24 research outputs found
Un microscope de champ magnétique basé sur le défaut azote-lacune du diamant : réalisation et application à l'étude de couches ferromagnétiques ultraminces
The ability to map the magnetic field at the nanometer scale would be a crucial advance to study the magnetic properties of solids as well as some transport phenomena, but also for fundamental studies in biology. This thesis deals with the realisation of a magnetic field microscope of a new kind, which promises a spatial resolution down to a few nanometres, a sensitivity of the order of a few nanoteslas, and operates under ambient conditions. This microscope is based on the nitrogen-vacancy defect in diamond, whose quantum properties can be harnessed to make an ultrasensitive, atomic-size magnetometre. In the first part, we will present the basic principles and the realisation of the nitrogen-vacancy defect microscope, which consists essentially in an atomic force microscope on the tip of which a diamond nanocrystal is grafted. We will test the microscope by imaging the stray field generated by a vortex core in a ferromagnetic microdisk. In the second part, we will apply the microscope to the study of ultrathin ferromagnets. These systems are interesting both from the physical point of view, as interface effects have been little explored so far, and for technology, as they are the cornerstone of several proposals for realising novel magnetic memory devices with low energy consumption. We will first study the nature of domain walls in these ultrathin ferromagnets, which will enable us to reveal the existence of an interface-related Dzyaloshinskii-Moriya interaction in some samples. Next, we will study the nanometric jumps of a domain wall induced by thermal fluctuations. In particular, we will demonstrate control over these jumps using a laser, which will allow us to visualise and explore the wall's energy landscape.La capacité à cartographier le champ magnétique à l'échelle nanométrique serait un atout crucial pour étudier les propriétés magnétiques des solides ainsi que certains phénomènes de transport, mais aussi pour des études fondamentales en biologie. Cette thèse porte sur la réalisation d'un microscope de champ magnétique d'un genre nouveau, qui promet une résolution spatiale de quelques nanomètres, une sensibilité de l'ordre du nanotesla, et fonctionne aux conditions ambiantes. Ce microscope est basé sur le défaut azote-lacune du diamant, dont les propriétés quantiques peuvent être exploitées pour en faire un magnétomètre ultrasensible de taille atomique. Dans un premier temps, nous présenterons le fonctionnement et la réalisation du microscope à défaut azote-lacune, qui consiste essentiellement en un microscope à force atomique sur la pointe duquel un nanocristal de diamant est attaché. Nous testerons le microscope en imageant le champ de fuite généré par un cœur de vortex dans un microdisque ferromagnétique. Dans un second temps, nous appliquerons le microscope à l'étude de couches ferromagnétiques ultraminces. Ces systèmes présentent un intérêt à la fois fondamental, les effets d'interfaces restant encore largement inexplorés à ce jour, et technologique, puisqu'ils sont à la base de propositions pour la réalisation de nouvelles mémoires magnétiques à basse consommation d'énergie. Nous étudierons d'abord la nature des parois de domaines dans ces couches ultraminces, ce qui nous permettra de révéler l'existence d'une interaction Dzyaloshinskii-Moriya d'origine interfaciale dans certains échantillons. Nous étudierons ensuite les sauts nanométriques d'une paroi de domaine induits par l'agitation thermique. Nous démontrerons en particulier le contrôle de ces sauts par un laser, ce qui nous permettra de visualiser et explorer le paysage énergétique de la paroi
Scanning nano-spin ensemble microscope for nanoscale magnetic and thermal imaging
Quantum sensors based on solid-state spins provide tremendous opportunities
in a wide range of fields from basic physics and chemistry to biomedical
imaging. However, integrating them into a scanning probe microscope to enable
practical, nanoscale quantum imaging is a highly challenging task. Recently,
the use of single spins in diamond in conjunction with atomic force microscopy
techniques has allowed significant progress towards this goal, but
generalisation of this approach has so far been impeded by long acquisition
times or by the absence of simultaneous topographic information. Here we report
on a scanning quantum probe microscope which solves both issues, by employing a
nano-spin ensemble hosted in a nanodiamond. This approach provides up to an
order of magnitude gain in acquisition time, whilst preserving sub-100 nm
spatial resolution both for the quantum sensor and topographic images. We
demonstrate two applications of this microscope. We first image nanoscale
clusters of maghemite particles through both spin resonance spectroscopy and
