Session W22: Focus Session: Magnetic Nanoparticles II

The Structure and Magnetic Properties of Nanoparticles and Their Arrays

The physics of magnetic nanoparticles and arrays is a very important topic of current research. Many questions remain. How does the structure of a single nanoparticle influence its magnetic properties? For instance, core/shell interactions can increase the coercivity and lead to exchange biasing, causing ferromagnetic rather than superparamagnetic behavior. At what thickness does the oxide shell begin to behave like an antiferromagnet? How do uncompensated surface spins affect the magnetic behavior? In addition, novel nanoparticle structures can lead to interesting physical behavior. In a bio-inspired approach, we are synthesizing \textit{highly} monodisperse oxide nanoparticles inside of protein cages. For these systems, magnetocrystalline anisotropy plays an important role; the surface anisotropy term becomes large, reducing the total particle moment. However, we find that the encapsulating protein shell reduces the surface anisotropy and increases the particle moment. Furthermore, we have synthesized mixed phase gamma-Fe$_{2}$O$_{3}$/CoO nanoparticles with large exchange biasing. Further questions arise for nanoparticle arrays. Dipole interactions modify the collective magnetic behavior. What are the strength and orientation of these interactions, and how do they depend on particle size, spacing, \textit{and} array ordering. Recent experiments have shown the importance of array order in determining the collective magnetic properties. The physics of magnetic nanoparticles is rich and complex, and depends upon both the structure of the individual particles and their assemblies. By using synchrotron based magnetic circular dichroism, small angle X-ray scattering and neutron scattering, we have been able to quantify many aspects of both nanoparticle and array structures. A quantitative understanding of these structural relationships has led to a better understanding of their magnetic behavior.   

Particle Size Control of Polyethylene Glycol Coated Fe Nanoparticles

Recent interest in Fe nanoparticles with high magnetization is driven by their potential use in biomedical applications such as targeted drug delivery, MRI contrast enhancement and hyperthermia treatment of cancer. This study looks at the use of a polyethylene glycol (PEG) solution to mediate the particle size and therefore control the coercivity of the resulting nanoparticles. Iron nanoparticles were synthesized using an aqueous sodium borohydride reduction of ferrous chloride by a simultaneous introduction of reagents in a Y- junction. The resulting product was collected in a vessel containing a 15 mg/ml carboxyl terminated polyethylene glycol (cPEG) in ethyl alcohol solution located under the Y junction. By varying the length of tubing below the Y junction, the particle size was varied from 5-25 nm. X-ray diffraction data indicates the presence of either amorphous Fe-B or crystalline alpha Fe, depending on the molar ratio of reagents. Magnetic measurements indicate the particles are ferromagnetic with values of coercivity ranging from 200-500 Oe and a saturation magnetization in range of 70-110 emu/g. The XRD shows that the particles are not affected by the polymer coating.   

Using Single Nanoparticle Devices to Investigate Nano-junction GMR and TMR

High-pressure sputtering / cluster-beam deposited single nanoparticle devices were fabricated to study giant magneto- resistance (GMR) and tunneling magneto-resistance (TMR) of nano- junctions. The junction used to investigate GMR was formed by the point contact of a nearly spherical $\sim$20 nm diameter ferromagnetic (FM) particle on a FM thin film deposited on a silicon substrate. The particles were covered with a thick Al$_ {2}$O$_{3}$ dielectric layer. The tunneling device is identical, except for an additional Al$_{2}$O$_{3}$ tunnel barrier sandwiched between the FM particle and film. Using a focused ion beam (FIB), small apertures were milled in the dielectric layer to expose individual particles and metallic contacts were subsequently deposited. Other contacts were made directly to the underlying FM film creating a simple two- contact measurement geometry. Temperature dependent TMR and GMR are presented for isolated particles that are independent of proximity and ensemble effects. This work is supported by the ACS Petroleum Research Fund.   

High Frequency Properties of Magnetodielectric Composites Consisting of Oriented Fe-based Flakes Embedded in a Polymeric Matrix

Magnetodielectric composites containing small ferromagnetic inclusions in a continuous dielectric matrix could be useful high frequency materials if relatively large and similar values of the permeability ($\mu )$ and permittivity ($\varepsilon )$ could be obtained. This potential, however, can not be fully achieved for spherical inclusions because their demagnetizing fields severely limit the effective permeability of the composite. Therefore, we have prepared and studied a series of magnetodielectric composites containing oriented Fe-based flakes. The flakes were produced by a mechanical deformation technique and were typically several 100 $\mu $m wide and several $\mu $m thick. These flakes were mixed with a styrene based liquid resin and aligned in an applied magnetic field prior to polymerizing the resin. X-ray diffraction and hysteresis loop measurements confirm a significant degree of alignment. Permeability and permittivity measurements indicate that values of $\mu $ and $\varepsilon $ in excess of 20 can be achieved in these samples with small losses when the loading fraction of the Fe flakes approaches 50{\%}.   

