Publications
442 results found
Mendonça AA, Ghivelder L, Bernardo PL, et al., 2023, Low hysteretic magnetostructural transformation in Cr-doped Ni-Mn-Ga Heusler alloy, Journal of Alloys and Compounds, Vol: 938, Pages: 1-8, ISSN: 0925-8388
Ni-Mn-Ga based alloys are widely studied due to their potential for practical applications, making use of their martensitic transformations. However, hysteresis is a long-standing drawback that reduces the chance of transferring these alloys from the laboratory to industry. In this work, we studied a Cr-doped Ni2.15Mn0.70Cr0.15Ga alloy. We arrived at this composition by integrating data obtained from the previous phase and hysteresis diagrams taken from the literature. The compound presents a magnetostructural transition at room temperature, with a ferromagnetic martensite phase, and moderate thermal hysteresis of approximately 4 K. The magnetocaloric and ferromagnetic shape memory effects were explored, showing a reversible entropy change of approximately 9 Jkg-1K-1 under 0–5 T field change, and a cyclical magnetic-field-induced deformation close to 0.81% under 0–9 T, an advance towards high reversibility for this family alloys.
Cohen L, 2023, Landau theory-based relaxational modelling of first-order magnetic transition dynamics in magnetocaloric materials, Journal of Physics D: Applied Physics, ISSN: 0022-3727
Balandin AA, Iwamoto S, Loi MA, et al., 2023, Last 60th salute to the journal, Applied Physics Letters, Vol: 122, ISSN: 0003-6951
Johnson F, Zázvorka J, Beran L, et al., 2023, Room temperature weak collinear ferrimagnet with symmetry driven, large intrinsic magneto-optic signatures, Physical Review B: Condensed Matter and Materials Physics, Vol: 107, ISSN: 1098-0121
Here we present a magnetic thin film with a weak ferrimagnetic (FIM) phase above the Néel temperature (TN=240K) and a noncollinear antiferromagnetic (AFM) phase below, exhibiting a small net magnetization due to strain-associated canting of the magnetic moments. A long-range ordered FIM phase has been predicted in related materials, but without symmetry analysis. We now perform this analysis and use it to calculate the magneto-optical Kerr effect (MOKE) spectra in the AFM and FIM phases. From the good agreement between the form of the measured and predicted MOKE spectra, we propose the AFM and FIM phases share the magnetic space group C2′/m′ and that the symmetry-driven magneto-optic and magneto-transport properties are maximized at room temperature in the FIM phase due to the nonzero intrinsic Berry phase contribution present in these materials. A room temperature FIM with large optical and transport signatures, as well as sensitivity to lattice strain and magnetic field, has useful prospects for high-speed spintronic applications.
Ferreira-Teixeira S, Vanstone A, Pires AL, et al., 2022, Electronic conduction channels engineered in topological insulator sputtered thin films, Physical Review B: Condensed Matter and Materials Physics, Vol: 4, Pages: 5789-5798, ISSN: 1098-0121
Sb2Te3 is a topological insulator (TI) material that can be used in a wide range of applications from energy harvesting to Spin-Orbitronics. In this paper, the structural, electrical, and thermal transport properties of nanocrystalline ion beam sputtered Sb2Te3 thin films were studied. Films with thicknesses between 35 and 300 nm, with nanocrystallites of sizes 10–20 nm, have a high resistivity between 0.072 and 2.03 Ω cm at 300 K, increasing with cooling. The Seebeck coefficient demonstrates the coexistence of n-type and p-type conduction, the latter being more prominent at high temperatures and in thicker films. The morphological and transport properties reveal that the films are constituted by two layers having different majority charge carriers, with the electronic bulk conduction being described by two semiconductor layers conducting in parallel, one p-type at the surface and another n-type, each described by an activation energy-dependent conductivity. Besides these bulk contributions, weak antilocalization (WAL) cusps are observed in the magnetoconductance below 10 K and at low magnetic fields. Analysis of the WAL hints that there is one two-dimensional conduction channel open at low temperatures for the thinner films, whereas for the thicker film, this 2D conduction appears to be masked by the bulk channels. However, a magnetic localization length LΦ between 62 and 90 nm at 2 K is observed for all thin films. This behavior suggests that as the bulk activation energy conductions freeze out at low temperatures, the electrical conduction is carried by the supposed 2D state, which appears to have some of the features of a TI surface state. Through these measurements, we demonstrate that the type of dominant conduction can be controlled by the Sb2Te3 film thickness in these large area sputtered films, while the conduction at low temperatures appears to be dominated by a robust TI state.
