Tuning interfacial spins in antiferromagnetic / ferromagnetic / heavy metal heterostructures via spin-orbit torque

Antiferromagnets are outstanding candidates for the next generation of spintronic applications, with great potential for downscaling and decreasing power consumption. Recently, the manipulation of bulk properties of antiferromagnets has been realized by several different approaches. However, the interfacial spin order of antiferromagnets is an important integral part of spintronic devices, thus the successful control of interfacial antiferromagnetic spins is urgently desired. Here, we report the high controllability of interfacial spins in antiferromagnetic / ferromagnetic / heavy metal heterostructure devices using spin-orbit torque (SOT) assisted by perpendicular or longitudinal magnetic fields. Switching of the interfacial spins from one to another direction through multiple intermediate states is demonstrated. The field-free SOT-induced switching of antiferromagnetic interfacial spins is also observed, which we attribute to the effective built-in out-of-plane field due to unequal upward and downward interfacial spin populations. Our work provides a precise way to modulate the interfacial spins at an antiferromagnet / ferromagnet interface via SOT, which will greatly promote innovative designs for next generation spintronic devices.

devices using spin-orbit torque (SOT) assisted by perpendicular or longitudinal magnetic fields. Switching of the interfacial spins from one to another direction through multiple intermediate states is demonstrated. The field-free SOT-induced reorientation of antiferromagnetic interfacial spins is also observed, which we attribute to the effective built-in out-of-plane field due to unequal upward and downward interfacial spin populations. Our work provides a precise way to modulate the interfacial spins at an antiferromagnet / ferromagnet interface via SOT, which will greatly promote innovative designs for next generation spintronic devices.

I. INTRODUCTION
Antiferromagnets have numerous advantageous properties for future spintronics applications: robustness against external field, no stray fields, and ultrafast spin dynamics [1,2]. Especially, the recent discovery of electrical switching of an antiferromagnet by spin-orbit torque (SOT) shows that antiferromagnets can be electrically manipulated in similar ways to their ferromagnetic (FM) counterparts [3], stimulating considerable research in antiferromagnetic (AFM) spintronics [4][5][6][7][8]. To date, most work has focused on electrical manipulation of bulk properties of AFM materials [3][4][5][6][7][8][9][10][11][12]. Conversely, from the point of view of expanding the functionality and the design flexibility in AFM spintronic devices, developing methods to tune the interfacial properties of AFM materials through SOT is a vitally significant issue.
Exchange bias (EB) refers to a shift in the hysteresis loop along the magnetic field axis due to the interfacial exchange coupling between adjacent FM/AFM layers. This 3 phenomenon has been extensively studied because of its technological importance, for example in read heads for magnetic storage or spin valves [13,14]. Moreover, it offers a unique tool to directly probe the AFM interfacial spin states and the interfacial exchange coupling. EB can be utilized to exert an internal effective field in a heavy metal (HM)/FM system to obtain deterministic SOT switching of a perpendicular magnetic anisotropy (PMA) magnetization [15][16][17][18][19][20] Here, we report the high tunability of AFM interfacial spins by SOT combined with perpendicular or longitudinal magnetic fields in a HM/FM/AFM system. We can effectively switch the AFM interfacial spins between multiple different states, using different combinations of pulsed electrical currents and magnetic fields. Moreover, an irreversible SOT-induced reorientation of AFM interfacial spins in zero magnetic field is demonstrated. The realization of AFM interfacial multi-state spin switching via SOT with or even without external fields will enlarge the designability of AFM spintronics.

II. EXPERIMENTAL
The stack structures of Ta (0.6)/Pt (3)/Co (0.8)/Ir25Mn75 (t)/Ta (2) (thickness in 4 nanometers) with t = 5, 6, 7, and 8 nm were deposited on thermally oxidized Si substrates by magnetron sputtering at room temperature. The bottom and top Ta layers were used for adhesion and capping layers, respectively. The base pressure was less than 1 × 10 -8 Torr before deposition, and the pressure of the sputtering chamber was 0.8 mTorr during deposition. No magnetic field was applied during the sputtering. The deposited rates for Ta, Pt, Co, and Ir25Mn75 films were controlled to be ≈ 0.016, 0.025, 0.012, and 0.015 nm/s, respectively. After that, the samples were patterned into Hall bar devices with channel widths of 10 μm by photolithography and Ar-ion etching. For field-annealing treatments, the fabricated devices were annealed at 250 o C for 30 min at a base vacuum of 1 × 10 -7 Torr under out-of-plane [along z direction in Fig. 1 magnetic field of 0.7 T, then were field-cooled to room temperature, by using oven for

