GdFeCo Thin Films Research Articles

Multi-temperature cooling performance of GdFeCo thin films with high magnetic entropy change and evolution of perpendicular magnetic anisotropy with thickness

Abstract To investigate the effect of thin film thickness on magnetic, magnetocaloric, and perpendicular magnetic anisotropy, we prepared Ta (5nm)/GdFeCo/Ta(5nm) heterostructure with GdFeCo thickness varying from 50 nm to 180 nm. The compensation temperature (Tcomp) and Curie temperature (Tc) shift significantly as the thickness increases. The temperature-dependent magnetization reveals that the Tcomp (224.74 K) is lower than room temperature for the 50 nm thin film, suggesting a CoFe-rich phase. The isothermal magnetic entropy change (|Sm|) is measured at both Tcomp and Tc corresponds to second order magnetic phase transition. |Sm| is found to be 0.24 J/kgK for 90 nm rises to 0.6 J/kgK for 180 nm. Doubling of the thickness from 90 nm to 180 nm results to increase in |Sm| of about 150%. In addition, a significant magnetic entropy change (0.31 J/kgK for 50 nm thin film @ 15 kOe field) is noticed near the Curie temperature for all thin films. For all higher thicknesses, a discernible change in magnetic entropy is observed at Tc. The magnetic anisotropy estimated using Hall effect and MOKE measurements demonstrates perpendicular magnetic anisotropy begins to dominate around room temperature for films with thickness greater than 90 nm as their compensation temperature falls around room temperature. We demonstrate GdFeCo thin films could potentially be suitable for multi-temperature magnetic cooling applications. 


Interface-Induced Conventional and Inverse Magnetocaloric Properties of GdFeCo Thin Films

Interface-Induced Conventional and Inverse Magnetocaloric Properties of GdFeCo Thin Films

Electrically Modulated Multilevel Optical Chirality in GdFeCo Thin Films.

This study introduces a simple approach to dynamically control multilevel optical ellipticity in ferrimagnetic GdFeCo alloys by switching the spin orientation through Joule heating induced by electrical current, with the assistance of a low magnetic field of 3.5 mT. It is demonstrated that selecting specific compositions of Gd x (FeCo)100-x alloys, with magnetic compensation temperatures near or above room temperature, allows for significant manipulation of the circular dichroism (CD) effect. This control enables the transformation of transmitted light from linearly polarized to elliptically polarized or the reversal of the rotation direction of elliptically polarized light across the photon energy range from visible (vis) to ultraviolet (UV). The efficacy of this method is rooted in the dominant contributions of FeCo to the CD effect in the vis-to-UV energy range. Because the magnetization of FeCo remains relatively independent of the temperature, substantial optical ellipticity is maintained for optical device applications, regardless of whether the compensation temperature is approached or crossed. Our results highlight the potential of GdFeCo thin films in chiral optics and demonstrate the selective contributions of rare-earth transition-metal elements to the CD effects, facilitating the design of advanced optical devices leveraging energy-resolved CD phenomena.

Ultra-low Gilbert damping and self-induced inverse spin Hall effect in GdFeCo thin films

Ferrimagnetic materials have garnered significant attention due to their broad range of tunabilities and functionalities in spintronics applications. Among these materials, rare earth-transition metal GdFeCo alloy films have been the subject of intensive investigation due to their spin-dependent transport properties and strong spin–orbit coupling. In this report, we present self-induced spin-to-charge conversion in single-layer GdFeCo films of different thicknesses via an inverse spin Hall effect. A detailed investigation of spin dynamics was carried out using broadband ferromagnetic resonance measurements. The anisotropy constant and the effective g-factor are found to decrease with thickness, and they become nearly constant for thicknesses beyond 25 nm. A remarkably low damping constant of 0.0029 ± 0.0003 is obtained in the 43 nm-thick film, which is the lowest among all previous reports on GdFeCo thin films. Furthermore, we have demonstrated a self-induced inverse spin Hall effect, which has not been reported so far in a single-layer of GdFeCo thin films. Our analysis shows that the inverse spin Hall effect becomes increasingly dominant over the spin rectification effect with increasing film thickness. The in-plane angular-dependent voltage measurement of the 43 nm-thick film reveals a spin pumping voltage of 1.64 μV. The observation of spin-to-charge current conversion could be due to the high spin–orbit coupling element Gd in the film as well as the interface between GeFeCo/Ti and substrate/GdFeCo of the films. Our findings underscore the potential of GdFeCo as a prime ferrimagnetic material for emerging spintronic technologies.

