Session E50: Magnetic Nanoparticles and Nanostructures

Spin correlations in nanostructured high-anisotropy magnetic materials.

The discovery and design of new complex materials with significant physical properties is a central issue in condensed matter and materials physics. This is especially true in the case of magnetic materials because of their important applications in energy and information-processing systems. Two parallel developments recently have provided impetus for progress, viz., computational studies predicting the structures and properties of complex ordered and disordered structures, and synthesis of such structures by non-equilibrium methods. This talk will focus on recent research directed towards the discovery of new materials with high magnetic anisotropy and high magnetization. Nanoscale and nanostructured magnets including spin correlations and exchange interactions will be discussed, with examples taken from our recent work on new phases and structures such as Co$_{\mathrm{2}}$Si[1], Mn$_{\mathrm{5}}$Si$_{\mathrm{3}}$[2], Co$_{\mathrm{3}}$Si[3], HfCo$_{\mathrm{1}}$ and Zr$_{\mathrm{2}}$Co$_{\mathrm{11}}$[4], and Fe$_{\mathrm{3+x}}$Co$_{\mathrm{3-x}}$X$_{\mathrm{2}}$ (X $=$ Ti, Nb) [5]. This research was done in collaboration with R. Skomski, B.~Balasubramanian, W. Zhang, B. Das, Y. Jin, X. Xu, P. Manchanda G. Hadjipanayis. [1] B. Balasubramanian et al., Appl. Phys. Lett. 106, 242401 (2015). [2] B. Das et al., Nano Lett. 16, 1132 (2016). [3] B. Balasubramanian et al., Appl. Phys. Lett. 108, 152406 (2016). [4] X. Zhao et al., Phys. Rev. Lett. 112, 045502 (2014); B. Balasubramanian et al., J. Phys. Cond. Matt. 26, 064204 (2014). [5] J. Zhang et al., J. Phys. D. 49, 175002 (2016); APL Mater. (in press); J. Phys. D. (in press).   

Magnetic Response of Cobalt--Carbide Nanoparticles with large Magnetocrystalline Anisotropy.

Recent experiments showed that Co$_{\mathrm{3}}$C particles \textasciitilde 8 nm diameter can exhibit stable long range ferromagnetic order up to 570 K. First principle theoretical investigation showed that the separation between the cobalt layers induced by the carbon atoms is responsible for large magnetocrystalline anisotropy energy, which gives Co$_{\mathrm{3}}$C this unique rare earth permanent magnet like characteristics [1]. In this work, we build the Hamiltonian for this system, theoretically study the evolution in its hysteretic magnetic response and compare these predictions against experimental magnetic behavior at various temperatures. [1] Ahmed A. El-Gendy et al., Appl. Phys. Lett. 104, 023111, 2014. Acknowledgement: S.N.K. acknowledges support from U.S. Department of Energy (DOE) through Grant No. DE-FG02-11ER16213.   

Anisotropy of Zr-Co and Hf-Co nanoclusters using the evolutionary algorithm

Nanostructures of Hf-Co and Zr-Co rare earth free magnetic material that exhibit a high room-temperature energy product. In our study, the evolutionary algorithm coupled with density functional (DFT) method is used to identify the global energy minimum atomic structure of Zr-Co and Hf-Co clusters. Using evolutionary crystal structure optimization algorithm, as implemented in USPEX, we studied the atomic structure, binding energies, magnetic properties, and anisotropy of ZrxCoy and HfxCoy (x$=$1,2 and y$=$5,7,11) clusters. A set of metastable and global minimum atomic structures are identified. Several new lower energy configurations were identified for Zr2Co11, Zr1Co5, Zr1Co7, Hf2Co11, Hf1Co5 and Hf1Co7clusters by our calculations. We discussed the magnetic interaction between the atoms of the clusters which is critical in finding the lowest energy structure. Our calculation show that Zr-Co and Hf-Co have ferromagnetic coupling and large magnetization. We will also discuss the magnetocrystalline anisotropy (MAE) variation in these clusters.   

Effect of nitrogen substitution on the magnetic property of cobalt clusters

We investigate the effect of nitrogen substitution on the magnetic property of cobalt clusters using a real-space formalism of pseudopotentials within the density-functional theory, i.e., the PARSEC code. We find that a few atomic percent of N substitution can enhance the net magnetic moment in comparison to that of pure Co cluster. An analysis of the spatial distribution of the local magnetic moment reveals the importance of a N-substitution site in Co cluster. By using the PARSEC code in a non-collinear magnetic mode, which enables us to explore various magnetic structures, the effect of N substitution on the magnetic anisotropy in Co cluster will also be discussed.   

