipulation of magnetic textures in situ, this microscope was modified by the implementation of a variable magnetic field (in collaboration with Caylar). This instrument has been used, for example, to image skyrmions in synthetic antiferromagnet multilayers (W. Legrand PhD thesis).

For some systems, the observation of nano-magnetism by interaction with a magnetic AFM tip reaches its limits both in terms of sensitivity (antiferromagnetic materials) and sample perturbation (low coercivity materials for magnonics). Scanning NV Magnetometer (SNVM) is a major advance in imaging nanomagnetism, since it offers a gain of 3 to 4 orders of magnitude in sensitivity compared with MFM, while remaining non-perturbative. This technique has been developed in France by Vincent Jacques’ team (L2C Univ. Montpellier), with whom the near-field platform works closely. We have initiated a collaboration between HORIBA (expertise in AFM with optical coupling), Qnami (NV centre tips) and our laboratory (expertise in AFM and nanomagnetism) with the aim of equipping the laboratory and encouraging the development of a commercial microscope. This project received support from the CNRS and the IdF region (‘ImageSpin’ Sésame project). The first instrument of this type is now fully operational in the laboratory. A second instrument with a variable magnetic field and even greater sensitivity (NV gradiometry) has just been acquired as part of the EQUIPEX E-DIAMANT program.

The Near Field Microscopy platform recently acquired a microscopy system dedicated to the analysis of 2D and molecular materials for spintronics. This instrument combines an AFM with co-located optical spectroscopy capabilities such as Raman spectroscopy and photoluminescence. The spatial resolution of these spectroscopies, normally restricted to several hundred nm by the diffraction limit, can in this case be exceeded thanks to optical methods known as “Tip-enhanced spectroscopies” using the phenomenon of exaltation of the local electromagnetic field by the tip of the AFM, giving access to “optical” spatial resolutions of typically few 10 nm.

The near-field microscopy platform is supported by a number of national and international collaborations:

• Academic collaborations: GEEPS Central Supelec (France), L2C Univ. Montpellier (France), Thales-RT (France), SPMS Centrale Supelec (France), C2N Univ. Paris Sacaly (France), LPS Univ Paris Saclay (France), Sextant Soleil (France), SPEC CEA (France), Univ. Canterburry (New Zealand), Univ. Arkansas (USA), ETH Zurich (Switzerland).
• Industrial collaborations (Instrumentation): HORIBA (France/Japan), BRUKER nano (Germany), Concept Scientific Instruments (France), Qnami (Switzerland), Qzabre (Switzerland), Park System (South Korea), Caylar (France).