This review is focused on describing state-of-the-art synthetic routes for the preparation of magnetic nanoparticles useful for biomedical applications. In addition to this topic, we have also described in some detail some of the possible applications of magnetic nanoparticles in the field of biomedicine with special emphasis on showing the benefits of using nanoparticles. Finally, we have addressed some relevant findings on the importance of having well-defined synthetic routes to produce materials not only with similar physical features but also with similar crystallochemical characteristics.

https://iopscience.iop.org/article/10.1088/0022-3727/36/13/202/pdf

The preparation of magnetic nanoparticles for applications in biomedicine

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

Exchange bias in nanostructures

Simple rules for the understanding of Heusler compounds

The last glacial cycle was characterized by substantial millennial-scale climate fluctuations1,2,3,4,5, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces6,7, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution8 or higher1. Records based on oxygen isotopes, however, contain uncertainties in the range of ±30 m, or ±1 °C in deep sea temperature9,10. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth—and hence global sea level—over the past 470,000 years (470 kyr). Our reconstruction is accurate to within ±12 m, and gives a centennial-scale resolution from 70 to 25 kyr before present. We find that sea-level changes of up to 35 m, at rates of up to 2 cm yr-1, occurred, coincident with abrupt changes in climate.

Sea-level fluctuations during the last glacial cycle

Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called 'superparamagnetic limit' in recording media. Here we show that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems can provide an extra source of anisotropy, leading to magnetization stability. We demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.

Beating the superparamagnetic limit with exchange bias

Nickel ferrite nanoparticles exhibit anomalous magnetic properties at low temperatures: low magnetization with a large differential susceptibility at high fields, hysteresis loops which are open up to 160 kOe, time-dependent magnetization in 70 kOe applied field, and shifted hysteresis loops after field cooling. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin-glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. Implications of this model for possible macroscopic quantum tunneling in these materials are discussed.

Surface Spin Disorder in NiFe2O4 Nanoparticles.

Antiferromagnetic NiO nanoparticles exhibit anomalous magnetic properties, such as large moments, and coercivities and loop shifts of up to 10 kOe. This behavior is difficult to understand in terms of 2-sublattice antiferromagnetic ordering which is accepted for bulk NiO. Numerical modeling of spin configurations in these nanoparticles yields 8-, 6-, or 4-sublattice configurations, indicating a new finite size effect, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the particle. The relatively weak coupling between the sublattices allows a variety of reversal paths for the spins upon cycling the applied field, resulting in large coercivities and loop shifts.

Finite Size Effects in Antiferromagnetic NiO Nanoparticles

We present a method for atomic-scale modeling of the magnetic behavior of ionic magnetic solids. Spin distributions and net magnetic moments are calculated for nanoparticles of ferrimagnetic ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ and $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3},$ and antiferromagnetic NiO as a function of applied field. Calculations incorporate crystal structures and exchange parameters determined from bulk data, bulk anisotropy for core spins, reasonable estimates for the anisotropy of surface spins, and finite temperatures simulated by random perturbations of spins. Surface spin disorder was found in the case of ferrimagnetic spinel nanoparticles, due to broken exchange bonds at the surface. The calculations also demonstrate that surface anisotropy enhances the coercivity of such particles only when surface spin disorder is present. Simulated thermal perturbations were used to characterize the distribution of energy barriers between surface spin states of such particles. The distribution of barriers can explain the macroscopic quantum tunneling like magnetic relaxation at low temperatures found experimentally. Calculations on NiO nanoparticles predict eight, six, or four-sublattice spin configurations in contrast to the two-sublattice configuration accepted for bulk NiO. Relatively weak coupling between the multiple sublattices allows a variety of reversal paths for the spins upon cycling the applied field, resulting in large coercivities and loop shifts, in qualitative agreement with experiment.

Atomic-scale magnetic modeling of oxide nanoparticles

The uncompensated spins on the surfaces of antiferromagnetic CoO films exhibit a thermoremanent magnetization after field cooling from ${T}_{N}$ that has the same temperature dependence as the exchange field of $\mathrm{Ni}{}_{81}\mathrm{Fe}{}_{19}/\mathrm{Co}\mathrm{O}$ bilayers after field cooling. This suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on interfacial grain size.

Interfacial Uncompensated Antiferromagnetic Spins: Role in Unidirectional Anisotropy in Polycrystalline Ni 81 Fe 19 / CoO Bilayers

Anomalous magnetic properties of organic coated NiFe2O4 nanoparticles have been reported previously (Berkowitz et al.).5 These properties included low magnetization with a large differential susceptibility at high fields and shifted hysteresis loops after field cooling, while Mossbauer spectra indicated that all of the material was magnetically ordered. In the present study, we find that the lack of saturation in high fields is accompanied by irreversibility (i.e., hysteresis loops are open) up to 160 kOe. In addition, the particles exhibit time dependent magnetization in 70 kOe applied field. The high field irreversibility and the loop shift both vanish above 50 K. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin- glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. The implications of this model for possible macroscopic quantum tunneling in these materials are discussed.

R. H. Kodama

Papers

Surface Spin Disorder in NiFe2O4 Nanoparticles.

Finite Size Effects in Antiferromagnetic NiO Nanoparticles

Atomic-scale magnetic modeling of oxide nanoparticles

Interfacial Uncompensated Antiferromagnetic Spins: Role in Unidirectional Anisotropy in Polycrystalline Ni 81 Fe 19 / CoO Bilayers

Surface spin disorder in ferrite nanoparticles (invited)