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A close look at the definitions of ''coercivity'' reveals two distinct notions: one referring to the hysteresis loop for B vs H and the other to that of M vs H. These are confused in several books. The properties of the M vs H loop are often ascribed to the B vs H loop, giving rise to various misconceptions. In this note we highlight a number of misconceptions and misinterpretations in the textbook presentations of the hysteresis loop for ferromagnets. This brief report is aimed at helping faculty become aware of the existence of problems in the current literature.

This review is the first one in a planned series on the systems potentially suitable for highmagnetic-field and high-frequency electron magnetic resonance (HMF-EMR) studies. Such systems must best exhibit large or very large zero-field splitting (ZFS). The transition metal 3d 4 and 3d 6 ions with an orbital singlet ground state (OSGS) with the spin S=2 in some crystals may provide most suitable examples for applications of the HMF-EMR techniques. The compounds for which an evidence of an OSGS (S=2) exists can be classified into the following groups: (a) magnetically ordered at low temperatures, (b) magnetically ordered at higher temperatures, and (c) not ordered magnetically down to very low temperatures. Our literature search has also identified other groups of the 3d 4 and 3d 6 ions: (d) magnetically ordered compounds but exhibiting substantial orbital degeneracy and (e) biological systems. The largest sub-class of the S=2 systems consists of the high-spin Fe 2+ ion systems. To conform to the main theme of the Workshop, this review focuses on the magnetically ordered high-spin S=2 Fe 2+ ion systems potentially suitable for HMF-EMR studies. The data for the group (a) are presented in the form of a detailed review. For each ion/compound system we provide (i) brief characteristics, (ii) survey of the experimental as well as theoretical methods employed so far, especially those used for determination of the ZFS parameters, and (iii) selected major references as 'pointers' for further studies and literature searches. The groups (b) and (d) are not of prime interest for the present study, however, summary tables with basic pertinent data are included for completeness. The groups (c) and (e) deserve separate reviews. The data collected and analysed in this review may provide guidance for future HMF-EMR studies as well as enable correlation of the experimental ZFS data with the predictions of the microscopic spin Hamiltonian theory developed by us.KEYWORDS: magnetic materials, high-magnetic-field and high-frequency electron magnetic resonance, Fe 2+ ions, orbital singlet ground state, high spin S=2, very large zero-field splitting