Spin-1/2 Heisenberg antiferromagnets Cs2CuCl4 and Cs2CuBr4 with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to Cs2CuCl4, yielding J/kB=4.7(2)  K, J'/kB=1.42(7)  K, [J'/J≃0.30] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs2CuBr4, we obtain J/kB=14.9(7)  K, J'/kB=6.1(3)  K, [J'/J≃0.41], providing exact and conclusive information on the exchange couplings in this frustrated spin system.

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R bFe(M oO 4)2 i s a rare exam pl e of a nearl y tw o-di m ensi onal H ei senberg anti ferrom agnet on a tri angul arl atti ce. M agneti c resonance spectra and m agneti zati on curvesrevealthatthesystem hasa l ayered spi n structure w i th si x m agneti c subl atti ces. T he subl atti cesw i thi n a l ayerare arranged i n a tri angul arm annerw i th the m agneti zati on vectors120 apart. T he H T phase di agram ,contai ni ng atl east ve di erentm agneti c phasesi sconstructed. In zero el d,R bFe(M oO 4)2 undergoesa phase transi ti on atT N = 3: 8 K i nto a non-col l i near tri angul ar spi n structure w i th al lthe spi ns con ned i n the basalpl ane. T he appl i cati on ofan i n-pl ane m agneti c el d i nduces a col l i near spi n state betw een the el ds H c1= 47 kO e and H c2= 71 kO e and produces a m agneti zati on pl ateau at one-thi rd ofthe saturati on m om ent. B oth the ESR and the m agneti zati on m easurem ents al so cl earl y i ndi cate an addi ti onal rst-order phase transi ti on i n a el d of35 kO e. T he exact nature ofthi s phase transi ti on i s uncertai n.PA C S num bers: 76.50+ g;75.50.E e;75.30.C r I. IN T R O D U C T IO N

Experiments in the antiferromagnetic phase of a quasi 2D S = 1/2 quasi-square lattice antiferromagnet Cu(pz)2(ClO4)2 reveal a biaxial type of the anisotropy, instead of the easy-plane one, considered before. The weak in-plane anisotropy, found by means of electron spin resonance spectroscopy and magnetization measurements, is about an order of magnitude weaker, than the off-plane anisotropy. The weak in-plane anisotropy results in a spin-flop phase transition for the magnetic field aligned along easy axis, and, thereby, in a bicritical point on the phase diagram. A remarkable feature of the weak in-plane anisotropy is the abrupt change of its sign at the spin-flop point. This anisotropy switching disappears at the tilting of magnetic field to the easy axis by the angle of 10• within the plane. The nature of the abrupt anisotropy reversal remains unclear. The phase diagram is characterized by the increase of the ordering temperature TN in the magnetic field used, except for a dip near the bicritical point.