Patent Application
20220406508
The present invention relates to the realization of a strong Dzyaloshinskii-Moriya Interaction (DMI) and perpendicular magnetic anisotropy (PMA) induced by chemisorbed species on a ferromagnetic layer.
The Dzyaloshinskii-Moriya interaction (DMI) is a spin-spin interaction that has finite values only in systems lacking inversion symmetry. Dzyaloshinskii proposed that the combination of low symmetry and spin-orbit coupling gives rise to an antisymmetric exchange interaction, and Moriya introduced how to calculate the antisymmetric exchange interaction for localized magnetic systems in a microscopic model. This picture was later used to successfully explain helical spin order as well as skyrmion lattices in MnSi and FeGe crystals lacking inversion symmetry. In addition, Fert and Levy proposed a DMI mechanism that involves magnetic and non-magnetic sites in spin glasses, which was extended to thin film surfaces and interfaces where inversion symmetry breaks along the surface normal direction.
This DMI mechanism may be invoked to explain the stability of preferred chirality in a large variety of systems featuring non-collinear spin textures, such as spin spirals, skyrmions or chiral domain walls (DWs). Its energy term −Dij·(Si×Sj) indicates that the sign of the DMI vector Dij determines the chirality of spin textures, i.e. being right- or left-handed, and the interplay between the magnitude of DMI and other magnetic interactions influences the size of spin textures. Intensive experimental and theoretical efforts have been made to explore the material dependence of the interfacial DMI and to exploit the flexibility of interface choices and stacking orders to enhance the effective DMI, with the goal of optimizing thin film and multilayer systems for the design of spin-orbitronic devices based on chiral spin textures.
Experimentally, most work on interfacial DMI systems has focused on magnetic layers adjacent to heavy metals, such as hafnium, tantalum, tungsten, iridium, platinum, palladium or ruthenium, where large differences of the DMI magnitude among those elements were attributed to the distinct degree of hybridization between 5d and 3d orbitals near the Fermi level. On the other hand, it is fundamentally interesting to explore effects of elements with low atomic number on the DMI. For instance, a significant magnitude of the DMI was observed at the Co/graphene interface and was attributed to the Rashba effect. The DMI at the Fe/oxygen interface has also been theoretically predicted. However, experimental measurement of the oxygen induced DMI remains unclear, partly due to the necessity of ultrahigh vacuum environment, which is not compatible with some commonly used approaches to quantify the DMI.
Accordingly, a need arises for improved techniques for the realization of strong Dzyaloshinskii-Moriya Interaction (DMI) and perpendicular magnetic anisotropy (PMA) induced by chemisorbed species on a ferromagnetic layer.
Embodiments may provide the realization of strong Dzyaloshinskii-Moriya Interaction (DMI) and perpendicular magnetic anisotropy (PMA) induced by chemisorbed species on a ferromagnetic layer. In the case of chemisorbed oxygen on ferromagnets, the sign of this DMI and its surprisingly large magnitude—despite the low atomic number of oxygen—are derived by examining the oxygen coverage dependent evolution of domain wall chirality. The oxygen induced DMI may be greater than the DMI induced at interfaces with many transition metals; it is sufficiently large to enable, e.g., the tailoring of skyrmion's winding number via oxygen chemisorption. This result extends the understanding of the DMI and supports chemisorption related design of spin-orbitronics devices.
For example, in an embodiment, an apparatus for generating a Dzyaloshinskii-Moriya interaction may comprise a ferromagnet comprising a single layer or multi-layers of materials made of metal, oxide or other types of magnetic films, and a substance chemisorbed on a surface of the ferromagnet to induce the Dzyaloshinskii-Moriya interaction at the interface between chemisorbed species and ferromagnet.
In embodiments, the Dzyaloshinskii-Moriya interaction may be controlled based on a thickness of at least one layer of metal. The Dzyaloshinskii-Moriya interaction may be controlled based on a substance chemisorbed on the surface of the ferromagnet. The Dzyaloshinskii-Moriya interaction may be controlled based on a thickness of the substance chemisorbed on the surface of the ferromagnet. The layers may be selected from transition metals, alkali metals, and lanthanides, including but not limited to Manganese, Iron, Cobalt, Nickel, Molybdenum, Ruthenium, Rhodium, Palladium, Cesium, Hafnium, Tantalum, Tungsten, Rhenium, Iridium, Platinum, Gadolinium, Terbium, Dysprosium, Holmium, and their alloys, or selected from a group of other non-metallic materials, including but not limited to ferrites, garnets, rare-earth oxides, Heusler alloys, CrO2, graphene, CrI3, and Cr2Ge2Te6. The substance may be selected from a group of substances comprising O2, H2, N2, F2, NH3, H2O, CH3, CH4, CO, CO2, fullerene (C60 and C70), bathocuproine (BCP), Tris(8-hydroxyquinoline)aluminum(III), and their ionic species such as O2−, H+, N3−, F− and OH−. The Dzyaloshinskii-Moriya interaction may be controlled so as to generate a skyrmion by changing a coverage of the chemisorbed substance. The substance coverage thickness may be in a range of about 0 to 100 nm.
In an embodiment, an apparatus for generating a Dzyaloshinskii-Moriya interaction may comprise a ferromagnet comprising Ni/Co/Pd/W multilayers or Ni/Co/W/Pd multilayers, and a substance chemisorbed on a surface Ni layer of the ferromagnet to induce the Dzyaloshinskii-Moriya interaction at the interface between the chemisorbed substance and ferromagnet, wherein the Dzyaloshinskii-Moriya interaction is controlled based on a thickness of a Pd layer or W layer.
In embodiments, the Dzyaloshinskii-Moriya interaction may be controlled based on the substance chemisorbed on the surface of the ferromagnet. The Dzyaloshinskii-Moriya interaction may be controlled based on a thickness of the substance chemisorbed on the surface of the ferromagnet. The substance may be selected from a group of substances comprising O2, H2, N2, F2, NH3, H2O, CH3, CH4, CO, CO2, fullerene (C60 and C70), bathocuproine, Tris(8-hydroxyquinoline)aluminum(III), and their ionic species such as O2−, H+, N3−, F− and OH−. The Dzyaloshinskii-Moriya interaction may be controlled so as to generate a skyrmion by introducing chemisorbed oxygen on top of ferromagnetic layers. A coverage of the oxygen coverage may determine the strength of the Dzyaloshinskii-Moriya interaction. The oxygen coverage thickness is in a range of about 0 to 100 nm.
In an embodiment, an apparatus for generating a perpendicular magnetic anisotropy may comprise a substance chemisorbed on a surface of a ferromagnet to induce perpendicular magnetic anisotropy at an interface between the chemisorbed substance and the ferromagnet.
In embodiments, the perpendicular magnetic anisotropy may be controlled based on a substance chemisorbed on the surface of the ferromagnet. The perpendicular magnetic anisotropy may be controlled based on a thickness of substance chemisorbed on the surface of the ferromagnet. The substance chemisorbed on the surface of the ferromagnet may further induce a Dzyaloshinskii-Moriya interaction at an interface between chemisorbed substance and the ferromagnet. The substance may be selected from a group of substances comprising bathocuproine, Tris(8-hydroxyquinoline)aluminum(III), and fullerene (C60 and C70).
In an embodiment, a method for generating a Dzyaloshinskii-Moriya interaction may comprise providing a ferromagnet comprising a single layer or multi-layers of materials made of metal, oxide or other types of magnetic films, and chemisorbing a substance on a surface of the ferromagnet to induce the Dzyaloshinskii-Moriya interaction at the interface between chemisorbed species and ferromagnet.
In embodiments, the method may further comprise controlling the Dzyaloshinskii-Moriya interaction based on a thickness of at least one layer of film. The method may further comprise controlling the Dzyaloshinskii-Moriya interaction based on a substance chemisorbed on the surface of the ferromagnet. The method may further comprise controlling the Dzyaloshinskii-Moriya interaction based on a thickness of the substance chemisorbed on the surface of the ferromagnet. The layers of the ferromagnet stack may be selected from transition metals, alkali metals, and lanthanides, including but not limited to Manganese, Iron, Cobalt, Nickel, Molybdenum, Ruthenium, Rhodium, Palladium, Cesium, Hafnium, Tantalum, Tungsten, Rhenium, Iridium, Platinum, Gadolinium, Terbium, Dysprosium, Holmium, and their alloys, or selected from a group of other non-metallic materials, including but not limited to ferrites, garnets, rare-earth oxides, Heusler alloys, CrO2, graphene, CrI3, and Cr2Ge2Te6. The substance may be selected from a group of substances comprising O2, H2, N2, F2, NH3, H2O, CH3, CH4, CO, CO2, fullerene (C60 and C70), bathocuproine, Tris(8-hydroxyquinoline)aluminum(III), and their ionic species such as O2−, H+, N3−, F− and OH−. The method may further comprise controlling the Dzyaloshinskii-Moriya interaction so as to generate a skyrmion by changing a coverage of the chemisorbed substance. The substance coverage thickness may be in a range of about 0 to 100 nm.
In an embodiment, a method for generating a Dzyaloshinskii-Moriya interaction may comprise providing a ferromagnet comprising Ni/Co/Pd/W multilayers or Ni/Co/W/Pd multilayers, chemisorbing a substance on a surface Ni layer of the ferromagnet to induce the Dzyaloshinskii-Moriya interaction at the interface between the chemisorbed substance and ferromagnet, and controlling the Dzyaloshinskii-Moriya interaction based on a thickness of a Pd layer or W layer.
In embodiments, the method may further comprise controlling the Dzyaloshinskii-Moriya interaction based on the substance chemisorbed on the surface of the ferromagnet. The method may further comprise controlling the Dzyaloshinskii-Moriya interaction based on a thickness of the substance chemisorbed on the surface of the ferromagnet. The substance may be selected from a group of substances comprising O2, H2, N2, F2, NH3, H2O, CH3, CH4, CO, CO2, fullerene (C60 and C70), bathocuproine, Tris(8-hydroxyquinoline)aluminum(III), and their ionic species such as O2−, H+, N3−, F− and OH−. The method may further comprise controlling the Dzyaloshinskii-Moriya interaction so as to generate a skyrmion by introducing chemisorbed oxygen on top of ferromagnetic layers. A coverage of the oxygen coverage may determine the strength of the Dzyaloshinskii-Moriya interaction. The oxygen coverage thickness may be in a range of about 0 to 100 nm.
In an embodiment, a method for generating a perpendicular magnetic anisotropy may comprise chemisorbing a substance on a surface of a ferromagnet to induce perpendicular magnetic anisotropy at an interface between the chemisorbed substance and the ferromagnet.
In embodiments, the method may further comprise controlling the perpendicular magnetic anisotropy based on a substance chemisorbed on the surface of the ferromagnet. The method may further comprise controlling the perpendicular magnetic anisotropy based on a thickness of substance chemisorbed on the surface of the ferromagnet. The substance chemisorbed on the surface of the ferromagnet may further induce a Dzyaloshinskii-Moriya interaction at an interface between chemisorbed substance and the ferromagnet. The substance may be selected from a group of substance comprising bathocuproine, Tris(8-hydroxyquinoline)aluminum(III), and fullerene (C60 and C70).
The details of the present invention, both as to its structure and operation, can best be understood by referring to the accompanying drawings, in which like reference numbers and designations refer to like elements.
Embodiments may provide the realization of large DMI and perpendicular magnetic anisotropy (PMA) induced by chemisorbed species on ferromagnets. A case in point is the large DMI induced by chemisorbed oxygen on Ni/Co/Pd/W(110) multilayers, which we measured using spin-polarized low energy electron microscopy (SPLEEM). We monitor the oxygen coverage dO dependent evolution of DW chirality in perpendicularly magnetized Ni/Co films on Pd/W(110), where the effective DMI can be tuned by precisely controlling the Pd spacer layer thickness dPd. We find that the chemisorbed oxygen can switch the DW chirality when the effective DMI of the bare (oxygen-free) Ni/Co/Pd/W(110) multilayer is Pd-like (left-handed) as a result of a relatively thick Pd spacer layer, but chemisorbed oxygen cannot switch the DW chirality when the Pd spacer layer is thinner and the effective DMI of the bare multilayer is tungsten-like (right-handed). A systematic measurement of the chirality in dPd-dO space allows us to quantify the DMI induced by chemisorbed oxygen. We find that the magnitude of the chemisorbed oxygen induced DMI is comparable to those induced at ferromagnet/heavy metal interfaces—despite the low atomic number of oxygen. We demonstrate that this oxygen induced DMI is sufficiently strong to tailor the topology of a magnetic bubble domain from a topologically trivial bubble to a skyrmion with topological charge 1.
Most notably, the observed large magnitude of the DMI induced by oxygen may be useful for the development of applications in the field of spintronics. Our results also highlight a strength of this experimental approach. Using the tunability of the DMI at the buried Pd/W interface allows precise quantification of the previously unknown DMI that is induced when another element—in this case oxygen—is chemisorbed on top of the multilayer. This approach may be a versatile method to measure unknown values of DMI induced at interfaces with other elements. For instance, chemisorption of many gases, such as H2, N2, F2, NH3, H2O, CH3, CH4, CO, CO2, or organic molecules such as bathocuproine or Tris(8-hydroxyquinoline)aluminum(III), occur on Ni(111) surface. In this work we also introduce the observation of induced DMI by chemisorbed hydrogen and bathocuproine.
A Tuneable Platform for Measuring Unknown DMI Contributions
One of the approaches to quantify the DMI in a layered system is to measure the DW spin texture as a function of layer thickness, where the sign and magnitude of the DMI can be determined by measuring the critical thickness where DW texture transitions from chiral Néel- to achiral Bloch textures. This can be done, e.g., by using SPLEEM or scanning electron microscopy with polarization analysis. If the addition of a new interface with unknown DMI to a layered system with well-understood DMI is found to switch the handedness of DWs, then the unknown DMI of the new interface can be measured in this way; for example the observation of right-handed chirality in Co/Ru(0001) and left-handed chirality in graphene/Co/Ru(0001) allowed unambiguous determination of left-handed DMI at the graphene/Co interface. The aim to generalize this experimental method motivates the development of DMI-tuneable platforms that combine pairs of buried interfaces with opposite DMI to provide magnetic surfaces with either left- or right-handed DW chirality, so that the sign of an unknown DMI at any new interfaces added to the structure can be unambiguously revealed.
In embodiments, tungsten and palladium may be chosen because they provide opposite DMI and because growth of Pd on W(110) results in high quality epitaxial films. The strong LEEM image intensity oscillations associated with the layer-by-layer growth allow the precise determination of the Pd film thickness, which permits the fine tuning of the effective DMI of the Pd/W system. Ni/Co bilayers grown on top provide perpendicular magnetic anisotropy, which allows the observation of DW chirality.
In the compound SPLEEM images 102, 106, 110, grey/black regions represent the down/up magnetization of the perpendicular magnetized domains, respectively, and colored boundaries show DW magnetization orientation according to the color wheel shown in the inset in panel 104. The histograms 104, 108, 112 of the angle α between DW magnetization m and DW normal direction n, as defined in the inset in 102, show the statistics of DW chirality. The single prominent peak in the histogram of α in the case of Pd thickness dPd=2.10 monolayer (ML), see 104, indicates left-handed DW chirality; the presence of two prominent peaks in the DW magnetization histogram at dPd=2.46 ML shown in 108 indicates achiral DW texture with left- and right-handed DW sections; and the single peak in the histogram shown in 112 indicates left-handedness of DW spin texture at dPd=2.90 ML, as the Pd-like DMI dominates the system at larger Pd thickness.
Examining Chemisorbed Oxygen Induced DW Chirality
Ni(111) is a well suited surface to study the role of oxygen on the DMI as the phase diagram of oxygen chemisorbed on Ni(111) is well understood from literature, showing that two-dimensional ONi(111) adsorbate layers can be realized in the range of 0-0.5 ML oxygen with respect to the planar atomic density of Ni(111). As a function of coverage, two long-range ordered structures can form: a p(2×2) phase at saturation coverage of ¼ ML, and a (√{square root over (3)}×√{square root over (3)})R30° phase at saturation coverage of ⅓ ML. In this work we focus on room temperature oxygen adsorption in the coverage regime up to 0.29 ML, where no formation of NiO is observed (see Methods).
The presence of adsorbed oxygen on Ni/Co/Pd/W(110) samples favors right-handed chirality. This is unambiguously demonstrated by utilizing the tuneability of the DMI in this multilayer, where the chirality of the magnetic layer can be adjusted from left-handed to achiral to right-handed, as a function of the thickness of the Pd spacer.
In the O-coverage dependent magnetization images 202-212, at dPd=2.76 ML, the a histogram derived from each image shows that the chirality evolves from left-handed at dO=0.12 ML to achiral near dO=0.19 ML, and to right-handed chirality at dO=0.22 ML. This trend can also be seen in samples with other Pd thickness, as shown in illustration 216: for Pd layer thickness dPd=2.46 ML the achiral state with essentially vanishing DMI occurs with the pristine Ni(111) surface and, as oxygen coverage is introduced, chirality gradually evolves to right-handedness at dO=0.22 ML. The left-to-righthanded chirality switch occurs at progressively larger O coverage as dPd is increased, as shown in 216 for dPd=2.60 ML, dPd=2.76 ML, dPd=2.83 ML. This is because the effective left-handed DMI increases with the Pd layer thickness and more O coverage is required to provide the balancing right-handed DMI.
Quantifying Chemisorbed Oxygen Induced DMI
Summarizing oxygen coverage dependent chirality at each Pd thickness in
In the following we discuss how to quantitatively extract the strengths of these DMI contributions. We start by measuring the DMI in [Ni/Co]n/Pd/W(110) with dPd=3.46 ML, which is 1 ML thicker than the zero-DMI case of dPd (2.46 ML). This measurement is based on observing DW configurations as a function of the thickness of the magnetic layer. The approach is to measure the DMI by tracking the competition between the interfacial DMI, which favours chiral Néel walls, and the dipolar interaction, which favours Bloch walls. Thus, one can indirectly estimate the DMI strength by calculating the dipolar energy penalty of Néel walls at the experimentally measured thickness of the magnetic film at which the Néel/Bloch DW texture transition occurs. In [Ni/Co]n/3.46 ML Pd/W(110) multilayers we find that this transition occurs between the thicknesses of n=3 and n=5 [Ni/Co] repeats, where the DWs are chiral Néel-type in the thinner Ni/Co multilayer and achiral Bloch-type in the thicker Ni/Co multilayer (304). Micromagnetic computation of the dipolar energy difference between Néel and Bloch DWs at this critical thickness yields an estimate of the effective DMI of 0.41±0.17 meV/atom in [Ni/Co]n/3.46 ML Pd/W(110) (as described by Yang, Chen, et al.). Taking into account that the effective DMI contribution from the upper [Ni/Co] repeats of the multilayers vanishes due to inversion symmetry, this estimated DMI value is attributed to the interface between the Co layer and the 3.46 ML Pd/W(110) film. Using the same approach, we further measured the DMI at the Co interface with bulk Pd in the [Ni/Co]n/Pd(111) system, finding DCo/Pd=1.44±0.15 meV/atom. These results show that the bulk Pd induced DMI is much larger than the DMI variation induced by a small Pd thickness change in the Co/Pd/W(110) system, similar to the smooth thickness dependence observed in another heavy-metal induced DMI system. Consequently, for sub-monolayer variations of the Pd layer thickness we can approximate that the DMI varies linearly with Pd thickness. Under this approximation, the DMI change induced by a Pd layer thickness variation of ΔdPd=0.37 ML is about 37% of the DMI change induced by a Pd layer thickness variation of ΔdPd=1 ML which, as described above, amounts to 0.41±0.17 meV/atom. As our experiments show that the DMI variation induced by ΔdPd=0.37 ML equals that induced by adding 0.24 ML oxygen onto the Ni surface, these measurements allow an estimate of the DMI at Ni/oxygen as
/atom for 1 ML of oxygen coverage.
It is interesting to compare this value of the oxygen-induced DMI on this Ni surface to a number to other DMI values as summarized at 306 and 308. Here we compare only measurements derived using the same experimental approach, as described in Chen et al. and Jiang et al., in order to avoid possible variations associated with potential systematic biases due to the use of different methods. Comparing to another light element, the DMI at the oxygen/Ni interface is approximately four times larger than that at the graphene/Co interface, where Dgraphene/Co=0.16±0.05 meV/atom. The chemisorbed oxygen induced DMI is comparable to the DMI induced at interfaces with many transition metals. For interfaces with Ni (306) DNi/Pt=1.05±0.18 meV/atom, DNi/Ir=0.12±0.04 meV/atom , DNi/W≈0.24 meV/atom, and DNi/Cu+Fe/Ni=0.15±0.02 meV/atom. For interfaces with Co (308): DCo/Ir=0.36±0.08 meV/atom, DCo/Ru=0.05±0.01 meV/atom, and DCo/Pd=1.44±0.15 meV/atom as measured in this work.
Tailoring Chirality of Spin Textures Via Oxygen
The large DMI induced by oxygen opens up new possibilities for designing chiral spin textures without using heavy metals. In the following we demonstrate experimentally that chemisorbed oxygen can be used to tailor the spin texture of a magnetic bubble.
Note that the DMI of oxygen adsorbed on top of Ni favors right-handed DW textures, which is the same handedness as Pt/Ni and Pd/Ni, suggesting that earth-abundant oxygen could potentially be used as an alternative to replace those rare noble metals in device applications. The large magnitude of the DMI at the oxygen/Ni interface may be sufficient to stabilize magnetic chirality in a few nm thick magnetic films, for instance, the chirality in typical perpendicular magnetic anisotropy multilayers [Co1mL/Ni2ML]n might be stabilized up to n=5 (roughly 3 nm thick). While the physical origin of the large magnitude of the oxygen induced DMI is an open question, it is plausible that it may be linked to the charge transfer at the oxygen/metal interface, which suggests that some other light elements may also induce significant DMI.
Reversible Chemisorption/Desorption of Hydrogen on Ni(111) and Co(0001) Surfaces
Measuring the work function on solid surfaces has been widely used to quantitatively understand hydrogen chemisorption. The work function shift Δφ upon hydrogen chemisorption on Ni(111) and Co(0001) allows the determination of the hydrogen coverage.
The LEEM is a powerful tool to measure the work function of material surfaces by fitting LEEM IV curves (502). We observed a work function increase of Δφ≈120 meV on a (111) oriented Ni film upon 0.9 Langmuir (L) hydrogen exposure (180 seconds at 5×10−9 torr) at room temperature (502). This significant work function shift is in excellent agreement with prior work, where a shift of Δφ≈135 meV was reported to occur upon hydrogen adsorption on a Ni(111) surface at 41° C. with hydrogen pressure set to 5×10−9 torr.
To explore the possible reversibility at room temperature, the evolution of Δφ is monitored during cycles of ON/OFF states of hydrogen at 5×10−9 torr/base pressure (see Methods). For the hydrogen covered Ni(111) surface, prior work identified two desorption maxima around 310 K (β1 state) and 380 K (β2 state) using the flash desorption approach, and only the (β2 state was found to get filled at small hydrogen coverage. Note that the atomic hydrogen occupies three-fold hollow sites with Ni—H bond length 1.84±0.06 Å on Ni(111), corresponding to an overlayer-substrate spacing of 1.15±0.1 A. Because the desorption temperature of the Δφ state is just above room temperature, spontaneous hydrogen desorption at room temperature is expected during evacuation of the vacuum chamber. An example of the work function shift Δφ on a Ni(111) surface as a function of time over four ON(3min)/OFF(l0min) cycles is shown in 504. The plot shows the gradual work function increase of Δφ≈120 meV during the first hydrogen exposure (0.9 L), and reversible oscillations of Δφ during the subsequent ON/OFF cycles with an amplitude of about ±40 meV. The known dependence of Δφ on the hydrogen coverage , indicates that chemisorption of hydrogen on Ni(111) is indeed partly reversible at room temperature, and desorption is likely limited to the β1 state. Consistent with prior literature, our result indicates that roughly one third of hydrogen can be reversibly chemisorbed/desorbed on a Ni(111) film surface at room temperature and under ultrahigh vacuum (UHV) conditions. Note that this coverage ratio may vary with a different hydrogen dose and pressure.
Hydrogen chemisorption also occurs on the Co(0001) surface, where temperature programmed thermal desorption measurements indicated desorption maxima with coverage dependent positions around 325-370 K (β1 state) and 400-420 K (β2 state), somewhat resembling the case of Ni(111). Similar to Ni(111), we found that cyclical hydrogen chemisorption/desorption on a Co(111) film is associated with a reversible work function change, albeit the amplitude is smaller with Δφ≈±20 meV. In 506, an example of a plot of time-dependent Δφ measurements over four ON (3 min at 5×10−9 torr)/OFF(10 min) cycles is shown. The observed spontaneous hydrogen desorption from Co(0001) films at room temperature is consistent with the detailed thermal desorption study of this system reported in Huesges and Christmann [Z. Phys. Chem. 227, 881 (2013)].
For DMI measurements described in detail below, we will use Ni/Co/Pd/W(110) multilayer samples. Here we first discuss the hydrogen chemisorption properties of such structures. Interestingly, we find that the hydrogen coverage ratio that results in cyclical chemisorption/desorption at room temperature can be greatly enhanced on these multilayer structures, compared to the single-element films described above. An example of the evolution of Δφ on the surface of a N(1)/Co(3)/Pd(2)/W(110) multilayer is shown in 508, where the numbers 1 ML and 3 ML stand for layer thickness in atomic monolayer (ML) of the Ni and Co layers, respectively. In 508 identical hydrogen ON/OFF cycles as shown in panels 504 and 506 were used.
We find that the initial work function rise of Δφ≈125 meV upon hydrogen exposure (3 min at 5×10−9 torr) is comparable to Δφ observed on Ni(111) (˜120 meV). However, the amplitude of work function oscillations during the subsequent hydrogen pressure cycles is around 80 meV, about two thirds of the initial Δφ. This amplitude is almost twice that observed in the thicker (6 ML) Ni(111) film (504). The element Pd is known for its large bulk hydrogen adsorption capacity and one might surmise that the presence of 2 ML Pd underneath the Ni/Co bilayer has something to do with the observed enhancement of hydrogen induced work function change. However, using a N(1)/Co(3)/Pd(20)/W(110) sample with a ten-fold thicker Pd layer, we observe that the Δφ evolution induced by identical hydrogen ON/OFF cycles is almost identical as in the sample with just 2 ML Pd. This suggests that the large Δφ ON/OFF ratio originates from the top Ni/Co bilayer, and not from the Pd layer. An even greater Δφ ON/OFF ratio can be achieved on the same N(1)/Co(3)/Pd(2)/W(110) structure at elevated temperatures. In 510 it is shown that when the sample is held at 90° C. then in the hydrogen OFF part of the cycles the work function nearly fully recovers to the initial value of the hydrogen-free surface. As a result, the ratio of hydrogen coverage extrema in the ON/OFF cycles is on the order of ˜90% of the initial work function rise. This observation is consistent with the reported observation of the two desorption maxima at 310 K (β1 state) and 380 K (β2 state) in the hydrogen/Ni(111) system. Note that our observed initial work function rise of Δφ≈50 meV at 90° C. is also in reasonable agreement with the value of Δφ≈40 meV reported in Christmann et al. [J. Chem. Phys. 60, 4528 (1974)] for Ni(111) at 89° C. in 5×10−9 torr hydrogen.
Exploring Interfacial DMI Induced by Chemisorbed Hydrogen
Direct measurement of magnetic chirality is one of the major approaches to unravelling the interfacial DMI. For instance, ground-breaking observations of cycloidal spin spirals using spin-polarized scanning tunneling microscopy have revealed the role of the interfacial DMI on magnetic chirality as well as the period of the spin spirals. More recently, observation of magnetic chirality in magnetic domain walls also allows the quantification of the magnitude and sign of the interfacial DMI. A particularly versatile approach to measure the DMI at the top interfaces of magnetic multilayers emerges when the magnitude and sign of the effective DMI induced at buried interfaces within the structure can be tuned predictably and accurately. This can be done by using hybrid substrates composed of a bulk crystal coated with a spacer layer where the crystal and spacer induce a DMI of opposite sign, such as Ir/Pt(111), or Pd/W(110). The advantage of using a tunable-DMI substrate in this fashion was previously demonstrated in quantifying the DMI induced by chemisorbed oxygen on the Ni(111) surface. Here we test the DMI induced by chemisorbed hydrogen on the top surface of N(1)/Co(3)/Pd(dPd)/W(110), where the effective DMI in the buried interfaces favors left-handed Néel chirality (Pd-like) at thick Pd thickness dPd, and right-handed Néel chirality (W-like) at thin dPd.
What makes this method advantageous for quantifying even rather weak DMI contributions is the fact that the magnitude and sign of the effective DMI of the buried interfaces can be fine-tuned right around the point of null-DMI. Here we tracked the magnetic chirality evolution upon hydrogen chemisorption on various samples with different initial chirality. We observe a clear hydrogen-induced chirality switching in samples with Pd spacer layer thickness dPd≠2.09 ML, where the effective DMI of the hydrogen-free multilayer is weakly Pd-like (left-handed). In 602, a SPLEEM image of the sample in the as-grown state is shown, where the domain wall magnetization preferentially points from grey domain (−Mz) to the black domain (+Mz), corresponding to left-handed Néel chirality. Upon hydrogen chemisorption, it is shown that the same domain wall evolves to right-handed Néel chirality (now the domain wall magnetization predominantly points from black domain (+Mz) to grey domain (−Mz) at 604). We denote this switching of the magnetic chirality as the chirality transition. In 606, 608, it is shown that, before/after an 0.9 Langmuir hydrogen exposure, the peak at α˜0°, in 606, indicates left-handed Néel chirality, whereas the peak at α˜180°, in 608, indicates right-handed Néel structure. This statistical approach allows quantification of the chirality transition as shown in 610, where the average domain wall chirality before and after 0.9 Langmuir hydrogen exposure is plotted for several samples, as a function of Pd spacer layer thickness dPd. Note the hydrogen coverage resulting from this dose at room temperature can be roughly estimated as dH=(0.6±0.1) ML with respect to the planar atomic density of Ni(111) (Methods). When the Pd spacer layer is too thin and the effective DMI remains W-like (right-handed), as in the dPd=2.00 ML and dPd=2.05 ML measurements, then the domain wall chirality remains completely unaffected by hydrogen chemisorption. Likewise, when the Pd spacer layer is too thick, as in the dPd=2.15 ML sample, then the Pd-like effective DMI (left-handed) is sufficiently strong to dominate the domain wall spin texture, and the spin texture of the wall remains unaffected even after hydrogen chemisorption. However, when the initial DMI is sufficiently weak, as in the samples with dPd=2.08 ML, 2.09 ML and 2.10 ML, then hydrogen chemisorption induces a transition of the domain wall chirality, clearly revealing the right-handed DMI induced at the hydrogen/Ni(111) top interface. Note that the typical Néel- to Bloch-wall transition near zero DMI is suppressed because a weak in-plane uniaxial magnetic anisotropy in this system prevents Bloch-like alignment of domain wall magnetization along the W[1-10] direction. The Néel components of the wall magnetization, however, are clearly sensitive to the sign of the DMI. These results show that chemisorbed hydrogen on top of the Ni(111) surface introduces finite DMI favoring right-handed spin structures, i.e. the sign of the DMI induced by an overlayer is the same as for Pt, Pd or oxygen.
