Storing magnetic information in IrMn/MgO/Ta tunnel junctions via field-cooling

Tunneling junctions containing no ferromagnetic elements have been fabricated and we show that distinct resistance states can be set by field cooling the devices from above the N\'eel along different orientations. Variations of the resistance up to 10% are found upon field cooling in applied fields of 2T, in-plane or out of plane. Below TN, we found that the metastable states are insensitive to magnetic fields thus constituting a memory element robust against external magnetic fields. Our work provides the demonstration of an electrically readable magnetic memory device, which contains no ferromagnetic elements and stores the information in an antiferromagnetic active layer.

2 Magnetic tunnel junctions used in modern hard-drive read heads and magnetic random access memories comprise two ferromagnetic electrodes whose relative magnetization orientations can be switched between parallel and antiparallel configurations, yielding the tunneling magnetoresistance effect. 1 Recently, large magnetoresistance signals have been observed on NiFe/IrMn/MgO/Pt stacks with an antiferromagnet (AFM) on one side and a non-magnetic metal on the other side of the tunnel barrier. 2,3 In these devices, ferromagnetic moments in NiFe are reversed by external magnetic field and the exchange-spring effect of NiFe on IrMn induces the rotation of the AFM moments in IrMn. This is then electrically detected via the measurement of the AFM tunneling anisotropic magnetoresistance (TAMR). The work has experimentally demonstrated the feasibility of a spintronic concept 4,5 in which the device transport characteristics are governed by an AFM.
The lack of magnetic stray fields and the relative insensitivity to external magnetic fields make AFM materials potentially fruitful complements to ferromagnets in the design of spintronic devices.
The zero net moment of compensated AFMs, however, also implies that weak magnetic fields of the order of the typical magnetic anisotropy fields in magnets cannot be directly applied to rotate the AFM moments. In the devices reported in Refs. 2, 3 the problem was circumvented by attaching an exchange-coupled ferromagnet to the AFM electrode to form an exchange-spring. 6 7 have demonstrated that room-temperature AFM TAMR can be achieved in exchange-spring tunnel junctions only in a narrow window of AFM thickness. A subtle balance is required between a thin enough AFM to allow for the exchange-spring rotation of AFM moments across the entire width of the AFM and a thick enough AFM to avoid the decrease of the Néel temperature T N below room temperature by the size effects. We also point out that as a memory element, the exchange-spring AFM tunnel junctions can be disturbed by weak magnetic field perturbations as they still contain a ferromagnetic element.
To fully exploit the potential robustness of the AFM based spintronic device against magnetic fields, we have fabricated magnetic tunnel junctions analogous to those in Ref. 2,3, but without the auxiliary ferromagnetic NiFe layer. In these antiferromagnetic tunnel junctions (ATJs) we show that metastable states can be set by cooling the sample and crossing the Néel temperature in external magnetic fields with different orientations. These metastable states can be detected electrically, due to an analogous effect to the AFM TAMR reported in Refs. 2,3,7. Since our field-cooling approach for writing does not require any ferromagnetic layer, the limitation on the AFM thickness is As calorimetry measurements showed that T N of a 2 nm thick IrMn is reduced to ∼173 K (see Fig.   3 and discussion thereafter), we performed the field cooling procedure from room temperature down to 120 K, thus covering a sufficiently large temperature range around T N . In the case of data reported in the main panel of Fig. 2, we applied an external field µ 0 H z of ±2T oriented 5 perpendicular to the sample surface (scattered and red curves) and µ 0 H x = +2T oriented in the plane of the sample, along the [100] axis of the STO substrate (thick green curve). The RA product (measured at a fixed bias of 20 mV) is identical for the three field orientations in the temperature interval from 300 K to approximately 170 K, while below this temperature the RA traces for out-ofplane and in-plane field orientations split. At 120 K the difference is more than 10%. Remarkably, these states are metastable at temperatures sufficiently below T N , as illustrated in the inset of Fig. 2.
In the mesurement, the higher resistance state was prepared by in-plane field cooling and then the temperature was stabilized at 120 K. The RA product (at 20 mV bias) was then monitored while continuously sweeping the magnetic field in the out-of-plane direction (H z ) and also in the two orthogonal in-plane directions (H x and H y ) between +2 T and -2 T, for 10 hours. No changes in the tunneling resistance are observed within the experimental noise, which is much smaller than the difference between the higher and lower resistance states observed in the main panel of Fig. 2. This demonstrates that the state prepared by field cooling is metastable and insensitive to relatively large external magnetic fields. Note that metastable states showing different tunneling resistances at zero magnetic field were also observed in Ref. 2, where the configuration of the AFM moments in IrMn was controlled below T N using the exchange-spring effect of a ferromagnet. Noteworthy, the observed field-cool induced magnetotransport effect shows the key signatures of an anisotropic magnetoresistance. In Fig. 2, while the temperature-dependent resistance traces for the in-plane and out-of-plane fields split below T N , we observe no difference between field-cool measurements performed at fields with the opposite polarity.
