Introduction

Oscillators are basic components of many communication, navigation and measurement systems. They provide a periodic output that can be used to generate electromagnetic energy for radiation, for high speed digital systems, clocking functions and to enable frequency up and down conversion if used as a local oscillator in communication circuits and for many other applications. There is a strong interest, driven by cost and performance, to develop improved microwave oscillators for on-chip integration and the spin-transfer nano-oscillator (STNO) is a promising candidate for this due to its combination of characteristics such as frequency tunability, nanoscale size, broad range of working temperature and relatively easy integration with complementary metal-oxide-semiconductor (CMOS) technology1,2,3,4,5,6,7. The STNO is not only one of the more complex non-linear dynamical systems (of great interest from fundamental point of view), but it is also important being one of the smallest auto-oscillators known in nature (hence of great interest from technological point of view). The remaining challenges that need to be addressed are the simultaneous optimization of a variety of properties, including high frequency operation, frequency tunability, narrow spectral linewidth, large output power and operation in both absence of external magnetic fields and with low applied current densities.

Here, we demonstrate that the interfacial perpendicular anisotropy (IPA) between the ferromagnetic electrodes and the tunnel barrier of magnetic tunnel junctions (MTJs) employing the material combination of CoFeB/MgO is a key ingredient to improve the dynamic properties of STNOs8. In general, IPA can be used for the design of high efficiency spintronic devices. It permits the realization of MTJs for storage applications with high thermal stability and low switching current8,9 and STNOs with very large emitted power (i.e. > 0.95 μW delivered to a matched load)10. In previous studies, IPA was used to orient the magnetization of both the free and polarizer layers out of the film plane8, or to reduce the out-of-plane demagnetizing field (Hd) while maintaining the orientation of both of the two magnetizations in the film plane10. In this work, we designed STNOs with an asymmetric MTJ consisting of an in-plane polarizer and a perpendicular free layer achieved by controlling the trade-off between IPA and Hd. This orthogonal magnetic configuration is different from the one in our previous study10 and is more efficient in order to excite large angle free layer precession (large output power) in absence of a bias magnetic field. The main objective of our experiment is to solve the key technological issues related to the integration of STNOs with CMOS technology: elimination of the need for an external magnetic bias field and reduction of drive current densities. The former goal will allow for eliminating the current lines necessary to create the magnetic field for the STNO biasing, while the latter is necessary to reduce the size of the current driving transistors (small currents allow for smaller transistors, hence making the overall oscillator smaller. For high-current oscillators, the actual size of the oscillator would be determined by the transistor size, rather than by the MTJ) and to control power dissipation.

Results

Spin-transfer torque oscillator

The samples studied have a core magnetic stack consisting of a synthetic antiferromagnet (SAF) Co70Fe30/Ru/Co40Fe40B20 layer and a Co20Fe60B20 free layer (FL) separated by a MgO insulator as shown in Fig. 1a. We introduce a Cartesian coordinate system where the x-axis is the direction of the polarizer (+x), the y-axis and the z-axis are the hard in-plane and the out-of-plane axes respectively. The SAF layer is designed to have an in-plane easy axis9,10 serving as a polarizer, while the Fe-rich Co20Fe60B20 FL with a thickness of t = 1.60 ~ 1.62 nm was chosen to achieve the proper IPA, which favors the out-of-plane (perpendicular) magnetic configuration in the free layer (see Methods). Electron-beam lithography and ion milling were used to define and etch the MTJs resulting in pillar-shaped devices with nominal dimensions of 150 nm × 70 nm. The samples are different from the 50–70 nm diameter point contact spin valves with out-of-plane magnetized CoNi free layers studied in refs. 11, 12. A d.c. bias current is injected into the sample through a bias Tee as shown in the measurement setup in our previous work10, where we define the positive current I as electrons flowing from the polarizer to the free layer. A time-varying voltage produced by the oscillations of the magnetization via the tunneling magnetoresistance (TMR) effect is recorded using a 9 kHz–26.5 GHz spectrum analyzer. The measurements were carried out at room temperature. We show detailed data from a single device with free layer thickness t = 1.60 nm, however similar results have been achieved for more than 5 devices for each free layer thickness.

