Subhankar Bedanta

Collective states of interacting ferromagnetic nanoparticles

Abstract Discontinuous magnetic multilayers [CoFe/Al 2 O 3 ] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co 80 Fe 20 nanoparticles are embedded in a diamagnetic insulating a-Al 2 O 3 matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.

The origin of ferromagnetism in 57Fe ion-implanted semiconducting 6H-polytype silicon carbide

Semiconducting (mostly p-doped) single crystals of the 6H-polytype of α-SiC(0001) were implanted with 57Fe ions with a nominal dose of 1.0 × 1016, 2.0 × 1016, 3.0 × 1016 or 2.0 × 1017 cm−2 (high-dose sample p-hd) at 100 or 200 keV ion energy in order to produce diluted magnetic semiconductors (DMSs). After implantation all samples (except p-hd) were subject to rapid thermal annealing at 1000 °C for 2 min. The structure was investigated by x-ray diffraction, high-resolution cross-sectional transmission electron microscopy and sputter-Auger depth profiling. The magnetic properties were obtained from superconducting quantum interference device (SQUID) magnetometry and 57Fe conversion electron Mossbauer spectroscopy (CEMS) at room temperature (RT) and 4.2 K. Our combined results obtained by several techniques prove unambiguously that ferromagnetism in 57Fe-implanted SiC for Fe concentrations above 3% originates mostly from epitaxial superparamagnetic Fe3Si (and possibly a small fraction of Fe nanoparticles) in the SiC matrix. We find a wide range of blocking temperatures, TB, which start from 400 K for a dose of 2.0 × 1016 cm−2, and shift downwards to ~220 K for 3.0 × 1016 cm−2. For the lowest dose of 1.0 × 1016 cm−2 at 200 keV, we find evidence of ferromagnetism below 20 K via weak magnetic hyperfine interaction. Our measurements suggest that for a maximum Fe concentration in the range of 1–3%, which corresponds to this lowest Fe dose, the possibility exists to obtain a DMS in Fe-implanted SiC, prepared at lower or equal implantation doses.

Magnetic nanoparticles: a subject for both fundamental research and applications

Single domain magnetic nanoparticles (MNPs) have been a vivid subject of intense research for the last fifty years. Preparation of magnetic nanoparticles and nanostructures has been achieved by both bottom-up and top-down approaches. Single domain MNPs show Neel-Brown-like relaxation. The Stoner-Wohlfarth model describes the angular dependence of the switching of the magnetization of a single domain particle in applied magnetic fields. By varying the spacing between the particles, the interparticle interactions can be tuned. This leads to various supermagnetic states such as superparamagnetism, superspin glass, and superferromagnetism. Recently, the study of the magnetization dynamics of such single domain MNPs has attracted particular attention, and observations of various collective spin wave modes in patterned nanomagnet arrays have opened new avenues for on-chip microwave communications. MNPs have the potential for various other applications such as future recording media and in medicine. We will discuss the various aspects involved in the research on MNPs.

Spin cluster glass and magnetoelectricity in Mn-doped KTaO3

In ceramics of KTaO3 doped with 3 at. % of Mn the dielectric response is dominated by the polydispersive behavior of Mn2+ centered polar regions, whereas the magnetic and magnetoelectric (ME) behaviors reflect an intimate coupling between A-site substituted Mn2+ ions and minute amounts of Mn3O4 precipitates mediated by the polar host material. This becomes apparent by the common onset at Tc≈42 K of the ordering of ferrimagnetic Mn3O4 and of a spin cluster glass, which is characterized by memory and rejuvenation effects. The composite magnetic system exposed to external magnetic and electric dc fields shows large third order ME susceptibility with a sharp anomaly at Tc and 1/T2 dependence as T→0.

Superferromagnetism in dipolarly coupled L10 FePt nanodots with perpendicular magnetization

The magnetization reversal for perpendicularly magnetized L10-FePt (001) nanodots with different interdot distances was studied by magnetic domain observation. We show the results for two kinds of dot arrays: (i) the dots with physical percolation leading to direct exchange coupling, and (ii) the dots which are fully isolated experiencing only dipolar interaction. For the physically percolated dot array, ferromagnetic domains were observed in which domain expanded with magnetic field. On the other hand, the array with the isolated FePt dots also exhibited domain like features resembling to the percolated dots, indicating the existence of superferromagnetism in the array of FePt nanodots.

