Session G14: Focus Session: Magnetic Nanoparticles II

Strong exchange coupling in conventional and inverse ferrimagnetic hard/soft and soft/hard core/shell heterostructured nanoparticles

Bi-magnetic core/shell nanoparticles are becoming increasingly appealing for diverse fields such as for permanent magnets, microawave absortion, biomedical applications, sensing applications, or future magnetic recording media. Ferrromagnetic (FM)/ antiferromagnetic (AFM) core/shell nanoparticles (or inverted AFM/FM) have been extensively studied. However, exchange coupled hard/soft, or inverse soft/hard, core/shell nanoparticles have been far less investigated. Interestingly, most bi-magnetic core/shell systems are derived by simple partial oxidation of the core, e.g., Co/CoO (FM/AFM) or FePt/Fe$_{3}$O$_{4}$ (hard/soft) and only few studies of heterostructured (where core and shell are formed by different magnetic ions) can be found in the literature. We have investigated conventional hard/soft and inverted soft/hard core/shell hetroestructured nanoparticles based on magnetically soft iron oxide (Fe$_{3}$O$_{4})$ and magnetically hard manganese oxide (Mn$_{3}$O$_{4})$. The core/shell samples were synthesized by seeded growth using either Fe$_{3}$O$_{4}$ or Mn$_{3}$O$_{\mathrm{4}}$ nanoparticles as seeds. Subsequently, thin layers of the complementary material were grown by thermal decomposition of the corresponding metallorganic precursors. The structure characterization (X-ray diffraction and electron diffraction) confirms the presence of cubic (Fe$_{3}$O$_{4})$ and tetragonal (Mn$_{3}$O$_{4})$ phases both at the bulk and local levels. In addition, high resolution transmission electron microscopy (HR-TEM) with electron energy loss spectroscopy (EELS) mapping confirms the core/shell structure of the nanopartciles. Magnetic characterization and element-selective hysteresis loops obtained by x-ray magnetic circular dichroism (XMCD) reveal a strong exchange coupling between the core and the shell which results in homogeneous loops with moderate coercivity. Moreover, the magnetic properties can be tuned by controlling the core diameter or shell thickness. However, the results depend only weakly on the hard/soft or inverse soft/hard morphology.   

Magnetic Reversal of Onion-Like Fe$_{3}$O$_{4}$\textbar MnO\textbar $\gamma $-Mn$_{2}$O$_{3}$ Core\textbar Shell\textbar Shell Nanoparticles

Magnetic nanoparticles offer potential for biomedical and data storage applications, especially with exchange bias to overcome the superparamagnetic limit. Here we study the role of an antiferromagnetic layer sandwiched between a soft ferrimagnetic core and hard ferrimagnetic shell. The nanoparticles studied consist of 3 nm (diameter) Fe$_{3}$O$_{\mathrm{4}}$ \textbar 50-60 nm thick MnO shell \textbar 5 nm thick $\gamma $-Mn$_{2}$O$_{3}$ shell [1]. Small-angle neutron scattering (SANS) probes both structural and magnetic morphology. SANS reveals that during reversal from 5 T to -5 T at 5 K, there is an increase in spins oriented perpendicular to the applied field. As the temperature is increased to 150 K (above the 123 K N\'{e}el temperature of MnO) evidence of an enhanced magnetism from within the MnO shell is observed. Finally, the scattering pattern shifts (indicating a change in the relative magnetism as a function of radius) between 5 K and 50 K. \\[4pt] [1] A. L\'{o}pez-Ortega \textit{et al}., Nanoscale 4, 5138 (2012); Salazar-Alvarez \textit{et al}., J. Am. Chem. Soc., 133, 16738 (2011)   

The Heisenberg Pentamer: Understanding the inelastic neutron scattering selection rules for magnetic clusters

Assuming Heisenberg interactions and the symmetric case of a spin S-S' pentamer, the energy eigenstates can be determined exactly. With the energies known, the inelastic neutron scattering intensities are then calculated for the special case of a 1-1/2 pentamer. Through an analysis of these results, two main insights are gained. (1) Because of symmetry constraints, not all $\Delta S_{tot}$ = $\pm$1 transitions are accessible by inelastic neutron scattering (INS). This constrains the standard selections rules for magnetic excitations. (2) The INS signatures of magnetic clusters are directly dependent on the state and component that is excited.   

Chemical attachment of magnetic nanoparticles through ``click chemistry''

Iron nanoparticles were used as a test system to explore the functionalization and attachment of magnetic nanoparticles with two different functionalities through ``click chemistry.'' Two different samples of iron nanoparticles were modified with 5-azidopentanoic acid and with 5-hexynoic acid, respectively. This modification was followed by click chemistry to change the morphology of agglomeration. A combination of density functional theory calculations, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy was used to monitor each step of the process. Spectroscopies confirmed the success and completion of click reaction. Scanning electron microscopy images showed the change in size and morphology of the iron nanoparticles before and after click chemistry. Vibrating sample magnetometer study showed the majority of the magnetic properties were retained following functionalization and click reaction. Exploring similar approach for two types of materials with functionalization and attachment of hard magnetic materials and soft magnetic materials will be presented based on our initial studies of SmCo nanoparticles in a combination with iron nanoparticles.   