Estimation of the Strength of Chemisorbed Hydrogen Induced DMI
The systematic dPd spacer layer thickness-dependent chirality studies summarized in 610 allows estimation of the magnitude of the hydrogen induced DMI. The chirality evolution towards right-handedness is observed between 2.08 ML and 2.10 ML during 0.6 ML hydrogen chemisorption. Above 2.10 ML Pd, no significant chirality change can be observed as the initial effective Pd-like DMI now dominates and hydrogen induced DMI at the Ni(111) surface can no longer affect the chirality. This approach provides an opportunity to quantify the hydrogen induced DMI by linking it to the dependence of the initial DMI on the Pd spacer layer thickness dPd. Without hydrogen the achiral state of domain walls, where the effective DMI is essentially zero, occurs at dPd≈2.08 ML. Upon chemisorption of 0.6 ML hydrogen the achiral state shifts to dPd=(2.095±0.004) ML. The relative change of the DMI in the Ni/Co/Pd/W(110) system as a function of the Pd layer thickness dPd was previously quantified as (0.41±0.17) meV/atom per monolayer ΔdPd=1 ML. The measurements summarized in 610 show that the change of effective DMI induced by 0.6 ML hydrogen chemisorption on top of the Ni/Co/Pd/W(110) multilayer is equivalent to the change of the DMI induced by increasing the Pd spacer layer thickness by dPd=(2.095−2.08) ML=(0.015±0.004) ML in the absence of hydrogen. Therefore, the DMI induced by the chemisorbed hydrogen on Ni/Co/Pd/W can be estimated as
meV/atom for 1 ML equivalent hydrogen coverage.
In 612 is shown a comparison of the DMI induced by various elements adjacent to Ni. For instance, the chemisorbed hydrogen induced DMI is much weaker than the chemisorbed oxygen induced DMI on Ni, which is (0.63±0.26) meV/atom at 1 ML equivalent oxygen coverage. The strength of the hydrogen induced DMI is one to two orders of magnitude smaller than the DMI induced at Ni/transition metal interfaces, for example, DNi/Cu+Fe/Ni=(0.15±0.02) meV/atom, DNi/W≈0.24 meV/atom, DNi/Ir=(0.12±0.04) meV/atom, DNi/Pt=(1.05±0.18) meV/atom. The hydrogen induced DMI is also much weaker than the DMI induced at the Co/graphene interface, which is (0.16±0.05) meV/ atom. Note that here we only compare DMI measured in SPLEEM-based experiments using methods, to avoid possible systematic measurement biases resulting from the use of different methods.
Hydrogen-Assisted Reversible Control of the Chirality
The observation of substantial reversibility of hydrogen chemisorption by desorption in clean UHV at room temperature, together with the observed hydrogen induced switching of domain wall chirality, suggests the possibility to reversibly switch the domain wall chirality by hydrogen chemisorption/desorption cycles. To test this possibility, we use SPLEEM to continuously monitor the domain wall magnetization in a Ni(1 ML)/Co(3 ML)/Pd(2.09 ML)/W(110) multilayer, while periodically cycling between 5×10−9 torr hydrogen pressure for 3 minutes and negligible hydrogen pressure (UHV base pressure) for 10 minutes (Methods).
In 702, it is shown that the evolution of the domain wall chirality in the four cycles, where the chirality switched from predominantly left-handed to predominantly right-handed upon the 1 hydrogen chemisorption (see the definition of the chirality in 702), and the chirality partially evolves toward left-handedness/right-handedness during “H-off”/ “H-on” states for the rest of the cycles. In 702, it is shown that the statistics of this domain wall switching experiment, tracking reversibility of the chirality over four cycles at room temperature (Methods). This magnetic chirality measurement is correlated with a hydrogen coverage measurement, as monitored by tracking the work function change of +120 meV for the H-on state and ±80 meV for the subsequent cycles. These results indicate that the hydrogen coverage changes shown in 508 indeed reversibly affect the DMI of the system so as to switch the sign of the effective DMI as well as the domain wall chirality. In this experiment, the chirality reversal during the H-off state is imperfect in the sense that a small fraction of domain wall sections remains in the right-handed state corresponding to the hydrogen induced DMI. It is plausible that these minor imperfections in the chirality switching are due to a combination of defect-induced pinning and the weaker DMI associated with residual hydrogen coverage due to incomplete desorption in the 10-minute OFF cycles.
Origin of Hydrogen Induced DMI
The physical origin of the finite DMI induced by chemisorbed hydrogen might be related to the electric surface dipole moment induced by charge transfer at the interface , where the charge transfer can be explained by the difference in electronegativity between hydrogen and 3d ferromagnets (hydrogen has a Pauling electronegativity of 2.20 and 3d ferromagnets have a Pauling electronegativity of about 1.9). The presence of a hydrogen-induced electric dipole moment on the Ni surface is consistent with the significant work function change (about 120 meV) induced by the hydrogen adlayer (
Writing/Deleting Magnetic Skyrmions via Hydrogen Chemisorption
Magnetic skyrmions are a promising type of information carrier in spintronics devices with ultra-low energy consumption, and the creation/annihilation of skyrmions is a key step toward skyrmion-based devices. Here we demonstrate that chemisorption is a new way to write/delete magnetic skyrmions.
