Arrays Of Nanomagnets Research Articles

Electrically Driven Reversible Magnetic Rotation in Nanoscale Multiferroic Heterostructures.

Electrically driven magnetic switching (EDMS) is highly demanded for next-generation advanced memories or spintronic devices. The key challenge is to achieve repeatable and reversible EDMS at sufficiently small scale. In this work, we reported an experimental realization of room-temperature, electrically driven, reversible, and robust 120° magnetic state rotation in nanoscale multiferroic heterostructures consisting of a triangular Co nanomagnet array on tetragonal BiFeO3 films, which can be directly monitored by magnetic force microscope (MFM) imaging. The observed reversible magnetic switching in an individual nanomagnet can be triggered by a small electric pulse within 10 V with an ultrashort time of ∼10 ns, which also demonstrates sufficient switching cycling and months-long retention lifetime. A mechanism based on synergic effects of interfacial strain and exchange coupling plus shape anisotropy was also proposed, which was also verified by micromagnetic simulations. Our results create an avenue to engineer the nanoscale EDMS for low-power-consumption, high-density, nonvolatile magnetoelectric memories and beyond.

Ferromagnetic resonance of a two-dimensional array of nanomagnets: Effects of surface anisotropy and dipolar interactions

We develop an analytical approach for studying the FMR frequency shift due to dipolar interactions and surface effects in two-dimensional arrays of nanomagnets with (effective) uniaxial anisotropy along the magnetic field. For this we build a general formalism on the basis of perturbation theory that applies to dilute assemblies but which goes beyond the point-dipole approximation as it takes account of the size and shape of the nano-elements, in addition to their separation and spatial arrangement. The contribution to the frequency shift due to the shape and size of the nano-elements has been obtained in terms of their aspect ratio, their separation and the lattice geometry. We have also varied the size of the array itself and compared the results with a semi-analytical model and reached an agreement that improves as the size of the array increases. We find that the red-shift of the ferromagnetic resonance due to dipolar interactions decreases for smaller arrays. Surface effects may induce either a blue-shift or a red-shift of the FMR frequency, depending on the crystal and magnetic properties of the nano-elements themselves. In particular, some configurations of the nano-elements assemblies may lead to a full compensation between surface effects and dipole interactions.

Multi-directional emission and detection of spin waves propagating in yttrium iron garnet with wavelengths down to about 100 nm

We performed broadband spin-wave spectroscopy on 200 nm thick yttrium iron garnet containing arrays of partially embedded magnetic nanodisks. Using integrated coplanar waveguides (CPWs), we studied the excitation and transmission of spin waves depending on the presence of nanomagnet arrays of different lateral extensions. By means of the grating coupler effect, we excited spin waves propagating in multiple lateral directions with wavelengths down to 111 nm. They exhibited group velocities of up to 1 km/s. Detection of such short-wavelength spin waves was possible only in symmetrically designed emitter/detector configurations, not with a bare CPW. We report spin waves propagating between grating couplers under oblique angles exhibiting a wave vector component parallel to the CPW. The effective propagation distance amounted to about 80 μm. Such transmission signals were not addressed before and substantiate the versatility of the grating coupler effect for implementing nanomagnonic circuits.

Realization of ground state in artificial kagome spin ice via topological defect-driven magnetic writing.

Arrays of non-interacting nanomagnets are widespread in data storage and processing. As current technologies approach fundamental limits on size and thermal stability, enhancing functionality through embracing the strong interactions present at high array densities becomes attractive. In this respect, artificial spin ices are geometrically frustrated magnetic metamaterials that offervast untapped potential due to their unique microstate landscapes, with intriguing prospects in applications from reconfigurable logic to magnonic devices or hardware neural networks. However, progress in such systems is impeded by the inability to access more than a fraction of the total microstate space. Here, we demonstrate that topological defect-driven magnetic writing-a scanning probe technique-provides access to all of the possible microstates in artificial spin ices and related arrays of nanomagnets. We create previously elusive configurations such as the spin-crystal ground state of artificial kagome dipolar spin ices and high-energy, low-entropy 'monopole-chain' states that exhibit negative effective temperatures.

Tailoring the magnetization states in 2D arrays of multiresponse ferromagnetic nanomagnets

We have fabricated nanomagnet arrays as a function of several geometrical parameters like the spacing between nanostructures, the aspect ratio and the layers thicknesses. The nanomagnets consist in two magnetic layers, separated by a non magnetic interlayer, that interact through magnetostatic coupling. They present a multiresponse hysteresis loops with two different switching fields. We have performed micromagnetic simulations to discern the role play by the different interactions. The spacing in the array strongly modifies the saturating field along the short axis and the magnetization reversal mechanisms from coherent rotation to domain wall nucleation. A small asymmetry between the two magnetic layers favors a magnetization reversal mechanism along the long axis with two different switching fields. These fields can be tailored through the thickness of the layers or the inter-element spacing in the array. In trilayers with the same magnetic layer thicknesses, the asymmetry can be induced by growing the two magnetic layers with a different anisotropy. The well-defined reversal fields make these nanomagnets potentially useful for magnetic tagging.