III. ANTIFERROMAGNETIC LAYER THICKNESS DEPENDENCE
Experiments were performed on Ta(0.6)/ Pt(3)/ Co(0.8)/ Ir25Mn75(tIrMn)/ Ta(2) (in nm) stacks, with tIrMn = 5, 6, 7 and 8 nm, as schematically illustrated in Fig. 1(a).  The as-grown Hall bar samples were then subjected to a sequence of current pulses along the x direction, of varying amplitude Ip and fixed width 50 ms, in a longitudinal applied field Bx = 0.1 T [ Fig. 1(b)]. Through the spin Hall effect (SHE), a charge current in the ± x direction should produce a spin polarization along the ± y direction for the positive spin-Hall angle of Pt [25]. The resulting spin current can switch the magnetization of PMA Co between the ± z directions, provided that both the current density and Bx are large enough. Moreover, the absorption of transverse spin currents is found to vary with the FM thickness with a characteristic saturation length of 1.2 nm [26]. Thus in our devices, not only the 0.8 nm thick Co layer but also the AFM interfacial spins can be directly affected by SOT. Fig. 1 Fig. S1). The SOT induced EB switching is also found for tIrMn = 7 nm but not for tIrMn = 5 and 6 nm (see Supplementary Fig. S2). Two-step hysteresis loops, similar to the one shown in Fig. 1(f), are commonly observed in as-deposited or zero-field cooled FM/AFM bilayers. They are related to the occurrence of a bi-domain state, in which the two domain populations are oppositely exchange biased due to opposite orientations of the uncompensated AFM spins at the FM/AFM interface [27,28]. The switching behavior observed in Fig. 1(h) is consistent with a change in the populations of the two domain types, due to a reorientation of interfacial AFM spins during the current pulse.
The effect of the Joule heating on the exchange bias reversal must be considered [20]. To estimate the temperature rise due to Joule heating, the resistance of the sample was measured during the current pulse for the sample with tIrMn = 7 nm. By comparing this to the measured temperature-dependence of resistance, a temperature rise of around 35 K was estimated for a 26 mA 50 ms current pulse (see Supplementary Fig. S3). In contrast, the blocking temperature for the tIrMn = 7 nm sample, defined as the temperature where the EB disappears, is around 450 K (see Supplementary Fig. S4).
Therefore, we rule out a significant role of Joule heating in the observed switching.
Furthermore, IrMn alloys were reported to have a spin Hall angle with the same sign as that of Pt but with smaller value [29,30]. From the SOT switching data, the dominant contribution is from the bottom Pt layer, moreover, the resistivity of IrMn is about one order bigger than that of Pt, so that the current density in the Pt layer is about ten times larger. Thus the spin current contribution from IrMn can be ignored in this system, We attribute the observed switching to the direct effect of the current-induced SOT on the uncompensated AFM spins at the FM/AFM interface. The spin current due to the SHE in the Pt layer induces a damping-like torque m × (σ × m) (along y direction) and a field-like torque m × σ (along x direction), where m is the interfacial spin moment and σ is the spin polarization of the spin current [31][32][33][34][35][36][37]. When the interfacial spins are deflected from the z direction due to SOT, switching into the direction of the FM layer magnetization will occur. The latter is determined by the relative alignments of Ip and Bx ( Supplementary Fig. S1).

PERPENDICULAR FIELDS
Further investigations were focused on the tunability of AFM interfacial spins through SOT with the assistance of Bx or Bz. Figure 2    The opposite trend can then be seen in Fig. 2(h) and Fig. 2(i), as compared to that in Correspondingly the step in the RH vs. Bz loops gradually moves to higher RH values 11 [ Fig. 3(b)]. The opposite direction of Ip under the same Bx induces the opposite shift of the magnetization step, as shown for the Ip = -20 mA loop in Fig. 3(b).
The shape of the RH vs. Bz loop can be further controlled via SOT with varying Bz, as shown in Fig. 3(c). Here, the initial state was set by applying , is plotted versus Bz in Fig. 3(d). Two distinct behaviors are observed: the switched fraction increases sharply to~82 % with Bz from 1 to 5 mT (region I), and then gradually increases to 100 % with further increasing Bz (region II). The curve's slope for region I is about two orders of magnitude higher than for region II.
Significantly, the switched fraction of 82% marked by the dashed line in Fig. 3(d) at the boundary between regions I and II is close to that for Ip  22 mA with longitudinal field, seen in Fig. 3(b). Therefore, the SOT induced switching under Bx is only effective up to the upper limit of region I.
To understand the reason for the formation of these two regions, it is necessary to consider the influence of the antiferromagnetic domain structure of the IrMn bulk. This may result in the pinning of a part of the interfacial spins in directions deviated from z.
The interfacial spins with effective spin moments along in-plane direction cannot be changed to perpendicular direction through SOT under Bx, resulting in a clear step in the RH vs. Bz curves in Fig. 3(b) for saturated Ip. Whereas, the SOT under large Bz can 12 flip all the spins to z direction with assistance of the strong Zeeman interaction. Hence, the interfacial spins which remain unswitched under in-plane fields, corresponding to the boundary between regions I and II, could be related to in-plane pinning by IrMn domains.
Furthermore, we found that positive and negative Ip have nearly the same effect on the switching under Bz [see Supplementary Fig. S5(a)]. This is consistent with our interpretation of the switching as being due to the direct effect of the SOT on the AFM interfacial spins. A deflection of the interfacial spins from the perpendicular direction due to a current-induced SOT of either sign will enable their switching into the direction of Bz, in order to minimize the Zeeman energy. Accordingly, a smaller SOT (due to smaller pulsed current) requires a larger Bz to flip the interfacial spins, as shown in Supplementary Fig. S6. The effect of SOT with varying Bx on the interfacial spin configuration is also observed. As shown in Fig. 3(e) and Fig. 3(f), with changing Bx from -100 to -10 mT, the RHstays nearly constant while the RH + gradually reduces. Therefore, the SOT switching is gradually reduced with decreasing negative Bx. Similarly, the RH vs. Ip curves with varying Bx from 100 to 10 mT exhibit a constant RH + and a gradual change of RH -(see Supplementary Fig. S7). However, after annealing the sample in a magnetic field along z, the switching is found to be only weakly dependent on Bx in the range 10-100 mT (see Supplementary Fig. S8), because the field-annealing induces an outof-plane effective field which can assist the SOT switching. Therefore, the switching can take place in quite small Bx in the field-annealed case.

