8 research outputs found
In-plane magnetic domains and N\'eel-like domain walls in thin flakes of the room temperature CrTe van der Waals ferromagnet
The recent discovery of magnetic van der Waals materials has triggered a
wealth of investigations in materials science, and now offers genuinely new
prospects for both fundamental and applied research. Although the catalogue of
van der Waals ferromagnets is rapidly expanding, most of them have a Curie
temperature below 300 K, a notable disadvantage for potential applications.
Combining element-selective x-ray magnetic imaging and magnetic force
microscopy, we resolve at room temperature the magnetic domains and domains
walls in micron-sized flakes of the CrTe van der Waals ferromagnet.
Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are
observed. Upon annealing the material above its Curie point (315 K), the
magnetic domains disappear. By cooling back down the sample, a different
magnetic domain distribution is obtained, indicating material stability and
lack of magnetic memory upon thermal cycling. The domain walls presumably have
N\'eel texture, are preferentially oriented along directions separated by 120
degrees, and have a width of several tens of nanometers. Besides microscopic
mapping of magnetic domains and domain walls, the coercivity of the material is
found to be of a few mT only, showing that the CrTe compound is
magnetically soft. The coercivity is found to increase as the volume of the
material decreases
Stability of the In-Plane Room Temperature van der Waals Ferromagnet Chromium Ditelluride and Its Conversion to Chromium-Interleaved CrTe Compounds
Van der Waals magnetic materials are building blocks for novel kinds of
spintronic devices and playgrounds for exploring collective magnetic phenomena
down to the two-dimensional limit. Chromium-tellurium compounds are relevant in
this perspective. In particular, the 1 phase of CrTe has been argued to
have a Curie temperature above 300~K, a rare and desirable property in the
class of lamellar materials, making it a candidate for practical applications.
However, recent literature reveals a strong variability in the reported
properties, including magnetic ones. Using electron microscopy, diffraction and
spectroscopy techniques, together with local and macroscopic magnetometry
approaches, our work sheds new light on the structural, chemical and magnetic
properties of bulk 1-CrTe exfoliated in the form of flakes having a
thickness ranging from few to several tens of nanometers. We unambiguously
establish that 1-CrTe flakes are ferromagnetic above room temperature,
have an in-plane easy axis of magnetization, low coercivity, and we confirm
that their Raman spectroscopy signatures are two modes,
(103.5~cm) and (136.5~cm). We also prove that
thermal annealing causes a phase transformation to monoclinic CrTe and,
to a lesser extent, to trigonal CrTe. In sharp contrast with
1-CrTe, none of these compounds have a Curie temperature above room
temperature, and they both have perpendicular magnetic anisotropy. Our findings
reconcile the apparently conflicting reports in the literature and open
opportunities for phase-engineered magnetic properties
Room temperature 2D ferromagnetism in few-layered 1-CrTe
Spin-related electronics using two dimensional (2D) van der Waals (vdW)
materials as a platform are believed to hold great promise for revolutionizing
the next generation spintronics. Although many emerging new phenomena have been
unravelled in 2D electronic systems with spin long-range orderings, the
scarcely reported room temperature magnetic vdW material has thus far hindered
the related applications. Here, we show that intrinsic ferromagnetically
aligned spin polarization can hold up to 316 K in a metallic phase of
1-CrTe in the few-layer limit. This room temperature 2D long range
