- Email: [email protected]
Influence of nanoparticular impurities on the magnetic anisotropy of self-assembled magnetic Co-nanoparticles
Journal of Magnetism and Magnetic Materials 326 (2013) 112–115
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of nanoparticular impurities on the magnetic anisotropy of self-assembled magnetic Co-nanoparticles a ¨ A. Regtmeier a, J. Meyer a,n, N. Mill a, M. Peter c, A. Weddemann b, J. Mattay c, A. Hutten a
Department of Physics, Thin Films and Physics of Nanostructures, Bielefeld University, PB 100131, 33501 Bielefeld, Germany Laboratory for Electromagnetic and Electronic Systems, Massachusetts Institute of Technology, 77 Mass Ave, Cambridge, MA 02139, USA c Department of Chemistry, Organic Chemistry I, Bielefeld University, PB 100131, 33501 Bielefeld, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 June 2012 Received in revised form 14 August 2012 Available online 5 September 2012
A suspension of monodisperse Au-particles of either 3 or 6 nm were mixed with a dilution of 6 nm Co-particles. The resulting mixture was employed for the formation of granular films and the transport properties of these assemblies were analyzed. An increased granular giant magnetoresistive response was observed for samples with a high content of Au-particles. The experimental data were compared to numeric solutions of the Landau–Lifshitz–Gilbert equation for discrete magnetic moments. The alteration of the magnetic properties can be related to the formation of a nanoparticular structure resulting from the minimization of the particle stray fields. & 2012 Elsevier B.V. All rights reserved.
Keywords: Co-nanoparticles Self assembly Local anisotropy Giant magnetoresistance
1. Introduction During the last decades magnetic nanoparticles have been thoroughly studied due to their many promising applications in chemical, physical and medical fields [1,2]. Magnetic nanoparticles embedded in a non-magnetic metallic matrix form a granular system that shows spin dependent transport phenomena such as the Giant Magnetoresistance Effect (GMR effect) [3,4] originally found and studied in magnetic multilayer systems [5,6]. The investigation of this effect starting from the modification of the nanocomponents is key for the realization of innovative magnetoresistive sensor devices capable of detecting and monitoring other magnetic sources. Different nanoparticle species can result from various intrinsic properties of the synthesis procedure. The presence of two ligands during the chemical formation may e.g. entail particles of two distinct sizes due to different dissociation constants and, thus, different growth properties. The resulting self-assembled patterns often show chain-like substructures. Larger particles form onedimensional substructures, which are embedded within smaller filler components [7–9]. In most of these cases, however, magnetic response or transport properties do not vary significantly from the single species systems. The features given by the geometric substructure is dominated by the material properties, which differ only slightly among the species due to finite size
n
Corresponding author. E-mail address: [email protected] (J. Meyer).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.08.048
effects [10]. In this work, the response properties of arrays of magnetic nanoparticles are altered by the formation of a microstructure. Mixing Co- with Au-nanoparticles produces local anisotropic suspensions of magnetic particles. Such an approach has various advantages. All particle species may be synthesized in independent process steps, which minimizes the chances for undesired side products. Furthermore, each product suspension may be treated by consequent refinement steps.
2. Experimental realization and results The magnetic Co-nanoparticles employed for the experiments were synthesized in a thermolysis procedure according to the approach of Puntes et al. [11]. The average particle diameter is 6 nm with a standard deviation of 1.14 nm. Particles are crystallized in the e–Co structure. Further details about the particle preparation can be found in [12]. Upon synthesis, the particles were redispersed in 1,2-dichlorobenzene. Suspensions were mixed with Co-particles and Au-particles of sizes 3 nm and 6 nm at mixture ratios of Co:Au—3:1, 2:1 and 1:1. A volume of 1 ml of the mixtures was deposited on a TEM-grid. After evaporation of the carrier liquid, the samples were analyzed by a transmission electron microscope (TEM). TEM-images of different samples are shown in Fig. 1. Fig. 1(a) shows the structure of an array of pure Co-nanoparticles. From (b) to (d), the number of Au-nanoparticles was increased from the mixture ratio 3:1 to 1:1, (e) shows a magnification of (c). For these examples, Co- and Au-particles can easily be seen due to
A. Regtmeier et al. / Journal of Magnetism and Magnetic Materials 326 (2013) 112–115
113
Fig. 1. TEM-images of evaporated samples of the pure Co-nanoparticle suspension (a) and the Co–Au-mixture with 3 nm Au-particles of ratios 3:1 (b), 2:1 (c) and 1:1 (d) are displayed. (e) is a magnification of Fig. 1(c). On TEM-images, it is not possible to distinguish between 6 nm Co- and Au-particles. (f) is a bright field image. A brightness contrast can be reported on pictures taken in the HAADF-mode (g) due to the different molar masses of Co and Au.