Interplay of uniaxial and cubic anisotropy in epitaxial Fe thin films on MgO (001) substrate

Epitaxial Fe thin films were grown on annealed MgO(001) substrates at oblique incidence by DC magnetron sputtering. Due to the oblique growth configuration, uniaxial anisotropy was found to be superimposed on the expected four-fold cubic anisotropy. A detailed study of in-plane magnetic hysteresis for Fe on MgO thin films has been performed by Magneto Optic Kerr Effect (MOKE) magnetometer. Both single step and double step loops have been observed depending on the angle between the applied field and easy axis i.e. along ⟨100⟩ direction. Domain images during magnetization reversal were captured by Kerr microscope. Domain images clearly evidence two successive and separate 90° domain wall (DW) nucleation and motion along cubic easy cum uniaxial easy axis and cubic easy cum uniaxial hard axis, respectively. However, along cubic hard axis two 180° domain wall motion dominate the magnetization reversal process. In spite of having four-fold anisotropy it is essential to explain magnetization reversal mechanism in 0°< ϕ < 90° span as uniaxial anisotropy plays a major role in this system. Also it is shown that substrate rotation can suppress the effect of uniaxial anisotropy superimposed on four-fold anisotropy.

Spacer layer and temperature driven magnetic properties in multilayer structured FeTaC thin films

We report the effects of thickness of spacer layers and temperature on the magnetic properties of multilayer structured [FeTaC(50 nm)/Ta(x nm)]n=3/FeTaC(50 nm)/substrate amorphous thin films. A transcritical loop with a large coercivity (HC) of 25 Oe was observed for x = 0 film, but the loop shape was changed to flat loop along with a rapid decrease in HC (< 0.25 Oe) by introducing Ta spacer layers. This is attributed to loss of perpendicular anisotropy causing a transition from stripe domain structure to in-plane orientation of spins. Magnetic hysteresis loops measured at different temperatures exhibited a spacer layers’ thickness dependent multistep magnetization reversal processes for temperature below 80 K. Thermomagnetization curves obtained under zero-field-cooled and field-cooled conditions displayed a bifurcation between them for x = 0 film. However, the bifurcation point was shifted to lower temperatures with increasing x and disappeared eventually for films with x 4. High-temperature magnetization data revealed no significant changes in the magnetic properties up to Curie temperature. The observed results are elucidated on the basis of change in magnetic structure with the thickness of the spacer layers, pinhole effects at the interface, and temperature, instigating an effective reduction in perpendicular anisotropy and magnetic disorder, and thereby enhancing magnetic properties in multilayer thin films. (Some figures may appear in colour only in the online journal)

FeSi diffusion barriers in Fe/FeSi/Si/FeSi/Fe multilayers and oscillatory antiferromagnetic exchange coupling

We study the diffusion of 57 Fe probe atoms in Fe/FeSi/Si/FeSi/Fe multilayers on Si(111) prepared by molecular beam epitaxy by means of 57 Fe conversion electron Mossbauer spectroscopy (CEMS). We demonstrate that the application of FeSi boundary layers successfully inhibits the diffusion of 57 Fe into the Si layer. The critical thickness for the complete prevention of Fe diffusion takes place at a nominal FeSi thickness of tFeSi = 10-12 u A, which was confirmed by the evolution of the isomer shift δ of the crucial CEM subspectrum. The formation of the slightly defective c-FeSi phase for thicker FeSi boundary layers (∼20 u A)

Synthesis, Properties, and Applications of Single-Domain Magnetic Nanoparticles

1 School of Physical Sciences, National Institute of Science Education and Research (NISER), IOP Campus, Bhubaneswar 751005, India 2Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake City, Kolkata 700098, India 3 Department of Physics, University Duisburg-Essen, 47057 Duisburg, Germany 4 Juelich Centre for Neutron Science JCNS and Peter Gruenberg Institute (PGI), JARA-FIT, Forschungszentrum Juelich GmbH, 52425 Juelich, Germany 5 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Dynamic magnetization properties of a superferromagnetic metal-insulator multilayer observed by magneto-optic Kerr microscopy

Longitudinal magneto-optical Kerr effect (MOKE) magnetometry and microscopy have been performed on a superferromagnetic (SFM) metal-insulator multilayer of [Co80Fe20(tn=1.3 nm)/Al2O3(3 nm)]10 to understand the dynamic behavior of the SFM domains. Frequency dependent hysteresis loops measured by MOKE show that the coercive field Hc increases as the rate (frequency) of magnetization reversal is increased similar to dynamic hysteresis loops observed on conventional ferromagnetic ultrathin films. This dynamic hysteresis behavior arises due to SFM friction-controlled domain wall motion which is observed by MOKE microscopy. We see that the nucleation field for SFM domains increases for hysteresis loops with higher frequency as expected for a ferroic system.