Erol Girt

Exchange stiffness in thin film Co alloys

The exchange stiffness (A(ex)) is one of the key parameters controlling magnetization reversal in magnetic materials. We used a method based on the spin spiral formation in two ferromagnetic films ...

A method for measuring exchange stiffness in ferromagnetic films

An exchange stiffness, A(ex), in ferromagnetic films is obtained by fitting the M(H) dependence of two ferromagnetic layers antiferromagnetically coupled across a nonmagnetic spacer layer with a si ...

Spin dynamics and magnetic anisotropies at the Fe/GaAs(001) interface

20Au/(d)Fe/GaAs(001) structures were deposited using molecular beam epitaxy, where Fe thickness du2009=u2009(5...90) atomic layers. Interface anisotropies were investigated using the in-plane angular dependence of ferromagnetic resonance (FMR). Intrinsic and extrinsic contributions to magnetic damping were investigated using FMR linewidth (ΔH) measurements at 9, 24, 36, and 72 GHz (in-plane configuration) and 9, 24, and 36 GHz (perpendicular configuration). The in-plane cubic and uniaxial perpendicular anisotropies were well described by the bulk and interface contributions indicating that the Fe films have a high lattice coherence and high critical Curie temperature Tc. The in-plane uniaxial anisotropy is more complex and will be discussed in detail. The frequency dependence of ΔH(f) was analyzed using the Gilbert damping, two magnon scattering, and long range magnetic inhomogeneity contributions. The thickness dependence of the Gilbert damping parameter α was found to be well described by the bulk and interface...

Interface magnetism of iron grown on sulfur and hydrogen passivated GaAs(001)

Sulfur (S) and hydrogen (H) atom passivated GaAs(001) templates were used for deposition of ultrathin crystalline Fe films using molecular beam epitaxy, where the Fe thickness ranged from 10 to 45 atomic layers. Reflection high-energy electron diffraction patterns showed that the S- and H-passivated surfaces had no and very weak (1u2009×u20092) superlattice reconstructions, respectively. This indicates that these GaAs(001) templates have a square-like symmetry. Magnetic anisotropies were investigated using the in-plane angular dependence of ferromagnetic resonance at 36u2009GHz. The in-plane cubic and uniaxial anisotropies and perpendicular uniaxial field were described by bulk and interface contributions, indicating that the Fe films have a high lattice coherence. The magnetic properties of the Fe films were compared to those grown on more commonly used GaAs(001) templates having a (4u2009×u20096) reconstruction with an As-rich in-plane uniaxial symmetry. The Fe films grown on S-passivated templates exhibited unique magneti...

Tunable Magnetization and Damping of Sputter-Deposited, Exchange Coupled Py|Fe Bilayers

We report on magnetic damping of exchange coupled, polycrystalline Py(Ni80Fe20)|Fe and Fe|Py bilayers, prepared by sputter-deposition on an amorphous 3u2009nm Ta seed layer. FMR measurements are performed on varying thicknesses of the individual Py and Fe layers while keeping the total bilayer structure thickness fixed. When Fe is grown directly on Ta, there is large magnetic inhomogeneity and damping. However, when a Py layer is deposited between Fe and Ta, both the magnetic inhomogeneity and damping significantly decrease even if Fe is covered by Ta. The intrinsic damping of the Ta|Py|Fe film can be further lowered by increasing the Fe to Py ratio. SQUID measurements show a linear increase in saturation magnetization with increasing ratio of Fe to Py. A combination of in-plane and out-of-plane X-ray diffraction measurements show that Py is textured along the 〈111〉 directions and Fe is textured along the 〈110〉, with Fe texture significantly improving if it is deposited on Ta|Py instead of Ta. By improving the texture of Fe by introducing a thin Py layer between Fe and Ta, one can grow Fe thin films with zero in-plane anisotropy, tunable magnetic moment, and low magnetic damping, approaching that of the best single crystal Fe.

FMR study of interlayer exchange coupling in FeCoB|Ta|FeCoB trilayers with in-plane anisotropy

In this work, we present a detailed ferromagnetic resonance (FMR) study of two FeCoB layers coupled across a Ta spacer. The structures of studied samples are FM1/Ta(d)/FM2 where FM1 is a magnetic layer composed of Fe/FeCoB, FM2 is a magnetic layer composed of FeCoB/NiFe, d is the thickness of the Ta layer in nm, and d is varied from 0.375u2009nm to 4u2009nm. The FeCoB within FM1 is strongly coupled to high saturation magnetization Fe, and the FeCoB in FM2 is strongly coupled to lower saturation magnetization NiFe in order to separate the FMR resonance positions of these two layers. This is required to determine the strength of interlayer exchange coupling (J) between FM1 and FM2. We solved a system of coupled Landau Lifshitz Gilbert equations, representing the coupled magnetic layers FM1 and FM2, and used it to fit the FMR data and determine J, magnetic anisotropy, Gilbert damping, and g-factor of each magnetic layer as a function of Ta spacer layer thickness and annealing temperature. This study reveals that the...

