Rare-earth antiferromagnets on a pyrochlore fcc-lattice are a well known example of strongly frustrated spin structures, where conventional magnetic ordering is suppressed due to the nontrivial topology of the nearest neighbour exchange bonds. Ordered states appear in these systems as a result of a competition between weaker interactions or may be stabilised by fluctuations. A fine balance of acting forces induces a variety of phase transitions of different origins, which may be effectively probed by measuring transverse magnetisation (TM) and electron spin resonance (ESR) spectra. Gadolinium titanate Gd2Ti2O7 is a realisation of a Heisenberg antiferromagnet whose magnetic structure has remained a challenge for more than a decade. Our studies have revealed that a sequence of phase transitions induced by magnetic field H||[100] is remarkably different from those observed in any other field direction. Breaking of the initial cubic symmetry of the magnetic structure is found to occur at a finite field of 10 kOe via a first order transition. A possible relationship between this transition and crystal lattice distortions is discussed.

Another compound in this series, Er2Ti2O7, is known for its nontrivial magnetic ordering which is supposedly stabilised by fluctuations (“order by disorder effect”). The proximity of the system to an XY model was assumed to explain this effect theoretically. Our measurements show that the TM is induced in low field ordered phases by an external field applied in all directions except for [100]. The field dependence of the TM allows one to model the magnetisation process of the spin structure and to reliably determine the g-tensor components and the other spin-Hamiltonian parameters of the system. These measurements, in combination with ESR spectra, confirm that an XY model is applicable to interpret the zero field magnetic structure of Er2Ti2O7, but fails to describe its evolution under magnetic field.