Sputtered L10-FePd and its Synthetic Antiferromagnet on Si/SiO2 Wafers for Scalable Spintronics.

As a promising alternative to the mainstream CoFeB/MgO system with interfacial perpendicular magnetic anisotropy (PMA), L10-FePd and its synthetic antiferromagnet (SAF) structure with large crystalline PMA can support spintronic devices with sufficient thermal stability at sub-5 nm sizes. However, the compatibility requirement of preparing L10-FePd thin films on Si/SiO2 wafers is still unmet. In this paper, we prepare high-quality L10-FePd and its SAF on Si/SiO2 wafers by coating the amorphous SiO2 surface with an MgO(001) seed layer. The prepared L10-FePd single layer and SAF stack are highly (001)-textured, showing strong PMA, low damping, and sizeable interlayer exchange coupling, respectively. Systematic characterizations, including advanced X-ray diffraction measurement and atomic resolution-scanning transmission electron microscopy, are conducted to explain the outstanding performance of L10-FePd layers. A fully-epitaxial growth that starts from MgO seed layer, induces the (001) texture of L10-FePd, and extends through the SAF spacer is observed. This study makes the vision of scalable spintronics more practical.

Effect of oblique versus normal deposition orientation on the properties of perpendicularly magnetized L10 FePd thin films.

Materials such as L10 Fe-based alloys with perpendicular magnetic anisotropy derived from crystal structure have the potential to deliver higher thermal stability of magnetic memory elements compared to materials whose anisotropy is derived from surfaces and interfaces. A number of processing parameters enable control of the quality and texture of L10 FePd among them, including substrate, deposition temperature, pressure and seed and buffer layer. The angle of inclination between the substrate and the sputtering target can also impact the texture of L10 crystallization of sputtered Fe-Pd and magnetic properties of the derived thin films. This study examines the difference between FePd layers that have been magnetron sputter deposited on Cr(15 nm)/Pt, Ir, or Ru(4 nm)/FePd (8 nm)/Ru(2 nm)/Ta(3 nm) substrate layers at an oblique angle (30° tilt from the sputtering target) versus normal incidence (target facing the substrate). X-ray diffraction, ferromagnetic resonance spectroscopy and vibrating sample magnetometry were used to compare the degree of L10 order and static and dynamic properties of films deposited under both conditions. The films grown using the oblique orientation exhibit a stronger degree of L10 orientation, a larger magnetic anisotropy energy and a lower Gilbert damping, on all three buffer layers.

Static and dynamic magnetic properties of sputtered Fe-Ga thin films.

We present measurements of the static and dynamic properties of polycrystalline iron-gallium films, ranging from 20 nm to 80 nm and sputtered from an Fe0.8Ga0.2 target. Using a broadband ferromagnetic resonance setup in a wide frequency range, perpendicular standing spin-wave resonances were observed with the external static magnetic field applied in-plane. The field corresponding to the strongest resonance peak at each frequency is used to determine the effective magnetization, the g-factor and the Gilbert damping. Furthermore, the dependence of spin-wave mode on field-position is observed for several frequencies. The analysis of broadband dynamic properties allows determination of the exchange stiffness A = (18 ± 4) pJ/m and Gilbert damping α = 0.042 ± 0.005 for 40 nm and 80 nm thick films. These values are approximately consistent with values seen in epitaxially grown films, indicating the potential for industrial fabrication of magnetostrictive FeGa films for microwave applications.

Low Gilbert damping and high perpendicular magnetic anisotropy in an Ir-coupled L10-FePd-based synthetic antiferromagnet.

Thin ferromagnetic films possessing perpendicular magnetic anisotropy derived from the crystal lattice can deliver the requisite magnetocrystalline anisotropy density for thermally stable magnetic memory and logic devices at the single-digit-nm lateral size. Here, we demonstrate that an epitaxial synthetic antiferromagnet can be formed from L10 FePd, a candidate material with large magnetocrystalline anisotropy energy, through insertion of an ultrathin Ir spacer. Tuning of the Ir spacer thickness leads to synthetic antiferromagnetically coupled FePd layers, with an interlayer exchange field upwards of 0.6 T combined with a perpendicular magnetic anisotropy energy of 0.95 MJ/m3 and a low Gilbert damping of 0.01. Temperature-dependent ferromagnetic resonance measurements show that the Gilbert damping is mostly insensitive to temperature over a range of 20 K up to 300 K. In FePd|Ir|FePd trilayers with lower interlayer exchange coupling, optic and acoustic dynamic ferromagnetic resonance modes are explored as a function of temperature. The ability to engineer low damping and large interlayer exchange coupling in FePd|Ir|FePd synthetic antiferromagnets with high perpendicular magnetic anisotropy could prove useful for high performance spintronic devices.

