Core shell superparamagnetic iron cobalt alloy nanoparticles with functional metal silicate core shell interface and a magnetic core containing the nanoparticles

Abstract

Core shell nanoparticles of an iron-cobalt alloy core, a silicon dioxide shell and a metal silicate interface between the core and the shell are provided. The magnetic properties of the nanoparticles are tunable by control of the interface thickness. A magnetic core of high magnetic moment obtained by compression sintering the thermally annealed superparamagnetic core shell nanoparticles is also provided. The magnetic core has little core loss due to hysteresis or eddy current flow.

Claims

1. A superparamagnetic core shell nanoparticle, comprising: a superparamagnetic core comprising an iron cobalt alloy; a shell of a silicon dioxide coating the core; and a metal silicate interface layer between the core and the silicon dioxide shell; wherein a diameter of the iron cobalt alloy core is 200 nm or less, the core shell particle is obtained by a process comprising: wet chemical precipitation of the core; coating of the core with a wet chemical silicate synthesis to form the silicon dioxide shell, and the thickness of the metal silicate interface is controlled by the time of the wet chemical synthesis.

2. The superparamagnetic core shell nanoparticle according to claim 1, wherein the metal silicate of the interface layer comprises at least one of iron silicate and cobalt silicate.

3. The superparamagnetic core shell nanoparticle according to claim 1, wherein the thickness of the metal silicate interface layer is from 0.5 nm to 15 nm.

4. The superparamagnetic core shell nanoparticle according to claim 1, wherein the superparamagnetic core consists of an iron cobalt alloy.

5. The superparamagnetic core shell nanoparticle according to claim 1, wherein the diameter of the iron cobalt core is from 2 to 75 nm.

6. A magnetic core, comprising: a plurality of the superparamagnetic core shell nanoparticles according to claim 1; wherein the magnetic core is a monolithic structure of superparamagnetic core grains of an iron cobalt alloy directly bonded by the silicon dioxide shells, which form a silica matrix, and a layer comprising at least one of iron silicate and cobalt silicate interfaces the core to the matrix.

7. The magnetic core according to claim 6, wherein a space between individual superparamagnetic iron cobalt alloy nanoparticles is occupied substantially only by the silicon dioxide and the at least one of iron silicate and cobalt silicate.

8. The magnetic core according to claim 7, wherein the superparamagnetic core consists of an iron cobalt alloy.

9. The magnetic core according to claim 7, wherein at least 97% by volume of the space between the superparamagnetic core grains of iron cobalt alloy is occupied by silicon dioxide and the at least one of iron silicate and cobalt silicate.

10. The magnetic core according to claim 7, wherein an average grain size of the superparamagnetic core grains of iron cobalt alloy is from 2 to 160 nm.

11. A method to prepare a monolithic magnetic core, the magnetic core comprising superparamagnetic core shell particles having a particle size of less than 200 nm; the method comprising sintering the superparamagnetic core shell nanoparticles of claim 1 under heat and pressure and under flow of an inert gas to obtain a monolithic structure; wherein the core comprises a superparamagnetic iron cobalt alloy nanoparticle in a matrix of a silicon dioxide matrix and an interface layer between the core and the shell comprises at least one of iron silicate and cobalt silicate.

12. The method according to claim 11, further comprising heating the core shell nanoparticles at a temperature of from 125° C. to 600° C. for from 3 to 7200 seconds before the sintering under heat and pressure.

13. An electrical/magnetic conversion device, which comprises a magnetic core according to claim 6.

14. An electrical/magnetic conversion device, which comprises a magnetic core according to claim 7.

15. A vehicle part comprising the electrical/magnetic conversion device according to claim 13, wherein the part is selected from the group consisting of a motor, a generator, a transformer, an inductor and an alternator.

16. A vehicle part comprising the electrical/magnetic conversion device according to claim 14, wherein the part is selected from the group consisting of a motor, a generator, a transformer, an inductor and an alternator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a TEM image of the nanoparticles prepared in the Example.

(2) FIG. 2 shows a X-ray photoelectron spectroscopy performed over the Si2p binding energies for the nanoparticles prepared in the Example.

(3) FIG. 3 shows the relationship of magnetic saturation with increase of SiO.sub.2 shell thickness.

