Surface-induced melting of metal nanoclusters

We investigate the size effect on melting of metal nanoclusters by molecular dynamics simulation and thermo dynamic theory based on Kofman's melt model. By the minimization of the free energy of metal nanoclusters with respect to the thickness of the surface liquid layer, it has been found that the nanoclusters of the same metal have the same premelting temperature T-pre = T-0 - T-0(gamma(su) - gamma(lv) - gamma(sl))/(rhoLxi) (T-0 is the melting point of bulk metal, gamma(sv) the solid-vapour interfacial free energy, gamma(sl) the liquid-vapour interfacial free energy, gamma(sl),l the solid-liquid interfacial free energy, p the density of metal, L the latent heat of bulk metal, and xi the characteristic length of surface-interface interaction) to be independent of the size of nanoclusters, so that the characteristic length of a metal can be obtained easily by T-pre, which can be obtained by experiments or molecular dynamics (MD) simulations. The premelting temperature T-pre of Cu is obtained by AID simulations, then xi is obtained. The melting point T-cm is further predicted by free energy analysis and is in good agreement with the result of our MD simulations. We also predict the maximum premelting-liquid width of Cu nanoclusters with various sizes and the critical size, below which there is no premelting.

The magnetic and structure properties of room-temperature ferromagnetic semiconductor (Ga,Mn)N

Diluted magnetic semiconductor (Ga,Mn)N were prepared by the implantation of Mn ions into GaN/Al2O3 substrate. Clear X-ray diffraction peak from (Ga,Mn)N is observed. It indicates that the solid solution (Ga,Mn)N phase was formed with the same lattice structure as GaN and different lattice constant. Magnetic hysteresis-loops of the (Ga,Mn)N were obtained at room temperature (293 K) with the coercivity of about 2496.97 A m(-1). (C) 2003 Elsevier B.V. All rights reserved.

Magnetization reversal and exchange bias effects in hard/soft ferromagnetic bilayers with orthogonal anisotropies

21 p.

Fluorescent Ag nanoclusters in glass induced by an infrared femtosecond laser

We report a method for the selective introduction of fluorescent Ag nanoclusters in glass. Extinction and photoluminescence spectra show that a fraction of the Ag atoms are generated through femtosecond laser induced multiphoton reduction and then aggregate to form Ag nanoclusters after heat treatment. Red luminescence from the irradiated region is observed under blue or green laser excitation. The fluorescence can be attributed to interband transitions within Ag nanoclusters. This method provides a novel route to fabricate fluorescent nanomaterials in 3D transparent materials. (c) 2007 Elsevier B.V. All rights reserved.

Structure and properties of an amorphous ferromagnetic alloy

The structure and the electrical and magnetic properties of an amorphous alloy containing approximately 80 at .% iron, 13 at.% phos phorus and 7 at.% carbon (Fe_(80)Fe_(13)C_7) obtained by rapid quenching from the liquid state have been studied. Transmission electron diffraction data confirm the amorphous nature of this alloy. An analysis of the radial distribution function obtained from X-ray diffraction data indicates that the number of nearest neighbors is approximately seven, at a distance of 2.6A. The structure of the alloy can be related to that of silicate glasses and is based on a random arrangement of trigonal prisms of Fe_2P and Fe_3C types in which the iron atoms have an average ligancy of seven. Electrical resistance measurements show that the alloys are metallic. A minimum in the electrical resistivity vs. temperature curve is observed between 10° K to 50° K depending on the specimen, and the temperature at which the minimum occurs is related to the degree of local ordering. The Fe-P-C amorphous alloys are ferromagnetic. The Curie temperature measured by the induction method and by Mossbauer spectroscopy is 315° C. The field dependence of the magneto-resistance at temperatures from liquid helium to room temperature is similar to that found in crystalline iron. The ordinary Hall coefficient is approximately 10^(-11) volt-cm/amp-G. The spontaneous Hall coefficient is about 0.6 x 10^(-9) volt-cm/amp-G and is practically independent of temperature from liquid helium temperature up to 300° c.

Ferromagnetic Resonance Force Microscopy

A new approach to magnetic resonance was introduced in 1992 based upon detection of spin-induced forces by J. Sidles [1]. This technique, now called magnetic resonance force microscopy (MRFM), was first demonstrated that same year via electron paramagnetic resonance (EPR) by D. Rugar et al. [2]. This new method combines principles of magnetic resonance with those of scanned probe technology to detect spin resonance through mechanical, rather than inductive, means. In this thesis the development and use of ferromagnetic resonance force microscopy (FMRFM) is described. This variant of MRFM, which allows investigation of ferromagnetic samples, was first demonstrated in 1996 by Z. Zhang et al. [3]. FMRFM enables characterization of (a) the dynamic magnetic properties of microscale magnetic devices, and (b) the spatial dependence of ferromagnetic resonance within a sample. Both are impossible with conventional ferromagnetic resonance techniques.