Emergent dynamic chirality in a thermally driven artificial spin ratchet.

Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice can lead to specific collective behaviour, including emergent magnetic monopoles, charge screening and transport, as well as magnonic response. Here, we demonstrate a spin-ice-based active material in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation. Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization. This opens the possibility of implementing a magnetic Brownian ratchet, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells.

Viewpoint on ‘Optical measurement of damping in nanomagnet arrays using magnetoelastically driven resonances’

Viewpoint on ‘Optical measurement of damping in nanomagnet arrays using magnetoelastically driven resonances’

Image Processing With Dipole-Coupled Nanomagnets: Noise Suppression and Edge Enhancement Detection

Hardware based image processing offers speed and convenience not found in software-centric approaches. Here, we show theoretically that a two-dimensional periodic array of dipole-coupled elliptical nanomagnets, delineated on a piezoelectric substrate, can act as a dynamical system for specific image processing functions. Each nanomagnet has two stable magnetization states that encode pixel color (black or white). An image containing black and white pixels is first converted to voltage states and then mapped into the magnetization states of a nanomagnet array with magneto-tunneling junctions (MTJs). The same MTJs are employed to read out the processed pixel colors later. Dipole interaction between the nanomagnets implements specific image processing tasks such as noise reduction and edge enhancement detection. These functions are triggered by applying a global strain to the nanomagnets with a voltage dropped across the piezoelectric substrate. An image containing an arbitrary number of black and white pixels can be processed in few nanoseconds with very low energy cost.

Optical measurement of damping in nanomagnet arrays using magnetoelastically driven resonances

Surface acoustic waves (SAWs) are optically excited in periodic nanomagnet arrays and drive the magnetization precession via magnetoelastic coupling. The frequency of this mechanically induced magnetic response is pinned at the SAW frequency over an extended range of applied fields. First, we show by experimental and numerical investigation of materials with different combinations of damping and magnetoelastic coupling strengths that the field-dependent width of this pinned resonance depends only on the effective damping αeff. Second, we derive an analytical expression for determining αeff from the Lorentzian lineshape of the field-dependent Fourier amplitude of this resonance. We show that the intrinsic Gilbert damping can be determined in the high field limit by analyzing multiple pinned resonances at different applied fields. This demonstrates that intrinsic damping can be extracted all-optically, despite interactions with nonmagnetic degrees of freedom. We find damping values of 0.027, 0.028 and 0.25 for Ni, Co and TbFe respectively. Finally, the validity of the experimental results is verified by excellent agreement with micromagnetic simulations incorporating the magnetoelastic coupling, which shows that the pinning width is unaffected by the magnetoelastic coupling constant over three orders of magnitude. This finding has implications for the rational design of spintronic devices that involve magnetoelastic effects.

Tailoring the magnetic order in a supermagnetic metamaterial

The emergent magnetism in close-packed assemblies of interacting superparamagnetic particles is commonly referred to as supermagnetism. The magnetic characteristics of such systems are determined by the dipolar coupling between the nanomagnets, rather than the exchange interaction responsible for ferro- and antiferromagnetism in continuous material. The dipolar coupling facilitates tuning of the magnetism, which renders supermagnetic ensembles suitable model systems for exploration of new physics. In this work, we discuss micromagnetic simulations of regular arrays of thin film nanomagnets, with magnetic material parameters typical of the ferromagnetic oxide La0.7Sr0.3MnO3. The ground state supermagnetic order in these systems is primarily determined by the lattice configuration, in that a square lattice results in antiferromagnetic order, whereas a triangular lattice shows ferromagnetic order. We found that a square lattice of circular nanomagnets may be switched from superferromagnetic to superantiferromagnetic order by a small external field applied in the appropriate direction.

