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Spin pumping in magnetic trilayer structures with an MgO barrier
Views: 1,717Citations: 1More detailArticle Open
AbstractWe present a study of the interaction mechanisms in magnetic trilayer structures with an MgO barrier grown by molecular beam epitaxy. The interlayer exchange coupling, Aex, is determined using SQUID magnetometry and ferromagnetic resonance (FMR), displaying an unexpected oscillatory behaviour as the thickness, tMgO, is increased from 1 to 4nm. Transmission electron microscopy confirms the continuity and quality of the tunnelling barrier, eliminating the prospect of exchange arising from direct contact between the two ferromagnetic layers. The Gilbert damping is found to be almost independent of the MgO thickness, suggesting the suppression of spin pumping. The element specific technique of x ray detected FMR reveals a small dynamic exchange interaction, acting in concert with the static interaction to induce coupled precession across the multilayer stack. These results highlight the potential of spin pumping and spin transfer torque for device applications in magnetic tunnel junctions relying on commonly used MgO barriers.
IntroductionMuch attention is currently devoted to the study of spin transfer torque (STT)1,2,3,4, through which it is possible to realize spontaneous magnetization precession and switching. Spin pumping from a ferromagnet (FM) into a non fake Van Cleef & Arpels Clover necklace black magnetic (NM) material is one of the most promising candidates for these applications5. Spin pumping has been under intense scrutiny since it was first proposed in 20026, studying the generation of pure spin currents by ferromagnetic resonance (FMR). Of particular importance is the transmission of spins across NM barriers, such as the conductors Ag (ref. 7), Au (ref. 8), and Cu (ref. 9), or insulators like MgO (ref. 10) and SrTiO3 (ref. 11). The efficacy of spin pumping is governed by the spin diffusion length of the NM layer, and the spin mixing conductance of the FM/NM interface12. Spin pumping is heavily suppressed in insulators, leading to very short spin coherence lengths, often under a nm11. However, coupling of the spin and charge degrees of freedom, and associated charge pumping in tunnelling heterostructures with an insulating barrier, can complicate interpretation of the results5,13,14. It is useful to investigate spin pumping through insulating layers using probes that are insensitive to charge based effects.
Here, we present a study of spin pumping and static exchange coupling in MgO based magnetic heterostructures grown by molecular beam epitaxy (MBE). These magnetic tunnel junctions (MTJs) are well established high tunnelling magnetoresistance ratio elements15,16. The strength and character of the interactions is determined in Co50Fe50/MgO/Ni magnetic heterostructures using superconducting quantum interference device (SQUID) magnetometry and vector network analyser (VNA) FMR. The structures are of high crystalline quality, as demonstrated by transmission electron microscopy (TEM). Static interlayer exchange is observed through shifts in the resonant field. X ray detected FMR (XFMR) measurements confirm the presence of static exchange coupling, but demonstrate that for the thinnest MgO barrier there is also a component of spin pumping. These results show the importance of spin transfer in technologically relevant MTJs, demonstrating that torques can be achieved for suitably thin barriers.
Ferromagnetic ResonanceIn the classical limit the spin dynamics of a ferromagnet are governed by the damped Landau Lifshitz Gilbert (LLG) equation of motion, describing the precession of the magnetisation about an effective field arising from internal and external fields:
with the gyromagnetic ratio, m is the unit magnetisation vector of the material, the dimensionless Gilbert damping parameter, and Heff the effective magnetic field. Through Heff the various energy terms, such as the exchange energy, the demagnetisation energy, the in plane cubic and uniaxial anisotropies, the static exchange, and the Zeeman energy enter the equation.
The static interlayer exchange coupling is a general term for any interaction that acts to (anti )align the magnetisations of the two layers in a magnetic trilayer structure such as a spin valve or an MTJ. Examples of such interactions include Ruderman Kittel Kasuya Yosida (RKKY), superexchange, Nel or orange peel coupling, and direct exchange through a discontinuous spacer layer. The presence of a static interaction modifies the LLG equation with an additional term17:
where is the interlayer exchange, and i, j index magnetic layers. The interlayer exchange is defined as18,19:
with Aex the interlayer exchange constant, t the thickness of the magnetic layer, M the equilibrium orientation of the magnetization, and i, j indices label the magnetic layers. The sign of Aex indicates whether the interaction favours parallel (positive) or antiparallel (negative) alignment.
