J. Gómez

In-plane anisotropic effect of magnetoelectric coupled PMN-PT/FePt multiferroic heterostructure: Static and microwave properties

The effects of the electric and magnetic field variation on multiferroic heterostructure were studied in this work. Thin films of polycrystalline Fe50Pt50 (FePt) were grown by dc-sputtering on top of the commercial slabs of lead magnesium niobate-lead titanate (PMN-PT). The sample was a (011)-cut single crystal and had one side polished. In this condition, the PMN-PT/FePt operates in the L-T (longitudinal magnetized-transverse polarized) mode. A FePt thin film of 20 nm was used in this study to avoid the characteristic broad microwave absorption line associated with these films above thicknesses of 40 nm. For the in-plane easy magnetization axis (01-1), a microwave magnetoelectric (ME) coupling of 28 Oe cm kV −1 was estimated, whereas a value of 42 Oe cm kV −1 was obtained through the hard magnetization axis (100). Insight into the effects of the in-plane strain anisotropy on the ME coupling is obtained from the dc-magnetization loops. It was observed that the trend was opposite along the easy and hard magnetic directions. In particular, along the easy-magnetic axis (01-1), a square and narrow loop with a factor of Mr /MS of 0.96 was measured at 10 kV/cm. Along the hard-magnetic axis, a factor of 0.16 at 10 kV/cm was obtained. Using electric tuning via microwave absorption at X-band (9.78 GHz), we observe completely different trends along the easy and hard magnetic directions; Multiple absorption lines along the latter axis compared to a single and narrower absorption line along the former. In spite of its intrinsic complexity, we propose a model which gives good agreement both for static and microwave properties. These observations are of fundamental interest for future ME microwave components, such as filters, phase-shifters, and resonators.

Angular and frequency dependence of standing spin waves in FePt films

We present a detailed analysis of the dynamic response of the magnetization in as-made FePt thin films, particularly studying the angular dependence of standing spin waves that can be observed when the external field is applied close to the film normal. We have found that the field separation between the uniform and the first excited mode depends strongly on angle and microwave frequency. To explain the observed behavior we have adopted the surface inhomogeneity model in the circular precession approximation. Using this model the experimental data could be very well fitted assuming that spins are not totally pinned at the surfaces by introducing a finite surface anisotropy. The experimental angular behavior of the resonance field at three different frequencies could be fitted with a single set of parameters indicating that the reported changes in the surface anisotropy as a function of film thickness are intrinsic to the samples.

Correlation between magnetic interactions and domain structure in A1 FePt ferromagnetic thin films

We have investigated the relationship between the domain structure and the magnetic interactions in a series of FePt ferromagnetic thin films of varying thickness. As-made films grow in the magnetically soft and chemically disordered A1 phase that may have two distinct domain structures. Above a critical thickness dcr ∼ 30 nm the presence of an out of plane anisotropy induces the formation of stripes, while for d < dcr planar domains occur. Magnetic interactions have been characterized using the well known DC demagnetization - isothermal remanent magnetization remanence protocols, δM plots, and magnetic viscosity measurements. We have observed a strong correlation between the domain configuration and the sign of the magnetic interactions. Planar domains are associated with positive exchange-like interactions, while stripe domains have a strong negative dipolar-like contribution. In this last case we have found a close correlation between the interaction parameter and the surface dipolar energy of the stri...

Critical thickness for stripe domain formation in FePt thin films: Dependence on residual stress

Magnetically soft FePt thin films of varying thickness (20 nm ≤ d ≤ 100 nm) were sputter-deposited at different Ar pressures in order to systematically modify the residual stress and hence the magnetic anisotropy. The magnetic domain structure of FePt thin films showed a transition from planar to nearly parallel stripes above a critical thickness, dcr, which was found to depend on an anisotropy contribution perpendicular to the film plane, originated essentially in magnetoelastic effects. A careful structural characterization was made in order to obtain the strain and the stress induced magnetic anisotropy in the samples. Vibrating sample magnetometry and magnetic force microscopy were used to investigate the changes occurring in the magnetic domain structure and the critical thickness of each set of films. Joining together structural and magnetic results, we have been able to construct a phase diagram that divided regions of different domain structures, either by changing the film thickness or the perpen...

