In order to investigate the field stability of EB-SAF coupled samples, the relaxation effect as explained in section 2.3.1 is investigated. Figure 4.14 shows an overview of the different fields used during the relaxation measurement. A magnetic field is applied for 1 hour, after which a hysteresis loop is measured. This is repeated up to 12 hours at a measuring temperature of 100° C in order to compare the measurements to previous measurements done at FNA. Figure 4.15 shows an example of two magnetic loops measured during the relaxation measurement. Figure 4.15 shows that the hysteresis loop of the bottom Co layer shifts to lower fields, while the shape of the loop remains unchanged. This shift of the loop is the result of the reduction in EB, and by calculating this shift over time the reduction of EB over time can be extracted. The stability of the EB is investigated for fields at different locations in the hysteresis loop as shown in figure 4.14. Three field values are used to investigate the EB stability for zero, small negative and small positive fields (-18 mT, 0 mT and 20 mT). Two field values where spin flop might occur (-27.5 mT and 62.5 mT) are used to investigate the effect of spin flop on the EB. Finally, two large field values where the magnetization is saturated are used (+/- 1 T) in order to compare the EB stability with previous studies on EB stability of IrMn/Co layers. For these
μ0H) Field)cooling)direc5on)
trelaxa5on)=)0)hours) trelaxa5on)=)12)hours) ΔHeb)
µ0H (mT)
Figure 4.15: Relaxation measurement results when a 1 T field is applied. Over time, the shift of the plateau decreases due to the reduction in EB. This shift (∆Heb) can be measured by calculating the shift of the hysteresis loop and is used to calculate the reduction in EB field.
measurements, a large Ta(2)/Pt(5)/IrMn(12)/Co(3)/Ru(2.8)/Co(3)/Ta(5) sample is grown and cut into smaller samples. Every measurement is done on a different small sample so that samples are used only once and annealing effects due to multiple annealing sequences are prevented from influencing the measurements.
The measurements during which an applied field of 62.5 mT and 1 T is used shows a reduction of EB over time. During all other measurements the EB does not change over time. Figure 4.16 shows the EB fields and coercive fields over time. A clear reduction of the EB field is observed. During the relaxation measurement with an applied field of 62.5 mT, a decay rate of 6.3% ± 0.3% per hour is measured. The measurement using an applied field of 1 T showed a decay rate of 27.0% ± 0.6% per hour. The decay rate is significantly higher than previous results obtained during EB studies done at FNA, as these measurements resulted in a decay rate between 0 and 5% per hour for Ta(2)/Pt(5)/IrMn(12)/Co(5)/Ta(10)-samples. For these samples, EB fields between 22 and 26 mT are found, which is lower than the EB measured for our samples (27.2 mT and 32.8 mT). The large difference in decay rate and EB can not be explained at this point. The coercive fields do not change significantly over time. This is in agreement with the theory behind the field stability of EB as explained in section 2.2.3.
62.5)mT)applied)field) 1)T)applied)field)
Figure 4.16: Change in EB (top left) and coercive field values (bottom left) during the relaxation measurements where a 62.5 mT (black) and 1 T (red) is applied. The Coercive fields do not show a significant change over time. In order to extract the decay rate over time, the EB is normalized and plotted versus a logaritmic time scale (top right). Other measurements using different applied fields as shown in figure 4.14 do not show any change of the EB.
In order to explain the difference in decay rates and the abscence of EB reduction for an applied field of -27.5 mT, the energies of the AF grains are considered. As explained in section 2.2.3, the energy of an AF grain is given by
EAF = tgKAFsin2(α) − cJebcos(β − α) (4.2) in which the angle β of the bottom Co layer plays an important role. depending on the orientation of the magnetization of the bottom Co-layer, the energy landscape of the AF grain changes. In order to illustrate the change of the landscape when β is varied, the order of magnitude of the parameters is estimated and used to plot energy landscapes. The parameters used are tg = 10−9 m, KAF = 106 J/m3, Jeb = 5 ∗ 10−5 J/m2 and c is set to 0.5. Figure 4.17 shows an example of the energy landscapes of an AF grain for β = 0 and β = π.
For 0 < β < π the energy landscape changes gradually. For example, consider β = 16π and β = 23π. The energy of an AF grain as a function of the magnetization angle of the AF grains is shown in figure 4.18.