Figure 1
figure 1

Sample structure and properties.

(a) Schematic of the sample layer structure consisting of an in-plane magnetized fixed (polarizer) layer and an out-of-plane magnetized free layer. (b) Resistance as a function of in-plane magnetic field (H∥) and perpendicular magnetic field (H⊥) for sample 1 (t = 1.60 nm), inset in (b) is the resistance as a function of H⊥, the black (red) arrow denotes the magnetization direction of the reference (free) layer. (c) Resistance-Current curve at zero applied magnetic field, AP (P) denotes the antiparallel (parallel) configurations between the free and fixed layers. (d) Microwave spectra as a function of d.c. current bias I at zero applied magnetic field, the curves are offset by approximately 20 nW GHz−1 along the vertical axis for clarity. Inset: full width at half maximum (FWHM, or linewidth) (triangles) and f0 (circles) of the STNO sample 1 as a function of I.

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Magneto-resistance transport properties

Figure 1b shows the resistance of a typical nanopillar device as a function of the external in-plane field (H||) at a sub-critical bias current of I = 10 μA, revealing a TMR ratio of 79%. As H|| increases from −700 Oe to +1100 Oe, the resistance increases gradually as the magnetization of the free layer changes gradually from parallel to antiparallel relative to the polarizer. One source of asymmetry of the field scan is the dipolar coupling between the polarizer and the free layer, which (at low bias currents and zero external field) induces the free layer magnetization to tilt at a small angle away from z-axis (Fig. 1a). The additional switching near H|| = −700 Oe and +1100 Oe corresponds to reorienting of the polarizer magnetization. The resistance curve scan as function of the out-of-plane field (inset of Fig. 1b) clearly indicates the perpendicular free layer.

Spin-transfer torque dynamics

The devices exhibit dynamical behavior in a range of external fields (see Supplementary Fig. S1 online), but in the rest of the paper we will focus on the data measured at zero bias magnetic field, being the most important result of this experiment. Fig. 1c shows the resistance versus applied current I, an intermediate resistance (IR) state is seen in the range of I = −0.34 to + 0.65 mA, while the high resistance state (AP) for I < −0.34 mA and the low resistance state (P) for I > 0.65 mA are observed. The large-power microwave emission is only observed in the IR state and for negative currents, i.e. B1 regime. We focused on frequency-domain measurements to establish the existence and study the characteristics (oscillation frequency f0, linewidth Δf0 and output power) of these STNOs.

Fig. 1d displays typical microwave power density spectra obtained for different values of I for sample 1 (t = 1.60 nm). For all of the current values (|I| ≤ 0.3 mA), a single peak (with Lorentzian line shape) is observed in the GHz-range that is characterized by the excitation of an out-of-plane oscillation mode as indicated by the micromagnetic simulations (see Methods). The oscillation frequency exhibits a red shift, consistent with previous studies in point contact geometries with in-plane polarizer and out of plane FL11, with a slope of 1.75 GHz mA−1 and a minimum linewidth of 28 MHz (inset Fig. 1d). The estimated threshold current Ic is ~ −45 μA (see Supplementary Note 2 online)13, which corresponds to a current density of Jc = −5.4 × 105 A/cm2, this value can be further reduced by controlling the thickness of the free layer t, e.g. at t = 1.62 nm Jc = −1.2 × 105 A/cm2 (Ic = −10 μA), which is to the best of our knowledge the smallest value measured to date. The observation of the ultralow Jc value can be explained as follows. In the presence of the IPA the Jc is approximately proportional to the effective demagnetizing field14 (, where is the IPA in the FL, which increases with decreasing the FL thickness8,9). In our case, the strong IPA in the FL significantly counteracts the effect of Hd, resulting in the ultralow Jc and is responsible for the strong dependence of Jc on the FL thickness. As can be observed, the effect of the FL thickness on IPA has a non-trivial influence on the dynamical properties of the STNOs (for example a larger thickness corresponds to a smaller critical current for ) and a complete systematic study will be presented elsewhere. Fig. 2 displays a comparison between the oscillation power as a function of the current for the two thicknesses t = 1.60 and 1.62 nm. A complete set of data for t = 1.62 is included in the Supplementary Note 3 online (the dynamical properties of the STNO for t = 1.60 and 1.62 nm are summarized in the last two rows of Table 1). The maximum oscillation power is larger at t = 1.60 nm with the best result achieved for a current I = −0.3 mA where the measured output power is 18 nW (> 60 nW delivered to a matched load, see Supplementary Note 4 online) with an oscillation frequency and a linewidth of 850 MHz and 73 MHz, respectively. In the absence of external fields, an additional property is the reduction of total power dissipation, at the best working condition for t = 1.60 nm the dissipated power is of the order of 50–70 μW which is a very small value for oscillators at sub-micrometer size. Hence, these results open the possibility to design nanoscale oscillators with improved dynamic performance and lower power dissipation.

