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This work bridges two gaps in the magnetic separation of rare-earth ions. 1) A material property database is provided for the solutal expansion coefficient and the magnetic susceptibility of eleven out of seventeen trivalent rare-earths. 2) A novel protocol is developed to enhance and resolve the magnetic term of the Kelvin force. For that purpose, an assembly of partition magnets is created where the individual magnets function in the first quadrant of their magnetic hysteresis loop. The mutual reinforcement is quantified in a particle magnetic levitation system. Thus, compared to exisiting magnetic assemblies, an enhancement in $\frac{\partial B^2}{2 \mu_0 \partial z}$ as high as 2 orders of magnitudes is realized that covers 90\% of the normalized spatial scale and requires 1 order of magnitude less magnet mass. Modeling the energy density field makes it possible to quantify the equilibrium position of the particle cloud at rest, which is attained via magnetophoresis of the particles regardless of their initial position. This enables the magnetic trapping and manipulation of particles with small hydrodynamic diameters. Optically tracking the transient magnetophoresis enables a high-fidelity, sub-mm resolution of $\frac{\partial \bm{B}^2}{2 \mu_0 \partial z}$ which is further used to quantify the magnetic susceptibility of Ho(III), Tb(III), Er(III) and Gd(III).

For a magnetic material, the easy and hard magnetic axes describe the directions of favourable respectively unfavourable alignment of the magnetisation. In this article, we describe how to determine these axes for cubic magnetic crystals. Usually it is assumed without further reasoning that they coincide with some principal symmetry directions of the crystal [Bozorth, Phys. Rev. 50, 1076–1081 (1936)], which is however invalid in general. In contrast, we present a full and elementary analysis using symmetric polynomial inequalities, which are well suited to the symmetry of the problem.

Spin-based technologies can operate at terahertz frequencies but require manipulation techniques that work at ultrafast timescales to become practical. For instance, devices based on spin waves, also known as magnons, require efficient generation of high-energy exchange spin waves at nanometre wavelengths. To achieve this, a substantial coupling is needed between the magnon modes and an electro-magnetic stimulus such as a coherent terahertz field pulse. However, it has been difficult to excite non-uniform spin waves efficiently using terahertz light because of the large momentum mismatch between the submillimetre-wave radiation and the nanometre-sized spin waves. Here we improve the light–matter interaction by engineering thin films to exploit relativistic spin–orbit torques that are confined to the interfaces of heavy metal/ferromagnet heterostructures. We are able to excite spin-wave modes with frequencies of up to 0.6 THz and wavelengths as short as 6 nm using broadband terahertz radiation. Numerical simulations demonstrate that the coupling of terahertz light to exchange-dominated magnons originates solely from interfacial spin–orbit torques. Our results are of general applicability to other magnetic multilayered structures, and offer the prospect of nanoscale control of high-frequency signals.

Nonvolatile and energy-efficient spin-based technologies call for new prospects for realization of logic devices able to operate at terahertz (THz) frequencies. For instance, magnonic elements require an efficient generation of high-energy exchange spin waves at nanometer wavelengths. For this to happen, a substantial coupling of the electro-magnetic stimulus, such as coherent THz field pulses, to the magnon modes is needed. However, the efficient excitation of non-uniform spin waves using THz light is elusive, due to the large momentum mismatch between the millimeter-wave radiation and the exchange-dominated, nanometer-sized, spin waves. Here, we overcome this fundamental problem of light-matter interaction via thin film engineering, exploiting relativistic spin-orbit torques confined to the interfaces of heavy-metal/ferromagnet heterostructures. Using single-cycle broadband THz radiation, we are able to excite spin-wave modes with a frequency of up to 0.6 THz and a wavelength as short as 6 nm. Numerical simulations demonstrate that the coupling of THz light with the exchange-dominated magnons originates solely from interfacial spin-orbit torques. Our results are of general applicability to other magnetic multilayered structures, and offer the prospective of nanoscale control of high-frequency signals.

Despite the numerous reports over the last two decades dedicated to the study of interfacial thermal transport, physics of thermal transport across nanoscale metallic multilayers is less explored. ...

