Magnetization, Anomalous Hall Effect and Single Pulse Partial All-optical Switching in Amorphous Rare-earth Transition-metal Thin Films

Abstract

Research on rare-earth (R) transition-metal (T) thin films has a long history. A revival of interest in this field was sparked by the discovery of single pulse all-optical switching (SP-AOS) in a-GdFeCo1 alloy thin films by sub-picosecond laser pulses without the presence of a magnetic field, approximately a decade ago. This reopens some questions about amorphous metals that were not perfectly resolved in the last century: • How do Co moments in the a-R-Co system change with composition and temperature? Will the Co orbital moment be quenched in amorphous materials? • How will the noncollinear magnetic structure a-DyxCo1−x and a-TbxCo1−x evolve with temperature? • Will the noncollinear magnetic structure on the R sites in a-DyxCo1−x and a-TbxCo1−x contribute to the anomalous Hall effect (AHE)? • Does the noncollinear magnetic structure on the R sites in a-DyxCo1−x and a-TbxCo1−x influence the SP-AOS? Driven by the non-magnetic properties of Y and its similar metallic radius to the other rare earths like Gd, Dy and Tb, the binary a-YxCo1−x was chosen to answer the first question listed above. The work on sputtered thin films of binary a-YxCo1−x with 0 ≤ x ≤ 0.54 and thickness ≈ 15 nm provides a basis for understanding the ferromagnetism of cobalt in other amorphous R- Co alloy thin films. All compositions below the critical value xc = 0.5 are very soft ferromagnets with almost no coercivity at room temperature. Above xc, the Co moment vanishes. In these thin films, the ferromagnetic easy axis is in-plane when x < 0.45. The average total moment is 1.63 μB in a-YCo3, consisting of 1.31 μB spin moment and 0.32 μB orbital moment, deduced from X-ray magnetic circular dichroism measurements. Because the orbital moment is similar to that in crystalline YCo5 with a big uniaxial anisotropy constant, the local anisotropy of a-YCo3 is expected to be similar. The strong ferromagnetic exchange ensures that deviations from collinear ferromagnetism due to random anisotropy are negligible because of exchange averaging. The Co moments are almost temperature-independent when x < 0.4 due to large exchange interaction. Based on the research conducted on a-YxCo1−x, the temperature dependence of the aver- aged magnetic moment of the noncollinear R subnetwork is deduced in the a-DyxCo1−x and a-TbxCo1−x sputtered thin films. For Dy, the z-component of magnetic moment changes from 7.3 μB (4 K) to 3.6 μB (300 K); For Tb, the z-component of magnetic moment changes from 7.0 μB (4 K) to 4.0 μB (300 K). The cone angle also decreases from ∼ 50○ with increasing temperature. A spin-flop transition near compensation temperature, with a spin-flop field of approximately 2 T, was observed in the anomalous Hall effect measurement of a-DyCo3 in 14 T field. The temperature dependence of the Hall voltage of a-DyCo3 is used to infer that the Co subnetwork dominates the AHE. In order to answer the final question listed above, the all-optical switching experiments were performed in a-DyxCo1−x and a-TbxCo1−x with x ≈ 0.25, irradiated with single 200 fs pulses of 800 nm laser light. A new type of partial and random SP-AOS, non-uniform on a length scale of about 150 nm, as measured by XMCD-PEEM, is found in a-DyxCo1−x for temperatures both below and above magnetization compensation temperature. This switching behavior is distinct from that in a-GdFeCo (with a complete switching and a well-defined switching boundary). Ringlike switched domain patterns appear at higher fluence, but below the fluence at which the anisotropy begins to change towards in-plane. Optical control of coercivity is demonstrated with a possible application in magnetic logic. These three main parts of the work help to obtain a better understanding of the behavior of Co magnetic moments in binary amorphous metals a-RxCo1−x and to further clarify the influence of the noncollinear magnetic structure of the Dy and Tb ions on the magnetization, the AHE and the partial and random SP-AOS in a-DyxCo1−x and a-TbxCo1−x.