Magnetic recording is today the dominant storage technology and CoCrPt:SiO2-based systems with perpendicular magnetic anisotropy represent a very important resource for hard disk drives (HDDs) realisation. Generally the HDD recording layer consists of single-domain CoCrPt grains with perpendicular magnetic anisotropy, separated by Si-oxide. The Si-oxide at the grain boundaries should completely prevent the ferromagnetic exchange coupling among neighbouring grains. However, in real systems, the actual quality of grain isolation, which strongly affects the performance of the recording medium, also depends in a quite complex way on the characteristics of the interlayers and seed-layers used in the complete stacking sequence [1] as well as on the film thickness [2,3]. In the present work, the correlation between the structural properties of granular CoCrPt:SiO2 films with variable thickness and the magnetization reversal mechanisms are investigated. [(Co90Cr10)80Pt20]92:(SiO2)8 thin films with a nominal thickness of 7.5, 10 and 17.5 nm (hereafter referred to as ML7.5, ML10 and ML17.5) were deposited by magnetron sputtering on 2.5-inch hard disk HOYA substrates, with a complex underlayer structure - Cr(2.5nm)/Ru(8nm)/Ru(12nm) - aimed to promote a perpendicular anisotropy and good microstructural properties; films were covered by a 4 nm protective overcoat of diamond-like carbon. Structural characterization were carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. XRD measurements were performed by a Bruker D8 Advance diffractometer operating with a Cu-K? radiation source. TEM analysis was carried out by a Philips CM200 electron microscope operating at 200 kV. For TEM cross-sectional and plan-view observations, samples were prepared by a conventional thinning procedure. Final thinning was carried out by an ion beam system (Gatan PIPS) using Ar ions at 5 kV. Magnetic measurements were carried out by using a vector vibrating sample magnetometer (vVSM, ADE-Technologies Model 10) equipped with 4 pick-up coils and an electromagnet which supplies a maximum field of 2 T. Micromagnetic simulations of room temperature magnetization loops were performed by numerically solving (finite difference method) the standard Landau-Lifshitz-Gilbert-Langevin equation for the time evolution of the magnetization, using the commercial LLG Micromagnetics SimulatorTM [4]. X-ray diffraction patterns of the samples show the presence of only two peaks. One peak labelled as Ru (002), is due to the underlying dual Ru layer, while the second peak is attributed to the (002) reflection of the hexagonal CoCrPt recording layer. The absence of any other Ru or CoCrPt peak suggests a preferential growth of both layers with the c-axis perpendicular to the substrate. The degree of preferential growth along the c-axis of CoCrPt and Ru layers was estimated by measuring the full width at half maximum (FWHM) of the corresponding rocking curves. For each sample, the FWHM values of the CoCrPt and the Ru layers are comparable, suggesting a good epitaxial growth of the magnetic layer on the Ru interlayer. TEM plan view observations reveal the granular nature of the CoCrPt films. Fine grains with an average size of 7 nm, well separated by amorphous-like material (Si-oxide), are clearly visible for all samples, Figure 1. The analysis of angular and time-dependent magnetic measurements indicates that the magnetization reversal mechanism is determined by the coexistence of coherent and incoherent processes, with a tendency toward more coherent reversal processes (i.e. Stoner-Wohlfarth character) as the thickness of the magnetic layer increases. The change in the predominant reversal mechanism is coherent with the observed reduction of the vact/Vgr ratio (activation volume vact and physical volume Vgr of the grains) with the increase of thickness. It is important to stress that the switch between coherent and incoherent process is connected to a lower or higher ferromagnetic exchange coupling among neighbouring grains. Looking at the plan view images of the samples the CoCrPt grains have the same lateral average size and degree of separation regardless of the film thickness. So to understand the nature of magnetic results, TEM cross section observations of the samples were performed, Figure 2. Comparing the three different samples one can observe that the upper part of the films is very similar in terms of lateral grain dimension and separation among grains, as already observed in the plan view images. On the contrary, different microstructural features can be observed in films with different thickness. All the samples can be considered as formed by an initial thin layer at the interface with the Ru underlayer, consisting of nanograins physically in contact among them; on top of such layer, well separated columnar islands grow, whose height increases with increasing thickness , with an intergrain distance remaining roughly constant through the entire film thickness. Such peculiar microstructural features are consistent with magnetic results. Incoherent switching behaviour of the magnetization reversal mechanism is due to the initial layer consisting of grains in close contact; the increasing portion of the region with well separated grains enhances the coherent behaviour with the increase of magnetic layer thickness. It is important to stress that numerical simulation of perpendicular hysteresis loops of samples ML17.5 well reproduces the experimental findings, taking into account a finite level of intergrain exchange coupling (Aint) lower than the bulk exchange constant (Abulk ~ 1·10-11 J/m), even in this sample. In particular, the simulated loop for a CoCrPt film with fully exchange decoupled grains (Aint = 0) shows larger coercivity values and loop closure at fields higher than the experimentally observed values. 1.K. Srinivasan, S.N. Piramanayagam, R.W. Chantrell, Y.S. Kay, J. Magn. Magn. Mat. 320 (2008) 3036 2.H.S. Jung, M. Kuo, S.S. Malhotra, G. Bertero, Appl. Phys.Lett. 91 (2007) 212502 3.H.S. Jung, U. Kwon, M. Kuo, E.M.T. Velu, S.S. Malhotra, W. Jiang, G. Bertero, IEEE Trans. Magn. 43 (2007) 615 4.http://llgmicro.home.mindspring.com
Microstructural investigation of magnetic CoCrPt:SiO2 films
G Varvaro;E Agostinelli;AM Testa;
2013
Abstract
Magnetic recording is today the dominant storage technology and CoCrPt:SiO2-based systems with perpendicular magnetic anisotropy represent a very important resource for hard disk drives (HDDs) realisation. Generally the HDD recording layer consists of single-domain CoCrPt grains with perpendicular magnetic anisotropy, separated by Si-oxide. The Si-oxide at the grain boundaries should completely prevent the ferromagnetic exchange coupling among neighbouring grains. However, in real systems, the actual quality of grain isolation, which strongly affects the performance of the recording medium, also depends in a quite complex way on the characteristics of the interlayers and seed-layers used in the complete stacking sequence [1] as well as on the film thickness [2,3]. In the present work, the correlation between the structural properties of granular CoCrPt:SiO2 films with variable thickness and the magnetization reversal mechanisms are investigated. [(Co90Cr10)80Pt20]92:(SiO2)8 thin films with a nominal thickness of 7.5, 10 and 17.5 nm (hereafter referred to as ML7.5, ML10 and ML17.5) were deposited by magnetron sputtering on 2.5-inch hard disk HOYA substrates, with a complex underlayer structure - Cr(2.5nm)/Ru(8nm)/Ru(12nm) - aimed to promote a perpendicular anisotropy and good microstructural properties; films were covered by a 4 nm protective overcoat of diamond-like carbon. Structural characterization were carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. XRD measurements were performed by a Bruker D8 Advance diffractometer operating with a Cu-K? radiation source. TEM analysis was carried out by a Philips CM200 electron microscope operating at 200 kV. For TEM cross-sectional and plan-view observations, samples were prepared by a conventional thinning procedure. Final thinning was carried out by an ion beam system (Gatan PIPS) using Ar ions at 5 kV. Magnetic measurements were carried out by using a vector vibrating sample magnetometer (vVSM, ADE-Technologies Model 10) equipped with 4 pick-up coils and an electromagnet which supplies a maximum field of 2 T. Micromagnetic simulations of room temperature magnetization loops were performed by numerically solving (finite difference method) the standard Landau-Lifshitz-Gilbert-Langevin equation for the time evolution of the magnetization, using the commercial LLG Micromagnetics SimulatorTM [4]. X-ray diffraction patterns of the samples show the presence of only two peaks. One peak labelled as Ru (002), is due to the underlying dual Ru layer, while the second peak is attributed to the (002) reflection of the hexagonal CoCrPt recording layer. The absence of any other Ru or CoCrPt peak suggests a preferential growth of both layers with the c-axis perpendicular to the substrate. The degree of preferential growth along the c-axis of CoCrPt and Ru layers was estimated by measuring the full width at half maximum (FWHM) of the corresponding rocking curves. For each sample, the FWHM values of the CoCrPt and the Ru layers are comparable, suggesting a good epitaxial growth of the magnetic layer on the Ru interlayer. TEM plan view observations reveal the granular nature of the CoCrPt films. Fine grains with an average size of 7 nm, well separated by amorphous-like material (Si-oxide), are clearly visible for all samples, Figure 1. The analysis of angular and time-dependent magnetic measurements indicates that the magnetization reversal mechanism is determined by the coexistence of coherent and incoherent processes, with a tendency toward more coherent reversal processes (i.e. Stoner-Wohlfarth character) as the thickness of the magnetic layer increases. The change in the predominant reversal mechanism is coherent with the observed reduction of the vact/Vgr ratio (activation volume vact and physical volume Vgr of the grains) with the increase of thickness. It is important to stress that the switch between coherent and incoherent process is connected to a lower or higher ferromagnetic exchange coupling among neighbouring grains. Looking at the plan view images of the samples the CoCrPt grains have the same lateral average size and degree of separation regardless of the film thickness. So to understand the nature of magnetic results, TEM cross section observations of the samples were performed, Figure 2. Comparing the three different samples one can observe that the upper part of the films is very similar in terms of lateral grain dimension and separation among grains, as already observed in the plan view images. On the contrary, different microstructural features can be observed in films with different thickness. All the samples can be considered as formed by an initial thin layer at the interface with the Ru underlayer, consisting of nanograins physically in contact among them; on top of such layer, well separated columnar islands grow, whose height increases with increasing thickness , with an intergrain distance remaining roughly constant through the entire film thickness. Such peculiar microstructural features are consistent with magnetic results. Incoherent switching behaviour of the magnetization reversal mechanism is due to the initial layer consisting of grains in close contact; the increasing portion of the region with well separated grains enhances the coherent behaviour with the increase of magnetic layer thickness. It is important to stress that numerical simulation of perpendicular hysteresis loops of samples ML17.5 well reproduces the experimental findings, taking into account a finite level of intergrain exchange coupling (Aint) lower than the bulk exchange constant (Abulk ~ 1·10-11 J/m), even in this sample. In particular, the simulated loop for a CoCrPt film with fully exchange decoupled grains (Aint = 0) shows larger coercivity values and loop closure at fields higher than the experimentally observed values. 1.K. Srinivasan, S.N. Piramanayagam, R.W. Chantrell, Y.S. Kay, J. Magn. Magn. Mat. 320 (2008) 3036 2.H.S. Jung, M. Kuo, S.S. Malhotra, G. Bertero, Appl. Phys.Lett. 91 (2007) 212502 3.H.S. Jung, U. Kwon, M. Kuo, E.M.T. Velu, S.S. Malhotra, W. Jiang, G. Bertero, IEEE Trans. Magn. 43 (2007) 615 4.http://llgmicro.home.mindspring.comI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
