A study on all-optical helicity-independent switching state diagram in GdFeCo alloys

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Microwave generation in GMR/TMR structures

As discussed in 1.1.1, the resistance of a SV/MTJ is determined by the angle between magnetizations of two ferromagnetic layers for a given device. Thus a steady magnetization precession in free layer can result in a periodic change of resistance of SV/TMR. Meanwhile, the injected current is constant. As a consequence, a high-frequency AC voltage is obtained in this case. Figure 1-10 schematically show the process of microwave generation.

STNO with different magnetization configurations In-plane STNO

Based on the magnetization configuration of MTJ or SV, STNOs can be divided into three categories. Initially, most research works focused on in-plane STNOs in which free and fixed FM layers have an easy-plane magnetic anisotropy, meaning that the magnetic moments lie in plane if no external magnetic field is applied. Co, CoFe and NiFe are commonly used materials with in-plane anisotropy. In this case, as discussed above, when a proper DC current is injected, the magnetic moments of the free layer will precess continuously. But it should be noted that the angle θ between moments of free and fixed layer remains unchanged during the precession, as shown in Figure 1-14. In this case, the injected current and device resistance are both constant, meaning that there is no AC signal output. Only when a large non-collinear external magnetic field is applied, the symmetry of the magnetic moments between the free layer and the fixed layer is broken; microwave signals could then be obtained.

Ultrafast laser-induced magnetization dynamics

In 1984, Agranat et al. first reported the evidence of ultrafast demagnetization in nickel films induced by ultra-short laser pulses [78]. Their results show that the nickel film with a 50-100 nm thickness demagnetized after exposure to a laser pulse with a 1-40 ns pulse duration. However, such a phenomenon was not observed when the pulse width is 5-20 ps. Consequently, they concluded that the demagnetization time in nickel film is on a time scale of 1-40 ns. In 1990, with the help of time-resolved spin-polarized emission spectroscopy, Vaterlaus et al. observed the evolution of magnetization of Gd after the action of a 10 ns laser pulse. Using 30 ps probe pulses, they showed that the spin relaxation in gadolinium took place within 100±80 ps [79]. Based on these studies, it was generally believed that the characteristic time of interaction between light and magnetization is similar to that of spin-lattice, which is on nanosecond time scale.
With the improvement in semiconductor technology, the generation of femtosecond laser pulse became possible. In 1996, Beaurepaire et al. reported similar results via the time-resolved magneto-optical Kerr effect. They found that a 60 fs laser pulse resulted in a significant demagnetization of a Ni film [80]. Surprisingly, the demagnetization time of Ni films is only about 1ps, as shown in Figure 1-21. Afterwards, many research groups obtained similar results in various materials such as Fe and Co. Ultrafast demagnetization was confirmed to be a universal effect.

All-optical helicity-independent switching

To date, all-optical magnetization switching has been observed in a large variety of materials such as ferromagnets, ferrimagnets and multilayers [10-15, 81-85]. Moreover, two different types of AOS have been identified. The first one is All-Optical Helicity-Independent Switching (AO-HIS) in which case one laser pulse is enough to reverse the magnetization. However, AO-HIS is mostly observed in Gd-based materials. For most other materials, it requires multiple pulses to obtain complete reversal, named All-Optical Helicity-Dependent Switching (AO-HDS). The final magnetization state in this case is determined by the laser helicity. In this part, we will give a brief review of AO-HIS and discuss its underlying mechanism.
The first observation of AO-HIS was reported by Radu et al. in 2011[10]. They found that a single 60-fs linearly polarized laser pulse resulted in the reversal of magnetization of GdFeCo. With element-specific X-ray magnetic circular dichroism (XMCD) technique, they present the temporal evolution of magnetization of Gd and Fe after the action of one laser pulse. As shown in Figure 1-23(a) and (b), the switching process of GdFeCo can be divided into two stages. First, the magnetizations of both sublattices rapidly decrease with different demagnetization rate. The Gd magnetic moment is quenched within 1.5 ps, while the moment of Fe sublattice is reduced to 0 within 400 fs. Second, the magnetizations of both sublattices switch their directions and recover slowly. Surprisingly, it is seen that the Gd and Fe magnetic moments are aligned in parallel between 400 fs and 2 ps, indicating the formation of a ferromagnetic-like state [schematically shown in Figure 1-23(c)].

