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Growth of half-metallic CrO2 nanostructures for superconducting spintronic applications

Superconductor-ferromagnet (S-F) hybrids based on half-metallic ferromagnets, such as CrO2, are excellent candidates for superconducting spintronic applications. This is primarily due to their fully spin polarized nature, which produces significantly enhanced long-range triplet proximity effects.. However, reliable production of CrO2-based Josephson junctions (JJs) is challenging, mainly because of the difficulty to control the transparency of the S/F interface. We have grown CrO2 nanowires by chemical vapor deposition on TiO2 substrates combined with selective area growth in trenches defined with amorphous SiOx. This allows us to create lateral JJs, with the nanowire as the weak link. We show that the nature of the growth is highly anisotropic, and that the morphology of the CrO2 nanostructructures changes systematically during the growth process, depending on the width of the trench. The detailed growth study enables us to synthesise multifaceted and highly homogeneous CrO2 wires. These are utilized to fabricate JJs with high S/F interface transparency, leading to large supercurrents. The well-defined geometry of the device allows us to reliably estimate an exceptionally high critical current density Jc = 10^9~Am^{-2} over a distance of 600~nm.

Superconductor/Ferromagnet (S/F) hybrids have attracted great interest due their potential application in spintronic devices to minimize the dissipation/Joule heating, which is the biggest challenge for current-driven processes at the nanoscale [1][2][3]. The novelty of such hybrids lies in the generation of spin polarized triplet Cooper pairs in the ferromagnet [4][5][6]. They are generated by subjecting singlet Cooper pairs at the S/F interface to spin mixing and rotation [7], which is usually achieved by introducing another thin ferromagnetic layer at the interface [8][9][10][11]. Since triplet Cooper pairs are not affected by the exchange field of the ferromagnet, they can survive over considerably long distances, thereby giving rise to spin-polarized supercurrents. Fully spin polarized CrO 2 is an ideal ferromagnet for such applications. The spatial extent of induced triplet correlations in CrO 2 is of the order of a micron [12,13], while it is limited to few tens of nm for standard ferromagnets (Co, Ni, Fe). Furthermore, the efficiency of the triplet generation is sigficantly enhanced, as demonstrated in recent experiments in a spin valve geometry [14,15]. Though large supercurrents have been observed in CrO 2 films [16], a serious bottleneck for further studies and applications of such Josephson junctions (JJs) has been the poorly controlled interface transparency, together with ill-defined current paths in the full film. Etching wire structures out of films proved no solution since the resist mask and the etch step are detrimental to the surface and bring extra disorder in the material. To address this issue, we have made a detailed investigation into the growth of CrO 2 nanowires by selective area chemical vapor deposition (CVD).
A salient feature of the CVD growth of CrO 2 is that it is only known to grow epitaxially on TiO 2 or sapphire substrates. This is an advantage for selective area growth (SAG), whereby a patterned SiO x mask can be used to define the geometry of the CrO 2 nanostructures. Previous reports on SAG of CrO 2 focussed on the morphology of thick CrO 2 structures but did not give insight into the nucleation and growth kinetics at the early stage of growth [17,18]. Here, we demonstrate the effect of mask confinement, lattice mismatch anisotropy, and trench aspect ratio on the magnetic and structural morphology of CrO 2 nanostructures at different stages of growth, covering a wide range of film thicknesses. A better understanding of SAG allows us to create high quality uniform CrO 2 nanowires with well-defined facets resulting in transparent interfaces. In addition, we show that the magnetization state of the nanowire can be controlled by exploiting its shape anisotropy. This robust control over the magnetization and transparency enables us to reproducibly fabricate CrO 2 -based JJs with large critical current densities. More generally, we have developed a device platform which will allow for a large variety of different wire, contact and magnetic domain geometries.