Borri P, Diez LH, Hu Q, et al., 2022, We are 60!, APPLIED PHYSICS LETTERS, Vol: 121, ISSN: 0003-6951
Wilkinson LA, Bennett TLR, Grace IM, et al., 2022, Assembly, structure and thermoelectric properties of 1,1 '-dialkynylferrocene 'hinges', CHEMICAL SCIENCE, ISSN: 2041-6520
Johnson F, Kimak J, Zemen J, et al., 2022, Identifying the octupole antiferromagnetic domain orientation in Mn3NiN by scanning anomalous Nernst effect microscopy, APPLIED PHYSICS LETTERS, Vol: 120, ISSN: 0003-6951
Cohen L, Johnson F, 2022, Antiferromagnetic domain orientation in Mn3NiN by scanning Anomalous Nernst Effect microscopy, Applied Physics Letters, ISSN: 0003-6951
The intrinsic anomalous Nernst effect in a magnetic material is governed by the Berry curvature at the Fermi energy and can be realized in non-collinear antiferromagnets with vanishing magnetization. Thin films of (001)-oriented Mn3NiN have their chiral antiferromagnetic structure located in the (111) plane facilitating the anomalous Nernst effect unusually in two orthogonal in-plane directions. The sign of each component of the anomalous Nernst effect is determined by the local antiferromagnetic domain state. In this work, a temperature gradient is induced in a 50 nm thick Mn3NiN two micron-size Hall cross by a focused scanning laser beam, and the spatial distribution of the anomalous Nernst voltage is used to image and identify the octupole macrodomain arrangement. Although the focused laser beam width may span many individual domains, cooling from room temperature through the antiferromagnetic transition temperature in an in-plane magnetic field prepares the domain state producing a checkerboard pattern resulting from the convolution of contributions from each domain. These images together with atomistic and micromagnetic simulations suggest an average macrodomain of the order of 1 μm2.
Bennett TLR, Alshammari M, Au-Yong S, et al., 2022, Multi-component self-assembled molecular-electronic films: towards new high-performance thermoelectric systems, CHEMICAL SCIENCE, Vol: 13, Pages: 5176-5185, ISSN: 2041-6520
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Bader SD, Lake RK, Cohen LF, 2022, Applied Physics Letters 2022 60th Anniversary Editorial, Applied Physics Letters, Vol: 120, Pages: 110401-110401, ISSN: 0003-6951
Bower R, Wells MP, Johnson F, et al., 2021, Tunable double epsilon-near-zero behavior in niobium oxynitride thin films, Applied Surface Science, Vol: 569, Pages: 150912-150912, ISSN: 0169-4332
Johnson F, Boldrin D, Zemen J, et al., 2021, Strain dependence of Berry-phase-induced anomalous Hall effect in the non-collinear antiferro-magnet Mn3NiN, Applied Physics Letters, Vol: 119, ISSN: 0003-6951
The anomalous Hall effect (AHE) has been shown to be present in certain non-collinear antiferromagnets due to their symmetry-breaking magnetic structure, and its magnitude is dependent primarily on the non-zero components of the Berry curvature. In the non-collinear antiferromagnet Mn3NiN, the Berry phase contribution has been predicted to have strong strain dependence, although in practice, direct observation may be obscured by other strain-related influences—for instance, magnetic phase transitions mediated by strain. To unravel the various contributions, we examine the thickness and temperature dependence of the AHE for films grown on the piezoelectric substrate BaTiO3. We observe a systematic reduction in TN due to increased compressive strain as film thickness is reduced and a linear decrease in the AHE magnitude as the films are cooled from their ferrimagnetic phase above TN to their antiferromagnetic phase below. At 190 K, we applied an electric field across a 0.5 mm thick BaTiO3 substrate with a 50 nm thick Mn3NiN film grown on top and we demonstrate that at the coercive field of the piezoelectric substrate, the tensile in-plane strain is estimated to be of the order of 0.15%, producing a 20% change in AHE. Furthermore, we show that this change is, indeed, dominated by the intrinsic strain dependence of the Berry curvature.