Identifying weakly-interacting single domain states in Ni nanowire arrays by FORC

The control and understanding of magnetization reversal mechanisms and magnetostatic interactions in nanomagnet arrays is a critical point that has to be overcome in order to reach industrial applications. However, usual magnetic characterization techniques can only provide information on the overall array behaviour and are not able to discriminate relevant local properties. In this work, we show that the first-order reversal curve (FORC) method is a unique tool to identify weakly-interacting uniaxial single domain (SD) particles in systems with complex mixed magnetic states. We compare the FORC diagrams of two sets of Ni nanowire (NW) arrays electrodeposited in hexagonally ordered nanoporous alumina templates: A) with a uniform length distribution (interacting SD particles), and B) with a non-uniform length distribution. For non-uniform length distributions, regions of isolated NWs occur in the array, creating a complex mixture of strongly and weakly-interacting SD particles. These weakly-interacting NWs are here identified by a characteristic ridge along the coercive field axis of the FORC diagram. Micromagnetic simulations confirmed the presence of this weakly-interacting magnetic behaviour in the array with non-uniform length distributions. Combining FORC results with micromagnetic simulations, we show that the magnetization reversal of interacting wires occurs by the nucleation of transverse domain walls at the NWs' ends, while isolated wires nucleate vortex domain walls. This work thus highlights the importance of the FORC method for the accurate identification and understanding of local magnetic behaviours in nanomagnet arrays.

Size-Controlled Growth of High-Density Ordered Nanomagnet Arrays by Template-Assisted Method

Size-Controlled Growth of High-Density Ordered Nanomagnet Arrays by Template-Assisted Method

Extensive degeneracy, Coulomb phase and magnetic monopoles in artificial square ice.

Artificial spin-ice systems are lithographically patterned arrangements of interacting magnetic nanostructures that were introduced as way of investigating the effects of geometric frustration in a controlled manner. This approach has enabled unconventional states of matter to be visualized directly in real space, and has triggered research at the frontier between nanomagnetism, statistical thermodynamics and condensed matter physics. Despite efforts to create an artificial realization of the square-ice model-a two-dimensional geometrically frustrated spin-ice system defined on a square lattice-no simple geometry based on arrays of nanomagnets has successfully captured the macroscopically degenerate ground-state manifold of the model. Instead, square lattices of nanomagnets are characterized by a magnetically ordered ground state that consists of local loop configurations with alternating chirality. Here we show that all of the characteristics of the square-ice model are observed in an artificial square-ice system that consists of two sublattices of nanomagnets that are vertically separated by a small distance. The spin configurations we image after demagnetizing our arrays reveal unambiguous signatures of a Coulomb phase and algebraic spin-spin correlations, which are characterized by the presence of 'pinch' points in the associated magnetic structure factor. Local excitations-the classical analogues of magnetic monopoles-are free to evolve in an extensively degenerate, divergence-free vacuum. We thus provide a protocol that could be used to investigate collective magnetic phenomena, including Coulomb phases and the physics of ice-like materials.

Intrinsic spin dynamics in optically excited nanoscale magnetic tunnel junction arrays restored by dielectric coating

We report the all-optical observation of intrinsic spin dynamics and extraction of magnetic material parameters from arrays of sub-100 nm spin-transfer torque magnetic random access memory (STT-MRAM) devices with a CoFeB/MgO interface. To this end, the interference of surface acoustic waves with time-resolved magneto-optic signals via magneto-elastic coupling was suppressed using a dielectric coating. The efficacy of this method is demonstrated experimentally and via modeling on a nickel nanomagnet array. The magnetization dynamics for both coated nickel and STT-MRAM arrays shows a restored field-dependent Kittel mode from which the effective damping can be extracted. We observe an increased low-field damping due to extrinsic contributions from magnetic inhomogeneities and variations in the nanomagnet shape, while the intrinsic Gilbert damping remains unaffected by patterning. The data are in excellent agreement with a local resonance model and have direct implications for the design of STT-MRAM devices as well as other nanoscale spintronic technologies.

Binary information propagation in circular magnetic nanodot arrays using strain induced magnetic anisotropy

Nanomagnetic logic has emerged as a potential replacement for traditional Complementary Metal Oxide Semiconductor (CMOS) based logic because of superior energy-efficiency (Salahuddin and Datta 2007 Appl. Phys. Lett. 90 093503, Cowburn and Welland 2000 Science 287 1466–68). One implementation of nanomagnetic logic employs shape-anisotropic (e.g. elliptical) ferromagnets (with two stable magnetization orientations) as binary switches that rely on dipole−dipole interaction to communicate binary information (Cowburn and Welland 2000 Science 287 1466–8, Csaba et al 2002 IEEE Trans. Nanotechnol. 1 209–13, Carlton et al 2008 Nano Lett. 8 4173–8, Atulasimha and Bandyopadhyay 2010 Appl. Phys. Lett. 97 173105, Roy et al 2011 Appl. Phys. Lett. 99 063108, Fashami et al 2011 Nanotechnology 22 155201, Tiercelin et al 2011 Appl. Phys. Lett. 99, Alam et al 2010 IEEE Trans. Nanotechnol. 9 348–51 and Bhowmik et al 2013 Nat. Nanotechnol. 9 59–63). Normally, circular nanomagnets are incompatible with this approach since they lack distinct stable in-plane magnetization orientations to encode bits. However, circular magnetoelastic nanomagnets can be made bi-stable with a voltage induced anisotropic strain, which provides two significant advantages for nanomagnetic logic applications. First, the shape-anisotropy energy barrier is eliminated which reduces the amount of energy required to reorient the magnetization. Second, the in-plane size can be reduced (∼20 nm) which was previously not possible due to thermal stability issues. In circular magnetoelastic nanomagnets, a voltage induced strain stabilizes the magnetization even at this size overcoming the thermal stability issue. In this paper, we analytically demonstrate the feasibility of a binary ‘logic wire’ implemented with an array of circular nanomagnets that are clocked with voltage-induced strain applied by an underlying piezoelectric substrate. This leads to an energy-efficient logic paradigm orders of magnitude superior to existing CMOS-based logic that is scalable to dimensions substantially smaller than those for existing nanomagnetic logic approaches. The analytical approach is validated with experimental measurements conducted on dipole coupled Nickel (Ni) nanodots fabricated on a PMN-PT (Lead Magnesium Niobate-Lead Titanate) sample.

