# Benchmarking Cantilever Torque Magnetometry as a Platform for Characterizing Molecular Qubits: A Case Study on Ni(II) Complexes

## TL;DR

This paper shows that cantilever torque magnetometry is a precise and accessible method for studying molecular qubits, particularly for Ni(II) complexes.

## Contribution

CTM is introduced as a new, accessible method for determining spin Hamiltonian parameters with high sensitivity and minimal sample requirements.

## Key findings

- CTM enables precise determination of spin Hamiltonian parameters from microgram-scale single crystals.
- CTM results show qualitative consistency but quantitative differences compared to high-frequency EPR spectroscopy.
- CTM is established as a complementary technique for characterizing low-anisotropy spin systems.

## Abstract

Precise and experimentally
accessible determination of
the electronic
structure of transition metal complexes remains a challenge in the
development of molecular qubits, particularly for leading candidates
with integer spin. Existing techniques often require large-scale facilities
and substantial sample quantities or offer limited spectral access
and sensitivity to subtle anisotropies. Here, we demonstrate that
cantilever torque magnetometry (CTM) overcomes these limitations by
combining high sensitivity to magnetic anisotropy with wide sample
compatibility, minimal sample demands, and true laboratory-scale accessibility.
By exploiting the distinct temperature dependences of g-tensor anisotropy and zero-field splitting (ZFS), CTM enables their
experimental decoupling, yielding exceptionally precise bulk-mean
value determination of spin Hamiltonian parameters from microgram-scale
single crystals. The parameters extracted by CTM were found to be
qualitatively consistent but quantitatively different from those determined
using high-frequency electron paramagnetic resonance spectroscopy
(∼1% for g and ∼5–15% for ZFS),
highlighting that perfect agreement between magnetometric and resonance
techniques is not guaranteed. Our study establishes CTM as a powerful
and broadly accessible complement to magnetic resonance methods, opening
new routes for high-precision characterization of low-anisotropy spin
systems in molecular quantum information science.