# Exchange coupling of a perpendicular ferromagnet to a half-metallic compensated ferrimagnet via a thin hafnium interlayer

## TL;DR

This study demonstrates that a thin hafnium layer can serve as both a diffusion barrier and an exchange coupling medium between a perpendicular ferromagnet and a compensated ferrimagnet, with the coupling sign changing at the ferrimagnet's compensation temperature.

## Contribution

It introduces hafnium as an effective interlayer for exchange coupling in magnetic heterostructures involving zero-moment half metals, overcoming limitations of traditional materials like ruthenium.

## Key findings

- Hafnium acts as a diffusion barrier at 0.7 nm thickness.
- Exchange coupling energy reaches 0.11 mJ/m^2 with thin Hf layers.
- Coupling sign reverses at the ferrimagnet's compensation temperature.

## Abstract

A thin Hafnium film is shown to act both as an effective diffusion barrier for manganese at a thickness of 0.7 nm, and as an effective exchange coupling layer in a sandwich structure with perpendicular magnetic anisotropy. The magnetic layers are Co$_{20}$Fe$_{60}$B$_{20}$ and the low moment ferrimagnet Mn$_2$Ru$_x$Ga (MRG). The coupling changes sign at the compensation temperature of MRG and the exchange energy reaches 0.11 mJm$^{-2}$ for the thinnest Hf interlayers. Ruthenium, the usual metal of choice for coupling ferromagnetic layers in thin film heterostructures, cannot be used with the zero-moment half metal MRG because of Ru interdiffusion. Due to its large coercivity near compensation, the MRG can act as an effective source of exchange pinning.