Origin and Optical Switching of Perpendicular Magnetization for Co100-xGdx/Pt Multilayers

Abstract

Introduction: Magnetization manipulation on ultrashort timescales is one of the major issues for developing high speed spintronic devices. All-optical switching (AOS) of magnetization by femtosecond laser pulses has attracted much attention as a route for ultrafast magnetization manipulation. Following the pioneering demonstrations of all-optical helicity dependent switching (AO-HDS) [Refs.1,2], another type of AOS was reported in Gd-Fe-Co [Ref.3], which is called AO helicity independent switching (AO-HIS). The AO-HIS is observed for the limited materials such as Gd-Fe-Co [Ref.3] and Co/Gd multilayer [Ref.4]. On the other hand, recent studies demonstrated that the AO-HDS was achieved for several ferromagnetic alloys, ferrimagnetic alloys and multilayers [Refs.5,6], suggesting that exploiting a variety of materials is beneficial to elucidate the underlying mechanism of AOS. The perpendicularly magnetized Gd-based alloys are representative materials not only for the AOS study, but also for the research of antiferromagnetic spintronics. However, the origin of perpendicular magnetic anisotropy (PMA) has not been fully understood yet. For example, in the case of Co-Gd, the thick Co-Gd single layer film sometimes exhibits the PMA [Ref.7] whereas the thin Co-Gd single layer often shows the in-plane magnetization [Ref.8], and the condition to obtain the PMA is not clear. Nevertheless, many spintronics and AOS studies exploited the combination of Gd-based alloy and Pt without addressing the origin of PMA. In addition, although a magnetic moment may be induced in Pt due to the proximity effect, the role of interface magnetism on the magneto-optical response has not been elucidated yet. These facts make the combination of Gd-based alloy and Pt a unique platform to investigate the correlation between the fundamental magnetic properties and the AOS behavior, which will provide knowledge to gain insight into the AOS.In this study, we paid attention to the combination of Co-Gd and Pt. The origin of PMA and the role of induced magnetic moment were first investigated for the Co-Gd/Pt multilayers with various Co-Gd layer thicknesses (t) and alloy compositions (Co100-xGdx). Then, the AOS experiments were carried out in order to find the composition dependence of AOS probability. Experimental Results: Thin films with the stacking structure of [Pt (2.0)/Co100-xGdx (t)]×3/Pt (2.0) (in nanometer) were prepared on sapphire (11-20) substrates using an ultrahigh vacuum compatible magnetron sputtering system. All the layers were deposited at room temperature.By tuning the sputtering powers of Co and Gd targets, the Gd concentration x at. % was widely varied in the range from x = 12 to 37 at. %. The alloy compositions were determined by the electron probe x-ray microanalysis. The film structures were characterized using x-ray reflectivity (XRR) and reflection high energy electron diffraction. Magnetic properties were measured using a superconducting quantum interference device magnetometer, a vibrating sample magnetometer and a polar magneto-optical Kerr effect set-up. The AOS experiment was carried out at room temperature with right and left circularly polarized laser pulses (center wavelength of 514 nm, pulse duration of ~200 fs, and repetition rate of 10 kHz) under zero magnetic field. The typical laser power was ~2.8 mW and the corresponding laser fluence applied was 0.9~2.1 mJ/cm2. A single laser pulse with a diameter of ~30 mm was illuminated on the surface of films (demonstration of helicity dependence) and/or the center of the microfabricated Hall cross (quantification of AOS probability).The XRR profiles were well fitted for all the samples with the assumption of the [Pt/Co-Gd]×3/Pt layer stack, meaning that the designed layered structures were successfully prepared. From the thickness and composition dependences of magnetization and magnetic anisotropy energy, we found that the magnetic anisotropy coming from the Co-Gd/Pt interface plays the major role for obtaining PMA while the contribution from the bulk properties of Co-Gd is negligibly small. This is of different origin for Gd-Fe-Co, in which the bulk magnetic anisotropy leads to the PMA. The proximity-induced magnetic moment in Pt remarkably appeared for the thin Co-Gd layers, which coupled with the Co magnetic moment in parallel. It is noted that the proximity-induced Pt moment largely affected the condition for the magnetization compensation of Co-Gd. The Co-Gd/Pt multilayers clearly exhibited the single laser pulse AO-HIS in the wide Co-Gd composition range, and the maximum AOS probability was obtained around the compensation composition of Co-Gd. The above findings are useful to improve the performance of AOS-based device. **