Magnetothermodynamics of Antiferromagnetic αMnCl2·4H2O. IV. Reversibility Conditions in the a, b, and p Regions with H ‖ c Axis. Spin Flop, an Inappropriate Term

Abstract

Detailed magnetothermodynamic measurements have been made on a 3.934‐cm‐diam spherical single crystal of α‐MnCl₂·4H₂O, at temperatures below 1.7°K and with fields below 25 kG directed along the $c$ crystallographic axis. Forty‐one adiabatics, selected to cross the so‐called boundaries in the magnetic regional diagram of antiferromagnetic MnCl₂·4H₂O were measured. Values of the temperature and magnetic moment along adiabatics were recorded automatically at very close magnetic field intervals. Equilibrium measurements of magnetic moment between 5 and 10 kG were made after the sample had been stabilized at constant field and temperature. The measurements were made by moving the sample slowly with respect to coils to avoid disturbing the equilibrium moment. Heat capacity measurements, supplementing those of Reichert and Giauque [J. Chem. Phys. 50, 4205 (1969)], were made at close intervals of field and temperature across the magnetic regional boundaries. The combined data made it possible to measure the entropy production on adiabatics and to locate the conditions under which it was produced. The gradual cooperative transition between the a and b regions was found to be under the equilibrium control of temperature and field. The reorientation does not occur irreversibly after a definite field enables the process to start. The equilibrium data, and particularly the magnetic moment measurements, show that the polarization in going between the a and b regions occurs over a broad band of fields. Since the word flop implies lack of control, the term spin flop for the gradual transition is quite inappropriate. No evidence was found to support any transition in the phase rule sense in this system and the term magnetic phase diagram is also undesirable usage. The term magnetic regional diagram is suggested for this very useful type of description in order to avoid the purposeless downgrading of a respected term in thermodynamics.