Session Q13: Focus Session: Magnetic Nanostructures-Nanoparticle Synthesis and Magnetism

Synthesis and properties of magnetic ceramic nanoparticles

Magnetic ceramic nanoparticles of the type xIn2O3-(1-x)alpha-Fe2O3, xV2O5-(1-x)alpha-Fe2O3 and xZnO-(1-x)alpha-Fe2O3 (x=0.1-0.7) were synthesized from the mixed oxides using mechanochemical activation for 0-12 hours. X-ray diffraction was used to derive the phase content, lattice constants and particle size information as function of ball milling time. Mossbauer spectroscopy results correlated with In3+, V5+ and Zn2+ substitution of Fe3+ in the hematite lattice. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced systems with a wide range of particle size distribution, from nanometer particles to micrometer agglomerates, but with a uniform distribution of the elements. Simultaneous DSC-TGA investigations up to 800 degrees C provided information on the heat flow, weight loss and the enthalpy of transformation in the systems under investigation. This study demonstrates the formation of a nanostructured solid solution for the indium oxide, an iron vanadate (FeVO4) for the vanadium oxide, and of the zinc ferrite (ZnFe2O4) for the zinc oxide. The transformation pathway for each case can be related to the oxidation state of the metallic specie of the oxide used in connection with hematite.   

Magnetic Interactions in Binary Nanocrystal Superlattices

The recent development\footnote{A. Dong, J. Chen, P. M. Vora, J. M. Kikkawa, and C. B. Murray, Nature 466, 474 (2010).} of highly ordered three-dimensional assemblies of magnetic nanocrystals (NCs) poses new questions for the study of collective magnetic dipolar interactions. Prior literature has focused on changes in blocking temperature in infinite, random assemblies. Here, we study ordered assemblies with two different magnetic NCs and complex superlattice unit cells. Monte Carlo simulations are compared with data to clarify magnetic behaviors in AB$_{13}$ superlattices consisting of two different-sized (14.3 nm and 7.2 nm) Fe$_{3}$O$_{4}$ NCs, which are superparamagnetic in isolation. This strategy illuminates the individual sublattice properties and their interaction. We show that under certain circumstances, the magnetic response of smaller magnetic NCs may be quenched by the random magnetic field of larger magnetic NCs,\footnote{J. Chen, A. Dong, J. Cai, X. Ye, Y. Kang, J. M. Kikkawa, and C. B. Murray, Nano. Lett. 10, 5103 (2010).} and that trends in blocking temperature with interaction strength depend strongly on both unit cell geometry and boundary conditions.   

Fabrication of Iron Oxide Nanoparticle Monolayers by Electrophoretic Deposition

Magnetic nanoparticle (NP) films are potentially useful in a variety of applications, such as magnetic storage media and ultra-strong permanent magnets. Monolayers of magnetic NPs are specifically interesting as the monolayer geometry maximizes film interactions with dissimilar materials below and above the monolayer. However, many potential commercial and industrial applications of NP films rely on fabrication techniques that are facile, rapid, and site-selective which create homogenous, densely packed, defect-free thin films. Electrophoretic deposition (EPD) is a technique for forming thin films that meets all of these criteria. This work shows, for the first time, EPD's utility in forming monolayers of magnetic NPs. Iron oxide NPs ($\sim $14nm) have been synthesized using a solution phase synthesis technique. Repeated centrifugation of the particles prepares the NPs for EPD. The particles are then deposited onto silicon substrates with EPD using dc electric fields. Analysis of the films using scanning electron microscopy and atomic force microscopy shows the particles deposit as NP monolayers. The monolayer density and deposition rate are controlled by varying the suspension concentration and the deposition time. Future research will focus on creating long-range order within the monolayers.   

Curie temperature reduction in SiO$_{2}$-coated ultrafine Fe$_{3}$O$_{4}$ nanoparticles: Quantitative agreement with a finite-size scaling law

We report high-temperature magnetic measurements for SiO$_{2}$-coated ultrafine Fe$_{3}$O$_{4}$ nanoparticles. The Curie temperatures of the ultrafine Fe$_{3}$O$_{4}$ nanoparticles are significantly reduced and follow a finite-size scaling law predicted from Monte Carlo simulations. Our current result provides the first quantitative confirmation of the finite-size scaling law for quasi-zero-dimensional magnetic systems.   