Significant Dzyaloshinskii-Moriya Interaction and Perpendicular Magnetic Anisotropy Induced by Chemisorbed Organic Molecules
Similar chemisorption processes could also lead to induced perpendicular magnetic anisotropy (PMA). PMA refers to the preference of a magnetic thin film to have its magnetic moment oriented normal to the film plane, instead of being in the plane of the film. This property is important in modern nanomagnetic devices such as magnetic recording media and magnetic memory and logic devices. PMA is usually achieved via interface magnetic anisotropy in magnetic multilayer thin films. The invention reported herein offers a new route using chemisorption to induce PMA.
The chemisorption of materials on the surface of magnetic materials can be realized by growing organic molecules on Ni(111) surface. To test the possible interface effect induced by organic molecules, we chose 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, also known as bathocuproine (BCP), which can be chemisorbed on the surface of metal films such as Magnesium. We have deposited BCP on the surface of Ni/Co/Pd/W, where the layer-by-layer growth associated LEEM oscillation allows the calibration of BCP layer thickness. The BCP growth can also be evident by the work function shift from ˜5.3 eV on 8 ML Ni/W(110) to ˜3.8 eV on 1 ML BCP/8 ML Ni/W(110).
We first study the possible DMI induced by the chemisorbed BCP layer. We find that the presence of half monolayer BCP can switch the magnetic chirality of domain walls in Ni/Co/Pd/W from left-handed Néel type (902) to right-handed Néel type (904), indicating a significant DMI induced at BCP/Ni interface that favors right-handed chirality. We find that the DMI change due to 1 ML BCP on Ni/Co/Pd/W is roughly equal to the DMI change of 0.1 ML Pd in Ni/Co/Pd/W, which indicates that the DMI at BCP/Ni is ˜18 times smaller than the DMI at the O/Ni interface (see section discussing how to quantitatively extract the strengths of these DMI contributions above and
We further test the role of BCP on PMA, by measuring the difference of critical ferromagnetic layer thickness at the spin reorientation transition (SRT). This approach has been used to explore the role of PMA at a given interface, such as graphene/Co.
In the present Ni/Co/Pd/W system, we have investigated Ni thickness-dependent domain structures, and found that the SRT occurs at thicker Ni film thickness in the Ni/Co/Pd/W system with BCP overlayer (1004), in contrast to the bare Ni/Co/Pd/W case (1002). This observation demonstrates the existence of induced PMA at the BCP/Ni interface.
The significant DMI and PMA induced by BCP may bring exciting opportunities for designing magnetic multilayer structures without heavy metals. Because BCP can be prepared on magnetic films at room temperature, and it is very different from the graphene case where the sample has to be annealed to at least 400-500° C. for the graphene growth, which may likely destroy the magnetic multilayers. Moreover, BCP is one of the most-common materials used between acceptor and electrode in organic photovoltaic cell, and BCP has been used to achieve air-stable BCP-based spin valves at room temperature, which may trigger the design of novel functionality of devices. For example, BCP may allow the combination of skyrmions and spin valves, as PMA could greatly enhance the performance of magnetic tunnel junction.
Discussions and Device Applications
The DMI induced by materials with strong spin-orbit coupling based on the Fert-Levy model has been experimentally observed at many interfaces between ferromagnets and heavy metals. The charge transfer and hybridization are expected to generally occur at interfaces between ferromagnets and the chemisorbed species. Although the induced DMI and PMA, the writing of magnetic skyrmions and switching of domain wall chirality due to the change of the effective DMI are demonstrated in specific systems (chemisorbed O2, H2 and bathocuproine) in this work, these effects may be generally applicable in other systems, including N2, F2, NH3, H2O, CH3, CH4, CO, CO2, fullerene (C60 and C70), organic molecules (Tris(8-hydroxyquinoline)aluminum(III), TMTES-Pentacene, Rubrene C42H28, B2PymPm), carbon nanotubes, carbon nanoribbons, and their ionic species such as O2−, H+, N3−, F− and OH− as layers that provide the DMI, and fullerene (C60 and C70), organic molecules (Tris(8-hydroxyquinoline)aluminum(III), TMTES-Pentacene, Rubrene C42H28, B2PymPm), carbon nanotubes, carbon nanoribbons as layers that provide the PMA. The choice of solid layers could be selected from transition metals, alkali metals, and lanthanides, including but not limited to Manganese, Iron, Cobalt, Nickel, Molybdenum, Ruthenium, Rhodium, Palladium, Cesium, Hafnium, Tantalum, Tungsten, Rhenium, Iridium, Platinum, Gadolinium, Terbium, Dysprosium, Holmium and their alloys, or selected from a group of other non-metallic materials, including but not limited to ferrites, garnets, rare-earth oxides, Heusler alloys, CrO2, graphene, CrI3, and Cr2Ge2Te6.
The sign change of the DMI due to the chemisorbed species effectively triggers the chirality switching of domain walls or skyrmions, as shown in
For example, the sensitive and reversible switching of the DMI and chiral spin texture via chemisorption is highly relevant for chiral spintronics, such as “racetrack” type of magnetic memories [see Parkin et al, Science 320, 190-194 (2008)] where the magnetic state is stored in domain walls or skyrmions propagating along a track, as shown in 1102 of
Another example is for gas-sensing, as the chemisorption induced effects are extremely sensitive to trace amount of chemisorbed species, down to a fraction of a monolayer of atoms. By measuring the spin texture change caused by exposure to certain gases such as oxygen or hydrogen, e.g., switching of perpendicular magnetization in-plane or vice versa, or toggling of the domain wall chirality, or manipulation of magnetic skyrmions, which can be read out electrically through the aforementioned magnetic memory and logic devices, e.g., via magnetoresistance response, one can sensitively detect the presence of these gases. Given the differences in the induced effect sizes, e.g., oxygen induced DMI is more than an order of magnitude larger than that induced by hydrogen, one can differentiate different gases, yielding gas selectivity. Such gas sensing capabilities may have large economic importance in various commercial applications including hydrogen-based energy storage and energy conversion systems. For example, in such energy applications hydrogen gas and oxygen gas coming into unintended contact may cause combustion or explosion risks, therefore prevention of such risks necessitates sensors capable to detect oxygen contamination in hydrogen vessels and vice-versa.
