P. Fannin - Academia.edu (original) (raw)

Papers by P. Fannin

Physica B: Condensed Matter, 2007

ABSTRACT Measurements are presented of the frequency and field dependent complex magnetic suscept... more ABSTRACT Measurements are presented of the frequency and field dependent complex magnetic susceptibility, of three different magnetic fluids, over the frequency and polarizing field (H) ranges of 100 MHz to 6 GHz and 0–122 kA/m, respectively. The investigated samples were Isopar M-based magnetic fluids, with magnetite particles (sample S1), Ni0.5Zn0.5Fe2O4 particles (sample S2) and Mn0.66Zn0.34Fe2O4 particles (sample S3). In the investigated frequency range ferromagnetic resonance occurs at a frequency, fres, and a corresponding maximum absorption occurs at a frequency fmax. From a plot of fres against H, average values of the anisotropy constant, , anisotropy field, , and gyromagnetic ratio, γ are determined. From a plot of fmax/fres against H the damping parameter, α, is obtained. Also, for each sample, an estimate is made of the particles magnetic viscosity, ηm, together with τ0, the pre-exponential factor of Brown's equation for the Néel relaxation time, τN, of the magnetic moment of superparamagnetic particles and τN itself.

Physica B: Condensed Matter, 2007

ABSTRACT Measurements are presented of the frequency and field dependent complex magnetic suscept... more ABSTRACT Measurements are presented of the frequency and field dependent complex magnetic susceptibility, of three different magnetic fluids, over the frequency and polarizing field (H) ranges of 100 MHz to 6 GHz and 0–122 kA/m, respectively. The investigated samples were Isopar M-based magnetic fluids, with magnetite particles (sample S1), Ni0.5Zn0.5Fe2O4 particles (sample S2) and Mn0.66Zn0.34Fe2O4 particles (sample S3). In the investigated frequency range ferromagnetic resonance occurs at a frequency, fres, and a corresponding maximum absorption occurs at a frequency fmax. From a plot of fres against H, average values of the anisotropy constant, , anisotropy field, , and gyromagnetic ratio, γ are determined. From a plot of fmax/fres against H the damping parameter, α, is obtained. Also, for each sample, an estimate is made of the particles magnetic viscosity, ηm, together with τ0, the pre-exponential factor of Brown's equation for the Néel relaxation time, τN, of the magnetic moment of superparamagnetic particles and τN itself.

AIP Conference Proceedings, 2009

The frequency (f) and polarizing field (H) dependence of the complex magnetic permeability μ(f,H)... more The frequency (f) and polarizing field (H) dependence of the complex magnetic permeability μ(f,H) = μ′(f,H)−iμ″(f,H), of different magnetic fluid samples, over the range 100 MHz to 6 GHz and 0 to 102.4 kA∕m, respectively, were analyzed. Starting from an initial magnetic fluid sample (sample A) with magnetite particles dispersed in kerosene and stabilized with oleic acid, having particle concentration n = 19.16⋅1022 m−3, three samples were obtained by successive dilution with kerosene (with a dilution ratio 2:3) (samples A1, A2, and A3). Based on the complex magnetic permeability measurements of each sample, and for each field value, values of the specific magnetic loss power were obtained. We have also studied the dependence on particle concentration of the magnetic loss power, both in zero polarizing field and in the presence of the polarizing field.

Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 2000

The influence of the spheroidal Co-Fe magnetic particles on the orientation of discotic liquid cr... more The influence of the spheroidal Co-Fe magnetic particles on the orientation of discotic liquid crystal molecules in the presence of minor magnetic field (∼0.3 Tesla) has been demonstrated using FTIR spectroscopy.

Journal of Physics: Condensed Matter, 2003

Starting from the Landau-Lifshitz equation, with resonant frequency f 0 = ω 0 /2π, it is demonstr... more Starting from the Landau-Lifshitz equation, with resonant frequency f 0 = ω 0 /2π, it is demonstrated that, in the case of a magnetic fluid, the measured resonant frequency, f res is always different from f 0 , except for the case of pure resonance (i.e. zero damping parameter of Landau-Lifshitz equation) where f res = f 0. It is also shown that f res and the corresponding maximum absorption frequency, f max , are different, thus supporting the deductions of Scaife, who arrived at this conclusion using an alternative theoretical approach. Furthermore, based on complex magnetic susceptibility measurements, over the frequency range 100 MHz-6 GHz, the dependence of the ratio f max / f res on an external polarizing magnetic field, H pol , over the approximate range 0 and 1.3 kOe and on particle concentration has been examined for different magnetic fluid samples. It is demonstrated how the ratio f max / f res tends to unity both by (i) increasing the polarizing field and (ii) decreasing the particle concentration of the samples.