Ultrafast demagnetization in diverse materials has sparked immense research activities due to its captivating richness and contested underlying mechanisms. Among these, the two most celebrated mechanisms have been the spin-flip scattering (SFS) and spin transport (ST) of optically excited carriers. In this work, we have investigated femtosecond laser-induced ultrafast demagnetization in perpendicular magnetic anisotropy-based synthetic antiferromagnets (p-SAFs) where [Co/Pt]n−1/Co multilayer blocks are separated by Ru or Ir spacers. Our investigation conclusively shows that the ST of optically excited carriers can have a significant contribution to the ultrafast demagnetization in addition to SFS processes. Moreover, we have also achieved an active control over the individual mechanisms by specially designing the SAF samples and altering the external magnetic field and excitation fluence. Our study provides a vital understanding of the underlying mechanism of ultrafast demagnetization in synthetic antiferromagnets, which will be crucial in future research and applications of antiferromagnetic spintronics.

The reversal of magnetic bubble helicity through topologically trivial transient states provides an additional degree of freedom that promises the development of multidimensional magnetic memories. A key requirement for this concept is the stabilization of bubble states at ambient conditions on application-compatible substrates. In the present work we demonstrate a stabilization routine for remanent bubble states in high perpendicular magnetic anisotropy [(Co(0.44\,nm)/Pt(0.7\,nm)]$_X$, X = 48, 100, 150 multilayers on Si/SiO$_2$ substrates by exploring the effect of external magnetic fields ($H_\mathrm{m}$) of different strength and angles ($\theta$) with respect to the film surface normal. By systematic variation of these two parameters, we demonstrate that remanent bubble density and mean bubble diameter can be carefully tuned and optimized for each sample. Our protocol based on magnetometry only reveals the densest remanent bubble states at $H_\mathrm{m} = 0.87 H_\mathrm{s}$ ($H_\mathrm{s}$ is the magnetic saturation field) and $\theta=60^\circ - 75^\circ$ for all $X$ with a maximum of 3736 domains/100 $\mu$m$^2$ for the X = 48 sample. The experimental observations are supported by micromagnetic simulations taking into account the nanoscale lateral grain structure of multilayers synthesized by magnetron sputter deposition, and thus helping understand the different density of the bubble states found in these systems., Comment: Regular Article: 10 pages, 6 figures, 1 table

The adsorption of chiral molecules was recently shown to trigger a change in the magnetisation of mesoscopic magnetic domains in a ferromagnetic underlayer. In this work, we investigated the macroscopic (magneto-)optical response of chemisorbed α-helical polyalanine self-assembled monolayers (SAMs) on a gold and gold-capped-cobalt thin film on Au substrates using spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy and microscopy. The optical and magneto-optical spectra reveal selective chemisorption of the α-helical polyalanine molecules depending on the orientation of the substrate remanent magnetisation during the SAMs process. Moreover, a sign change of the magneto-optical response was observed in some of the magnetic substrates after the chiral SAMs formation.

Polyalanine molecules (PA) with an α-helix conformation have recently attracted a great deal of interest, as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy under ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al2O3/Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the PA molecules was observed. The highest spin polarization values, P, were found for well-ordered self-assembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current-induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the chiral molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap results in a higher electron mobility, yielding a higher P value. By switching the magnetization direction of the Co layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a submolecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change in the magnetoresistance of such hybrid structures.

We present a frequency-domain study of the dynamic behavior of a magnetic vortex core within a single Permalloy disk by means of electrical detection and micromagnetic simulations. When exciting the vortex core dynamics in a non-linear regime, the lineshape of the rectified dc signal reveals a resonance peak splitting which depends on the excitation amplitude. Using micromagnetic simulations, we show that at high excitation power the peak splitting originates from the nanosecond time scale quasi-periodic switching of the vortex core polarity. Using lock-in detection, the rectified voltage is integrated over a ms time scale, so that the net signal detected between the two resonant peaks for a given range of parameters cancels out. The results are in agreement with the reported effects of the in-plane static field magnitude on the gyration dynamics, and complement them by detailed analysis of the effects of the rf current amplitude and the azimuthal angle of the in-plane bias magnetic field. Systematic characterization shows that a transition from linear to nonlinear dynamical regime can be controlled by rf current as well as by varying the magnitude and the direction of the bias magnetic field., 9 pages, 6 figures, 24 references

