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Fe trilayers
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 2353–2355 www.elsevier.com/locate/jmmm
Ion-beam induced magnetic nanopatterning of interlayer exchange coupled Fe/Cr/Fe trilayers S. Blomeiera,, P. Candeloroa, B. Hillebrandsa, B. Reuscherb, A. Brodyanskib, M. Kopnarskib a
Fachbereich Physik and Forschungsschwerpunkt MINAS, TU Kaiserslautern, Erwin-Schro¨dinger-Strasse 56, 67663 Kaiserslautern, Germany b Institut fu¨r Oberfla¨chen- und Schichtanalytik, TU Kaiserslautern, Erwin-Schro¨dinger-StraX e 56, 67663 Kaiserslautern, Germany Available online 28 November 2006
Abstract We demonstrate that antiferromagnetically coupled Fe/Cr/Fe trilayers can be magnetically patterned on the micrometer and submicrometer scale by irradiation with keV Ga+ ions, which induces a local transition to ferromagnetic coupling due to direct contact between the two Fe layers via ferromagnetic pinholes. The irradiated areas act as ferromagnetic elements which are embedded into a surrounding, antiferromagnetically coupled trilayer. We also demonstrate that the magnetic properties of the elements fabricated by this method are size dependent, including a transition from multi-domain to single-domain configurations with decreasing element size, and we determine an effective intrinsic limit to the lateral resolution of this patterning technique. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70. I; 75.70.Cn; 61.80.Jh Keywords: Ion irradiation; Interlayer exchange coupling; Magnetic nanopatterning
1. Introduction In recent years, ion irradiation has developed into a convenient tool for magnetic patterning. Material systems such as CoPt multilayers with out-of-plane anisotropy [1], exchange bias bilayers [2] and other magnetic thin film systems [3,4] have been magnetically patterned through local irradiation with keV ions. A major advantage of this technique is that the fluence ranges which are necessary to achieve a magnetic patterning effect are usually low enough that the topography of the bombarded systems is largely preserved. This property makes ion irradiation a promising candidate for technological applications. Previous publications [5,6] have shown that antiferromagnetically coupled trilayer systems can also be magnetically patterned by means of local irradiation with keV ions. In this case, the ions penetrating the bombarded multilayer stack cause structural damage to the interlayer, Corresponding author. Tel.: +49 631 2054203; fax: +49 631 2054095.
E-mail address: [email protected] (S. Blomeier). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.11.103
which is supposed to lead to the formation of ferromagnetic pinholes and induces a transition from antiferromagnetic to ferromagnetic coupling for sufficiently high ion fluences [7]. Moreover, for epitaxial Fe/Cr/Fe trilayers it was demonstrated that this irradiation can be performed locally in such a way that the irradiated areas exhibit the properties of small ferromagnetic elements [6]. In particular, the magnetic moments within those areas were found to form a Landau-type domain structure if the areas have a square geometry with their edges aligned parallel to the easy axes of the fourfold magnetocrystalline anisotropy of the Fe layers. However, the elements studied so far were not smaller than 20 20 mm2. In the work presented here, we investigate elements of smaller sizes and determine an effective intrinsic spatial resolution limit of the patterning method used here. 2. Experimental technique The investigated samples were prepared by electron beam evaporation onto MgO(1 0 0) substrates covered by a
ARTICLE IN PRESS 2354
S. Blomeier et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2353–2355
1 nm Fe seed layer and a 125 nm Ag buffer layer. All trilayers were of the form Fe(10 nm)/Cr(0.7 nm)/Fe(10 nm) and were found to be highly epitaxial according to in situ low-energy electron diffraction measurements. Each sample was covered by a Cr cap layer of 2 nm thickness to prevent oxidation. The exact growth procedure is described elsewhere in more detail [6]. Magnetometric investigations by means of longitudinal magneto-optical Kerr effect confirmed that the samples exhibited strong antiferromagnetic coupling before irradiation. A FEI ALTURA 865 dual-beam focused ion beam (FIB) source emitting 30 keV Ga+ ions was used to create square irradiated areas of different sizes between 