(4) FIG. 4A shows a TEM image of FeCo/SiO.sub.2 (3 nm) nanoparticles.

(5) FIG. 4B shows a high resolution XPS scan over the Fe2p binding energies for FeCo/SiO.sub.2 (3 nm) nanoparticles.

(6) FIG. 4C shows a high resolution XPS scan over the Co2p binding energies for FeCo/SiO.sub.2 (3 nm) nanoparticles.

(7) FIG. 4D shows a comparison of the Fe2p XPS scan with different SiO.sub.2 thickness.

(8) FIG. 5 shows Zero-field-cooled (ZFC) and field-cooled (FC) 5 mT DC susceptibility (χ) scans as a function of temperature for the three FeCo/SiO.sub.2 nanoparticle systems (3 nm, 4 nm and 6 nm).

(9) FIG. 6 shows AC susceptibility as a function of temperature and frequency for the three FeCo/SiO.sub.2 nanoparticle samples (3 nm, 4 nm and 6 nm) collected at 10, 100, 500, and 1000 Hz.

(10) FIG. 7 shows the temperature dependence of coercivity of the FeCo/SiO.sub.2 nanoparticles.

DETAILED DESCRIPTION OF THE INVENTION

(11) Throughout this description all ranges described include all values and sub-ranges therein, unless otherwise specified.

(12) Additionally, the indefinite article “a” or “an” carries the meaning of “one or more” throughout the description, unless otherwise specified.

(13) The inventors have discovered that the formation of individual FeCo alloy nanoparticles coated with silica shells of various thicknesses may be achieved via a scalable wet chemical process. Surprisingly, the inventors have discovered that formation of interfacial metal silicates may alter significantly the nanomagnetism in these ultra-high surface area FeCo alloy nanoparticle systems. Evidence that an interfacial layer of metal silicates had formed was observed in x-ray photoelectron spectra collected over the 2p transitions of Fe and Co; and as the thickness of the silica shell was increased (by altering the duration of the silica reaction) a thicker interfacial metal silicate layer was formed, increasing the nanoparticles' overall magnetic anisotropy, as evidenced by increased blocking temperatures and altered coercivities. Thus the inventors have surprisingly discovered that by producing superparamagnetic iron cobalt alloy nanoparticles that are encapsulated in silica shells with varying degree of wet synthesis treatment time, core shell FeCo nanoparticles having differing nanomagnetic properties may be obtained. Compacting and sintering these nanoparticles with tuned magnetic properties into a monolithic nanomaterial provides a core having zero (or very low) hysteresis and very low eddy current formation.

(14) Thus, the first embodiment of the present invention includes a superparamagnetic core shell nanoparticle, comprising: a superparamagnetic core of an iron cobalt alloy; a shell of a silicon dioxide coating the core; and a metal silicate interface layer between the core and the silicon dioxide shell; wherein a diameter of the iron cobalt alloy core is 200 nm or less, the core shell particle is obtained by a process comprising: wet chemical precipitation of the core; coating of the core with a wet chemical silicate synthesis to form the silicon dioxide shell, and the thickness of the metal silicate interface is controlled by the time of the wet chemical synthesis. In certain embodiments the diameter of the iron cobalt alloy nanoparticle core is 100 nm or less, and in further embodiments the diameter of the iron cobalt alloy nanoparticle core is from 2 nm to 50 nm.

(15) According to the invention, the iron cobalt alloy nanoparticle grains are of or approaching the size of the single particle magnetic domain of the iron cobalt alloy and thus are superparamagnetic. While not being constrained to theory, the inventors believe control of grain size to approximately that of the particle magnetic domain is a factor which contributes to the reduced hysteresis of a magnetic core according to the present invention. Moreover, the presence of insulating silica shells about the core grains is a factor which contributes to the low eddy current formation of a magnetic core according to the present invention.

(16) It is conventionally known that the range of particle size for which single domain particles exhibit superparamagnetism has an upper boundary characteristic of the particle chemical composition.

(17) The inventors have discovered that during synthesis of the silicon dioxide shell a metal silicate thin layer interface is coincidently formed. Evidence that an interfacial layer of metal silicates had formed was observed in x-ray photoelectron spectra collected over the 2p transitions of Fe and Co; and as the thickness of the silica shell was increased (by altering the duration of the silica reaction) a thicker interfacial metal silicate layer was formed, increasing the nanoparticles' overall magnetic anisotropy, as evidenced by increased blocking temperatures and altered coercivities. The inventors have recognized that an understanding of the effect of this interfacial metal silicate layer to control magnetic properties is a key element to effective utility of these materials in applications as low-loss transformer cores.