Ever since its discovery ultrafast demagnetization has remained one of the most intriguing research areas in magnetism. Here, we demonstrate that in [Co (tCo )/ Pd (0.9 nm)] 8 multilayers, the characteristic decay time in femtosecond time- scale varies non-monotonically with tCo in the range 0.07 nm B tCo B 0.75 nm. Further investigation reveals higher spin fluctuation at higher ratio of electron to Curie temperature to be responsible for this. Microscopic three-temperature modelling unravels a similar trend in the spin–lattice interaction strength, which strongly supports our experimental observation. The knowledge of the fem- tosecond magnetization decay mechanism in ultrathin ferromagnetic films is unique and important for the advancement of fundamental magnetism besides their potential applications in ultrahigh speed spintronic devices.

We study the charge and spin dependent scattering in a set of CoFeB thin films whose crystalline order is systematically enhanced and controlled by annealing at increasingly higher temperatures. Terahertz conductivity measurements reveal that charge transport closely follows the development of the crystalline phase, with increasing structural order leading to higher conductivity. The terahertz-induced ultrafast demagnetization, driven by spin-flip scattering mediated by the spin-orbit interaction, is measurable in the pristine amorphous sample and much reduced in the sample with highest crystalline order. Surprisingly, the largest demagnetization is observed at intermediate annealing temperatures, where the enhancement in spin-flip probability is not associated with an increased charge scattering. We are able to correlate the demagnetization amplitude with the magnitude of the in-plane magnetic anisotropy, which we characterize independently, suggesting a magnetoresistance-like description of the phenomenon., 5 pages, 4 figures

Not only since the progressive reduction of structure sizes in modern micro- and nanotechnology, surface and interface effects have played an ever-increasing role and nowadays often dominate the behavior of entire systems. Therefore, understanding the nature of surface and interface effects and being able to fully control them is of fundamental importance, in particular in modern thin-film technology. In this study, it is revealed how Co/Pt multilayer-based synthetic antiferromagnets (SAFs) with perpendicular magnetic anisotropy in the regime of dominating antiferromagnetic interlayer exchange can be employed to control the collective magnetic reversal via systematically altering surface and interface effects. The specifically designed samples and experiments highlight the superior tunability of synthetic systems as compared to their intrinsic stoichiometric counterparts, where the antiferromagnetism is directly tied to the indivisible discrete atomic nature and crystal structure of the materials. Thus, it is demonstrated that in SAFs, it becomes possible to energetically heal the broken magnetic symmetry at the surface, thereby enabling either on demand suppression or controlled enhancement of respective surface and interface effects, as demonstrated here in this study for the surface spin-flop and the exchange bias effect.

We systematically analyze the influence of 5 nm thick metal interlayers inserted at the interface of several sets of different metal-dielectric systems to determine the parameters that most influence interface transport. Our results show that despite the similar Debye temperatures of Al2O3 and AlN substrates, the thermal boundary conductance measured for the Au/Al2O3 system with Ni and Cr interlayers is ∼2× and >3× higher than the corresponding Au/AlN system, respectively. We also show that for crystalline SiO2 (quartz) and Al2O3 substrates having highly dissimilar Debye temperature, the measured thermal boundary conductance between Al/Al2O3 and Al/SiO2 are similar in the presence of Ni and Cr interlayers. We suggest that comparing the maximum phonon frequency of the acoustic branches is a better parameter than the Debye temperature to predict the change in the thermal boundary conductance. We show that the electron–phonon coupling of the metallic interlayers also alters the heat transport pathways in a metal-dielectric system in a nontrivial way. Typically, interlayers with large electron–phonon coupling strength can increase the thermal boundary conductance by dragging electrons and phonons into equilibrium quickly. However, our results show that a Ta interlayer, having a high electron–phonon coupling, shows a low thermal boundary conductance due to the poor phonon frequency overlap with the top Al layer. Our experimental work can be interpreted in the context of diffuse mismatch theory and can guide the selection of materials for thermal interface engineering.

