Vanishing Magnetic Interactions in Ferromagnetic Thin Films

We have used element-specific hysteresis measurements, based on the x-ray magnetic circular dichroism technique, to investigate magnetic trilayer structures composed of Fe and Ni layers. Within a critical regime we have discovered a class of structures in which the exchange interaction, the mechanism responsible for the macroscopic magnetism, can become vanishingly small. The experimental observations are supported by first principles theory and are explained as arising from a cancellation of several competing magnetic interactions. Hence, we have discovered a system with a novel exchange interaction between magnetic layers in direct contact that replaces the conventional exchange interaction in ferromagnets.

Figure 1

The observed magnetic hysteresis data for the ${\mathrm{Ni}}_2{\mathrm{Fe}}_2{\mathrm{Ni}}_8/\mathrm{Cu}(001)$ trilayer system. The reflected intensity of circularly polarized soft x rays, with energies corresponding to the $L_3$ absorption edges of Fe (solid line) and Ni (dashed line), is measured as an applied magnetic field is cycled by means of an in situ solenoid. In this way we obtain elementally specific hysteresis loops.

Figure 2

An illustration of the geometry used in the theoretical calculations. The angle between the magnetic moments in the Fe and Ni layers is denoted $\Theta$. In the noncollinear geometry half of the Fe magnetic moments rotate with $\Theta$ and the other half with $-\Theta$.

Figure 3

The calculated exchange energy for the ${\mathrm{Fe}}_2/{\mathrm{Ni}}_8$ multilayer as a function of $\Theta$, the angle between the magnetization direction of the Fe and Ni layers. (See Fig. 2.)

Figure 4

The calculated energy difference between ferromagnetic and antiferromagnetic coupling of Fe and Ni trilayers as a function of the lattice constant.