Observation of Magnetic Droplets in Magnetic Tunnel Junctions

Magnetic droplets, a class of highly non-linear magnetodynamical solitons, can be nucleated and stabilized in nanocontact spin-torque nano-oscillators where they greatly increase the microwave output power. Here, we experimentally demonstrate magnetic droplets in magnetic tunnel junctions (MTJs). The droplet nucleation is accompanied by a power increase of over 300 times compared to its ferromagnetic resonance modes. The nucleation and stabilization of droplets are ascribed to the double-CoFeB free layer structure in the all-perpendicular MTJ which provides a low Zhang-Li torque and a high pinning field. Our results enable better electrical sensitivity in the fundamental studies of droplets and show that the droplets can be utilized in MTJ-based applications.

2 still a large challenge for conventional STNOs 7 . Magnetic and magnetodynamic structures [13][14][15] , such as vortices 16 , bullets 17,18 , and droplets 19 , may provide one way to increase their power emission as they maximize the use of the available magnetoresistance thanks to their high precession amplitude. Especially, magnetic droplets are observed in nano-contact (NC) STNOs with a strong perpendicular magnetic anisotropy (PMA) 20,21 . The microwave output power of the magnetic droplet mode was reported as 40 times higher than that of the normal ferromagnetic resonance (FMR) mode, mainly due to its large precession angle 7,19,20 . However, all experimental work on magnetic droplets has until now focused on spin-valve (SV) structures 19,20,[22][23][24] , and spin-hall nano oscillators (SHNOs) 25,26 . The very low magnetoresistance (MR) (~1%) in SVs and SHNOs limits the power emission and hence any further use in STNO-based applications. Comparatively, magnetic tunneling junctions with strong PMA (pMTJs) have presented a high tunneling magnetoresistance (TMR) 27 , reaching 249% especially for double-CoFeB free layer (DFL) pMTJs 28 which becomes the main structure in MTJs based MRAM. Therefore, one might expect to observe magnetic droplets in pMTJ based NC-STNOs. Nevertheless, our previous experiment shows that it is difficult to format a stable droplet in a single-free layer (SFL) MTJ 29 , which may result from the large Zhang-Li torque induced by the lateral current. In contrast, the DFL pMTJs 30 are expected to suppress this large Zhang-Li torque and therefore benefit to format a stable magnetic droplet.
Here we experimentally observe and investigate stable magnetic droplets in a DFL pMTJ and find that the droplet enhances the emission power by more than two orders of magnitude with respect to the FMR mode precession in the same device. Furthermore, using micromagnetic simulations, we argue that the stable magnetic droplets in MTJs are mainly due to the combination of the low Zhang-Li torque and the strong pinning field in the DFL. Our findings provide a comprehensive understanding of the nucleation of magnetic droplets in MTJs. It will pave the way for further optimization for use of magnetic droplets in MTJs. Figure 1a shows a schematic and stack information of our NC-STNO device. The film stack is composed of a [Co/Pt]-based pinned layer (PL), a CoFeB reference layer (RL) and a CoFeB/W/CoFeB coupled DFL, both with strong PMA. Figure 1b shows the out-of-plane (OOP) and in-plane (IP) magnetization hysteresis loops of a corresponding unpatterned film. The IP hysteresis loop shows a typical hard-axis response while the OOP hysteresis loop exhibits three distinct switching fields corresponding to the switching of the RL, DFL, and PL, respectively. The saturation magnetization Ms of the DFL is about 987 kA/m. The PMA field µ0Hk is 120.9(8) mT (see Figure S1 in Supporting Information). A TMR ratio of 12.6% is measured in the fully processed STNO (Figure 1c).
After having confirmed the static behavior, we study their magnetodynamics in and subsequent re-nucleation 31 . We found that the low-f noise, weak dependence of field and current, presents an obvious peak at ~200 MHz, which is far away from the 1/f noise caused by the spin-transfer torque (STT) induced incoherent precession 32,33 .
