G. C. Hadjipanayis

Beating the superparamagnetic limit with exchange bias

Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called ‘superparamagnetic limit’ in recording media. Here we show that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems can provide an extra source of anisotropy, leading to magnetization stability. We demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.

Science and Technology of Nanostructured Magnetic Materials

Thin Films, Surfaces and Interfaces -- Electronic Structure and Magnetism of Metal Surfaces, Overlayers and Interfaces -- Growth and Magnetic Properties of Metastable Structures -- Spin-Resolved Photoemission -- Correlation of Crystalline and Electronic Structure in Epitaxial FCC-Cobalt Monolayers on Cu(100) -- Hybrid Ferromagnetic/Semiconductor Structures -- Mossbauer Studies of Ultrathin Magnetic Films of Fe/Ag(100) -- Spin-Dependence of Absorbed and Reflected Current on Fe(110) -- MBE Growth of Metal/Semiconductor Interfaces -- Surface and Interface Magnetism -- Ferromagnetic Resonance Studies of BCC Epitaxial Ultrathin Fe(001)/Cu(001) Bilayers and Fe(001)/Cu(001)/Fe(001) Trilayers -- Laser Ablation Deposition of Metallic Thin Films -- Exchange Coupled Films for Magneto-Optic Applications -- Temperature Dependence of Micromagnetic Domain Structure in Cobalt Films -- Hyperfine Interaction Techniques Applied to the Study of Surfaces and Interfaces -- Surface Magnetostriction -- Multilayers -- Magnetic Rare Earth Artificial Metallic Superlattices -- X-Ray Characterization of Magnetic Multilayers and Superlattices -- The Characterization of Interface Roughness and Other Defects in Multilayers by X-Ray Scattering -- Magnetism of Nanostructured Rare Earth Multilayers -- FMR Studies of Metallic Magnetic Thin Films in Layered Structures -- Compositionally Modulated Magnetic Multilayers: Temperature- and Modulation-Dependent Properties -- Structural and Magnetic Properties of Epitaxial Co/Pd Superlattices -- First-Principles Calculation of the Magnetocrystalline Anisotropy Energy of ConPdm Multilayers -- Structural and Magnetic Studies in Co-Pt Multilayers -- Magnetic Properties of Hexagonal Fe/Ru Superlattices With Short Periodicity -- Magnetic Studies of Fe-Si Compositionally Modulated Thin Films -- Mossbauer Spectroscopy of the Fe/Ni Interface -- Analysis of Amorphous Dysprosium-Transition Metal Nanoscale Magnetic Multilayers -- Transport Properties of Thin Metallic Films and Multilayers -- Domain Walls, Magnetic Domains And Techniques For Their Observation -- Micromagnetics of Longitudinal Recording Media -- MO-Recording: The Switching Process and Its Relation to the Magnetic Properties of Thin Films -- Micromagnetic Computations of Magnetization Configurations -- Domain Walls and Wall Structure -- Domain Wall Multiplication in Amorphous Ferromagnetic Alloys -- Electron Microscope Methods for Imaging Internal Magnetic Fields at High Spatial Resolution -- Scanning Tunneling Microscopy and Force Microscopy Applied to Magnetic Materials -- Special Session on Spin-Polarized Vacuum Tunneling -- Magnetic Imaging Via Scanning Electron Microscopy with Polarization Analysis -- Atomic Scale Probe into High-Tc Superconductors Using Scanning Tunnelling Microscopy -- Magnetic Anisotropy and Random Magnets -- Magnetic Anisotropy -- Random Anisotropy in Magnetic Materials -- Perpendicular and In-Plane Anisotropy in Amorphous Tb-Fe -- Magnetostriction in Amorphous Ferromagnets -- Anderson Localization in 3-Dimensional Amorphous Alloys: Evolution with the Content of Magnetic Ions -- On the Law of Approach to Saturation in the Series of Amorphous Alloys a-Dyxd1?xNi -- Magnetoresistance of Amorphous U1?xSbx Films -- Absence of Temperature-Driven First-Order Phase Transitions in Systems with Random Bonds -- Magnetic Semiconductors and Intermetallic Compounds -- Magnetic Behavior of Diluted Magnetic Semiconductors -- Intermetallic Compounds and Crystal Field Interactions -- Crystal-Field and Exchange Interactions in Hard Magnetic Materials -- First Order Magnetization Processes -- Structure and Properties of Novel Ternary Fe-Rich Rare-Earth Carbides -- Fine Particles -- Granular Solids -- Ultrafine Magnetic Particles -- Magnetic Nanometer Systems and Mossbauer Spectroscopy -- Some Topics in Fine Particle Magnetism -- Mossbauer Studies of Fine Fe-Based Particles -- Mossbauer Studies of Fine Particles of Fe-Cr-B -- Chemical Preparation of Amorphous Fe-Cr-B Particles -- Composition and Structure of Fe-Ni-B Alloy Particles Prepared by Chemical Reduction with NaBH4 -- Quantum Effects in Ultrafine Nd-Fe-B Particles -- Magnetization Reversal in Clusters of Magnetic Particles -- Electric and Magnetic Properties of Small Systems -- Existence of Frequency Cut-Off in the Spin Wave Spectrum of Small Magnetic Particles -- Magnetic Hysteresis and Permanent Magnets -- Mechanically Alloyed Permanent Magnets -- Melt-Spun Magnets -- Solid NdFeB Magnets Made by Gas Atomization and Extrusion -- The Role of Microstructure in Permanent Magnets -- Lorentz Microscopy Studies in Permanent Magnets -- Coercivity in Hard Magnetic Materials -- Micromagnetism and Magnetization Processes in Modern Magnetic Materials -- Micromagnetic Approach to Magnetic Hysteresis -- Magnetic Hysteresis in Disordered Magnets -- Coercivity of Nanostructured Materials -- Magnetic Hysteresis of CoPt Films -- Technology and Applications of Permanent Magnets -- Author Index.

