Experimental methods in chemical engineering: Magnetometry

Abstract

Magnetometry is a non‐invasive technique to characterize the behaviour of magnetic materials of which the most common contain Fe, Co, and Ni. Vibrating sample magnetometers (VSMs) vibrate samples at a fixed frequency in a static, homogeneous magnetic field to induce a voltage in pick up coils according to Faraday's law of induction. This voltage is proportional to the magnetic moment of the sample, allowing magnetization to be studied. Magnetometers monitor active phases and particle size distribution changes of catalysts operando . Despite the huge potential for this technique in chemical engineering, it remains on the edge and is slowly becoming more popular with 80 articles a year indexed by Web of Science Core Collection™. It remains mostly in the realm of physics and method development: cantilever magnetometry, vector magnetometry, torque magentometry, SQUID magnetometry (superconduction quantum devices), atomic magnetometry, and magentoencephalography. The complimentary techniques in chemical engineering are nuclear magentic resonance (NMR), Mössbauer spectroscopy, atomic force microscopy (AFM), and elecron paramagnetic resonance/electron spin resonance (EPR/ESR). The main limitation is that only strong ferro or ferrimagnetic samples can be studied, while the weaker paramagnetic or antiferromagnetic phases are obscured by the background contribution, due to the lack of sensitivity. Magnetometry is a bulk technique that measures only the overall magnetization, and subsequently it is problematic to identify contributions from materials comprising multiple phases. Other limitations are related to the effect of chemisorption on the magnetization, thermal blocking and domain structure that complicates determining the particle size distribution.