When β = 16π the energy barrier has decreased as compared to β = 0, but the grain is still in its global minimum for α = 0. When β increases, the energy barrier decreases, and the global energy minimum shifts towards α = π, as is observed for example for β = 23π. In this configuration, the energy barrier has decreased
β=0) Ferromagne5c)layer) )An5ferromagne5c)grain) )
E2)
E1)
E0)
ΔE) Field)cooling)direc5on)
E2)
E1)
E0)
ΔE)
β=π) Ferromagne5c)layer)
)An5ferromagne5c)grain) )
Field)cooling)direc5on)
Figure 4.17: The energy landscapes of an AF grain for β = 0 (top) and β = π (bottom). When β = 0, the AF grain is in its global energy minimum. When β = π, the grain needs to overcome an energy barrier ∆E to reach the global energy minimum.
enough for the grains to overcome the barrier. furthermore, the local and global energy minimum do not vary much in energies as compared to β = π, and if grains
switched grains over time as compared to β = π. This difference is experimentally observed, as the decay rate of the measurement using an applied field of 62.5 mT is much smaller than the decay rate for an applied field of 1T. The field stability measurements show promise for future applications of EB-SAF pinned multilayers in GMR/TMR sensors. The addition of an SAF increases the external field that is needed to switch the magnetization of the bottom Co layer as the field has to break both the EB and RKKY-coupling. However, when the magnetization is rotated for large applied fields opposite to the field cooling direction, a large decay rate is found. Only when the magnetization of the bottom FM layer rotates the EB becomes unstable over time. Furthermore, the suggested spin flop that can occur when a field is applied along the field cooling direction does not result in instability of the EB.
E2)
E1)
E0)
ΔE) β=π/6) Ferromagne5c)layer)
)An5ferromagne5c)grain) )
Field)cooling)direc5on)
E2)
E1)
E0)
ΔE)
Ferromagne5c)layer) )An5ferromagne5c)grain) )
Field)cooling)direc5on) β=2π/3)
Figure 4.18: The energy of an AF grain versus the angle α of the AF spins for β = π6 (top) and β = 23π (bottom). As β increases, the energy barrier decreases, and the location of the global energy minimum changes. When β is large enough, the thermal fluctuations can cause the AF grains to overcome the energy barrier and reach the global energy minimum, resulting in a switch of the AF grains.
Conclusions
Experimental work was done to investigate the magnetic properties of Co/NM/Co multilayers. Measurements were done on these multilayers containing wedged Ru, Ir and Cr spacer layer materials, for which the multilayers containing a Ru and Ir spacer layer showed both FM and AF coupling, depending on the thickness of the spacer layer. Simulations based on a macrospin model were done in order to reproduce and understand the magnetic behavior of these multilayers.
AF coupling peaks were found for tRu = 0.8 ∗ 10−9 m and tIr = 0.9 ∗ 10−9 m with JRKKY = 0.37 ∗ 10−3 J/m2 and JRKKY = 0.049 ∗ 10−3 J/m2 respectively. The RKKY-coupling strengths are much lower than RKKY-coupling strengths reported by Parkin. Furthermore, only 1 and 2 AF domains were found for the Co/Ru/Co and Co/Ir/Co respectively, while Parkin reports at least 3 AF domains. This suggests that the quality of the RKKY-coupling is far from ideal. In order to improve the RKKY-coupling, a growth study was conducted.
An experimental study on the growth conditions during the Co deposition was conducted. Co layers were grown at pressures varying between 7 sccm, 15 sccm and 30 sccm and at powers varying between 10 W, 20 W and 40 W. When Co was grown at 15 sccm, a significant increase in the coupling strength was observed as compared to Co grown at 7 sccm, as a maximum RKKY-coupling of 1.09 ∗ 10−3 J/m2 was found. This difference is explained by considering that at a higher pressure, the Co atoms that are deposited have a lower energy, resulting in less interdiffusion and better Ta/Co and Co/NM interfaces. SAFs containing Co layers grown at 30 sccm did not show any magnetic behavior. Varying the power used during the Co deposition resulted in varying RKKY-coupling strengths, although a clear pattern was not found. This power-dependence of the RKKY-coupling strength is unexpected and cannot be explained at this point.
Thermal stability measurements were performed on Co/Ru/Co and Co/Ir/Co SAFs containing a fixed Ru and Ir layer thickness in the first AF domain. The AF coupling significantly changes at Tannealing = 250° C and Tannealing = 190° C for the Co/Ru/Co and Co/Ir/Co SAFs respectively.
67
The thermal stability of a Co/Ru/Co sample containing a wedged Ru layer with Co layers grown at 15 sccm and 20 W was investigated. This sample showed two AF peaks at tRu = 1.06∗10−9m and tRu = 2.62∗10−9m. When the sample was annealed at 220° C, the RKKY-coupling strength of the first AF peak starts to decrease by 30%, and at an annealing temperature of 320° C the first AF peak has completely disappeared. The second AF domain is still present at these annealing temperatures.
Pinholes present at low Ru thickness can explain the thermal instability of the first AF peak. As the annealing temperature increases, the pinhole density increases.
When the annealing temperature is high enough, pinholes completely destroy the AF coupling in the first AF domain. The second AF domain is more stable as pinholes are mainly present at lower Ru thickness. Investigating the evolution of the hysteresis loops at low spacer layer thickness supports this theory.
Field stability measurements on an IrMn/Co/Ru/Co stack, containing both EB and an SAF were conducted and simulations were done in order to understand the behavior of the magnetizations of the Co layers. A reduction of EB was found only for applied fields of 62.5 mT and 1 T with a decay rate of 6.3% ± 0.3% per hour and 27.0% ± 0.6% per hour were found respectively. These decay rates are significantly higher than decay rates previously observed at FNA. Simulations suggested that when a field is applied along the field cooling direction, spin flop can occur, which can influence the stability of the EB. Relaxation measurements did not show instability of the EB for fields applied along the field cooling direction, which shows promise for future applications.
Outlook
The field stability measurements performed on an EB-SAF stack showed that only fields applied opposite to the field cooling direction influence the stability of the EB, and that the addition of RKKY-coupling increases the field that is needed to rotate the magnetization of the pinned Co layers. This is a promising result for the ap-plication of SAFs in the pinned multilayer of GMR/TMR sensors. However, during this study several problems were encountered, and in order to fully understand and optimize the magnetic properties of an EB-SAF stack, future research and experi-ments are required. In this section, suggestions for future research and experiexperi-ments are proposed.
First of all, the early SAFs grown at standard growth condtions resulted in a low RKKY-coupling strength and an unstable RKKY-coupling when relatively low annealing temperatures were used. Although a study on the growth conditions of the Co layers resulted in a higher RKKY-coupling strength and the presence of a second AF domain for Co/Ru/Co SAFs, the coupling strengths are still significantly lower than what Parkin obtained. In order to fully optimize the RKKY-coupling, growth studies on the remaining Ta and Ru layers could be conducted. However, even when the RKKY-coupling strength was improved by growing the Co layer at 15 sccm and 20 W, the first AF domain showed signs of pinhole coupling, and was thermally unstable due to these pinholes. Recently, a new sputter system was installed at FNA, with which samples can be grown under much better growth conditions. With this new system, the quality of the layers and interfaces can be significantly improved.
If the quality can be improved such that no pinholes are present at the first AF domain, a higher thermal stability could be achieved. During this study, in the EB-SAF stack, and EB-SAF with Ru thickness in the second AF domain was used, since the first AF domain was thermally unstable. If the thermal stability of the first AF domain can be improved such that it can be used in EB-SAF stacks, this will from an interesting next step for the application of EB-SAF multilayers in GMR/TMR sensors. Recently, the first SAFs have been grown with this new system, and the first results on the thermal stability of the first AF peak are promising.
69
The model used to simulate the magnetic loops of an EB-SAF cannot reproduce the experimentally obtained magnetic loops. In order to do this, the model needs to be improved. The model used during this work is a macrospin model, which cannot explain the linear behavior of the magnetization at small fields or the coercive field resulting from unstable grains, as the AF layer is not considered to contain grains. Implementing the grains within the model could be a big step to reproduce the experimentally observed loops and understand the magnetic behavior of the magnetizations of the different layers. Improving the model could also lead to a decent prediction of decay rates, helping to understand the process of EB reduction and predict parameter values that can improve the EB stability.
Different AF materials could be used. It is known that PtMn can result in a high EB and is thermally more stable. However, the annealing sequence to set the exchange bias for PtMn AF layers requires much higher temperatures. If the thermal stability issues of the SAFs can be solved, PtMn could be used in order to improve the EB.