The ferromagnetic resonance of a disordered A2 Fe60Al40 ferromagnetic stripe, of dimensions 5 µm × 1 µm × 32 nm, has been observed in two vastly differing surroundings: in the first case, the ferromagnetic region was surrounded by ordered B2 Fe60Al40, and in the second case it was free standing, adhering only to the oxide substrate. The embedded ferromagnet possesses a periodic magnetic domain structure, which transforms to a single domain structure in the freestanding case. The two cases differ in their dynamic response, for instance, the resonance field for the uniform (k = 0) mode at ~ 14 GHz excitation displays a shift from 209 to 194 mT, respectively for the embedded and freestanding cases, with the external magnetic field applied along the long axis. The resonant behavior of a microscopic ferromagnet can thus be finely tailored via control of its near-interfacial surrounding.

The surface spin flop, observed in synthetic antiferromagnets (SAFs) with uniaxial anisotropy and strong antiferromagnetic (AF) interlayer exchange coupling, can be considered as a laterally homogeneous, vertical AF domain wall pushed into the SAF from either the top or the bottom in the presence of a strong external vertical magnetic field. As a result, the AF domain wall can be described as a one-dimensional entity. In this work, we present a concept to stabilize laterally homogeneous vertical AF domain walls by local variation of the perpendicular magnetic anisotropy in $\mathrm{Co}/\mathrm{Pt}$-based SAFs. Our approach not only allows the stabilization of the vertical AF domain wall in the absence of any external magnetic field, but furthermore enables a deterministic selection among four different remanent states, each one stable within a broad external magnetic field range of almost one tesla. We also demonstrate an extension to our concept by stabilizing two coexisting vertical AF domain walls, thus yielding a system with a total of six different selectable (and reprogrammable) remanent states. The controlled stabilization of noncollinear AF textures in the form of vertical AF domain walls at remanence could be used as an infrastructure for propagating spin waves within the AF domain wall itself, as well as for tuning the dynamic behavior of perpendicular standing spin wave modes existing vertically across the SAF.

In this work, we investigate magneto-acoustic attenuation in thin film multiferroic Lamb wave delay lines. By leveraging magneto-acoustic interactions, multiferroics have potential to realize passive chip-scale alternatives to bulky ferrite devices. For the first time, magnetic field dependence of magneto-acoustic interactions in multiferroic Lamb wave devices is characterized. Multiferroic heterostructures of aluminum nitride and cobalt iron boron are fabricated into Lamb wave delay lines operating at 7.492 GHz to study the effect of strain nonuniformity on the multiferroic coupling. The attenuation of the Lamb waves is characterized as a function of the magnitude and angle of an applied in-plane bias magnetic field. It is found that the bias magnitude for peak attenuation is a strong function of angle, indicating that it is due to coupling between the Lamb waves and spin wave modes. This is in contrast with current models of attenuation in multiferroic SAW delay lines, where the uniform surface strains couple to ferromagnetic resonance, which has no angular dependence on the in-plane bias field magnitude. These results are the first steps towards passive chip-scale alternatives to ferrite devices utilizing Lamb wave devices, which have better coupling and scalability than their SAW counterparts. [2020-0114]

Antiferromagnets (AFMs) with zero net magnetization are proposed as active elements in future spintronic devices. Depending on the critical thickness of the AFM thin films and the measurement temperature, bimetallic Mn-based alloys and transition metal oxide-based AFMs can host various coexisting ordered, disordered, and frustrated AFM phases. Such coexisting phases in the exchange coupled ferromagnetic (FM)/AFM-based heterostructures can result in unusual magnetic and magnetotransport phenomena. Here, we integrate chemically disordered AFM IrMn3 thin films with coexisting AFM phases into complex exchange coupled MgO(001)/Ni3Fe/IrMn3/Ni3Fe/CoO heterostructures and study the structural, magnetic, and magnetotransport properties in various magnetic field cooling states. In particular, we unveil the impact of rotating the relative orientation of the disordered and reversible AFM moments with respect to the irreversible AFM moments on the magnetic and magnetoresistance properties of the exchange coupled heterostructures. We further found that the persistence of AFM grains with thermally disordered and reversible AFM order is crucial for achieving highly tunable magnetic properties and multi-level magnetoresistance states. We anticipate that the introduced approach and the heterostructure architecture can be utilized in future spintronic devices to manipulate the thermally disordered and reversible AFM order at the nanoscale., 27 pages and 9 figures; 19 supplementary figures and 1 table