Ferromagnetically coupled systems, however, pose unique challenges for force detection. In this thesis the attainable spatial resolution - and the underlying physical mechanisms that determine it - are established. We analyze the dependence of the magnetostatic modes upon sample dimensions using a series of microscale yttrium iron garnet (YIG) samples. Mapping of mode amplitudes within these sample is attained with an unprecedented spatial resolution of 15μm. The modes, never before analyzed on this scale, fit simple models developed in this thesis for samples of micron dimensions. The application of stronger gradient fields induces localized perturbation of the ferromagnetic resonance modes. The first demonstrations of this effect are presented in this study, and a simple theoretical model is developed to explain our observations. The results indicate that the characteristics of the locally-detected ferromagnetic modes are still largely determined by the external fields and dimensions of the entire sample, rather than by the localized interaction volume (i.e., the locale most strongly affected by the local gradient field). Establishing this is a crucial first step toward understanding FMRFM in the high gradient field limit where the dispersion relations become locally determined. In this high gradient field regime, FMRFM imaging becomes analogous with that of EPR MRFM.

FMRFM has also been employed to characterize magnetic multilayers, similar to those utilized in giant magnetoresistance (GMR) devices, on a lateral scale 40 x 40μm. This is orders of magnitude smaller than possible via conventional methods. Anisotropy energies, thickness, and interface qualities of individual layers have been resolved.

This initial work clearly demonstrates the immense and unique potential that FMRFM offers for characterizing advanced magnetic nanostructures and magnetic devices.

I. Anisotropy and crystal structure of ferromagnetic thin films. II. Investigations into magnetic microstructure by Lorentz microscopy

The induced magnetic uniaxial anisotropy of Ni-Fe alloy films has been shown to be related to the crystal structure of the film. By use of electron diffraction, the crystal structure or vacuum-deposited films was determined over the composition range 5% to 85% Ni, with substrate temperature during deposition at various temperatures in the range 25° to 500° C. The phase diagram determined in this way has boundaries which are in fair agreement with the equilibrium boundaries for bulk material above 400°C. The (α+ ɤ) mixture phase disappears below 100°C.

The measurement of uniaxial anisotropy field for 25% Ni-Fe alloy films deposited at temperatures in the range -80°C to 375°C has been carried out. Comparison of the crystal structure phase diagram with the present data and those published by Wilts indicates that the anisotropy is strongly sensitive to crystal structure. Others have proposed pair ordering as an important source of anisotropy because of an apparent peak in the anisotropy energy at about 50% Ni composition. The present work shows no such peak, and leads to the conclusion that pair ordering cannot be a dominant contributor.

Width of the 180° domain wall in 76% Ni-Fe alloy films as a function of film thickness up to 1800 Å was measured using the defocused mode of Lorentz microscopy. For the thinner films, the measured wall widths are in good agreement with earlier data obtained by Fuchs. For films thicker than 800 Å, the wall width increases with film thickness to about 9000 Å at 1800 Å film thickness. Similar measurements for polycrystalline Co films with thickness from 200 to 1500 Å have been made. The wall width increases from 3000 Å at 400 Å film thickness to about 6000 Å at 1500 Å film thickness. The wall widths for Ni-Fe and Co films are much greater than predicted by present theories. The validity of the classical determination of wall width is discussed, and the comparison of the present data with theoretical results is given.

Finally, an experimental study of ripple by Lorentz microscopy in Ni-Fe alloy films has been carried out. The following should be noted: (1) the only practical way to determine experimentally a meaningful wavelength is to find a well-defined ripple periodicity by visual inspection of a photomicrograph. (2) The average wavelength is of the order of 1µ. This value is in reasonable agreement with the main wavelength predicted by the theories developed by others. The dependence of wavelength on substrate deposition temperature, alloy composition and the external magnetic field has been also studied and the results are compared with theoretical predictions. (3) The experimental fact that the ripple structure could not be observed in completely epitaxial films gives confirmation that the ripple results from the randomness of crystallite orientation. Furthermore, the experimental observation that the ripple disappeared in the range 71 and 75% Ni supports the theory that the ripple amplitude is directly dependent on the crystalline anisotropy. An attempt to experimentally determine the order of magnitude of the ripple angle was carried out. The measured angle was about 0.02 rad. The discrepancy between the experimental data and the theoretical prediction is serious. The accurate experimental determination of ripple angle is an unsolved problem.