their different sizes. For low Au-concentrations, Co-particles are homogeneously distributed along the substrate surface. Au-particles can be found in the spaces between the larger colloids and increase the average distance between the Co-particles for higher Au-contributions as shown in Fig. 1(e). As can be seen in Fig. 1(f), it is difficult to distinguish between the Co- and the 6 nm Au-particles. When analyzed in the HAADF-mode, however, the particle species exhibit a strong brightness contrast. Au-particles appear brighter due to their higher molar mass of 196.967 g/mol compared to the Co-particles with the molar mass 58.933 g/mol. In Fig. 1(g), it can be seen that large Au-particles do not influence the symmetry of the assembly, but act as nonmagnetic impurities in the array. The influence of the microstructure on the magnetic response functions was analyzed by alternating gradient magnetometer (AGM) measurements at room temperature. The resulting response functions for the highest Au-particle concentrations in comparison with a pure Co-sample as a reference are shown in Fig. 2. The measured values for the coercive fields are summarized in Table 1. The reference sample (black line) shows the lowest coercive field and the steepest response behavior. An increasing coercive field and a decreasing susceptibility is observed from the mixtures from 3 nm Au-particles (red line) to 6 nm Au-particles (blue line) within the mixture ratio 1:1. The addition of Au-particles
Fig. 2. (Color online) Magnetic response functions are displayed. The comparison between the reference (pure Co-sample) and the mixture with a 1:1 ratio is shown. Numeric values for the coercive fields are summarized in Table 1.
results in an increased value of the coercive field HC and a decreased magnetic susceptibility. The values presented in Table 1 reveal a clear trend of increased coercive field values for higher Au contents in the sample. Moreover, GMR measurements at room temperature were performed using a 4-point probe method. It is interesting to note that adding Au-impurities will increase the GMR-ratio after multiple consecutive field sweeps
114
A. Regtmeier et al. / Journal of Magnetism and Magnetic Materials 326 (2013) 112–115
Table 1 The coercive fields of samples with different Co:Au ratios measured by AGM are shown. Particle species
We employed a backward differentiation formula of order 4. The external magnetic field at the location of each particle is given by the stray field of contiguous particles and the homogenous field contribution employed for the magnetic saturation of the assembly. Due to the assumed particle morphology, the particle stray field can be described by a dipole field and, hence, is
ratio Co:Au
HC in Oe
Co-particle
1:0
57.91
Co/Au (3 nm) Co/Au (3 nm) Co/Au (3 nm)
3:1 2:1 1:1
61.66 62.79 65.57
H iext ¼ H iext,hom þ
Co/Au (6 nm) Co/Au (6 nm) Co/Au (6 nm)
3:1 2:1 1:1
70.15 79.41 88.56
The electric transport properties within such a device depend on the relative orientation of adjacent magnetic moments. This granular GMR effect [3,4] can be described by[14]
X
H ijdipole
C GMR ¼ 1 /1 þcos yS2 2
Fig. 3. (Color online) Different GMR measurements are displayed. The dashed line shows the response of the 3 nm Au-particle system with a 1:1 mixture ratio and the dotted line the altered response of the same sample after 4 iterative field sweeps. The straight line resembles the behavior of the reference (pure Cosample).
(Fig. 3). This is contrary to the iterative behavior of pure granular Co-monolayers, where the amplitude of the ratio drops with the number of iterative measurements [9].
