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Magnetic resonance investigation of monodisperse oleoylsarcosine-coated magnetic fluid
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 289 (2005) 146–148 www.elsevier.com/locate/jmmm
Magnetic resonance investigation of monodisperse oleoylsarcosine-coated magnetic fluid A.R. Pereiraa,, F. Pelegrinia, P.C. Moraisb a Instituto de Fı´sica, Universidade Federal de Goia´s, 74001-970 Goiaˆnia-GO, Brazil Instituto de Fı´sica, Universidade de Brası´lia, Fı´sica Aplicada, 70919-970 Brası´lia-DF, Brazil
b
Available online 24 November 2004
Abstract Magnetic resonance experiments were used to investigate the occurrence of particle arrays induced by the application of a small steady field (50 G) during the frozen procedure. In order to skip from the size-dependent effects the sample was filtered to a very narrow distribution of particle sizes. Three well-defined components were chosen to fit the envelope resonance line. Analyses of the resonance field versus temperature of each component were used as a support for the occurrence of dimers and tetramers as the dominant magnetic arrays. r 2004 Published by Elsevier B.V. PACS: 75.50.Mm; 75.30.Gw; 76.50.+g Keywords: Magnetic fluid; Magnetic anisotropy; Magnetic resonance
Magnetic resonance has been used as a valuable tool in the investigation of many aspects of ionic [1], surfacted [2,3] and biocompatible [4,5] magnetic fluids (MFs). In particular, useful data concerning the particle–particle interaction in magnetic fluid samples have been obtained through the magnetic resonance experiments [5]. Dimer disruption, for instance, has been investigated by the temperature evolution of multicomponent resonance spectra [6]. Zero-field-frozen [7] and field-frozen [8,9] magnetic fluid samples have been studied using magnetic resonance experiments as well. Magnetic fields close to the sample saturation values (above 1.5 T), however, have been used to freeze the MF samples in order to carry on angular variation experiments. In other words, magnetic resonance has been used to investigate frozen MFs only in the two extreme Corresponding author. Tel./fax: +55 62 5211029.
E-mail addresses: [email protected], anarita@fis.ufg.br (A.R. Pereira). 0304-8853/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2004.11.042
limits; at zero-field- (zero magnetic texture) or high-fieldfrozen (high magnetic texture) conditions. On the other hand, the particle size polydispersity, characteristic of the chemically synthesized magnetic nanoparticles, play a key role in the interpretation of almost all experimental data, requiring the averaging of any sizedependent property. As a consequence of such sample characteristic, fine details in the experimental data could be blurred out by the particle size dispersity profile. In order to skip from this difficulty we have prepared a monodisperse MF sample based on magnetite nanoparticles surface-coated with oleoylsarcosine. The aqueousbased magnetic fluid sample, containing about 2 1017 particle/cm3 was prepared following the standard recipe described in the literature [10]. In order to reduce the size dispersity down to the monodisperse profile filtering of the as-prepared (polydisperse) magnetic fluid sample was performed using a magnetic field gradient procedure. The particle size histogram, obtained from the transmission electron microscopy
ARTICLE IN PRESS A.R. Pereira et al. / Journal of Magnetism and Magnetic Materials 289 (2005) 146–148
micrographs, were curve-fitted using a log-normal distribution function characterized by the average particle diameter of ð6:2 0:1Þ nm and diameter dispersion of 0:27 0:01: In addition, in order to induce only very small-sized particle arrays the MF sample was frozen from room-temperature down to 77 K in the presence of a steady 50 G field. After the field-frozen procedure the temperature was raised from 100 to 350 K while the X-band resonance spectra were recorded. The magnetic resonance spectra in the temperature range of 100–250 K, corresponding to the range where the MF sample is frozen and the nanoparticles firmly attached to the water-based matrix, was analyzed in terms of three well-characterized components. It is assumed here that the three resonance lines are related to three distinct nanoparticle arrays built from nearly identical magnetic units. Each of these nanoparticle arrays is expected to be well-characterized by a magnetic resonance signature. It is the purpose of this study to report on both the analysis of the magnetic resonance spectra in terms of their components and the temperature dependence of the resonance field of each magnetic resonance component obtained from the de-convolution procedure of the envelope resonance line. In addition, the rate at which the resonance field of each resonance component shifts as a function of the temperature will be used to identify a particular nanoparticle array. Typical magnetic resonance spectra analyses are shown in Fig. 1 at two different temperatures. Though the multi-component spectrum is visually obvious, either due to a smoothly broad and asymmetric envelope
Fig. 1. Typical analyses of the magnetic resonance spectra in terms of three components, at different temperatures.
