D. Baldomir

Learning from Nature to Improve the Heat Generation of Iron-Oxide Nanoparticles for Magnetic Hyperthermia Applications

The performance of magnetic nanoparticles is intimately entwined with their structure, mean size and magnetic anisotropy. Besides, ensembles offer a unique way of engineering the magnetic response by modifying the strength of the dipolar interactions between particles. Here we report on an experimental and theoretical analysis of magnetic hyperthermia, a rapidly developing technique in medical research and oncology. Experimentally, we demonstrate that single-domain cubic iron oxide particles resembling bacterial magnetosomes have superior magnetic heating efficiency compared to spherical particles of similar sizes. Monte Carlo simulations at the atomic level corroborate the larger anisotropy of the cubic particles in comparison with the spherical ones, thus evidencing the beneficial role of surface anisotropy in the improved heating power. Moreover we establish a quantitative link between the particle assembling, the interactions and the heating properties. This knowledge opens new perspectives for improved hyperthermia, an alternative to conventional cancer therapies.

Influence of dipolar interactions on hyperthermia properties of ferromagnetic particles

We show both experimental evidences and Monte Carlo modeling of the effects of interparticle dipolar interactions on the hysteresis losses. Results indicate that an increase in the intensity of dipolar interactions produce a decrease in the magnetic susceptibility and hysteresis losses, thus diminishing the hyperthermia output. These findings may have important clinical implications for cancer treatment.

Magnetocaloric effect in melt spun Ni50.3Mn35.5Sn14.4 ribbons

We determined the magnetic entropy change and refrigerant capacity of melt spun Ni50.3Mn35.5Sn14.4 ribbons around both the structural and the magnetic transitions for a field of 20kOe. The maximum entropy changes at the structural and magnetic transitions were of 4.1 and −1.1Jkg−1K−1. Ribbons studied show a larger refrigerant capacity around the magnetic transition (46Jkg−1) than around the structural transition (26Jkg−1), suggesting that the temperature range at the magnetic transition is more adequate for a refrigerant cycle than that at the structural transition.

Reduction of the bulk modulus at high pressure in CrN

Nitride coatings are increasingly demanded in the cutting- and machining-tool industry owing to their hardness, thermal stability and resistance to corrosion. These properties derive from strongly covalent bonds; understanding the bonding is a requirement for the design of superhard materials with improved capabilities. Here, we report a pressure-induced cubic-to-orthorhombic transition at approximately 1 GPa in CrN. High-pressure X-ray diffraction and ab initio calculations show an unexpected reduction of the bulk modulus, K0, of about 25% in the high-pressure (lower volume) phase. Our combined theoretical and experimental approach shows that this effect is the result of a large exchange striction due to the approach of the localized Cr:t3 electrons to becoming molecular-orbital electrons in Cr-Cr bonds. The softening of CrN under pressure is a manifestation of a strong competition between different types of chemical bond that are found at a crossover from a localized to a molecular-orbital electronic transition.

Thermal and magnetic field-induced martensite-austenite transition in Ni50.3Mn35.3Sn14.4 ribbons

Thermal and field-induced martensite-austenite transition was studied in melt spun Ni50.3Mn35.3Sn14.4 ribbons. Its distinct highly ordered columnarlike microstructure normal to ribbon plane allows the direct observation of critical fields at which field-induced and highly hysteretic reverse transformation starts (H=17kOe at 240K), and easy magnetization direction for austenite and martensite phases with respect to the rolling direction. Single phase L21 bcc austenite with TC of 313K transforms into a 7M orthorhombic martensite with thermal hysteresis of 21K and transformation temperatures of MS=226K, Mf=218K, AS=237K, and Af=244K.

Magnetocaloric effect in preferentially textured Mn50Ni40In10 melt spun ribbons

Inverse and direct magnetocaloric properties were evaluated in preferentially textured Mn50Ni40In10 ribbons applying the magnetic field H∥ along the ribbon length and perpendicular H⊥ to the ribbon plane (ΔH=30 kOe). Maximum magnetic entropy change, hysteretic losses, and refrigerant capacity were not significantly affected by crystallographic texture. Refrigeration capacity around structural transition is strongly reduced by the large hysteretic losses associated to the metamagnetic field-induced reverse martensitic transformation and narrower working temperature range making the interval around the magnetic transition more efficient for a refrigerant cycle (RCstruct=71 J kg−1 versus RCstructeff≈60 J kg−1, and RCmagn=89–86 J kg−1, for H∥ and H⊥, respectively).

Unexpected Magnetism of Small Silver Clusters

The ground-state electronic and magnetic properties of small silver clusters Ag{sub n} (2{<=}n{<=}22) have been studied using a linear combination of atomic Gaussian-type orbitals within the density functional theory. The results show that the silver atoms, which are diamagnetic in a bulk environment, can be magnetic when they are grouped together in clusters. The Ag{sub 13} cluster with icosahedral symmetry has the highest magnetic moment per atom among the silver clusters studied. The cluster symmetry and the reduced coordination number specific to small clusters are revealed as a fundamental factor for the onset of magnetism.

Structure of small silver clusters and static response to an external electric field

The static response properties and the structural stability of silver clusters in the size range

Structural and magnetic characterization of Co particles coated with Ag

1\le n \le 23

Model potential density functional study of small cobalt clusters, Con, n≤3

have been studied using a linear combination of atomic Gaussian-type orbitals within the density functional theory in the finite field approach. The Kohn-Sham equations have been solved in conjuction with a generalized gradient approximation (GGA) exchange-correlation functional. A proof that the finite basis set GGA calculation holds the Hellmann-Feynman theorem is also included in the Appendix. The calculated polarizabilities of silver clusters are compared with the experimental measurements and the jellium model in the spillout approximation. Despite the fact that the calculated polarizabilities are in good agreement with both of them, we have found that the polarizability appears to be strongly correlated to the cluster shape and the highest occupied-lowest unoccupied molecular-orbital gap.