Spin wave resonance in ferromagnetic films

The objective of this investigation has been a theoretical and experimental understanding of ferromagnetic resonance phenomena in ferromagnetic thin films, and a consequent understanding of several important physical properties of these films. Significant results have been obtained by ferromagnetic resonance, hysteresis, torque magnetometer, He ion backscattering, and X-ray fluorescence measurements for nickel-iron alloy films.

Taking into account all relevant magnetic fields, including the applied, demagnetizing, effective anisotropy and exchange fields, the spin wave resonance condition applicable to the thin film geometry is presented. On the basis of the simple exchange interaction model it is concluded that the normal resonance modes of an ideal film are expected to be unpinned. The possibility of nonideality near the surface of a real film was considered by means of surface anisotropy field, inhomogeneity in demagnetizing field and inhomogeneity of magnetization models. Numerical results obtained for reasonable parameters in all cases show that they negligibly perturb the resonance fields and the higher order mode shapes from those of the unpinned modes of ideal films for thicknesses greater than 1000 Å. On the other hand for films thinner than 1000 Å the resonance field deviations can be significant even though the modes are very nearly unpinned. A previously unnoticed but important feature of all three models is that the interpretation of the first resonance mode as the uniform mode of an ideal film allows an accurate measurement of the average effective demagnetizing field over the film volume. Furthermore, it is demonstrated that it is possible to choose parameters which give indistinguishable predictions for all three models, making it difficult to uniquely ascertain the source of spin pinning in real films from resonance measurements alone.

Spin wave resonance measurements of 81% Ni-19% Fe coevaporated films 30 to 9000 Å thick, at frequencies from 1 to 8 GHz, at room temperature, and with the static magnetic field parallel and perpendicular to the film plane have been performed. A self-consistent analysis of the results for films thicker than 1000 Å, in which multiple excitations can be observed, shows for the first time that a unique value of exchange constant A can only be obtained by the use of unpinned mode assignments. This evidence and the resonance behavior of films thinner than 1000 Å strongly imply that the magnetization at the surfaces of permalloy films is very weakly pinned. However, resonance measurements alone cannot determine whether this pinning is due to a surface anisotropy, an inhomogeneous demagnetizing field or an inhomogeneous magnetization. The above analysis yields a value of 4πM=10,100 Oe and A = (1.03 ± .05) x 10^-6 erg/cm for this alloy. The ability to obtain a unique value of A suggests that spin wave resonance can be used to accurately characterize the exchange interaction in a ferromagnet.

In an effort to resolve the ambiguity of the source of pinning of the magnetization, a correlation of the ratio of magnetic moment and X-ray film thickness with the value of effective demagnetizing field 4πNM as determined from resonance, for films 45 to 300 Å has been performed. The remarkable agreement of both quantities and a comparison with the predictions of five distinct models, strongly imply that the thickness dependence of both quantities is related to a thickness dependent average saturation magnetization, which is far below 10,100 Oe for very thin films. However, a series of complementary experiments shows that this large decrease of average saturation magnetization cannot be simply explained by either oxidation or interdiffusion processes. It can only be satisfactorily explained by an intrinsic decrease of the average saturation magnetization for very thin films, an effect which cannot be justified by any simple physical considerations.

Recognizing that this decrease of average saturation magnetization could be due to an oxidation process, a correlation of resonance measurements, He ion backscattering, X-ray fluorescence and torque magnetometer measurements, for films 40 to 3500 Å thick has been performed. On basis of these measurements it is unambiguously established that the oxide layer on the surface of purposefully oxidized 81% Ni-19% Fe evaporated films is predominantly Fe-oxide, and that in the oxidation process Fe atoms are removed from the bulk of the film to depths of thousands of angstroms. Extrapolation of results for pure Fe films indicates that the oxide is most likely α-Fe₂O₃. These conclusions are in agreement with results from old metallurgical studies of high temperature oxidation of bulk Fe and Ni-Fe alloys. However, X-ray fluorescence results for films oxidized at room temperature, show that although the preferential oxidation of Fe also takes place in these films, the extent of this process is by far too small to explain the large variation of their average saturation magnetization with film thickness.

Effects of magnetostriction and superlattice formation in ferromagnetic thin films

The contribution to the magnetic uniaxial perpendicular anisotropy which arises from substrate constraint through magnetostrictive effects has been measured in Ni-Fe and Ni-Co thin films evaporated on substrates at room temperature. This was accomplished by measuring the perpendicular anisotropy before and after removal of the film from the substrate. Data are given for the fcc crystal structure regions of both alloy systems, but data for Ni-Co include compositions with less than 60% Ni which have a small percentage of the hcp phase mixed with the fcc phase. The constraint contribution to the perpendicular anisotropy correlates well with the value of the bulk magnetostriction constant using the equation ∆K_˔=3/2λ_sσ. Measured values of isotropic stress for films thicker than 600 Å were 1.6 x 10¹⁰ dyn/cm². In films less than 600 Å thick the isotropic stress decreased with decreasing thickness. After removal of the films from the substrates, the measured perpendicular anisotropy deviated from the expected geometrical shape anisotropy near pure Ni in both alloys. This indicates that additional significant sources of anisotropy exist at these compositions.