147
resonance line at lower temperatures (250 K) or due to visible line structure at higher temperatures (300 K), decision concerning the number of features in a single, broad envelope line is not an easy task. Three components (L1, L2, and L3 lines) was the best scenario found in the whole range of temperature, including consistency of the chi-square test, temperature dependence of the resonance field separations and temperature dependence of the resonance linewidths. The temperature dependence of the resonance field of each resonance feature (L1, L2, and L3 lines) found from our spectra analysis is shown in Fig. 2. The resonance field data in Fig. 2 are restricted to the range of 100–275 K, slightly above the samples melting point (about 250 K). Three aspects related to the data shown in Fig. 2 call immediately our attention. First, all points in the temperature range of the frozen sample fall in well-defined straight lines. Second, the intermediate straight line (L2 line) in Fig. 2 has a slope quite different from the other two ones (L1 and L3 lines). Third, the external straight lines (lowest versus highest fields) show similar slopes, except for the sign (positive versus negative). Finally, in the temperature range of 100–275 K the three components have quite different line intensities. In this temperature range line L1 is the weakest signal whereas line L2 is the strongest signal. Immediately above the samples melting point (about 250 K) L2 increases in intensity (area under the resonance curve) whereas L3 reduces in intensity. In this small temperature range (250–275 K) the L1 intensity stays approximately unchanged. The model used to fit the data shown in Fig. 2 starts with the resonance frequency oR, i.e. the Larmor precession frequency of the nanoparticle magnetic moment in the presence of an effective magnetic field (HEFF). The resonance frequency is written as
Fig. 2. The resonance field versus temperature of the three lines L1, L2, and L3 in the temperature range of the frozen state. Symbols are experimental data whereas the straight lines are the best fit according to the equation H R ¼ B þ AT:
ARTICLE IN PRESS 148
A.R. Pereira et al. / Journal of Magnetism and Magnetic Materials 289 (2005) 146–148
oR ¼ gH EFFF ; where g is the gyromagnetic ratio and the effective magnetic field is a result of three main components; the external sweeping field (HE), the exchange anisotropy field (HX), and the effective anisotropy field (HA). At the resonance condition HE matches the resonance field HR. Then, the resonance field is given by H R ¼ oR =g2H X 2H A [11]. The exchange anisotropy field, on the other hand, is given by H X ¼ 4Cn2 =M S Db ; where C is the exchange constant, n are eigenvalues of the differential equation involving the spherical Bessel functions dj n ðnÞ=dn ¼ 0; MS is the saturation magnetization, D is the particle diameter, and b ¼ 2 in the absence of surface anisotropy [12]. Inspection of equation H R ¼ oR =g2H X 2H A ; however, reveals that the temperature dependence of the resonance field (HR) is mainly associated to the last term on the right-hand side (HA), through its dependence upon the effective magnetocrystalline anisotropy density (KEFF). The anisotropy field is written as H A ¼ 2K EFF =M S : In magnetic nanoparticles the effective anisotropy energy density has both bulk (KB) and surface (KS) components, i.e. K EFF ¼ K B þ K S : The surface component, however, is related to the surface-tovolume ratio K S ¼ ð6=DÞkS ; where kS is the surface anisotropy coefficient. In general, KEFF and MS are both temperature dependent. However, considering that our data were taken far below the Curie point (around 850 K for bulk magnetite), MS could be taken as approximately flat. Therefore, the