Session Z13: Focus Session: Magnetic Nanostructures-Nanoparticles and Granular Systems

Unexpected magnetization in highly pure metal oxide nanoparticles

We report the synthesis and characterization of a large set of highly pure metal oxide (CeO$_{2}$, SnO$_{2}$ and ZnO) nanoparticles of ultra-small size (2-10 nm). While the metal oxide systems in this study are non-magnetic as bulk materials, our prepared nanoparticles possess an unexpected small room-temperature ferromagnetic magnetization on the order of 0.001 emu/g. This magnetization is shown to not be a result of magnetic impurities, and is discussed in terms of modification of the electronic structure and crystal lattice. These nanoparticles were thoroughly characterized in their size and phase by x-ray diffraction, morphology by transmission electron microscopy, chemical state and elemental purity by x-ray photoelectron spectroscopy, electronic bandgap by UV-vis absorption spectroscopy, and magnetic properties by vibrating sample magnetometry and electron paramagnetic resonance.   

Magnetic Transitions in Granular FeRh

The relationship between the crystallographic lattice and magnetism in materials undergoing first order thermodynamic phase transitions, where structural and magnetic phase transitions occur simultaneously, are not fully understood. Nanostructuring of such materials offers a route to tailor these transitions through alteration of free energy terms dependent upon the surface area and volume of finite systems. Previous studies in melt spun ribbons of FeRh nanoprecipitates in a Cu matrix have shown a reduced phase transition compared to bulk [1]. In this study nanostructured FeRh films were obtained by RF sputter deposition in nonmagnetic matrices of Cu, Si and alumina. After vacuum annealing, the structure and magnetism of the samples were studied. Preliminary results highlight the relationships between chemistry, nanostructuring and magnetic response in the FeRh system. \\[4pt] [1] Evidence for Highly Suppressed Magnetostructural Transition Temperature in Nanostructured FeRh, R. Barua; F. Jimenez-Villacorta; H. Jiang; J.E. Shield; D. Heiman; L.H. Lewis, IEEE MMM Conference 2011, Scottsdale, Arizona, US, Abstract No CE-05   

Investigation of Local Structures and Magnetism in Mn-doped Y$_{2}$O$_{3}$ Nanocrystals

Nanocrystals of Mn-doped Y$_{2}$O$_{3}$ were prepared by thermal decomposition method and alternately annealed in oxygen and forming gas to vary the oxygen deficiency. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), x-ray absorption fine structures (XAFS), and superconducting quantum interference device (SQUID) techniques were applied before and after each annealing to monitor structural and magnetic variations of the sample. The XRD data show that these annealing treatments do not appreciably change the average particle size of the sample. An amorphous-to-crystalline long-range-order structural change was observed for the first annealing applied to the as-made samples. The short-range-order structure exhibited by XAFS reveals that O vacancies surrounding magnetic impurity atoms were appreciably increased by forming-gas-annealing and decreased by oxygen-annealing in the samples. The increase and decrease of O vacancies are accompanied by enhanced and reduced saturation magnetization as demonstrated by SQUID, respectively. Our experimental results demonstrate clear correlation between magnetism and O vacancies around magnetic ions and therefore strongly support the bound magnetic polaron model in these nanocrystal DMO samples.   

Preparation and Characterization of Mg Substituted NiCuZn Nano Ferrites for Multilayer Chip Inductors

The present paper examines the effect of magnesium substitution on structural and magnetic properties of NiCuZn nano ferrites synthesized by sol - gel method. Formation of single phase spinel structure was confirmed both from XRD and FTIR. The initial permeability shows decreasing trend with increasing Mg concentration due to reduced magnetization, grain size and increased magneto - crystalline anisotropy constant. At the same time, the cut off frequency increases with increasing Mg content. This is attributed to domain wall pinning arising due to the presence of non magnetic magnesium ions. Also the permeability is observed to be constant up to 10MHz frequency range showing compositional stability and quality of the material. The magnetic loss factor shows very low values at higher frequencies. It is concluded that even though both zinc and magnesium are non magnetic ions, substitution of one cation by another prone to influence the magnetic properties due to their change in dimension and cation distribution among the two available sites of a spinel system. These samples have advantages of low sintering temperature find applications in multilayer chip inductors due to their high and constant permeability even at higher frequencies.   