Magnetic properties of Fe nanoparticles: application of the DFT-Inhomogeneous-DMFT approach

Dynamical Mean-Field Theory (DMFT) in combination with Density Functional Theory (DFT) has been successfully applied to examine the properties of transition metal elements in which correlation plays an important role. It was recently shown that this approach can also be applied to study correlation effects in nanostructures [1]. Here we present results of a combined DFT-inhomogeneous-DMFT approach used to investigate the size dependent magnetic properties of small iron clusters containing 15 to 19 atoms. For the DMFT impurity solver we use the iterated-perturbation theory approximation. The numerical analysis with the code developed in our group allows one to study systems consisting up to several hundred atoms. The optimized structure of the Fe clusters is obtained from spin polarized DFT calculations. We find our approach to yield better agreement with experimental data [2] than that obtained using DFT and DFT+U, which generally overestimates the magnetization. \\[4pt] [1] V. Turkowski, et al, J. Phys.: Condens. Matt. 22, 462202 (2010).\\[0pt] [2] M. B. Knickelbein, Chem. Phys. Lett. 353, 221(2002).   

Finite-size scaling behavior and intrinsic critical exponents of nickel: Consistent with the three-dimensional Heisenberg model

We report high-temperature magnetic measurements of silica-coated nickel nanoparticles. The Curie temperature is found to decrease with decreasing particle size and follow a finite-size scaling relation with the correlation length exponent $\nu$ = 1.06$\pm$0.07. The measured exponent is in excellent agreement with the reported values for nickel nanowires and some nickel thin films. By carefully analyzing the reported thickness dependencies of the Curie temperatures for some nickel films, we show that the intrinsic $\nu$ value for nickel is 0.73$\pm$0.03 while the much larger $\nu$ values (about 1.0) found for some other samples might arise from the presence of long-range correlated disorder near the surface. The intrinsic $\nu$ value together with the experimental values of other critical exponents consistently shows that the three-dimensional Heisenberg model is sufficient to describe the ferromagnetism of nickel. Our current work thus resolves a long standing controversy in this field.   

Self-assembly of Superparamagnetic Nanoparticles with Permanent Magnetization

Magnetic nanoparticles (MNPs) exhibit superparamagnetism when thermal fluctuations overcome the potential barrier for spin reversal set by magnetocrystalline anisotropy. The magnetic moment in such a material oscillates between the easy axes leading to zero net magnetization. Stable colloidal dispersions of MNPs exploit this state to prevent agglomeration. Self-assembly of MNPs presents an excellent bottom up nanofabrication technique due to the wide range of structures that can be formed. A stable dispersion of MNPs is an essential starting point for good control of the process. In this study we explore the theoretical basis for a self-assembled MNP structure with permanent magnetization starting from a dispersion of superparamangetic MNPs. Magnetostatic coupling of dipole moments enhance the potential barrier for magnetization reversals. We use X-Ray microCT and TEM to visualize the self-assembled structures. We use a stochastic form of the Landau-Lifshitz-Gilbert equation to simulate the magnetization dynamics in each MNP. Permanent magnetization in self-assembled structures generated \textit{in situ} promise several significant applications such as targeted drug delivery, tissue engineering and novel soft composites.   

Rate-dependent hysteresis losses in ensembles of magnetic nanoparticle clusters

Hysteresis is ubiquitous in magnetic nanoparticle systems and understanding how it emerges from complex interactions and for different time scales is a long-standing issue in magnetism research. Understanding the phenomenon is most important for engineering magnetic nanoparticle structures of well-controlled properties in magnetic recording, hysteresis loss optimization in hyperthermia cancer treatment in biomedicine, or biological and chemical sensing, to name a few examples. In this work we address one of the general questions related to the influence of thermal activation processes on hysteresis loss. Employing large-scale computational modeling based on the master-equation framework we investigate the influence of dipolar interactions on thermal hysteresis loops in ensembles of magnetic nanoparticle chains and clusters. We show that the directional dependence of dipolar interactions results in enhanced or reduced hysteresis loss, depending on the distribution of particles' anisotropy axes and particle chain orientations with respect to the external field. Additional hysteresis loss reduction occurs in case of particle clusters due to possibility of the frustration phenomenon not present for topologically simpler chains.   