200 nm and 5 mm. The edges of these squares were aligned parallel to the easy axes of the fourfold magnetocrystalline anisotropy of the Fe layers. In all cases, an ion fluence of 2.7 1016 ions/cm2 was used. Subsequently, the irradiated structures were investigated by means of atomic force microscopy (AFM) and magnetic force microscopy (MFM), using a Solver NT-MDT magnetic force microscope in the tapping mode. 3. Results and discussion MFM investigations yielded that elements of sizes 1 1 mm2 or larger consistently exhibit a Landau-type domain structure in remanence, as demonstrated for the 20 20 mm2 element in Ref. [6]. Moreover, upon application of an external field, in each case a magnetization reversal process takes place which is qualitatively similar to the one displayed previously [6] (the field is applied inplane and parallel to the square edges). However, the saturation field of each element, i.e. the value of the external field which is necessary to saturate such an element in a magnetic single-domain state (as observed by MFM), was found to be dependent on its lateral size. Fig. 1(a) displays the dependence of the saturation field on the square size. For element sizes of 2 mm and larger, it is found that the saturation field decreases with increasing element size, which can be attributed to the demagnetizing field of these elements that is roughly inversely proportional to its lateral dimensions. However, for element sizes of 1 mm and smaller, the saturation field is found to decrease with decreasing element size. In particular, at an element size of 500 nm, a transition to a single-domain state in remanence (i.e. a saturation field of 0 Oe) can be observed for some (but not all) elements. This is shown in Figs. 1(b) and (c), which display an AFM image of an array of 500 500 nm2 elements with a 2 mm spacing and the corresponding MFM image in remanence, respectively. Upon close inspection it can be observed in Fig. 1(c) that some of the elements still exhibit a Landau domain pattern (which is barely visible even in an enlarged view), while the remaining elements have assumed a magnetic singledomain configuration that produces much stronger contrast. It can thus be deduced that a multi-domain to singledomain transition, which is characteristic for many
Fig. 1. (a) Dependence of the saturation field on the element size. (b) AFM image of an array of 500 500 nm2 or (d) 200 200 nm2 irradiated areas. (c)+(e) Corresponding MFM images in remanence.
magnetically patterned material systems, occurs at this element size. This is not an unexpected result, as element sizes in the sub-micron range are approaching the width of the 901 domain walls of the Landau domain patterns within such elements in remanent configurations. Correspondingly, it can be expected that below a certain threshold it is energetically more favorable for such an element to assume a single domain state in remanence instead of the multi-domain configurations observed for larger element sizes. If the element size is further reduced to 200 nm, all elements exhibit a single-domain configuration in remanence, which is shown in Fig. 1(e). Moreover, it can be observed from Fig. 1(e) that the magnetic configuration surrounding each element is noticeably larger than the element itself and is of the order of 500 nm to 1 mm in size. This configuration is caused by magnetic interactions in
ARTICLE IN PRESS S. Blomeier et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2353–2355
lateral direction which will be discussed in more detail within another upcoming publication. These interactions constitute an effective intrinsic limit to the spatial resolution of the patterning method investigated here.
Acknowledgment This work was supported by the Deutsche Forschungsgemeinschaft. References
4. Summary In summary, we have demonstrated the feasibility of ion beam-induced magnetic nanopatterning of interlayer exchange coupled Fe/Cr/Fe trilayers, and we have determined an intrinsic resolution limit of this patterning method.
2355
[1] [2] [3] [4] [5] [6] [7]
C. Chappert, et al., Science 280 (1998) 1919. J. Fassbender, et al., Phys. Stat. Sol. A 189 (2002) 439. D. McGrouther, et al., Appl. Phys. Lett. 87 (2005) 022507. J. McCord, et al., Appl. Phys. Lett. 86 (2005) 162502. V. Demidov, et al., Appl. Phys. Lett. 84 (2004) 2853. S. Blomeier, et al., J. Appl. Phys. 98 (2005) 093503. S.O. Demokritov, et al., Phys. Rev. Lett. 90 (2003) 097201.
