₀ordering parameter(S) of FePd is calculated to be 0.73±0.01, also comparable to the counterparts using MgO(001) or SrTiO₃(001) substrates. The MgO peaks are not observed due to its small thickness t_MgO=5 nm and low electron density, however the crystallinity of the MgO layer is later confirmed with STEM measurements. The unlabeled peaks with very high intensity and narrow spread are from the substrate lattice.

The microstructure of the MgO layer and the epitaxial relationship of the stack were studied by STEM. FIG. 5A shows a cross-sectional image of the stack under the high-angle annular dark field (HAADF) mode. The dashed line marks a grain boundary. It is clear that both the bilayer buffer and L1₀-FePd followed a grain-on-grain heteroepitaxial growth on textured MgO. The grain boundaries, one of which is marked by the dashed line, are shared by the MgO seed layer, bilayer buffer, as well as FePd. This epitaxial growth is established from MgO since it tends to form (001) texture on amorphous surfaces, of which CoFeB is a known example. A zoomed-in image around the FePd layer and the energy dispersive X-ray (EDX) profile is shown in FIG. 5B. The portion illustrates the L1₀-FePd and EDX elemental distribution. As illustrated, the interfaces are flat and sharp, showing no significant interdiffusion during the sputtering and annealing processes. By taking atomic-resolution images (FIG. 5C) and nanobeam diffraction (NBD) patterns (shown later), the lattice parameters of each layer were estimated, as illustrated in FIG. 5D. The lattice parameters are consistent with the values obtained from XRD measurements. The epitaxial relationship was inferred to be MgO (001)/Cr [110](001)/Pt [100](001)/L1₀-FePd [100](001), in which the Cr/Pt buffer can effectively release the lattice misfit between MgO and L1₀-FePd.

In addition, FePd-based p-SAFs on Si/SiO₂wafers were prepared by inserting a sub-nm Ir spacer (t_lris wedged from 4.5 to 6.0 Å) between two L1₀-FePd layers.

The samples were cleaved along the wedge direction and indexed from 1 to 12. Their M-H_exthysteresis loops, as shown in FIG. 6A and FIG. 6B, indicate strong PMA with all t_lrand markedly larger in-plane saturation field (by nearly 1 T magnitude) compared to the out-of-plane saturation field. That is, both the top and bottom FePd layers are (001)-textured. By increasing fir, the M-H_extloop gradually changes from a compensated antiferromagnetic configuration to a ferromagnetic-like configuration in which two FePd layers reverse almost coherently. That means, within the t_lrrange of this study, a monotonic decrease of IEC strength as t_lrincreases. FIG. 6A illustrates the perpendicular direction, while FIG. 6B illustrate the in-plane H_ext. FIG. 6C illustrates field-dependent f of HF and LF in Sample 1 (θ_H=) 85°. Circles and lines denote measurement data and theoretical fits, respectively. FIG. 6D is a chart illustrating −J_exof FePd-based p-SAFs vs. sample index (by number) and nominal tr. Inset: Schematic of high-fidelity TR-MOKE metrology developed to study p-SAFs. M₁, M₂, θ_M1, and θ_M2denote the magnetization of the top and bottom L1₀-FePd layers and their equilibrium angles under H_ext, respectively.

Based on SAF magnetization switching behaviors, the top FePd layer is found to switch first as it was prepared to be thinner (3 nm) than the bottom FePd layer (6 nm). In the M-H_extloops of low index samples, the gradual transition of the magnetizations from antiparallel to parallel is consistent with the magnetic free energy of a p-SAF with Ku near parity with −J_ex/t_FePd,top. Since overcoming PMA to rotate L1₀-FePd layers into the plane and IEC to force them aligned cost certain amounts of Zeeman energy, the anisotropy and exchange fields should be approximately additive. Assuming the intrinsic magnetic properties of L1₀-FePd layers remain constant among such a narrow fir range of only ˜1.5 Å, the areas contoured by two in-plane hysteresis curves should reflect their difference in the IEC energy. Therefore, it is also observed that −J_exdecreases as t_lrincreases. To quantitatively measure −J_ex, the TR-MOKE technique was employed to study the p-SAFs as a function of τ_lr. Capable of measuring dynamics up to the THz range, TR-MOKE is a powerful tool to characterize p-SAFs with high PMA and IEC. By fitting the field-dependent f in the high-(HF) and low-frequency (LF) modes, J_excan be calculated by J_ex=J₁+2J₂, with J₁and J₂the extracted bilinear and biquadratic exchange coupling constants, respectively.