Selective Area Growth We start with a rutile (100) TiO 2 substrate which has a tetragonal surface net (b = 0.459 nm, c = 0.296 nm), on which CrO 2 (b = 0.442 nm, c = 0.292 nm) can be grown isostructurally. The lattice mismatch is anisotropic, being -1.4 % along the b-axis and -3.8 % along the c-axis, respectively. This leads to anisotropic growth rates, which is important for nanowire growth. Substrates were cleaned with organic solvents (Acetone/IPA) followed by HF treatment, on which a 25 nm SiO x thin film was sputter-deposited. Using e-beam resist and lithography, openings of different sizes and shapes were created in the resist along different crystallographic directions. SiO x was selectively removed from the openings using RIE etching with a of SiO x while overetching damages the underlying TiO 2 substrate, which affects the film quality adversely. Next, CrO 2 was grown selectively in the SiO x trenches (trench depth 25 nm, trench width w trench ) by CVD in a twozone furnace where the substrate temperature was kept at 395 • C while the precursor CrO 3 was heated to 260 • C in the presence of a flow of 100 sccm O 2 carrier gas. By monitoring the growth times (t g ), samples with varying thicknesses were prepared. Each sample consists of CrO 2 structures with different sizes, shapes, aspect ratios and orientations. Scanning electron microscopy (SEM), Atomic force microscopy (AFM), and Magnetic force microscopy (MFM) were used to examine the structural and magnetic morphology of the CrO 2 nanostructures. In-trench growth Fig. 1a-e show the SEM images of L-shaped nanowires, grown for 20 minutes, with w trench varying from 230 nm to 2 µm. The time is chosen such that it fills the 25 nm deep trench. For the discussion, the width range is divided into two regimes, Regime I : w trench ≤ 2λ, and Regime II: w trench ≥ 2λ, where λ is a characteristic length bearing resemblance to a surface diffusion length, which is anisotropic due to the anisotropic lattice mismatch. For free films, growth results in the formation of rectangular CrO 2 grains with their long axis along [001] and with an average size of ∼300 nm×100 nm, which sets the scale for λ. In the strongly confined structures shown in Fig.1a which also is the unconfined direction. Individual grains grow faster and coalesce to form a continuous and homogeneous wire. Along [010] the wires grow in a peculiar fashion, namely from the sides rather than from the bottom. Fig. 1c shows a close-up of the 340 nm trench where a smooth surface is seen at the sides, and a hole in the middle where, at the bottom, growth has also started. Apparently, CrO 2 islands nucleate preferably at the SiO x /TiO 2 interface and grow laterally inward but also vertically upward. This dynamically inhibits the supply of the reactants towards the bottom of the trench, but it does allow inward lateral overgrowth starting higher up the wall. The result is a disordered structure with voids. This so-called nanopendeo-epitaxy overgrowth (nPEO) is known from semiconductor growth, in particular for GaN, in the fabrication of nanoairbridges and suspended structures of GaN with superior quality [19][20][21].
At w trench = 550 nm ( Fig.1d) Regime II starts and the growth changes to a homogeneous filling of the trench [22]. Islands nucleate along the walls and "quantum fortress"-like structures are formed irrespective of the orientation of trenches [23]. Increasing w trench to 1 µm and 2 µm (Fig.1e,f) the filling becomes inhomogeneous again, with the middle of the trench empty but now because the growth fronts propagating from the walls do not reach the middle. We assume that the size of the CrO 2 strip along the walls is determined by λ and we observe λ 010 ∼ 450 nm and λ 010 ∼300 nm independent of w trench . Note that these reults are quite different from earlier reports on SAG of CrO 2 , where it was assumed that CrO 2 nucleates and grows uniformly in the narrow trenches away from the SiO x side walls [18]. Our results present a very different picture, with important new aspects for CrO 2 nanowire growth.
Lateral overgrowth Next we focus on the lateral epitaxial overgrowth (LEO) above the SiO x mask at higher t g . Fig. 2a-c shows the evolution of the structural morphology of CrO 2 , grown in circular trenches, for different t g . For t g = 20 minutes, no overgrowth yet occurs, resulting in the formation of CrO 2 discs. With further increase in t g (25 minutes), CrO 2 starts to overgrow laterally on SiO x . Important to realize is that there is no epitaxial relation with the amorphous SiO x , so this is sideways growth starting from the structure coming out of the trench. The development of (011) facets can be observed, assumedly to minimize the total surface energy, although the facets formed at this stage are rough with step edges. For t g = 45 minutes, the CrO 2 islands exhibit fully developed facets with smooth surfaces. This type of facet evolution with growth time has been reported recently for SAG of GaN [22]. In order to isolate the effect of lattice mismatch and size on the issue of CrO 2 overgrowth, we also prepared rectangular 500 nm wide SiO x trenches oriented between the (010)-(θ = 0 • ) and (001)-directions (θ = 90 • ), at intervals of 15 • . The structural morphologies were characterized at different stages (thicknesses) of growth. In Fig. 3a schematic of the LEO is outlined and Fig 3b,c show the SEM images of CrO 2 nanowires, with t g = 20 minutes, and 45 minutes, respectively. Clearly, there is no anisotropy in the lateral growth for in the first 20 minutes of growth while it is highly anisotropic for t g = 45 minutes. As shown in Fig. 3c, the nanowires exhibit jagged facets at the intermediate angles which is attributed to the higher growth rate along (001) [17]. The anisotropy of lateral overgrowth with SAG has been reported earlier for thick CrO 2 structures [17,18]. However, from our data we can extract additional and quantitative information on the anisotropy in lateral as well as vertical overgrowth over a range of sizes and thicknesses. In Fig. 3d we plot the full width of the nanowire as measured from Fig.3c. This exhibits a maximum at θ = 60 • and becomes more pronounced with increasing t g . The direction coincides with the diagonal of the CrO 2 unit cell.
