Tuning magnetic properties of Mn₄ cluster with gold coating

The magnetic properties of transition metal clusters are a unique function of their size and differ from their bulk behavior due to quantum confinement. Here we show that surface modification provides another channel to tune their magnetic properties. This is demonstrated by taking Mn₄ as an example. Although Mn₄ carries a giant magnetic moment of 20 μ_B, the magnetic coupling can be tuned from ferromagnetic to ferrimagnetic by changing the number of gold atoms coated on its surface. We found that 26 gold atoms are needed to fully cover a Mn₄ cluster. When partially coated, the system exhibits ferromagnetic coupling with a total magnetic moment of 18 μ_B and it becomes ferrimagnetic with a moment of 8 μ_B when fully coated. This magnetic cross-over is caused by the shrinking of the Mn-Mn bond length, suggesting that the magnetic properties of a Mn₄ cluster can be tuned by controlling the surface coverage.