(18) In a study of the FeCo alloy core shell nanoparticles, the inventors have discovered that interfacial metal silicates formed during the silicon dioxide shell coating synthesis, alter the overall magnetic anisotropy of the nanoparticles as a higher anisotropy phase that is a combination of Fe- and Co-based silicates that acts to increase the ‘magnetically active volume’ of the nanoparticles compared to a bare FeCo nanoparticle.

(19) Binary alloy FeCo single-magnetic-domain nanoparticle samples were synthesized (see Example), with the exception of varying the duration of the SiO.sub.2 reaction times, which led to SiO.sub.2 shells of varying thickness: a 1 min reaction time produced a 3 nm thick shell, 10 minutes a 4 nm thick shell, and 20 minutes a 6 nm thick shell. The average FeCo nanoparticle diameter and SiO.sub.2 shell thickness were determined and for all three core/shell nanoparticle samples (FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm)), the average FeCo core diameter was found to be 4±1 nm indicating a high degree of reproducibility in the nanoparticle core synthesis. The thicknesses of the silica shells were determined in a similar manner and found to be 3±1 nm, 4±1 nm, and 6±1 nm for the FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm) samples, respectively. From the TEM images, it was observed that the FeCo cores were covered completely by the silica shells. Analysis of X-ray diffraction patterns indicated the presence of both Fe and Co silicates. However, the relative proportions appear to be variable and although not wishing to be constrained by theory, the inventors believe that metal silicate content may be related to the thermodynamic energy of formation of the metal silicate. The studies showed that Fe- and Co-silicates formed at the interface between the FeCo nanoparticle core and the SiO.sub.2 shell during the synthesis process. However, the relative integrated areas of the Fe.sup.0 and Co.sup.0 metallic peaks of the different core/shell nanoparticle systems indicated Fe-silicates may be formed preferentially over Co-silicates.

(20) To examine the effect of the metal silicate interface on the magnetic properties of the nanoparticles, the temperature dependence of the DC susceptibility, χ.sub.DC(T)=M(T)/μ.sub.0H, for the FeCo/SiO.sub.2 nanoparticle samples was measured in both the zero-field-cooled (ZFC) and field-cooled (FC) configurations using an applied field of 5 mT, (FIG. 5). Initially, the samples were ZFC from 400 to 2 K, well above their superparamagnetic blocking temperature (T.sub.B) to ensure that the nanoparticles' magnetizations were oriented randomly along their easy axis (set by the intrinsic magnetocrystalline anisotropy) so that no preferred orientation was set. As thermal energy was added to the system with warming from 2 K, a rapid increase in the overall susceptibility (their magnetizations underwent 180° orientation flips with respect to their intrinsic magnetocrystalline anisotropy axis; faster than the timescale of the measurement) and observed as a maximum in χ.sub.ZFC(T). With further addition of thermal energy from warming, the remaining larger nanoparticles became superparamagnetic and a decrease in the measured magnetization was observed as the (time-averaged) measured susceptibility decreased. In general, for a non-interacting monodisperse single-domain nanoparticle T.sub.B is indicated by maximum in the χ.sub.ZFC(T) that coincides with the divergence with χ.sub.FC(T). Since the TEM images indicated a reasonably monodisperse system, with all FeCo nanoparticles in their SiO.sub.2 shells having similar small size distributions, the observed differences in the χ.sub.ZFC(T) and χ.sub.FC(T) around a temperature which should nominally be a T.sub.B must be due to dipole-dipole interactions occurring between the nanoparticles. The maximum χ.sub.ZFC(T) and χ.sub.FC(T) temperature (T.sub.P), is representative of the temperature where the dipole-dipole interaction energies are on par with that provide by the applied field to reorient the nanoparticle's magnetization at that temperature. The T.sub.B's for the nanoparticle samples were determined to be ˜30 K, ˜30 K, and ˜43 K from the peaks in the χ.sub.ZFC(T) for the FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm) samples, respectively. These T.sub.B's are in good agreement with those determined from the temperature dependence of the coercivity. The observed increase in T.sub.B with silica shell thickness is an indication of an increased magnetic anisotropy due to the metal-silicate interfacial layer which altered the surface spin environment of the FeCo core nanoparticle. A reduction in surface spin disorder would increase the overall magnetic volume of the nanoparticle core (compared to a bare nanoparticle where the surface moments, a significant fraction of the overall magnetic moments, are disordered). Further evidence of the effects of the interactions between the interfacial metal silicate layer and the FeCo nanoparticle core were observed in the temperature dependence of the coercivity and frequency dependent AC susceptibility measurements.