spin interaction may be beneficial from an itinerant enhancement. Spin
transport measurements indicate an in-plane room temperature negative
anisotropic magnetoresistance (AMR) in few-layered CrTe, but a sign
change in the AMR at lower temperature, with -0.6 at 300 K and +5 at 10
K, respectively. This behavior may originate from the specific spin polarized
band structure of CrTe. Our findings provide insights into magnetism in
few-layered CrTe, suggesting potential for future room temperature
spintronic applications of such 2D vdW magnets.Comment: 9 Pages, 4 Figure
Modulation de la fréquence d'un oscillateur spintronique (STNO) pour des applications de communication sans fil
Spin Transfer Nano-Oscillators (STNOs) are a novel type of Radio Frequency (RF) oscillators that make use of the Spin Transfer Torque (STT) effect in a magnetic tunnel junction (MTJ) device to produce high-frequency auto-oscillations. STNOs are attractive for applications in wireless communications due to their nanometric size and their frequency tuning capabilities via either a dc current or an applied field. This frequency tuning permits to encode the information via frequency shift keying (FSK) by digital modulation of the current or applied field between two discrete values without the need of an external RF mixer, leading to potentially less complex RF components. In this thesis, the feasibility of the digital frequency modulation (frequency shift keying (FSK)) using in-plane magnetized MTJ STNOs has been studied. For this, the maximum modulation rate, up to which a signal can be modulated or the frequency can be shifted between two discrete values, is an important aspect that need to be characterized.The characterization of the maximum modulation rate for in-plane magnetized MTJ STNOs has been studied via numerical macrospin simulation for different modulation configurations, i.e. modulation by a sinusoidal RF current and a sinusoidal RF field. It revealed that the maximum modulation rate under RF current modulation is given by the amplitude relaxation frequency fp of the STNO. Under RF field modulation, i.e. an RF field applied parallel to the easy axis, an enhanced modulation rate above fp can be achieved since the frequency is modulated directly via the field and not via the amplitude. This suggests an important strategy for the design of STNO-based wireless communications and to achieve high data rates. Besides numerical simulation, experimental studies of frequency shift keying (FSK) by current modulation in STNOs have been also demonstrated. The first demonstration is the FSK in standalone STNOs. The analysis confirmed that the FSK was successfully observed with a frequency shift around 200MHz (the frequency shift between â8.9 GHz and â9.1 GHz) at the modulation rate of 10Mbps. This modulation rate is however less than the upper limit, which is given by the relaxation frequency fp of the STNO as predicted in the numerical simulation, because of the relatively high phase noise of the device measured. In order to test the feasibility of the STNO within microwave systems, the FSK modulation of STNOs was performed on a printed circuit board (PCB) emitter. FSK with a frequency shift around 300MHz (the frequency shift between â9 GHz and â9.3 GHz) was observed with a modulation rate of 20 Mbps. The data rate here was limited by characteristics of the PCB emitter and not intrinsic to the STNO. The simulation and experiment studies of frequency modulation of STNOs demonstrate that the data rate of is adequate for wireless communication used in WSN. However, further improvements in materials and nanofabrication of STNOs are required to enhance the output power and improve the spectral characteristics of the oscillations to push the data rates to higher values with large