Size limit of superparamagnetic inclusions in dust grains and difficulty of magnetic grain alignment in protoplanetary disks

TL;DR

This study examines the limitations of magnetic alignment of dust grains in protoplanetary disks, highlighting how superparamagnetic inclusions are constrained by physical processes, making alignment of larger grains unlikely.

Contribution

We analyze the size limits of superparamagnetic inclusions in dust grains and their impact on magnetic alignment in protoplanetary disks, revealing fundamental constraints.

Findings

Larmor precession timescale is much longer than damping time for millimeter-sized grains.

Maximum SPI sizes are limited by Nel's relaxation process and anisotropic energy constant.

Large grains (~100 μm) are unlikely to be aligned magnetically even with SPIs.

Abstract

Alignment of non-spherical grains with magnetic fields is an important problem as it lays the foundation of probing magnetic fields with polarized dust thermal emissions. In this paper, we investigate the feasibility of magnetic alignment in protoplanetary disks (PPDs). We use an alignment condition that Larmor precession should be fast compared with the damping timescale. We first show that the Larmor precession timescale is some three orders of magnitude longer than the damping time for millimeter-sized grains under conditions typical of PPDs, making the magnetic alignment unlikely. The precession time can be shortened by superparamagnetic inclusions (SPIs), but the reduction factor strongly depends on the size of the SPI clusters, which we find is limited by the so-called "N\'{e}el's relaxation process." In particular, the size limit of SPIs is set by the so-called "anisotropic energy constant" of the SPI material, which describes the energy barrier needed to change the direction of the magnetic moment of an SPI. For the most common iron-bearing materials, we find maximum SPI sizes corresponding to a reduction factor of the Larmor precession timescale of order $10^3$. We also find that reaching this maximum reduction factor requires fine-tuning on the SPI sizes. Lastly, we illustrate the effects of the SPI size limits on magnetic alignment of dust grains with a simple disk model, and we conclude that it is unlikely for relatively large grains of order 100 $\mu$m or more to be aligned with magnetic fields even with SPIs.