Exploring THz exchange resonances in synthetic ferrimagnets with ultrashort optically induced spin currents

Using spin currents generated by fs laser pulses, we demonstrate excitation of GHz ferromagnetic resonance and THz ferrimagnetic exchange resonances in Co/Gd/Co/Gd multilayers by time-resolved magneto-optic Kerr effect measurements. Varying the Gd layer thickness allows for a tuning of the resonance spectrum by manipulating the total angular momentum and strength of effective exchange fields between the antiferromagnetically coupled layers. Close to the compensation point of angular momentum, a minimum in the frequency of the exchange-dominated mode and a maximum in the frequency of the ferromagnetic resonance mode is observed. Finally, to gain better understanding of the excitation mechanism, we analyze the anomalous variation in the measured exchange mode amplitude as a function of its frequency. A peak in this amplitude in the vicinity of the compensation point of angular momentum is explained using a macrospin model, taking nonlinear effects at finite precession amplitudes into account

Ultrafast single-pulse all-optical switching in synthetic ferrimagnetic Tb/Co/Gd multilayers

In this work, we investigate single-shot all-optical switching (AOS) in Tb/Co/Gd/Co/Tb multilayers in an attempt to establish AOS in synthetic ferrimagnets with high magnetic anisotropy. In particular, we study the effect of varying Tb thicknesses to disentangle the role of the two rare earth elements. Even though the role of magnetic compensation has been considered to be crucial, we find that the threshold fluence for switching is largely independent of the Tb content. Moreover, we identify the timescale for the magnetization to cross zero to be within the first ps after laser excitation using time-resolved MOKE. We conclude that the switching is governed mostly by interactions between Co and Gd

Enhancing All-Optical Switching of Magnetization by He Ion Irradiation

All-optical switching (AOS) of magnetization by a single femtosecond laser pulse in Co/Gd based synthetic ferrimagnets is the fastest magnetization switching process. On the other hand, He ion irradiation has become a promising tool for interface engineering of spintronic material platforms, giving rise to significant modification of magnetic properties. In this paper, we explore the use of He ion irradiation to enhance single pulse AOS of Co/Gd bilayer-based synthetic ferrimagnets. The intermixing of the constituent magnetic layers by He ion irradiation was both numerically simulated and experimentally verified. We theoretically modelled the effects of intermixing on AOS using the layered microscopic 3-temperature model and found that AOS is enhanced significantly by breaking the pristine Co/Gd interface through intermixing. Following this notion, we studied the threshold fluence of AOS as a function of He ion irradiation fluence. We found that the AOS threshold fluence can be reduced by almost 30%. Our study reveals the control of AOS by He ion irradiation, which opens up an industrially compatible approach for local AOS engineering

Structure and magnetic properties of ferrimagnetic [Gd/Fe]n multilayer and GdxFe100−x thin films

The structural and magnetic properties of two series of [Gd(2, 4 nm)/Fe(t)]n multilayer films with varying Fe thickness were investigated and compared to those of amorphous ferrimagnetic GdFe alloys of the same corresponding composition. Transmission electron microscopy studies confirmed the high interface quality of both multilayer series. Furthermore, the microstructure was analyzed, revealing polycrystallinity in both Gd and Fe layers with strong (101̄0)-oriented textured growth of Gd particularly for the multilayer series with 2 nm Gd. Magnetic measurements confirm an out-of-plane magnetic easy axis in the alloy samples and an in-plane magnetic easy axis in all multilayer samples. Twisted spin states in samples with a low remanent magnetization were identified. Magnetic compensation points of both multilayer series are compared to those of the alloy samples. It was found that the dependence of the magnetic compensation point on effective Gd concentration in the series with 2 nm Gd closely resembles the strong dependence observed in the alloy counterparts. In contrast, a weaker dependence is revealed for the multilayer series with 4 nm Gd, which makes this system more robust against variations in composition required for applications

Effect of Gd addition on the structural and magnetic properties of L10-FePt alloy thin films