Meanwhile, the ~GHz signal, low-f noise could exist till -50 mT for -2. shown in Figs. 3a-b, respectively. When the field is swept in the negative direction, the magnetization of the DFL stays AP to the RL's magnetization from 30 till -5 mT without STTST as shown in Figure 1c. However, since the STT from the negative applied current favors the AP state, the magnetization instead tends to maintain the original AP state underneath the NC area, while only the DFL area outside of the NC switches to the P state. This partial switching of the magnetization induces an intermediate resistance state, which is slightly higher than the resistance of the droplet and is completely void of any accompanying low-f noise. This novel partially reversed state is hence not a precessing droplet, but more likely a "static magnetic bubble" with a reversed core, which is similar with SV-based NC-STNOs 35 , where the current density is not enough to keep the droplet in a state of precession. The bubble state remains stable until the external magnetic field is swept down to -14 mT. By increasing the fields, the pressure on the static bubble makes it shrink until it again transforms into a precessing droplet with a step-like decrease of resistance and the reappearance of lowf noise. The transformation from a static bubble to a precessing droplet may result from the STT becoming strong enough to again compensate for the damping torque as the magnetic field pushes the bubble perimeter into the NC region. Besides, the field sweep from negative to positive shows a similar property.
As for the current sweep in Figure 3b, the P state switches into a droplet state with a low-f noise and step-like increase of the resistanceat a threshold current of Ith = -1.57 mA. The small near ~1 GHz signals of the magnetic droplet also could be observed during the current sweep as shown in Figure 3b. The magnetic droplet is stable while the current is further decreased to -2.2 mA. When the current is swept back, the droplet still exists even though the current is much lower than the threshold current Ith of nucleation of a droplet. As the low-f noise damps out at around -0.9 mA, the dynamic droplet starts to slow down, and turns to a static bubble at further lower current with the disappearance of the ~1 GHz signal. The obvious hysteresis indicates the different magnitude of stimulus that is required for the droplet nucleation and annihilation 20 .
We note that the threshold current density of droplet nucleation in our devices can be as low as ~5 MA cm -2 with a magnetic field of ~20 mT which is quite less than that in the SVs (~130 MA cm -2 under 0.25 T) 22 . Such low current density could be associated with i) slightly lower damping (0.02 for CoFeB here, 0.03 for [Co/Ni] multilayers 22 ); ii) different spin-torque efficiency and iii) different device structures inducing different current distributions, and so on. We would also like to emphasize here that the lower threshold current density for droplets is critical in the MTJs with a power emission of up to 600 pW, providing a ~300 times enhancement (compared with the FMR-mode signal), which is more advantageous than that in SVs or SHNOs (see the chart in supplementary note 4).
In contrast to our previous work on an SFL MTJ 29 , the magnetic droplets are successfully observed in a DFL MTJ here. To clarify the essential difference, we 6 perform the micromagnetic simulations 23,24 to further analyze the stability of magnetic droplets. Usually, the size of the droplet is determined by a combination of factors, such as the external magnetic field, the Zhang-Li torque induced by realistic lateral current spreading 22 , and the pinning field of the free layer. For comparison, we calculated the current distribution in free layers of a SFL MTJ and an SV by COMSOL. The lateral current spread -Jx is much higher in MTJs than that in SVs 22  Moreover, an inhomogeneous thin W insert layer between double-free layers can cause local DMI and non-uniform RKKY distribution, which induces a much higher pinning field compared with an SFL MTJs 30 . To figure out the influence of this pinning field, we performed a simulation for a DFL MTJ with RKKY distribution and DMI in Figure 4g. The range for nucleation of magnetic droplets becomes wider, indicating that the higher pinning field in our DFL MTJ highly stabilizes the observed magnetic droplets.
By using the DFL structure in pMTJs, a magnetic droplet is generated for significant power enhancement. In DFL structures, the low Zhang-Li torque and high pinning field have been identified to be responsible to stabilize the magnetic droplet. The high TMR of MTJs, compared to the low MR of SVs, enables one order of magnitude smaller current density to induce a comparatively higher power emission. Furthermore, the generation of magnetic droplets has been observed to increase the microwave power by at least two orders of magnitude compared to FMR-like modes. The droplet implementation in MTJs provides an effective method to improve the power of STNOs for radio frequency electronics. This new finding could launch an alternative path towards exploring applications of spintronic devices.  with RKKY and DMI distribution, respectively. The horizontal arrows present the current range for stable magnetic droplets. All the simulations are carried out at -10 mT and 300 K. The initial state for the device is P state.