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

Exchange bias in nanostructures

Platinum-based nanostructured materials: synthesis, properties, and applications.

Due to finite size effects, such as the high surface-to-volume ratio and different crystal structures, magnetic nanoparticles are found to exhibit interesting and considerably different magnetic properties than those found in their corresponding bulk materials. These nanoparticles can be synthesized in several ways (e.g., chemical and physical) with controllable sizes enabling their comparison to biological organisms from cells (10–100 μm), viruses, genes, down to proteins (3–50 nm). The optimization of the nanoparticles’ size, size distribution, agglomeration, coating, and shapes along with their unique magnetic properties prompted the application of nanoparticles of this type in diverse fields. Biomedicine is one of these fields where intensive research is currently being conducted. In this review, we will discuss the magnetic properties of nanoparticles which are directly related to their applications in biomedicine. We will focus mainly on surface effects and ferrite nanoparticles, and on one diagnostic application of magnetic nanoparticles as magnetic resonance imaging contrast agents.

https://www.mdpi.com/1422-0067/14/11/21266/pdf

Magnetic Nanoparticles: Surface Effects and Properties Related to Biomedicine Applications

Core-shell magnetic nanoparticles have received significant attention recently and are actively investigated owing to their large potential for a variety of applications. Here, the synthesis and characterization of bimetallic nanoparticles containing a magnetic core and a gold shell are discussed. The gold shell facilitates, for example, the conjugation of thiolated biological molecules to the surface of the nanoparticles. The composite nanoparticles were produced by the reduction of a gold salt on the surface of pre-formed cobalt or magnetite nanoparticles. The synthesized nanoparticles were characterized using ultraviolet-visible absorption spectroscopy, transmission electron microscopy, energy dispersion X-ray spectroscopy, X-ray diffraction and super-conducting quantum interference device magnetometry. The spectrographic data revealed the simultaneous presence of cobalt and gold in 5.6 ± 0.8 nm alloy nanoparticles, and demonstrated the presence of distinct magnetite and gold phases in 9.2 ± 1.3 nm core-shell magnetic nanoparticles. The cobalt-gold nanoparticles were of similar size to the cobalt seed, while the magnetite-gold nanoparticles were significantly larger than the magnetic seeds, indicating that different processes are responsible for the addition of the gold shell. The effect on the magnetic properties by adding a layer of gold to the cobalt and magnetite nanoparticles was studied. The functionalization of the magnetic nanoparticles is demonstrated through the conjugation of thiolated DNA to the gold shell.

Synthesis of core-shell gold coated magnetic nanoparticles and their interaction with thiolated DNA

We report the observation of exchange bias phenomena in the hole-doped perovskite cobaltites ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ ($x=0.12$, 0.15, 0.18, and 0.30) in which a spontaneous phase separation occurs. When the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ samples are cooled in a static magnetic field through a freezing temperature, the magnetization hysteresis loops exhibit both horizontal and vertical shifts. We also observed training effect of the exchange bias, which can be interpreted by a spin configurational relaxation model. Moreover, exchange bias in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ is strongly dependent on the measuring field and the cooling field due to the influence of magnetic field on the relative proportion of the coexisting phases. These results suggest that the intrinsic phase inhomogeneity in a spontaneously phase-separated system may induce an interfacial exchange anisotropy.

Exchange bias associated with phase separation in the perovskite cobaltite La 1 − x Sr x CoO 3

We present a study of the magnetic properties of oxidized Co nanoparticles with an average grain size of 3nm, embedded in an amorphous Al2O3 matrix. These nanoparticles can be considered as imperfect Co-core CoO-shell systems. Magnetization measurements after magnetic field cooling show a vertical shift of the hysteresis loop, while no exchange bias is observed. With a simple model, we show that there is a critical grain size for hybrid ferromagnetic-antiferromagnetic particles, below which exchange bias is absent for any ratio of ferromagnetic and antiferromagnetic constituents. The reason is that the interfacial exchange energy dominates over other energies in the system due to a large surface-to-volume ratio in the nanoparticles.