Magnetic properties of GdFeCo thin films tailored by sputtering conditions

Magnetic properties of GdFeCo thin films tailored by sputtering conditions

Broad table-like magnetocaloric effect in GdFeCo thin-films for room temperature Ericsson-cycle magnetic refrigeration

We demonstrate magnetocaloric entropy change and compensation temperatures in ferrimagnetic Gdx(Fe10Co90)100−x amorphous thin films with transition metal-rich and rare earth-rich configurations. Thin films are sputtered with same Gd/FeCo elemental ratio at different thicknesses and of various Gd/FeCo ratios at a constant thickness to understand the effect of these two parameters on an antiferromagnetically coupled magnetic sub-lattice system. Temperature- and field-dependent magnetic measurements [M(H,T)] and magnetocaloric studies are performed over a broad range of temperature (70–600 K) by applying a magnetic field of ±15 kOe on sputter deposited 90 nm thin films of Gdx(Fe10Co90)1−x(x = 30,40,50,55,70). The compensation temperature is found to increase with increasing Gd concentration for thin films of the same thickness. A high magnetocaloric entropy change around 0.97 J/kg K (ΔH = ± 15 kOe) is observed for thin films having the same Gd/FeCo elemental ratio. Furthermore, we observed a “table-like” magnetocaloric entropy change in GdFeCo thin film stacks with a high operational window (60 K) at a low applied field for an Ericsson magnetic regenerator around room temperature. The studies will provide important insight into magnetocaloric studies for Ericsson-cycle refrigeration in thin films having antiferromagnetically coupled sublattices.

Temperature-dependence of anomalous Hall effect in Ta-(GdFeCo)δ-Ta films with vertical composition gradients

Temperature-dependence of anomalous Hall effect in Ta-(GdFeCo)δ-Ta films with vertical composition gradients

Cavity-dumping a single infrared pulse from a free-electron laser for two-color pump-probe experiments.

Electromagnetic radiation in the mid- to far-infrared spectral range represents an indispensable tool for the study of numerous types of collective excitations in solids and molecules. Short and intense pulses in this terahertz spectral range are, however, difficult to obtain. While wide wavelength-tunability is easily provided by free-electron lasers, the energies of individual pulses are relatively moderate, on the order of microjoules. Here, we demonstrate a setup that uses cavity-dumping of a free-electron laser to provide single, picosecond-long pulses in the mid- to far-infrared frequency range. The duration of the Fourier-limited pulses can be varied by cavity detuning, and their energy was shown to exceed 100 µJ. Using the aforementioned infrared pulse as a pump, we have realized a two-color pump-probe setup facilitating single-shot time-resolved imaging of magnetization dynamics. We demonstrate the capabilities of the setup first on thermally induced demagnetization and magnetic switching of a GdFeCo thin film and second by showing a single-shot time-resolved detection of resonant phononic switching of the magnetization in a magnetic garnet.

Universal critical exponents of the magnetic domain wall depinning transition

Magnetic-field-driven domain wall dynamics in a ferrimagnetic GdFeCo thin film with perpendicular magnetic anisotropy is studied using low-temperature magneto-optical Kerr microscopy. Measurements performed in a practically athermal condition allow for the direct experimental determination of the velocity ($\ensuremath{\beta}=0.30\ifmmode\pm\else\textpm\fi{}0.03$) and correlation length ($\ensuremath{\nu}=1.3\ifmmode\pm\else\textpm\fi{}0.3$) exponents of the depinning transition. The whole family of exponents characterizing the transition is deduced, providing evidence that the depinning of magnetic domain walls is better described by the quenched Edwards-Wilkinson universality class.

Domain-wall roughness in GdFeCo thin films: Crossover length scales and roughness exponents

Domain-wall dynamics and spatial fluctuations are closely related to each other and to universal features of disordered systems. Experimentally measured roughness exponents characterizing spatial fluctuations have been reported for magnetic thin films, with values generally different from those predicted by the equilibrium, depinning and thermal reference states. Here, we study the roughness of domain walls in GdFeCo thin films over a large range of magnetic field and temperature. Our analysis is performed in the framework of a model considering length-scale crossovers between the reference states, which is shown to bridge the differences between experimental results and theoretical predictions. We also quantify for the first time the size of the depinning avalanches below the depinning field at finite temperatures.

