Thin film epitaxial [111] Co[Formula: see text]Pt[Formula: see text]: structure, magnetisation, and spin polarisation

Abstract

Ferromagnetic films with perpendicular magnetic anisotropy are of interest in spintronics and superconducting spintronics. Perpendicular magnetic anisotropy can be achieved in thin ferromagnetic multilayer structures, when the anisotropy is driven by carefully engineered interfaces. Devices with multiple interfaces are disadvantageous for our application in superconducting spintronics, where the current perpendicular to plane is affected by the interfaces. Robust intrinsic PMA can be achieved in certain Co[Formula: see text]Pt[Formula: see text] alloys and compounds at any thickness, without increasing the number of interfaces. Here, we grow equiatomic Co[Formula: see text]Pt[Formula: see text] and report a comprehensive study on the structural, magnetic, and spin-polarisation properties in the [Formula: see text] and [Formula: see text] ordered compounds. Primarily, interest in Co[Formula: see text]Pt[Formula: see text] has been in the [Formula: see text] crystal structure, where layers of Pt and Co are stacked alternately in the [100] direction. There has been less work on [Formula: see text] crystal structure, where the stacking is in the [111] direction. For the latter [Formula: see text] crystal structure, we find magnetic anisotropy perpendicular to the film plane. For the former [Formula: see text] crystal structure, the magnetic anisotropy is perpendicular to the [100] plane, which is neither in-plane or out-of-plane in our samples. We obtain a value for the ballistic spin polarisation of the [Formula: see text] and [Formula: see text] Co[Formula: see text]Pt[Formula: see text] to be [Formula: see text].

Figure 1

Structural characterisation of Al${}_2$O${}_3$(sub)/Pt (4 nm)/CoPt (40 nm)/Pt (4 nm) sheet films grown at different set temperatures obtained by X-ray diffraction measurements. (a–c) X-ray diffraction data for samples at selected growth temperatures. The position of the main [111] structural peak for the CoPt layer is indicated. (d) The normalised CoPt [111] peak intensity. (e) The CoPt crystallite size determined from the CoPt [111] peak Gaussian full width half maximum value. (f) The CoPt c-plane spacing. Uncertainties in the peak intensity, crystallite size, and c-plane spacing are smaller than the data symbols.

Figure 2

Structural characterisation of Al${}_2$O${}_3$(sub)/Pt (4 nm)/CoPt (40 nm)/Pt (4 nm) sheet films grown at different set temperatures obtained by X-ray reflectivity measurements. (a,c,e) X-ray reflecitvity data for samples at selected growth temperatures. The reflectivity is fit to a best fit model, which is shown correspondingly in (b,d,f). (g) The surface roughness parameter extracted from the best fit model. (h) The interfacial roughness parameter from the model, indicating the interdiffusion between the CoPt and Pt layers.

Figure 3

Magnetisation characterisation of Al${}_2$O${}_3$(sub)/Pt (4 nm)/CoPt (40 nm)/Pt (4 nm) sheet films grown at different set temperatures with the applied field oriented out-of-plane ($▴$) and in-plane ($▶$). (a–c) Magnetic hysteresis loops acquired at a temperature of 300 K. The sample growth temperature is (a) $350{}^{\circ }\text{C}$, corresponding to $L{1}_1$ crystal structure, (b) $550{}^{\circ }\text{C}$, corresponding to disordered $A_1$ growth, and (c) $800{}^{\circ }\text{C}$, corresponding to $L{1}_0$ crystal structure. The diamagnetic contribution from the substrate has been subtracted. (d–f) Trends in the magnetic characterisation as a function of sample growth temperature, (d) magnetisation, (e) saturation field and (f) squareness ratio ($M_r/M_s$). Magnetisation is calculated from the measured total magnetic moments, the areas of the sample portions, and the nominal thickness of the CoPt layer (40 nm). The significant contribution to the uncertainty in magnetisation is from the area measurements of the sample portions.

Figure 4

In-plane to out-of-plane angular dependent Hall resistance for Al${}_2$O${}_3$(sub)/Pt (4 nm)/CoPt (40 nm)/Pt (4 nm). (a) Schematic of Hall bar devices, measurement geometry, and applied field direction. Hall resistance verses angle for (b) the $L{1}_1$ crystal structure ($350{}^{\circ }\text{C}$ growth temperature) at 0.4 T and (c) $L{1}_0$ crystal structure ($800{}^{\circ }\text{C}$) at 3 T. Data acquired at a temperature of 295 K.

Figure 5

In-plane rotation of the Al${}_2$O${}_3$(sub)/Pt (4 nm)/CoPt(128 nm)/Pt (4 nm) sample with $L{1}_0$ crystal structure ($800{}^{\circ }\text{C}$). The coercive field is extracted from the hysteresis loop measured at each rotator angle. The position of 0${}^{\circ }$ is arbitrary. The line shows a best fit sine function as a guide for the eye. Data acquired at a temperature of 300 K.

Figure 6

Magnetisation characterisation of Al${}_2$O${}_3$(sub)/Pt (4 nm)/CoPt($d_{\text{CoPt}}$)/Pt (4 nm) sheet films with the applied field oriented out-of-plane ($▴$) and in-plane ($▶$). (a,b) Magnetic hysteresis loops acquired at a temperature of 300 K for $d_{\text{CoPt}}=4$ nm. The sample growth temperature is (a) $350{}^{\circ }\text{C}$, corresponding to $L{1}_1$ crystal structure, (b) $800{}^{\circ }\text{C}$, corresponding to $L{1}_0$ crystal structure. The diamagnetic contribution from the substrate has been subtracted. (c,d) Magnetic moment per area versus $d_{\text{CoPt}}$ for growth temperature of (c) $350{}^{\circ }\text{C}$ (with zoomed region inset) and (d) $800{}^{\circ }\text{C}$. The significant contribution to the uncertainty in moment/area is from the area measurements of the sample portions. The magnetisation and uncertainty therein is determined from the fit to Eq. (1) (solid lines) and gives a best fit of $M=750\pm 50$ emu/cm${}^3$ and $M=520\pm 50$ emu/cm${}^3$ for the $L{1}_1$ and $L{1}_0$ crystal structures respectively. (e,f) The squareness ratio ($M_s/M_r$) versus $d_{\text{CoPt}}$ for growth temperature of (e) $350{}^{\circ }\text{C}$ and (f) $800{}^{\circ }\text{C}$.

Figure 7

Point contact Andreev reflection measurements on the Al${}_2$O${}_3$(sub)/Pt (4 nm)/CoPt(128 nm)/Pt (4 nm) samples grown at $350{}^{\circ }\text{C}$, corresponding to $L{1}_1$ crystal structure, and $800{}^{\circ }\text{C}$, corresponding to $L{1}_0$ crystal structure. (a) Exemplar conductance versus voltage curve for the $L{1}_0$ sample with best fit to the BKT model (see text). (b) The polarisation is shown as a function of the square of the barrier strength, Z${}^2$, with linear fits to the data.