Finally, the EB-SAF multilayers form part of a GMR/TMR sensor. This study did not cover any measurements on GMR/TMR sensors containing the EB-SAF multilayer, and for applicational purposes it could be interesting to characterize the properties of full GMR/TMR stacks.
[2] Asleep at the wheel, Washington’s blog, 19-04-2012
[3] T. R. McGuire and R.I. Potter, Anisotropic magnetoresistance in ferro-magnetic 3d Alloys, IEEE Transactions on ferro-magnetics, Vol. Mag-11 No.
4, July 1975
[4] H. Swagten, “ Magnetism and magnetic meterials”, 2011 Course syllabus for TU/e
[5] P. Bruno, “ Theory of interlayer exchange coupling”, Phys. Rev. B, vol.
52 1995
[6] P. Bruno and C. Chappert, Ruderman-Kittel theory of oscillatory inter-layer exchange coupling, Phys. Rev. B Vol. 46 1992
[7] Meiklejohn and Bean, New magnetic anisotropy, Phys. Rev., 102:1413 [8] J. Nogues and I.K. Schuller. Exchange bias. J. Magn. Magn. Mater.,
192:203 (1999)
[9] R. Coehoorn. Exchange anisotropy, Stoner-Wohlfarth model. Lecture Notes, Eindhoven University of Technology 2000-2001
[10] F. Radu and H. Zabel, Exchange Bias Effect of Ferro-/ Antiferromagnetic Heterostructures, Springer-Verslag Berlin Heidelberg 2007
[11] E. Fulcomer and S. H. Charap, Thermal fluctuation aftereffect model for some systems with ferromagnetic antiferromagnetic coupling, J. Appl.
Phys. 43, 4190 (1972)
[12] Andreas Biternas, “Study of the training effect in exchange bias”, A Thesis submitted for the degree of Doctor of Philosophy (2009)
[13] J. Nogues and I. Schuller, “Exchange bias”, Journal of magnetism and magnetic materials, vol. 192, 1999
[14] D. Paccard, C. Schlenker, O. Massenet, R. Montmory and A. Yelon, “A New property of Ferromagnetic-Antiferromagnetic Coupling”, Phys. stat.
sol. 16, 1966
[15] A. Hoffman, “Symmetry Driven Irreversibilities at Ferromagnetic- Anti-ferromagnetic Interfaces”, Phys. Rev. Lett. 93, 2004
[17] V.M. Uzdin, C. Demangeat, Pinhole defects in Fe/Cr trilayers, Journal of magnetism and magnetic materials col 164, 1997
[18] M. Ruderman and C. Kittel, Phys. Rev. 96 (1954) 99 [19] T. Kasuya, Prog. Theor. Phys. 16 (1956) 45
[20] K. Yosida, Phys. Rev. 106 (1957) 893
[21] M. L. M. Lalieu, Charging the interlayer exchange coupling, FNA Tu/e 2014
[22] Ultrathin magnetic structures II, B. Heinrich and J.A.C. Bland (Eds), Springer-Verslag Berlin Heidelberg 1994
[23] J.R. Cullen and K. B. Hathaway, Phys. Rev. B 47, 14 998 (1993) [24] J.C. Slonczewski, Origin of biquadratic exchange in magnetic multilayers,
J. Appl. Phys. 73, 5957 (1993)
[25] S. S. P. Parkin, Systematic Variation of the Strength and Oscillation Period of Indirect Magnetic Exchange Coupling through the 3d, 4d, and 5d Transition Metals, Phys. Rev. B, vol. 67 1991
[26] S. S. P. Parkin, N. More, K.P. Roche, Oscillations in Exchange Cou-pling and Magnetoresistance in Metallic Superlattice Structures: Co/Ru, Co/Cr, and Fe/Cr, Phys Rev. Lett., vol 64 1990
[27] J.-F. Bobo, M. Piecuch, E. Snoeck, Complex AF couplings for Cu-Co multilayers with low copper thickess, Journal of magnetism and magnetic materials, vol. 126 1993
[28] F. Stobiecki, T. Lucinski, J. Dubowik, B. Szymanski, M. Urbaniak, F. J.
Castano, T. Stobieki, The effect of pinholes on magnetic behavior of an-tiferromagnetically coupled Ni-Fe/Cu Multilayers, Journal of Magnetics 3, 1998