Single Nanomagnet Behaviour: Surface and Finite-Size Effects

Iglesias, Òscar; Kachkachi, Hamid

doi:10.1007/978-3-030-60473-8_1

Cited by 5 publications

(5 citation statements)

References 113 publications

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“…153 This has dramatic consequences in the magnetic properties of the NP that, in addition to finite-size effects, become dominated by surface effects due to the breaking of crystal-field symmetry at the boundaries of the NP. 154 Consequently, single-site anisotropy contributions to the energy of surface atoms are altered both in the direction of the local easy axes and magnitude of the anisotropy constant with respect to the core atoms, for which anisotropy can be assumed to be the same than in bulk.…”

Section: The Role Of the Surface Surface Anisotropy And Coatingmentioning

confidence: 99%

“…Usually, the Néel model is adopted assuming that surface atoms have anisotropy directions that tend to point along the direction of the missing neighbors, which, for spherical NP, is close to radial direction. 154,155 Atomic disorder and lattice reconstruction at the surface may also lead to locally disordered easy-axes that can cause surface spin disorder, [156][157][158] antiphase boundaries, 129,159 and frustration 160 . Then, a critical parameter for the magnetic characterization of NP is the surface anisotropy constant 𝐾 𝑆 , whose magnitude can usually exceed its core counterpart 𝐾 𝑉 .…”

Section: The Role Of the Surface Surface Anisotropy And Coatingmentioning

confidence: 99%

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Magnetic Nanoparticles: from the Nanostructure to the Physical Properties

Batlle,

Moya,

Escoda-Torroella

et al. 2023

2023 IEEE International Magnetic Conference - Short Papers (INTERMAG Short Papers)

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Some of the synthesis methods and physical properties of iron oxide-based magnetic nanoparticles such as Fe3-xO4 and CoxFe3-xO4 are reviewed because of their interest in health, environmental applications, and ultra-high density magnetic recording. Unlike high crystalline quality nanoparticles larger than a few nanometers that show bulk-like magnetic and electronic properties, nanostructures with increasing structural defects yield a progressive worsening of their general performance due to frozen magnetic disorder and local breaking of their crystalline symmetry. Thus, it is shown that single-crystal, monophasic nanoparticles do not exhibit significant surface or finite-size effects, such as spin canting, reduced saturation magnetization, high closure magnetic fields, hysteresis-loop shift or dead magnetic layer, features which are mostly associated with crystallographic-defective systems. Besides, the key role of the nanoparticle coating, surface anisotropy, and inter-particle interactions are discussed. Finally, the results of some single particle techniques -magnetic force microscopy, Xray photoemission electron microscopy, and electron magnetic chiral dichroism-that allow studying individual nanoparticles down to sub-nanometer resolution with element, valence and magnetic selectivity, are presented. All in all, the intimate, fundamental correlation of the nanostructure (crystalline, chemical, magnetic…) to the physical properties of the nanoparticles is ascertained.

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Section: The Role Of the Surface Surface Anisotropy And Coatingmentioning

confidence: 99%

Section: The Role Of the Surface Surface Anisotropy And Coatingmentioning

confidence: 99%

Magnetic Nanoparticles: from the Nanostructure to the Physical Properties

Batlle,

Moya,

Escoda-Torroella

et al. 2023

2023 IEEE International Magnetic Conference - Short Papers (INTERMAG Short Papers)

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“…In these cases, the response to a magnetic field as reflected in the loops must come from intrinsic properties of an individual core/shell particles. Surface spins may present frustration due to broken links or lack of coordination, and increased surface anisotropy [55], which distinguish them from those in the inner regions of the shell that are pinned by the coupling to core spins and do not contribute to the magnetic response [34]. Therefore, the absence of saturation at low temperatures with lower ϕ can be attributed to the progressive alignment of the outermost layer of the AFM shell spins towards the core magnetization.…”

Section: Zfc Hysteresis Loopsmentioning

confidence: 99%

Dependence of Exchange Bias on Interparticle Interactions in Co/CoO Core/Shell Nanostructures