Journal of Magnetism and Magnetic Materials 310 (2007) 2353–2355 www.elsevier.com/locate/jmmm
Ion-beam induced magnetic nanopatterning of interlayer exchange coupled Fe/Cr/Fe trilayers S. Blomeiera,, P. Candeloroa, B. Hillebrandsa, B. Reuscherb, A. Brodyanskib, M. Kopnarskib a
Fachbereich Physik and Forschungsschwerpunkt MINAS, TU Kaiserslautern, Erwin-Schro¨dinger-Strasse 56, 67663 Kaiserslautern, Germany b Institut fu¨r Oberfla¨chen- und Schichtanalytik, TU Kaiserslautern, Erwin-Schro¨dinger-StraX e 56, 67663 Kaiserslautern, Germany Available online 28 November 2006
Abstract We demonstrate that antiferromagnetically coupled Fe/Cr/Fe trilayers can be magnetically patterned on the micrometer and submicrometer scale by irradiation with keV Ga+ ions, which induces a local transition to ferromagnetic coupling due to direct contact between the two Fe layers via ferromagnetic pinholes. The irradiated areas act as ferromagnetic elements which are embedded into a surrounding, antiferromagnetically coupled trilayer. We also demonstrate that the magnetic properties of the elements fabricated by this method are size dependent, including a transition from multi-domain to single-domain configurations with decreasing element size, and we determine an effective intrinsic limit to the lateral resolution of this patterning technique. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70. I; 75.70.Cn; 61.80.Jh Keywords: Ion irradiation; Interlayer exchange coupling; Magnetic nanopatterning
1. Introduction In recent years, ion irradiation has developed into a convenient tool for magnetic patterning. Material systems such as CoPt multilayers with out-of-plane anisotropy [1], exchange bias bilayers [2] and other magnetic thin film systems [3,4] have been magnetically patterned through local irradiation with keV ions. A major advantage of this technique is that the fluence ranges which are necessary to achieve a magnetic patterning effect are usually low enough that the topography of the bombarded systems is largely preserved. This property makes ion irradiation a promising candidate for technological applications. Previous publications [5,6] have shown that antiferromagnetically coupled trilayer systems can also be magnetically patterned by means of local irradiation with keV ions. In this case, the ions penetrating the bombarded multilayer stack cause structural damage to the interlayer, Corresponding author. Tel.: +49 631 2054203; fax: +49 631 2054095.
E-mail address: [email protected] (S. Blomeier). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.11.103
which is supposed to lead to the formation of ferromagnetic pinholes and induces a transition from antiferromagnetic to ferromagnetic coupling for sufficiently high ion fluences [7]. Moreover, for epitaxial Fe/Cr/Fe trilayers it was demonstrated that this irradiation can be performed locally in such a way that the irradiated areas exhibit the properties of small ferromagnetic elements [6]. In particular, the magnetic moments within those areas were found to form a Landau-type domain structure if the areas have a square geometry with their edges aligned parallel to the easy axes of the fourfold magnetocrystalline anisotropy of the Fe layers. However, the elements studied so far were not smaller than 20 20 mm2. In the work presented here, we investigate elements of smaller sizes and determine an effective intrinsic spatial resolution limit of the patterning method used here. 2. Experimental technique The investigated samples were prepared by electron beam evaporation onto MgO(1 0 0) substrates covered by a
ARTICLE IN PRESS 2354
S. Blomeier et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2353–2355
1 nm Fe seed layer and a 125 nm Ag buffer layer. All trilayers were of the form Fe(10 nm)/Cr(0.7 nm)/Fe(10 nm) and were found to be highly epitaxial according to in situ low-energy electron diffraction measurements. Each sample was covered by a Cr cap layer of 2 nm thickness to prevent oxidation. The exact growth procedure is described elsewhere in more detail [6]. Magnetometric investigations by means of longitudinal magneto-optical Kerr effect confirmed that the samples exhibited strong antiferromagnetic coupling before irradiation. A FEI ALTURA 865 dual-beam focused ion beam (FIB) source emitting 30 keV Ga+ ions was used to create square irradiated areas of different sizes between 200 nm and 5 mm. The edges of