LiHoF$_4$: Cuboidal Demagnetizing Factor in an Ising Ferromagnet

TL;DR

This study measures the susceptibility of LiHoF$_4$ to test a recent theory on how demagnetizing factors depend on sample shape and material details, revealing the importance of correct transformations for accurate physical property determination.

Contribution

It provides experimental validation that the microscopic magnetic details influence the demagnetizing transformation, especially for Ising spins, and clarifies the intrinsic susceptibility in ferromagnetic phases.

Findings

Demagnetizing transformation depends on microscopic spin details.

Incorrect transformations lead to large measurement errors.

Intrinsic susceptibility in the ordered phase is infinite regardless of shape.

Abstract

The demagnetizing factor has an important effect on the physics of ferromagnets. For cuboidal samples it depends on susceptibility and the historic problem of determining this function continues to generate theoretical and experimental challenges. To test a recent theory, we measure the magnetic susceptibility of the Ising dipolar ferromagnet LiHoF$_4$, using samples of varying aspect ratio, and we reconsider the demagnetizing transformation necessary to obtain the intrinsic material susceptibility. Our experimental results confirm that the microscopic details of the material significantly affect the transformation, as predicted. In particular, we find that the uniaxial Ising spins require a demagnetizing transformation that differs from the one needed for Heisenberg spins and that use of the wrong demagnetizing transformation would result in unacceptably large errors in the measured physical properties of the system. Our results further shed light on the origin of the mysterious `flat' susceptibility of ordered ferromagnets by demonstrating that the intrinsic susceptibility of the ordered ferromagnetic phase is infinite, regardless of sample shape.