Goswami¹,

Gupta

Nayak

et al. 2022

Nanomaterials

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This article reports the dependence of exchange bias (EB) effect on interparticle interactions in nanocrystalline Co/CoO core/shell structures, synthesized using the conventional sol-gel technique. Analysis via powder X-Ray diffraction (PXRD) studies and transmission electron microscope (TEM) images confirm the presence of crystalline phases of core/shell Co/CoO with average particle size ≈ 18 nm. Volume fraction (φ) is varied (from 20% to 1%) by the introduction of a stoichiometric amount of non-magnetic amorphous silica matrix (SiO2) which leads to a change in interparticle interaction (separation). The influence of exchange and dipolar interactions on the EB effect, caused by the variation in interparticle interaction (separation) is studied for a series of Co/CoO core/shell nanoparticle systems. Studies of thermal variation of magnetization (M−T) and magnetic hysteresis loops (M−H) for the series point towards strong dependence of magnetic properties on dipolar interaction in concentrated assemblies whereas individual nanoparticle response is dominant in isolated nanoparticle systems. The analysis of the EB effect reveals a monotonic increase of coercivity (HC) and EB field (HE) with increasing volume fraction. When the nanoparticles are close enough and the interparticle interaction is significant, collective behavior leads to an increase in the effective antiferromagnetic (AFM) CoO shell thickness which results in high HC and HE. Moreover, in concentrated assemblies, the dipolar field superposes to the local exchange field and enhances the EB effect contributing as an additional source of unidirectional anisotropy.

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“…As the size reduces to the nanometer, the magnetic system, nowadays called a nanomagnet, exhibits a new phenomenon known as superparamagnetism [5,6] with a change in the relevant temperature by an order of magnitude and in the relaxation time by several orders of magnitude. Furthermore, the large surface contribution in nanoscaled magnetic systems leads to inhomogeneous atomic-spin configurations [see [7][8][9][10][11] and references therein] and thereby to a drastically different behavior in response to external stimuli. For example, new modes of magnetization switching play a crucial role in many new spintronic applications [12].…”

Section: Introductionmentioning

confidence: 99%

Spatial magnetization profile in spherical nanomagnets with surface anisotropy: Green's function approach

Adams,

Michels,

Kachkachi

2023

Phys. Scr.

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We consider a single spherical nanomagnet and investigate the spatial magnetization profile $\mathbf{m}\left(\mathbf{r}\right)$ in thecontinuum approach, using the Green's function formalism. The energy of the (many-spin) nanomagnet comprises an isotropic exchange interaction, a uniaxial anisotropy in the core and N'eel's surface anisotropy, and an external magnetic field. We derive a semi-analytical expression for the magnetization vector field $\mathbf{m}\left(\mathbf{r}\right)$ for an arbitrary position $\mathbf{r}$ within and on the boundary of the nanomagnet, as a solution of a homogeneous Helmholtz equation with inhomogeneous Neumann boundary conditions. In the absence of core anisotropy, we use the solution of this boundary problem and infer approximate analytical expressions for the components $m_{\alpha},\alpha=x,y,z$, as a function of the radial distance $r$ and the direction solid angle. Then, we study the effects of the nanomagnet's size and surface anisotropy on the spatial behavior of the net magnetic moment.In the presence of a core anisotropy, an approximate analytical solution is only available for a position $\mathbf{r}$ located on the surface. This solution yields the maximum spin deviation as a result of the competition between the uniaxial core anisotropy and N'eel's surface anisotropy. Along with these (semi-)analytical calculations, we have solved a system of coupled Landau-Lifshitz equations written for the atomic spins, and compared the results with the Green's function approach.For a plausible comparison with experiments, \emph{e.g.} using the technique of small-angle magnetic neutron scattering, we have averaged over the direction solid angle and derived the spatial profile in terms of the distance $r$. We believe that the predictions of the present study could help to characterize and understand the effects of size and surface anisotropy on the magnetization configurations in nanomagnet assemblies such as arrays of well-spaced platelets.

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Single Nanomagnet Behaviour: Surface and Finite-Size Effects

Cited by 5 publications

References 113 publications

Magnetic Nanoparticles: from the Nanostructure to the Physical Properties

Magnetic Nanoparticles: from the Nanostructure to the Physical Properties

Dependence of Exchange Bias on Interparticle Interactions in Co/CoO Core/Shell Nanostructures

Spatial magnetization profile in spherical nanomagnets with surface anisotropy: Green's function approach

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