Boldrin D, Cohen L, Mendive-Tapia E, et al., 2021, Barocaloric properties of quaternary Mn3(Zn,In)N for room temperature refrigeration applications, Physical Review B: Condensed Matter and Materials Physics, Vol: 104, Pages: 1-7, ISSN: 1098-0121
The magnetically frustrated manganese nitride antiperovskite family display significant changesof entropy under hydrostatic pressure that can be useful for the emerging field of barocaloric cool-ing. Here we show that barocaloric properties of metallic antiperovskite Mn nitrides can be tai-lored for room temperature application through quaternary alloying.We find an enhanced en-tropy change of ∆St= 37 J K−1kg−1at theTt= 300 K antiferromagnetic transition of quater-nary Mn3Zn0.5In0.5N relative to the ternary end-members.The pressure-driven barocaloric entropychange of Mn3Zn0.5In0.5N reaches ∆SBCE= 20 J K−1kg−1in 2.9 kbar.Our results open up a largephase space where new compounds with improved barocaloric properties may be found.
Wang X, Ismael A, Almutlg A, et al., 2021, Optimised power harvesting by controlling the pressure applied to molecular junctions, Chemical Science, Vol: 12, Pages: 5230-5235, ISSN: 2041-6520
A major potential advantage of creating thermoelectric devices using self-assembled molecular layers is their mechanical flexibility. Previous reports have discussed the advantage of this flexibility from the perspective of facile skin attachment and the ability to avoid mechanical deformation. In this work, we demonstrate that the thermoelectric properties of such molecular devices can be controlled by taking advantage of their mechanical flexibility. The thermoelectric properties of self-assembled monolayers (SAMs) fabricated from thiol terminated molecules were measured with a modified AFM system, and the conformation of the SAMs was controlled by regulating the loading force between the organic thin film and the probe, which changes the tilt angle at the metal-molecule interface. We tracked the thermopower shift vs. the tilt angle of the SAM and showed that changes in both the electrical conductivity and Seebeck coefficient combine to optimize the power factor at a specific angle. This optimization of thermoelectric performance via applied pressure is confirmed through the use of theoretical calculations and is expected to be a general method for optimising the power factor of SAMs.
Zverev VI, Gimaev RR, Miyanaga T, et al., 2021, Peculiarities of the phase transformation dynamics in bulk FeRh based alloys from magnetic and structural measurements, Journal of Magnetism and Magnetic Materials, Vol: 522, Pages: 1-10, ISSN: 0304-8853
We analyze coexistence of antiferromagnetic and ferromagnetic phases in bulk iron-rhodium and its alloys with palladium, Fe50,4Rh49,6, Fe49,7Rh47,4Pd2,9 and Fe48,3Rh46,8Pd4,9, using neutron diffraction, magnetization and scanning Hall probe imaging. Temperature dependencies of the lattice parameters, AFM and FM phase weight fractions, and Fe magnetic moment values were obtained on cooling and heating across the AFM-FM transition. Substantial thermomagnetic hysteresis for the phases’ weight fractions and a relatively narrow one for the unit cell volume has been observed on cooling-heating. A clear dependence of hysteretic behavior on Pd concentration has been traced. Additional direct magnetic measurements of the spatial distribution of the phase transition are acquired using scanning Hall probe microscopy, which reveals the length scale of the phase coexistence and the spatial progression of the transition in the presence of external magnetic field. Also, the magnetic phase diagram has been constructed for a series of Pd-doped FeRh alloys.
