Random field and random anisotropy O(N) spin systems with a free surface

Abstract

We study the surface scaling behavior of a semi-infinite $d$-dimensional O(N) spin system in the presence of quenched random field and random anisotropy disorders. It is known that above the lower critical dimension $d_{\mathrm{lc}}=4$ the infinite models undergo a paramagnetic-ferromagnetic transition for $N>N_c$ ($N_c=2.835$ for random field and $N_c=9.441$ for random anisotropy). For $N<N_c$ and $d<d_{\mathrm{lc}}$ there exists a quasi-long-range ordered phase with zero order parameter and a power-law decay of spin correlations. Using functional renormalization group we derive the surface scaling laws which describe the ordinary surface transition for $d>d_{\mathrm{lc}}$ and the long-range behavior of spin correlations near the surface in the quasi-long-range ordered phase for $d<d_{\mathrm{lc}}$. The corresponding surface exponents are calculated to one-loop order. The obtained results can be applied to the surface scaling of periodic elastic systems in disordered media and amorphous magnets.