Structural, electronic and magnetic properties of L10 ordered CoPt nanoparticles: An experimental and DFT study (original) (raw)
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Local Order and Magnetic Properties in Mass-Selected ${\rm L}1_{0}$ CoPt Nanoparticles
IEEE Transactions on Magnetics, 2011
Because of high magnetocrystalline anisotropy in bulk phase, binary nanoalloys from Co 3d and Pt 5d metal are expected to offer the opportunity to obtain room temperature nanomagnets. In this paper, we report a quantitative study combining X-ray absorption and magnetic measurements on well-defined size-selected CoPt clusters assemblies in the 2-4 nm range, diluted in an inert carbon matrix. The particles undergo A1 to L1 0 phase transition without coalescence upon annealing under vacuum. For the as-prepared samples, using a fcc chemically disordered A1 model, we have found a contracted lattice parameter in the whole sizes range, while the cobalt-platinum stacking in L1 0 chemically ordered phase corresponds to a larger tetragonalization at the nanoscale compared to the bulk phase. Such quantitative characterizations will be helpful for future theoretical calculations that may elucidate size and alloy effects on the magnetic behavior at nanoscale.
CoPt magnetic nanocrystals in the A1/L10 transformation
Journal of Applied Physics, 2011
CoPt magnetic nanocrystals have been synthesized in a range of size from 2 to 4 nm using a colloidal chemical synthetic method. As-synthesized nanocrystals exhibit an equiatomic composition and a crystalline phase of type alloy CoPt A1 disordered. The morphology and the magnetic properties were investigated for different crystal size. In order to reach the crystalline transition toward the L10 ordered phase, a thermal annealing has been carried out on CoPt nanocrystals of 4 nm. We showed that the use of HOPG substrate both limits the coalescence effects and promotes the structural ordering. The modification of the crystalline structure selected area electron diffraction (SAED) and the magnetic properties (superconducting quantum interference device) has been studied for different heat-treatment conditions. Ferromagnetic properties at room temperature were achieved for nanocrystals of 16 nm in average size with a coercivity of 4 kOe.
Magnetic properties and L1(0) phase formation in CoPt nanoparticles
Magnetism and Magnetic Materials V, 2012
The effect of the atomic disorder-order transformation on remanence, coercivity and magnetic anisotropy energy in CoPt nanoparticles prepared by thermal decomposition and annealed at 400 o C for 4 and 16 hours has been studied. The observed remanence and magnetic anisotropy energy enhancement versus annealing time are discussed in the terms of ordering domain formation inside nanoparticles.
Evidence of L10 chemical order in CoPt nanoclusters: Direct observation and magnetic signature
Physical Review B, 2008
We report the synthesis and characterization of well-defined CoPt clusters with a mean diameter of 3 nm, produced in ultrahigh vacuum conditions following a physical route. Samples made of diluted layers of CoPt clusters embedded in amorphous carbon have been studied by transmission electron microscopy. Highresolution observations have revealed the appearance of L1 0 chemical order upon annealing, even for clusters with a 2 nm diameter, without cluster coalescence. The magnetic properties of both chemically disordered and ordered CoPt clusters embedded in amorphous carbon have then been measured by x-ray magnetic circular dichroism and superconducting quantum interference device magnetometry. Despite a striking change of the Co magnetic moment, the magnetic anisotropy of chemically ordered nanoparticles increases, with respect to the chemically disordered A1 phase, in much lower proportions than what is observed for the bulk.
Magnetic Properties of Cobalt in Deposited CoPt Clusters and Nanoparticles
2010
Hamburg 2009 quthter der hisserttionX rofF hrF ilfried urth rofF hrF homs wöller quthter der hisputtionX rofF hrF ilfried urth rofF hrF idgr ekert htum der hisputtionX HSFHQFPHIH orsitzender des rüfungsusshussesX hrF ulus etermnn orsitzender des romotionsusshussesX rofF hrF oert ulnner hekn der pkultät wthemtikD snformtik und xturwissenshften rofF hrF rF qrener Magnetische Eigenschaften von Kobalt in Deponierten CoPt Clustern und Nanopartikeln KURZFASSUNG Die magnetischen Eigenschaften von in situ präparierten gröÿenselektierten sub-nm CoPt clustern auf magnetischen Unterlagen, sowie die Eigenschaften von naÿchemisch hergestellten 3.7 nm bis 8.4 nm durchmessenden CoPt Nanopartikeln deponiert auf Silizium Wafern in einem externen Magnetfeld bis zu 7 T, wurden mit zirkularem magnetischen Röntgendichroismus (XMCD) untersucht. Die magnetischen Momente wurden mit Hilfe der XMCD Summenregeln bestimmt, während einige elektronische Eigenschaften, speziell der Oxidationszustand der naÿchemisch erzeugten Partikel aus den Röntgenabsorptionsmessungen gewonnen werden konnten. Der Magnetismus kleiner deponierter Legierungs,-Cluster und -Partikel kann von deren Gröÿe, der Zusammensetzung und der Unterlage abhängen. Je kleiner die Cluster sind, desto stärker sind die zu erwartenden Eekte, daher wurden die gröÿenselektierten Cluster auf einer schwach und einer stark austauschwechselwirkenden Unterlage deponiert, während die Zusammensetzung und Gröÿe der Cluster variiert wurde. Bei den gröÿeren naÿchemisch erzeugten, mit organischen Liganden überzogenen Nanopartikeln wurde neben der Sättigungsmagnetisierung, der Oxidationszustand und das Alterungsverhalten in einem Zeitintervall von bis zu 290 Tagen untersucht. Die magnetischen Eigenschaften der sub-nm Cluster variieren stark mit der unterschiedlichen Austauschkopplung der Unterlage. Sie zeigen deutlich erhöhte Bahnmomente wenn man zu einem reinen Kobalt-Cluster Platin hinzu gibt; dieser Eekt ist bei geringerer Austauschkopplung der Unterlage stärker ausgeprägt. Die gröÿeren Partikel besitzen eine gröÿenunabhängige Magnetisierung, zeigen dafür jedoch deutliches Alterungsverhalten, welches mit einem Kern-Schalen-Modell beschrieben werden kann.