AIP Conference Proceedings, 2009

The frequency (f) and polarizing field (H) dependence of the complex magnetic permeability μ(f,H)... more The frequency (f) and polarizing field (H) dependence of the complex magnetic permeability μ(f,H) = μ′(f,H)−iμ″(f,H), of different magnetic fluid samples, over the range 100 MHz to 6 GHz and 0 to 102.4 kA∕m, respectively, were analyzed. Starting from an initial magnetic fluid sample (sample A) with magnetite particles dispersed in kerosene and stabilized with oleic acid, having particle concentration n = 19.16⋅1022 m−3, three samples were obtained by successive dilution with kerosene (with a dilution ratio 2:3) (samples A1, A2, and A3). Based on the complex magnetic permeability measurements of each sample, and for each field value, values of the specific magnetic loss power were obtained. We have also studied the dependence on particle concentration of the magnetic loss power, both in zero polarizing field and in the presence of the polarizing field.

Journal of Physics D: Applied Physics, 2002

ABSTRACT Measurements and analysis are presented of the incremental magnetic susceptibilities, χ∥... more ABSTRACT Measurements and analysis are presented of the incremental magnetic susceptibilities, χ∥(H0) and χ⊥(H0), of magnetic fluids subjected to an external polarizing field, H0. The technique of placing a fluid sample in a cylindrical coil positioned between the poles of a magnet and determining the susceptibility, at a fixed frequency, when the long axis of the sample is (1) parallel (χ∥(H0)) and (2) perpendicular (χ⊥(H0)), to H0, provides a convenient method for the determination of the magnetic moment, and hence the effective particle size, of nano-particles in magnetic fluids. Two magnetic fluids samples consisting of magnetite and cobalt-zinc ferrite particles, respectively, are investigated and the results obtained are found to be in good agreement with those of the manufacturer. As the samples are cylindrical in shape, the demagnetizing field, HD, for both the perpendicular and parallel orientations of the samples, is accounted for in calculating χ∥(H0) and χ⊥(H0).

Journal of Molecular Liquids, 2004

ABSTRACT Measurement of the frequency dependent, complex susceptibility, χ(ω)=χ′(ω)−iχ″(ω), over ... more ABSTRACT Measurement of the frequency dependent, complex susceptibility, χ(ω)=χ′(ω)−iχ″(ω), over the frequency range 100 Hz to 1 MHz, of a colloidal suspension of cobalt ferrite particles, by means of the toroidal technique, indicates the presence of aggregation in the samples: the aggregation being identified by the appearance of a low-frequency loss-peak in the χ″(ω) component of the susceptibility. By means of the application of a polarising field, H (A/m), the field dependence of the complex susceptibility of the aggregates, χ(ω, H), the frequency dependent loss tangent, tanδ and mean power loss, T (J/m3), are investigated for polarising fields over the approximate range 0–13.6 kA/m. It is shown that the plots of both χ(ω), H and tanδ may be fitted in terms of the Cole–Cole parameter, α, and it is demonstrated how the fits can be used to determine data outside the measurement frequency range. A novel method for determining the tanδ data is also presented. It is also shown how, with increase in polarising field, the magnetic losses of the fluid are reduced.

Journal of Magnetism and Magnetic Materials, 2005

Field and frequency-dependent, complex susceptibility, w(o, H) ¼ w 0 (o, H)-i w 00 (H, o), measur... more Field and frequency-dependent, complex susceptibility, w(o, H) ¼ w 0 (o, H)-i w 00 (H, o), measurements, obtained by means of the toroidal technique, over the range 10 Hz-1 MHz, are used to investigate the non-linear relaxational properties of a water-based magnetic fluid. The results obtained are discussed in terms of the magnetic analogue of the Coffey and Paranjape model as modified by De´jardin.

Journal of Magnetism and Magnetic Materials, 2005

Due to the magnetic anisotropy introduced by the Co2+ ion in octahedral sites of cubic spinel fer... more Due to the magnetic anisotropy introduced by the Co2+ ion in octahedral sites of cubic spinel ferrites, it is possible to tailor the magnetic properties by changing the cobalt content. Magnetic fluids with magnetite-cobalt ferrite nanoparticles given by the formula Co(x)Fe(3−x)O4 with x=0, 0.2 and 0.4 were prepared. Kerosene and oleic acid were used as liquid carrier and surfactant, respectively.