The impact of Gd addition on the structural and magnetic properties of L10-FePt alloy thin films, which were sputter-deposited on MgO(001) substrates at 800C, was investigated. A rapid deterioration of L10 chemical ordering along with a strong amorphization effect is observed with addition of Gd. At more than 20 at. % Gd, additional crystalline phases occur. Due to the diminishing L10 chemical order, the perpendicular magnetic anisotropy (PMA) gets strongly reduced from about 3 down to 1 MJ/m3 with addition of 14 at. % Gd. For higher Gd concentrations, the easy axis of magnetization turns fully in-plane due to dominating magnetic shape anisotropy. Furthermore, the saturation magnetization gets reduced with Gd addition due to the antiferromagnetic coupling between the Fe and Gd magnetic moments. Also, the Curie temperature can be lowered. Interestingly, with addition of Gd, a change in film morphology takes place, changing from an isolated island structure to a more continuous film morphology, which is of particular interest for samples with low Gd concentra- tion as these films exhibit still strong PMA but rather low coercive fields

Ultrafast single-pulse all-optical switching in synthetic ferrimagnetic Tb/Co/Gd multilayers

In this work, we investigate single-shot all-optical switching (AOS) in Tb/Co/Gd/Co/Tb multilayers in an attempt to establish AOS in synthetic ferrimagnets with high magnetic anisotropy. In particular, we study the effect of varying Tb thicknesses to disentangle the role of the two rare earth elements. Even though the role of magnetic compensation has been considered to be crucial, we find that the threshold fluence for switching is largely independent of the Tb content. Moreover, we identify the timescale for the magnetization to cross zero to be within the first ps after laser excitation using time-resolved MOKE. We conclude that the switching is governed mostly by interactions between Co and Gd

L1₀-ordered Fe100(1-x)CrxPt thin films: Phase formation, morphology, and spin structure

Chemically ordered L10 (Fe100−xCrx)Pt thin films were expitaxially grown on MgO(001) substrates by magnetron sputter-deposition at 770∘C. In this sample series, Fe was continuously substituted by Cr over the full composition range. The lattice parameter in the [001] growth direction steadily increases from L10-FePt toward L10-CrPt, confirming the incorporation of Cr in the lattice occupying Fe sites. With the observed high degree of chemical ordering and (001) orientation, strong perpendicular magnetic anisotropy is associated, which persists up to a Cr content of x=20 at. %. Similarly, the coercive field in the easy-axis direction is strongly reduced, which is, however, further attributed to a strong alteration of the film morphology with Cr substitution. The latter changes from a well-separated island microstructure to a more continuous film morphology. In the dilute alloy with low Cr content, isolated Cr magnetic moments couple antiferromagnetically to the ferromagnetic Fe matrix. In this case, all Cr moments are aligned parallel, thus forming a ferrimagnetic FeCrPt system. With increasing Cr concentration, nearest-neighbor Cr-Cr pairs start to appear, thereby increasing magnetic frustration and disorder, which lead to canting of neighboring magnetic moments, as revealed by atomistic spin-model simulations with model parameters based on first principles. At higher Cr concentrations, a frustrated ferrimagnetic order is established. With Cr substitution of up to 20 at. %, no pronounced change in Curie temperature, which is in the range of 700 K, was noticed. But with further addition the Curie temperature drops down substantially even down to room temperature at 47 at. % Cr. Furthermore, x-ray magnetic circular dichroism studies on dilute alloys containing up to 20 at. % of Cr revealed similar spin moments for Fe and Cr in the range between 2.1–2.5 μB but rather large orbital moments of up to 0.50 ±0.10μB for Cr. These results were also compared to ab initio calculations.publishe

Control of magnetoelastic coupling in Ni/Fe multilayers using He + ion irradiation

International audienceThis study reports the effects of post-growth He + irradiation on the magneto-elastic properties of a Ni/Fe multi-layered stack. The progressive intermixing caused by He + irradiation at the interfaces of the multilayer allows us to tune the saturation magnetostriction value with increasing He + fluences and even to induce a reversal of the sign of the magnetostrictive effect. Additionally, the critical fluence at which the absolute value of the magnetostriction is dramatically reduced is identified. Therefore, insensitivity to strain of the magnetic stack is nearly reached, as required for many applications. All the above-mentioned effects are attributed to the combination of the negative saturation magnetostriction of sputtered Ni and Fe layers and the positive magnetostriction of the Ni x Fe 1− x alloy at the intermixed interfaces, whose contribution is gradually increased with irradiation. Importantly, the irradiation does not alter the layer polycrystalline structure, confirming that post-growth He + ion irradiation is an excellent tool to tune the magneto-elastic properties of multilayer samples. An alternative class of spintronic devices can be envisioned with a material treatment able to arbitrary change the magnetostriction with ion-induced “magnetic patterning.