In FMR experiments, additional interaction mechanisms must be considered. As the magnetisation of a ferromagnetic layer precesses on resonance it acts as a spin battery, generating a pure spin current transverse to the axis about which it precesses. When the FM layer is thicker than the ferromagnetic coherence length, a pure spin current can be driven into an adjacent NM layer. A spin current can persist across the spacer layer, and in a trilayer structure either return to the first FM/NM interface, or else flow through to a second NM/FM interface. If it is not reflected here, it crosses the interface and is absorbed by the FM, inducing precession through the STT4.
The increased flow of spin momentum out of the FM layer acts as an additional channel for energy loss, leading to an increase in damping. This damping is linear with resonant frequency, and can thus be described in the same terms as Gilbert damping. The absorbed spin current leads to a comparable anti damping term in the off resonance layer.
With the addition of these two coupling mechanisms, the LLG for a trilayer system becomes:
where superscripts denote magnetic layers, and is the effective field acting on layer i. The intrinsic Gilbert damping is , while are spin source (m=n) and spin sink (mn) terms. The XFMR results can be modelled using this a linearised solution of this equation.
The presence of these coupling mechanisms alters the magnetodynamics of the on and off resonance layer. The static coupling shifts the resonant field of both layers, as it functions as an additional field term in the Kittel equation. Spin pumping, on the other hand, broadens the resonances by providing an additional energy loss mechanism. However, the most important change is the precession induced in the off resonance layer, wherein the exchange interactions transfer energy between the two layers. This leads to changes in both the phase and amplitude of precession, with the phase being the more sensitive probe.
ResultsStructural properties of the magnetic trilayer structuresThe structural properties of the grown magnetic heterostructures were studied using transmission electron microscopy (TEM). Figure 1b d shows cross sectional TEM images of the magnetic heterostructures. These images demonstrate epitaxial growth of the films, and confirm that the MgO barrier is continuous (down to the thinnest barrier thickness of 1nm, see Supplementary Fig. S1 for details).
Figure 1: Heterostructure growth and structural properties.(a) RHEED images of the MgO substrate (at 700C), CoFe layer, 3 nm thick MgO barrier (after annealing at 300C), and Ag capped Ni layer (from left to right). (b d) Cross sectional TEM view of the magnetic heterostructure with a 2 nm thick MgO barrier. (b) Low magnification high angle annular dark field image of the magnetic heterostructure showing uniform thickness of the ferromagnetic layers and the MgO tunnel barrier. (c) High resolution bright field scanning TEM showing the atomic structure of the substrate, ferromagnetic layers, and barrier viewed along [010]. (d) Interface region of ferromagnetic layer(s)/MgO barrier showing the atomically abrupt interfaces and the well structured MgO barrier textured along the [010] direction.
Figure 2 shows hysteresis loops measured by SQUID VSM for all four magnetic heterostructures, displaying a reduction in static interlayer coupling as a function of increasing tMgO. For the thinnest MgO barrier (tMgO=1nm, Fig. 2a) the two layers are strongly bound, and there is a single switching step, with a coercive field of 2mT. As the thickness of the barrier increases, the layers decouple and behave independently. For tMgO=2 and 3nm (Fig. 2bc), the coupling between the two layers appears to cause winding, leading to a smeared out transition as opposed to sharp steps. Nevertheless, two distinct steps in the hysteresis loop can be identified. Strong coupling for the thinnest barrier aligns the magnetizations of the two layers at all fields, but as barrier thickness increases they start to move independently, leading to two distinct steps in (d).
As the TEM measurements show that the MgO barrier appears to be continuous, the observed coupling cannot be due to direct, large area contact between the two ferromagnetic layers, but it is more likely to be resulting from tunnelling across the barrier. However, there is some evidence of surface roughness of the MgO, which could lead to a Nel orange peel coupling.
Lab based FMR measurementsVNA FMR measurements were performed to determine values for the interlayer exchange coupling and magnetocrystalline anisotropy parameters for all samples. Figure 1a shows a representative field frequency map for the sample with tMgO=1 nm. Fits to resonance fields were performed as a function of RF frequency and magnetization alignment using the Kittel equation20, including the interlayer exchange field17,18, using Eq. (3).