Ferromagnetic Resonance on Micro- and Nanoferrites in Millimeter Waves

Complex magnetic permeability and dielectric permittivity of micro- and nano-sized powdered barium ferrite (BaFe$_{12}$O$_{19})$ and strontium ferrite (SrFe$_{12}$O$_{19})$ have been studied in a broadband millimeter wave frequency range for the first time. Transmittance measurements have been performed using a free space quasi-optical millimeter wave spectrometer, equipped with a set of high power backward wave oscillators. Backward wave oscillators have been used as sources of tunable coherent radiation at each individual $Q$-, $V$- and $W$- frequency bands. Real and imaginary parts of dielectric permittivity for both types of micro- and nanoferrites have been calculated using analysis of recorded high precision transmittance spectra. Frequency dependences of the magnetic permeability have been obtained from Schl\"{o}mann's equation for partially magnetized ferrites. Tunable millimeter wave absorber, based on micro- and nano-sized powdered ferrite materials is presented.   

Polarized Magnetic Induced Broadening of Plasmon-photonics in Fe$_{3}$O$_{4}$- Silicone Elastomer Composite Films

We report systematic studies of polarization dependence of magneto-optical response of Fe$_{3}$O$_{4}$-silicone elastomer composite. The Fe$_{3}$O$_{4}$ particles were aligned in the elastomer matrix with static magnetic field. The optical response of two composites containing 5wt{\%} and 15wt{\%} of 20nm-30nm diameter Fe$_{3}$O$_{4}$ particle aligned in- and out-of-plane were measured with an absorption spectrometer. We observed a systematic redshift in the optical response of the out-of-plane samples with increasing static magnetic field. Furthermore, the observed redshift increases with increasing weight percent of Fe$_{3}$O$_{4}$ in the composite; obtaining a maximum shift of $\sim $ 174 nm at 600 Gauss in the 15wt{\%} Fe$_{3}$O$_{4}$-elastomer composite films. The observed redshift in the optical response of the out-of-plane composite is attributed to the effect of magnetic field strength and the metal particle/cluster size in the elastomer. However, 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.   

Precipitation of coherent FeRh nanoparticles with highly suppressed magnetostructural transition temperatures in rapidly solidified (FeRh)$_{5}$Cu$_{95}$ alloys

Magnetostructural phase transitions have the capability of delivering large functional effects in response to small excursions in magnetic field, temperature and strain; this potential might be amplified in nanostructured systems by virtue of large surface:volume ratios. Nanoprecipitates ($\sim$10nm) of FeRh, a well-known magnetostructural material, were studied with structural and magnetic probes in a rapidly solidified phase-separated system of (FeRh)$_{5}$Cu$_{95}$. Magnetization studies indicate a dramatic reduction in the magnetostructural phase transition temperature (T$_{t}$) of the nanoscaled FeRh phase relative to the bulk value ($\Delta$T=T$_{t,Bulk}$ - T$_{t,Nano}$ = 220 K). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) reveals a coherent orientational relationship between the FeRh (a$_{FeRh}$ = 3.09 {\AA})and Cu (a$_{Cu}$ = 3.78 {\AA}) phases. At the matrix/precipitate interface a constrained misfit strain of $\epsilon$ = 0.18 is observed. The reduction of the magnetostructural phase transition temperature and evolution of the magnetic properties with system annealing is analyzed in the context of the strain between the FeRh nanoparticles and the Cu matrix.   

Biomimetic Control of Magnetite Shape and Morphology using Polyaminoacids

Inspired by nature, this work explores the use of randomly sequenced poly(aminoacids)s to control the size, morphology and magnetic properties of magnetite via synthetic methods in a controlled manner as in the case of magnetotactic bacteria. Aqueous partial oxidation and chemical precipitation methods are employed for the synthesis of 7 - 50 nm iron oxide nanoparticles at room temperature. X -- ray diffraction (XRD) and Transmission Electron Microscopy (TEM) revealed formation of iron oxide nanoparticles both in the presence and absence of poly(amino acids). In the presence of random poly(amino acid)s with different compositions consisting of E, K and A amino acids the mean particle size for the chemical precipitation method is decreased regardless of amino acid composition. For partial oxidation method, mean particle size is also decreased and nanoparticle strings are observed while synthesized in the presence of poly(aspartic acid). Magnetic properties of particles obtained via different routes are also investigated. This provides a bio-inspired route for control over size, morphology and magnetic properties of magnetite nanoparticles.   

High Resolution Far Infrared Study of Antiferromagnetic Resonance Transitions in ${\alpha-}Fe_{2}O_{3}$ (hematite)

In this study, we report high resolution optical measurements of the temperature dependence of the antiferromagnetic (AFM) transition in ${\alpha-}Fe_{2}O_{3}$ (hematite) between $(0.5$ and $10)$ cm$^{-1}$. The absorption peak position, over a large temperature range, is found to be in agreement with a modified spin-wave model at both the high and low temperature phases, where the temperature is above and below the Morin transition temperature, respectively. The high spectral resolution optical measurements as demonstrated in this study allow unprecedented zero-field spectral analysis of the zone center AFM magnon in a previously challenging spectral region, giving insights into the role of temperature and strain on the exchange and anisotropy interactions in the system. The results also suggest that the frequency-resolved measurement platform could be extended for room-temperature non-destructive examination and imaging applications for antiferromagnetic materials and devices.   