Data repository for manuscript submitted to Physical Review Applied:Agility of spin Hall nano-oscillators. DATA fingerprint for resubmitted: md5:a9bbd503a5370963b835d3c40cdf8ba8 Ignore the older onesmd5:36e53eb278f8c3073a51de6c709f72c8 (Ignoremd5:3e2ddf76473149ad1d58cf100f90321f , I am unable to remove it, it is just an incomplete submission) Data organised on a figure by figure basis. The provided file-How to navigate the data- links all the data sets and data handling scripts utilised on each figure. Ipython notebook was used in the data handling and Omnigraffle was used to assemble the sub-figures and label the plots produced in via the Ipython notebooks. Data shown in the corresponding plots can be found in the .txt files with same labelling as figures. Abstract. We investigate the temporal response of constriction-based spin Hall nano-oscillators driven by pulsed stimuli using time-resolved Brillouin light scattering microscopy. The growth rate of the magnetization auto-oscillations, enabled by spin Hall effect and spin orbit torque, is found to vary with the amplitude of the input voltage pulses, as well as the synchronization frequency set by an external microwave input. The combination of voltage and microwave pulses allows to generate auto-oscillation signals with multi-level amplitude and frequency in the time-domain. Our findings suggest that the lead time of processes such as synchronization and logic using spin Hall nano-oscillators can be reduced to the nanosecond time-scale.

We demonstrate a novel type of spin Hall nano-oscillator (SHNO) that allows for efficient tuning of magnetic auto-oscillations over an extended range of gigahertz frequencies, using bipolar direct currents at constant magnetic fields. This is achieved by stacking two distinct ferromagnetic layers with a platinum interlayer. In this device, the orientation of the spin polarised electrons accumulated at the top and bottom interfaces of the platinum layer is switched upon changing the polarity of the direct current. As a result, the effective anti-damping required to drive large amplitude auto-oscillations can appear either at the top or bottom magnetic layer. Tuning of the auto-oscillation frequencies by several gigahertz can be obtained by combining two materials with sufficiently different saturation magnetization. Here we show that the combination of NiFe and CoFeB can result in 3 GHz shifts in the auto-oscillation frequencies. Bipolar SHNOs as such may bring enhanced synchronisation capabilities to neuromorphic computing applications.

We report on the controlled switching of domain-wall (DW) magnetization in aligned stripe-domain structures, stabilized in [$\mathrm{Co}$(0.44 nm)/$\mathrm{Pt}$(0.7 nm)]${}_{X}$($X=48$, 100, 150) multilayers with perpendicular magnetic anisotropy. The switching process, induced by an external magnetic field, is monitored by measuring the evolution of the in-plane magnetization. We show that the remanent in-plane magnetization originates from the polarization of the Bloch-type DWs. With micromagnetic simulations, we reveal that the reversal of the DW polarization is the result of the emergence and collapse of horizontal Bloch lines within the DWs at particular strengths of the external magnetic field, applied opposite to the DW polarization. Our findings are relevant for DW-based magnonics and bubble-skyrmion applications in magnetic multilayers.

Topological objects, such as magnetic vortices in soft-magnetic thin films, have attracted continued attention over the past 20 years for their unique magnetic reversal behavior and compelling potential applications. In exchange-coupled ferromagnetic/antiferromagnetic microdisks, high-resolution magnetic force microscopy imaging reveals a distorted ferromagnetic vortex structure due to randomly distributed (nonplanar) uncompensated spins at the antiferromagnetic/ferromagnetic interface. Further, in-field imaging and magneto-optical Kerr effect magnetometry demonstrate an unexpected increase in the stability of an otherwise somewhat volatile, intermediate vortex-antivortex state in exchange-coupled microdisks.

Artificial ferromagnetic (FM)/nonmagnetic multilayers, with large enough FM thickness to prevent the dominance of interface anisotropies, offer a straightforward insight into the understanding and control of perpendicular standing spin wave (PSSW) modes. Here we present a study of the static and dynamic magnetic properties of ${[\mathrm{Co}(3.0\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Au}(0.6\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{1\ensuremath{\le}N\ensuremath{\le}30}$ multilayers. Magnetometry reveals that the samples exhibit magnetization reversal properties typical of an effective single layer with weak perpendicular anisotropy, with the distinctive thickness-dependent magnetization reorientation transition from uniform in-plane to out-of-plane stripe domains at remanence. However, when such multilayer systems are out-of-plane saturated, the dynamic response reveals the existence of several different ferromagnetic resonances in the form of PSSW modes that strongly depend on the material modulation characteristics along the total thickness. These modes are induced by the layer stacking itself as the effective single layer model fails to describe the observed complex dynamics. In contrast to most systems considered in the past, described by a dynamic model of a single effectively homogeneous thick layer, the specific structures investigated here provide a unique platform for a large degree of tunability of the mode frequencies and amplitude profiles. We argue that the combination of periodic magnetic properties with vertical deformation gradients, arising from heteroepitaxial strain relaxation, converts the Au interlayer regions into a vertical regular array of magnetic pinning planes for the PSSW modes, which promotes the complex dynamics observed in this system.

