# Spintronic RF-Direct on-off Keying Modulation Using a Frequency Division Multiplex
## 2 Wireless Spintronic Communication  
The spin transfer torques of a free ferromagnetic layer are described in terms of magnetization dynamics, M (r,t), and can be derived by the Landau-Lifshitz-Gilbert (LLG) equation which calculates the precession of the spin-polarized magnetization from dipolar interactions and damping [12; 13]. The STNO is operated through the spin transfer torques in a spin nano-pillar by flowing the current across a spin nano-pillar, and/or applying a magnetic field around a spin nano-pillar, which is comprised of three layers: the free ferromagnetic layer, the spacer, and the pinned ferromagnetic layer. The current passing through the polarized magnetization can generate a spin wave signal with angular momentum in the free layer caused from an effective negative damping. For the wireless communication applying an STNO, a spin torque magnetic tunnel junction device is fabricated. Figure 1 shows a block diagram of a fabricated STNO structure, and the measurement setup. A stacking structure of Co\({}_{6}\)Fe\({}_{2}\)B\({}_{2}\) (2 nm depth) is deposited on the synthetic structure pinned layer by a magnetron sputtering process as a free layer. The pinned and free layers are separated by a thin MgO (1 nm depth) layer as a tunnel barrier. All of the stacked layers are manufactured by a single deposition process. After an e-beam masking process, ion milling etching process is carried out, and the etch is stopped in the middle of the CoFeB pinned layer. At that time, the junction size is decided as 90 \(\times\) 100 nm\({}^{2}\). A post annealing process is carried out over 30 min at 300 \({}^{\circ}\)C, under a 4 kOe magnetic field. Figure 1: A block diagram of a fabricated STNO structure, and the measurement setup. For measuring an STNO, a C-type magnetic closed circuit was placed at the center of the probe station for a function of the applied magnetic field. A constant current was applied to the electromagnet and a fringing field was generated at the core gap. The fabricated magnetic tunnel junction (MTJ) was located at the center of the core gap. Thus, magnetic flux line is aligned in parallel to the wafer plane. This magnetic flux line is also mechanically rotated from 0 to 360 degrees. The magnetoresistance (MR) of MTJ device has a dependence of the applied magnetic field. When the magnetic field is applied, the MR is appeared into one of two states, anti-parallel (AP) or parallel (P) [14]. The MR of the fabricated MTJ between AP and P relation of magnetization was found to be from 48 \(\Omega\) to 80 \(\Omega\) by a magnetic field from \(-\)250 Oe to 250 Oe. Figure 2a shows the fabricated STNO oscillation characteristics of frequency depending on the magnetic field. Frequency variation is from 2.1 GHz to 5.0 GHz when an external magnetic field is applied from 0 Oe to 350 Oe. The STNO frequency tuning ratio is around 80%, whereas the LC-VCO electrical circuit generally has difficulty obtaining a tuning range above 20%. Figure 2b shows the oscillation frequency can also be set by the rotation field angle at the one magnetic field condition. Figure 2 notices that if the frequency is set into 3.5 GHz and 4.2 GHz, the magnetic field could be adjusted into the 125 Oe and 175 Oe at the same rotation field angle, or the rotation field angle be into 0 degrees and 45 degrees at the 175 Oe same magnetic field. Figure 3 shows that the peak power level and signal linewidth vary between \(-\)71 dBm and \(-\)76 dBm and 130 MHz and 260 MHz, respectively, when an external magnetic field is applied from 0 Oe to 300 Oe at the condition of a zero-rotation field angle, and a 1.5 mA current DC supply flowing through the STNO. In particular, 130 MHz linewidth measured in our fabricated STNO is meaning the phase noise is \(-\)63 dBc at an offset of 130 MHz. Figure 3 shows that the STNO power level and signal purity should be improved for use in wireless communication.