Observation of Mermin-Wagner behavior in LaFeO$_3$/SrTiO$_3$ superlattices

TL;DR

This study demonstrates the suppression of long-range magnetic order in ultrathin LaFeO$_3$/SrTiO$_3$ superlattices with one monolayer, confirming the Mermin-Wagner theorem's predictions for two-dimensional systems.

Contribution

First experimental observation of Mermin-Wagner behavior in LaFeO$_3$/SrTiO$_3$ superlattices using muon spin rotation spectroscopy.

Findings

Superlattices with 3 and 2 monolayers show static antiferromagnetic order.

Superlattices with 1 monolayer exhibit no magnetic order and fluctuating moments.

Dimensionality controls magnetic ordering in ultrathin films.

Abstract

Two-dimensional magnetic materials attract a lot of attention since they potentially exhibit new magnetic properties due to, e.g., strongly enhanced spin fluctuations. However, the suppression of the long-range magnetic order in two dimensions due to long-wavelength spin fluctuations, as suggested by the Mermin-Wagner theorem, has been questioned for finite-size laboratory samples. Here we study the magnetic properties of a dimensional crossover in superlattices composed of the antiferromagnetic LaFeO$_3$ and SrTiO$_3$ that, thanks to their large lateral size, allowed examination using a sensitive magnetic probe - muon spin rotation spectroscopy. We show that the iron electronic moments in superlattices with 3 and 2 monolayers of LaFeO$_3$ exhibit a static antiferromagnetic order. In contrast, in the superlattices with single LaFeO$_3$ monolayer, the moments do not order and fluctuate to the lowest measured temperature as expected from the Mermin-Wagner theorem. Our work shows how dimensionality can be used to tune the magnetic properties of ultrathin films.