Two-pulse laser excitation of FePt nanolayers distinguishes ultrafast stresses of magnetic excitations from the Poisson effect., Invar-behavior occurring in many magnetic materials has long been of interest to materials science. Here, we show not only invar behavior of a continuous film of FePt but also even negative thermal expansion of FePt nanograins upon equilibrium heating. Yet, both samples exhibit pronounced transient expansion upon laser heating in femtosecond x-ray diffraction experiments. We show that the granular microstructure is essential to support the contractive out-of-plane stresses originating from in-plane expansion via the Poisson effect that add to the uniaxial contractive stress driven by spin disorder. We prove the spin contribution by saturating the magnetic excitations with a first laser pulse and then detecting the purely expansive response to a second pulse. The contractive spin stress is reestablished on the same 100-ps time scale that we observe for the recovery of the ferromagnetic order. Finite-element modeling of the mechanical response of FePt nanosystems confirms the morphology dependence of the dynamics.

Modern spectroscopic techniques for the investigation of magnetization dynamics in micro and nanostructures or thin films typically use microwave antennas. They are directly fabricated on top of the sample by means of electron-beam lithography (EBL). Following this approach, every magnetic structure on the sample needs its own antenna, resulting in additional EBL steps and layer-deposition processes. Here, we demonstrate an approach for magnetization excitation that is suitable for optical and nonoptical spectroscopy techniques. By patterning the antenna on a separate flexible glass cantilever and insulating it electrically, we solve the mentioned issues. Since we use flexible transparent glass as the antenna substrate, optical spectroscopy techniques like microfocused Brillouin-light-scattering microscopy ($\ensuremath{\mu}\mathrm{BLS}$), time-resolved magneto-optical Kerr-effect measurements, or optically detected magnetic resonance measurements can be carried out at visible laser wavelengths. As the antenna is detached from the sample it can be freely positioned in all three dimensions to address only the desired magnetic structures and to achieve an effective excitation. We demonstrate the functionality of these antennas using $\ensuremath{\mu}\mathrm{BLS}$ and compare coherently and thermally excited magnon spectra to reveal an enhancement of the signal by a factor of about 400 due to the strong excitation by the antenna. Moreover, we succeed in characterizing yttrium-iron-garnet thin films with spatial resolution using optical ferromagnetic resonance experiments. We analyze the spatial excitation profile of the antenna by measuring the magnetization dynamics in two dimensions. The technique is furthermore applied to investigate injection locking of spin Hall nano-oscillators in the most favourable geometry with the highest spin-torque efficiency.

CoFeB alloys are highly relevant materials for spintronic applications. In this work, the crystallization of CoFeB alloys triggered by thermal annealing was investigated by x-ray diffraction techniques and scanning electron microscopy, as well as spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy for annealing temperatures ranging from 300 to ${600}^{\ensuremath{\circ}}\mathrm{C}$. The transformation of \ensuremath{\sim}100-nm-thick ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{(80\ensuremath{-}x)}{\mathrm{B}}_{20}$ films from amorphous to polycrystalline was revealed by the sharpening of spectral features observed in optical and magneto-optical dielectric functions spectra. The influence of B on the dielectric function was assessed both experimentally and by optical modeling. By analyzing the Drude component of the optical dielectric function, a consistent trend between the charge-carrier scattering time/resistivity and the annealing temperature was observed, in agreement with the electrical investigations by means of the four-point-probe method.

