Exchange bias of ${\mathrm{TbPc}}_2$ molecular magnets on antiferromagnetic FeMn and ferromagnetic Fe films

Improving the magnetic stability of single-molecule magnets is a key challenge facing molecular spintronics. We use x-ray magnetic circular dichroism to explore the possibility of magnetically stabilizing ${\mathrm{TbPc}}_2$ molecules by attaching them to ultrathin Fe and FeMn films. We show that ${\mathrm{TbPc}}_2$ deposited on antiferromagnetic FeMn films exhibits magnetic hysteresis and exchange bias as a consequence of coupling to the uncompensated interfacial Fe spins. The FeMn-thickness dependence of the coercive field and exchange bias of ${\mathrm{TbPc}}_2$ is similar to that of inorganic ferromagnetic/antiferromagnetic systems. The magnetic remanence is comparable with the fraction of molecules attached to pinned interfacial Fe spins. The Tb magnetic moments are antiferromagnetically coupled to the Fe thin films as well as to the uncompensated Fe spins at the FeMn interface. The sign of the coupling changes from antiferromagnetic to ferromagnetic after doping the interface with electron-donor Li atoms.

Figure 1

STM micrographs of ${\mathrm{TbPc}}_2$/FeMn/Cu(100) for different thicknesses of the FeMn layer, taken at room temperature before and after ${\mathrm{TbPc}}_2$ deposition on different sample regions.

Figure 2

XAS and XMCD spectra of ${\mathrm{TbPc}}_2$/FeMn(6ML)/Cu(100) at (a) the $L_{2,3}$ Fe edge, (b) the $M_{4,5}$ Tb edge, and (c) the $L_{2,3}$ Mn edge. The data were taken at $T=8$ K in a field $B=4.5$ T applied perpendicular to the sample. The XAS signal was rescaled to have an $L_3$ edge jump equal to 1.

Figure 3

(a)–(c) Fe (blue) and Tb (red) magnetization curves of ${\mathrm{TbPc}}_2$/FeMn(t)/Cu(100) for $t=3$, 6, and 8 ML, respectively, following field cooling in $B=4.5$ T applied perpendicular to the sample. Insets: low-field region of the Tb magnetization curve. (d) Fe and Tb magnetization curves of ${\mathrm{TbPc}}_2$/FeMn(8 ML)/Cu(100) after Li deposition. All data were recorded at $T=8$ K. The XMCD signal was normalized by the jump in the averaged XAS signal at the corresponding edge.

Figure 4

Fe and Tb magnetization curves of ${\mathrm{TbPc}}_2$/Fe/Cu(100) (a),(b) before and (c),(d) after deposition of Li. The data were taken at $T=8$ K at oblique incidence $\mathrm{[}\theta ={73}^{\circ }$ for (a),(b); $\theta ={60}^{\circ }$ for (c),(d)], following field cooling in the zero field. Insets: Detail of the low-field region. The XMCD signal was normalized by the jump in the averaged XAS signal at the corresponding edge.