SWITCHINGREORIENTATION
We also observed a modification of the RH vs. Bz loop induced by current pulses in zero external field for the as-grown sample with tIrMn = 8nm [ Fig. 4(a)]. The initial state was set by applying Ip = 22 mA under Bz = -0.2 T, and subsequent loops were obtaining after applying Ip of varying magnitude under zero field. As shown in Fig. 4(a), after pulsing in zero field the RH vs. Bz behavior transforms from a single-step loop similar to Fig. 2(g) for the initial state, to a two-step loop similar to Fig. 2(b). The effect is much more pronounced for the field-annealed sample [ Fig. 4(b)]. Comparing the switched fractions versus Ip in Fig. 4(c), a smaller threshold Ip and a much larger switched fraction is observed for the field-annealed sample. The saturated state after zero-field SOT in Fig. 4(b) is close to the initial state of the as-grown device [see Fig.   2(a)], with nearly equal upward and downward parts of the loop.
In HM/FM systems with PMA, it is necessary to break the symmetry between up and down magnetization directions in order to generate deterministic switching using SOT. Typically this is achieved by applying an in-plane magnetic field collinear with the electric current, but a lateral asymmetry [33], tilted magnetic anisotropy [38], antiferromagnetic layer [15], polarized ferroelectric substrate [35], interlayer exchange coupling [16,39], interfacial spin-orbit interaction [40] or competing spin currents [41] have also been introduced to achieve field-free deterministic switching. In our system, the field-free SOT induced interfacial spin switching reorientation should be related to the inequivalent upward and downward domain populations, which produces an 15 effective out-of-plane field (Bz-eff). It can be considered as a training effect, in which the built-in Bz-eff assists the SOT to switch alter the interfacial spins from a metastable single-domain state, to an equilibrium state with incomplete alignment of the interfacial spins. As the net Bz-eff is zero for the equilibrium state, the field-free effect is not reversible on reversing the current, which is different from the field-free SOT magnetization switching in Refs. [15,16,33,35,[38][39][40][41]. Instead, it provides an efficient way to initialize the interfacial spins via SOT at zero field.  16 We have demonstrated a high controllability of the spin states at the FM/AFM interface via SOT. Multi-state switching is achieved using SOT in combination with external magnetic fields Bz or Bx, while field-free variation from the fully aligned state was also realized. Our work provides a very efficient scheme for tuning of the uncompensated antiferromagnetic interfacial spin states via SOT, which will expand the designability of spintronic devices. For instance, the SOT-magnetic random access memory (MRAM) can potentially be realized by varying the FM/AFM interface via SOT, in contrast to the conventional design. Multiple resistance states and thus high density storage may be achieved in this SOT-MRAM cell. Furthermore, combining with the conventional field-annealing and the pulsed electrical currents approaches will open up more potential applications in spintronic devices. For example, if the EB is initially set along a preferred in-plane axis (x or y) by field-annealing, the current pulses can induce EB in perpendicular direction without disturbing the in-plane EB. For magnetic sensors containing many cells with different exchange bias directions, the current-pulse offers a convenient approach to tune the EB in different directions, respectively. In addition, the precise control of interfacial spins at a FM/AFM interface by SOT might result in a multi-state perpendicular ferromagnet, which has a potential application in a synaptic emulator for neuromorphic computing.