Table 1 Comparison among the performance parameters (external field, critical current density, oscillation power, oscillation frequency and the minimum linewidth) of different STNO solutions and the ones reported in this work at zero field and for two thicknesses t (1.60 nm and 1.62 nm). Here Pmes is the measured maximum power integrated from the oscillation peak, Pmax is the maximum power delivered to a matched load and the hyphen (−) indicates cases where no data are available
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Figure 2
figure 2

Power of STNO samples.

Integrated power of fundamental signals at zero applied magnetic field as a function of I. Olive circles are for 1.60 nm sample and blue squares for 1.62 nm sample.

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Comparison with simulations

We performed micromagnetic simulations considering the same experimental framework to identify the origin of the steady-state magnetization oscillation. The simulations have been performed by solving the Landau-Lifshitz-Gilbert-Slonczewski equation (See Methods: Micromagnetic simulations). As in the experimental data, the magnetization precession is observed at negative currents and it is related to the excitation of a mode with an out-of-plane oscillation axis and a trajectory which is initially circular and expands (i.e. the output power increases) as the current increases (compare trajectories at −82 and −164 μA in Fig. 3 (left)). At large currents the oscillation axis moves towards the x-y plane (trajectory at −288 μA) and for values of I < −0.34 mA the AP-state is obtained as in the experimental data. The magnetization dynamics are characterized by the excitation of a spatially quasi-uniform mode (see also Supplementary Video online). Fig. 3 (right) shows two examples of snapshots at I = −0.3 mA, the arrows indicate the in-plane component of the magnetization while the colors are related to the mx (blue negative, red positive). At high currents (I < −0.3 mA), the simulations show that for some range of time the dynamics is switched off (see Supplementary Fig. S4 online). We believe that the presence of this behavior could be the origin of the non-uniform dynamics (experimental low frequency tail) measured in the high current regime (see Supplementary Fig. S3 online). We observed that this behavior is more evident in the range of thickness where the IPA is comparable with the out-of-plane demagnetizing field. The simulations also predict a decrease in the oscillation frequency as a function of I at a rate of ~1.8 GHz mA−1, this value is consistent with the experimentally observed rate of ~1.75 GHz mA−1 (Fig. 4).

Figure 3
figure 3

Micromagnetic simulations for sample 1.

Left. Trajectories of the average magnetization vector on the unit sphere as computed from micromagnetic simulations (I = −82, −164 and −288 μA). Right: example of two snapshots of the spatial distribution of the magnetization indicating the uniform dynamics (the color means the x-component of the magnetization blue negative, red positive).

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Figure 4
figure 4

Dependence of microwave frequencies on current for sample 1.

The blue circles show experimental data as a function of current bias at zero applied magnetic field. The black squares show the results from micromagnetic simulations.