temperature dependence of the anisotropy field would follow mainly the temperature dependence of the effective magnetic anisotropy. Inspection of the experimental data in Fig. 2 shows a linear relationship between HR and T for all three resonance lines (L1, L2, and L3). In other words, the effective magnetic anisotropy would be empirically represented by K EFF ¼ K 0 þ kEFF T: K0 is a constant and kEFF is a size-dependent coefficient, expressed in units of erg/cm3 K. Straight lines in Fig. 2 represent the best fit of the data using the empirical relation: H R ¼ B þ AT; where B ¼ ðoR =g22K 0 =M S 24Cn2 =M S Db Þ is the intercept constant and A ¼ 22kEFF =M S the slope. The slopes found from the fitting of the data shown in Fig. 2 were A1 ¼ ð1:26 0:03Þ G/K, A2 ¼ ð0:46 0:04Þ G/K and A3 ¼ ð1:11 0:06Þ G/K for lines L1, L2, and L3, respectively. Comparing the empirical relation K EFF ¼ K 0 þ kEFF T with K EFF ¼ K B þ K S one sees that K0 scales with KB and kEFF T scales with ð6=DÞkS : Note that the ratio A1 =A2 ¼ 2:7 0:3 is expected to scale with the ratio of the typical sizes (for instance the chain length) associated with the magnetic arrays probed by the resonance lines L1 and L2. This is very much close to the simplest dimer structure and combination of two dimers to form tetramers. Obviously, this picture requires a
certain particle-to-particle distance along the chain-like structure. It is reasonable to assume that the spacer thickness has to do more with hydrodynamic values instead of only the molecular surface-coating thickness. Assuming the spacer of the order of the particle diameter, we found the tetramer/dimer size-ratio of 73 ¼ 2:3: Slopes A1 and A3 are just about the same, except for the sign. The sign difference could be due to the two possible arrays in a dimer structure, i.e. coherent and fanning associations. In conclusion, the analysis of the resonance line shape and the temperature dependence of the resonance fields of a monodisperse-like magnetic fluid sample, frozen under a small applied field, were used to draw conclusions about the built-in particle arrays. Three components in the envelop resonance line were identified with distinct particle arrays. Values of the ratio of the resonance field versus temperature slopes support the existence of dimers and tetramers as the more likely particle arrays in the sample. Negative and positive slopes (resonance field versus temperature curves) suggest distinct couplings of the nanoparticles in the dimer structure, though more investigations need to be performed to check such hypothesis. The Brazilian agencies CNPq, FINEP, and FINATEC have supported this work.
References [1] A.L. Tronconi, P.C. Morais, F. Pelegrini, F.A. Tourinho, J. Magn. Magn. Mater. 122 (1993) 90. [2] P.C. Morais, G.R.R. Goncalves, K. Skeff Neto, F. Pelegrini, N. Buske, IEEE Trans. Magn. 38 (2002) 3225. [3] K. Parekh, R.V. Upadhyay, R.V. Mehta, D. Srinivas, J. Appl. Phys. 88 (2000) 2799. [4] L.B. Silveira, J.G. Santos, F. Pelegrini, C. Gansau, N. Buske, P.C. Morais, IEEE Trans. Magn. 39 (2003) 2642. [5] C. Wilhelm, F. Gazeau, J.C. Bacri, Eur. Biophys. J. 31 (2002) 118. [6] P.C. Morais, G.R.R. Goncalves, A.F. Bakuzis, K. Skeff Neto, F. Pelegrini, J. Magn. Magn. Mater. 225 (2001) 84. [7] P.C. Morais, M.C.F.L. Lara, A.L. Tronconi, F.A. Tourinho, A.R. Pereira, F. Pelegrini, J. Appl. Phys. 79 (1996) 7931. [8] A.F. Bakuzis, P.C. Morais, F. Pelegrini, J. Appl. Phys. 85 (1999) 7480. [9] R.V. Upadhyay, K. Parekh, R.V. Mehta, Phys. Rev. B 68 (2003) 224434. [10] T. Goetze, C. Gansau, N. Buske, M. Roeder, P. Gornert, M. Bahr, J. Magn. Magn. Mater. 152 (2002) 399. [11] O. Silva, E.C.D. Lima, P.C. Morais, J. Appl. Phys. 93 (2003) 8456. [12] A. Aharoni, J. Appl. Phys. 69 (1991) 7762.