frequency shift.Les Oscillateurs Spintronique (STNO) sont un nouveau type d'oscillateurs Ă frĂ©quence radio (RF) qui utilisent l'effet « Spin Transfer Torque (STT) » dans un dispositif de jonction tunnel magnĂ©tique (MTJ) pour produire des oscillations entretenues Ă haute frĂ©quence. Les STNO fournissent des solutions compactes pour la communication sans fil utilisĂ©es dans « wireless sensor network (WSN) » car leur frĂ©quence peut ĂȘtre rĂ©glĂ©e via un courant continu. Ce rĂ©glage de frĂ©quence permet de coder l'information via « Frequency shift keying (FSK) » par modulation numĂ©rique entre deux valeurs discrĂštes sans besoin d'un RF mixer, ce qui conduit Ă des composants RF potentiellement moins complexes. Dans cette thĂšse, la faisabilitĂ© de FSK a Ă©tĂ© Ă©tudiĂ©e pour des STNO MTJ Ă aimantation dans le plan en vue des communications sans fil utilisĂ©es dans les WSN. Les paramĂštres abordĂ©s dans cette Ă©tude sont le dĂ©calage de frĂ©quence et le taux de modulation maximum, auquel la frĂ©quence peut ĂȘtre dĂ©calĂ©e entre deux valeurs discrĂštes.Pour caractĂ©riser le taux de modulation maximum, des simulations macrospin et des Ă©tudes expĂ©rimentales ont Ă©tĂ© rĂ©alisĂ©es. Les simulations rĂ©vĂšlent que le taux de modulation maximum pour FSK par courant est limitĂ© par la frĂ©quence de relaxation du STNO, qui est de l'ordre de quelques centaines de MHz pour les STNO Ă aimantation dans le plan. Cela signifie que le taux de modulation maximum est limitĂ© Ă quelques centaines de Mbps, ce qui est ciblĂ© ici pour une communication sans fil Ă dĂ©bit de donnĂ©es modĂ©rĂ© utilisĂ©es dans les WSN. Des Ă©tudes expĂ©rimentales du FSK par modulation de courant dans les STNO ont Ă©tĂ© effectuĂ©es pour des STNO autonomes et pour des STNO intĂ©grĂ©s dans des systĂšmes hyperfrĂ©quences. Le FSK sur les STNO autonomes montre un dĂ©calage de frĂ©quence autour de 200 MHz (le dĂ©calage de frĂ©quence entre â 8,9 GHz et â9,1 GHz) au taux de modulation de 10Mbps. Ce taux de modulation est infĂ©rieur Ă la limite supĂ©rieure donnĂ©e par la frĂ©quence de relaxation du STNO comme prĂ©vu dans la simulation numĂ©rique en raison du bruit de phase relativement Ă©levĂ© du dispositif mesurĂ©. Afin de tester la faisabilitĂ© du STNO dans les systĂšmes hyperfrĂ©quences, la modulation FSK des STNO a Ă©tĂ© effectuĂ©e sur un Ă©metteur de carte de circuit imprimĂ© (PCB). L'Ă©metteur de PCB a Ă©tĂ© rĂ©alisĂ© et dĂ©veloppĂ© par le partenaire du projet Mosaic FP7, TUD University. L'analyse confirme qu'un changement de frĂ©quence autour de 300 MHz (le dĂ©calage de frĂ©quence entre â9 GHz et â9,3 GHz) a Ă©tĂ© observĂ© avec un taux de modulation de 20 Mbps. Le taux de donnĂ©es est limitĂ© par les caractĂ©ristiques de l'Ă©metteur de PCB et non intrinsĂšque au STNO. Les Ă©tudes de simulation et d'expĂ©rience de la modulation de frĂ©quence des STNO dĂ©montrent que le dĂ©bit de donnĂ©es est adĂ©quat pour la communication sans fil utilisĂ©e dans WSN.Cependant, d'autres amĂ©liorations dans les matĂ©riaux et la nanofabrication de STNO sont nĂ©cessaires pour amĂ©liorer la puissance de sortie et amĂ©liorer les caractĂ©ristiques spectrales des oscillations pour pousser les dĂ©bits de donnĂ©es Ă des valeurs plus Ă©levĂ©es avec un grand dĂ©calage de frĂ©quenc
Frequency modulation of spin torque nano-oscillators (STNOs) for wireless communication applications
