Magnetic properties of mixed triangle-based ladder magnets (C1TET-TTF)(FeBr4)(1-x)(FeCl4)(x) (C1TET-TTF : 4,5-ethylenedithio-4 ',5 '-bis(methylthio)tetrathiafulvalene)

Isostructural magnets (C1TET-TTF)FeX4 (X = Br, CI) are featured with a tria ngle-based ladder lattice comprised of an Fe3+ zigzag chain where a donor d imer bridges between nest nearest neighboring Fe3+ sites. The FeCl4- salt i s an antiferromagnet with its easy axis tilted from tile ladder sheet, the FeBr4- salt shows weak ferromagnetism, with the easy axis of its antiferrom agnetic behavior lying on the sheet, We investigate the magnetic properties of mixed crystal (C1TET-TTF)(FeBr4)(1-x) (FeCl4)(x) in order to reveal the origin of the differences in magnetism. The strengths of the exchange inte r actions varying with x can be understood on the basis of the inter-anion/ donor atomic contact lengths in the crystal. Weak ferromagnetism survives w ith its easy asis tilted from the sheet in the concentration region x appro ximate to 0.2. Then weak ferromagnetism disappears above x greater than or equal to 0.42. In the high concentration range x similar to 0.4-0.8, the or dered state deviates from an ordinary antiferromagnet, due to the competiti on between two constituent exchange interactions. However, the possibility of a spiral spin which configuration theory predicts is not conclusive. The magnetic anisotropy, whose direction varies depending on x, is a consequen ce of the competition between the dipole-dipole interaction and the single- ion anisotropy.