A revolution in novel nanoparticles and colloidal building blocks has been enabled by recent breakthroughs in particle synthesis These new particles are poised to become the ‘atoms’ and ‘molecules’ of tomorrow’s materials if they can be successfully assembled into useful structures Here, we discuss the recent progress made in the synthesis of nanocrystals and colloidal particles and draw analogies between these new particulate building blocks and better-studied molecules and supramolecular objects We argue for a conceptual framework for these new building blocks based on anisotropy attributes and discuss the prognosis for future progress in exploiting anisotropy for materials design and assembly

/pdf/anisotropy-of-building-blocks-and-their-assembly-into-2quept9gqn.pdf

Anisotropy of building blocks and their assembly into complex structures

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

We present a detailed study of static and dynamic magnetic behavior of Fe3O4 nanoparticles with average particle sizes 〈d〉 ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, TV, shifts down to ∼20 K for 〈d〉=50 nm and is no longer observable for smaller particles. The smallest particles (〈d〉=5 nm) display superparamagnetic behavior at room temperature, with transition to a blocked state at TB∼45 K, which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization MS at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with 〈d〉=5 nm, dynamic ac susceptibility measurements show...

Static and dynamic magnetic properties of spherical magnetite nanoparticles

In the light of recent experimental as well as theoretical studies, we summarize our present understanding of polar oxide surfaces and examine fundamental issues regarding their stability. The focus is on the surface atomic configurations (relaxations, reconstructions, non-stoichiometry, etc) obtained under specific preparation conditions and their associated electronic structure. We discuss several mechanisms at work on polar surfaces, such as relaxation effects, change of covalency in the surface layers, partial filling of surface states, and stoichiometry variations, and try to assess their actual efficiency for cancelling the polarity.

https://iopscience.iop.org/article/10.1088/0953-8984/12/31/201/pdf

Polar oxide surfaces

Epitaxial growth and properties of thin film oxides

The blocking temperature T(B) has been determined as a function of the antiferromagnetic layer thickness in the Fe3O4/CoO exchange biased system. For CoO layers thinner than 50 A, T(B) is reduced below the Neel temperature T(N) of bulk CoO (291 K), independent of crystallographic orientation or film substrate ( alpha-Al2O3, SrTiO3, and MgO). Neutron diffraction studies show that T(B) does not track the CoO ordering temperature and, hence, that this reduction in T(B) does not arise from finite-size scaling. Instead, the ordering temperature of the CoO layers is enhanced above the bulk T(N) for layer thicknesses approximately less than or equal to 100 A due to the proximity of magnetic Fe3O4 layers.

Difference between blocking and Néel temperatures in the exchange biased Fe3O4/CoO system.

On the construction of an Fe3O4 based all-oxide spin-valve

The magnetic interlayer coupling between two magnetic layers separated by an insulator was investigated on ${\mathrm{Co}}_{0.2}$${\mathrm{Fe}}_{2.8}$${\mathrm{O}}_{4}$/${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$/MgO/${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ samples grown by means of molecular beam epitaxy on (001) ${\mathrm{MgAl}}_{2}$${\mathrm{O}}_{4}$ substrates. The samples were designed to observe interlayer coupling of either sign. Hysteresis loop measurements show that the ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ layers are ferromagnetically coupled in the thickness range 0--45 nm MgO. Below a MgO spacer thickness of 1.3 nm, the coupling strength increases drastically with decreasing MgO thickness and is ascribed to the existence of ferromagnetic bridges through the MgO spacer. The small ferromagnetic coupling above 1.3 nm seems to arise from a magnetostatic coupling due to correlated interface irregularities.

/pdf/interlayer-coupling-between-fe3o4-layers-separated-by-an-2fzh4wc2xs.pdf

Interlayer coupling between Fe3O4 layers separated by an insulating nonmagnetic MgO layer

The magnetic interlayer coupling of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ across NiO is studied using ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}/\mathrm{NiO}/{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ trilayers epitaxially grown on (001) MgO substrates. For NiO thicknesses between 0.7 and 5 nm, the magnetic moments of the two ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ layers are directed perpendicularly with respect to each other. The 90\ifmmode^\circ\else\textdegree\fi{} coupling strength is determined to be $0.35\ifmmode\pm\else\textpm\fi{}0.08\mathrm{mJ}/{\mathrm{m}}^{2}$ for a 1.4-nm-thick NiO spacer. The 90\ifmmode^\circ\else\textdegree\fi{} coupling can be understood from the effect of an antiferromagnetic spacer in the presence of interface roughness.

/pdf/evidence-for-roughness-driven-90-coupling-in-fe3o4-nio-fe3o4-4vkm561ums.pdf

JM Gaines

Papers

Difference between blocking and Néel temperatures in the exchange biased Fe3O4/CoO system.

On the construction of an Fe3O4 based all-oxide spin-valve

Interlayer coupling between Fe3O4 layers separated by an insulating nonmagnetic MgO layer

Evidence for roughness driven 90° coupling in Fe3O4/NiO/Fe3O4 trilayers

A ferromagnetic resonance study on ultra-thin Fe3O4 layers grown on (0 0 1)MgO