In Fig.3e we plot the lateral overgrowth (defined as the difference between the total width of the CrO 2 nanowire and the trench width) as function of θ for different trench widths up to 500 nm (Regime I). The Figure shows that the lateral overgrowth hardly depends on trench width for the various angles, confirming that it is mainly determined by the thickness and anisotropic strain. Finally, to study the anisotropy in vertical growth, AFM was used to determine the thickness of the nanowires grown for different times up to t g = 45 minutes. Fig. 3f again shows suggests that during the first 20 minutes the CrO 2 growth front just reaches the top of the SiO 2 trench and the thickness is isotropic. For higher growth times the vertical overgrowth becomes anisotropic, now with the minimum along θ = 60 • : lateral overgrowth clearly goes at the expense of vertical overgrowth, with the largest effect around 60 o from the (010)-direction. This is in contrasts with the previous reports on SAG of CrO 2 structures where vertical overgrowth was reported to be isotropic [17]. More formally, the correlation between the vertical and the lateral overgrowth, can be understood from the mass transport mechanism i.e. absorption or desorption of adatoms on different facets of CrO 2 during the CVD growth. The vertical growth depends on the impingent flux on top and on the sides of the growing surface and also on the diffusion flux from the substrate to the top surface [24]. The size dependence of the vertical growth rate is given by the following expression, where H is the height, D is the width of the structures, γ top , γ sw , and γ sub are the constants related to surface impingement on top and side wall and diffusion from the substrate, respectively. In this simple picture, the maximum lateral overgrowth corresponds to largest D and hence lowest vertical overgrowth. Furthermore, with change in orientation of the wire, γ top remains constant while the contribution from γ sw , and γ sub decreases due to the formation of jagged edges at the intermediate angles. Nonuniform / ragged edges prevent the surface diffusion of adatom on the side walls and also from the substrates, slowing down the vertical overgrowth. Magnetic domains Next we discuss the magnetic domain structures of our nanowires. Domain behavior has been studied both in in early SAG investigations [18] and in etched wires [25,26] and it is well known that for CrO 2 films on TiO 2 the [001] direction is the magnetic easy axis while the [010] direction is the magnetic hard axis, as a consequence of the strain. Fig.4a shows the magnetic structure of wires of different size along different crystallographic orientations as measured by using magnetic force microscopy (MFM). The wires oriented along [001] are in a single domain state, as seen from the absence of any contrast in MFM image, and in the cross-section shown in the lowest panel of Fig. 4b. When the wire is rotated away from the easy axis, striplike domains start appearing in the wires to minimize the total magnetic energy. This can be seen by dark/bright contrast the MFM micrograph (Fig. 4a) and as oscillations in the magnetic contrast (top and middle panel Fig. 5b). We observe that the domain size is also direction dependent with a minimum along the [001] axis, which is the hard axis. The wires therefore behave as expected.
