Magnetic Trilayer Structures Research Articles

Enhanced optical mode coherence in exchange coupled soft magnetic multilayers

We report on an all-optical investigation of coupled spin excitation modes in a series of magnetic trilayer structures. Using time-resolved magneto-optic Kerr effect (tr-MOKE) magnetometry, we observe multi-mode coherent spin excitations in Ni81Fe19/Ru/Co49Fe49V2 multilayers even though the tr-MOKE optical detection is sensitive only to the Co49Fe49V2 magnetization dynamics. Frequency shifts of the different modes indicate that the coupling between the Ni81Fe19 and Co49Fe49V2 layers varies from anti-ferromagnetic to ferromagnetic to uncoupled as the Ru spacer layer thickness is increased from 8 Å to 200 Å. The lifetime of the high frequency coherent oscillations in the Co49Fe49V2 layer increases by over 200%–300% even in the case of uncoupled Ni81Fe19 and Co49Fe49V2 layers with a 200 Å thick Ru spacer. The results suggest an additional method to decrease the damping of high-moment alloys in layered magnetic nanostructures.

Tuneable exchange-spring stiffness in amorphous magnetic trilayer structures

We investigate the magnetic properties of amorphous Sm10Co90/Co60(Al70Zr30)40/Co85(Al70Zr30)15 exchange-spring magnet trilayers. The magnetically soft Co85(Al70Zr30)15 layer is coupled to the magnetically hard Sm10Co90 layer through the weakly magnetic low-T c Co60(Al70Zr30)40 spacer layer. The strength of the coupling can be controlled with temperature and the coupling persists above the intrinsic T c of the spacer layer due to a long-range magnetic proximity effect. Polarized neutron reflectivity is used to examine the magnetic profile of the trilayers during magnetization reversal. A two-step switching occurs, with the switching angle of the soft layer strongly dependent on the strength of the coupling. In the strong coupling regime a magnetic state can be achieved where the soft layer magnetization is perpendicular to the hard layer whereas in the weak coupling regime the soft layer reverses fully.

Spin pumping in magnetic trilayer structures with an MgO barrier.

We 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 4 nm. 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.

Large spin pumping effect in antisymmetric precession of Ni79Fe21/Ru/Ni79Fe21

In magnetic trilayer structures, a contribution to the Gilbert damping of ferromagnetic resonance arises from spin currents pumped from one layer to another. This contribution has been demonstrated for layers with weakly coupled, separated resonances, where magnetization dynamics are excited predominantly in one layer and the other layer acts as a spin sink. Here, we show that trilayer structures in which magnetizations are excited simultaneously, antisymmetrically, show a spin-pumping effect roughly twice as large. The antisymmetric (optical) mode of antiferromagnetically coupled Ni79Fe21(8 nm)/Ru/Ni79Fe21(8 nm) trilayers shows a Gilbert damping constant greater than that of the symmetric (acoustic) mode by an amount as large as the intrinsic damping of Py (Δα≃0.006). The effect is shown equally in field-normal and field-parallel to film plane geometries over 3–25 GHz. The results confirm a prediction of the spin pumping model and have implications for the use of synthetic antiferromagnets (SAF)-structures in GHz devices.

Interface structure in magnetic multilayers using x-ray standing waves

It is demonstrated that interface structure in thin film nanostructures can be studied with a depth resolution of a fraction of a nanometer by using x-ray standing waves generated by a multilayer mirror used as a substrate. Two interfaces of a few nm thick Fe layer in magnetic trilayer structures $\mathrm{Tb}∕\mathrm{Fe}∕\mathrm{Tb}$ and $\mathrm{Cr}∕\mathrm{Fe}∕\mathrm{Cr}$ could be clearly resolved using x-ray standing waves generated by an underlying $\mathrm{W}∕\mathrm{Si}$ multilayer mirror. It is found that in both the cases rms roughness of the two interfaces Fe-on$\text{\ensuremath{-}}\mathrm{Tb}(\mathrm{Cr})$ and $\mathrm{Tb}(\mathrm{Cr})$-on-Fe are not equal. For example, roughness of $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{on}\text{\ensuremath{-}}\mathrm{Tb}$ interface is $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, while that of $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{on}\text{\ensuremath{-}}\mathrm{Fe}$ interface is $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The technique is particularly suitable in systems in which x-ray scattering contrast between adjacent layers is poor.

Vanishing Magnetic Interactions in Ferromagnetic Thin Films

We have used element-specific hysteresis measurements, based on the x-ray magnetic circular dichroism technique, to investigate magnetic trilayer structures composed of Fe and Ni layers. Within a critical regime we have discovered a class of structures in which the exchange interaction, the mechanism responsible for the macroscopic magnetism, can become vanishingly small. The experimental observations are supported by first principles theory and are explained as arising from a cancellation of several competing magnetic interactions. Hence, we have discovered a system with a novel exchange interaction between magnetic layers in direct contact that replaces the conventional exchange interaction in ferromagnets.

Magnetically soft, high moment grain-refined Fe films: Application to magnetic tunnel junctions

The effect of N-doping on the microstructure and magnetic properties of thin Fe layers has been employed to construct all Fe-electrode magnetic tunnel junctions that displayed the tunneling magnetoresistance (TMR) effect. Using low nitrogen doses, a reduction in coercivity was achieved due to grain refinement, without a concurrent decrease in the saturation magnetization of the Fe films caused by the formation of crystalline iron nitride phases. It was demonstrated that this N-induced grain refinement can be applied beneficially to control the switching field of the “free” layer in magnetic trilayer structures. In general the ability to control magnetic softness without reducing saturation magnetization will prove important for incorporating high spin-polarized materials into spin valves and TMR devices.

Properties of (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As magnetic trilayer structures

Magnetoresistance effect due to the spin-dependent scattering and the spin-polarized tunneling as well as the interlayer coupling in (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As semiconductor-based magnetic trilayer structures were studied. Both current-in-plane resistance and current-perpendicular-to-plane tunneling resistances are shown to depend on the relative magnetization directions of the two ferromagnetic (Ga,Mn)As layers. The interlayer coupling between the two (Ga,Mn)As layers is always ferromagnetic and the magnitude is weak (<0.5 μJ/ m 2) .

Magnetoresistance effect and interlayer coupling of (Ga, Mn)As trilayer structures

We have investigated the magnetic and magnetotransport properties of (Ga, Mn)As/(Al, Ga)As/(Ga, Mn)As semiconductor-based magnetic trilayer structures. We observe a weak ferromagnetic interlayer coupling between the two ferromagnetic (Ga, Mn)As layers as well as magnetoresistance effects due to spin-dependent scattering and to spin-dependent tunneling. Both the coupling strength and the magnetoresistance ratio decrease with the increase of temperature and/or the increase of Al composition of the nonmagnetic (Al, Ga)As layer.

Electron spectroscopic evidence of metastable alloy formation of Ag and Au in Co

High-resolution x-ray photoemission spectroscopy measurements of the conduction bands of dilute noble metal/Co metastable alloy films show a narrowing of the noble-metal d bands at concentrations below 6 at. % of noble metal consistent with coordination of the noble metal atoms with Co. Such alloying contrasts strongly with Ag and Au precipitation at higher concentrations, and with conventional metallurgical experience. It is consistent, however, with recent observations of modified interlayer exchange coupling in magnetic trilayer structures doped with noble-metal atoms [U. Ebels et al., Phys. Rev. B 58, 6367 (1998)].