Les Oscillateurs Spintronique (STNO) sont un nouveau type d'oscillateurs Ă frĂ©quence radio (RF) qui utilisent l'effet « Spin Transfer Torque (STT) » dans un dispositif de jonction tunnel magnĂ©tique (MTJ) pour produire des oscillations entretenues Ă haute frĂ©quence. Les STNO fournissent des solutions compactes pour la communication sans fil utilisĂ©es dans « wireless sensor network (WSN) » car leur frĂ©quence peut ĂȘtre rĂ©glĂ©e via un courant continu. Ce rĂ©glage de frĂ©quence permet de coder l'information via « Frequency shift keying (FSK) » par modulation numĂ©rique entre deux valeurs discrĂštes sans besoin d'un RF mixer, ce qui conduit Ă des composants RF potentiellement moins complexes. Dans cette thĂšse, la faisabilitĂ© de FSK a Ă©tĂ© Ă©tudiĂ©e pour des STNO MTJ Ă aimantation dans le plan en vue des communications sans fil utilisĂ©es dans les WSN. Les paramĂštres abordĂ©s dans cette Ă©tude sont le dĂ©calage de frĂ©quence et le taux de modulation maximum, auquel la frĂ©quence peut ĂȘtre dĂ©calĂ©e entre deux valeurs discrĂštes.Pour caractĂ©riser le taux de modulation maximum, des simulations macrospin et des Ă©tudes expĂ©rimentales ont Ă©tĂ© rĂ©alisĂ©es. Les simulations rĂ©vĂšlent que le taux de modulation maximum pour FSK par courant est limitĂ© par la frĂ©quence de relaxation du STNO, qui est de l'ordre de quelques centaines de MHz pour les STNO Ă aimantation dans le plan. Cela signifie que le taux de modulation maximum est limitĂ© Ă quelques centaines de Mbps, ce qui est ciblĂ© ici pour une communication sans fil Ă dĂ©bit de donnĂ©es modĂ©rĂ© utilisĂ©es dans les WSN. Des Ă©tudes expĂ©rimentales du FSK par modulation de courant dans les STNO ont Ă©tĂ© effectuĂ©es pour des STNO autonomes et pour des STNO intĂ©grĂ©s dans des systĂšmes hyperfrĂ©quences. Le FSK sur les STNO autonomes montre un dĂ©calage de frĂ©quence autour de 200 MHz (le dĂ©calage de frĂ©quence entre â 8,9 GHz et â9,1 GHz) au taux de modulation de 10Mbps. Ce taux de modulation est infĂ©rieur Ă la limite supĂ©rieure donnĂ©e par la frĂ©quence de relaxation du STNO comme prĂ©vu dans la simulation numĂ©rique en raison du bruit de phase relativement Ă©levĂ© du dispositif mesurĂ©. Afin de tester la faisabilitĂ© du STNO dans les systĂšmes hyperfrĂ©quences, la modulation FSK des STNO a Ă©tĂ© effectuĂ©e sur un Ă©metteur de carte de circuit imprimĂ© (PCB). L'Ă©metteur de PCB a Ă©tĂ© rĂ©alisĂ© et dĂ©veloppĂ© par le partenaire du projet Mosaic FP7, TUD University. L'analyse confirme qu'un changement de frĂ©quence autour de 300 MHz (le dĂ©calage de frĂ©quence entre â9 GHz et â9,3 GHz) a Ă©tĂ© observĂ© avec un taux de modulation de 20 Mbps. Le taux de donnĂ©es est limitĂ© par les caractĂ©ristiques de l'Ă©metteur de PCB et non intrinsĂšque au STNO. Les Ă©tudes de simulation et d'expĂ©rience de la modulation de frĂ©quence des STNO dĂ©montrent que le dĂ©bit de donnĂ©es est adĂ©quat pour la communication sans fil utilisĂ©e dans WSN.Cependant, d'autres amĂ©liorations dans les matĂ©riaux et la nanofabrication de STNO sont nĂ©cessaires pour amĂ©liorer la puissance de sortie et amĂ©liorer les caractĂ©ristiques spectrales des oscillations pour pousser les dĂ©bits de donnĂ©es Ă des valeurs plus Ă©levĂ©es avec un grand dĂ©calage de frĂ©quenceSpin Transfer Nano-Oscillators (STNOs) are a novel type of Radio Frequency (RF) oscillators that make use of the Spin Transfer Torque (STT) effect in a magnetic tunnel junction (MTJ) device to produce high-frequency auto-oscillations. STNOs are attractive for applications in wireless communications due to their nanometric size and their frequency tuning capabilities via either a dc current or an applied field. This frequency tuning permits to encode the information via frequency shift keying (FSK) by digital modulation of the current or applied field between two discrete values without the need of an external RF mixer, leading to potentially less complex RF components. In this thesis, the feasibility of the digital frequency modulation (frequency shift keying (FSK)) using in-plane magnetized MTJ STNOs has been studied. For this, the maximum modulation rate, up to which a signal can be modulated or the frequency can be shifted between two discrete values, is an important aspect that need to be characterized.The characterization of the maximum