Size dependence and thermal stability of chiral states in ferromagnetic nanoparticles deposited on a ferromagnetic substrate

Chiral magnetization profiles are observed in many low dimensional systems such as nanoscale particles, thin films, or bulk systems such as multiferroic samples, that are characterized by a lack of inversion symmetry. These spiraling spin structures are often size-dependent and can be attributed to the competition between exchange and anisotropic, or parity breaking Dzyaloshinskii-Moriya (DM) terms. To utilize such spiraling magnetization profiles in novel spintronic devices, it is necessary to understand the mechanisms under which these spiral spin configurations form and how they can be harnessed via an external field. Here we present exact analytic solutions for the magnetization profiles and the associated energies and energy barriers for ferromagnetic nanoparticles deposited on a ferromagnetic substrate. Our method allows us to determine the critical length at which spiral solutions are supported in such samples, which we find to be $l_c = (\pi /2) \sqrt{A/K_e}$ for vanishing applied field and a misfit angle of $\pi/2$ between substrate and nanoparticle anisotropy axis. We also demonstrate that in absence of further pinning effects, the nanoparticles are exclusively in a uniform state for $l < l_c$. We show that our theory is in good agreement with recent experiments.   

Magnetic and magnetocaloric properties of NdMnO$_{3}$ nanoparticles

Recently nanosized manganites have attracted considerable attention as the reduction of particle size has exotic effect on their properties. We have studied the magnetic properties and magnetocaloric effect (MCE) of NdMnO$_{3 }$nanoparticles with particle size $\sim $20 and 30nm (denoted as S20 and 30 respectively). In temperature dependence of magnetization, M(T), a paramagnetic to ferromagnetic transition is observed at T$_{C}\sim $ 70K for S20, which is 5K higher than that for S30 indicating enhancement of ferromagnetic interaction with particle size reduction. In addition to this, an anomaly in M(T) is observed at 20K (T$_{CA})$ for both S20 and S30, which is attributed to the stabilization of a canted magnetic state (CMS) due to the ordering of Nd$^{3+}$ . The magnetic entropy change [ -$\Delta $S$_{M}$(T)] is calculated from isothermal magnetization curves using Maxwell relation. There are two maximas in -$\Delta $S$_{M}$(T) at T$_{C}$ and T$_{CA}$ indicating large MCE over both temperature regions. Interestingly, the relative cooling power is enhanced in case of smaller particle size in which the influence of stabilization of CMS on MCE is less pronounced.   

Effect of the size distributions of magnetic nanoparticles on metastability using phonon-assisted transition rates

Experiments show that magnetic nanoparticles have distributions of sizes and shapes, and that the distributions greatly influence static and dynamic properties of the nanoparticles. Therefore, it is critical to understand their properties as a function of the distributions. Previously, we studied an effect of particle size distributions on metastability in magnetization relaxation, using spin $S=1$ Blume-Capel model with Glauber transition rates. The size distributions were simulated using distributions of magnetic anisotropy parameter $D$ with spins fixed. We found that the lifetime of the metastable state is governed by the smallest particle in a given system. In this talk, we present the effect of size distributions on metastability in magnetization relaxation with phonon-assisted transition rates. These transition rates differ from Glauber dynamics and are derived from weak spin-phonon coupling. In the phonon-assisted transition rates, spin-flips occur via emission or absorption of phonons, and so transitions are forbidden between degenerate states. We investigate magnetization relaxation with distributions of $D$ using kinetic Monte Carlo simulations, when the distributions include values with which such forbidden transitions are expected.   