3. Numerical calculations and discussion In order to gain a better understanding of these findings, numerical calculations were carried out. Each particle was approximated by a perfect sphere of radius R¼ 3 nm with a saturation magnetization MS ¼900 kA/m. The latter value differs from the Co-bulk magnetization value due to finite size effects that occur in magnetic nanoparticles [10]. For the calculation, all contributions depending on time derivatives of the magnetization vector may be omitted since we are only interested in the stationary solutions of the particle system. Under these assumptions, the governing Landau–Lifshitz–Gilbert equations reduce to a set of ordinary equations [13] m H eff ¼ 0
ð1Þ
with 9m9 ¼ 1 and the effective magnetic field Heff H eff ¼
df ani þH ext dm
ð3Þ
j
ð2Þ
with fani the anisotropy energy functional. Geometric anisotropy effects introduced by the microstructured spatial particle distribution influence the magnetic properties of the system due to an altered inter-particle dipolar coupling. To keep the focus on this particular contribution, the magnetocrystalline anisotropy was set to fani 0 in the following.
ð4Þ
where the constant C is a measure for the spin dependence of electron scattering and y the angle between adjacent magnetic moments. For the sake of simplicity, we will set C ¼1 in the following. Furthermore, the influence of the Au-particles on the magnetic equilibrium state of the Co-particle assembly can be neglected since the Au-particles show a diamagnetic response behavior. Due to steric repulsion, however, they form obstacles during the self-assembly process. In particular, as already shown in Fig. 1(g), they take a place within the ordered assembly without breaking the spatial symmetry if they have a similar size as the Co-particles. Therefore, from the perspective of the Co-superstructure, they introduce vacancies to the perfectly ordered particle lattice. An example of the magnetic equilibrium state of 100 particles assembled in a hexagonal symmetry is shown in Fig. 4. The color code represents the in-plane components of the magnetic moments. Out-of-plane components are usually very small for two-dimensional assemblies and will not be taken into account here. The highlighted regions show the impact of the vacancies on the magnetic equilibrium state. Individual moments align parallel to the geometric structure to minimize the magnetic stray field contribution. Even though the sample is a homogeneous mixture of Co- and Au-particles, the Au-nanocrystals introduce a local anisotropy. The presence of vacancies increases the inner boundaries of areas with Co-particles. Along these boundaries, a magnetization alignment parallel to the Co–Au-interface is energetically most favorable because of a reduction of the magnetic stray field. Consequently, the corresponding magnetic states are of a higher stability than the magnetic equilibrium state of a pure Comonolayer. The configuration shown in Fig. 4 allows us to understand the different effects observed in the experiments. The reduction of the GMR-ratio in the presence of Au-impurities is a direct result from the strong correlation between local anisotropy and magnetic equilibrium. Areas which favor a parallel alignment of contiguous particles (Fig. 4(a), e.g. blue arrays), reduce the scattering probability of conducting electrons due to their high degree of order [9] as they also change the magnetization as a whole in the sense of a magnetic domain. For pure granular Co-monolayers, iterative field sweeps result in increased local order where magnetic particle moments form domains of parallel alignment. In the system considered here, the local one-dimensional substructures show a tendency to align their magnetic moments with the geometric direction but antiparallel to each other. Such a configuration results in an increased GMR-amplitude after several iterative measurements. A typical example of such a state can be found on the left side of the particle assembly in Fig. 4(a) (discontinuous 1801-transition, pink-turquoise).
A. Regtmeier et al. / Journal of Magnetism and Magnetic Materials 326 (2013) 112–115
115
Fig. 4. (Color online) (a) The equilibrium state of a magnetic assembly of 100 particles with 50 vacancies was calculated. The correlation between spatial particle distribution and magnetic configuration is highlighted. (b) Comparison between the assembly shown in (a) and a full 10 10-assembly obtained by numerical calculations. The introduction of vacancies entails increased coercive fields and decreased susceptibilities.
4. Conclusion It could be shown that the addition of impurities by Auparticles to assemblies of Co-particles increases the local anisotropy along the microstructure of the spatial particle distribution. A higher Au-content resulted in increased coercive field values and lowered magnetic susceptibilities. The effects could be related to an increased degree of boundary areas where more stable magnetic configurations can be attained. The controlled introduction of impurities offers an additional degree of freedom to tailor the properties of particle monolayers to specific functional tasks. On the other hand, we can also conclude that whenever a very soft switching of granular films is desired, impurities and vacancies need to be reduced at any cost since they entail stable magnetic configurations and, therefore, increased values for the coercive magnetic field and magnetic stiffness.
