Terahertz Emission From an Exchange-Coupled Synthetic Antiferromagnet

We report on terahertz emission from $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}/\mathrm{Ru}/\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ and $\mathrm{Pt}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Ru}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Pt}$ synthetic antiferromagnet (SAF) structures upon irradiation by a femtosecond laser; the former is via the anomalous Hall effect, whereas the latter is through the inverse spin Hall effect. The antiparallel alignment of the two ferromagnetic layers leads to a terahertz emission peak amplitude that is almost double that of a corresponding single-layer or bilayer emitter with the same equivalent thickness. In addition, we demonstrate by both simulation and experiment that terahertz emission provides a powerful tool to probe the magnetization reversal processes of individual ferromagnetic layers in a SAF structure, as the terahertz signal is proportional to the vector difference $({\mathbf{M}}_1-{\mathbf{M}}_2)$ of the magnetizations of the two ferromagnetic layers.

Figure 1

Schematic illustration of the (a) $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ SAF and (b) $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ SAF structures used in this study. (c) Schematic illustration of the coordinate systems used for terahertz detection.

Figure 2

Simulated longitudinal (along $y$ axis) and transverse (along $x$ axis) components of total (i.e., $\mathbf{M}={\mathbf{M}}_1+{\mathbf{M}}_2$) and differential (i.e., $\mathrm{\Delta }\mathbf{M}={\mathbf{M}}_1\mathbf{-}{\mathbf{M}}_2$) components of the magnetization of a $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ SAF versus a sweeping external field (H). (a)–(d), $M_y=M_{y1}+M_{y2}$; (e)–(h), $\mathrm{\Delta }{M}_y=M_{y1}-M_{y2}$; (i)–(l), $-\mathrm{\Delta }{M}_x=M_{x2}-M_{x1}$. The four different cases in each row correspond to the four different combinations of rotation directions of ${\mathbf{M}}_1$ and ${\mathbf{M}}_2$ when the SAF is subjected to the sweeping field, as detailed in Appendix pp1. The solid and dashed lines represent forward and backward sweeping, respectively. The blue and red arrows indicate the directions of ${\mathbf{M}}_1$ and ${\mathbf{M}}_2$ (right, +y direction; left, −y direction).

Figure 3

Simulated longitudinal (along $y$ axis) and transverse (along $x$ axis) components of total (i.e., $\mathbf{M}={\mathbf{M}}_1+{\mathbf{M}}_2$) and differential (i.e., $\mathrm{\Delta }\mathbf{M}={\mathbf{M}}_1-{\mathbf{M}}_2$) components of the magnetization of a $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ SAF versus a sweeping external field (H). (a)–(d), $M_y=M_{y1}+M_{y2}$; (e)–(h), $-\mathrm{\Delta }{M}_y=M_{y2}-M_{y1}$; (i)–(l), $\mathrm{\Delta }{M}_x=M_{x1}-M_{x2}$. The four different cases in each row correspond to the four different combinations of rotation directions of ${\mathbf{M}}_1$ and ${\mathbf{M}}_2$ when the SAF is subjected to the sweeping field, as detailed in Appendix pp1. The solid and dashed lines represent forward and backward sweeping, respectively. The blue and red arrows indicate the directions of ${\mathbf{M}}_1$ and ${\mathbf{M}}_2$ (right, +y direction; left, −y direction).

Figure 4

Magnetic properties of the SAFs. The solid lines represent forward sweeping, and the dashed lines represent backward sweeping. (a) M-H curves of the (d₁, d₂) $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ SAFs. (b) Temperature dependence of the M-H loop of the (4, 4) $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ SAF. (c) M-H curves of the (d₁, d₂) $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ SAFs. Inset: M-H curves in the field region from −200 Oe to 200 Oe. (d) Temperature dependence of the M-H loops of the (4, 3) $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ SAF. Inset: M-H curves in the field region from −100 Oe to 100 Oe.

Figure 5

Time-domain terahertz emission waveforms from the SAF structures. (a) Terahertz waveforms from the (4, 4) $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ SAF at 200 Oe (dashed line) and 0 Oe (solid line), and from a $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$(5 nm) single-layer emitter (dotted line) at a positive magnetic field. (b) Terahertz waveforms from the (4, 3) $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ SAF at 2000 Oe (dashed line) and 0 Oe (solid line), and from a $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$(3 nm)/$\mathrm{Pt}$(3 nm) bilayer emitter (dotted line) at a negative magnetic field. (c) Pump-fluence dependence of the terahertz peak amplitude for the $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ (diamonds) and $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ (squares) SAFs.

Figure 6

Magnetic field dependence of the terahertz emission. The solid lines represent forward sweeping, and the dashed lines represent backward sweeping. (a) E_x-H curve for the (4, 4) $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ SAF at pumping fluences from 110 to 385 µJ/cm²; (b) E_y-H curve for the (4, 4) $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ SAF at a pumping fluence of 330 µJ/cm²; (c) E_x-H curve for the (4, 3) $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ SAF at fluences from 110 to 550 µJ/cm²; (d) E_y-H curve for the (4, 3) $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ SAF at a pumping fluence of 330 µJ/cm².

Figure 7

Simulated φ₁ and φ₂ values for (a)–(d) $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ SAF and (e)–(h) $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ SAF under a sweeping magnetic field. The four sets of results correspond to the four different combinations of rotation directions of ${\mathbf{M}}_1$ and ${\mathbf{M}}_2$ summarized in Table 2. φ₁ and φ₂ are positive for counterclockwise rotation and negative for clockwise rotation. The solid lines represent forward sweeping, and the dashed lines represent backward sweeping. (i)–(l) Schematic illustrations of the initial magnetization rotation direction during field sweeping for the four different cases (upper half, backward sweeping; lower half, forward sweeping).

Figure 8

(a)–(c) Subprocesses on different time scales after ultrafast laser excitation of a metallic layer. (a) Excitation of nonequilibrium electrons. (b) High-temperature Fermi-Dirac distribution of electron subsystem. (c) Nearly thermal equilibrium state among electrons, phonons, and magnons. (d) Schematic illustration of nonthermal electron reflection at quartz/FM and FM/$\mathrm{Mg}\mathrm{O}$ interfaces, the directions of spin polarity in each $\mathrm{Fe}\text{−}\mathrm{Mn}\text{−}\mathrm{Pt}$ layer, the formation of backflow currents, and the directions of the charge currents.