Journal of Magnetism and Magnetic Materials 289 (2005) 146–148 www.elsevier.com/locate/jmmm
Magnetic resonance investigation of monodisperse oleoylsarcosine-coated magnetic fluid A.R. Pereiraa,, F. Pelegrinia, P.C. Moraisb a Instituto de Fı´sica, Universidade Federal de Goia´s, 74001-970 Goiaˆnia-GO, Brazil Instituto de Fı´sica, Universidade de Brası´lia, Fı´sica Aplicada, 70919-970 Brası´lia-DF, Brazil
b
Available online 24 November 2004
Abstract Magnetic resonance experiments were used to investigate the occurrence of particle arrays induced by the application of a small steady field (50 G) during the frozen procedure. In order to skip from the size-dependent effects the sample was filtered to a very narrow distribution of particle sizes. Three well-defined components were chosen to fit the envelope resonance line. Analyses of the resonance field versus temperature of each component were used as a support for the occurrence of dimers and tetramers as the dominant magnetic arrays. r 2004 Published by Elsevier B.V. PACS: 75.50.Mm; 75.30.Gw; 76.50.+g Keywords: Magnetic fluid; Magnetic anisotropy; Magnetic resonance
Magnetic resonance has been used as a valuable tool in the investigation of many aspects of ionic [1], surfacted [2,3] and biocompatible [4,5] magnetic fluids (MFs). In particular, useful data concerning the particle–particle interaction in magnetic fluid samples have been obtained through the magnetic resonance experiments [5]. Dimer disruption, for instance, has been investigated by the temperature evolution of multicomponent resonance spectra [6]. Zero-field-frozen [7] and field-frozen [8,9] magnetic fluid samples have been studied using magnetic resonance experiments as well. Magnetic fields close to the sample saturation values (above 1.5 T), however, have been used to freeze the MF samples in order to carry on angular variation experiments. In other words, magnetic resonance has been used to investigate frozen MFs only in the two extreme Corresponding author. Tel./fax: +55 62 5211029.
E-mail addresses: [email protected], anarita@fis.ufg.br (A.R. Pereira). 0304-8853/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2004.11.042
limits; at zero-field- (zero magnetic texture) or high-fieldfrozen (high magnetic texture) conditions. On the other hand, the particle size polydispersity, characteristic of the chemically synthesized magnetic nanoparticles, play a key role in the interpretation of almost all experimental data, requiring the averaging of any sizedependent property. As a consequence of such sample characteristic, fine details in the experimental data could be blurred out by the particle size dispersity profile. In order to skip from this difficulty we have prepared a monodisperse MF sample based on magnetite nanoparticles surface-coated with oleoylsarcosine. The aqueousbased magnetic fluid sample, containing about 2 1017 particle/cm3 was prepared following the standard recipe described in the literature [10]. In order to reduce the size dispersity down to the monodisperse profile filtering of the as-prepared (polydisperse) magnetic fluid sample was performed using a magnetic field gradient procedure. The particle size histogram, obtained from the transmission electron microscopy
ARTICLE IN PRESS A.R. Pereira et al. / Journal of Magnetism and Magnetic Materials 289 (2005) 146–148
micrographs, were curve-fitted using a log-normal distribution function characterized by the average particle diameter of ð6:2 0:1Þ nm and diameter dispersion of 0:27 0:01: In addition, in order to induce only very small-sized particle arrays the MF sample was frozen from room-temperature down to 77 K in the presence of a steady 50 G field. After the field-frozen procedure the temperature was raised from 100 to 350 K while the X-band resonance spectra were recorded. The magnetic resonance spectra in the temperature range of 100–250 K, corresponding to the range where the MF sample is frozen and the nanoparticles firmly attached to the water-based matrix, was analyzed in terms of three well-characterized components. It is assumed here that the three resonance lines are related to three distinct nanoparticle arrays built from nearly identical magnetic units. Each of these nanoparticle arrays is expected to be well-characterized by a magnetic resonance signature. It is the purpose of this study to report on both the analysis of the magnetic resonance spectra in terms of their components and the temperature dependence of the resonance field of each magnetic resonance component obtained from the de-convolution procedure of the envelope resonance line. In addition, the rate at which the resonance field of each resonance component shifts as a function of the temperature will be used to identify a particular nanoparticle array. Typical magnetic resonance spectra analyses are shown in Fig. 1 at two different temperatures. Though the multi-component spectrum is visually obvious, either due to a smoothly broad and asymmetric envelope
Fig. 1. Typical analyses of the magnetic resonance spectra in terms of three components, at different temperatures.