The extracted interlayer coupling is shown in Fig. 3b, while values for all the copy van cleef jewelry fitting parameters are given in Table 1. 21). 18. Note that in spin dependent tunnelling such an oscillatory thickness dependence has been observed for MgO barriers in Fe/MgO/Fe tunnelling structures22. (b) Interlayer exchange coupling, Aex, as a function of MgO barrier thickness extracted from frequency dependence of the resonance field, using the Kittel equation. (c) Gilbert damping extracted from linewidth of resonance using Eq. (5). Results for both FM layers are plotted as a function of MgO barrier thickness in Fig. 3c. Within the error bars, there is no change in the Gilbert damping of the Ni layer, which supports the assertion that an insulating layer such as MgO does not permit a spin current to flow. The data for the CoFe layer is more complicated, showing at first a slight increase, then a drop in damping. Microscopy and magnetic characterization gave no evidence of a change in the structural or magnetic properties of the CoFe layer to account for this finding.
While a drop in damping with increasing interlayer thickness is usually indicative of spin pumping, the trend here does not match the exponential decay observed in previous studies7,11,24. Measurements on comparable bare CoFe layers yielded a Gilbert damping of 3103. This suggests that at least some of the spins pumped by the van cleef clover jewelry replica precessing magnetization of the CoFe are absorbed. The lower damping in the case of the thinnest MgO layer could be in part due to the strong static exchange coupling observed.
Synchrotron based FMR measurementsXFMR measurements were performed to study off resonance precession induced by static exchange coupling and the STT exerted by pumped spins. Figure 4 shows the amplitude and phase of precession at 4GHz for each layer in the sample with tMgO=1nm, with the magnetic field applied along the easy axis of the CoFe. The resonance in the Ni layer is easily identified, with a peak in amplitude at 28mT. Off resonance precession is induced in the CoFe layer; the resulting dynamic XMCD signal is approximately ten times smaller than that of the Ni. There is also a small phase feature in the Co, which is predominantly unipolar in character. Taken together with the bipolar shape of the Co amplitude variation, this indicates that the static interaction is primarily responsible for induced precession in the CoFe.
Views: 1,717Citations: 1More detailArticle Open
AbstractWe present a study of the interaction mechanisms in magnetic trilayer structures with an MgO barrier grown by molecular beam epitaxy. The interlayer exchange coupling, Aex, is determined using SQUID magnetometry and ferromagnetic resonance (FMR), displaying an unexpected oscillatory behaviour as the thickness, tMgO, is increased from 1 to 4nm. Transmission electron microscopy confirms the continuity and quality of the tunnelling barrier, eliminating the prospect of exchange arising from direct contact between the two ferromagnetic layers. The Gilbert damping is found to be almost independent of the MgO thickness, suggesting the suppression of spin pumping. The element specific technique of x ray detected FMR reveals a small dynamic exchange interaction, acting in concert with the static interaction to induce coupled precession across the multilayer stack. These results highlight the potential of spin pumping and spin transfer torque for device applications in magnetic tunnel junctions relying on commonly used MgO barriers.
IntroductionMuch attention is currently devoted to the study of spin transfer torque (STT)1,2,3,4, through which it is possible to realize spontaneous magnetization precession and switching. Spin pumping from a ferromagnet (FM) into a non fake Van Cleef & Arpels Clover necklace black magnetic (NM) material is one of the most promising candidates for these applications5. Spin pumping has been under intense scrutiny since it was first proposed in 20026, studying the generation of pure spin currents by ferromagnetic resonance (FMR). Of particular importance is the transmission of spins across NM barriers, such as the conductors Ag (ref. 7), Au (ref. 8), and Cu (ref. 9), or insulators like MgO (ref. 10) and SrTiO3 (ref. 11). The efficacy of spin pumping is governed by the spin diffusion length of the NM layer, and the spin mixing conductance of the FM/NM interface12. Spin pumping is heavily suppressed in insulators, leading to very short spin coherence lengths, often under a nm11. However, coupling of the spin and charge degrees of freedom, and associated charge pumping in tunnelling heterostructures with an insulating barrier, can complicate interpretation of the results5,13,14. It is useful to investigate spin pumping through insulating layers using probes that are insensitive to charge based effects.
