Practical limits for detection of ferromagnetism using highly sensitive magnetometry techniques
Résumé
Over the last ten years, signatures of high temperature ferromagnetism have been found in thin films and nanoparticles of various materials which are non-ferromagnetic in bulk, from semiconductors to superconductors. These studies often involve state-ofthe-art magnetometers working close to the limits of their sensitivity, where magnetic contaminations and measurement artefacts become non-negligible. Because such spurious effects may be involved, the reliability of magnetometry techniques for the detection of ferromagnetism in these new magnetic nanomaterials has been questioned. In this paper, we present a detailed study on magnetic contamination arising from sample processing and handling, describing how it may occur and how it can be avoided or otherwise removed. We demonstrate that, when proper procedures are followed, extrinsic magnetic signals can be reproducibly kept below 5 × 10 −7 emu (5 × 10 −10 Am 2). We also give an overview of the expected levels of contamination when such optimum conditions can not be guaranteed and analyze the characteristics of the resulting magnetic behaviour, discussing which features may or may not be used as criteria to distinguish it from intrinsic ferromagnetism. Although the measurements were performed using superconducting quantum interference device (SQUID) magnetometers, most of what we describe can also be applied when using alternating-gradient force (AGFM) and vibrating-sample (VSM) magnetometers.
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