As a representative example, FIG. 6C shows the field-dependence off in both precessional modes of Sample 1. The fitting curve gives −J_ex=3.48±1.09 mJ m⁻², which is ˜34% larger compared to the former result. In FIG. 6D, −J_exis plotted as a function of sample index and nominal t_lr. In agreement with VSM results, −J_exdecreases as t_lrincreases, indicating the coverage of t_lrby the first (and strongest) antiferromagnetic range of Ir's Ruderman-Kittel-Kasuya-Yosida (RKKY) oscillation. A sketch of the p-SAF trilayer and TR-MOKE metrology is shown as the inset of FIG. 6D. Given the multiple advantages of p-SAFs over single-magnet counterparts, the FePd-based p-SAFs prepared on Si/SiO₂wafers can further advance the application of scalable spintronic devices.

One example for the enhancement of −J_exis attributed to the strong RKKY interaction of Ir as well as proper seed and buffer layers of L1₀-FePd on amorphous SiO₂surface. To elucidate the high performance of L1₀-FePd/Ir/L1₀-FePd p-SAFs, high-resolution transmission electron microscopy (HRTEM) and STEM were performed to investigate their microstructure. As shown in FIG. 7A, the high crystallinity of both the top and bottom FePd layers is confirmed. Though the Ir spacer is ultrathin, it is still observable. By using the STEM-HAADF mode, the Ir spacer can be imaged with better contrast as shown in FIG. 7B, where the two FePd slabs are separated by a continuous Ir band. To illustrate the epitaxial relationship between each layer, especially the two FePd layers, NBD measurements to a single grain were conducted. The results, as presented in FIG. 7C, show a grain-on-grain heteroepitaxy through the Ir layer. Specifically, the SAF trilayer is textured as L1₀-FePd [100](001)/Ir/L1₀-FePd [100](001). Such an epitaxial growth, together with the sharp and flat interfaces between each layer, enables the considerable improvement in −J_ex.

FIG. 8 is a cross-sectional diagram illustrating example device 800. Example device may be an example of the bottom stack of FIG. 2A. Device 800 includes silicon wafer substrate 802. Silicon wafer substrate 802 includes a plurality of metal layers 804A, 804B, and 804C (“metal layers 804”). Metal layers 804 may combine with silicon wafer substrate 802 to define a plurality of pre-patterned transistor structures (“CMOS structures”). Top metal layer 804A may be covered by seed layer 806, which may be configured to allow epitaxial growth of further layers as described above. For example, device 800 may include one or more bridge layers 808 (buffer layers). Device 800 may include PMA structure 810. PMA structure 810 may include at least one of iron-palladium (FePd) alloy, iron-platinum alloy (FePt), or iron-platinum-palladium alloy (Fe(Pt_xPd_(1-x)).

In one or more examples, device 800 is a CMOS device, such as may be part of an integrated circuit. For instance, PMA structure 810 and bridge layers 808 may together form part of a MTJ structure used for bit storage. As another example, PMA structure 810 and bridge layers 808 may together form part of the integrated circuit to control impedance, be coupled to transistors, from part of a memory device, form part of a logic device, etc. In some examples, device 800 may be a (SOT) MRAM device or a logic device. In some examples, device 800 may include silicon wafer substrate 802, seed layer 806 formed on a surface of silicon wafer substrate 802, perpendicular magnetic anisotropy (PMA) structure 810 exhibiting crystalline PMA formed on the seed layer, wherein PMA structure 810 includes at least one of iron-palladium alloy (FePd), iron-platinum alloy (FePt), or iron-platinum-palladium alloy (Fe(Pt_xPd_(1-x)). In some examples, device 800 may further include a magnetic tunnel junction bridge layer including a MgO tunnel barrier (not illustrated in FIG. 8) formed on PMA structure 810 and a hard perpendicular magnetic anisotropy (PMA) structure (not illustrated in FIG. 8). The hard PMA structure may exhibit PMA, ad may be formed on the magnetic tunnel junction bridge layer so that it acts as a tunnel barrier with a pinned magnetization orientation. Device 800 may further include a conducting layer (not illustrated in FIG. 8) formed on the hard PMA structure. A current delivered through the conducting layer, hard PMA structure, and the PMA structure may set a magnetization state of the PMA structure. In such examples, electrical stimulation of the SOT MRAM device may include measuring the tunneling magnetoresistance of the MRAM device and/or may include writing the orientation of the PMA structure.

FIG. 9 is a flowchart illustrating an example technique for forming devices according to the present disclosure. The technique includes forming a seed layer on a surface of the silicon wafer substrate (902). In some examples, the seed layer may include at least one of magnesium oxide (MgO) or strontium titanate (SrTiO₃) having an (001) texture. The technique also includes forming a structure which exhibits crystalline perpendicular magnetic anisotropy (“PMA structure”) on the seed layer (904). In some examples, the PMA structure includes iron-palladium (FePd) alloy.

As used herein, “formed on” and “on” means a layer or coating that is formed on top of another layer or coating, and encompasses both a first layer or coating formed immediately adjacent a second layer or coating and a first layer or coating formed on top of a second layer or coating with one or more intermediate layers or coatings present between the first and second layers or coatings. In one or more examples, “formed directly on” and “directly on” denote a layer or coating that is formed immediately adjacent another layer or coating, i.e., there are no intermediate layers or coatings.

In summary, high-quality L1₀-FePd(001) thin films were deposited on industry-compatible Si/SiO₂wafers. Without expensive MgO(001) or SrTiO₃(001) substrates, the FePd thin films still possess strong PMA and low damping constant (α) with suitable seed and buffer layers (which may also be called “bridge layers”). Furthermore, by adding an Ir spacer, p-SAFs with markedly improved IEC were also achieved. The high performance of both the L1₀-FePd single layer and its SAF indicates high crystallinity, strong (001) texture, as well as an epitaxial growth on amorphous SiO₂surface. Comprehensive characterizations were performed to reveal the lattice details of the MgO seed layer, Cr/Pt bilayer buffer, L1₀-FePd, and its SAF and the epitaxial relationship throughout the stack. As MgO, the crucial seed layer material in this study, has been widely used by the spintronics community and is deposited by conventional RF sputtering, our results removed a key obstacle of applying L1₀-FePd for deeply-scalable spintronic devices.

Examples of the disclosure are illustrated by the following clauses and claims:

Clause 1. A device, comprising: a silicon wafer substrate; a seed layer formed on a surface of the silicon wafer substrate; and a perpendicular magnetic anisotropy (PMA) structure exhibiting crystalline PMA formed on the seed layer, wherein the PMA structure comprises iron-palladium (FePd) alloy.

Clause 2. The device of clause 1, wherein the device comprises at least a portion of a memory device or a logic device.

Clause 3. The device of any of clause 1 or clause 2, wherein the device comprises a magnetic tunnel junction (MTJ), and wherein the PMA structure is a free layer.

Clause 4. The device of any of clauses 1-3, wherein the silicon wafer comprises both elemental silicon (Si) and silicon dioxide (SiO₂).

Clause 5. The device of any of clauses 1-4, wherein the surface of the silicon wafer substrate silicon wafer is an amorphous SiO₂surface.

Clause 6. The device of clause 5, wherein the surface of the silicon wafer substrate defines one or more metallized contacts, and the one or more metallized contacts comprise copper, aluminum, tungsten, or other metals.

Clause 7. The device of any of clauses 1-6, wherein the seed layer comprises at least one of magnesium oxide (MgO) or strontium titanate (SrTiO₃) having an (001) texture.

Clause 8. The device of any of clauses 1-7, wherein the seed layer is polycrystalline.

Clause 9. The device of any of clauses 1-8, wherein the seed layer is a thin film having a thickness measured normal to the surface of the silicon wafer substrate of from about 0.5 nanometers to about 20 nanometers, or from about 1 nanometer to about 8 nanometers, or from about 2 nanometers to about 7 nanometers.

Clause 10. The device of any of clauses 1-9, wherein the FePd alloy defines an L1₀-phase structure.

Clause 11. The device of any of clauses 1-10, further comprising at least one bridge layer between the seed layer and the PMA structure.

Clause 12. The device of clause 11, wherein the at least one bridge layer comprises two bridge layers including a first bridge layer and a second bridge layer.

Clause 13. The device of clause 12, wherein the first bridge layer comprises chromium and the second bridge layer comprises platinum.