Komori S, Devine-Stoneman JM, Ohnishi K, et al., 2021, Spin-orbit coupling suppression and singlet-state blocking of spin-triplet Cooper pairs, Science Advances, Vol: 7, Pages: 1-7, ISSN: 2375-2548
An inhomogeneous magnetic exchange field at a superconductor/ferromagnet interface converts spin-singlet Cooper pairs to a spin-aligned (i.e. spin-polarized) triplet state. Although the decay envelope of such triplet pairs within ferromagnetic materials is well studied, little is known about their decay in non-magnetic metals and superconductors, and in particular in presence of spin-orbit coupling. Here we report devices in which triplet supercurrents are created and are injected into the s-wave superconductor Nb. In the normal state of Nb, triplet pairs decay over a distance of 5 nm, which is an order of magnitude smaller than the decay of zero spin singlet pairs due to the spin-orbit coupling interacting with the spin associated with a triplet supercurrent. In the superconducting state of Nb, triplet supercurrents are blocked by the lack of available equilibrium states in the singlet superconducting gap. The results offer new insight into the dynamics between s-wave singlet and triplet states.
Mendonca AA, Ghivelder L, Bernardo PL, et al., 2020, Experimentally correlating thermal hysteresis and phase compatibility in multifunctional Heusler alloys, Physical Review Materials, Vol: 4, ISSN: 2475-9953
Thermal hysteresis is recognized as one of the main drawbacks for cyclical applications of magnetocaloric and ferromagnetic shape memory materials with first-order transformations. As such, the challenge is to develop strategies that improve the compatibility between the phases involved in the transitions and study its influence on thermal hysteresis. With this purpose, we explore the thermal, structural, and magnetic properties of the Ni2Mn1−xCuxGa0.84Al0.16 Heusler alloys. The alloys present a thermal hysteresis reduction of ∼60% when the Cu content in the compound varies from x=0.10 to x=0.25, with a minimum hysteresis width of 6 K being achieved. We applied the geometric nonlinear theory of martensite to address the phase compatibility, quantified by the parameter λ2, the middle eigenvalue of the transformation stretch tensor, and found that the minimum of hysteresis is associated with a better crystallographic compatibility (λ2 closer to 1) between the austenite and martensite phases. In addition, we show that the valleylike properties of hysteresis found in the Ni2Mn1−xCuxGa0.84Al0.16 compounds is present in several other alloys in the literature. These results provide pathways to understand as well as to master the phase compatibility and ultimately achieve a low thermal hysteresis in multifunctional Heusler alloys.
Cohen LF, 2020, To boldly go: new frontiers for APL, Applied Physics Letters, Vol: 117, ISSN: 0003-6951
Jeon K-R, Montiel X, Komori S, et al., 2020, Tunable pure spin supercurrents and the demonstration 3 of their gateability in a spin-wave device, Physical Review X, Vol: 10, ISSN: 2160-3308
Recent ferromagnetic resonance experiments and theory of Pt/Nb/Ni8Fe2 proximity-coupled structures strongly suggest that spin-orbit coupling (SOC) in Pt in conjunction with a magnetic exchange field in Ni8Fe2 are the essential ingredients to generate a pure spin supercurrent channel in Nb. Here, by substituting Pt for a perpendicularly magnetized Pt/Co/Pt spin-sink, we are able to demonstrate the role of SOC, and show that pure spin supercurrent pumping efficiency across Nb is tunable by controlling the magnetization direction of Co. By inserting a Cu spacer with weak SOC between Nb and Pt/(Co/Pt) spin-sink, we also prove that Rashba-type SOC is key for forming and transmitting pure spin supercurrents across Nb. Finally, by engineering these properties within a single multilayer structure, we demonstrate a prototype superconducting spin-wave (SW) device in which lateral SW propagation is gateable via the opening or closing of a vertical pure spin supercurrent channel in Nb.