${\mathrm{He}}^{+}$-ion irradiation enables controlled postdeposition modification of layered magnetic thin-film systems. The degree of modification and its depth profile can be tuned by the irradiation dose and energy. Here, we use magnetometry and magnetic force microscopy to explore the impact of gentle ${\mathrm{He}}^{+}$-ion irradiation on synthetic antiferromagnets, consisting of ferromagnetic $\mathrm{Co}/\mathrm{Pt}$ multilayers with perpendicular magnetic anisotropy, which are antiferromagnetically (AF) coupled via $\mathrm{Ru}$ interlayers. This system shows a rich variety of magnetic domain patterns due to the strong competition between different magnetic energies. We show that AF interlayer exchange and perpendicular interface anisotropy energy are gradually reduced by the ion irradiation while the demagnetization energy is mainly preserved, which thus results in multiple successive magnetic-phase transitions.

Interfaces separating ferromagnetic (FM) layers from non-ferromagnetic layers offer unique properties due to spin-orbit coupling and symmetry breaking, yielding effects such as exchange bias, perpendicular magnetic anisotropy, spin-pumping, spin-transfer torques, conversion between charge and spin currents and vice-versa. These interfacial phenomena play crucial roles for magnetic data storage and transfer applications, which require forming FM nano-structures embedded in non-ferromagnetic matrices. Here, we investigate the possiblity of creating such nano-structures by ion-irradiation. We study the effect of lateral confinement on the ion-irradiation-induced reduction of non-magnetic metal oxides (e.g., antiferro- or paramagnetic) to form ferromagnetic metals. Our findings are later exploited to form 3-dimensional magnetic interfaces between Co, CoO and Pt by spatially-selective irradiation of CoO/Pt multilayers. We demonstrate that the mechanical displacement of the O atoms plays a crucial role during the reduction from insulating, non-ferromagnetic cobalt oxides to metallic cobalt. Metallic cobalt yields both perpendicular magnetic anisotropy in the generated Co/Pt nano-structures, and, at low temperatures, exchange bias at vertical interfaces between Co and CoO. If pushed to the limit of ion-irradiation technology, this approach could, in principle, enable the creation of densely-packed, atomic scale ferromagnetic point-contact spin-torque oscillator (STO) networks, or conductive channels for current-confined-path based current perpendicular-to-plane giant magnetoresistance read-heads., Comment: This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials and Interfaces copyright \c{opyright} American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https:// pubs.acs.org/articlesonrequest/AOR-rPGr2e7nZ7tzzwK3tnBZ

The understanding of how spins move and can be manipulated at pico- and femtosecond timescales has implications for ultrafast and energy-efficient data-processing and storage applications. However, the possibility of realizing commercial technologies based on ultrafast spin dynamics has been hampered by our limited knowledge of the physics behind processes on this timescale. Recently, it has been suggested that inertial effects should be considered in the full description of the spin dynamics at these ultrafast timescales, but a clear observation of such effects in ferromagnets is still lacking. Here, we report direct experimental evidence of intrinsic inertial spin dynamics in ferromagnetic thin films in the form of a nutation of the magnetization at a frequency of ~0.5 THz. This allows us to reveal that the angular momentum relaxation time in ferromagnets is on the order of 10 ps. Inertial dynamics are observed in a ferromagnet. Specifically, a nutation is seen on top of the usual spin precession that has a lifetime on the order of 10 picoseconds.

Magnetic skyrmions have been suggested as information carriers for future spintronic devices. As the first material with experimentally confirmed skyrmions, B20-type MnSi was the research focus for more than a decade. Although B20–MnSi films have been successfully grown on Si(111) substrates, there is no report about B20–MnSi films on Si(100) substrates, which would be more preferred for practical applications. In this letter, we present the first preparation of B20–MnSi on Si(100) substrates. It is realized by sub-second solid-state reaction between Mn and Si via flash-lamp annealing at ambient pressure. The regrown layer shows an enhanced Curie temperature of 43 K compared with bulk B20–MnSi. The magnetic skyrmion signature is obtained in our films by magnetic and transport measurements. The millisecond-range flash-lamp annealing provides a promising avenue for the fabrication of Si-based skyrmionic devices.