Statistically meaningful measure of domain-wall roughness in magnetic thin films

Domain walls in magnetic thin films display a complex dynamical response when subject to an external drive. It is claimed that different dynamic regimes are correlated with the domain-wall roughness, i.e., with the fluctuations of domain-wall position due to the inherent disorder in the system. Therefore, key to understanding the dynamics of domain walls is to have a statistically meaningful measure of the domain-wall roughness. Here we present a thorough study of the roughness parameters, i.e., roughness exponent and roughness amplitude, for domain walls in a ferrimagnetic GdFeCo thin film in the creep regime. Histograms of roughness parameters are constructed with more than 40 independent realizations under the same experimental conditions, and the average values and standard deviations are compared in different conditions. We found that the most prominent feature of the obtained distributions is their large standard deviations, which is a signature of large fluctuations. We show that even if the roughness parameters for a particular domain wall are well known, these parameters are not necessarily representative of the underlying physics of the system. In the low field limit, within the creep regime of domain-wall motion, we found the average roughness exponent and roughness amplitude to be around 0.75 and 0.45 $\ensuremath{\mu}{\mathrm{m}}^{2}$, respectively. When an in-plane magnetic field is applied we observed that, even though the distributions are wide, changes in the mean values of roughness parameters can be identified; the roughness exponent decreasing to values around 0.72 while the roughness amplitude increases to 0.65 $\ensuremath{\mu}{\mathrm{m}}^{2}$. Our results call for a careful consideration of statistical averaging over different domains walls when reporting roughness exponents.

From single to multiple pulse all-optical switching in GdFeCo thin films

All-optical switching refers to the magnetization reversal induced by ultrashort laser pulses. Both single-pulse all-optical helicity-independent switching and multiple pulses all-optical helicity-dependent switching have been reported for GdFeCo thin films. In this paper, we demonstrate that for two GdFeCo compositions, the transition between these two behaviors can appear by increasing the thickness of the Pt capping or by tuning the laser fluence. The two types of switching cannot be observed for the same set of parameters, which supports that their mechanisms are uncorrelated. Our calculations indicate that the transition may be induced by a strong inhomogeneity of the electronic temperature within the thickness of the GdFeCo layer in the subpicosecond timescale.

Surface engineering of magnetic and mechanical properties of Ta/Pt/GdFeCo/IrMn/Pt heterostructures by femtosecond laser pulses

Surface engineering of magnetic and mechanical properties of Ta/Pt/GdFeCo/IrMn/Pt heterostructures by femtosecond laser pulses

Ultrafast magnetization reversal by picosecond electrical pulses

The field of spintronics involves the study of both spin and charge transport in solid-state devices. Ultrafast magnetism involves the use of femtosecond laser pulses to manipulate magnetic order on subpicosecond time scales. We unite these phenomena by using picosecond charge current pulses to rapidly excite conduction electrons in magnetic metals. We observe deterministic, repeatable ultrafast reversal of the magnetization of a GdFeCo thin film with a single sub-10-ps electrical pulse. The magnetization reverses in ~10 ps, which is more than one order of magnitude faster than any other electrically controlled magnetic switching, and demonstrates a fundamentally new electrical switching mechanism that does not require spin-polarized currents or spin-transfer/orbit torques. The energy density required for switching is low, projecting to only 4 fJ needed to switch a (20 nm)3 cell. This discovery introduces a new field of research into ultrafast charge current-driven spintronic phenomena and devices.

Role of electron and phonon temperatures in the helicity-independent all-optical switching of GdFeCo

Ultrafast optical heating of the electrons in ferrimagnetic metals can result in all-optical switching (AOS) of the magnetization. Here we report quantitative measurements of the temperature rise of GdFeCo thin films during helicity-independent AOS. Critical switching fluences are obtained as a function of the initial temperature of the sample and for laser pulse durations from 55 fs to 15 ps. We conclude that non-equilibrium phenomena are necessary for helicity-independent AOS, although the peak electron temperature does not play a critical role. Pump-probe time-resolved experiments show that the switching time increases as the pulse duration increases, with 10 ps pulses resulting in switching times of ~sim 13 ps. These results raise new questions about the fundamental mechanism of helicity-independent AOS.

