Effective exchange interaction for terahertz spin waves in iron layers

TL;DR

This paper investigates the exchange stiffness in ultrathin iron layers, revealing a 20% reduction compared to bulk iron and an interface-related decrease below 10 nm, using ultrafast spin wave excitation and analytical modeling.

Contribution

It provides new insights into the mode-dependent interface effects on exchange stiffness in ultrathin iron layers through experimental and theoretical analysis.

Findings

Exchange stiffness in ultrathin iron is 20% smaller than bulk value.

Interface effects cause a reduction in exchange stiffness below 10 nm thickness.

The reduction is mode-dependent, as shown by analytical modeling.

Abstract

The exchange stiffness is a central material parameter of all ferromagnetic materials. Its value controls the Curie temperature as well as the dynamic properties of spin waves to a large extend. Using ultrashort spin current pulses we excite perpendicular standing spin waves (PSSW) in ultrathin epitaxial iron layers at frequencies of up to 2.4 THz. Our analysis shows that for the PSSWs the observed exchange stiffness of iron is about 20% smaller compared to the established iron bulk value. In addition, we find an interface-related reduction of the effective exchange stiffness for layers with the thickness below 10 nm. To understand and discuss the possible mechanisms of the exchange stiffness reduction we develop an analytical 1D-model. In doing so we find that the interface induced reduction of the exchange stiffness is mode-dependent.