The European Physical Journal Applied Physics, 2002

Measurements of the complex susceptibility, χ(ω) = χ (ω) − iχ (ω), as a function of the frequency... more Measurements of the complex susceptibility, χ(ω) = χ (ω) − iχ (ω), as a function of the frequency (100 Hz-18 GHz) and polarizing field (0-90 kA m −1) at room temperature together with static magnetic measurements over the temperature range 4-300 K, are reported for a colloidal suspension of cobalt nanoparticles. The transition of the cobalt particles to the superparamagnetic state are supported by the temperature dependencies of field cooling (FC) and zero field cooling (ZFC) magnetization measurements. From these measurements, which show a typical blocking behaviour of an assembly of superparamagnetic particles with a wide distribution of blocking temperatures, the exponential pre-factor τ0 of Brown's equations for Néel relaxation, is found to be equal to 9.2 × 10 −10 s. Measurement of the complex susceptibility χ(ω) over this broad frequency range, with an upper frequency value corresponding to three times that previously reported in our measurements on cobalt, has enabled the presence of both resonance and Néel relaxation mechanisms to be identified. From the Néel component, a further value for τ0 was evaluated and shown to be in close agreement with that obtained from the ZFC data. Data on the after-effect function, realised by Fourier transformation of the χ (ω) component, is also presented.

Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. ... more Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. Sample A was a magnetic fluid with mixed ferrite particles of Ni-Zn type and sample B was a magnetic fluid with mixed ferrite particles of Mn-Fe type. Sample C was a composite magnetic fluid obtained by mixing a part of sample A with a part

Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. ... more Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. Sample A was a magnetic fluid with mixed ferrite particles of Ni-Zn type and sample B was a magnetic fluid with mixed ferrite particles of Mn-Fe type. Sample C was a composite magnetic fluid obtained by mixing a part of sample A with a part

Journal of Physics D: Applied Physics, 2003

Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. ... more Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. Sample A was a magnetic fluid with mixed ferrite particles of Ni-Zn type and sample B was a magnetic fluid with mixed ferrite particles of Mn-Fe type. Sample C was a composite magnetic fluid obtained by mixing a part of sample A with a part

Journal of Molecular Liquids, 2007

ABSTRACT Magnetic properties of ZnxCo1−xFe2O4 (x=0.3, 0.5 and 0.7) nanoparticles in ferrofluids h... more ABSTRACT Magnetic properties of ZnxCo1−xFe2O4 (x=0.3, 0.5 and 0.7) nanoparticles in ferrofluids have been investigated by means of Mössbauer spectroscopy in the temperature range 4.2–300 K. The mean sizes of the particles were 10.2, 7.4 and 5.2 nm, respectively. The transition temperatures to the superparamagnetic state and values of crystal magnetic anisotropy have been evaluated. The transition to the superparamagnetic state was observed at temperatures below the freezing point of the carrier liquid for the 5.2 nm and 7.4 nm particles and above its freezing point for the 10.2 nm particles. A multi-level model is used in the analysis of the Mössbauer spectra and in the calculation of the hyperfine magnetic field.

ABSTRACT Measurements of the permittivity, ε(ω)=ε′(ω)−iε″(ω), and conductivity, σ(ω)=ωε0ε″(ω), of... more ABSTRACT Measurements of the permittivity, ε(ω)=ε′(ω)−iε″(ω), and conductivity, σ(ω)=ωε0ε″(ω), of two ferrofluid samples of ferrite particles in a hydrocarbon carrier, isopar M, over the frequency range 0.1–6GHz are presented. It is shown that the sample with the highest concentration of particles has the highest permittivity and by means of profile fitting it is demonstrated, for the first time, that σ(ω) has a Debye-type profile.The frequency limitation of the used measurement technique is highlighted and a possible alternative technique capable of measuring in the 20–30GHz region is presented.

Journal of Magnetism and Magnetic Materials, 1993

ABSTRACT Measurements are reported of the permittivity tensors of a number of magnetic colloids s... more ABSTRACT Measurements are reported of the permittivity tensors of a number of magnetic colloids subjected to a constant magnetic field up to 0.3 T over the frequency range 50 kHz to 1 MHz. Magnetically-induced anisotropy of the dielectric permittivity was only significant in one of the fluids and its dependence on the magnitude of the magnetic field was in accord with theory.