A Dual-Colour Architecture for Pump-Probe Spectroscopy of Ultrafast Magnetization Dynamics in the Sub-10-femtosecond Range

Current time-resolution-limited dynamic measurements clearly show the need for improved techniques to access processes on the sub-10-femtosecond timescale. To access this regime, we have designed and constructed a state-of-the-art time-resolved magneto-optic Kerr effect apparatus, based on a new dual-color scheme, for the measurement of ultrafast demagnetization and precessional dynamics in magnetic materials. This system can operate well below the current temporal ranges reported in the literature, which typically lie in the region of around 50 fs and above. We have used a dual-colour scheme, based on ultra broadband hollow-core fibre and chirped mirror pulse compression techniques, to obtain unprecedented sub-8-fs pump and probe pulse durations at the sample plane. To demonstrate the capabilities of this system for ultrafast demagnetization and precessional dynamics studies, we have performed measurements in a ferrimagnetic GdFeCo thin film. Our study has shown that the magnetization shows a sudden drop within the first picosecond after the pump pulse, a fast recovery (remagnetization) within a few picoseconds, followed by a clear oscillation or precession during a slower magnetization recovery. Moreover, we have experimentally confirmed for the first time that a sub-10-fs pulse is able to efficiently excite a magnetic system such as GdFeCo.

Femtosecond X-ray magnetic circular dichroism absorption spectroscopy at an X-ray free electron laser.

X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L(3,2)-edges. The high brightness of the X-ray free electron laser combined with high accuracy detection of incident and transmitted X-rays enables ultrafast X-ray magnetic circular dichroism studies of unprecedented sensitivity. This new capability is applied to a study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature.

Nanoscale sub-100 picosecond all-optical magnetization switching in GdFeCo microstructures.

Ultrafast magnetization reversal driven by femtosecond laser pulses has been shown to be a promising way to write information. Seeking to improve the recording density has raised intriguing fundamental questions about the feasibility of combining ultrafast temporal resolution with sub-wavelength spatial resolution for magnetic recording. Here we report on the experimental demonstration of nanoscale sub-100 ps all-optical magnetization switching, providing a path to sub-wavelength magnetic recording. Using computational methods, we reveal the feasibility of nanoscale magnetic switching even for an unfocused laser pulse. This effect is achieved by structuring the sample such that the laser pulse, via both refraction and interference, focuses onto a localized region of the structure, the position of which can be controlled by the structural design. Time-resolved photo-emission electron microscopy studies reveal that nanoscale magnetic switching employing such focusing can be pushed to the sub-100 ps regime.

Heat-Assisted Magnetization Dynamics in GdFeCo Using Field-Induced TR-MOKE

A custom microcoil was used to generate a magnetic field pulse and a continuous wave (CW) laser was used as a heating source to study the heat-assisted effect in magnetization switching on GdFeCo thin films. Four different pulsed field strengths were chosen, and the power of the CW laser heating was varied from 0 to 180 mW. The heat-assisted enhancement was not observed when the pulsed field strength was 2400 Oe. The enhancement was visible when the pulsed field strength was reduced to 2160 Oe, and became obvious when the pulsed field strength was 1750 and 1350 Oe. These interesting results prove that CW laser heating could provide a marked improvement to the magnetization switching when the switching field is relatively small, thus making it a more applicable and economic alternative to pulsed laser heating. Simulations based on the Landau-Lifshitz-Bloch equation have also been performed and agreed well with the experimental data. The CW laser heating scheme provides a more economic and practical way to realize heat-assisted magnetization switching, which could be used for potential future heat-assisted magnetic recording technology.

Nanostructuring of GdFeCo Thin Films for Laser Induced Magnetization Switching

The effect of structuring GdFeCo thin films has been investigated in view of laser induced magnetic recording applications. It is found that patterning the substrate via electron beam lithography combined with reactive ion etching before the actual magnetic layer deposition can produce a significant magnetic de-coupling between the obtained structures and their immediate magnetic surrounding. However, upon laser heating, dipolar coupling with this surrounding is found to determine completely the magnetization orientation of the structures, a situation which could be problematic for laser induced recording applications. An alternative structuring approach, based on a lift-off technique of the magnetic layer, is immune to this problem since no magnetic surrounding remains.