Strong perpendicular magnetic anisotropy energy density at Fe alloy/HfO2 interfaces

We report on the perpendicular magnetic anisotropy (PMA) behavior of heavy metal (HM)/ Fe alloy/MgO thin film heterostructures after an ultrathin HfO2 passivation layer is inserted between the Fe alloy and the MgO. This is accomplished by depositing one to two atomic layers of Hf onto the Fe alloy before the subsequent rf sputter deposition of the MgO layer. This Hf layer is fully oxidized during the subsequent deposition of the MgO layer, as confirmed by X-ray photoelectron spectroscopy measurements. As the result a strong interfacial perpendicular anisotropy energy density can be achieved without any post-fabrication annealing treatment, for example 1.7 erg/cm^2 for the Ta/Fe60Co20B20/HfO2/MgO heterostructure. Depending on the HM, further enhancements of the PMA can be realized by thermal annealing to at least 400C. We show that ultra-thin HfO2 layers offer a range of options for enhancing the magnetic properties of magnetic heterostructures for spintronics applications.

The realization of robust perpendicular magnetic anisotropy (PMA) in heavy metal (HM)/Fe alloy/MgO thin-film heterostructures 1,2 , where typically the Fe alloy is Fe 80-x Co x B 20 (FeCoB), has enabled a pathway for the implementation of high density memory elements based on the spin transfer torque switching of perpendicularly magnetized tunnel junctions (MTJs) [2][3][4] . Strong PMA is also required to create the perpendicularly magnetized nanowire structures needed to enable manipulation of domain walls and novel magnetic chiral structures such as skyrmions by the spin Hall effect [5][6][7][8] . At present the only viable FM/oxide combination that yields the strong PMA and low damping required for practical devices is Fe 80-x Co x B 20 (FeCoB)/MgO where the PMA originates from the strong spin-orbit interaction in the hybridized 3d Fe-2p O bonding at the FeCoB/MgO interface 9,10 . Even there obtaining significant PMA requires an annealing step [1][2][3][4] that can compromise the layers in the magnetic heterostructure.
In previous research, we studied the modification of field-like spin orbit torque at the FeCoB/MgO interface via tuning the PMA there by introducing an ultra thin Hf oxide layer 11 .
Here we report a systematic study that this addition to the surface of FeCoB of as little as 0.2 nm of Hf "dusting", which is oxidized to HfO2 during the subsequent MgO deposition process, can yield strong PMA without any post-fabrication annealing treatment. Depending on the HM, the system can also, if that is desired, be annealed to at least o 400 C to further enhance the PMA.  Fig. 1(a), the HfO 2 4f 7/2 and 4f 5/2 peaks are clearly displayed at 17.1 eV and 18.8 eV, with only a very small sub-oxide peak at ~ 16.0 eV and no evidence for the Hf metallic 4f 7/2 peak at 14.3 eV. To achieve strong PMA it is also required that the Fe alloy not be oxidized beyond the interfacial Fe-O bonds. Fig. 1(b) shows for the same sample the XPS 2p 3/2 peak of Fe at 706.0 eV, which can be well fit with the narrow asymmetric spin-split peak function characteristic of metallic Fe 12 . For a series C sample without the Hf dusting layer, the Fe 2p 3/2 peak is much broader with a high energy tail indicative of substantial oxidation of the surface Fe during the direct deposition of MgO by rf sputtering 13 ( Fig. 1(b)). We also examined the Fe XPS signal for a series B sample with Ta as the dusting layer (0.3 nm). That yielded a metallic signal indistinguishable from the Hf case, again indicating protection of the ferromagnetic surface from significant oxidation. However, the magnetic characteristics of these heterostructures are quite different.
In Fig. 2  Since H c of such PMA samples depends on both the anisotropy field and its uniformity, which together act to set the depinning field for magnetic reversal, further enhancement in H c should be expected with refinements in the smoothness and uniformity of such heterostructures.
We measured the perpendicular anisotropy fields H a as a function of HfO 2 thicknesses in a different set of samples Ta (6) . This deterioration may be due to the diffusion of Ta from the base layer since such diffusion has been known to damage the interfacial PMA in the Ta based PMA systems 3,16 .
We have also examined whether this Ta in-diffusion problem can be avoided by the use of other heavy metal base layers, especially those with strong spin Hall effects, e.g. W and Pt. In we obtain H a > 1.6 T for a sufficiently thick HfO 2 passivation layer, indicative of an interfacial anisotropy energy density ≥ 1.5 ergs/cm 2 . When we use a 1 nm Ta seeding layer before the deposition of the W layer, it results in the W being smoother and in it also being in the lower resistivity alpha-phase. As shown in Fig. 3 amorphous Hf(0.5) spacer between the Ta base layer and the NiFe, which presumably helps to accommodate the crystalline mismatch between the Ta and the NiFe. In Fig. 3(c)  An important question in terms of application is whether MTJ's with a HfO 2 passivation layer at the tunnel barrierfree layer interface can provide sufficiently high TMR to useful for STT and other spintronics applications. As reported previously 26 , a TMR of 80% has been achieved with an in-plane magnetized Pt/Hf/FeCoB (1.6)/MgO(1.6)/FeCoB/Ru/Ta MTJ structure annealed at 300 C, where analytical STEM reveals substantial Hf within the tunnel barrier and the greatly reduced demagnetization field, ≈ 4 kOe, indicates a substantial K s . As will be reported elsewhere 27 , we are now obtaining similar results with W base layer MTJs, so hybrid HfO 2 MgO tunnel barriers may well provide sufficient TMR to be of technology interest.
In summary, we have demonstrated that perpendicular magnetic anisotropy in HM/Fe alloy/MgO heterostructures can be dramatically strengthened by incorporating a very thin HfO 2 dusting layer at the Fe alloy/MgO interface. In HM/FeCoB/MgO devices, the dusting layer enables strong PMA even in the absence of the post-deposition annealing step that has previously been necessary. When annealing is desired, the dusting layer allows the PMA to remain strong for annealing temperatures even above 400 C  , provided a proper base layer is utilized, a much higher limit than for some current STT-MRAM prototype technologies. This can allow easier integration with Si circuitry. The HfO 2 dusting can also create robust PMA using magnetic materials for which previously this has been impossible, thereby expanding the portfolio of magnetic materials available for PMA technologies beyond just FeCoB. In particular we have demonstrated PMA with thin-film Ni 80 Fe 20 , a material that is attractive for its low damping and low magnetostriction. Overall, the strengthening of PMA with the use of HfO 2 dusting layers has great promise both for enhancing the performance of spin-transfer-torque magnetic memory based on PMA magnetic tunnel junctions and also for improving control of chiral domain walls and skyrmion structures within PMA HM/Fe alloy/MgO structures 5,6,28-30 .