Physica B: Condensed Matter, 2007

ABSTRACT Measurements are presented of the frequency and field dependent complex magnetic suscept... more ABSTRACT Measurements are presented of the frequency and field dependent complex magnetic susceptibility, of three different magnetic fluids, over the frequency and polarizing field (H) ranges of 100 MHz to 6 GHz and 0–122 kA/m, respectively. The investigated samples were Isopar M-based magnetic fluids, with magnetite particles (sample S1), Ni0.5Zn0.5Fe2O4 particles (sample S2) and Mn0.66Zn0.34Fe2O4 particles (sample S3). In the investigated frequency range ferromagnetic resonance occurs at a frequency, fres, and a corresponding maximum absorption occurs at a frequency fmax. From a plot of fres against H, average values of the anisotropy constant, , anisotropy field, , and gyromagnetic ratio, γ are determined. From a plot of fmax/fres against H the damping parameter, α, is obtained. Also, for each sample, an estimate is made of the particles magnetic viscosity, ηm, together with τ0, the pre-exponential factor of Brown's equation for the Néel relaxation time, τN, of the magnetic moment of superparamagnetic particles and τN itself.

Physica B: Condensed Matter, 2007

ABSTRACT Measurements are presented of the frequency and field dependent complex magnetic suscept... more ABSTRACT Measurements are presented of the frequency and field dependent complex magnetic susceptibility, of three different magnetic fluids, over the frequency and polarizing field (H) ranges of 100 MHz to 6 GHz and 0–122 kA/m, respectively. The investigated samples were Isopar M-based magnetic fluids, with magnetite particles (sample S1), Ni0.5Zn0.5Fe2O4 particles (sample S2) and Mn0.66Zn0.34Fe2O4 particles (sample S3). In the investigated frequency range ferromagnetic resonance occurs at a frequency, fres, and a corresponding maximum absorption occurs at a frequency fmax. From a plot of fres against H, average values of the anisotropy constant, , anisotropy field, , and gyromagnetic ratio, γ are determined. From a plot of fmax/fres against H the damping parameter, α, is obtained. Also, for each sample, an estimate is made of the particles magnetic viscosity, ηm, together with τ0, the pre-exponential factor of Brown's equation for the Néel relaxation time, τN, of the magnetic moment of superparamagnetic particles and τN itself.

AIP Conference Proceedings, 2009

The frequency (f) and polarizing field (H) dependence of the complex magnetic permeability μ(f,H)... more The frequency (f) and polarizing field (H) dependence of the complex magnetic permeability μ(f,H) = μ′(f,H)−iμ″(f,H), of different magnetic fluid samples, over the range 100 MHz to 6 GHz and 0 to 102.4 kA∕m, respectively, were analyzed. Starting from an initial magnetic fluid sample (sample A) with magnetite particles dispersed in kerosene and stabilized with oleic acid, having particle concentration n = 19.16⋅1022 m−3, three samples were obtained by successive dilution with kerosene (with a dilution ratio 2:3) (samples A1, A2, and A3). Based on the complex magnetic permeability measurements of each sample, and for each field value, values of the specific magnetic loss power were obtained. We have also studied the dependence on particle concentration of the magnetic loss power, both in zero polarizing field and in the presence of the polarizing field.

Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 2000

The influence of the spheroidal Co-Fe magnetic particles on the orientation of discotic liquid cr... more The influence of the spheroidal Co-Fe magnetic particles on the orientation of discotic liquid crystal molecules in the presence of minor magnetic field (∼0.3 Tesla) has been demonstrated using FTIR spectroscopy.

Journal of Physics: Condensed Matter, 2003

Starting from the Landau-Lifshitz equation, with resonant frequency f 0 = ω 0 /2π, it is demonstr... more Starting from the Landau-Lifshitz equation, with resonant frequency f 0 = ω 0 /2π, it is demonstrated that, in the case of a magnetic fluid, the measured resonant frequency, f res is always different from f 0 , except for the case of pure resonance (i.e. zero damping parameter of Landau-Lifshitz equation) where f res = f 0. It is also shown that f res and the corresponding maximum absorption frequency, f max , are different, thus supporting the deductions of Scaife, who arrived at this conclusion using an alternative theoretical approach. Furthermore, based on complex magnetic susceptibility measurements, over the frequency range 100 MHz-6 GHz, the dependence of the ratio f max / f res on an external polarizing magnetic field, H pol , over the approximate range 0 and 1.3 kOe and on particle concentration has been examined for different magnetic fluid samples. It is demonstrated how the ratio f max / f res tends to unity both by (i) increasing the polarizing field and (ii) decreasing the particle concentration of the samples.