Electrical characterization

In this thesis, the performance of STNO is measured by an electrical method. Figure 2-8 shows the schematic of the experimental set-up. The main parts of this set-up include a spectrum analyzer, a nano-voltmeter, a DC current source and a Bias Tee. The STNO is connected to the DC source via ground-signal-ground (GSG) probe. We measure the static transport properties at room temperature under a constant DC current of 10μA. A Rohde & Schwarz spectrum analyzer is used to characterize the power spectrum density of STNO output. The Bias Tee is a three-terminal device consisting of an inductor and a capacitor, which is used to extract the AC component of the STNO output. As mentioned above, once IDC reaches a certain value, the damping torque in free layer is completely offset by STT exerted by the polarized spin current, leading to a steady magnetization precession and thus a high-frequency AC output. It is important to note that the output power scales with the precession angle θ (marked in Fig 1-7). Output signals were recorded after 40 dB amplification and the background noise, measured at zero dc current, was subtracted from the power spectra. All the RF measurements were performed at room temperature with a projected field electromagnet that allows us to control the field angle continuously.

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PCB holder for memristor

The high frequency PCB board (HF-PCB) was made considering two factors: impedance matching and board design. Impedance is defined as the combined effect of capacitance, inductance, and resistance that a circuit offers a signal at a given frequency. When high-frequency signals are carried on transmission lines of any significant length, care must be taken that the transmission medium is matched to its terminations. The source and load impedances should be as close as possible to the characteristic impedance of the transmission line, as this minimizes signal reflection which causes serious power loss. As for the board design, stripline and microstrip are the two most popular RF/microwave transmission-line formats. Microstrip routing is a transmission line trace routed on an external layer of the board. Because of this, it is separated from a single ground plane by a dielectric material. Stripline and microstrip have different benefits, here, we choose microstrip routing since its transmission delay is smaller which benefits the signal modulation [125-132]. A schematic view of a microstrip PCB and the PCB board we used are shown in Figure 3-2.

Basic characterization for memristor

Recently we developed a memristor stack of TiN/TaOx/HfO2/TiN with high performance [133]. The TaOx layer was shown to act as an in-built current compliance layer in the memristor device and improve thermal efficiency, which resulted in a much better reliability compared to the well-studied memristor stacks such as TiN/Ti/HfO2/TiN.
In general, memristors exhibit two switching modes, the unipolar switching and the bipolar switching. For unipolar switching, we change the amplitude rather than the polarity of the applied voltage to manipulate the resistance of memristors. Thus, set/reset process takes place at the same polarity. For bipolar switching, the applied voltage bias for reset and set are opposite. Figure 3-5 (a) shows a typical I-V curve of our device, showing classical bipolar switching behaviors. When the positive applied voltage exceeds a certain threshold, the resistance decreases dramatically, which is generally attributed to oxygen ions drifting to the anode interface, the so-called formation of conductive filaments in metal oxide. By contrast, the conductive filaments break under reversed voltage bias, returning the memory cell to a high resistance state [134]. Such switching mechanism has been explained more comprehensively in another recent work [135]. Based on this, memristors also exhibit multilevel resistance states when setting different current limits as shown in Figure 3-5 (b). Applying voltage pulses with different amplitudes lead to different resistance states. Such operation method is similar as that for Phase Change Memory (PCM). Specifically, a small voltage leads to the formation of thin conductive filaments while a larger voltage results in thick ones.

Table of contents :

1 Fundamentals
1.1 Spin transfer torque nano-oscillators (STNOs)
1.1.1 Magnetoresistance effect
1.1.2 Spin-transfer-torque-based phenomena
1.1.3 Microwave generation in GMR/TMR structures
1.2 All optical magnetization switching
1.2.1 Ultrafast laser-induced magnetization dynamics
1.2.2 All-optical helicity-independent switching
1.2.3 All-optical helicity-dependent switching
2 Research methods
2.1 Experimental tools
2.1.1 Sample deposition and fabrication
2.1.2 Kerr imaging set-up
2.1.3 Electrical characterization
2.2 Atomistic modeling
3 Magnetization dynamics in in-plane Spin Nano-oscillators
3.1 Introduction
3.2 Samples and measurement set-up
3.3 Basic Characterization for STNO
3.4 Basic characterization for memristor
3.5 Microwave Modulation based on MSN
3.6 Summary and discussion
4 Magnetization dynamics in all perpendicular spin nano-oscillators with composite Free Layer
4.1 Introduction
4.2 Samples and experimental details
4.3 Results
4.3.1 PSD measured under external magnetic field at different direction
4.3.2 PSD measured at different currents
4.4 Summary and discussion
5 A study on all-optical helicity-independent switching state diagram in GdFeCo alloys
5.1 Introduction
5.2 Samples and measurement set-up
5.3 Results
5.3.1 Magnetization state diagram of GdFeCo
5.3.2 Atomistic modeling for single-shot AO-HIS
5.3.3 AO-HIS state diagrams as a function of the GdFeCo concentration
5.4 Summary and discussion
6 A study on all-optical helicity-dependent switching state diagram in Co/Pt multilayers
6.1 Introduction
6.2 Samples and measurement set-up
6.3 Results
6.3.1 AO-HDS state diagram
6.3.2 AO-HDS process
6.4 Summary and discussion
References

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