Magneto optical Kerr effect study of close packed arrays of cobalt nanostructure

Nanopatterned cobalt thin films were fabricated on silicon substrate using nanosphere lithography. Nanospheres of different diameters, 200 nm, 600 nm, and 900 nm, were used to pattern the surface of the cobalt thin films. The surface morphology of thin films were characterized using high powered optical microscope, scanning electron microscope and the atomic force microscope. The nanopatterned thin film is highly uniform at macroscopic scale while several distinct nanostructure morphologies are observed at microscopic scale. Magnetic switching behavior was investigated by the magneto optical Kerr effect (MOKE) measurement. Modification to the surface of cobalt thin film was found to extensively change its magnetization behavior due to influence of the shape anisotropy and magnetic domain wall. Coercivity increased in nanopatterned cobalt thin film as compared to the continuous thin film. Nanostructure sample using nanospheres template less than 600 nm was additionally found to show a various features in the MOKE hysteresis loops including the multi-steps, multi-loops and shearing features. The multi-steps and multi-loops behaviors are more prominent as the size of nanosphere is reduced.   

Tracking the Magnetization Evolution in $\gamma$-Fe$_2$O$_3$ / Metallic Fe Core-Shell Nanoparticle Variants

Iron-core magnetic nanoparticles (MNPs) with oxide shells exhibit varying magnetic properties due to the different ordering temperatures of the core and shell spins, as well as the coupling across the metal/oxide interface. While spin coupling across two dimensional interfaces has been well explored, less is known about three dimensional interfaces such as those presented in the MNPs. In this work, MNPs were synthesized with a bcc Fe core and $\gamma$-Fe$_2$O$_3$ shell and placed in an oxygen rich environment to encourage the transition from cores shell (CS) to core void shell (CVS) to hollow (H) structures. Static magnetic measurements (MvT) and AC magnetometry were performed to explore the magnetic behavior of the various synthesized structures. To further understand the nature of the spin coupling in the MNPs, TEM and conventional magnetometry as well as variable-temperature small angle neutron scattering (SANS), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy were performed. Modeling of the x-ray spectra and SANS data will enable us to develop a cohesive picture of spin coupling, freezing and frustration along the three-dimensional metal / oxide interface.   

The effects of intraparticle and interparticle interactions on the magnetic hysteresis loop of frozen suspensions of bionized nanoferrite particles

Bionized nano-ferrite (iron oxide/dextran) nanoparticles have been shown to have a large heating response in an alternating magnetic field, making them very promising for applications in magnetic nanoparticle hyperthermia cancer treatment. Magnetic hysteresis loop measurements of these particles provide insight into the magnetic reversal behavior of these particles, and thus their heating response. Measurements have been performed on frozen suspensions of nanoparticles dispersed in H$_2$O, which have been frozen in a range of applied fields in order to tune the interparticle dipolar interactions through formation of linear chains. These experimental results are compared with micromagnetic models of both monolithic (single-domain) and internally structured (multi-grain) particles. It is found that the internal structure of the nanoparticles, which are made up of parallelepiped-shaped grains, is important for describing the magnetic reversal behavior of the particles and the resulting shape of the hysteresis loops. In addition to this, interparticle interactions between particles in a linear chain modify the reversal behavior and thus the shape of the hysteresis loop.   

Fe3O4 Nano-clusters for Ultrafast Hyperthermia

In this work, we studied the hyperthermia behavior of chemically synthesized Fe3O4 nano-clusters (NCs). The PVP concentration was found to significantly affect the shape, size and magnetic properties of the NCs. TEM images show that the NCs have a mean diameter of $\le $100nm and consist of isolated Fe3O4 nanoparticles with a mean diameter of 6-8nm. Magnetic measurements show that the room-temperature saturation magnetization (Ms) value increases from 20.2 to 61.7emu/g with increase in PVP concentration from 0.5g to 4g. To increase the water solubility and stability of the NCs, the as-made materials were first functionalized by PVP and then by citrate ions. HR-TEM images suggest a flower-like morphology with 30-40nm mean diameter. Magnetic measurements revealed that these NCs are superparamagnetic with Ms of 39emu/g and 58emu/g for the PVP and citrate coated materials, respectively. Hyperthermia measurements on NCs in water solution (10mg/ml), with a 10kA/m applied field at 150kHz frequency, showed a temperature increase from 293 to 338K after only 79s of field exposure with a high rate of 0.57C/s for the citrate functionalized particles and a rate of 0.31C/s for the PVP coated particles.   