Acknowledgments The authors would like to thank the FOR 945 and the SFB 613 for financial support in the framework of project 3 and K3, respectively, and BMBF-Bead.Plus for funding under the grant number 16SV5403.
Alexander Weddemann gratefully acknowledges financial funding from the Alexander von Humboldt foundation. References ¨ [1] G. Reiss, A. Hutten, Nature Materials 4 (2005) 725–726. [2] A. Weddemann, C. Albon, A. Auge, F. Wittbracht, P. Hedwig, D. Akemeier, ¨ K. Rott, D. Meißner, P. Jutzi, A. Hutten, Biosensors and Bioelectronics 26 (2010) 1152. [3] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, ¨ F.T. Parker, A. Hutten, G. Thomas, Physical Review Letters 68 (1992) 3745. [4] J.Q. Xiao, J. Samuel Jiang, C.L. Chien, Physical Review Letters 68 (1992) 3749. [5] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, Physical Review Letters 61 (1988) 2472. ¨ [6] G. Binasch, P. Grunberg, F. Saurenbach, W. Zinn, Physical Review B 39 (1989) 4828. [7] A. Ghazali, J. Levy, Physical Review B 67 (2003) 064409. [8] D. Levesque, J.J. Weis, Physical Review B 49 (1994) 5131–5140. ¨ [9] A. Weddemann, I. Ennen, A. Regtmeier, C. Albon, A. Wolff, K. Eckstadt, N. Mill, ¨ M.K.H. Peter, J. Mattay, C. Plattner, N. Sewald, A.Hutten. Beilstein, Nanotechnology 1 (2010) 75–93. [10] I. Ennen, Ph.D. Thesis, Bielefeld University, 2008. PUB-ID: 2306267 Available from: /http://pub.uni-bielefeld.de/publication/2306267S. [11] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115–2117. [12] A. Regtmeier, Ph.D. Thesis, Bielefeld University, 2011. PUB-ID: 2451342 Available from: /http://pub.uni-bielefeld.de/publication/2451342S. ¨ [13] A. Weddemann, A. Auge, D. Kappe, F. Wittbracht, A. Hutten, Journal of Magnetism and Magnetic Materials 322 (2010) 643–646. [14] N.J. Wiser, Journal of Magnetism and Magnetic Materials 159 (1996) 119–124.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of nanoparticular impurities on the magnetic anisotropy of self-assembled magnetic Co-nanoparticles a ¨ A. Regtmeier a, J. Meyer a,n, N. Mill a, M. Peter c, A. Weddemann b, J. Mattay c, A. Hutten a
Department of Physics, Thin Films and Physics of Nanostructures, Bielefeld University, PB 100131, 33501 Bielefeld, Germany Laboratory for Electromagnetic and Electronic Systems, Massachusetts Institute of Technology, 77 Mass Ave, Cambridge, MA 02139, USA c Department of Chemistry, Organic Chemistry I, Bielefeld University, PB 100131, 33501 Bielefeld, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 June 2012 Received in revised form 14 August 2012 Available online 5 September 2012
A suspension of monodisperse Au-particles of either 3 or 6 nm were mixed with a dilution of 6 nm Co-particles. The resulting mixture was employed for the formation of granular films and the transport properties of these assemblies were analyzed. An increased granular giant magnetoresistive response was observed for samples with a high content of Au-particles. The experimental data were compared to numeric solutions of the Landau–Lifshitz–Gilbert equation for discrete magnetic moments. The alteration of the magnetic properties can be related to the formation of a nanoparticular structure resulting from the minimization of the particle stray fields. & 2012 Elsevier B.V. All rights reserved.