AIP Conference Proceedings, 2009

The frequency (f) and polarizing field (H) dependence of the complex magnetic permeability μ(f,H)... more The frequency (f) and polarizing field (H) dependence of the complex magnetic permeability μ(f,H) = μ′(f,H)−iμ″(f,H), of different magnetic fluid samples, over the range 100 MHz to 6 GHz and 0 to 102.4 kA∕m, respectively, were analyzed. Starting from an initial magnetic fluid sample (sample A) with magnetite particles dispersed in kerosene and stabilized with oleic acid, having particle concentration n = 19.16⋅1022 m−3, three samples were obtained by successive dilution with kerosene (with a dilution ratio 2:3) (samples A1, A2, and A3). Based on the complex magnetic permeability measurements of each sample, and for each field value, values of the specific magnetic loss power were obtained. We have also studied the dependence on particle concentration of the magnetic loss power, both in zero polarizing field and in the presence of the polarizing field.

Journal of Physics D: Applied Physics, 2002

ABSTRACT Measurements and analysis are presented of the incremental magnetic susceptibilities, χ∥... more ABSTRACT Measurements and analysis are presented of the incremental magnetic susceptibilities, χ∥(H0) and χ⊥(H0), of magnetic fluids subjected to an external polarizing field, H0. The technique of placing a fluid sample in a cylindrical coil positioned between the poles of a magnet and determining the susceptibility, at a fixed frequency, when the long axis of the sample is (1) parallel (χ∥(H0)) and (2) perpendicular (χ⊥(H0)), to H0, provides a convenient method for the determination of the magnetic moment, and hence the effective particle size, of nano-particles in magnetic fluids. Two magnetic fluids samples consisting of magnetite and cobalt-zinc ferrite particles, respectively, are investigated and the results obtained are found to be in good agreement with those of the manufacturer. As the samples are cylindrical in shape, the demagnetizing field, HD, for both the perpendicular and parallel orientations of the samples, is accounted for in calculating χ∥(H0) and χ⊥(H0).

Journal of Molecular Liquids, 2004

ABSTRACT Measurement of the frequency dependent, complex susceptibility, χ(ω)=χ′(ω)−iχ″(ω), over ... more ABSTRACT Measurement of the frequency dependent, complex susceptibility, χ(ω)=χ′(ω)−iχ″(ω), over the frequency range 100 Hz to 1 MHz, of a colloidal suspension of cobalt ferrite particles, by means of the toroidal technique, indicates the presence of aggregation in the samples: the aggregation being identified by the appearance of a low-frequency loss-peak in the χ″(ω) component of the susceptibility. By means of the application of a polarising field, H (A/m), the field dependence of the complex susceptibility of the aggregates, χ(ω, H), the frequency dependent loss tangent, tanδ and mean power loss, T (J/m3), are investigated for polarising fields over the approximate range 0–13.6 kA/m. It is shown that the plots of both χ(ω), H and tanδ may be fitted in terms of the Cole–Cole parameter, α, and it is demonstrated how the fits can be used to determine data outside the measurement frequency range. A novel method for determining the tanδ data is also presented. It is also shown how, with increase in polarising field, the magnetic losses of the fluid are reduced.

Journal of Magnetism and Magnetic Materials, 2005

Field and frequency-dependent, complex susceptibility, w(o, H) ¼ w 0 (o, H)-i w 00 (H, o), measur... more Field and frequency-dependent, complex susceptibility, w(o, H) ¼ w 0 (o, H)-i w 00 (H, o), measurements, obtained by means of the toroidal technique, over the range 10 Hz-1 MHz, are used to investigate the non-linear relaxational properties of a water-based magnetic fluid. The results obtained are discussed in terms of the magnetic analogue of the Coffey and Paranjape model as modified by De´jardin.

Journal of Magnetism and Magnetic Materials, 2005

Due to the magnetic anisotropy introduced by the Co2+ ion in octahedral sites of cubic spinel fer... more Due to the magnetic anisotropy introduced by the Co2+ ion in octahedral sites of cubic spinel ferrites, it is possible to tailor the magnetic properties by changing the cobalt content. Magnetic fluids with magnetite-cobalt ferrite nanoparticles given by the formula Co(x)Fe(3−x)O4 with x=0, 0.2 and 0.4 were prepared. Kerosene and oleic acid were used as liquid carrier and surfactant, respectively.

