MAGNETIC-X-RAY SCATTERING STUDY OF RANDOM-FIELD EFFECTS IN MN0.75ZN0.25F2

We report a detailed synchrotron X-ray scattering study of the magneti c correlations in two samples of Mn0.75Zn0.25F2 as a function of tempe rature and applied field. The critical behavior of this system is beli eved to be isomorphic with that of the three-dimensional random field Ising model (RFIM). On cooling in an external magnetic field (FC), the first sample exhibits a transition to long range order (LRO) in the n ear-surface region, at a field dependent temperature, T(N)(H). In cont rast, bulk neutron scattering studies show a long lived metastable dom ain state forming below a metastability boundary, T(M)(H). The transit ion temperature T(N)(H), lies below the metastability boundary, T(M)(H ). The temperature difference, T(M)(H)-T(N)(H), increases with increas ing field and agrees closely with the value deduced from an extrapolat ion from above T(M)(H) of earlier, equilibrium neutron scattering resu lts on the spin-spin correlation length. On cooling, the order paramet er exponent is found to be large, beta=0.30+/-0.05. We speculate that there is an imbalance in the random fields in the neighborhood of line ar surface defects (scratches) in this sample, and that the consequent net staggered field initiates a regular random Ising transition. The second sample was cut from the first and underwent a more extensive po lishing process, resulting in a smoother surface with a small density of visible defects. Interestingly, it does not attain a LRO state on c ooling, but rather it forms a domain state consistent with that observ ed by neutron diffraction. Both samples may be prepared in a LRO state , either by cooling in zero field and subsequently applying a field (Z FC) or, at high fields, by heating from the XY phase (FHXY). We have s tudied the evolution of the metastable LRO state in each sample on war ming. We find universal behavior in both samples at all fields studied . Specifically there is a power-law-like decay of the order parameter with exponent beta=0.20+/-0.05, and a rounded transition region which may be described by a Gaussian distribution of transition temperatures . The width of this distribution scales as H-2. A scaling plot of all the warming data as a function of the scaling variable (T-T(C)(H))/H-2 is constructed. We label this non-equilibrium pseudo-critical behavio r, ''trompe l'oeil critical behaviour.'' Phenomenologically, these res ults enable us to explain many previous, apparently contradictory, res ults in the literature.