(21) The effects of the interfacial metal silicate layer on the overall magnetocrystalline anisotropy of the core/shell nanoparticles were determined by performing frequency dependent AC susceptibility measurements (χ.sub.AC(T,ν)) using a 0.25 mT drive field at frequencies from 10 to 1000 Hz (FIG. 6). For example, the FeCo/SiO.sub.2(3 nm) nanoparticle system, after ZFC down to 10 K, presented a gradual increase in χ.sub.AC(T) with warming, from the increased thermal energy which enabled the single domain FeCo/SiO.sub.2(3 nm) nanoparticle's magnetizations to oscillate with increasing rates. At T.sub.B=30 K, the nanoparticle's magnetizations started to become superparamagnetic, undergoing 180° orientation flips. As the occurrence of these superparamagnetic fluctuations increased, with continued warming, the time-averaged nanoparticle magnetization decreased, resulting in the gradual reduction of χ.sub.AC(T) along with no observable frequency independence as the fluctuations occurred beyond the measuring time window. For the FeCo/SiO.sub.2(4 nm) and FeCo/SiO.sub.2(6 nm) nanoparticle samples, similar trends in the temperature and frequency dependence of χ.sub.AC(T,ν) were observed and T.sub.B's of 30 and 43 K were determined. Describing the observed frequency dependence of T.sub.B from the in-phase χ′.sub.AC(T,ν) using the Vogel-Fulcher law enabled the nanoparticles overall magnetocrystalline anisotropies to be calculated. A value of (6±1)×10.sup.5 J/m.sup.3, (5±1)×10.sup.5 J/m.sup.3, and (12±2)×10.sup.5 J/m.sup.3 were obtained for the FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm) samples, respectively. As the thickness of the silica shells increased the overall magnetocrystalline anisotropy increased as well, indicating that the metal silicate interface layer significantly affected the surface spin environment of the FeCo core.

(22) Hysteresis loops (inset of FIG. 7) were collected after the samples were initially field-cooled (FC) in 5 T from 300 K (a temperature above T.sub.B) to 5 K. Field-cooling was done to establish whether even a monolayer of a Fe- or Co-oxide had formed around the FeCo nanoparticle core's that would result in a field shift from zero of the hysteresis loops—none was observed, indicating clearly that any oxidation had not occurred. It was determined that as the thickness of the silica shell was increased from 3 to 6 nm, the measured H.sub.C also increased. Although not wishing to be constrained by theory this behavior may be a result of the increased magnetic volume of the core as more interface Fe and Co atomic spins in the FeCo nanoparticles were recaptured to provide additional magnetic response. A similar increase was observed in the saturation magnetization measured at 5 K, where M.sub.S=54.6±0.9 emu/g, 56.8±0.1 emu/g, and 65.8±0.2 emu/g for FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm), respectively.

(23) Nanoparticles of Fe—Co/SiO.sub.2 may be synthesized by the ethanolic reaction of sodium borohydride with iron dichloride and cobalt dichloride in a solution of sodium hydroxide and tetraoctylammonium bromide. The obtained nanoparticles may be treated with tetraethyl orthosilicate, in water ethanol mixture using triethylamine as the base-catalyst, to form silica shells. These particles may then be purified using an aqueous ethanol rinse.

(24) As indicated, the length of the treatment of the Fe—Co nanoparticles determines the width of the silicon dioxide coating and correspondingly, the width of the metal silicate layer. The longer the treatment time, the greater the amount of the coating and the greater the width of the metal silicate layer.

(25) The synthesis may be conducted for such time as necessary to prepare a metal silicate layer of 0.5 to 20 nm, preferably 0.8 to 10 nm and most preferably 1.0 to 8 nm.