The European Physical Journal Applied Physics, 2002

Measurements of the complex susceptibility, χ(ω) = χ (ω) − iχ (ω), as a function of the frequency... more Measurements of the complex susceptibility, χ(ω) = χ (ω) − iχ (ω), as a function of the frequency (100 Hz-18 GHz) and polarizing field (0-90 kA m −1) at room temperature together with static magnetic measurements over the temperature range 4-300 K, are reported for a colloidal suspension of cobalt nanoparticles. The transition of the cobalt particles to the superparamagnetic state are supported by the temperature dependencies of field cooling (FC) and zero field cooling (ZFC) magnetization measurements. From these measurements, which show a typical blocking behaviour of an assembly of superparamagnetic particles with a wide distribution of blocking temperatures, the exponential pre-factor τ0 of Brown's equations for Néel relaxation, is found to be equal to 9.2 × 10 −10 s. Measurement of the complex susceptibility χ(ω) over this broad frequency range, with an upper frequency value corresponding to three times that previously reported in our measurements on cobalt, has enabled the presence of both resonance and Néel relaxation mechanisms to be identified. From the Néel component, a further value for τ0 was evaluated and shown to be in close agreement with that obtained from the ZFC data. Data on the after-effect function, realised by Fourier transformation of the χ (ω) component, is also presented.

Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. ... more Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. Sample A was a magnetic fluid with mixed ferrite particles of Ni-Zn type and sample B was a magnetic fluid with mixed ferrite particles of Mn-Fe type. Sample C was a composite magnetic fluid obtained by mixing a part of sample A with a part

Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. ... more Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. Sample A was a magnetic fluid with mixed ferrite particles of Ni-Zn type and sample B was a magnetic fluid with mixed ferrite particles of Mn-Fe type. Sample C was a composite magnetic fluid obtained by mixing a part of sample A with a part

Journal of Physics D: Applied Physics, 2003

Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. ... more Three kerosene-based magnetic fluid samples, denoted A, B and C have been used for measurements. Sample A was a magnetic fluid with mixed ferrite particles of Ni-Zn type and sample B was a magnetic fluid with mixed ferrite particles of Mn-Fe type. Sample C was a composite magnetic fluid obtained by mixing a part of sample A with a part

Journal of Molecular Liquids, 2007

ABSTRACT Magnetic properties of ZnxCo1−xFe2O4 (x=0.3, 0.5 and 0.7) nanoparticles in ferrofluids h... more ABSTRACT Magnetic properties of ZnxCo1−xFe2O4 (x=0.3, 0.5 and 0.7) nanoparticles in ferrofluids have been investigated by means of Mössbauer spectroscopy in the temperature range 4.2–300 K. The mean sizes of the particles were 10.2, 7.4 and 5.2 nm, respectively. The transition temperatures to the superparamagnetic state and values of crystal magnetic anisotropy have been evaluated. The transition to the superparamagnetic state was observed at temperatures below the freezing point of the carrier liquid for the 5.2 nm and 7.4 nm particles and above its freezing point for the 10.2 nm particles. A multi-level model is used in the analysis of the Mössbauer spectra and in the calculation of the hyperfine magnetic field.

ABSTRACT Measurements of the permittivity, ε(ω)=ε′(ω)−iε″(ω), and conductivity, σ(ω)=ωε0ε″(ω), of... more ABSTRACT Measurements of the permittivity, ε(ω)=ε′(ω)−iε″(ω), and conductivity, σ(ω)=ωε0ε″(ω), of two ferrofluid samples of ferrite particles in a hydrocarbon carrier, isopar M, over the frequency range 0.1–6GHz are presented. It is shown that the sample with the highest concentration of particles has the highest permittivity and by means of profile fitting it is demonstrated, for the first time, that σ(ω) has a Debye-type profile.The frequency limitation of the used measurement technique is highlighted and a possible alternative technique capable of measuring in the 20–30GHz region is presented.

Journal of Magnetism and Magnetic Materials, 1993

ABSTRACT Measurements are reported of the permittivity tensors of a number of magnetic colloids s... more ABSTRACT Measurements are reported of the permittivity tensors of a number of magnetic colloids subjected to a constant magnetic field up to 0.3 T over the frequency range 50 kHz to 1 MHz. Magnetically-induced anisotropy of the dielectric permittivity was only significant in one of the fluids and its dependence on the magnitude of the magnetic field was in accord with theory.