Metallic iron nanoparticles for MRI contrast enhancement and local hyperthermia.

Current magnetic-nanoparticle technology is challenging due to the limited magnetic properties of iron oxide nanoparticles (IONPs). Increasing the saturation magnetization of magnetic nanoparticles may permit more effective development of multifunctional agents for simultaneous targeted cell delivery, magnetic resonance imaging (MRI) contrast enhancement, and targeted cancer therapy in the form of local hyperthermia. We describe the synthesis and characterization of novel iron-based nanoparticles (FeNPs) coated with biocompatible bis-carboxyl-terminated poly(ethylene glycol) (cPEG). In comparison to conventional IONPs similar in size (10 nm), FeNPs particles have a much greater magnetization and coercivity based on hysteresis loops from sample magnetometry. Increased magnetization afforded by the FeNPs permits more effective generation of local hyperthermia than IONPs when subjected to an oscillating magnetic field in a safe frequency range. Furthermore, FeNPs have a much stronger shortening effect on T2 relaxation time than IONPs, suggesting that FeNPs may be more effective MRI contrast agents. Next-generation FeNPs with a biocompatible coating may in the future be functionalized with the attachment of peptides specific to cancer cells for imaging and therapy in the form of local hyperthermia.

Cobalt‐free permanent magnet materials based on iron‐rare‐earth alloys (invited)

The magnetic properties of rapidly quenched FeRM alloys where R=La,Y,Pr,Nd,Gd and M=B,Si,Al,Ga,Ge have been investigated over a wide range of chemical compositions. The samples are generally magnetically soft in the as‐quenched state. Magnetic hardening is produced by annealing the samples around 700 °C. The best properties have been obtained in samples containing Pr and Nd together with B and Si. An energy product of 13 MGOe and a coercive field of 15 kOe have been obtained in a Fe76Pr16B5Si3 sample. The higher Fe content samples appear to be more promising with a potential energy product of 49 MGOe. Thermomagnetic data show that a structural transformation takes place upon heating the samples to 700 °C. The Curie temperature of the as‐quenched phase is around 160 °C while that of the new phase is around 320 °C. Transmission electron microscope studies show fine precipitates (∼100 A) dispersed in a matrix of different chemical composition. X‐ray and electron diffraction data indicate that the precipitate...