(26) In another embodiment, the present invention includes a magnetic core, comprising: the thermally annealed iron-cobalt alloy core shell nanoparticles having a particle size of less than 200 nm, preferably less than 100 nm and most preferably from 2 to 50 nm; wherein the magnetic core is a monolithic structure of superparamagnetic core grains of an iron cobalt alloy directly bonded by the silicon dioxide shells, which form a silica matrix, and a layer comprising at least one of iron silicate and cobalt silicate interfaces the core to the matrix.

(27) The core according to the present invention is monolithic, having the space between the iron-cobalt alloy nanoparticle grains occupied by the silicon dioxide. Preferably at least 97% of the space between the grains, preferably 98% and most preferably 99% of the space is silicon dioxide. According to the present invention neither any binder nor any resin is contained in the matrix of the monolithic core. The monolithic core according to the present invention is obtained by a process comprising sintering a powder of the superparamagnetic core shell particles having a particle size of less than 200 nm under pressure under flow of an inert gas to obtain a monolithic structure; wherein the core of the core/shell particle consists of superparamagnetic iron-cobalt alloy, and the shell consists of silicon dioxide. Because a magnetic material is only superparamagnetic when the grain size is near or below the magnetic domain size, the nanoparticle core must be maintained as small as possible, or the sample will become ferromagnetic, and express magnetic hysteresis. Therefore, the most mild and gentle sintering conditions that still yield a monolithic sample that is robust enough to be machined into a toroid are desired, because more aggressive sintering conditions will promote unwanted grain growth and potentially, loss of superparamagnetic performance.

(28) The magnetic core as described herein may be employed as a component in an electrical/magnetic conversion device, as known to one of ordinary skill in the art. In particular the magnetic core according to the present invention may be a component of a vehicle part such as a motor, a generator, a transformer, an inductor and an alternator, where high magnetic moment is advantageous.

(29) Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. Skilled artisans will recognize the utility of the devices of the present invention as a battery as well as the general utility of the electrolyte system described herein.

Example

(30) Core/Shell Iron-Cobalt/Silica-Coated Nanoparticles

(31) FeCo/silica core/shell nanoparticles were synthesized, in an inert argon atmosphere, as follows: FeCl.sub.2.4H.sub.2O (10.9229 g), CoCl.sub.2.6H.sub.2O (12.0466 g), trioctylammonia bromide (17.9006 g), and NaOH (0.4906 g) were dissolved in ethanol (250 mL). A solution of NaBH.sub.4 (12.2580 g) in ethanol (450 mL) was added drop-wise. The product was then washed three times with a solution of 70% H.sub.2O and 30% ethanol (500 mL) that had been bubbled with N.sub.2 to remove dissolved O.sub.2. A solution of triethylamine (2 mL) and H.sub.2O (625 mL) bubbled with N.sub.2 to remove dissolved O.sub.2 was added to the FeCo nanoparticle suspension and allowed to mix fully. Tetraethyl orthosilicate (TEOS, 0.5 mL) in N.sub.2 bubbled ethanol (390 mL) was added to this nanoparticle suspension, and stirred for 15 minutes. The silica-coated iron cobalt nanoparticles were collected from solution using a rare-earth magnet, and then washed with N.sub.2 bubbled ethanol (500 mL) several times. The solution was then dried under vacuum for several hours.

(32) X-ray photoelectron spectroscopy experiments (FIG. 1) performed over the Si2p binding energies indicated the presence of metal silicate phases (e.g. Fe—SiO.sub.2, Co—SiO.sub.2, or [Fe,Co]—SiO.sub.2). The metal silicate peak was located at a binding energy of 102.51 eV and a SiO.sub.2 peak was located at 100.70 eV, in agreement with previously viewed nanoscale SiO.sub.2 Transmission electron microscopy images (FIG. 3) showed clearly the core/shell nature of the nanoparticles. The FeCo of the nanoparticle cores were observed to remained apparently unchanged (4±1 nm in diameter) with the SiO.sub.2 shell thickness increasing systematically with the TEOS reaction time (3±1 nm for a 1 minute reaction, 4±1 nm for a 10 minute reaction and 6±1 nm for a 20 minute reaction).