modulation rate for in-plane magnetized MTJ STNOs has been studied via numerical macrospin simulation for different modulation configurations, i.e. modulation by a sinusoidal RF current and a sinusoidal RF field. It revealed that the maximum modulation rate under RF current modulation is given by the amplitude relaxation frequency fp of the STNO. Under RF field modulation, i.e. an RF field applied parallel to the easy axis, an enhanced modulation rate above fp can be achieved since the frequency is modulated directly via the field and not via the amplitude. This suggests an important strategy for the design of STNO-based wireless communications and to achieve high data rates. Besides numerical simulation, experimental studies of frequency shift keying (FSK) by current modulation in STNOs have been also demonstrated. The first demonstration is the FSK in standalone STNOs. The analysis confirmed that the FSK was successfully observed with a frequency shift around 200MHz (the frequency shift between â8.9 GHz and â9.1 GHz) at the modulation rate of 10Mbps. This modulation rate is however less than the upper limit, which is given by the relaxation frequency fp of the STNO as predicted in the numerical simulation, because of the relatively high phase noise of the device measured. In order to test the feasibility of the STNO within microwave systems, the FSK modulation of STNOs was performed on a printed circuit board (PCB) emitter. FSK with a frequency shift around 300MHz (the frequency shift between â9 GHz and â9.3 GHz) was observed with a modulation rate of 20 Mbps. The data rate here was limited by characteristics of the PCB emitter and not intrinsic to the STNO. The simulation and experiment studies of frequency modulation of STNOs demonstrate that the data rate of is adequate for wireless communication used in WSN. However, further improvements in materials and nanofabrication of STNOs are required to enhance the output power and improve the spectral characteristics of the oscillations to push the data rates to higher values with large frequency shift
Stability of the In-Plane Room Temperature van der Waals Ferromagnet Chromium Ditelluride and Its Conversion to Chromium-Interleaved CrTe<sub>2</sub> Compounds
Van der Waals magnetic materials are building blocks
for novel
kinds of spintronic devices and playgrounds for exploring collective
magnetic phenomena down to the two-dimensional limit. Chromiumâtellurium
compounds are relevant in this perspective. In particular, the 1T phase of CrTe2 has been argued to have a Curie
temperature above 300 K, a rare and desirable property in the class
of lamellar materials, making it a candidate for practical applications.
However, recent literature reveals a strong variability in the reported
properties, including magnetic ones. Using electron microscopy, diffraction,
and spectroscopy techniques, together with local and macroscopic magnetometry
approaches, our work sheds new light on the structural, chemical,
and magnetic properties of bulk 1T-CrTe2 exfoliated in the form of flakes having a thickness ranging from
few to several tens of nanometers. We unambiguously establish that
1T-CrTe2 flakes are ferromagnetic above
room temperature, have an in-plane easy axis of magnetization, and
low coercivity, and we confirm that their Raman spectroscopy signatures
are two modes: E2g (103.5 cmâ1) and A1g (136.5 cmâ1). We also prove that thermal
annealing causes a phase transformation to monoclinic Cr5Te8 and, to a lesser extent, to trigonal Cr5Te8. In sharp contrast with 1T-CrTe2, none of these compounds have a Curie temperature above room
temperature, and they both have perpendicular magnetic anisotropy.
Our findings reconcile the apparently conflicting reports in the literature
and open opportunities for phase-engineered magnetic properties