In Fig. 3 we highlight that the onset of the splitting of the RA traces for cooling in fields with different directions coincides with the transition to the ordered AFM state in the 2 nm IrMn film.
Side by side we plot in the figure the normalized variation of the tunneling resistance (R(H x )-R(H z ))/R(H z ) in one of our 2 nm IrMn pillar devices and the differential specific heat of a 2nm IrMn layer as a function of temperature. Quasi-adiabatic nanocalorimetry (QAnC) is an ideal technique 6 for investigating the Néel temperature of thin IrMn films. This technique allows for a direct measurement of the specific heat of the sample, enabling the observation of the critical behavior in the specific heat near T N . Identical multilayers to those used in the tunnel junctions were sputtered onto self-standing silicon nitride membranes that form the nanocalorimetric cells. A twin calorimeter loaded with a reference multilayer sample without IrMn was used for differential measurements of the specific heat. 8 The Néel temperature inferred from the inflexion point of the specific heat singularity 8 is approximately 173 K, i.e., it is significantly reduced in the 2 nm IrMn film as compared to the bulk IrMn, having T N > 1000 K. This is in agreement with previous observations in case of other AFMs, e.g. CoO. 8 The reproducibility of the specific heat method has been confirmed in different samples prepared in separate growth runs under the same growth conditions. Moreover, the correspondence between T N and the onset of the field-cool AFM TAMR has been confirmed by independent measurements using the PPMS and the vector magnet cryostat, and studying different ATJs with the same nominal layer structure. All samples show a negligible magnetoresistance in the paramagnetic phase and a reproducible splitting of the RA traces when continuing the field-cooling below T N with in-plane and out-of-plane magnetic fields. The percentage difference between the two metastable resistance states obtained at 120 K varies from 2% to 10% in different ATJ samples. Higher values were found in devices with larger RA, thus indicating that tunneling is the origin of the observed magnetoresistance. The last one simply decreases in devices with thinner barriers, where defects can creates parallel conductive paths partially masking the effect of anisotropic tunneling.
We point out that the observed magnetoresistance cannot be ascribed to magnetizationindependent tunneling transport phenomena due, e.g., to Lorentz force effects of the magnetic fields applied along different directions with respect to the tunneling current direction. These types of phenomena can be excluded since the field-cooling magnetoresistance disappears above T N and since we observe a negligible resistance variation upon application of external fields when the 7 temperature is stabilized below T N , as shown in the inset of Fig. 2. For the same reason, we exclude that possible Mn interdiffusion wthin the oxide layer is responsible for the observed metastable magnetic states. The coincidence between the Néel temperature and the onset of the resistance splitting is a strong evidence for the linking of the observed phenomena to the antiferromagnetism of the IrMn layer and not to the magnetic behavior of some Mn atoms dispersed in the tunneling barrier. We also note that the microscopic mechanism which yields the field-cool TAMR in IrMn is distinct from the high-field magnetotransport effects previously observed in iron pnictide AFMs. 9 In the latter materials the phenomenon has been ascribed to field-induced selection of structural crystal twin domains. 9 IrMn does not undergo a crystal phase change near T N and we therefore ascribe the distinct metastable states realized by field-cooling purely to distinct AFM configurations of uniform IrMn film.
The precise microscopic identification of these states requires a detailed study, which is beyond the proof-of-concept work presented in this paper. Here we recall the theoretical study 10 on IrMn which for Ir 20 Mn 80 identified two non-collinear AFM phases 2Q and 3Q (confining the magnetic spins in the plane or yielding an out-of-plane component, respectively) with an energy difference of only ~0.25 mRy/atom, and a collinear phase whose energy is ~1.25 mRy/atom higher. We surmise that depending on the direction and strength of the applied field, the field-cooling procedure starting from temperatures above T N can favor spin configurations with different proportion of these distinct metastable AFM phases. Finally we remark that in the previously studied NiFe/IrMn exchangespring AFM tunnel junctions, 2,3 the formation of the distinct magnetic configurations affecting the tunnel transport could be ascribed to bulk properties of the AFM or to the interface effects with the ferromagnet. From this perspective, our present experiments provide valuable complementary evidence showing that the interface with another magnetic layer is not required for stabilizing distinct states in the IrMn AFM.
To summarize, we have demonstrated the storage of information in an AFM/insulator/normal-8 metal tunneling device comprising no ferromagnetic elements. Different metastable configurations, yielding the high and low resistance states of the ATJ, can be set by cooling the AFM from above T N in magnetic fields with different orientations. By increasing the AFM layer thickness, the Néel temperature of the AFM film is expected to increase, virtually allowing setting the T N above room temperature. The absence of stray fields and the robustness against magnetic field perturbations are the key features of these devices, which hold potential for the development of novel spintronic devices without ferromagnets. FIG. 1. (color online) High resolution Z contrast image of the heterostructure studied here. In the inset a high resolution image of the Ta/MgO/IrMn tunneling junction is shown. The reference system reported on the right is that used for indexing the magnetic fields during field cooling.