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Discussion

Microwave emission with large output power (in the presence of external magnetic fields and large drive current densities) has been already measured in STNOs with IPA10,15. However, to the best of our knowledge this work is the first experimental demonstration of large oscillation power without bias field and with ultralow current density Jc below 6 × 105 A/cm2. For microwave generation in the absence of external magnetic fields, various other solutions have been recently proposed. For example, the idea based on a wave-like angular dependence of the spin torque16, the incorporation of a perpendicular polarizer into spin-valve structures17, magnetic vortex oscillations6,18 or a tilted free layer19, but the resulting microwave power in those STNOs is smaller (<1 nW) and the excitation current is higher (>106 A/cm2)11,16,17,18. When compared with the best results obtained from previous field-free STNOs, the data in this work show microwave emission with output power at least one order of magnitude larger and critical current densities one order of magnitude smaller. Table 1 compares the dynamical properties of STNO solution achieved without magnetic field. Importantly, the oscillation frequency is of the same order as that obtained from other types of STNOs. Finally, the STNOs in this work exhibit a tunability of ~1.75 GHz mA−1 that is substantially larger than the ones measured in spin-transfer driven vortex oscillations (0.03 GHz mA−1 in Ref. 6 and 0.08 GHz mA−1 in Ref. 18). This, however, comes at the cost of the larger linewidth compared to those obtained in magnetic vortex self-oscillations6,18. The origin of the linewidth is related primarily to thermal fluctuations and to the coupling between the oscillator phase and power as in other STNOs13. One possible remedy is the use of these STNOs as a basis of an array of phase locked STNOs, in that scenario it is expected that a significant decrease in the linewidth and an increase in the output power to over the tens of μW20,21 could be achieved. A different solution would be the application of a low frequency current modulation as demonstrated in Ref. 22.

The possibilities opened by these results will eliminate some of the key issues related to on-chip integration of STNOs with CMOS technology. This direct integration and the reduced power consumption may potentially open applications in portable electronic devices and wireless modules such as embedded communications and power efficient local clock signal generation in digital systems. Our findings also provide a key ingredient in the development of ultralow–critical-current and zero-field spin-wave sources in magnonic logic devices23.

Methods

Sample preparation

The magnetic stacks were deposited by sputtering in a Singulus TIMARIS PVD system and contain a layer structure: PtMn (15)/Co70Fe30 (2.3)/Ru (0.85)/Co40Fe40B20 (2.4)/MgO (0.8)/Co20Fe60B20 (1.5 ~ 2.0) (the thicknesses are in nm). Details of the growth and fabrication process are similar to that in our previous work9. Note that the IPA gradually increases as the Co20Fe60B20 FL thickness (t) decreases8,9. The direction of the FL magnetic moments is mainly determined by the competition between the IPA () and the Hd. For a thicker FL, e.g. t = 1.70 nm10, it has been verified that the magnetization is in-plane and it requires an external magnetic field to produce microwave signals. By contrast, the thinner t < 1.60 nm results in a larger and consequently a larger critical current density. We find that, the microwave oscillations with no external field exist in a thickness range from 1.55 to 1.65 nm and that in particular, for the FL thickness between 1.60 and 1.62 nm, better microwave performance in terms of power and drive current density is achieved.

Micromagnetic simulation

We numerically solve the Landau-Lifshitz-Gilbert-Slonczewski equation which also includes the field-like torque term TOP24,25. TOP is considered dependent on the square of the bias voltage26 up to a maximum value of 25% of the in-plane torque computed for a current density |J| = 6.0 × 106 A/cm2. The total torque, including also the in-plane component TIPis:

where g is the gyromagnetic splitting factor, is the gyromagnetic ratio, is the Bohr magneton, q(V) is the voltage dependent parameter for the perpendicular torque, is the spatial dependent current density, V is the voltage (computed from Fig. 1c in the main text), t is the thickness of the free layer and e is the electron charge27. The effective field takes into account the standard micromagnetic contributions (exchange, self-magnetostatic) and the magnetic coupling between free and polarizer layer as well as the Oersted field due to I. The presence of the IPA has been modeled as an additional contribution to the effective field. The parameters used for the CoFeB have been: saturation magnetization Ms = 1.1 × 106 A/m, IPA constant ku = 7.4 × 105 J/m3, exchange constant A = 2.0 × 10−11 J/m, damping parameter α = 0.01. The polarization function has been computed by Slonczewski28,29, being not dependent on the bias voltage, where m and mp are the normalized magnetizations of the free and polarizer layer, respectively. To fit the critical current density, we use for the spin-polarization the value 0.5730. Micromagnetic simulations provide useful insight into the nature of the excited mode in the STNO with IPA. They indicate the excitation of an out-of plane mode characterized by a quasi-uniform spatial distribution of the magnetization.