Enhancement of Curie Temperature in Ferromagnetic Insulator-Topological Insulator Heterostructures

TL;DR

This paper provides a theoretical analysis of how topological insulator surface states enhance the Curie temperature in ferromagnetic insulator-topological insulator heterostructures, explaining experimental observations of increased Tc.

Contribution

It introduces a dominant short-range interaction mechanism and links Tc enhancement to Van Vleck susceptibility, advancing understanding of magnetic ordering in TI-FMI systems.

Findings

Short-range Bloembergen-Rowland interaction dominates over RKKY.

Tc enhancement is proportional to Van Vleck susceptibility.

Out-of-plane ferromagnetic order arises due to spin-momentum locking.

Abstract

We theoretically analyze the topological insulator (TI) surface state mediated interactions between local moments in a proximate 2D ferromagnetic insulator (FMI) motivated by recent experiments that show a significant increase in the Curie temperature Tc of FMI-TI heterostructures. Such interactions have been investigated earlier with a focus on dilute magnetic dopants in TIs. Our problem involves a dense set of moments for which we find that the short range Bloembergen-Rowland interaction, arising from virtual particle-hole transitions between the valence and conduction bands, dominates over the oscillatory Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. We show that the Tc enhancement is proportional to the Van Vleck susceptibility and that the spin-momentum locking of surface states leads to out-of-plane ferromagnetic order in the FMI. We investigate how the hybridization between top and bottom surfaces in a thin TI film impacts Tc enhancement, and show how our results can help understand recent experiments on atomically thin Cr2Te3-(Bi,Sb)2Te3.