High temperature magnetic stabilization of cobalt nanoparticles by an antiferromagnetic proximity effect

Thermal activation tends to destroy the magnetic stability of small magnetic nanoparticles, with crucial implications in ultra-high density recording among other applications. Here we demonstrate that low blocking temperature ferromagnetic (FM) Co nanoparticles (TB<70 K) become magnetically stable above 400 K when embedded in a high N\'eel temperature antiferromagnetic (AFM) NiO matrix. The origin of this remarkable TB enhancement is due to a magnetic proximity effect between a thin CoO shell (with low N\'eel temperature, TN; and high anisotropy, KAFM) surrounding the Co nanoparticles and the NiO matrix (with high TN but low KAFM). This proximity effect yields an effective AFM with an apparent TN beyond that of bulk CoO, and an enhanced anisotropy compared to NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations via core-shell exchange-bias coupling, leading to the observed TB increase. Mean-field calculations provide a semi-quantitative understanding of this magnetic- proximity stabilization mechanism

Exchange biasing of single-domain Ni nanoparticles spontaneously grown in an antiferromagnetic MnO matrix

Exchange biased composites of ferromagnetic single-domain Ni nanoparticles embedded within large grains of MnO have been prepared by reduction of Ni$_x$Mn$_{1-x}$O$_4$ phases in flowing hydrogen. The Ni precipitates are 15-30 nm in extent, and the majority are completely encased within the MnO matrix. The manner in which the Ni nanoparticles are spontaneously formed imparts a high ferromagnetic- antiferromagnetic interface/volume ratio, which results in substantial exchange bias effects. Exchange bias fields of up to 100 Oe are observed, in cases where the starting Ni content $x$ in the precursor Ni$_x$Mn$_{1-x}$O$_4$ phase is small. For particles of approximately the same size, the exchange bias leads to significant hardening of the magnetization, with the coercive field scaling nearly linearly with the exchange bias field.Comment: 6 pages PDFLaTeX with 9 figure

A Multilevel Magnetic Synapse Based on Voltage-Tuneable Magnetism by Nitrogen Ion Migration

Altres ajuts: acords transformatius de la UABAltres ajuts: European Union NextGenerationEU/PRTR (grant CNS2022-135230)Advanced synaptic devices with simultaneous memory and processor capabilities are envisaged as core elements of neuromorphic computing (NC) for low-power artificial intelligence. So far, most synaptic devices are based on resistive memories, where the device resistance is tuned with applied voltage or current. However, the use of electric current in such resistive devices causes significant power dissipation due to Joule heating. Higher energy efficiency has been reported in materials exhibiting voltage control of magnetism (VCM). In particular, voltage-driven ion motion to modulate magnetism (magneto-ionics) is an emerging VCM mechanism that can offer new prospects for low-power implementation of NC. In the present work, voltage-driven nitrogen ion motion is exploited in transition metal nitride (CoFeN) thin films (i.e., nitrogen magneto-ionics) to emulate biological synapses. In the proposed device, distinct multilevel non-volatile magnetic states for analog computing and multi-state storage are realized. Moreover, essential synaptic functionalities of the human brain are successfully simulated. The device exhibits an excellent synapse with a remarkable retention time (≈6 months), high switching ratio and large endurance (≈103), for hardware implementation of NC. This research provides new insight into exploiting magneto-ionic-based synaptic devices for spin-based neuromorphic systems

Magnetic relaxation of exchange biased (Pt/Co) multilayers studied by time-resolved Kerr microscopy

Magnetization relaxation of exchange biased (Pt/Co)5/Pt/IrMn multilayers with perpendicular anisotropy was investigated by time-resolved Kerr microscopy. Magnetization reversal occurs by nucleation and domain wall propagation for both descending and ascending applied fields, but a much larger nucleation density is observed for the descending branch, where the field is applied antiparallel to the exchange bias field direction. These results can be explained by taking into account the presence of local inhomogeneities of the exchange bias field.Comment: To appear in Physical Review B (October 2005

Highly cyclable voltage control of magnetism in cobalt ferrite nanopillars for memory and neuromorphic applications

Tuning the properties of magnetic materials by voltage-driven ion migration (magneto-ionics) gives potential for energy-efficient, non-volatile magnetic memory and neuromorphic computing. Here, we report large changes in the magnetic moment at saturation (mS) and coercivity (HC), of 34% and 78%, respectively, in an array of CoFe2O4 (CFO) epitaxial nanopillar electrodes (∼50 nm diameter, ∼70 nm pitch, and 90 nm in height) with an applied voltage of −10 V in a liquid electrolyte cell. Furthermore, a magneto-ionic response faster than 3 s and endurance >2000 cycles are demonstrated. The response time is faster than for other magneto-ionic films of similar thickness, and cyclability is around two orders of magnitude higher than for other oxygen magneto-ionic systems. Using a range of characterization techniques, magnetic switching is shown to arise from the modulation of oxygen content in the CFO. Also, the highly cyclable, self-assembled nanopillar structures were demonstrated to emulate various synaptic behaviors, exhibiting non-volatile, multilevel magnetic states for analog computing and high-density storage. Overall, CFO nanopillar arrays offer the potential to be used as interconnected synapses for advanced neuromorphic computing applications

