Full ferromagnetic saturation of a two-dimensional quantum antiferromagnet.

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Title:	Full ferromagnetic saturation of a two-dimensional quantum antiferromagnet.
Authors:	Coldea, R., Tennant, D.A., Habicht, K., Smeibidl, P., Tylczynski, Z.
Source:	Applied Physics A: Materials Science & Processing. Dec2002 Supplement, Vol. 74 Issue 6, ps904. 1p.
Subjects:	Antiferromagnetism, Ferromagnetic materials
Abstract:	Cs[sub 2]CuCl[sub 4] is a 2D frustrated quantum magnet that has recently been shown to display a very unusual quantum spin liquid state. The excitations are not spin-1 magnons as observed in other un-frustrated 2D quantum magnets but instead are spinons carrying a fractional spin of ½, as predicted by a resonating-valence-bond picture. Here we use high magnetic fields to induce a transition from this fractional quantum spin-liquid phase to the fully-polarized phase where spins are ferromagnetically aligned along the field. In this field-induced phase all quantum fluctuations are quenched by the external field and the system is expected to behave like a classical magnet with spin-1 magnon excitations. Measurements are made in fields up to 12 T and temperatures below 0.2 K. Ferromagnetic saturation is observed at the critical field B[sub c] = 8.44(1)T a. Above B[sub c] the measured excitations lineshapes show well-defined, almost resolutionlimited peaks as expected for spin-1 magnons and are gapped throughout the zone. The gap to the lowest energy excitation decreases linearly upon decreasing field and closes at B[sub c], below which the system orders into a cone phase. This transition provides an opportunity to study how ordered phases arise from the condensation of excitations. [ABSTRACT FROM AUTHOR]
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Database:	Engineering Source

Description
Abstract:	Cs[sub 2]CuCl[sub 4] is a 2D frustrated quantum magnet that has recently been shown to display a very unusual quantum spin liquid state. The excitations are not spin-1 magnons as observed in other un-frustrated 2D quantum magnets but instead are spinons carrying a fractional spin of ½, as predicted by a resonating-valence-bond picture. Here we use high magnetic fields to induce a transition from this fractional quantum spin-liquid phase to the fully-polarized phase where spins are ferromagnetically aligned along the field. In this field-induced phase all quantum fluctuations are quenched by the external field and the system is expected to behave like a classical magnet with spin-1 magnon excitations. Measurements are made in fields up to 12 T and temperatures below 0.2 K. Ferromagnetic saturation is observed at the critical field B[sub c] = 8.44(1)T a. Above B[sub c] the measured excitations lineshapes show well-defined, almost resolutionlimited peaks as expected for spin-1 magnons and are gapped throughout the zone. The gap to the lowest energy excitation decreases linearly upon decreasing field and closes at B[sub c], below which the system orders into a cone phase. This transition provides an opportunity to study how ordered phases arise from the condensation of excitations. [ABSTRACT FROM AUTHOR]
ISSN:	09478396
DOI:	10.1007/s003390101163