Magnetic Quenching of Plasmon-photonic Activities in Fe$_3$O$_4$-Elastomer Composite

We report for the first time, a systematic study of polarization dependence and the effect of particle size on the optical response of Fe$_{3}$O$_{4}$-silicone elastomer composites in the presence of external magnetic field. The optical response of composites containing 2wt{\%}, 5wt{\%} and 15wt{\%} of 20nm$\le $d $\le $30nm, 40 nm$\le $d$\le $ 60nm and d$\le $ 500nm Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ particles were aligned in- and out-of-plane in the elastomer host. We observed a systematic redshift in the optical response of the out-of-plane composite samples (containing nanoparticles 20nm$\le $d$\le $30nm) with increasing static magnetic field strength, which saturated near 600 Gauss. There were no observable shifts in the in-plane samples, suggesting that the orientation (polarization) of the magnetic dipole and the induced electric dipole play a crucial role in the optical response. However, we observed a dramatic suppression to near quenching of the plasmonic activities in the micron size particles (d \textless\ 500nm) elastomer composite, suggesting particle size limitations in modulation of plasmon-photonics by external magnetic field. Dipole approximation model is used to explain the quenching phenomenon.   

Magnetic properties of Fe and Fe-Pt nanoparticles: application of nano-DFT$+$DMFT

We apply a combined density-functional theory and dynamical mean-field theory (DFT $+$ DMFT) approach [1] to handle reliably nanosized systems which display strong electron correlations. The code that we have recently developed allows one to examine systems containing several hundred atoms with feasible computational time. In particular, we calculate the magnetization of iron and iron-platinum nanoparticles by changing the system size (from 27 to 147 atoms), shape and composition. We demonstrate that the experimentally observed non-monotonous dependence of the magnetization as function of nanoparticle size can be rather accurately reproduced within DFT$+$DMFT, contrary to DFT and DFT$+$U approaches.\\[4pt] [1] V. Turkowski, A. Kabir, N. Nayyar and T.S. Rahman J. Phys.: Condens. Matter 22, 462202 (2010); J. Chem. Phys. 136, 114108 (2012).   

A comprehensive study of the structure and magnetic properties of Gd13 Cluster

Several experimental and theoretical studies of Gd13 cluster have led to confusing results. While experimental studies using Stern-Gerlach technique yield different magnetic moments, theoretical studies provide different spin orientations and structures. We have carried out a comprehensive study of the structure-magnetic property relationship of Gd13 cluster by examining different isomers. Our calculations are based on density functional theory with GGA$+$U and takes into account spin-orbit interactions and spin canting. The cluster with icosahedra structure and collinear spins has the lowest energy irrespective of the level of theory used. However, the magnetic coupling between the central and surface atoms does depend upon the value of U. For U$=$0 the magnetic coupling in the ground state structure is antiferromagnetic between the central and surface atoms. The coupling changes to ferromagnetic when U \textgreater 4. The effect of temperature on the observed magnetic moment is also studies using Monte Carlo simulation.   

Microwave absorption properties of BaGd$_{\mathrm{x}}$Fe$_{\mathrm{12-x}}$O$_{19}$ nanoparticles synthesized by wet milling process

It is a big demand to have a wide band, easy to synthesize microwave absorption materials with a high absorption ratio according to their weight. As a solution, nanoparticles are used for the couple of years because of their tunable frequencies by just changing their particle size. Most interesting nano structures for this objective are ferrites. In this work as a microwave absorber, BaFe12O19 and BaGd2Fe10O19 nanoparticles with different particles size are synthesized by the wet milling process. Their crystal structure analyzed by XRD, mean particle sizes were calculated from XRD patterns using rietveld analysis and from TEM images. Magnetic properties are analyzed by using Quantum design VSM. Microwave absorption properties are measured by using coaxial transmission method with an Agilent E5071 VNA. With the change of the last milling time from 0 to 20-hour crystalline sizes are changed from 48 nm to 13 nm. Decrease of particle size give rise to a decrease at coercivity and saturation magnetization of the samples. Change at the hysteresis loops gives a clue to the change of the microwave absorption frequency which is directly observed from the microwave measurements.   

Investigation of Local Structures and Magnetism in (Y, Co) codoped CeO2 Nanoparticles

Nanocrystals of (Y, Co) codoped CeO2 with different Y concentration prepared by a Polyol method were studied by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), Raman spectroscopy, x-ray absorption fine structures (XAFS), and superconducting quantum interference device (SQUID) techniques to monitor the structural and magnetic variations of the samples. As revealed by the XRD data, all nanocrystal samples under investigation have similar average particle size. The concentration of O vacancies in the samples was found to increase with Y doping level as indicated by the Raman spectroscopy and XAFS data. Such increase of O vacancies is also accompanied by enhanced ferromagnetism as observed by SQUID measurements. Our experimental results demonstrate clear correlation between magnetism and O vacancies induced by Y doping and therefore are consistent with the bound magnetic polaron model.   

Ferrofluid based micro-electrical energy harvesting

Innovations in energy harvesting have seen a quantum leap in the last decade. With the introduction of low energy devices in the market, micro energy harvesting units are being explored with much vigor. One of the recent areas of micro energy scavenging is the exploitation of existing vibrational energy and the use of various mechanical motions for the same, useful for low power consumption devices. Ferrofluids are liquids containing magnetic materials having nano-scale permanent magnetic dipoles. The present work explores the possibility of the use of this property for generation of electricity. Since the power generation is through a liquid material, it can take any shape as well as response to small acceleration levels. In this work, an electromagnet-based micropower generator is proposed to utilize the sloshing of the ferrofluid within a controlled chamber which moves to different low frequencies. As compared to permanent magnet units researched previously, ferrofluids can be placed in the smallest of containers of different shapes, thereby giving an output in response to the slightest change in motion. Mechanical motion from 1- 20 Hz was able to give an output voltage in mV's. In this paper, the efficiency and feasibility of such a system is demonstrated.