Sm–Co–Cu–Ti high-temperature permanent magnets

A class of promising permanent-magnet materials with an appreciable high-temperature coercivity of 8.6 kOe at 500 °C is reported. The Sm–Co–Cu–Ti magnets are prepared by arc melting and require a suitable heat treatment. Magnetization measurements as a function of temperature and x-ray diffraction patterns indicate that the samples are two-phase mixtures of 2:17 and 1:5 structures. Depending on heat treatment and composition, some of the magnets exhibit a positive temperature coefficient of coercivity. The promising high-temperature behavior of the coercivity is ascribed to the temperature dependence of the domain-wall energy, which affects the curvature of the walls and the pinning behavior.

Nanocomposite R2Fe14B/Fe exchange coupled magnets

We have studied the crystallization, crystal structure, microstructure and magnetic properties of R‐Fe‐B (R=Nd,Pr,Dy,Tb) based melt‐spun ribbons consisting of a mixture of R2Fe14B and α‐Fe phases. All the samples crystallize first to α‐Fe and a metastable phase (Y3Fe62B14 for R=Nd,Pr,Dy and TbCu7 for R=Tb) before they finally transform to 2:14:1 and α‐Fe. The highest values of coercivity and reduced remanence, 4.5 and 0.63 kOe, respectively, were obtained in a Nd3.85Tb2(Fe‐Nb‐B)94.15 sample. These properties are the result of a fine grain microstructure consisting of a mixture of α‐Fe and 2:14:1 having an average grain size of 30 nm.

CoPt and FePt thin films for high density recording media

Highly anisotropic face-centered-tetragonal (fct) CoPt and fct FePt nanoparticles have been prepared and embedded in a C or BN matrix by cosputtering from pure Co50Pt50, BN, and Fe50Pt50, C solid targets using a tandem deposition mode. The as-made films show a disordered face-centered-cubic structure which is magnetically soft and have low coercivity ( 107 erg/cm3. Transmission electron microscope studies showed FePt particles embedded in C matrix with a size increasing from below 3 nm in the as-made state to about 8 nm in the optimum annealed state. These results are very promising and make these materials potential candidates for high-density magnetic recording.

New iron‐rare‐earth based permanent magnet materials

The magnetic and structural properties of rapidly quenched FePr(BSi) alloys are presented. The samples are usually magnetically soft in the as‐quenched state. However, large coercive fields in the range of 5–20 kOe are developed upon heating the samples to a temperature around 700 °C. The best properties have been obtained on a Fe76Pr16B5Si3 sample with a maximum energy product (BH)m≂12 MGOe. This value is much better than that of AlNiCo and makes these materials outstanding as cobalt‐free permanent magnets.

CoPt and FePt nanowires by electrodeposition

In this study we have fabricated by electrodeposition CoPt(FePt) nanowires embedded inside an array of empty holes in anodized aluminum disks. By adjusting the current density and solution composition, one can control the composition of the film to 50:50 in order to obtain the high anisotropy CoPt and FePt face centered tetragonal (L10) phases. The as-made films are magnetically soft. Magnetic hardening is developed after annealing at 700 °C with coercivity typically in the range of 3–6 kOe. The scanning electron microscopy and transmission electron microscopy studies showed that nanowire structured CoPt and FePt with near stoichiometric composition inside an array of holes which are about 25–100 nm in diameter. A preferred perpendicular anisotropy is observed in the CoPt nanowires.

Preparation of manganese ferrite fine particles from aqueous solution

Fine manganese ferrite particles have been prepared by a coprecipitation method and subsequent digestion process (below 100°C). Manganous salts mixed with either ferric or ferrous salts were coprecipitated with sodium hydroxide. Particles produced from ferric salts were MnFe2O4 with relatively smaller sizes (5 to 25 nm) while ferrous salts created MnxFe3−xO4 (0.2 < × < 0.7) with bigger sizes up to 180 nm. In either case, the particle size appeared to be a unique function of the ratio of metal ion concentration to hydroxide ion concentration when the digestion conditions were fixed. For the system with ferric salts, the undigested samples were polycrystals with crystallite sizes of about 2 nm. Digestion, which could be described as an Ostwald ripening process, did not change the crystalline structure but increased both the crystallite size and the particle size. A basic solution was essential for an effective digestion process in this system. The system with ferrous salts, on the contrary, needed an acidic solution to create a single ferrite phase. Digestion changed both the crystalline structure and the particle size of the precipitated precursors. This process involved a dissolution and renucleation/growth mechanism. Cation and anion effects on the particle size and the evolution during digestion were also studied.