Plasmonic nanodevice with magnetic funcionalities: fabrication and characterization

We have designed and fabricated a nanodevice exhibiting simultaneously ferromagnetic properties of nanostructures with plasmonic properties of continuous films. Our device consists of an array of nanomagnets on top of a continuous plasmonic film. The patterned nanomagnets magnetic state is single domain and well-defined shape anisotropy. Despite the presence of the patterned media on top of the Au film, the system exhibits surface plasmon resonance characteristics of a continuous film, i.e., propagating surface plasmon-polaritons.

Arrays of Single-Domain Nanomagnets As Memory- Logic-Hybrid Device - a Nonvolatile Magnetic Memory That Can Perform Logical Operations

With nonvolatile NAND flash memory becoming the dominant memory type the former champion for mass data storage – magnetic memory – has now become a niche player. This is – at least to some extent – unjustified, as magnetic data storage has still its advantages such as radiation-hardness, and long data storage time. In this work we celebrate a renaissance of magnetic data storage with a new concept that allows not only storing of information but also data processing with nanomagnets. We use focused electron beam induced deposition of single domain nanomagnets to fabricate arrays of magnetic nanowires and magnetic nanopillars. If the single nanomagnets are in close vicinity, then the stray fields allow to influence neighboring nanomagnets. With a majority gate such a nanomagnet assembly can even perform logical calculations. In this work we will present the fabrication process for such nanomagnetic devices, characterize their magnetic properties by magnetic force microscop, and design and fabricate nanomagnet arrays. With this dual functionality as memory and as logic unit these directly-written nanomagnet devices promise a promising future potential. Further applications of advanced device structures based on single domain nanomagnetswill be discussed. We will report on the optimization of the device fabrication process and chemical and structural analysis of nanomagnets. The functional characteristics of these hybrid devices will be described and future system applications will be discussed. Figure 1

ChemInform Abstract: Magnetic Nanoparticles: a Subject for Both Fundamental Research and Applications

AbstractReview: 167 refs.

Growth of Co Nanomagnet Arrays with Enhanced Magnetic Anisotropy.

A trigon structure formed by submonolayer gadolinium deposition onto Au(111) is revealed as a robust growth template for Co nanodot arrays. Scanning Tunneling Microscopy and X‐Ray Magnetic Circular Dichroism measurements evidence that the Co nanoislands behave as independent magnetic entities with an out‐of‐plane easy axis of anisotropy and enhanced magnetic anisotropy values, as compared to other self‐organized Co nanodot superlattices. The large strain induced by the lattice mismatch at the interface between Co and trigons is discussed as the main reason for the increased magnetic anisotropy of the nanoislands.

Cobalt double-ring and double-dot structures: Magnetic properties

The magnetization reversal mechanism of nanostructures of cobalt double-rings (D-rings) and double-dots (D-dots) is investigated in the framework of micromagnetic simulations. The arrays contain two identical coupled rings (wide and narrow) or dots with outer diameter of 200nm and thicknesses ranging from 2–20nm. Hysteresis loops, dipole–dipole and exchange energies are systematically calculated for the cases of the structures touching and the structures with a 50-nm inter-magnet separation; moreover, magnetization states along the hysteresis curve are analyzed. The results of both dot and ring D-magnets are compared with the corresponding individual magnets. Our results reveal that all D-ring (in contact and separated) arrays containing narrow rings exhibit non-null remanent magnetization; furthermore, higher coercive fields are promoted when the magnet thickness is increased. It is observed that the magnetization reversal is driven mainly by a clockwise rotation of onion-states, followed by states of frustrated vortices. Our results could help improve the understanding of the magnetic interactions in nanomagnet arrays.