High field level crossing studies on spin dimers in the low dimensional quantum spin system Na$_2$T$_2$(C$_2$O$_4$)$_3$(H$_2$O)$_2$ with T=Ni,Co,Fe,Mn

TL;DR

This study uses high magnetic fields to investigate the magnetic properties of low-dimensional spin systems in a series of compounds with varying transition metal ions, revealing field-driven ground state changes and the influence of spin and anisotropy.

Contribution

It demonstrates the effectiveness of high-field techniques and a dimer model to analyze complex spin ladder compounds with different transition metals.

Findings

Magnetic field induces ground state transitions in all compounds.

A dimer model accurately describes the magnetic behavior despite ladder structure.

Increased spin quantum number correlates with decreased intradimer exchange.

Abstract

In this paper we demonstrate the application of high magnetic fields to study the magnetic properties of low dimensional spin systems. We present a case study on the series of 2-leg spin-ladder compounds Na$_2$T$_2$(C$_2$O$_4$)$_3$(H$_2$O)$_2$ with T = Ni, Co, Fe and Mn. In all compounds the transition metal is in the $T^{2+}$ high spin configuation. The localized spin varies from S=1 to 3/2, 2 and 5/2 within this series. The magnetic properties were examined experimentally by magnetic susceptibility, pulsed high field magnetization and specific heat measurements. The data are analysed using a spin hamiltonian description. Although the transition metal ions form structurally a 2-leg ladder, an isolated dimer model consistently describes the observations very well. This behaviour can be understood in terms of the different coordination and superexchange angles of the oxalate ligands along the rungs and legs of the 2-leg spin ladder. All compounds exhibit magnetic field driven ground state changes which at very low temperatures lead to a multistep behaviour in the magnetization curves. In the Co and Fe compounds a strong axial anisotropy induced by the orbital magnetism leads to a nearly degenerate ground state and a strongly reduced critical field. We find a monotonous decrease of the intradimer magnetic exchange if the spin quantum number is increased.