Reduction-Induced Magnetic Behavior in LaFeO3-δ Thin Films.

The effect of oxygen reduction on the magnetic properties of LaFeO3-δ (LFO) thin films was studied to better understand the viability of LFO as a candidate for magnetoionic memory. Differences in the amount of oxygen lost by LFO and its magnetic behavior were observed in nominally identical LFO films grown on substrates prepared using different common methods. In an LFO film grown on as-received SrTiO3 (STO) substrate, the original perovskite film structure was preserved following reduction, and remnant magnetization was only seen at low temperatures. In a LFO film grown on annealed STO, the LFO lost significantly more oxygen and the microstructure decomposed into La- and Fe-rich regions with remnant magnetization that persisted up to room temperature. These results demonstrate an ability to access multiple, distinct magnetic states via oxygen reduction in the same starting material and suggest LFO may be a suitable materials platform for nonvolatile multistate memory.

Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers.

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3 [Formula: see text] A/cm2.

Effect of gas composition during Pt sputtering on structural and magnetic properties of CoFeB thin films.

Ultrathin Ta/CoFeB/Pt trilayer structures are relevant to a wide range of spintronic applications, from magnetic tunnel junctions to skyrmionics devices. Controlling the perpendicular magnetic anisotropy, Gilbert damping and Dzyaloshinskii-Moriya interaction in the CoFeB layer is key for these applications. We examine the role of sputter gas composition during the Pt overlayer deposition of a Ta/CoFeB/Pt trilayer in Ar, Kr and Xe working gas environments during direct current magnetron sputtering. Decreasing density of the Pt layer (from 21 g/cm3 to 15 g/cm3) was apparent in specular x-ray reflectivity measurements of the trilayer films when increasing the molecular weight of the sputtering gas from Ar to Kr to Xe. Significant effects on the Gilbert damping and the interfacial Dzyaloshinskii-Moriya interaction (DMI) energy were observed, with increases in the damping from 0.037(1) to 0.042(1) to 0.048(1), and reductions in the interfacial DMI from 0.47(4) mJ/m2 to 0.45(5) mJ/m2 to 0.39(4) mJ/m2. The ability to control the perpendicular magnetization and DMI strength of these materials through judicious interfacial control is a means toward magnetic devices with better stability at smaller lateral dimension, key to device scaling for spintronic device arrays.

Low Gilbert damping and high thermal stability of Ru-seeded L10-phase FePd perpendicular magnetic thin films at elevated temperatures.

Bulk perpendicular magnetic anisotropy materials are proposed to be a promising candidate for next-generation ultrahigh density and ultralow energy-consumption spintronic devices. In this work, we experimentally investigate the structure, thermal stability, and magnetic properties of FePd thin films seeded by a Ru layer. An fcc-phase Ru layer induces the highly-ordered L10-phase FePd thin films with perpendicular magnetic anisotropy (K u ~ 10.1 Merg/cm3). The thermal stability of FePd samples is then studied through the annealing process. It is found that a K u ~ 6.8 Merg/cm3 can be obtained with the annealing temperature of 500 °C. In addition, the damping constant α, an important parameter for switching current density, is determined as a function of the testing temperature. We observe that α increases from 0.006 to 0.009 for as-deposited FePd sample and from 0.006 to 0.012 for 400 °C-annealed FePd sample as the testing temperature changes from 25 °C to 150 °C. These results suggest that Ru-seeded FePd provides great potential in scaling perpendicular magnetic tunnel junctions below 10 nm for applications in ultralow energy-consumption spintronic devices.

Capturing magnetic bead-based arrays using perpendicular magnetic anisotropy.