Fundamentals of interface phenomena in advanced bulk nanoscale materials

The review is devoted to a study of interface phenomena influencing advanced properties of nanoscale materials processed by means of severe plastic deformation, high-energy ball milling and their combinations. Interface phenomena include processes of interface defect structure relaxation from a highly nonequilibrium state to an equilibrium condition, grain boundary phase transformations and enhanced grain boundary and triple junction diffusivity. On the basis of an experimental investigation, a theoretical description of the key interfacial phenomena controlling the functional properties of advanced bulk nanoscale materials has been conducted. An interface defect structure investigation has been performed by TEM, high-resolution x-ray diffraction, atomic simulation and modeling. The problem of a transition from highly non-equilibrium state to an equilibrium one, which seems to be responsible for low thermostability of nanoscale materials, was studied. Also enhanced grain boundary diffusivity is addressed. Structure recovery and dislocation emission from grain boundaries in nanocrystalline materials have been investigated by analytical methods and modeling

Out-of-plane magnetic patterning on austenitic stainless steels using plasma nitriding

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.A correlation between the grain orientation and the out-of-plane magnetic properties of nitrogen-enriched polycrystalline austenitic stainless steel surface is performed. Due to the competition between the magnetocrystalline anisotropy, the exchange and dipolar interactions, and the residual stresses induced by nitriding, the resulting effective magnetic easy-axis can lay along unusual directions. It is also demonstrated that, by choosing an appropriate stainless steel texturing, arrays of ferromagnetic structures with out-of-plane magnetization, embedded in a paramagnetic matrix, can be produced by local plasma nitriding through shadow masks

Optimizing magneto-ionic performance in structure/composition-engineered ternary nitrides

Magneto-ionics, an emerging approach to manipulate magnetism that relies on voltage-driven ion motion, holds the promise to boost energy efficiency in information technologies such as spintronic devices or future non-von Neumann computing architectures. For this purpose, stability, reversibility, endurance, and ion motion rates need to be synergistically optimized. Among various ions, nitrogen has demonstrated superior magneto-ionic performance compared to classical species such as oxygen or lithium. Here, we show that ternary Co1−xFexN compound exhibits an unprecedented nitrogen magneto-ionic response. Partial substitution of Co by Fe in binary CoN is shown to be favorable in terms of generated magnetization, cyclability and ion motion rates. Specifically, the Co0.35Fe0.65N films exhibit an induced saturation magnetization of 1,500 emu/cm3, a magneto-ionic rate of 35.5 emu/(cm3·s) and endurance exceeding 103 cycles. These values significantly surpass those of other existing nitride and oxide systems. This improvement can be attributed to the larger saturation magnetization of Co0.35Fe0.65 compared to individual Co and Fe, the nature and size of structural defects in as-grown films of different composition, and the dissimilar formation energies of Fe and Co with N in the various developed crystallographic structures

Spontaneously formed porous and composite materials

In recent years, a number of routes to porous materials have been developed which do not involve the use of pre-formed templates or structure-directing agents. These routes are usually spontaneous, meaning they are thermodynamically downhill. Kinetic control, deriving from slow diffusion of certain species in the solid state, allows metastable porous morphologies rather than dense materials to be obtained. While the porous structures so formed are random, the average architectural features can be well-defined, and the porosity is usually highly interconnected. The routes are applicable to a broad range of functional inorganic materials. Consequently, the porous architectures have uses in energy transduction and storage, chemical sensing, catalysis, and photoelectrochemistry. This is in addition to more straightforward uses deriving from the pore structure, such as in filtration, as a structural material, or as a cell-growth scaffold. In this feature article, some of the methods for the creation of porous materials are described, including shape-conserving routes that lead to hierarchical macro/mesoporous architectures. In some of the preparations, the resulting mesopores are aligned locally with certain crystallographic directions. The coupling between morphology and crystallography provides a macroscopic handle on nanoscale structure. Extension of these routes to create biphasic composite materials are also described

Structural and magnetic characterization of batch-fabricated nickel encapsulated multi-walled carbon nanotubes

We report on the growth and fabrication of Ni-filled multi-walled carbon nanotubes (Ni-MWNTs) with an average diameter of 115 nm and variable length of 400 nm-1μm. The Ni-MWNTs were grown using template-assisted electrodeposition and low pressure chemical vapor deposition (LPCVD) techniques. Anodized alumina oxide (AAO) templates were fabricated on Si using a current controlled process. This was followed by the electrodeposition of Ni nanowires (NWs) using galvanostatic pulsed current (PC) electrodeposition. Ni NWs served as the catalyst to grow Ni-MWNTs in an atmosphere of H2/C2H2 at a temperature of 700º C. Time dependent depositions were carried out to understand the diffusion and growth mechanism of Ni-MWNTs. Characterization was carried out using scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), Raman spectroscopy and energy dispersive x-ray spectroscopy (EDX). TEM analysis revealed that the Ni nanowires possess a fcc structure. To understand the effects of the electrodeposition parameters, and also the effects of the high temperatures encountered during MWNT growth on the magnetic properties of the Ni-MWNTs, vibrating sample magnetometer (VSM) measurements were performed. The template-based fabrication method is repeatable, efficient, enables batch fabrication and provides good control on the dimensions of the Ni-MWNT