Supercurrents As mentioned in the introduction, fully spin-polarized ferromagnetic nanowires are of interest for superspintronics applications, and here we show that we can fabricate Josephson junctions with the CrO 2 nanowire as weak link. First we grow a nanowire along the easy axis [001] in order to have the magnetization along the wire axis. Next we use lift-off and sputtering to deposit rectangularly shaped contacts of (Cu or Ag)(5 nm)/Ni(1.5 nm)/MoGe(125 nm) trilayer stacks, with their long axis perpendicular to CrO 2 wire, i.e. along the [010] direction as shown in Fig. 5a. The distance between the contacts is typically 500 nm. The shape anisotropy aligns the magnetization of the Ni layer in the contact stack along the crystal [010]) direction, which is very effective in providing maximum non-collinearity at the interface, a key ingredient for triplet generation. In the trilayer stack we used either Cu or Ag to to prevent exchange coupling between the CrO 2 and Ni layers. Amorphous MoGe is the superconductor used to induce the proximity effect. Since at the surface CrO 2 inevitably reduces to the antiferromagnetic insulator Cr 2 O 3 we clean it in-situ with a brief Ar-etch prior to the trilayer contact deposition. In this report, we present the results obtained on two JJ devices, one with Cu and one with Ag as spacer layer, named J1 and J2, respectively. The other parameters of J1 and J2 are provided in the table below. Fig.5b shows a plot of resistance R versus temperature (T ) for J1 measured in a quasi-4-point geometry, meaning  that the interface resistance is part of the total normal state resistance. R disappears around 6.5 K, but there is a finite resistance tail down to 5 K, where R reaches 0 (c.q. the measurement limit). Below 5 K, supercurrents can be measured. Fig.5c shows the current-voltage (I-V) characteristics of J1 at different temperatures from which the critical current of the weak link (the proximized CrO 2 ) can be determined. Interestingly, despite its lower junction length (see Table I), J1 has a lower critical current density than J2, which is probably due to lower interface transparency. Our earlier studies did not use Ag, but the result indicates that the use of Cu might lead to degradation of the interface by the formation of some oxide. Fig. 5d displays the variation of critical current density as a function temperature for J1 and J2, and as expected, I c increases monotonically with the decrease in temperature. The measured resistance of the device is dominated by the Cu layer in the leads, since the Cu specific resistivity ρ 0,Cu of the order of 5 µΩcm is much smaller than ρ 0,M oGe ≈ 200µΩcm. Taking a length of 70 µm per lead, a width of 2.5 µm and a thickness of 5 nm, we find 560 Ω. The normal resistance of the CrO 2 weak link we estimate using ρ 0,Cro2 ≈ 6 µΩcm (the thin films value for the easy axiss) to be 0.32 Ω. This leaves about 20 Ω as the contact resistance. We note in Fig.5b that the low-temperature R is well below 0.32 Ω, and in Fig.5c that the voltage increase above I c starts with a small steep slope, but then settles to a slope which is close to 0.3 Ω, quite as expected for the CrO 2 weak link. Analyzing the junction behavior a bit further, in the long junction limit I c is proportional to T 3/2 exp(2πk B T /E th ) −1/2 where E th = D/L 2 is the Thouless energy of the junction with diffusion coefficient D and junction length L. Therefore, the slope of the plot of ln(I c ) − 3/2ln(T ) vs T 1/2 , shown in the inset, can be used to estimate the Thouless energy E T h which is 11 µV. From this number we can estimate D to be 2.7 × 10 −3 m 2 /s, which is close to numbers obtained before. Using a Fermi velocity of 2.2 × 10 5 m/s [27] it corresponds to a mean free path of 36 nm which suggests that our JJS are in long (diffusive) junction limit.
Conclusions To conclude, we have made a detailed study of growth kinetics of halfmetallic ferromagnetic CrO 2 nanowires by selective area growth on TiO 2 , in 25 nm deep trenches defined by a SiO x mask. At an early stage of growth (t g =20 minutes), for trench width below 500 nm we find that thin homogeneous wires can be grown along [001], while along [010] the growth is highly disordered, with voids, formed in a fashion called nanopendeo epitaxy. Above a trench width of 500 nm, the trench is lined by CrO 2 strips formed along the edges with nonuniformity at the center. By increasing the growth time, uniform CrO 2 structures are formed due to the lateral overgrowth over SiO x mask. Wires grown along the magnetic easy axis, which is the [001] direction, were contacted with stacks of Cu/Ni/MoGe and Ag/Ni/MoGe, which allowed us to study triplet supercurrents. This upholds the earlier observation of very high supercurrent densities of order 10 9 A/m 2 at 4.2 K over a distance of 600 nm. CrO 2 wire based Josephson junctions will make it now possible to characterize the in- terfaces fully, make reliable measurements of the dependence of critical current on junction length, and study the influence of magnetic domains in the supercurrent path.