High performance electronic device for the measurement of the inverse spin Hall effect

We have developed a high performance analog electronic device that can be used for the measurement of the inverse spin Hall effect (ISHE) as a function of the applied magnetic field. The electronic circuit is based on the synchronous detection technique with a careful selection of the active components in order to optimize the response in this application. The electronic accessory was adapted for the simultaneous measurement of the ISHE signal and the microwave absorption in an electron spin resonance spectrometer and tested with a bilayer sample of 5 nm of permalloy (Ni80Fe20) and 5 nm of tantalum. The response of the electronic device was characterized as a function of the microwave power, the amplitude and frequency of the modulation signal, and the relative phase between signal and reference. This last characterization reveals a simple method to put in phase the signal with the reference. The maximum signal to noise ratio was achieved for a modulation frequency between 6 and 12 kHz, for the largest possible values of field modulation amplitude and microwave power.

Magnetoelectric control of spin currents

The ability to control the spin current injection has been explored on a hybrid magnetoelectric system consisting of a (011)-cut ferroelectric lead magnesium niobate-lead titanate (PMNT) single crystal, a ferromagnetic FePt alloy, and a metallic Pt. With this PMNT/FePt/Pt structure we have been able to control the magnetic field position or the microwave excitation frequency at which the spin pumping phenomenon between FePt and Pt occurs. We demonstrate that the magnetoelectric heterostructure operating in the L-T (longitudinal magnetized-transverse polarized) mode couples the PMNT crystal to the magnetostrictive FePt/Pt bilayer, displaying a strong magnetoelectric coefficient of ∼140 Oe cm kV−1. Our results show that this mechanism can be effectively exploited as a tunable spin current intensity emitter and open the possibility to create an oscillating or a bistable switch to effectively manipulate spin currents.

Spin pumping and inverse spin Hall effect in antiferromagnetic exchange coupled [Co/Ru/Co]/Pt heterostructures

We report the observation of spin pumping and inverse spin Hall effects in antiferromagnetically coupled [Co/Ru/Co]/Pt heterostructures. The “spin-flop” magnetization process observed in antiferromagnetically exchange coupled Co layers combined with spin pumping and inverse spin Hall effects allowed us to detect both transversal and longitudinal charge accumulations. By controlling the exchange coupling strength and the spin flop transition in the magnetization process, it was possible to produce spin currents polarized in different directions.

Pure spin current manipulation in antiferromagnetically exchange coupled heterostructures

We present a model to describe the spin currents generated by ferromagnet/spacer/ferromagnet exchange coupled trilayer systems and heavy metal layers with strong spin-orbit coupling. By exploiting the magnitude of the exchange coupling (oscillatory RKKY-like coupling) and the spin-flop transition in the magnetization process, it has been possible to produce spin currents polarized in arbitrary directions. The spin-flop transition of the trilayer system originates pure spin currents whose polarization vector depends on the exchange field and the magnetization equilibrium angles. We also discuss a protocol to control the polarization sign of the pure spin current injected into the metallic layer by changing the initial conditions of magnetization of the ferromagnetic layers previously to the spin pumping and inverse spin Hall effect experiments. The small differences in the ferromagnetic layers lead to a change in the magnetization vector rotation that permits the control of the sign of the induced voltage components due to the inverse spin Hall effect. Our results can lead to important advances in hybrid spintronic devices with new functionalities, particularly, the ability to control microscopic parameters such as the polarization direction and the sign of the pure spin current through the variation of macroscopic parameters, such as the external magnetic field or the thickness of the spacer in antiferromagnetic exchange coupled systems.We present a model to describe the spin currents generated by ferromagnet/spacer/ferromagnet exchange coupled trilayer systems and heavy metal layers with strong spin-orbit coupling. By exploiting the magnitude of the exchange coupling (oscillatory RKKY-like coupling) and the spin-flop transition in the magnetization process, it has been possible to produce spin currents polarized in arbitrary directions. The spin-flop transition of the trilayer system originates pure spin currents whose polarization vector depends on the exchange field and the magnetization equilibrium angles. We also discuss a protocol to control the polarization sign of the pure spin current injected into the metallic layer by changing the initial conditions of magnetization of the ferromagnetic layers previously to the spin pumping and inverse spin Hall effect experiments. The small differences in the ferromagnetic layers lead to a change in the magnetization vector rotation that permits the control of the sign of the induced voltage ...

Magnetoelectric tuning of the inverse spin-Hall effect

Fil: Vargas, Jose Marcelo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Patagonia Norte; Argentina. Comision Nacional de Energia Atomica. Gerencia del Area de Energia Nuclear. Instituto Balseiro; Argentina. Comision Nacional de Energia Atomica. Centro Atomico Bariloche; Argentina