Here, we present a study of spin pumping and static exchange coupling in MgO based magnetic heterostructures grown by molecular beam epitaxy (MBE). These magnetic tunnel junctions (MTJs) are well established high tunnelling magnetoresistance ratio elements15,16. The strength and character of the interactions is determined in Co50Fe50/MgO/Ni magnetic heterostructures using superconducting quantum interference device (SQUID) magnetometry and vector network analyser (VNA) FMR. The structures are of high crystalline quality, as demonstrated by transmission electron microscopy (TEM). Static interlayer exchange is observed through shifts in the resonant field. X ray detected FMR (XFMR) measurements confirm the presence of static exchange coupling, but demonstrate that for the thinnest MgO barrier there is also a component of spin pumping. These results show the importance of spin transfer in technologically relevant MTJs, demonstrating that torques can be achieved for suitably thin barriers.
Ferromagnetic ResonanceIn the classical limit the spin dynamics of a ferromagnet are governed by the damped Landau Lifshitz Gilbert (LLG) equation of motion, describing the precession of the magnetisation about an effective field arising from internal and external fields:
with the gyromagnetic ratio, m is the unit magnetisation vector of the material, the dimensionless Gilbert damping parameter, and Heff the effective magnetic field. Through Heff the various energy terms, such as the exchange energy, the demagnetisation energy, the in plane cubic and uniaxial anisotropies, the static exchange, and the Zeeman energy enter the equation.
The static interlayer exchange coupling is a general term for any interaction that acts to (anti )align the magnetisations of the two layers in a magnetic trilayer structure such as a spin valve or an MTJ. Examples of such interactions include Ruderman Kittel Kasuya Yosida (RKKY), superexchange, Nel or orange peel coupling, and direct exchange through a discontinuous spacer layer. The presence of a static interaction modifies the LLG equation with an additional term17:
where is the interlayer exchange, and i, j index magnetic layers. The interlayer exchange is defined as18,19:
with Aex the interlayer exchange constant, t the thickness of the magnetic layer, M the equilibrium orientation of the magnetization, and i, j indices label the magnetic layers. The sign of Aex indicates whether the interaction favours parallel (positive) or antiparallel (negative) alignment.
In FMR experiments, additional interaction mechanisms must be considered. As the magnetisation of a ferromagnetic layer precesses on resonance it acts as a spin battery, generating a pure spin current transverse to the axis about which it precesses. When the FM layer is thicker than the ferromagnetic coherence length, a pure spin current can be driven into an adjacent NM layer. A spin current can persist across the spacer layer, and in a trilayer structure either return to the first FM/NM interface, or else flow through to a second NM/FM interface. If it is not reflected here, it crosses the interface and is absorbed by the FM, inducing precession through the STT4.
The increased flow of spin momentum out of the FM layer acts as an additional channel for energy loss, leading to an increase in damping. This damping is linear with resonant frequency, and can thus be described in the same terms as Gilbert damping. The absorbed spin current leads to a comparable anti damping term in the off resonance layer.
With the addition of these two coupling mechanisms, the LLG for a trilayer system becomes:
where superscripts denote magnetic layers, and is the effective field acting on layer i. The intrinsic Gilbert damping is , while are spin source (m=n) and spin sink (mn) terms. The XFMR results can be modelled using this a linearised solution of this equation.
The presence of these coupling mechanisms alters the magnetodynamics of the on and off resonance layer. The static coupling shifts the resonant field of both layers, as it functions as an additional field term in the Kittel equation. Spin pumping, on the other hand, broadens the resonances by providing an additional energy loss mechanism. However, the most important change is the precession induced in the off resonance layer, wherein the exchange interactions transfer energy between the two layers. This leads to changes in both the phase and amplitude of precession, with the phase being the more sensitive probe.
ResultsStructural properties of the magnetic trilayer structuresThe structural properties of the grown magnetic heterostructures were studied using transmission electron microscopy (TEM). Figure 1b d shows cross sectional TEM images of the magnetic heterostructures. These images demonstrate epitaxial growth of the films, and confirm that the MgO barrier is continuous (down to the thinnest barrier thickness of 1nm, see Supplementary Fig. S1 for details).
