Critical Behavior of a Canted Antiferromagnet

Abstract

Previous investigations of the magnetic properties of the diammonium family (CH,), (NH,),FeCl,, with n = 2 , 3 , ..., 7 showed that all members studied become antiferromagnetic near liquid nitrogen temperature [ 1 to 51. This strong antiferromagnetism is associated with a weak ferromagnetic interaction and is reflected as a sharp spike in the susceptibility-temperature relationship. Behavior of this type is encountered in what is known as a canted spin arrangement [6]. The presence of such a sharp spike over a small temperature interval (~2 to 3 K) offers a good opportunity for determining the critical point exponents and helps to provide a better description of these systems.This note is concerned with the above compound when n = 6, referred to as HDFC, where HD stands for hexane diammonium. Magnetic susceptibility measurements near the magnetic transition temperature are presented and the critical point exponents are determined. The sample used was in the form of a powder. It was prepared as described elsewhere [2j in a stream of nitrogen gas and handled under a dry N, atmosphere.The crystal structure of this compound has not been solved yet, but it is known that members of this family are isomorphic to the corresponding M2+ series for which the structure is known [7]. It consists of layers of metal chloride octahedra MC1, sharing corners and extending in a two-dimensional network. The octahedra are tilted back and forth resulting in a puckered structure. These puckered layers are bound together by the hexane diammonium chain.The magnetic susceptibility was measured using a vibrating sample magnetometer. The specimens were cooled in zero magnetic field down to liquid nitrogen temperature. This was accomplished by applying a magnetic field equal in magnitude and opposite in direction to the earth's field. The temperature was measured with a pure Cu resistance thermometer placed in contact with the sample holder. Differential temperature measurements were made with an accuracy of 0.02 K. Magnetic moment measurements were carried out at a constant field value while varying the temperature at a constant rate between 99 and 103 K. Different fields were then used which ranged from 19 up to 50 Oe. Fig. 1 shows the magnetization as a function of temperature for several different magnetic field values. The magnetization near the maximum value increases rapidly as the field increases reflecting the ferromagnetic behavior in the critical region.