Keywords: Co-nanoparticles Self assembly Local anisotropy Giant magnetoresistance
1. Introduction During the last decades magnetic nanoparticles have been thoroughly studied due to their many promising applications in chemical, physical and medical fields [1,2]. Magnetic nanoparticles embedded in a non-magnetic metallic matrix form a granular system that shows spin dependent transport phenomena such as the Giant Magnetoresistance Effect (GMR effect) [3,4] originally found and studied in magnetic multilayer systems [5,6]. The investigation of this effect starting from the modification of the nanocomponents is key for the realization of innovative magnetoresistive sensor devices capable of detecting and monitoring other magnetic sources. Different nanoparticle species can result from various intrinsic properties of the synthesis procedure. The presence of two ligands during the chemical formation may e.g. entail particles of two distinct sizes due to different dissociation constants and, thus, different growth properties. The resulting self-assembled patterns often show chain-like substructures. Larger particles form onedimensional substructures, which are embedded within smaller filler components [7–9]. In most of these cases, however, magnetic response or transport properties do not vary significantly from the single species systems. The features given by the geometric substructure is dominated by the material properties, which differ only slightly among the species due to finite size
n
Corresponding author. E-mail address: [email protected] (J. Meyer).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.08.048
effects [10]. In this work, the response properties of arrays of magnetic nanoparticles are altered by the formation of a microstructure. Mixing Co- with Au-nanoparticles produces local anisotropic suspensions of magnetic particles. Such an approach has various advantages. All particle species may be synthesized in independent process steps, which minimizes the chances for undesired side products. Furthermore, each product suspension may be treated by consequent refinement steps.
2. Experimental realization and results The magnetic Co-nanoparticles employed for the experiments were synthesized in a thermolysis procedure according to the approach of Puntes et al. [11]. The average particle diameter is 6 nm with a standard deviation of 1.14 nm. Particles are crystallized in the e–Co structure. Further details about the particle preparation can be found in [12]. Upon synthesis, the particles were redispersed in 1,2-dichlorobenzene. Suspensions were mixed with Co-particles and Au-particles of sizes 3 nm and 6 nm at mixture ratios of Co:Au—3:1, 2:1 and 1:1. A volume of 1 ml of the mixtures was deposited on a TEM-grid. After evaporation of the carrier liquid, the samples were analyzed by a transmission electron microscope (TEM). TEM-images of different samples are shown in Fig. 1. Fig. 1(a) shows the structure of an array of pure Co-nanoparticles. From (b) to (d), the number of Au-nanoparticles was increased from the mixture ratio 3:1 to 1:1, (e) shows a magnification of (c). For these examples, Co- and Au-particles can easily be seen due to
A. Regtmeier et al. / Journal of Magnetism and Magnetic Materials 326 (2013) 112–115
113
Fig. 1. TEM-images of evaporated samples of the pure Co-nanoparticle suspension (a) and the Co–Au-mixture with 3 nm Au-particles of ratios 3:1 (b), 2:1 (c) and 1:1 (d) are displayed. (e) is a magnification of Fig. 1(c). On TEM-images, it is not possible to distinguish between 6 nm Co- and Au-particles. (f) is a bright field image. A brightness contrast can be reported on pictures taken in the HAADF-mode (g) due to the different molar masses of Co and Au.
their different sizes. For low Au-concentrations, Co-particles are homogeneously distributed along the substrate surface. Au-particles can be found in the spaces between the larger colloids and increase the average distance between the Co-particles for higher Au-contributions as shown in Fig. 1(e). As can be seen in Fig. 1(f), it is difficult to distinguish between the Co- and the 6 nm Au-particles. When analyzed in the HAADF-mode, however, the particle species exhibit a strong brightness contrast. Au-particles appear brighter due to their higher molar mass of 196.967 g/mol compared to the Co-particles with the molar mass 58.933 g/mol. In Fig. 1(g), it can be seen that large Au-particles do not influence the symmetry of the assembly, but act as nonmagnetic impurities in the array. The influence of the microstructure on the magnetic response functions was analyzed by alternating gradient magnetometer (AGM) measurements at room temperature. The resulting response functions for the highest Au-particle concentrations in comparison with a pure Co-sample as a reference are shown in Fig. 2. The measured values for the coercive fields are summarized in Table 1. The reference sample (black line) shows the lowest coercive field and the steepest response behavior. An increasing coercive field and a decreasing susceptibility is observed from the mixtures from 3 nm Au-particles (red line) to 6 nm Au-particles (blue line) within the mixture ratio 1:1. The addition of Au-particles
Fig. 2. (Color online) Magnetic response functions are displayed. The comparison between the reference (pure Co-sample) and the mixture with a 1:1 ratio is shown. Numeric values for the coercive fields are summarized in Table 1.