147
resonance line at lower temperatures (250 K) or due to visible line structure at higher temperatures (300 K), decision concerning the number of features in a single, broad envelope line is not an easy task. Three components (L1, L2, and L3 lines) was the best scenario found in the whole range of temperature, including consistency of the chi-square test, temperature dependence of the resonance field separations and temperature dependence of the resonance linewidths. The temperature dependence of the resonance field of each resonance feature (L1, L2, and L3 lines) found from our spectra analysis is shown in Fig. 2. The resonance field data in Fig. 2 are restricted to the range of 100–275 K, slightly above the samples melting point (about 250 K). Three aspects related to the data shown in Fig. 2 call immediately our attention. First, all points in the temperature range of the frozen sample fall in well-defined straight lines. Second, the intermediate straight line (L2 line) in Fig. 2 has a slope quite different from the other two ones (L1 and L3 lines). Third, the external straight lines (lowest versus highest fields) show similar slopes, except for the sign (positive versus negative). Finally, in the temperature range of 100–275 K the three components have quite different line intensities. In this temperature range line L1 is the weakest signal whereas line L2 is the strongest signal. Immediately above the samples melting point (about 250 K) L2 increases in intensity (area under the resonance curve) whereas L3 reduces in intensity. In this small temperature range (250–275 K) the L1 intensity stays approximately unchanged. The model used to fit the data shown in Fig. 2 starts with the resonance frequency oR, i.e. the Larmor precession frequency of the nanoparticle magnetic moment in the presence of an effective magnetic field (HEFF). The resonance frequency is written as
Fig. 2. The resonance field versus temperature of the three lines L1, L2, and L3 in the temperature range of the frozen state. Symbols are experimental data whereas the straight lines are the best fit according to the equation H R ¼ B þ AT:
ARTICLE IN PRESS 148
A.R. Pereira et al. / Journal of Magnetism and Magnetic Materials 289 (2005) 146–148
oR ¼ gH EFFF ; where g is the gyromagnetic ratio and the effective magnetic field is a result of three main components; the external sweeping field (HE), the exchange anisotropy field (HX), and the effective anisotropy field (HA). At the resonance condition HE matches the resonance field HR. Then, the resonance field is given by H R ¼ oR =g2H X 2H A [11]. The exchange anisotropy field, on the other hand, is given by H X ¼ 4Cn2 =M S Db ; where C is the exchange constant, n are eigenvalues of the differential equation involving the spherical Bessel functions dj n ðnÞ=dn ¼ 0; MS is the saturation magnetization, D is the particle diameter, and b ¼ 2 in the absence of surface anisotropy [12]. Inspection of equation H R ¼ oR =g2H X 2H A ; however, reveals that the temperature dependence of the resonance field (HR) is mainly associated to the last term on the right-hand side (HA), through its dependence upon the effective magnetocrystalline anisotropy density (KEFF). The anisotropy field is written as H A ¼ 2K EFF =M S : In magnetic nanoparticles the effective anisotropy energy density has both bulk (KB) and surface (KS) components, i.e. K EFF ¼ K B þ K S : The surface component, however, is related to the surface-tovolume ratio K S ¼ ð6=DÞkS ; where kS is the surface anisotropy coefficient. In general, KEFF and MS are both temperature dependent. However, considering that our data were taken far below the Curie point (around 850 K for bulk magnetite), MS could be taken as approximately flat. Therefore, the