Magnetic phase transition induced by bond randomness in mixed alternating Heisenberg chains<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CHNH</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">u</mml:mi><mml:mo>(</mml:mo><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">l</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Br</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi>−</mml:mi><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>observed by magnetization process
Abstract
Mixed magnetic systems $({\mathrm{CH}}_{3}{)}_{2}{\mathrm{CHNH}}_{3}\mathrm{Cu}({\mathrm{Cl}}_{x}{\mathrm{Br}}_{1\ensuremath{-}x}{)}_{3}$ are good candidates for experimental studies of the effect of bond randomness in both the Haldane state and the singlet dimer state. We investigated the magnetization process in these mixed compounds with various values of $x (0<~x<1)$ under an external field up to 41 T at 1.7 K. As a result, the existence of a finite energy gap was confirmed for $0.87<~x<~1,$ while a typical spin flop transition, which indicates absence of the energy gap, was detected for $0.44<x<0.87.$ From the variation of the spin flop transition field with x, the expected value of spins $〈{S}_{i}^{z}〉$ was found to reach its maximum at $x\ensuremath{\simeq}0.71,$ the point at which bond randomness is highest. For $0<~x<~0.44,$ on the other hand, the magnetization curves showed indistinct increases, which made it difficult to ascertain whether the energy gap exists.
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