(33) The average FeCo nanoparticle diameter and SiO.sub.2 shell thickness were determined using ImageJ analysis of transmission electron microscopy (TEM) images (FIG. 2) collected using a Hitachi 9500 operating at 120 kV. For all three core/shell nanoparticle samples (FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm)), the average FeCo core diameter was found to be 4±1 nm indicating a high degree of reproducibility in the nanoparticle core synthesis. The thicknesses of the silica shells were determined in a similar manner and found to be 3±1 nm, 4±1 nm, and 6±1 nm for the FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm) samples, respectively. From the TEM images, it was observed that the FeCo cores were covered completely by the silica shells and formed agglomerations of core/shell nanoparticles. X-ray diffraction patterns collected using a Rigaku SmartLab x-ray diffractometer showed a broad amorphous reflections centered at ˜20°2θ from the silica shells and a less intense broadened reflection at ˜44°2θ attributed the FeCo nanoparticles main reflection (the [110] reflection). X-ray photoelectron spectroscopy (XPS) spectra were collected using a PHI Versaprobe II (after Ar.sup.+ ion sputtering to remove any possible surface contamination) on the three core/shell nanoparticle samples at the binding energies associated with the Fe2p, Co2p, Si2p, C1s, and O1s transitions. After correcting the spectra for charge effects using the 285 eV C1s peak, the Fe2p and Co2p XPS spectra were described using individual Gaussian-Lorentzians with constrained widths and shapes, and an iterated Shirley background function. Metallic Fe and Co 2p.sup.3/2 peaks (see FIGS. 4,B-D) centered at ˜707 eV and 778 eV, respectively, were observed for all three samples. Peaks at higher binding energies resulting from the 2+ and 3+ oxidation states of Fe and Co were observed, indicating the presence of both Fe and Co silicates. Effects of the proximity to Fe and Co ions at the interface between core and shell on the Si2p peak were observed by way of silica and metallic silicate spectral features required to describe correctly the Si2p peak spectral features. Fe- and Co-silicates formed at the interface between the FeCo nanoparticle core and the SiO.sub.2 shell during the synthesis process. Further evidence was provided by way of comparison between the relative integrated areas of the Fe.sup.0 and Co.sup.0 metallic peaks of the different core/shell nanoparticle systems that indicated Fe-silicates formed preferentially over Co-silicates.

(34) To examine the effect the metal silicate interface formed during SiO.sub.2 shell synthesis had on the magnetism, the temperature dependence of the DC susceptibility, χ.sub.DC(T)=M(T)/μ.sub.0H, for the FeCo/SiO.sub.2 nanoparticle samples was measured using a Quantum Design (QD) superconducting quantum interference device (SQUID) magnetometer in both the zero-field-cooled (ZFC) and field-cooled (FC) configurations using an applied field of 5 mT, shown in FIG. 5. Initially, the samples were ZFC from 400 to 2 K, well above their superparamgnetic blocking temperature (T.sub.B) to ensure that the nanoparticles' magnetizations were oriented randomly along their easy axis (set by the intrinsic magnetocrystalline anisotropy) so that no preferred orientation was set. As thermal energy was added to the system with warming from 2 K, a rapid increase in the overall susceptibility (their magnetizations underwent 180° orientation flips with respect to their intrinsic magnetocrystalline anisotropy axis; faster than the timescale of the measurement) and observed as a maximum in χ.sub.ZFC(T). With further addition of thermal energy with warming, the remaining larger nanoparticles became superparamagnetic and a decrease in the measured magnetization was observed as the (time-averaged) measured susceptibility decreased. In general, for a non-interacting monodisperse single-domain nanoparticle T.sub.B is indicated by maximum in the χ.sub.ZFC(T) that coincides with the divergence with χ.sub.FC(T). Since the TEM images indicated a reasonably monodisperse system, with all FeCo nanoparticles in their SiO.sub.2 shells having similar small size distributions, the observed differences in the χ.sub.ZFC(T) and χ.sub.FC(T) around a temperature which should nominally be a T.sub.B must be due to dipole-dipole interactions occurring between the nanoparticles. The maximum χ.sub.ZFC(T) and χ.sub.FC(T) temperature (T.sub.P), is representative of the temperature where the dipole-dipole interaction energies are on par with that provided by the applied field to reorient the nanoparticle's magnetization at that temperature. The T.sub.B's for the nanoparticle samples were determined to be ˜30 K, ˜30 K, and ˜43 K from the peaks in the χ.sub.ZFC(T) for the FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm) samples, respectively. These T.sub.B's were in good agreement with those determined from the temperature dependence of the coercivity.