These chemisorption-based results are also relevant to the emerging field of magneto-ionics, which uses ionic motion across magnetic heterostructure interfaces to transform those interfaces and their physical and chemical properties. They not only significantly expand on the magnetic functionalities that can be controlled magneto-ionically, but also offer exciting potentials for completely reversible and energy-efficient switching. So far much of the progress has been based on oxygen ions and vacancies. Tan et al. have demonstrated H+-based reversible magneto-ionic switching at room temperature where electric field-controlled hydrogenation at the buried Co/GdO interface is used to toggle the perpendicular magnetic anisotropy (PMA) [Nature Materials 18, 35 (2019)]. Hydrogen based magneto-ionics is particularly appealing, comparing to mostly oxygen-based systems studied so far, due to the superior reversibility and speed. However, besides PMA, other hydrogen-induced magneto-ionic functionalities remain largely unexplored.
Embodiments may be used in multilayer heterostructures, where the chemisorption onto ferromagnet surface takes place under buried interfaces, as illustrated in 1202 of
Summary
In summary, our experiments demonstrate significant DMI and PMA induced by chemisorbed species on ferromagnet films. We show that the DMI induced at an oxygen/Ni interface is comparable to that induced at interfaces with most heavy metals. We find this large chemisorbed oxygen induced DMI can be used to write magnetic skyrmions. We also show that a tuneable DMI systems such as the Pd/W(110) system employed here open up a useful and broadly applicable way to quantify the magnitude of the DMI in thin film systems. We show that the DMI induced at a hydrogen/Ni interface allows sensitive and reversible switching of domain wall chirality in Ni/Co/Pd/W system. We anticipate that significant DMI may also be induced by chemisorbed oxygen, hydrogen, or other species on other magnetic surfaces such as chromium, manganese, iron or cobalt. The observation of the significant DMI and PMA induced by chemisorbed species, along with the possibility of voltage-controlled ionic migration in multilayer systems, may create new possibilities in the field of spintronics and magneto-ionics.
Methods
Sample preparation. The experiments were conducted in the SPLEEM instrument at the National Center for Electron Microscopy of Lawrence Berkeley National Laboratory. All samples were prepared under ultra-high vacuum conditions in the SPLEEM chamber, with a base pressure better than 4.0×10−11 torr. The W(110) substrate was cleaned by flashing to 1,950° C. in 3.0×10−8 torr O2, and final annealing at the same temperature under ultrahigh vacuum to remove oxygen. Ni, Co and Pd layers were deposited at room temperature by physical vapour deposition from electron beam evaporators, and the film thicknesses of Ni, Co and Pd layers were controlled by monitoring the LEEM image intensity oscillations associated with atomic layer-by-layer growth.
Oxygen exposures were done by controlled leaking of high-purity oxygen at pressure in the range of 5×10−9 torr to 1×10−8 torr, and the surface contaminations from other residual gases (base pressure <4×10−11 torr) is estimated to be at least two order of magnitude less, which is insufficient to influence the result. The oxygen coverage is estimated based on the kinetics relation previously reported in by Kortan and Park [Phys. Rev. B 23, 6340 (1981)], and our measurements of oxygen-coverage dependent work function change as well as the low-energy electron diffraction pattern shows excellent agreement with the previous measurement. The low-energy electron diffraction pattern is taken at total dose of 2L on the surface of 1 ML Ni/3 ML Co/2.5 ML Pd/W(110) system, at electron energy of 80 eV.
Hydrogen exposures were realized by leaking of high-purity hydrogen (99.999%) at a pressure of at 5×10−9 torr. The pressure of hydrogen reading of the ionization gauge has been corrected by a factor of 0.46. No noticeable change was observed in the LEED pattern upon hydrogen chemisorption at room temperature. On Ni(111), the maximum work function shift occurs at the hydrogen coverage of 0.5-0.6 ML , and volumetric measurements reveals that the saturation coverage of chemisorbed hydrogen on Ni(111) is ˜0.7 ML at room temperature. Therefore, the hydrogen coverage on the surface of Ni/Co/Pd/W(110) is estimated based on the work function shift measurement with a maximum work function shift (Δφ=125 meV), which roughly corresponds to 0.5-0.7 ML hydrogen overlayer.
Time-Dependent Work Function Measurement
The work function is determined by fitting the LEEM IV spectrum (image intensity vs incident energy of electrons, see 502) with a complementary error function erfc (Start voltage). The value where the drop-off occurs, VSO, represents a measurement of the sample work function given by ϕsample=VSO+ECO, where a represents the peak of the electron distribution emitted from our photocathode (p-type GaAs crystal activated with CsO). The emission of the GaAs cathode of SPLEEM is set to 100 nA to optimize the energy spread to about 180 meV (full width at half maximum) and ECO˜1.4-1.5 eV measuring a reference surface such as Highly Oriented Pyrolytic Graphite (HOPG). Time-dependent work function measurements were performed by recording the reflectivity of low energy electrons while sweeping the start voltage in a loop. In order to record the work function changes during hydrogen adsorption/desorption at the surface, the start voltage was swept from 1.5 V below to about 2 V above the intensity drop-off using 50 mV voltage steps and an image integration time of 250 ms. Relative changes in the work function over time can be detected with very-high sensitivity down to about 5 mV given by the shift of the centroid of the gaussian distribution extracted by the erfc (Start voltage) fitting.
Time-dependent in-plane domain wall analysis Due to the noise present in the individual in-plane domain wall images, we used standard image denoising methods to provide a more accurate estimate for the magnetization presented in
Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims.
This application claims the benefit of U.S. Provisional Application No. 62/888,691, filed Aug. 19, 2019, the contents of which are incorporated herein in their entirety.
This invention was made with government support under Grant Numbers DMR-1610060 and DMR-1905468, awarded by the National Science Foundation; Grant Number MRP-17-454963 awarded by the University of California Office of the President Multi-campus Research Programs; Contract Number DE-AC02-05CH11231 awarded by the Office of Science, Office of Basic Energy Sciences, U.S. Department of Energy; and Grant Number 2018-NE-2861, awarded by nCORE, a Semiconductor Research Corporation program, sponsored by the National Institute of Standards and Technology (NIST). The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2020/046125 | 8/13/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62888691 | Aug 2019 | US |