Cohen L, 2020, Tuning the thermoelectrical properties of anthracene-based self-assembled monolayers, Chemical Science, Vol: 11, Pages: 6836-6841, ISSN: 2041-6520
It is known that the electrical conductance of single molecules can be controlled in a deterministic manner by chemically varying their anchor groups to external electrodes. Here, by employing synthetic methodologies to vary the terminal anchor groups around aromatic anthracene cores, and by forming self-assembled monolayers (SAMs) of the resulting molecules, we demonstrate that this method of control can be translated into cross-plane SAM-on-gold molecular films. The cross-plane conductance of SAMs formed from anthracene-based molecules with four different combinations of anchors are measured to differ by a factor of approximately 3 in agreement with theoretical predictions. We also demonstrate that the Seebeck coefficient of such films can be boosted by more than an order of magnitude by an appropriate choice of anchor groups and that both positive and negative Seebeck coefficients can be realised. This demonstration that the thermoelectric properties of SAMs are controlled by their anchor groups represents a critical step towards functional ultra-thin-film devices for future molecular-scale electronics.
Wang X, Bennett TLR, Ismael A, et al., 2020, Scale-up of room-temperature constructive quantum interference from single molecules to self-assembled molecular-electronic films, Journal of the American Chemical Society, Vol: 142, Pages: 8555-8560, ISSN: 0002-7863
The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up of QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on CQI effects within the core. We also demonstrate that for molecules with thioether anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ∼50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step toward functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.
Doiron B, Gusken NA, Lauri A, et al., 2020, Hot Carrier Optoelectronics with Titanium Nitride, Lasers and Electro-Optics Society Annual Meeting-LEOS, ISSN: 1092-8081
© 2020 OSA. Titanium oxynitride enables a range of plasmonic and optoelectronic functionality using long-lived photo-generated hot carriers. We explore the time scale of hot carriers in TiN and their use in photochemical reduction and Schottky detectors.
Guo L, Lovell E, Tang Q, et al., 2020, Fine control of Curie temperature of magnetocaloric alloys La(Fe,Co,Si)(13) using electrolytic hydriding, Scripta Materialia, Vol: 175, Pages: 33-37, ISSN: 1359-6462
This work demonstrates precision control of hydrogen content in La(Fe,Co,Si)13Hδ for the development of environmentally friendly magnetocaloric-based cooling technologies, using an electrolytic hydriding technique. We show the Curie temperature, a critical parameter which directly governs the temperature window of effective cooling, can be varied easily and reproducibly in 1 K steps within the range 274 K to 402 K. Importantly, both partially (up to 10%) and fully hydrided compositions retain favorable entropy change values comparable to that of the base composition. Crucially, we show in these second-order phase transition compounds, partial hydriding is stable and not susceptible against phase separation.
Cohen LF, 2020, Applied Physics Letters welcomes papers in quantum technologies, Applied Physics Letters, Vol: 116, ISSN: 0003-6951
Doiron B, Güsken NA, Lauri A, et al., 2020, Hot carrier optoelectronics with titanium nitride
Titanium oxynitride enables a range of plasmonic and optoelectronic functionality using long-lived photo-generated hot carriers. We explore the time scale of hot carriers in TiN and their use in photochemical reduction and Schottky detectors.
