We report on magnetic resonance studies within the magnetically ordered phase of the quasi-onedimensional antiferromagnet LiCuVO 4 . Our studies reveal a spin reorientational transition at a magnetic field H c1 Ϸ 25 kOe applied within the crystallographic ab plane in addition to the recently observed one at H c2 Ϸ 75 kOe ͓M. G. Banks et al., J. Phys.: Condens. Matter 19, 145227 ͑2007͔͒. Spectra of the antiferromagnetic resonance along low-frequency branches can be described in the framework of a macroscopic theory of exchange-rigid planar magnetic structures. These data allow us to obtain the parameter of the anisotropy of the exchange susceptibility together with a constant of the uniaxial anisotropy. Spectra of 7 Li nuclear magnetic resonance ͑NMR͒ show that, within the magnetically ordered phase of LiCuVO 4 in the low-field range H Ͻ H c1 , a planar spiral spin structure is realized with the spins lying in the ab plane, in agreement with neutron-scattering studies of Gibson et al. ͓Physica B 350, 253 ͑2004͔͒. Based on NMR spectra simulations, the transition at H c1 can well be described as a spin-flop transition, where the spin plane of the magnetically ordered structure rotates to be perpendicular to the direction of the applied magnetic field. For H Ͼ H c2 Ϸ 75 kOe, our NMR spectra simulations show that the magnetically ordered structure exhibits a modulation of the spin projections along the direction of the applied magnetic field H.

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The XY-pyrochlore antiferromagnet Er2Ti2O7 is studied by heat capacity measurements and electron spin resonance spectroscopy performed on single crystal samples. The magnetic phase diagrams are established for two directions of applied field, H[100] and H [111]. In the magnetically ordered phase observed below TN = 1.2 K, the magnetic excitation spectrum consists of a Goldstone mode acquiring an isotropic gap in an applied field, and another mode with a gap softening in the vicinity of a field-induced phase transition. This second-order transition takes place at a critical field Hc above which the magnetization process is accompanied by a canting of the magnetic moments off their local "easy-planes". The specific heat curves for H [100] (H ≫ Hc) are well described by a model presuming a single dispersionless excitation mode with the energy gap obtained from the spectroscopic measurements.

We have studied the magnetic and thermodynamic properties as well as the NMR spectra of the quasi two-dimensional Heisenberg antiferromagnet RbFe(MoO4)2. The observed temperature dependence of the order parameter, the critical indices and the overall magnetic H − T phase diagram are all in a good agreement with the theoretical predictions for a 2D XY model. The temperature dependence of the specific heat at low temperature demonstrates a crossover from a T 2 law characteristic of a two-dimensional antiferromagnet to a three-dimensional T 3 law.

An adiabatic demagnetization process is studied in Gd2Ti2O7, a geometrically frustrated antiferromagnet on a pyrochlore lattice. In contrast to conventional paramagnetic salts, this compound can exhibit a temperature decrease by a factor of ten in the temperature range below the Curie-Weiss constant. The most efficient cooling is observed in the field interval between 120 and 60 kOe corresponding to a crossover between saturated and spin-liquid phases. This phenomenon indicates that a considerable part of the magnetic entropy survives in the strongly correlated state. According to the theoretical model, this entropy is associated with a macroscopic number of local modes remaining gapless till the saturation field. Monte Carlo simulations on a classical spin model demonstrate good agreement with the experiment. The cooling power of the process is experimentally estimated with a view to possible technical applications. The results for Gd2Ti2O7 are compared to those for Gd3Ga5O12, a well-known material for low temperature magnetic refrigeration.

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

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