Surface effect on the magnetism of a $\mathrm{Mn}{\mathrm{Pt}}_3$-type ordered surface alloy on Pt(001)

We grow a $\mathrm{Mn}{\mathrm{Pt}}_3$-type trilayer ordered surface alloy on Pt(001), $c(2\times 2)$ $\mathrm{Mn}\text{−}\mathrm{Pt}∕1\mathrm{ML}$ $\mathrm{Pt}∕c(2\times 2)$ $\mathrm{Mn}-\mathrm{Pt}∕\mathrm{Pt}\left(001\right)$, of which atomic structure is investigated by dynamic low energy electron diffraction analysis. Magneto-optic Kerr effect measurements of the surface alloy at $45\mathrm{K}$ show no hysteresis, although bulk $\mathrm{Mn}{\mathrm{Pt}}_3$ ordered alloy is ferromagnetic. Ab initio total energy calculations based on density functional theory, rather, predict that ferromagnetic order is not the ground state for the surface alloy. We propose that the nonferromagnetic order of the surface alloy originates mainly from surface effects in regards to nonferromagnetic ground state also found for the near-surface layers of the bulk-terminated $\mathrm{Mn}{\mathrm{Pt}}_3\left(001\right)$.

Figure 1

Unit cell of bulk $\mathrm{Mn}{\mathrm{Pt}}_3$ ordered alloy. Black sphere is Pt, and gray sphere is Mn. Every other layer forms checkerboardlike, ordered MnPt alloy.

Figure 2

(a) MEED spot intensity oscillation while growing Mn film at substrate temperatures, $373\mathrm{K}$, $193\mathrm{K}$, and $143\mathrm{K}$. Periodic oscillation is found only for the growth temperature of $143\mathrm{K}$ after $\sim 6\mathrm{ML}$ of Mn deposition. Averaged period of such oscillation is taken to calibrate the thickness of the deposited amount of Mn. Incident electron energy for MEED calibration is between $560\mathrm{eV}$ and $580\mathrm{eV}$. (b) Mn films are deposited on Pt(001) at room temperature in two independent experiments. Thicknesses of the films in one case are determined by self-consistent LEED-AES calibration (noted as LEED calibration in the figure), and those in the other one by MEED calibration. Auger intensity ratios of Mn $\left(589\mathrm{eV}\right)$ to Pt $\left(64\mathrm{eV}\right)$ as a function of Mn thickness show good agreement for the two independently prepared and differently calibrated films.

Figure 3

Experimental (solid line) and theoretical (doted line) LEED I/V curves for the $\mathrm{Mn}{\mathrm{Pt}}_3$-type surface alloy on Pt(100).

Figure 4

The best-fit structure for the $\mathrm{Mn}{\mathrm{Pt}}_3$-type surface alloy on Pt(001). The filled circles represent Mn atoms and the empty circles represent Pt atoms.