Designing and implementing means of locally trapping magnetic beads and understanding the factors underlying the bead capture force are important steps toward advancing the capture-release process of magnetic particles for biological applications. In particular, capturing magnetically labeled cells using magnetic microstructures with perpendicular magnetic anisotropy (PMA) will enable an approach to cell manipulation for emerging lab-on-a-chip devices. Here, a Co (0.2 nm)/Ni (0.4 nm) multilayered structure was designed to exhibit strong PMA and large saturation magnetization (M s ). Finite element simulations were performed to assess the dependence of the capture force on the value of M s . The simulated force profile indicated the largest force at the perimeter of the disks. Arrays of Co/Ni disk structures of (4-7) µm diameter were fabricated and tested in a microchannel with suspended fluorescent magnetic beads. The magnetic beads were captured and localized to the edge of the disks as predicted by the simulations. This approach has been demonstrated to enable uniform assembly of magnetic beads without external fields and may provide a pathway toward precise cell manipulation methods.

Large Interfacial Magnetostriction in (Co/Ni)4/Pb(Mg1/3Nb2/3)O3-PbTiO3 Multiferroic Heterostructures.

The magnetoelastic behavior of multiferroic heterostructures-coupling of magnetic anisotropy or domain dynamics to structural deformations-has been intensively studied for developing materials for energy-efficient, spin-based applications. Here, we report on a large, interface-dominated magnetostriction in (Co/Ni)4/Pb(Mg1/3Nb2/3)O3-PbTiO3 multiferroic heterostructures. Ferromagnetic resonance spectroscopy under voltage-induced strains enabled estimation of the saturation magnetostriction as a function of Ni thickness. The volume and the interface components to the saturation magnetostriction are (6.6 ± 0.9) × 10-6 and (-2.2 ± 0.2) × 10-14 m, respectively. Similar to perpendicular magnetic anisotropy in Co/Ni, the large, negative magnetostriction originates from the Co/Ni interfaces. This interfacial functionality delivers an effect over 300% larger than the bulk contribution and can enable low-energy, nanoelectronic devices that combine the tunable magnetic and magnetostrictive properties of Co/Ni multilayers with the ferroelectric properties of Pb(Mg1/3Nb2/3)O3-PbTiO3.

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal.

When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation1. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties2, 3 as well as for various applications including magneto-optical recording4. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts5-9. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure10-14. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of M ~0.002 µB/Mn, the non-collinear AF metal Mn3Sn15 exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Néel state16 may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.

Nanoscale imaging of magnetization reversal driven by spin-orbit torque.

We use scanning electron microscopy with polarization analysis to image deterministic, spin-orbit torque-driven magnetization reversal of in-plane magnetized CoFeB rectangles in zero applied magnetic field. The spin-orbit torque is generated by running a current through heavy metal microstrips, either Pt or Ta, upon which the CoFeB rectangles are deposited. We image the CoFeB magnetization before and after a current pulse to see the effect of spin-orbit torque on the magnetic nanostructure. The observed changes in magnetic structure can be complex, deviating significantly from a simple macrospin approximation, especially in larger elements. Overall, however, the directions of the magnetization reversal in the Pt and Ta devices are opposite, consistent with the opposite signs of the spin Hall angles of these materials. Our results elucidate the effects of current density, geometry, and magnetic domain structure on magnetization switching driven by spin-orbit torque.

Reversible strain control of magnetic anisotropy in magnetoelectric heterostructures at room temperature.

The ability to tune both magnetic and electric properties in magnetoelectric (ME) composite heterostructures is crucial for multiple transduction applications including energy harvesting or magnetic field sensing, or other transduction devices. While large ME coupling achieved through interfacial strain-induced rotation of magnetic anisotropy in magnetostrictive/piezoelectric multiferroic heterostructures has been demonstrated, there are presently certain restrictions for achieving a full control of magnetism in an extensive operational dynamic range, limiting practical realization of this effect. Here, we demonstrate the possibility of generating substantial reversible anisotropy changes through induced interfacial strains driven by applied electric fields in magnetostrictive thin films deposited on (0 1 1)-oriented domain-engineered ternary relaxor ferroelectric single crystals with extended temperature and voltage ranges as compared to binary relaxors. We show, through a combination of angular magnetization and magneto-optical domain imaging measurements, that a 90° in-plane rotation of the magnetic anisotropy and propagation of magnetic domains with low applied electric fields under zero electric field bias are realized. To our knowledge, the present value attained for converse magnetoelectric coupling coefficient is the highest achieved in the linear piezoelectric regime and expected to be stable for a wide temperature range, thus representing a step towards practical ME transduction devices.