The exchange interaction effects on magnetic properties of the nanostructured CoPt particles
Journal of Magnetism and Magnetic Materials
Various manifestations of the exchange interaction effects in magnetization curves of the CoPt nanostructured particles are demonstrated and discussed. The inter-grain exchange constant A in the sponge-like agglomerates of crystallites is estimated as А = (7 ± 1) pJ/m from the approach magnetization to saturation curves that is in good agreement with А = (6.6 ± 0.5) pJ/m obtained from Bloch T 3/2 law. The fractal dimensionality of the exchange coupled crystallite system in the porous media of the disordered CoPt alloy d = (2.60 ± 0.18) was estimated from the approach magnetization to saturation curve. Coercive force decreases with temperature as Hc ~ T 3/2 which is assumed to be a consequence of the magnetic anisotropy energy reduction due to the thermal spin wave excitations in the investigated CoPt particles.
Interface Effects on the Magnetism of CoPt-Supported Nanostructures
Nano Letters, 2011
The magnetism of CoPt nanostructures supported on the MgO(100) surface is investigated via first-principles simulations using 1D models. Nanostructures with L1 0 chemical ordering and cube-on-cube epitaxy are predicted to possess large magnetic moments and easy magnetization axis perpendicular to the surface. However, their magnetic anisotropy energy is roughly halved with respect to the bulk alloy due to a peculiar mixing of particle and support electronic states. The general factors at play in determining this behavior and the implications of these findings are discussed in view of designing room-temperature magnetic bits.
IOP conference series, 2018
The structural and magnetic properties of both L10 ordered FePt and CoPt nanoparticles make them potential candidates for optical-electronic and magneto-optical devices. First, we carried out an ab initio total energy minimization study to find the geometrical optimization of both L10 phases of FePt and CoPt nanoparticles. Then, we investigated the magnetocrystalline anisotropy energy (MAE) of both systems along special line joining the points of high symmetry (A,B and C points) using super-cell slap approach with alternating layers Fe/Co and Pt along the (001) direction. We found that the point (A) has the highest MAE value for both systems, where the value of MAE in FePt is 8.89 × 10 7 erg/cm 3 and in CoPt is 6.40 × 10 7 erg/cm 3. Our spin density based calculations indicate that large spin-orbit interaction and the hybridization between Pt 5d states and Fe/Co 3d states are the dominant factors in determining the MAE in both systems.
Annealing-Induced High Ordering and Coercivity in Novel L10 CoPt-Based Nanocomposite Magnets
Metals, 2018
A novel class of quaternary intermetallic alloys based on CoPt is investigated in view of their interesting magnetic properties induced by the presence of hard magnetic L1 0 phase. A Co 48 Pt 28 Ag 6 B 18 alloy has been prepared by rapid solidification from the melt and subjected to various isothermal annealing procedures. The structure and magnetism of both as-cast and annealed samples as well as the phase evolution with temperature are investigated by means of thermal analysis, X-ray, and selected area electron diffraction, scanning and high-resolution electron microscopy, and magnetic measurements. The X-ray diffraction (XRD) analysis shows that both the as-cast alloy and the sample annealed at 400 • C (673 K) have a nanocrystalline structure where fcc CoPt phase predominates. Annealing at 473 • C promotes the formation of L1 0 phase triggered by the disorder-order phase transformation as documented in the differential scanning calorimetry results. The sample annealed at 670 • C (943 K) shows full formation of L1 0 CoPt as revealed by XRD. Magnetic measurements showed coercivity values ten times increased compared to the as-cast state. This confirms the full formation of L1 0 CoPt in the annealed samples. Moreover, detailed atomic resolution HREM images and SAED patterns show the occurrence of the rarely seen (003) superlattice peaks, which translated into a high ordering of the L1 0 CoPt superlattice. Such results spur more interest in finding novel classes of nanocomposite magnets based on L1 0 phase.
Synthesis and magnetic properties of CoPt nanoparticles
2004
High magnetocrystalline anisotropy CoPt particles with an average size of 8 nm were synthesized by the superhydride reduction of CoCl 2 and Pt͑acac) 2 at a high temperature. As-made particles showed a disordered face-centered cubic lattice and were superparamagnetic. Upon heat treatment at temperatures above 600°C, the particles transformed to the L1 0 phase, as indicated by the appearance of the superlattice peaks in the x-ray diffraction and high magnetocrystalline anisotropy. The temperature dependence of the coercivity of nanoparticles annealed at 650°C was measured from 10 to 300 K and analyzed using a Sharrock formula. After annealing at 650°C, the anisotropy of the nanoparticles was Kϳ1.7ϫ10 7 erg/cm 3 .