Nowadays, the development of sustainable and wearable energy harvesters is gaining an increasing interest. Herein, an approach used to develop a high performance flexible nanogenerator based on hybrid piezoelectric composite is reported. The approach consists first of employing solution mixing method with different solvents to determine the suitable solvent for achieving higher piezoelectric property of the piezoceramic polymer composites. Then, integration of conductive silver nanoparticles is done to boost the performance of the nanogenerator (NG). Different aspects are considered which are the homogeneity of particles distribution within PVDF-HFP and the crystallinity of the composite using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). This study demonstrates the efficiency of dimethyl formamide (DMF) solvent to increase the rate of crystalline phases due to their moderate evaporation rate and their high dipole moment that leads to enhanced piezoelectric performance. This approach proves its effectiveness to strengthen the piezoelectric performance especially by doping with silver nanoparticles (Ag NPs). The composite exhibits improved output voltage around 2.21 V and an output power of 0.22 μW, which are, respectively, around three times and 9 times higher than the composite without Ag NPs. In addition, the NG shows good stability over 900 cycles illustrating their robustness. The followed approach extends the performance limits of PVDF-HFP based NGs and their potential applications. Also, we have proved the potential of the optimized NG to harvest mechanical energy from human activities, with ability to generate around 3.56 V by striking with a palm hand.

The phase shift of the electron wave is a useful measure for the projected magnetic flux density of magnetic objects at the nanometer scale. More important for materials science, however, is the knowledge about the magnetization in a magnetic nano-structure. As demonstrated here, a dominating presence of stray fields prohibits a direct interpretation of the phase in terms of magnetization modulus and direction. We therefore present a model-based approach for retrieving the magnetization by considering the projected shape of the nano-structure and assuming a homogeneous magnetization therein. We apply this method to FePt nano-islands epitaxially grown on a SrTiO3 substrate, which indicates an inclination of their magnetization direction relative to the structural easy magnetic [001] axis. By means of this real-world example, we discuss prospects and limits of this approach.

© 2017 American Chemical Society. Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle X-ray scattering at an X-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, 1 order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer.

We report the optical and magneto-optical properties of amorphous and crystalline Co

We investigate the ultrafast magnetization dynamics of FePt in the L10 phase after an optical heating pulse, as used in heat assisted magnetic recording. We compare continuous and nanogranular thin films and emphasize the impact of the finite size on the remagnetization dynamics. The remagnetization speeds up significantly with increasing external magnetic field only for the continuous film, where domain wall motion governs the dynamics. The ultrafast remagnetization dynamics in the continuous film are only dominated by heat transport in the regime of high magnetic fields, whereas the timescale required for cooling is prevalent in the granular film for all magnetic field strengths. These findings highlight the necessary conditions for studying the intrinsic heat transport properties in magnetic materials., 7 pages, 4 Figures

In the growing field of $m\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}s$, spin waves (magnons) are manipulated as information carriers, particularly for data processing. Thus controlling the propagation of spin waves is of paramount interest here. This investigation considers a ferromagnetic bilayer with dipolar coupling as a potential nonreciprocal magnonic device, in which frequency dispersion is reciprocal for parallel yet nonreciprocal for antiparallel orientation of layer magnetizations. This is progress in the technological implementation of magnonic devices with asymmetric transmission properties, since reconfigurability can be easily achieved just by switching, without any rotation of the applied magnetic field.

Antiferromagnetic (AF) domain walls have recently attracted revived attention, not only in the emerging field of AF spintronics, but also more specifically for offering fast domain wall velocities and dynamic excitations up to the terahertz frequency regime. Here we introduce an approach to nucleate and stabilize an AF domain wall in a synthetic antiferromagnet (SAF). We present experimental and micromagnetic studies of the magnetization reversal in [(Co/Pt)$_{X-1}$/Co/Ir]$_{N-1}$(Co/Pt)$_X$ SAFs, where interface induced perpendicular magnetic anisotropy (PMA) and AF interlayer exchange coupling (IEC) are completely controlled via the individual layer thicknesses within the multilayer stack. By combining strong PMA with even stronger AF IEC, the SAF reveals a collective response to an external magnetic field applied normal to the surface, and we stabilize the characteristic surface spin flop (SSF) state for an even number N of AF-coupled (Co/Pt)$_{X-1}$/Co multilayer blocks. In the SSF state our system provides a well-controlled and fully tunable vertical AF domain wall, easy to integrate as no single crystal substrates are required and with uniform 2D-magnetization in the film plane for further functionalization options, such as for example lateral patterning via lithography., 15 pages, 3 figures