temperature dependence of the anisotropy field would follow mainly the temperature dependence of the effective magnetic anisotropy. Inspection of the experimental data in Fig. 2 shows a linear relationship between HR and T for all three resonance lines (L1, L2, and L3). In other words, the effective magnetic anisotropy would be empirically represented by K EFF ¼ K 0 þ kEFF T: K0 is a constant and kEFF is a size-dependent coefficient, expressed in units of erg/cm3 K. Straight lines in Fig. 2 represent the best fit of the data using the empirical relation: H R ¼ B þ AT; where B ¼ ðoR =g22K 0 =M S 24Cn2 =M S Db Þ is the intercept constant and A ¼ 22kEFF =M S the slope. The slopes found from the fitting of the data shown in Fig. 2 were A1 ¼ ð1:26 0:03Þ G/K, A2 ¼ ð0:46 0:04Þ G/K and A3 ¼ ð1:11 0:06Þ G/K for lines L1, L2, and L3, respectively. Comparing the empirical relation K EFF ¼ K 0 þ kEFF T with K EFF ¼ K B þ K S one sees that K0 scales with KB and kEFF T scales with ð6=DÞkS : Note that the ratio A1 =A2 ¼ 2:7 0:3 is expected to scale with the ratio of the typical sizes (for instance the chain length) associated with the magnetic arrays probed by the resonance lines L1 and L2. This is very much close to the simplest dimer structure and combination of two dimers to form tetramers. Obviously, this picture requires a
certain particle-to-particle distance along the chain-like structure. It is reasonable to assume that the spacer thickness has to do more with hydrodynamic values instead of only the molecular surface-coating thickness. Assuming the spacer of the order of the particle diameter, we found the tetramer/dimer size-ratio of 73 ¼ 2:3: Slopes A1 and A3 are just about the same, except for the sign. The sign difference could be due to the two possible arrays in a dimer structure, i.e. coherent and fanning associations. In conclusion, the analysis of the resonance line shape and the temperature dependence of the resonance fields of a monodisperse-like magnetic fluid sample, frozen under a small applied field, were used to draw conclusions about the built-in particle arrays. Three components in the envelop resonance line were identified with distinct particle arrays. Values of the ratio of the resonance field versus temperature slopes support the existence of dimers and tetramers as the more likely particle arrays in the sample. Negative and positive slopes (resonance field versus temperature curves) suggest distinct couplings of the nanoparticles in the dimer structure, though more investigations need to be performed to check such hypothesis. The Brazilian agencies CNPq, FINEP, and FINATEC have supported this work.
References [1] A.L. Tronconi, P.C. Morais, F. Pelegrini, F.A. Tourinho, J. Magn. Magn. Mater. 122 (1993) 90. [2] P.C. Morais, G.R.R. Goncalves, K. Skeff Neto, F. Pelegrini, N. Buske, IEEE Trans. Magn. 38 (2002) 3225. [3] K. Parekh, R.V. Upadhyay, R.V. Mehta, D. Srinivas, J. Appl. Phys. 88 (2000) 2799. [4] L.B. Silveira, J.G. Santos, F. Pelegrini, C. Gansau, N. Buske, P.C. Morais, IEEE Trans. Magn. 39 (2003) 2642. [5] C. Wilhelm, F. Gazeau, J.C. Bacri, Eur. Biophys. J. 31 (2002) 118. [6] P.C. Morais, G.R.R. Goncalves, A.F. Bakuzis, K. Skeff Neto, F. Pelegrini, J. Magn. Magn. Mater. 225 (2001) 84. [7] P.C. Morais, M.C.F.L. Lara, A.L. Tronconi, F.A. Tourinho, A.R. Pereira, F. Pelegrini, J. Appl. Phys. 79 (1996) 7931. [8] A.F. Bakuzis, P.C. Morais, F. Pelegrini, J. Appl. Phys. 85 (1999) 7480. [9] R.V. Upadhyay, K. Parekh, R.V. Mehta, Phys. Rev. B 68 (2003) 224434. [10] T. Goetze, C. Gansau, N. Buske, M. Roeder, P. Gornert, M. Bahr, J. Magn. Magn. Mater. 152 (2002) 399. [11] O. Silva, E.C.D. Lima, P.C. Morais, J. Appl. Phys. 93 (2003) 8456. [12] A. Aharoni, J. Appl. Phys. 69 (1991) 7762.