High Magnetic Hardness for the Canted Antiferromagnetic, Ferroelectric, and Ferroelastic Layered Perovskite-like (C₂H₅NH₃)₂[Fe^IICl₄]

An unusual high magnetic hardness for the layered perovskite-like (C₂H₅NH₃)₂­[Fe^IICl₄], in addition to its already found canted antiferromagnetism, ferroelasticity, and ferroelectricity, which are absent for (CH₃NH₃)₂­[Fe^IICl₄], has been observed. The additional CH₂ in the ethylammonium compared to methylammonium allows more degrees of freedom and therefore numerous phase transitions which have been characterized by single-crystal structure determinations from 383 to 10 K giving the sequence from tetragonal to orthorhombic to monoclinic (I4/mmm ↔ P4₂/ncm ↔ Pccn ↔ Pcab ↔ C2/c) accompanied by both tilting and rotation of the FeCl₆ octahedra. The magnetic properties on single crystal and powder samples at high temperature are similar for both compounds, but at T_N (C₂H₅NH₃)₂­[Fe^IICl₄] is a proper canted antiferromagnet unlike the hidden canting observed for (CH₃NH₃)₂­[Fe^IICl₄]. The canting angle is 0.6° toward the c-axis, and thus the moments lie in the easy plane of the iron-chloride layer defined by a critical exponent β = 0.18. The isothermal magnetizations for the field along the three orthogonal crystallographic axes show wider hysteresis for H ∥ c and is present at all temperature below 98 K. The coercive field increases as the temperature is lowered, and at T < 20 K it is difficult to reverse all the moments with the available 50 kOe of the SQUID for both single crystal and powder samples. The shape of the virgin magnetization after zero-field-cool (ZFC) indicates that the high coercive field is due to domain wall pinning. Thus, there are unusual associated anomalies such as asymmetric hysteresis and history dependence. The difference in magnetic hardness of the two compounds suggests that magnetic, electric, and elastic domains are intricately manifested and therefore raise the key question of how the different domains interact.