Fabrication of Ordered Mesoporous Silica with Encapsulated Iron Oxide Particles using Ferritin-Doped Block Copolymer Templates

Recently, two-dimensional arrays of iron oxide clusters were fabricated by dip-coating a silica substrate into an aqueous solution. Here we report the encapsulation of ferritin in 3D mesoporous silica structures by the replication of block copolymer templates in supercritical CO$_{2}$. In our approach, preparation of the highly ordered, doped template via spincasting and microphase separation and silica network formation occur in discreet steps. A solution of an amphiphilic PEO-PPO-PEO triblock copolymer (Pluronic) template, horse spleen ferritin and a low concentration of PTSA acid was prepared and spin-coated onto a Si wafer. Upon drying the block copolymer microphase separates resulting in partitioning of the acid catalyst and ferritin to the hydrophilic domain. The polymer template was then exposed to a solution of supercritical carbon dioxide and tetraethyl orthosilicate (TEOS) at 125 bar and 40$^{o}$C. Equilibrium limited CO$_{2}$ sorption in the block copolymer template resulted in modest dialation of the microphase segregated structure. Under these conditions, the precursor was readily infused into the copolymer and reacted within the hydrophilic domain containing the acid catalyst. The resultant film was calcined in air at 400$^{o}$C for 6 hours producing a well-ordered iron oxide-doped mesoporous silica film. TEM and XRD revealed crystalline iron oxide structures within the mesoporous silica supports. Magnetic properties were analyzed using a superconducting quantum intereference device (SQUID).   

Synthesis of Barium hexaferrite nanoparticles for functional multilayers

Magnetic barium ferrite (BaFe$_{12}$O$_{19 }$or BaM) nanoparticles were synthesized by a two system microemulsion process. X-ray diffraction of these nanoparticles confirmed the presence of a dominant hexagonal BaM phase. The magnetic characterization of the nanoparticles was performed using a Physical Properties Measurement System (PPMS). The M-H hysteresis of the BaM, at 5K and 300K, displays a saturation magnetization of $\sim $ 68 emu/g, 48 emu/g and large coercivities of $\sim $ 2300 Oe, 3100 Oe respectively, consistent with bulk BaM. The zero field cooled (ZFC) and field cooled (FC) curves illustrate that superparamagnetism was not present in the BaM below 300K. These particles will be used to prepare multilayers of ferroelectric and ferromagnetic films by depositing on a ferroelectric polymer (polyvinylidene fluoride) matrix using the Langmuir-Blodgett technique. The functional properties of these multilayers will be discussed. Work supported by NSF grant {\#}CTS-0408933 and NSF Integrated Interdisciplinary Nanoscience REU DMR 0243997.   

Theory of Brillouin Light Scattering from Ferromagnetic Nanospheres

We develop the theory of Brillouin light scattering (BLS) from spin waves in ferromagnetic nanospheres, within a framework that incorporates the spatial variation of the optical fields within the sphere. Through use of our recent theory [1] of exchange dipole spin wave modes of the sphere, we develop a method which properly normalizes the eigenvectors. We then describe the BLS spectrum associated with the first few dipole/exchange spin wave modes with emphasis on their relative intensity. We also discuss the stokes/anti stokes ratio. \newline \newline References: \newline [1] Rodrigo Arias, Ping Chu and D. L. Mills, Phys. Rev.B71, 224410 (2005).   

Self Assembled CoFe$_2$O$_4$ Nanoparticles within Block Copolymer Films: Structural and Magnetic Properties

Nanosize CoFe$_2$O$_4$ particles have been synthesized by self-assembly within diblock co-polymers, through a room-temperature templating strategy, amenable to large scale fabrication. XRD, TEM, SQUID and Mossbauer studies are combined in order to explore the morphological, structural, micromagnetic and interfacial characteristics of this nanocomposite system. TEM micrographs indicate low polydispersity, with particle size of 9.6 nm diam. Low temperature Mossbauer studies predict average sub lattice saturation hyperfine magnetic fields H (A) =501 kOe and H [B] = 527 kOe, respectively, for the tetrahedral and octahedral iron coordination sites of the ferrite spinel structure. Superparamagnetic relaxation processes, analyzed within a cubic magnetic anisotropy model, give a magnetic anisotropy density K = $3.23 x 10^5$J/m$^3$, while SQUID magnetometry predicts a saturation coercivity of 6.1 kOe. Deviations from bulk CoFe$_2$O$_4$ and unsupported CoFe$_2$O$_4$ nanoparticles are discussed in terms of finite-size effects and interfacial interactions.   