Roughness-induced domain structure in perpendicular Co/Ni multilayers

Abstract We investigate the correlation between roughness, remanence and coercivity in Co/Ni films grown on Cu seed layers of varying thickness. Increasing the Cu seed layer thickness of Ta/Cu/8×[Co/Ni] thin films increases the roughness of the films. In-plane magnetization loops show that both the remanence and coercivity increase with increasing seed layer roughness. Polar Kerr microscopy and magnetic force microscopy reveal that the domain density also increases with roughness. Finite element micromagnetic simulations performed on structures with periodically modulated surfaces provide further insight. They confirm the connection between domain density and roughness, and identify the microsocpic structure of the domain walls as the source of the increased remanence in rough films. The simulations predict that the character of the domain walls changes from Bloch-like in smooth films to Neel-like for rougher films.

Spin Torque Switching in Nanopillars With Antiferromagnetic Reference Layer

Spin-transfer-torque-induced switching is investigated in 200 nm diameter circularly shaped, perpendicularly 10 magnetized nanopillars. A synthetic antiferromagnet, consisting of two Co/Ni multilayers coupled antiferromagnetically 11 across a Ru layer, is used as a reference layer to minimize the dipolar field on the free layer. The free layer is a single 12 4 × [Co/Ni] multilayer. The use of Pt and Pd was avoided to lower the spin-orbit scattering in magnetic layers and intrinsic 13 damping in the free layer, and therefore, reduce the critical current required for spin-transfer-torque switching. The intrinsic 14 Gilbert damping constant of a continuous 4 × [Co/Ni] multilayer film was measured by ferromagnetic resonance to be 15 α = 0.022, which is significantly lower than in Ptor Pd-based magnetic multilayers. In zero magnetic field, the critical 16 current required to switch the free layer from parallel to antiparallel alignment is 5.2 mA, and from antiparallel to parallel 17 alignment is 4.9 mA. Given the volume of the free layer, VFL = 1.01 × 10-22 m3, the switching efficiency, Ic/(VFL × μ0Hc), 18 is 5.28 × 1020 A/T·m3, twice as efficient as any previously reported device with a similar structure.

Micro Magnetic Exchange Interaction Tensor and Magnetization Reversal of

We investigate the effect of crystal symmetry driven tensor form of micromagnetic exchange stiffness A on switching properties of L10 FePt composite nano-structures. The exchange interaction term in micromagnetic energy functional have been constructed to capture the results of the first principle calculations for magnetic excitations in [001] and [100] directions. We find that out-of-plane (OP) exchange component (A||) is about two times weaker as compared to the in-plane (IP) component (A⊥). We study the effect of this difference between principal axis elements of A tensor on the M-H loops using micromagnetic simulations. We compare two cases for IP and OP exchange constants: (a) A⊥ = A|| (limited case) and (b) A⊥ = 0.5 A|| (based on ab-initio calculations), and find that the a thermal coercive force for 30 × 30 nm bi-layer rectangular dots differs for these two cases. Further, we find that the coercive force difference between two cases decreases as the external field goes from OP to IP.

{\rm L}1_{0}

Ferromagnetic coupling strength through Pt is experimentally determined using ferromagnetic resonance studies of Py | Pt ( d Pt ) | [ Py | Fe ] for Pt thicknesses, dPt, between 0.5 and 2.2u2009nm. The coupling strength decreases exponentially with the Pt thickness from 4.5 mJ/m2 for dPt = 0.5u2009nm and reduced to less than 0.02 mJ/m2 for dPt = 2.2u2009nm. The mechanism mediating exchange coupling is assumed to originate from the induced magnetization of Pt due to its proximity to ferromagnetic Py. The fitting thickness dependence of coupling with this model yields a characteristic coupling length scale of ξ = 0.31u2009±u20090.01u2009nm. Additionally, the molar susceptibility of proximity induced Pt is found to be 1.4u2009×u200910−7 ± 0.2u2009×u200910−7 m3/mol, an enhancement of ∼100 times as compared to bulk Pt. Ruderman-Kittel-Kasuya-Yosida type oscillations with a period of ∼0.8u2009nm are also observed as a small contribution of the total coupling.Ferromagnetic coupling strength through Pt is experimentally determined using ferromagnetic resonance studies of Py | Pt ( d Pt ) | [ Py | Fe ] for Pt thicknesses, dPt, between 0.5 and 2.2u2009nm. The coupling strength decreases exponentially with the Pt thickness from 4.5 mJ/m2 for dPt = 0.5u2009nm and reduced to less than 0.02 mJ/m2 for dPt = 2.2u2009nm. The mechanism mediating exchange coupling is assumed to originate from the induced magnetization of Pt due to its proximity to ferromagnetic Py. The fitting thickness dependence of coupling with this model yields a characteristic coupling length scale of ξ = 0.31u2009±u20090.01u2009nm. Additionally, the molar susceptibility of proximity induced Pt is found to be 1.4u2009×u200910−7 ± 0.2u2009×u200910−7 m3/mol, an enhancement of ∼100 times as compared to bulk Pt. Ruderman-Kittel-Kasuya-Yosida type oscillations with a period of ∼0.8u2009nm are also observed as a small contribution of the total coupling.