  A numerical study is presented of the 3d Gaussian Random Field Ising Model at
T=0 driven by an external field. Standard synchronous relaxation dynamics is
employed to obtain the magnetization versus field hysteresis loops. The focus
is on the analysis of the number and size distribution of the magnetization
avalanches. They are classified as being non-spanning, 1d-spanning, 2d-spanning
or 3d-spanning depending on whether or not they span the whole lattice in the
different space directions. Moreover, finite-size scaling analysis enables
identification of two different types of non-spanning avalanches (critical and
supercritical) and two different types of 3d-spanning avalanches (critical and
subcritical), whose numbers increase with L as a power-law with different
exponents. We conclude by giving a scenario for the avalanches behaviour in the
thermodynamic limit.