Magnetization of iron clusters from first-principles calculations

The magnetic moment of Fe clusters as function of number of atoms has been observed to show a slow decrease from the isolated atom value (4 Bohr magnetons) to its bulk value of 2.2 Bohr magnetons per atom. In addition, a series of peaks has been observed, for which the causes are not yet fully understood (see Billas, Chatelain, and de Heer, Science, 1994). We analyze the dependence of total magnetic moment, local magnetic moment, cohesive energy and other physical quantities in iron clusters Fe$_n$ ( $1 < n < 250$ ), and compare these results with available experimental data. We use a real-space method, pseudopotentials and first-principles DFT to obtain the properties of the cluster in its ground state. Calculations are done using the PARSEC code ( www.ices.utexas.edu/parsec ). We also discuss some of the recently developed capabilities of PARSEC.   

Chern-number spin Hamiltonians for magnetic nanoclusters by ab-initio methods

Combining field-theory methods and ab-initio calculations, we construct an effective Hamiltonian with a single giant-spin degree of freedom, capable of describing the low-energy spin dynamics of ferromagnetic metal nanoclusters consisting of up to a few tens of atoms. In our procedure, the magnetic moment direction of the Kohn-sham SDFT wave-function is constrained by means of a penalty functional, allowing us to explore the entire parameter space of directions, and to extract the magnetic anisotropy energy and the Berry curvature functionals. The average of the Berry curvature over all magnetization directions is a Chern number, a topological invariant that can only take on values equal to multiples of half-integers, which represents the dimension of the Hilbert space of the effective spin system. The spin Hamiltonian is obtained by quantizing the classical anisotropy-energy functional, after a change of variables which yields a constant Berry curvature. We illustrate this procedure by explicitly constructing the Hamiltonian for dimers and trimers of Co and Cr, whose spin dynamics has been recently investigated experimentally by STM methods.   

The coupling of magnetic and dielectric properties in magnetic nanoparticles

The low frequency dielectric properties of \textit{$\gamma $}-Fe$_{2}$O$_{3}$ and MnFe$_{2}$O$_{4}$ magnetic nanoparticles have been investigated. These samples showed frequency dependent dielectric anomalies near their respective magnetic blocking temperatures suggesting a coupling between the magnetic and dielectric properties of the systems. In addition, the samples exhibited considerable magnetocapacitance above the magnetic blocking temperature. The magnetic field induced change in the dielectric constant was shown to be proportional to the square of the magnetization, suggesting that the dielectric properties of these systems are strongly connected to the distribution of magnetic moments in the samples. The results will be discussed in the framework of a theory explaining how magnetodielectric effects can arise from magnetoresistance in a Maxwell-Wagner capacitance model.   

A new magnetization-reversal strategy for Stoner particles

A new strategy is proposed aimed at substantially reducing the minimal magnetization switching field for a Stoner particle. Unlike the normal method of applying a static magnetic field which must be larger than the magnetic anisotropy, a much weaker field, proportional to the damping constant in the weak damping regime, can be used to switch the magnetization from one state to another if the field is along the motion of the magnetization. The concept is to constantly supply energy to the particle from the time-dependent magnetic field to allow the particle to climb over the potential barrier between the initial and the target states.   

Structural, Magnetic and Dynamical Properties of Dipolar Nanoparticles

We investigated the structural, magnetic and collective properties of dipolar nanoparticles. The dynamic of the systems is determined by differential equations for the translational and rotational degrees of freedom, which are studied using molecular dynamics. The interaction potential of the particles consists of both an anisotropic dipolar interaction and an isotropic hard-sphere potential. Dependent on the temperature and external magnetic field, the system is found to be in different states. These states can be characterized by their respective structural ordering, that is closely related to the magnetic and energetic properties of the assembly of particles. In the ground state the particles arrange themselves in closed rings due to the anisotropic nature of the interaction. Besides this structure also the formation of metastable chains and network-like structures can be observed. Thermal excitations lead to a destabilization while the influence of an external magnetic field depends on its relative orientation with respect to the structures. In this work the phase diagrams in two and three dimensions of the various structures are determined as a function of temperature and external field.