The effect of substrate constraint on the crystalline anisotropy K₁ of Ni-Fe epitaxial films has been studied by use of a film removal technique, which involves the evaporation of an epitaxial layer of LiF on MgO, the epitaxial growth of the metallic film on the LiF, and the stripping of the film with water soluble tape. Films ranging in composition from 50% to 100% Ni have been studied. For compositions below 90% Ni the experimental values agree reasonably well with the first order theoretical prediction, ∆K₁=[-9/4(C₁₁-C₁₂)λ² 100+9/2C₄₄λ²111].

In order to compare the magnetic properties of epitaxial thin films more completely with the properties of bulk single crystals, Ni-Fe films ranging in composition from 60% to 90% Ni, which were evaporated epitaxially on (100) MgO substrates, have been subsequently annealed at 400°C in a vacuum of less than 10^-7 Torr to form the ordered Ni₃Fe structure near the 75% composition. This ordered structure has been confirmed by electron diffraction.

The saturation magnetization at Ni₃Fe increased about 6% with ordering which is in good agreement with previous bulk data. Measurements of the magnetocrystalline anisotropy energy K₁ for the epitaxial films show the same large changes with ordering as observed in bulk single crystal samples. In the (001) plane the magnetostriction constants ^λ100, ^λ111 are directly related to the induced anisotropy due to a uniform uniaxial strain in the [100] and [110] directions respectively. Assuming that the elastic constants of a film are the same as in bulk material and are unchanged by ordering, the changes in strain sensitivity with ordering for the epitaxial films are found to be in good agreement with values predicted from bulk data. The exchange constant A as measured by ferromagnetic resonance has been measured at the Ni₃Fe composition and found to increase 25% with ordering. This seems to indicate a significant increase in the Curie temperature which has only been inferred indirectly for bulk material.

An Embedded Stress Sensor for Concrete SHM Based on Amorphous Ferromagnetic Microwires

A new smart concrete aggregate design as a candidate for applications in structural health monitoring (SHM) of critical elements in civil infrastructure is proposed. The cement-based stress/strain sensor was developed by utilizing the stress/strain sensing properties of a magnetic microwire embedded in cement-based composite (MMCC). This is a contact-less type sensor that measures variations of magnetic properties resulting from stress variations. Sensors made of these materials can be designed to satisfy the specific demand for an economic way to monitor concrete infrastructure health. For this purpose, we embedded a thin magnetic microwire in the core of a cement-based cylinder, which was inserted into the concrete specimen under study as an extra aggregate. The experimental results show that the embedded MMCC sensor is capable of measuring internal compressive stress around the range of 1-30 MPa. Two stress sensing properties of the embedded sensor under uniaxial compression were studied: the peak amplitude and peak position of magnetic switching field. The sensitivity values for the amplitude and position within the measured range were 5 mV/MPa and 2.5 mu s/MPa, respectively.

Higher order vortex gyrotropic modes in circular ferromagnetic nanodots

Magnetic vortex that consists of an in-plane curling magnetization configuration and a needle-like core region with out-of-plane magnetization is known to be the ground state of geometrically confined submicron soft magnetic elements. Here magnetodynamics of relatively thick (50-100 nm) circular Ni80Fe20 dots were probed by broadband ferromagnetic resonance in the absence of external magnetic field. Spin excitation modes related to the thickness dependent vortex core gyrotropic dynamics were detected experimentally in the gigahertz frequency range. Both analytical theory and micromagnetic simulations revealed that these exchange dominated modes are flexure oscillations of the vortex core string with n = 0,1,2 nodes along the dot thickness. The intensity of the mode with n = 1 depends significantly on both dot thickness and diameter and in some cases is higher than the one of the uniform mode with n = 0. This opens promising perspectives in the area of spin transfer torque oscillators.

Enhancement of ferromagnetic properties in In1.99Co0.01O3 by additional Cu doping

This paper reports room-temperature ferromagnetism in Co- and Cu-doped In2O3 samples synthesized by a solid-state reaction method. Structure and composition analyses revealed that Co and Cu were incorporated into the In2O3 lattices. Photoluminescence measurement revealed an additional emission at 520 urn from these doped samples. The magnetic measurement showed that additional Cu doping greatly enhanced the ferromagnetism of In1.99Co0.01O3 bulk samples. The implication of the effects of additional Cu doping is also discussed. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.