Unusual Magnetic Spin Arrangements in Manganese Ferrite Nanoparticle Assemblies

Magnetic nanoparticles are of interest for many applications, yet the relevant magnetic structures are often difficult to predict \textit{a priori} or even measure directly. In previous work, polarization analyzed small-angle neutron scattering (PASANS) measurements revealed an unusual core-shell magnetic structure for Fe$_3$O$_4$ nanoparticles governed primarily by the competition of exchange vs. Zeeman energy [1]. For CoFe$_2$O$_4$, the substantially larger anisotropy led to a uniform magnetic structure within each nanoparticle with sizeable canting in an applied magnetic field [2]. Here, we report new PASANS results investigating dense assemblies of 7.5 nm diameter manganese ferrite nanoparticles. For this system, we see a significant component of magnetic scattering perpendicular to the applied magnetic field, indicating important intra and now \textit{inter-particle} effects. These results are interpreted considering the expected weaker exchange and anisotropy for MnFe$_2$O$_4$ relative to Fe$_3$O$_4$ and CoFe$_2$O$_4$, respectively and the internal structure of the individual nanoparticles. [1] K.L. Krycka, et al., \textit{Phys. Rev. Lett.} \textbf{113}, 147203 (2014). [2] K. Hasz et al., \textit{Phys. Rev. B} \textbf{90}, 180405 (R) (2014).   

Magnetic and Structural Properties of Mn$_{\mathrm{3}}$Ge$_{\mathrm{\thinspace }}$Nanoparticles

In this work, we have investigated the magnetic and structural properties of Mn$_{\mathrm{3}}$Ge nanoparticles prepared by the cluster-beam deposition technique. The composition, crystal structure and magnetic properties of the nanoparticles have been characterized by energy dispersive x-ray spectroscopy (EDS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and magnetometry measurements with the PPMS. Particles made with 1.5 Torr Argon pressure and power of 60 W have an average size of 13 nm and a highly disordered crystal structure. However, after a short annealing at 700 $^{\mathrm{o}}$C, the particle's structure is transformed to the hexagonal Ni$_{\mathrm{3}}$Sn-type structure with space group P6$_{\mathrm{3}}$/mmc(194) which is the same as in bulk. Hysteresis loop measurements showed that the annealed particles are ferromagnetic at room temperature showing a coercivity of 0.3 kOe at 50 K. The effects of particle size and temperature on the magnetic and structural properties are currently being studied and the results will be reported and discussed.   

Deciphering chemical order/disorder and magnetic properties of FePt nanoparticle at the single-atom level

Iron-Platinum (FePt) alloy exhibits large magnetocrystalline anisotropy energy (MAE) with prominent potential for magnetic storage media application. However, a full 3D atomic structure determination of real system has not been accomplished, therefore a fundamental understanding of their magnetic properties remains elusive. Here, we determined the 3D coordinates of 6,569 iron and 16,627 platinum atoms in a model FePt nanoparticle system with 22 pm precision via atomic electron tomography. We identified rich structural variety and chemical order/disorder including 3D atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. The atomic structure was used as direct input for first principles calculations to determine atomic magnetic moments and local MAE. This work not only opens the door to determining 3D atomic arrangements and chemical order/disorder of a wide range of nanostructured materials, but also will transform our understanding of structure-property relationships at the most fundamental level [arXiv:1607.02051].   

Size-effect driven ferromagnetism in gold nanoparticles

Bulk gold (Au) is well-known to be a diamagnetic material. The occurrence of ferromagnetism in nanostructured gold would clearly be an interesting example of novel quantum phenomenon that emerges in materials with reduced dimensionality. Recently, several reports have suggested that gold is ferromagnetic in the nano-regime, particularly when capped with certain thiol-containing ligands. We have employed pulsed laser deposition (PLD) to prepare multiple layers of bare gold nanoparticles buried in nonmagnetic, disordered alumina thin films. Transmission electron microscopy (TEM) measurements show that, depending on conditions, the resulting particle sizes range from \textasciitilde 1nm to \textasciitilde 20nm. The size-dependent magnetic response of such samples was characterized using a SQUID magnetometer. Our experimental results provide direct evidence that size-effect driven ferromagnetism occurs in gold together with the critical size required for observing ferromagnetism. These results can be used as a guide for exploring magnetism in other metals that are diamagnetic in the bulk.   

Structural and magnetic properties of magneto-luminescent ZnO-iron oxide core-shell nanoparticles.

Oxide core-shell nanoparticles have attracted considerable interest for its multifunctional properties with high electron correlation. Luminescent ZnO and ferromagnetic iron oxide have been exploited to develop magneto-luminescent nanomaterials. ZnO-Iron oxide core-shell nanoparticles have been developed using physiochemical method. Magnetic properties have been varied while keeping the luminescence intact in core-shell nanoparticles. Structural and physical properties have been investigated using XRD, TEM, RAMAN, XPS, PL, DLS, and SQUID magnetometer. DLS and TEM data show acceptable dispersion for synthesized core shell nanoparticles. The phase of the shell region has been controlled through annealing the samples at different environment. Rietvield refinement of XRD data clearly show a phase change of iron oxide in the shell region due to annealing. Temperature and field dependent magnetization data establish the ferromagnetism in core-shell nanoparticles. Variation in coercive field and remnant and saturation magnetization further confirms the presence of different iron oxides in the shell region. This research work is supported by National Cancer Institute (1R15 CA139390-01).