spin relaxometry, under ambient conditions. Our images reveal fast magnetic
field fluctuations in addition to a static component, indicating the presence
of both superparamagnetic and ferromagnetic particles. We next demonstrate a
new imaging modality where the nano-spin ensemble is used as a thermometer. We
use this technique to map the photo-induced heating generated by laser
irradiation of a single gold nanoparticle in a fluid environment. This work
paves the way towards new applications of quantum probe microscopy such as
thermal/magnetic imaging of operating microelectronic devices and magnetic
detection of ion channels in cell membranes.Comment: 22 pages including Supporting Information. Changes to v1:
affiliations and funding information updated, plus minor revisions to the
main tex
Detection of paramagnetic spins with an ultrathin van der Waals quantum sensor
Detecting magnetic noise from small quantities of paramagnetic spins is a
powerful capability for chemical, biochemical, and medical analysis. Quantum
sensors based on optically addressable spin defects in bulk semiconductors are
typically employed for such purposes, but the 3D crystal structure of the
sensor inhibits the sensitivity by limiting the proximity of the defects to the
target spins. Here we demonstrate the detection of paramagnetic spins using
spin defects hosted in hexagonal boron nitride (hBN), a van der Waals material
which can be exfoliated into the 2D regime. We first create negatively charged
boron vacancy (V) defects in a powder of ultrathin hBN nanoflakes
(~atomic monolayers thick on average) and measure the longitudinal spin
relaxation time () of this system. We then decorate the dry hBN nanopowder
with paramagnetic Gd ions and observe a clear quenching, under
ambient conditions, consistent with the added magnetic noise. Finally, we
demonstrate the possibility of performing spin measurements including
relaxometry using solution-suspended hBN nanopowder. Our results highlight the
potential and versatility of the hBN quantum sensor for a range of sensing
applications, and pave the way towards the realisation of a truly 2D,
ultrasensitive quantum sensor.Comment: 19 pages, 11 figure
Stray magnetic field imaging of thin exfoliated iron halides flakes
Magnetic van der Waals materials are often proposed for use in future
spintronic devices, aiming to leverage the combination of long-range magnetic
order and near-atomic thinness to produce energy-efficient components. One
class of material that has been discussed in this context are the iron halides
FeCl and FeBr, which are A-type antiferromagnets with strong uniaxial
magnetocrystalline anisotropy. However, despite characterization of the bulk
materials, the possibility for sustaining the magnetic behaviors that would
underpin such applications in thin flakes has not been investigated. In this
work, we use nitrogen-vacancy (NV) center microscopy to quantitatively image
magnetism in individual exfoliated flakes of these iron halides, revealing the
absence of magnetic remanence, a weak induced magnetization under bias field
and variable behavior versus temperature. We show that our results are
consistent with the antiferromagnetic behavior of the bulk material with a soft
ferromagnetic uncompensated layer, indicating that extended (m)
ferromagnetic domains are not sustained even at low temperatures (down to 4 K).
Finally, we find that the magnetic order is strongly affected by the sample
preparation, with a surprising diamagnetic order observed in a thin, hydrated
sample.Comment: 15 pages, 13 figure
Stray magnetic field imaging of thin exfoliated iron halides flakes
Magnetic van der Waals materials are often proposed for use in future spintronic devices, aiming to leverage the combination of long-range magnetic order and near-atomic thinness to produce energy-efficient components. One class of material that has been discussed in this context are the iron halides FeCl2 and FeBr2, which are A-type antiferromagnets with strong uniaxial magnetocrystalline anisotropy. However, despite characterization of the bulk materials, the possibility for sustaining the magnetic behaviors that would underpin such applications in thin flakes has not been investigated. In this work, we use nitrogen-vacancy center microscopy to quantitatively image magnetism in individual exfoliated flakes of these iron halides, revealing the absence of magnetic remanence, a weak induced magnetization under bias field, and variable behavior versus temperature. We show that our results are consistent with the antiferromagnetic behavior of the bulk material with a soft ferromagnetic uncompensated layer, indicating that extended (>1µm) ferromagnetic domains are not sustained even at low temperatures (down to 4 K). Finally, we find that the magnetic order is strongly affected by the sample preparation, with a surprising diamagnetic behavior observed in a thin, hydrated sample.<br/