Fabrication of Spin-Transfer Nano-Oscillator by Colloidal Lithography

We fabricate nanoscale spin-transfer oscillators (STOs) by utilizing colloidal nanoparticles as a lithographicmask. By this approach, high quality STO devices can be fabricated, and as an example the fabricated STO devices using MgO magnetic tunnel junction as the basic cell exhibit current-induced microwave emission with a large frequency tunability of 0.22GHz/mA. Compared to the conventional approaches that involve a step of defining nanoscale elements by means of electron beam lithography, which is not readily available for many groups, our strategy for STO fabrication does not require the sophisticated equipment (∼million dollars per unit) and expensive lithography resist, while being cost-effective and easy to use in laboratory level. This will accelerate efforts to implement STO into on-chip integrated high-radio frequency applications.


Introduction
Spin-transfer oscillators (STOs) have recently aroused tremendous and continuous research interest due to their nanoscale size, frequency tunability, broad temperature operation range, and Si technology compatibility [1][2][3].STO devices utilize spin-transfer torque (STT) generated by a spinpolarized current traversing a nanoscale magnetic multilayer [4,5], such as spin valve and magnetic tunnel junctions (MTJs) that consist of a free layer and reference layer separated by a nonmagnetic metal or insulator layer.STT can excite steady processions of the free layer magnetization, leading to emission of a microwave signal [6][7][8][9][10].In 2003, the microwave signal generated by STT was firstly demonstrated in Co/Cu/Co spin valve nanopillars [6].However, the output power is far low (on the level of pW).Since that, many efforts, including optimization of device geometry and material structure, and the control of applied magnetic field directions, have been made to improve STO performances, that is, enhancing output power, narrowing spectral linewidth, or removing the need of the external magnetic field [11][12][13][14][15][16][17].
For most experiments performed to date, in order to fabricate nanoscale STO devices, the high-performance electron beam lithography (EBL) system was required [6].However, the expensive EBL system is not readily available to many groups.Furthermore, EBL process is time consuming and limited to small scales.These issues of EBL may partly hinder STO research.
It is well known that the colloidal nanospheres are widely used to fabricate nanostructures with a wide range of sizes [18].Recently, Wang et al. [19] demonstrated that magnetic tunnel junction device can be fabricated by using nanospheres (∼400 nm in diameter) as the lithographic mask.The fabricated MTJs showed an obvious switching for magnetic random access memory.They also mentioned that it is challenging to produce small nanopillars (∼100 nm) due to difficulties in the lift-off of nanospheres after SiO 2 deposition.However, for spin-torque nano-oscillators, in order to observe microwave signal, it is generally required to fabricate the smaller nanopillars (∼100 nm in lateral) for enabling large d.c.current density to excite magnetization.In this study, we used the nanospheres with a diameter of 160 nm as a mask and have fabricated successfully smaller nanopillars by optimizing the lift-off process.The STOs fabricated by this approach exhibited a steady-state magnetization procession, which can be controlled by d.c.current or magnetic field.Our findings show that the strategy of combination of colloidal lithography and optical lithography provides an alternative, simple, and cost-effective method for STO research and applications.

Experimental Details
The Fe 60 B 20 free layer has also an in-plane easy axis at zero external magnetic field, but with strong perpendicular magnetic anisotropy at the interface of the CoFeB layer with the MgO tunnel barrier [11].This perpendicular anisotropy can partially cancel the demagnetization field in the free layer [13]; thus this structure in essence enables to excite large-amplitude magnetization precession under a small bias current.
Standard photolithography and Ar ion milling were used to pattern the contact pads and the 6 × 50 m 2 mesas as shown in Figure 1(a).Then the second photolithography step was introduced to open a window that allows the polystyrene nanospheres to adhere to the top of the mesas.The photoresist was also used to protect the contact pads from the nanospheres and SiO 2 insulator deposition.The next step is a key part of the fabrication process that defines the MTJ nanopillars by colloidal lithography.The nanospheres with a diameter of 160 nm (see Figure 2(a)) were suspended in a deionized-(DI-) based solvent by ultrasound 3 minutes in order to improve dispersity.Before coating spheres, the wafer was coated with hexamethyldisilazane (HMDS) that strengthens adhesion between wafer and nanospheres; the spheres were spin-coated on the surface to act as etching mask for subsequent etching (Figure 1(b)).After that the Argon ion milling was used to etch the materials without protection by photoresist and nanospheres (Figures 1(c) and 2(b)).This was followed by depositing a 50 nm SiO 2 insulator layer by Inductively Coupled Plasma CVD (ICP-CVD) system at low temperature (∼75 ∘ C).Next, the wafer was dipped in dichloromethane and resist-remover to remove the spheres as well as the photoresist (Figures 1(d) and 2(c)).The fabrication of devices is completed by contacting the nanopillars and the two ground contacts to form a Ti (10 nm)/Au (100 nm) ground-signal-ground (GSG) waveguide, deposited using electron beam evaporation and patterned using optical lithography and lift-off (Figure 1(e)).The final device is shown in Figure 2(d).
The measurement setup of STT-induced microwave oscillation is similar to that described in the previous work [13].In the experiments, the d.c.current was applied to the device through a bias Tee, where we define the positive current  d.c. as electrons flowing from the polarizer to the free layer.The microwave voltages generated by STT were recorded by a spectrum analyzer with an amplification of 40 dB and a baseline taken at  d.c.= 0 mA was subtracted from the measured spectral data.All measurements were carried out at room temperature.