results in an increased value of the coercive field HC and a decreased magnetic susceptibility. The values presented in Table 1 reveal a clear trend of increased coercive field values for higher Au contents in the sample. Moreover, GMR measurements at room temperature were performed using a 4-point probe method. It is interesting to note that adding Au-impurities will increase the GMR-ratio after multiple consecutive field sweeps
114
A. Regtmeier et al. / Journal of Magnetism and Magnetic Materials 326 (2013) 112–115
Table 1 The coercive fields of samples with different Co:Au ratios measured by AGM are shown. Particle species
We employed a backward differentiation formula of order 4. The external magnetic field at the location of each particle is given by the stray field of contiguous particles and the homogenous field contribution employed for the magnetic saturation of the assembly. Due to the assumed particle morphology, the particle stray field can be described by a dipole field and, hence, is
ratio Co:Au
HC in Oe
Co-particle
1:0
57.91
Co/Au (3 nm) Co/Au (3 nm) Co/Au (3 nm)
3:1 2:1 1:1
61.66 62.79 65.57
H iext ¼ H iext,hom þ
Co/Au (6 nm) Co/Au (6 nm) Co/Au (6 nm)
3:1 2:1 1:1
70.15 79.41 88.56
The electric transport properties within such a device depend on the relative orientation of adjacent magnetic moments. This granular GMR effect [3,4] can be described by[14]
X
H ijdipole
C GMR ¼ 1 /1 þcos yS2 2
Fig. 3. (Color online) Different GMR measurements are displayed. The dashed line shows the response of the 3 nm Au-particle system with a 1:1 mixture ratio and the dotted line the altered response of the same sample after 4 iterative field sweeps. The straight line resembles the behavior of the reference (pure Cosample).
(Fig. 3). This is contrary to the iterative behavior of pure granular Co-monolayers, where the amplitude of the ratio drops with the number of iterative measurements [9].
3. Numerical calculations and discussion In order to gain a better understanding of these findings, numerical calculations were carried out. Each particle was approximated by a perfect sphere of radius R¼ 3 nm with a saturation magnetization MS ¼900 kA/m. The latter value differs from the Co-bulk magnetization value due to finite size effects that occur in magnetic nanoparticles [10]. For the calculation, all contributions depending on time derivatives of the magnetization vector may be omitted since we are only interested in the stationary solutions of the particle system. Under these assumptions, the governing Landau–Lifshitz–Gilbert equations reduce to a set of ordinary equations [13] m H eff ¼ 0
ð1Þ
with 9m9 ¼ 1 and the effective magnetic field Heff H eff ¼
df ani þH ext dm
ð3Þ
j
ð2Þ
with fani the anisotropy energy functional. Geometric anisotropy effects introduced by the microstructured spatial particle distribution influence the magnetic properties of the system due to an altered inter-particle dipolar coupling. To keep the focus on this particular contribution, the magnetocrystalline anisotropy was set to fani 0 in the following.
ð4Þ
where the constant C is a measure for the spin dependence of electron scattering and y the angle between adjacent magnetic moments. For the sake of simplicity, we will set C ¼1 in the following. Furthermore, the influence of the Au-particles on the magnetic equilibrium state of the Co-particle assembly can be neglected since the Au-particles show a diamagnetic response behavior. Due to steric repulsion, however, they form obstacles during the self-assembly process. In particular, as already shown in Fig. 1(g), they take a place within the ordered assembly without breaking the spatial symmetry if they have a similar size as the Co-particles. Therefore, from the perspective of the Co-superstructure, they introduce vacancies to the perfectly ordered particle lattice. An example of the magnetic equilibrium state of 100 particles assembled in a hexagonal symmetry is shown in Fig. 4. The color code represents the in-plane components of the magnetic moments. Out-of-plane components are usually very small for two-dimensional assemblies and will not be taken into account here. The highlighted regions show the impact of the vacancies on the magnetic equilibrium state. Individual moments align parallel to the geometric structure to minimize the magnetic stray field contribution. Even though the sample is a homogeneous mixture of Co- and Au-particles, the Au-nanocrystals introduce a local anisotropy. The presence of vacancies increases the inner boundaries of areas with Co-particles. Along these boundaries, a magnetization alignment parallel to the Co–Au-interface is energetically most favorable because of a reduction of the magnetic stray field. Consequently, the corresponding magnetic states are of a higher stability than the magnetic equilibrium state of a pure Comonolayer. The configuration shown in Fig. 4 allows us to understand the different effects observed in the experiments. The reduction of the GMR-ratio in the presence of Au-impurities is a direct result from the strong correlation between local anisotropy and magnetic equilibrium. Areas which favor a parallel alignment of contiguous particles (Fig. 4(a), e.g. blue arrays), reduce the scattering probability of conducting electrons due to their high degree of order [9] as they also change the magnetization as a whole in the sense of a magnetic domain. For pure granular Co-monolayers, iterative field sweeps result in increased local order where magnetic particle moments form domains of parallel alignment. In the system considered here, the local one-dimensional substructures show a tendency to align their magnetic moments with the geometric direction but antiparallel to each other. Such a configuration results in an increased GMR-amplitude after several iterative measurements. A typical example of such a state can be found on the left side of the particle assembly in Fig. 4(a) (discontinuous 1801-transition, pink-turquoise).