Figure 1: Heterostructure growth and structural properties.(a) RHEED images of the MgO substrate (at 700C), CoFe layer, 3 nm thick MgO barrier (after annealing at 300C), and Ag capped Ni layer (from left to right). (b d) Cross sectional TEM view of the magnetic heterostructure with a 2 nm thick MgO barrier. (b) Low magnification high angle annular dark field image of the magnetic heterostructure showing uniform thickness of the ferromagnetic layers and the MgO tunnel barrier. (c) High resolution bright field scanning TEM showing the atomic structure of the substrate, ferromagnetic layers, and barrier viewed along [010]. (d) Interface region of ferromagnetic layer(s)/MgO barrier showing the atomically abrupt interfaces and the well structured MgO barrier textured along the [010] direction.
Figure 2 shows hysteresis loops measured by SQUID VSM for all four magnetic heterostructures, displaying a reduction in static interlayer coupling as a function of increasing tMgO. For the thinnest MgO barrier (tMgO=1nm, Fig. 2a) the two layers are strongly bound, and there is a single switching step, with a coercive field of 2mT. As the thickness of the barrier increases, the layers decouple and behave independently. For tMgO=2 and 3nm (Fig. 2bc), the coupling between the two layers appears to cause winding, leading to a smeared out transition as opposed to sharp steps. Nevertheless, two distinct steps in the hysteresis loop can be identified. Strong coupling for the thinnest barrier aligns the magnetizations of the two layers at all fields, but as barrier thickness increases they start to move independently, leading to two distinct steps in (d).
As the TEM measurements show that the MgO barrier appears to be continuous, the observed coupling cannot be due to direct, large area contact between the two ferromagnetic layers, but it is more likely to be resulting from tunnelling across the barrier. However, there is some evidence of surface roughness of the MgO, which could lead to a Nel orange peel coupling.
Lab based FMR measurementsVNA FMR measurements were performed to determine values for the interlayer exchange coupling and magnetocrystalline anisotropy parameters for all samples. Figure 1a shows a representative field frequency map for the sample with tMgO=1 nm. Fits to resonance fields were performed as a function of RF frequency and magnetization alignment using the Kittel equation20, including the interlayer exchange field17,18, using Eq. (3).
The extracted interlayer coupling is shown in Fig. 3b, while values for all the copy van cleef jewelry fitting parameters are given in Table 1. 21). 18. Note that in spin dependent tunnelling such an oscillatory thickness dependence has been observed for MgO barriers in Fe/MgO/Fe tunnelling structures22. (b) Interlayer exchange coupling, Aex, as a function of MgO barrier thickness extracted from frequency dependence of the resonance field, using the Kittel equation. (c) Gilbert damping extracted from linewidth of resonance using Eq. (5). Results for both FM layers are plotted as a function of MgO barrier thickness in Fig. 3c. Within the error bars, there is no change in the Gilbert damping of the Ni layer, which supports the assertion that an insulating layer such as MgO does not permit a spin current to flow. The data for the CoFe layer is more complicated, showing at first a slight increase, then a drop in damping. Microscopy and magnetic characterization gave no evidence of a change in the structural or magnetic properties of the CoFe layer to account for this finding.
While a drop in damping with increasing interlayer thickness is usually indicative of spin pumping, the trend here does not match the exponential decay observed in previous studies7,11,24. Measurements on comparable bare CoFe layers yielded a Gilbert damping of 3103. This suggests that at least some of the spins pumped by the van cleef clover jewelry replica precessing magnetization of the CoFe are absorbed. The lower damping in the case of the thinnest MgO layer could be in part due to the strong static exchange coupling observed.
Synchrotron based FMR measurementsXFMR measurements were performed to study off resonance precession induced by static exchange coupling and the STT exerted by pumped spins. Figure 4 shows the amplitude and phase of precession at 4GHz for each layer in the sample with tMgO=1nm, with the magnetic field applied along the easy axis of the CoFe. The resonance in the Ni layer is easily identified, with a peak in amplitude at 28mT. Off resonance precession is induced in the CoFe layer; the resulting dynamic XMCD signal is approximately ten times smaller than that of the Ni. There is also a small phase feature in the Co, which is predominantly unipolar in character. Taken together with the bipolar shape of the Co amplitude variation, this indicates that the static interaction is primarily responsible for induced precession in the CoFe.
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