Low-remanence criterion for helicity-dependent all-optical magnetic switching in ferrimagnets

We demonstrate that a low-remanent sample magnetization $M_{\mathrm{R}}$ is crucial for all-optical magnetic switching (AOS) in ferrimagnets and ferrimagnet heterostructures. $M_{\mathrm{R}}$ may be devised by the composition of the material. However, it can also be controlled in situ by changing the sample temperature because it affects $M_{\mathrm{R}}$. We show that increasing the lattice temperature by laser pulses or a simple heating resistor enables AOS in a ferrimagnetic Tb-Fe film, which does not exhibit AOS at room temperature. We reconcile earlier contradicting results for AOS in heated and cooled magnetic films by applying the low-remanence criterion. It also applies to other existing rare-earth transition-metal (RE-TM) alloys, such as GdFeCo or Tb-Co, to ferrimagnet heterostructures, and to RE-free synthetic ferrimagnets.

Figure 1

Heatable sample holder and layer stack. The sample is sandwiched between the heated aluminum top plate and a copper fastener. The heater is a 5-W, 330-mΩ heating resistor connected to an adjustable high-current power supply. To ensure proper heat conduction between the sample holder and the sample, it is embedded into a conductive paste. The temperature close to the sample position is measured by two thermocouple probes located on opposite sides of the sample.

Figure 2

Beam geometry for (a) Faraday imaging and (b) Kerr magnetometry.

Figure 3

All-optical helicity-dependent magnetic switching (AOS) in $\mathrm{T}{\mathrm{b}}_{34}\mathrm{F}{\mathrm{e}}_{66}$ at a pulse repetition rate of 250 or 10 kHz. The left column shows the initial homogeneously magnetized state, the middle column after exposure with right-handed circularly polarized $\left({\sigma }^{+}\right)$ laser pulses, and the right column images the state after exposure with left-handed circularly polarized $\left({\sigma }^{-}\right)$ laser pulses at the same fluence as used in the corresponding previous image. (a–c) 250 kHz pulse repetition rate and substrate temperature of $T=300\mathrm{K}$. The threshold fluence for all-optical switching (AOS) is $F=(1.0\pm 0.3)\mathrm{mJ}/{\mathrm{cm}}^2$. (d–l) 10 kHz repetition rate and different substrate temperatures. (d–f) Pure thermal demagnetization (PTD) at a substrate temperature of $T=300\mathrm{K}$ and $F=(1.4\pm 0.2)\mathrm{mJ}/{\mathrm{cm}}^2$. (g–i) AOS onset at a substrate temperature of $T=345\mathrm{K}$ and $F=(0.9\pm 0.1)\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$. (j–l) AOS at a substrate temperature of $T=370\mathrm{K}$ and $F=(0.5\pm 0.1)\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$.

Figure 4

Hysteresis loops of $\mathrm{T}{\mathrm{b}}_{34}\mathrm{F}{\mathrm{e}}_{66}$ measured by magneto-optical Kerr magnetometry (MOKE): (a) With laser irradiation at a pulse repetition rate of $\nu =250\mathrm{kHz}$ at the AOS threshold fluence of $F=(1.0\pm 0.3)\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$. The substantial decrease of the coercivity and remanence due to irradiation with the switching pulses is visible. Solid lines represent averaged values. (b) With laser irradiation of $\nu =10\mathrm{kHz}$ pulse repetition rate at the PTD threshold fluence of $F=(1.4\pm 0.2)\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$. Here, the magnetic properties are unchanged with and without laser irradiation.

Figure 5

Hysteresis loops of $\mathrm{T}{\mathrm{b}}_{34}\mathrm{F}{\mathrm{e}}_{66}$ at various sample temperatures of $T=300,$ 345, and 370 K, measured by a MOKE and by SQUID-VSM. For elevated sample temperatures the coercivity as well as the remanence decreases, which facilitates AOS at a pulse repetition rate of $\nu =10\mathrm{kHz}$ at $T=370\mathrm{K}$.

Figure 6

All-optical helicity-dependent magnetic switching (AOS) in $\mathrm{T}{\mathrm{b}}_{29}\mathrm{F}{\mathrm{e}}_{71}$ at $\nu =10\mathrm{kHz}$. Substrate temperature is $T=300\mathrm{K}$. (a) Initial homogeneously magnetized state $M^{-}$. (b) AOS at a threshold fluence of $F=(1.7\pm 0.2)\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$, after irradiation with right-handed circularly polarized laser pulses $\left({\sigma }^{+}\right)$. (c) Erasing of the written domain structure at the same fluence as in (b) by using left-handed circularly polarized laser $\left({\sigma }^{-}\right)$ pulses.

Figure 7

Overview of all-optical switching (AOS) in $\mathrm{G}{\mathrm{d}}_x\mathrm{F}{\mathrm{e}}_{100-x-y}\mathrm{C}{\mathrm{o}}_y$ [6, 7, 8, 11, 22, 26, 27], $\mathrm{T}{\mathrm{b}}_x\mathrm{F}{\mathrm{e}}_{100-x}$ [10], $\mathrm{T}{\mathrm{b}}_x\mathrm{C}{\mathrm{o}}_{100-x}$ [24], and [Co/Ir/Co/Ni/Pt/Co/Ir]×5 [25]. Red dots and light-red area indicate AOS. Black squares and light-gray area denote pure thermal demagnetization (PTD). Red (black) arrows represent the decrease (increase) of sample magnetization due to heating; the solid red arrow shows our result obtained in $\mathrm{T}{\mathrm{b}}_{34}\mathrm{F}{\mathrm{e}}_{66}$. The dashed red and black arrows visualize the trend in $\mathrm{T}{\mathrm{b}}_x\mathrm{C}{\mathrm{o}}_{100-x}$. (See main text for details.)

Figure 8

Remanent sample magnetization $M_{\mathrm{R}}$ versus temperature for a $\mathrm{T}{\mathrm{b}}_{19}\mathrm{C}{\mathrm{o}}_{81}$ amorphous alloy film of 20 nm thickness, measured by SQUID-VSM magnetometry in the temperature range $T=4-370\mathrm{K}$. $M_{\mathrm{R}}$ amounts to $300\mathrm{kA}/\mathrm{m}(300\mathrm{emu}/\mathrm{cc})$ at room temperature and increases for rising temperatures.