/pdf/critical-size-for-exchange-bias-in-ferromagnetic-23q7eagwh6.pdf

Critical size for exchange bias in ferromagnetic-antiferromagnetic particles

We present a study on the production of iron-platinum bimetallic clusters with a laser vaporization cluster source, and subsequent low-energy cluster beam deposition in ultrahigh-vacuum conditions. Time-of-flight mass spectrometry investigations of the clusters in the gas phase show that FePt is a stable alloy at the nanoscale. Magnetic and structural properties of the deposited $2.25\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ cluster-assembled films with different Fe:Pt ratios are investigated with vibrating sample and superconducting quantum interference device magnetometry, Rutherford backscattering spectrometry, transmission electron microscopy, and x-ray diffraction. The as-deposited cluster films are highly porous and magnetically soft. A systematic study of the influence of annealing on the sample properties revealed that the chemically disordered-ordered phase transition only occurs in the cluster films with Fe:Pt ratio close to 1:1, and at temperatures higher than $500\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Combining magnetization and structural investigations we distinguish between the phase transition inside a single cluster and the coalescence of clusters, which starts to dominate at temperatures above $550\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, leading to complete cluster intermixing on the atomic level.

Atomic-scale modification of hybrid FePt cluster-assembled films

Ensembles of nanometer-sized Au, Co, Er, and FePt clusters were generated by a laser vaporization source and embedded in a MgO matrix grown on a mica substrate. The size distribution of the clusters was measured before sample preparation by time-of-flight mass spectrometry and afterwards by transmission electron microscopy. These well-characterized samples were used for investigations by small-angle X-ray scattering (SAXS). The scattering data was evaluated by Guinier analysis providing the average radii of the embedded clusters. The results are in good agreement with the cluster sizes obtained from the mass spectra as well as the dimensions determined from the transmission electron micrographs. Furthermore, other samples which were produced at an elevated substrate temperature of 500 °C exhibited an increased average cluster radius in the SAXS measurements. This behavior is attributed to diffusion and coalescence emerging at higher deposition temperatures.

Measurement of the size of embedded metal clusters by mass spectrometry, transmission electron microscopy, and small-angle X-ray scattering

We compare the magnetic properties of Co cluster assembled films with different degrees of oxidation. Clusters with grain size $(2.3\ifmmode\pm\else\textpm\fi{}0.7)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ are produced in a laser vaporization cluster source and soft-landed in ultrahigh vacuum conditions, forming highly porous nanogranular films. After exposure to air for different periods of time, the Co clusters oxidize and the sample may be considered as a thin antiferromagnetic Co oxide matrix containing ferromagnetic Co clusters. Magnetization measurements were performed in a temperature range from 300 down to $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, at applied magnetic fields up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. The exchange bias value at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the strongly oxidized sample is $4.8\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ against the value of $0.75\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ for the less oxidized sample. The mean values of the thicknesses of the Co oxide layers are estimated to be 0.6 and $0.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the more and less oxidized sample, respectively. We propose a method of measuring the exchange bias inducing temperature, i.e., the temperature at which exchange anisotropy is established. We determined the mean inducing temperatures for both samples, which are 55 and $25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, for the more and less oxidized samples. Both temperatures are well below the bulk CoO N\'eel temperature of $292\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A low value of the inducing temperature of the Co oxide layer is a consequence of its subnanometer thickness, while a large exchange bias value is a consequence of different dimensionality of Co clusters and Co oxide matrix.

Influence of finite size effects on exchange anisotropy in oxidized Co nanocluster assembled films

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Papers

Critical size for exchange bias in ferromagnetic-antiferromagnetic particles

Atomic-scale modification of hybrid FePt cluster-assembled films

Measurement of the size of embedded metal clusters by mass spectrometry, transmission electron microscopy, and small-angle X-ray scattering

Influence of finite size effects on exchange anisotropy in oxidized Co nanocluster assembled films