these squares were aligned parallel to the easy axes of the fourfold magnetocrystalline anisotropy of the Fe layers. In all cases, an ion fluence of 2.7 1016 ions/cm2 was used. Subsequently, the irradiated structures were investigated by means of atomic force microscopy (AFM) and magnetic force microscopy (MFM), using a Solver NT-MDT magnetic force microscope in the tapping mode. 3. Results and discussion MFM investigations yielded that elements of sizes 1 1 mm2 or larger consistently exhibit a Landau-type domain structure in remanence, as demonstrated for the 20 20 mm2 element in Ref. [6]. Moreover, upon application of an external field, in each case a magnetization reversal process takes place which is qualitatively similar to the one displayed previously [6] (the field is applied inplane and parallel to the square edges). However, the saturation field of each element, i.e. the value of the external field which is necessary to saturate such an element in a magnetic single-domain state (as observed by MFM), was found to be dependent on its lateral size. Fig. 1(a) displays the dependence of the saturation field on the square size. For element sizes of 2 mm and larger, it is found that the saturation field decreases with increasing element size, which can be attributed to the demagnetizing field of these elements that is roughly inversely proportional to its lateral dimensions. However, for element sizes of 1 mm and smaller, the saturation field is found to decrease with decreasing element size. In particular, at an element size of 500 nm, a transition to a single-domain state in remanence (i.e. a saturation field of 0 Oe) can be observed for some (but not all) elements. This is shown in Figs. 1(b) and (c), which display an AFM image of an array of 500 500 nm2 elements with a 2 mm spacing and the corresponding MFM image in remanence, respectively. Upon close inspection it can be observed in Fig. 1(c) that some of the elements still exhibit a Landau domain pattern (which is barely visible even in an enlarged view), while the remaining elements have assumed a magnetic singledomain configuration that produces much stronger contrast. It can thus be deduced that a multi-domain to singledomain transition, which is characteristic for many
Fig. 1. (a) Dependence of the saturation field on the element size. (b) AFM image of an array of 500 500 nm2 or (d) 200 200 nm2 irradiated areas. (c)+(e) Corresponding MFM images in remanence.
magnetically patterned material systems, occurs at this element size. This is not an unexpected result, as element sizes in the sub-micron range are approaching the width of the 901 domain walls of the Landau domain patterns within such elements in remanent configurations. Correspondingly, it can be expected that below a certain threshold it is energetically more favorable for such an element to assume a single domain state in remanence instead of the multi-domain configurations observed for larger element sizes. If the element size is further reduced to 200 nm, all elements exhibit a single-domain configuration in remanence, which is shown in Fig. 1(e). Moreover, it can be observed from Fig. 1(e) that the magnetic configuration surrounding each element is noticeably larger than the element itself and is of the order of 500 nm to 1 mm in size. This configuration is caused by magnetic interactions in
ARTICLE IN PRESS S. Blomeier et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2353–2355
lateral direction which will be discussed in more detail within another upcoming publication. These interactions constitute an effective intrinsic limit to the spatial resolution of the patterning method investigated here.
Acknowledgment This work was supported by the Deutsche Forschungsgemeinschaft. References
4. Summary In summary, we have demonstrated the feasibility of ion beam-induced magnetic nanopatterning of interlayer exchange coupled Fe/Cr/Fe trilayers, and we have determined an intrinsic resolution limit of this patterning method.
2355
[1] [2] [3] [4] [5] [6] [7]
C. Chappert, et al., Science 280 (1998) 1919. J. Fassbender, et al., Phys. Stat. Sol. A 189 (2002) 439. D. McGrouther, et al., Appl. Phys. Lett. 87 (2005) 022507. J. McCord, et al., Appl. Phys. Lett. 86 (2005) 162502. V. Demidov, et al., Appl. Phys. Lett. 84 (2004) 2853. S. Blomeier, et al., J. Appl. Phys. 98 (2005) 093503. S.O. Demokritov, et al., Phys. Rev. Lett. 90 (2003) 097201.