Magnoelastic coupling in magnetic oxide nanoparticles

Phonons are exquisitely sensitive to finite length scale effects in a wide variety of materials. To investigate confinement in combination with strong magnetoelastic interactions, we measured the infrared vibrational properties of MnO and CoFe$_2$O$_4$ nanoparticles and their parent compounds. For MnO, a charge and bonding analysis reveals that Born effective charge, local effective charge, total polarizability, and the force constant are overall lower in the nanoparticles compared to the bulk. We find that the spin-lattice coupling drops from $\sim$7 cm$^{-1}$ in the single crystal to $<$1 cm$^{-1}$ in the nanoparticles. For CoFe$_2$O$_4$, the spectroscopic response is sensitive to the size-induced crossover to the superparamagnetic state, which occurs between 7 and 10 nm, and a spin-phonon coupling analysis supports the core-shell model. Moreover, it provides an estimate of the thickness of the magnetically disordered shell, increasing from 0.4 nm in the 14 nm particles to 0.8 nm in the 5 nm particles, demonstrating that the associated local lattice distortions take place on the length scale of the unit cell. These findings are important for understanding finite length scale effects in magnetic oxides and other more complex functional oxides.   

The specific edge effects of 2D core/shell model for spin-crossover nanoparticles

We analyzed the size effect of spin-crossover nanoparticles at the edges of the 2D square lattices core/shell model, where the edge atoms are constrained to the high spin (HS) state. We performed MC simulations using the Ising-like Hamiltonian, \[ H=-J\sum\limits_{(i,j)} {\sum\limits_{\begin{array}{l} i'=\pm 1; \\ j'=\pm 1 \\ \end{array}} {S\left( {i,j} \right)S\left( {i+i',j+j'} \right)} +\left( {\frac{\Delta }{2}-\frac{k_B T}{2}\ln g} \right)\sum\limits_{(i,j)} {S\left( {i,j} \right)} } \mbox{ } \] The molar entropy change is $\Delta $S$\approx $50J/K/mol, ln$g=\Delta $S/R$\approx $6 (R is the perfect gas constant), energy gap is $\Delta $=1300K. The HS fixed edges were based on the observation of an increasing residual HS fraction at low temperature upon particle size reduction. This specific boundary condition acts as a negative pressure which shifts downwards the equilibrium temperature. The interplay between the equilibrium temperature (=$\Delta $/k$_{B}$ln$g)$ variation and the expected variation of the effective interactions in the system leads to a non-monotonous dependence of the hysteresis loop width upon the particle size. We described how the occurrence condition of the first-order transition has to be adapted to the nanoscale.   

Magnetic Properties of Single Co Nanoparticles Probed by Tunneling and Microwaves

We present tunneling studies of magnetic hysteresis loops of single Co nanoparticles. The magnetic switching field at mK-temperature is strongly reduced as a function of bias voltage. At 10mV bias voltage, the switching field is reduced by 15\%, while the magnetization can be switched by applying a voltage pulse of 10mV. The strong reduction of the switching field is not an artifact due to charge noise or Joule heating, nor it is a result of the electric field dependence of the surface anisotropy. Instead, the reduction represents the case of magnetic excitation driven by the tunnel current. The strength of the effect indicates strongly enhanced coupling between magnetic excitations and the tunnel current in ferromagnets with strongly reduced dimensions. We also present the first measurement of the magnetic relaxation time in a single Co nanoparticle ($\sim$microsecond) , obtained by combining tunneling spectroscopy and microwave pumping.   

Exchange Bias Studies in Core/Shell Structures Nanoparticles

This study is focused on a comparison of magnetic properties of chemically synthesized core/shell structured iron/iron-oxide nanoparticles with different core sizes and comparable shell thickness. Particles were synthesized by high temperature decomposition of iron organometallic compounds. Thermomagnetic data revealed that particles are superparamagnetic at room temperature. The field cooled hysteresis loops showed interesting features of enhanced coercivity and horizontal and vertical shifts along directions of the cooling field, all strongly depend on temperature, indicative of an exchange-bias-like phenomenon. These effects were more pronounced in smaller core size nanoparticles with an exchange bias field of 4098 Oe. The spin-glass-like phase with high-field irreversibility in the iron oxide shells played the role of the fixed phase in the core/shell system and provided the pinning force to the reversible spins. The magnetic domains and higher contributions from the surface anisotropy in the hollow nanoparticles caused enormous magnetic frustration that is the origin of high field irreversibility and vertical shift of hysteresis loop in these particles.