Gusken NA, Lauri A, Li Y, et al., 2020, IR hot carrier based photodetection in titanium nitride oxide thin film-Si junctions, MRS Advances, Vol: 5, Pages: 1843-1850, ISSN: 2059-8521
Hot carrier based methods constitute a valuable approach for efficient and silicon compatible sub-bandgap photodetection. Although, hot electron excitation and transfer have been studied extensively on traditional materials such as Au and Ti, reports on alternative materials such as titanium nitride (TiN) are rare. Here, we perform hot hole photodetection measurements on a p-Si/metal thin film junction using Ti, Au and TiN. This material is of interest as it constitutes a refractory alternative to Au which is an important property for plasmonic applications where high field intensities can occur. In contrast to Au, a TiN/Si junction does not suffer from metal diffusion into the Si, which eases the integration with current Si-fabrication techniques. This work shows that a backside illuminated p-Si/TiN system can be used for efficient hot hole extraction in the IR, allowing for a responsivity of 1 mA/W at an excitation wavelength of 1250 nm and at zero bias. Via a comparison between TiN and other commonly used materials such as Au, the origin of this comparably high photoresponse can be traced back to be directly linked to a thin TiO2-x interfacial layer allowing for a distinct hot-hole transfer mechanism. Moreover, the fabrication of TiN nanodisk arrays is demonstrated which bears great promise to further boost the device efficiency.
Cohen L, Boldrin D, Johnson F, et al., 2019, The biaxial strain dependence of magnetic order in spin frustrated mn3nin thin films, Advanced Functional Materials, Vol: 29, ISSN: 1616-301X
Multi-component magnetic phase diagrams are a key property of functional materials for a variety of uses, such as manipulation of magnetisation for energy efficient memory, data storage and cooling applications. Strong spin-lattice coupling extends this functionality further by allowing electric-field-control of magnetisation via strain coupling with a piezoelectric . Here we explore the magnetic phase diagram of piezomagnetic Mn3NiN thin films, with a frustrated non-collinear antiferromagnetic (AFM) structure, as a function of the growth induced biaxial strain. Under compressive strain the films support a canted AFM state with large coercivity of the transverse anomalous Hall resistivity, ρxy, at low temperature, that transforms at a well-defined Néel transition temperature (TN) into a soft ferrimagnetic-like (FIM) state at high temperatures. In stark contrast, under tensile strain the low temperature canted AFM phase transitions to a state where ρxy is an order of magnitude smaller and therefore consistent with a low magnetisation phase. Neutron scattering confirms that the high temperature FIM-like phase of compressively strained films is magnetically ordered and the transition at TN is 1st-order. Our results open the field towards future exploration of electric-field driven piezospintronic and thin film caloric cooling applications in both Mn3NiN itself and the broader Mn3AN family.
Cohen L, Boldrin D, 2019, Anomalous Hall effect in noncollinear antiferromagnetic Mn 3NiN thin films, Physical Review Materials, Vol: 3, ISSN: 2475-9953
We have studied the anomalous Hall effect (AHE) in strained thin lms of the frustrated anti-ferromagnet Mn3NiN. The AHE does not follow the conventional relationships with magnetizationor longitudinal conductivity and is enhanced relative to that expected from the magnetization inthe antiferromagnetic state belowTN= 260 K. This enhancement is consistent with origins fromthe non-collinear antiferromagnetic structure, as the latter is closely related to that found in Mn3Irand Mn3Pt where a large AHE is induced by the Berry curvature. As the Berry phase inducedAHE should scale with spin-orbit coupling, yet larger AHE may be found in other members of thechemically exible Mn3AN structure.
Dion T, Arroo DM, Yamanoi K, et al., 2019, Tunable magnetization dynamics in artificial spin ice via shape anisotropy modification, Physical Review B, Vol: 100, ISSN: 2469-9950
Ferromagnetic resonance (FMR) is performed on kagome artificial spin ice (ASI) formed of disconnected Ni80 Fe20 nanowires. Here we break the threefold angular symmetry of the kagome lattice by altering the coercive field of each sublattice via shape anisotropy modification. This allows for distinct high-frequency responses when a magnetic field is aligned along each sublattice and additionally enables simultaneous spin-wave resonances to be excited in all nanowire sublattices, unachievable in conventional kagome ASI. The different coercive field of each sublattice allows selective magnetic switching via global field, unlocking novel microstates inaccessible in homogeneous-nanowire ASI. The distinct spin-wave spectra of these states are detected experimentally via FMR and linked to underlying microstates using micromagnetic simulation.
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