Structural and magnetic properties of thin cobalt films with mixed hcp and fcc phases

Gauravkumar Patel, Fabian Ganss, Ruslan Salikhov, Sven Stienen, Lorenzo Fallarino, Rico Ehrler, Rodolfo A. Gallardo, Olav Hellwig, Kilian Lenz, and Jürgen Lindner

Phys. Rev. B 108, 184429 – Published 27 November 2023

Abstract

Cobalt is a magnetic material that finds extensive use in various applications, ranging from magnetic storage to ultrafast spintronics. Usually, it exists in two phases with different crystal lattices, namely in hexagonal-close-packed (hcp) or face-centered-cubic (fcc) structure. The crystal structure of Co films significantly influences their magnetic and spintronic properties. We report on the thickness dependence of the structural and magnetic properties of sputter-deposited Co on a Pt seed layer. It grows in an hcp lattice at low thicknesses, while for thicker films it becomes a mixed hcp-fcc phase due to a stacking fault progression along the growth direction. The x-ray-based reciprocal space map technique has been employed to distinguish and confirm the presence of both phases. Moreover, the precise determination of Landé's $g$ -factor by ferromagnetic resonance provides valuable insights into the structural properties. In our detailed experiments, we observe that a structural variation results in a nonmonotonic variation of the magnetic anisotropy along the thickness. This careful study reveals the fundamental physics, but also provides important insight for potential applications of thin Co films with perpendicular magnetic anisotropy.

Received 15 May 2023
Revised 11 August 2023
Accepted 30 October 2023

DOI:https://doi.org/10.1103/PhysRevB.108.184429

Physics Subject Headings (PhySH)

Ferromagnetism Magnetic anisotropy Magnetometry

Ferromagnetic resonance Magnetic force microscopy X-ray scattering

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Gauravkumar Patel ^1,2, Fabian Ganss¹, Ruslan Salikhov ¹, Sven Stienen¹, Lorenzo Fallarino^1,3, Rico Ehrler⁴, Rodolfo A. Gallardo⁵, Olav Hellwig ^1,4, Kilian Lenz ¹, and Jürgen Lindner ¹

¹Helmholtz-Zentrum Dresden–Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany
²Faculty of Physics, Dresden University of Technology, D-01062 Dresden, Germany
³CIC energiGUNE, Parque Tecnológico de Álava, Albert Einstein 48, 01510 Vitoria-Gasteiz, Spain
⁴Institute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany
⁵Departamento de Física, Universidad Técnica Federico Santa María, Avenida España 1680, 2390123 Valparaíso, Chile

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 108, Iss. 18 — 1 November 2023

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) Sketches of the sample stack with and without the 20 nm Pt seed layer. The Co thickness $X$ was varied between 4 and 110 nm. (b) A schematic diagram of the RSM measurement technique as applied in this study.
Reuse & Permissions
Figure 2
(a) $θ / 2 θ$ XRD scans of the 35-nm-thick Co film with the Pt seed layer (blue curve) and without (red curve). The vertical black solid line and the magenta dashed line indicate the peak positions for fcc Co (111) (or Co (222)) and hcp Co (0002) (or Co (0004)), respectively. The inset magnifies the Co (0002)/(111) peak region indicated by the black rectangle. A small bump around $2 θ = 40^{\circ}$ in the red curve originates from the Pt cap layer. The Cu and W spectral lines of the x-ray source are labeled. The slight mismatch observed in the XRD data for the Si (004) substrate peaks in the two different samples is caused by the miscut of the Si wafer surface and the alignment routine referring to the sample surface. Since the films are grown on a 100-nm-thick thermally oxidized surface of Si and have a much broader texture, the exact orientation of the Si lattice does not significantly affect the film reflections, and it can be disregarded, as Fig. S1 [44] demonstrates in more detail. (b) Rocking curve measurements for a Co thickness of 35 nm around the Co (0002)/(111) Bragg peak. For Co without Pt seed (red curve), the measured intensity is scaled by a factor of 3. (c) $d$ -spacing of the Co (blue line) and Pt (magenta line) lattice planes as a function of the Co thickness. The dashed horizontal lines indicate reference $d$ -spacing values for the hcp [45] and fcc [46] phases of Co. (d) FWHM of the Co (0004)/(222) diffraction peak for the thickness series with Pt seed.
Reuse & Permissions
Figure 3
Reciprocal space maps with black contour lines of (a) 13 nm, (b) 35 nm, (c) 43-nm-thick Co with Pt seed, and (d) a 35-nm-thick Co film without Pt seed. Magenta dots denote the angles where the peaks of ideal fcc (002) [denoted as “ ${Co}_{fcc}$ (002)”] or hcp ( $01 \bar{1} 1$ ) [denoted as “ ${Co}_{hcp}$ ( $01 \bar{1} 1$ )”] lattices are expected, respectively. The central red region in (a), (b), and (c) stems from the 20 nm Pt seed layer.
Reuse & Permissions
Figure 4
(a) $f (H)$ plots for both thickness series. The same color indicates the same thickness; solid and dashed lines indicate samples with and without Pt seed, respectively. (b) Effective magnetization ( $μ_{0} M_{eff}$ ) of Co with (blue curve) and without (red curve) Pt seed as well as effective anisotropy ( $K_{eff}$ ) of Co with Pt seed (black curve) are plotted as functions of the Co thickness. $K_{eff}$ is determined from the area enclosed between in-plane and out-of-plane hysteresis loops in the positive field cycle as highlighted in Fig. 5. (c) $g$ -factor of Co with (blue curve) and without (red curve) Pt seed is plotted as a function of the thickness of Co. The horizontal dotted and dashed lines are the references for the $g$ -factors of hcp and fcc phase of Co, respectively.
Reuse & Permissions
Figure 5
(a)–(e) In-plane (red) and out-of-plane (black) magnetization reversal (hysteresis) loops measured by VSM at room temperature for selected Co thicknesses with the Pt seed layer. The gray area in the top right quadrant of (a) is used to determine the effective magnetic anisotropy ( $K_{eff}$ ). $K_{eff}$ is calculated for all samples with Pt seed and plotted in Fig. 4. The inset of (a) shows the enlarged IP hysteresis loop for an external field range between $\pm 0.05$ T with a field step of 0.5 mT. (f)–(j) Corresponding MFM images taken at remanence after out-of-plane saturation. All the MFM images are plotted with the same color scale, which corresponds to the phase change.
Reuse & Permissions
Figure 6
Coercive field ( $H_{c}$ ) of both thickness series as determined from IP hysteresis loops, plotted vs film thickness.
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics