Magnetic surface anisotropy of cobalt and surface roughness effects within Neel's model

A Perspective on Modelling Metallic Magnetic Nanoparticles in Biomedicine: From Monometals to Nanoalloys and Ligand-Protected Particles

The focus of this review is on the physical and magnetic properties that are related to the efficiency of monometallic magnetic nanoparticles used in biomedical applications, such as magnetic resonance imaging (MRI) or magnetic nanoparticle hyperthermia, and how to model these by theoretical methods, where the discussion is based on the example of cobalt nanoparticles. Different simulation systems (cluster, extended slab, and nanoparticle models) are critically appraised for their efficacy in the determination of reactivity, magnetic behaviour, and ligand-induced modifications of relevant properties. Simulations of the effects of nanoscale alloying with other metallic phases are also briefly reviewed.

The role of electron confinement in Pd films for the oscillatory magnetic anisotropy in an adjacent Co layer

We demonstrate the interplay between quantum well states in Pd and the magnetic anisotropy in Pd/Co/Cu (0 0 1) by combined scanning tunneling spectroscopy (STS) and magneto optical Kerr effect (MOKE) measurements. Low temperature scanning tunneling spectroscopy reveals occupied and unoccupied quantum well states (QWS) in atomically flat Pd films on Co/Cu (0 0 1). These states give rise to sharp peaks in the differential conductance spectra. A quantitative analysis of the spectra reveals the electronic dispersion of the Pd (0 0 1) d-band ([Formula: see text]-type) along the [Formula: see text]-X direction. In situ MOKE experiments on Pd/Co/Cu (1, 1, 13) uncover a periodic variation of the in-plane uniaxial magnetic anisotropy as a function of Pd thickness with a period of 6 atomic layers Pd. STS shows that QWS in Pd cross the Fermi level with the same periodicity of 6 atomic layers. Backed by previous theoretical work we ascribe the variation of the magnetic anisotropy in Co to QWS in the Pd overlayer. Our results suggest a novel venue towards tailoring uniaxial magnetic anisotropy of ferromagnetic films by exploiting QWS in an adjacent material with large spin-orbit coupling.

Impact of buffer layer and Pt thickness on the interface structure and magnetic properties in (Co/Pt) multilayers

The influence of Pt thickness on the interface structure (roughness / intermixing) and magnetic properties has been investigated for (Co / Pt) multilayers sputtered on a Pt or a thin oxide (MgO or AlO x ) buffer layer. When Pt thickness increases from 1.2 nm-2.2 nm, we observe that the effective anisotropy increases with the Pt thickness, simultaneously with the decrease of roughness, i.e. the occurrence of sharper interfaces. Perpendicular magnetic anisotropy (PMA) is still achieved on the oxide buffer layers, but with a lower effective anisotropy correlated to more perturbed interfaces. The detailed analysis of the saturation magnetization shows that: (i) M s is significantly enhanced in the case of rough/intermixed interfaces, which is attributed to and discussed in the framework of Pt induced polarization, (ii) the change in volume dipolar anisotropy is the main factor responsible for the reduction of K eff for systems grown on oxides. Beyond the major role of volume dipolar contribution that reduces PMA, a supplemental positive contribution promoting PMA can be invoked for rough interfaces and large M s (deposit on oxide). This contribution is consistent with a dipolar surface anisotropy term and increases for rough interfaces, in contrast to the Néel surface anisotropy. These opposite variations may interestingly lead to an enhanced anisotropy in (Co / Pt) stackings grown on oxides compared to systems deposited on Pt, i.e. with sharper interfaces.

Proximity-Driven Enhanced Magnetic Order at Ferromagnetic-Insulator-Magnetic-Topological-Insulator Interface

Magnetic exchange driven proximity effect at a magnetic-insulator-topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. Here we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS/Sb(2-x)V(x)Te3 hybrid heterostructure, where V doping is used to drive the TI (Sb2Te3) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

Magnetic reversal in Mn5Ge3 thin films: an extensive study

We present a comprehensive study of magnetization reversal process in thin films of Mn5Ge3. For this investigation, we have studied the magnetic anisotropy of Mn5Ge3 layers as a function of the film thickness using VSM and SQUID magnetometers. The samples grown by molecular beam epitaxy exhibit a reorientational transition of the easy axis of magnetization from in-plane to out-of-plane as the film thickness increases. We provide evidence that above a critical thickness estimated as 20 nm, the magnetic structure is most probably constituted of stripes with out-of-plane magnetization pointing alternately up and down. We have analyzed our results using different phenomenological models and all the calculations converge towards values for magnetocrystalline anisotropy constant and saturation magnetization that are in excellent agreement with the reported values for bulk Mn5Ge3. This study has also led to the first estimation in Mn5Ge3 of the exchange constant, the surface energy of domain walls as well as their width. These parameters are essential for determining whether this material can be used in the next generation of spintronic devices.