A. Regtmeier et al. / Journal of Magnetism and Magnetic Materials 326 (2013) 112–115
115
Fig. 4. (Color online) (a) The equilibrium state of a magnetic assembly of 100 particles with 50 vacancies was calculated. The correlation between spatial particle distribution and magnetic configuration is highlighted. (b) Comparison between the assembly shown in (a) and a full 10 10-assembly obtained by numerical calculations. The introduction of vacancies entails increased coercive fields and decreased susceptibilities.
4. Conclusion It could be shown that the addition of impurities by Auparticles to assemblies of Co-particles increases the local anisotropy along the microstructure of the spatial particle distribution. A higher Au-content resulted in increased coercive field values and lowered magnetic susceptibilities. The effects could be related to an increased degree of boundary areas where more stable magnetic configurations can be attained. The controlled introduction of impurities offers an additional degree of freedom to tailor the properties of particle monolayers to specific functional tasks. On the other hand, we can also conclude that whenever a very soft switching of granular films is desired, impurities and vacancies need to be reduced at any cost since they entail stable magnetic configurations and, therefore, increased values for the coercive magnetic field and magnetic stiffness.
Acknowledgments The authors would like to thank the FOR 945 and the SFB 613 for financial support in the framework of project 3 and K3, respectively, and BMBF-Bead.Plus for funding under the grant number 16SV5403.
Alexander Weddemann gratefully acknowledges financial funding from the Alexander von Humboldt foundation. References ¨ [1] G. Reiss, A. Hutten, Nature Materials 4 (2005) 725–726. [2] A. Weddemann, C. Albon, A. Auge, F. Wittbracht, P. Hedwig, D. Akemeier, ¨ K. Rott, D. Meißner, P. Jutzi, A. Hutten, Biosensors and Bioelectronics 26 (2010) 1152. [3] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, ¨ F.T. Parker, A. Hutten, G. Thomas, Physical Review Letters 68 (1992) 3745. [4] J.Q. Xiao, J. Samuel Jiang, C.L. Chien, Physical Review Letters 68 (1992) 3749. [5] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, Physical Review Letters 61 (1988) 2472. ¨ [6] G. Binasch, P. Grunberg, F. Saurenbach, W. Zinn, Physical Review B 39 (1989) 4828. [7] A. Ghazali, J. Levy, Physical Review B 67 (2003) 064409. [8] D. Levesque, J.J. Weis, Physical Review B 49 (1994) 5131–5140. ¨ [9] A. Weddemann, I. Ennen, A. Regtmeier, C. Albon, A. Wolff, K. Eckstadt, N. Mill, ¨ M.K.H. Peter, J. Mattay, C. Plattner, N. Sewald, A.Hutten. Beilstein, Nanotechnology 1 (2010) 75–93. [10] I. Ennen, Ph.D. Thesis, Bielefeld University, 2008. PUB-ID: 2306267 Available from: /http://pub.uni-bielefeld.de/publication/2306267S. [11] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115–2117. [12] A. Regtmeier, Ph.D. Thesis, Bielefeld University, 2011. PUB-ID: 2451342 Available from: /http://pub.uni-bielefeld.de/publication/2451342S. ¨ [13] A. Weddemann, A. Auge, D. Kappe, F. Wittbracht, A. Hutten, Journal of Magnetism and Magnetic Materials 322 (2010) 643–646. [14] N.J. Wiser, Journal of Magnetism and Magnetic Materials 159 (1996) 119–124.