(35) To determine the effects of the interfacial metal silicate layer on the overall magnetocrystalline anisotropy of the core/shell nanoparticles, frequency dependent AC susceptibility measurements (χ.sub.AC(T,ν)) using a 0.25 mT drive field at frequencies from 10 to 1000 Hz were performed (see FIG. 6). For example, the FeCo/SiO.sub.2(3 nm) nanoparticle system, after ZFC down to 10 K, presented a gradual increase in χ.sub.AC(T) with warming, from the increased thermal energy which enabled the single domain FeCo/SiO.sub.2(3 nm) nanoparticle's magnetizations to oscillate with increasing rates. At T.sub.B=30 K (obtained from the frequency dependent peak of the in-phase and out-of-phase component of the χ.sub.AC(T)), the nanoparticle's magnetizations started to become superparamagnetic, undergoing 180° orientation flips. As the occurrence of these superparamagnetic fluctuations increased, with continued warming, the time-averaged nanoparticle magnetization decreased, resulting in the gradual reduction in χ.sub.AC(T) along with no observable frequency independence as the fluctuations occurred beyond the measuring time window. For the FeCo/SiO.sub.2(4 nm) and FeCo/SiO.sub.2(6 nm) nanoparticle samples, similar trends in the temperature and frequency dependence of χ.sub.AC(T,ν) were observed and T.sub.B's of 30 and 43 K were determined. Describing the observed frequency dependence of T.sub.B from the in-phase χ′.sub.AC(T,ν) using the Vogel-Fulcher law enabled the nanoparticles overall magnetocrystalline anisotropies to be calculated. A value of (6±1)×10.sup.5 J/m.sup.3, (5±1)×10.sup.5 J/m.sup.3, and (12±2)×10.sup.5 J/m.sup.3 were obtained for the FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm) samples, respectively. As the thickness of the silica shells increased the overall magnetocrystalline anisotropy increased as well, indicating that the metal silicate interface layer affected significantly the surface spin environment of the FeCo core.

(36) Hysteresis loops (inset of FIG. 6) were collected after the samples were initially FC in 5 T from 300 K (a temperature above T.sub.B) to 5 K. Field-cooling was done to establish whether even a monolayer of a Fe- or Co-oxide had formed around the FeCo nanoparticle core's that would result in a field shift from zero of the hysteresis loops—none was observed, indicating clearly that any oxidation had not occurred. Interesting, the hysteresis loops of all three FeCo/silica core/shell systems had a paramagnetic component at high fields, likely from the complex metal silicates. Above the saturating field, all of the nanoparticle's magnetic moments aligned with the applied field, reaching saturation, thus any measured susceptibility reflected only the non-ferromagnetic contributions (e.g. paramagnetic and diamagnetic). With an increased thickness of the silica shell, a decrease in the high field susceptibility was observed as the overall contribution from the diamagnetic silica increased relative to the paramagnetic contribution from the complex metal silicates (the silica shell accounted for a large percentage of the overall mass, >80%, and volume, >90%, of the nanoparticle sample). The temperature dependence of the measured coercivities normalized to the respective blocking temperatures to account for the different (temperature) energy-scales due to the intrinsic differences in magnetocrystalline anisotropies described above, H.sub.C(T/T.sub.B), shown in FIG. 6, were observed to decrease rapidly as the temperature was increased with H.sub.C(T=T.sub.B)=0 in good agreement with both susceptibility measurements (χ.sub.AC, and χ.sub.DC). It was observed that as the thickness of the silica shell was increased from 3 to 6 nm, the measured H.sub.C also increased, likely as a result of the increased magnetic volume of the core as more interface Fe and Co atomic spins in the FeCo nanoparticles were recaptured to provide additional magnetic response. A similar increase was observed in the saturation magnetization measured at 5 K, where M.sub.S=54.6±0.9 emu/g, 56.8±0.1 emu/g, and 65.8±0.2 emu/g for FeCo/SiO.sub.2(3 nm), FeCo/SiO.sub.2(4 nm), and FeCo/SiO.sub.2(6 nm), respectively.