Magnetic and Transport Behavior of Af-Coupled Layers with a Limited Number of Repetitions

We investigated the magnetization behavior of [(Co/Pd) 4-Ru]x samples with perpendicular anisotropy and a limited number of repetitions (x = 1,‥,22). In these systems the Co/Pd Multilayers behave as single magnetic entities. A detailed analysis and comparison of the magnetization curves observed by MOKE and VSM permits us to observe the magnetization reversal and hysteresis of the individual layers and to determine the antiferromagnetic coupling J between each pair of layers. A gradual increase in J is observed in all samples when going from the bottom layer to the top layer. Magnetoresistance curves show the same sharp transitions as the magnetization curves. A clear distinction can be made between an outer layer and an inner layer. These results will be compared with model calculations.

Anisotropy Energies and Critical Behavior of Ultrathin Ni(111) Films Grown on Smooth and Rough W(110) (invited)

6 to 80 Å thin Ni(111) films were prepared on smooth and rough W(110) substrates in UHV and characterized by LEED and Auger spectroscopies. The measurements of the magnetic properties were carried out in situ by ferromagnetic resonance at 9 GHz between 300 and 600 K. We found that the effective anisotropies, which consist of surface, crystal, and stress induced anisotropy, increase with decreasing film thickness and temperature. The roughness of the substrate results in the drastic decrease of the effective anisotropy. This is attributed to the change of the surface structure and the stress within the Ni films. Furthermore we found that the Curie temperature Tc and the critical exponent β of Ni films on the smooth and rough substrates show no change.

Magnetic Domain Imaging of Epitaxial thin Films by Spin-Polarized Scanning Electron Microscopy

The formation of magnetic domains in thin epitaxial Co/Au(111) films is investigated by spin-polarized scanning electron microscopy. Three-monolayer films are shown to decay into out-of-plane domains of micrometer size. The transition from out-of-plane to in-plane magnetization at a crossover thickness of 4.5 layers is followed by imaging the domains, and the transition is shown to occur as a continuous rotation of the magnetization. The domain size in field-free-grown perpendicular films depends linearly on film thickness. From high-resolution line scans across magnetization reversals we determine the resolution in magnetic imaging to be better than 40 nm.

Dominant role of the epitaxial strain in the magnetism of core-shell Co/Au self-organized nanodots

Self-organized Co nanodots on a Au(111) surface have been surrounded by controlled Au rings that progressively cap the entire dots. The magnetic susceptibility of these dots has been measured in situ as a function of the Au coverage. The blocking temperature increases when the Co bilayer dots are surrounded by the first Au atomic layer and decreases with the subsequent capping. This result cannot be explained by interfacial anisotropy which is generally assumed to be the dominant term in the magnetic anisotropy of nanostructures. Using molecular dynamics simulations, we evidence that the large strain inside the Co clusters is the main driving force for the anisotropy changes during the Au encapsulation.

The relationship between interface structure, conformality and perpendicular anisotropy in CoPd multilayers

The relationship between the interface structure and perpendicular anisotropy in sputtered Co/Pd multilayers has been investigated using grazing incidence x-ray scattering and vibrating sample magnetometry. Using fits to a self-affine fractal model of the interfaces, the variation in in-plane correlation length, fractal parameter and conformality has been determined as a function of the number of repeats in the Co/Pd bilayers. As the number of interfaces rises, the roughness becomes predominantly non-conformal and the in-plane length scale associated with the roughness increases as a power law with multilayer thickness. It is suggested that the loss of conformality, characterized by a relatively short out-of-plane correlation length, may be the cause of the reduction in anisotropy energy per interface observed for high numbers of bilayer repeats. There is a weak association between fractal parameter and interface anisotropy; a reduction in the fractal dimension of the interface appears to result in a higher surface anisotropy.

Temperature dependence of the surface anisotropy of Fe ultrathin films on Cu(001)

We report an experimental approach to separate temperature dependent reversible and irreversible contributions to the perpendicular magnetic anisotropy of Fe films grown at low temperatures on Cu(001) substrates. The surface anisotropy K(S)(T) is found to decrease linearly with temperature, causing a thermally induced spin reorientation into the plane. The irreversible shift of the spin reorientation transition and the coercivity of the iron films are directly correlated to the increasing Fe island size during annealing. The increased coercivity is discussed in terms of domain wall energy inhomogeneities provided by the islands.