Mapping Nanomagnetic Fields Using a Radical Pair Reaction

We visualized the magnetic field around ferromagnetic nanostructures using a combination of a standard epifluorescence microscope and a fluorescence chemical indicator of magnetic field (H. Lee et al., Nano Lett. DOI: 10.1021/nl202950h). The indicator was a chain-linked electron donor-acceptor molecule, phenanthrene-(CH$_{2})_{12}$-O-(CH$_{2})_{2}$-dimethylaniline, that forms spin-correlated radical pairs upon photoexcitation. The magnetic field altered the coherence spin dynamics, yielding an 80{\%} increase in exciplex fluorescence in a 0.1 T magnetic field. The magnetic field distributions were quantified to precision of 1.8 $\times $ 10$^{-4}$ T by image analysis and agreed with finite-element nonmagnetic simulations.   

Electrostatic Force Microscopy of Fe$_3$O$_4$ nanoparticles

The electronic compressibility is a fundamental property that characterizes the electronic properties of materials submitted to an external electric field. In metals (insulators), the electronic compressibility is large (small) and leads to a small (large) screening length. Variations of the screening length can be observed through measurements of the ``quantum'' capacitance between one material and a metallic counter-electrode. Using an Electrostatic Force Microscope (EFM), we measured maps of the local capacitance of 8 nm magnetite nanoparticles synthesized following the ``benzyl alcohol route'' deposited on a metallic substrate. Magnetite, an inverse spinel structure of composition Fe$_3$O$_4$, is a material with strongly correlated electronic properties and presents a metal-insulator transition at 120 K, the so-called Verwey transition. We present EFM measurements of these nanoparticles as a function of tip-sample distance and temperature.   

Nano-scaled magnetic domains in CMR-manganites

La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) is one of interesting materials with strongly-correlated electrons, wherein a complex variety of ground states are generated depending on the Sr doping concentration $x$. In this work, we have microscopically investigated changes of the magnetic states by applying magnetic fields in single crystals of LSMO by using Lorentz transmission electron microscopy. In the specimen with $x$ = 0.175, the magnetic stripe domains appear at regular intervals of about 200 nm as a magnetic ground state in zero magnetic field at 110 K. Importantly, we have clarified that magnetic domains as large as 100 nm are generated in the magnetic stripe domains in vertical magnetic fields and take a form of the magnetic vortex with tilted magnetic components. To the best of our knowledge, these magnetic domains are new kinds of magnetic ground states (spin textures) in manganites. In the presentation, we will explain detailed responses of magnetic vortices to various experimental parameters of external magnetic fields and discuss the nucleation and growth mechanism of magnetic vortices in the magnetic stripe domains and the expected functionality of magnetic vortices in manganites.   

Anomalous thermal hysteresis in the high-field magnetic moments of magnetic nanoparticles embedded in multi-walled carbon nanotubes

We report high-temperature (300-1120 K) magnetic properties of Fe and Fe$_{3}$O$_{4}$ nanoparticles embedded in multi-walled carbon nanotubes. We unambiguously show that the magnetic moments of Fe and Fe$_{3}$O$_{4}$ nanoparticles are seemingly enhanced by a factor of about 3 compared with what they would be expected to have for free (unembedded) magnetic nanoparticles. What is more intriguing is that the enhanced moments were completely lost when the sample was heated up to 1120 K and the lost moments at 1120 K were completely recovered through several thermal cycles below 1020 K. The anomalous thermal hysteresis of the high-field magnetic moments is unlikely to be explained by existing physical models except for the high-field paramagnetic Meissner effect due to the existence of ultrahigh temperature superconductivity in the multi-walled carbon nanotubes.   

Carbon Nanostraws with Novel Magnetic Properties for Microwave Devices and Biomedical Applications

Carbon nanotubes (CNTs) have stirred interest in many areas of current research because of their unique electrical properties and potential use in the biomedical field. Here, we report on the synthesis, structural, and magnetic characterization of ``nanostraws,'' which consist of nanoparticle-filled CNTs made by a template-assisted chemical vapor deposition method. In this study, the nanoparticle fillers are magnetite, cobalt ferrite, and nickel ferrite. These high-aspect ratio magnetic nanostructures have a tunable anisotropy in addition to enhanced magnetic interactions amongst the CNT-encapsulated magnetic nanoparticles. Enhanced magnetic interactions include higher saturation magnetization and higher blocking temperature. These properties are desirable for microwave devices and biosensing applications.