Results and Discussion
Before microwave signal generation study, we measured the transport properties under an external magnetic field for understanding the magnetization states.Figure 3(a) presents a magnetoresistance loop corresponding to the switching of the free layer between the low-resistance ( P ) parallel state and the high-resistance ( AP ) antiparallel alignment with a bias current  d.c.= 10 A.The TMR ratio, defined as TMR = 100 × ( AP −  P )/ P , and the resistance-area (RA) product in the parallel state were 93% and 1.1 Ωm 2 , respectively.The low RA allows the sufficient d.c.current to excite the microwave oscillations [11].A small loop shift was observed, which is due to the ferromagnetic coupling through barrier roughness and antiferromagnetic dipolar interaction between layers [9].All devices exhibited microwave signal generation when driven with sufficiently large d.c.current, even without the application of external magnetic field.For the device presented above, we observed obvious high-frequency signal for currents above  d.c.= 0.3 mA, which corresponds to a critical threshold current density ( th ) of 1.5 MA/cm 2 for microwave generation.Note that the  c value can be reduced if the thinner CoFeB free layer is used since the interfacial perpendicular anisotropy between the CoFeB electrodes and the tunnel barrier of the MTJ increases with decreasing the thickness of the CoFeB free layer [20].For currents above 0.3 mA, two peaks with Lorentzian line shape were observed for each current as shown in Figure 3(b).The lowerfrequency peak may be attributed to a centre precession mode while the higher-frequency peak probably arises from a mode with large amplitude at the sides of the nanopillar.Quantitative data are gathered in Figure 4, where we report the frequencies, powers, and linewidths of the centre precession mode as a function of applied current.First, a blueshift in the oscillation frequencies with increasing the d.c.current is observed.For 0.5 mA ≤  d.c.≤ 1.5 mA, the frequencies increase rapidly, resulting in a current modulation capability of 0.22 GHz/mA.Further increasing current, the frequencies exhibit a slight increase.The similar trend has been also observed in other systems [21], which suggests that the steady-state excitation corresponds to an out-ofplane precession.Second, the integrated power of the centre mode increases gradually with increasing the d.c.current and reaches 5 nW at  d.c.= 4 mA.
Figure 4(c) shows the linewidth of the centre mode as a function of the d.c.current.We observed a relatively broad linewidth of 0.74 GHz just above threshold at low currents.For 0.5 mA ≤  d.c.≤ 1.5 mA, the linewidth narrows quasilinearly as the current is increased, while the linewidth narrows slightly above 1.5 mA.This trend can be understood as follows.According to the nonlinear theories [2,22], the linewidth Δ in the above threshold regime depends not only on the frequency nonlinearity  = d/d d.c., but also on the oscillation energy that is proportional to the output power ; that is, Δ ∝ /.It can be seen from Figure 4(a) that the frequency shift decreases, suggesting the frequency nonlinearity  decreases with increasing the currents.Meanwhile, the output power has an ascending dependence on the currents as shown in Figure 4(b).As a result, it is explicit that the current dependence of Δ observed in this study is reasonable.Finally, we study the dependence of the microwave signal on the external magnetic field ( ext ). Figure 5 displays the magnetic field dependence of the central mode frequency at  d.c.= 3 mA.It can be seen that the oscillation frequency can be controlled by adjusting the amplitude of the applied magnetic field.Assuming that the free layer magnetization aligns with applied magnetic field direction in all measurements, the relationship between the frequency  and  ext is expressed by the Kittel formula [23]: where  = 2.8 MHz/Oe is the gyromagnetic ratio and   and 4 eff are the coupling field and the effective

Conclusion
In summary, we have successfully fabricated nanoscale STOs by colloidal lithography and optical lithography instead of the expensive e-beam lithography.The fabricated STO device showed microwave signal generation under d.c.currents.Moreover, the generated microwave signal can be controlled by adjusting the d.c.current or magnetic field.We believe that such a simple and affordable bottom-up approach will drastically speed up the basic research of spin-torque devices and their potential applications.

Figure 2 :
Figure 2: (a) Scanning electron microscopy (SEM) image of polystyrene nanospheres on wafer.(b) SEM image for MTJ nanopillar with nanosphere after Argon ion etching, (c) SEM image of nanopillar after removing nanosphere and SiO 2 , and (d) optical image of the final STO device.

Figure 3 :
Figure 3: (a)  The device resistance as a function of applied magnetic field alone in-plane easy axis under bias current of 10 uA.The orange/blue arrow represents the magnetization direction of the reference/free layer.(b) Current-induced magnetoresistance oscillations for one representative device for positive current between 0.5 A and 4 A with 0.5 A steps at zero magnetic field.A vertical offset of 2 nW/GHz is applied to the curves for visual clarity.

Figure 4 :
Figure 4: (a) Frequency, (b) integrated power, and (c) linewidth of the centre mode as a function of applied current.

Figure 5 :
Figure 5: The oscillation frequency as a function of applied magnetic field at  d.c.= 3 mA.