Macroscopic　hysteresis　loops　and　microscopic　magnetic　moment　distributions　have　been　determined　by　a　three-dimensional　(3D)　model　for　exchange-coupled　Sm-Co/alpha-Fe/Sm-Co　trilayers　with　in-plane　collinear　easy　axes.　These　results　are　carefully　compared　with　the　popular　one-dimensional　(1D)　micromagnetic　models　and　recent　experimental　data.　It　is　found　that　the　results　obtained　from　the　two　methods　match　very　well,　especially　for　the　remanence　and　coercivity,　justifying　the　calculations.　Both　nucleation　and　coercive　fields　decrease　monotonically　as　the　soft　layer　thickness　L-S　increases　while　the　largest　maximum　energy　product　(roughly　50　MGOe)　occurs　when　the　thicknesses　of　hard　and　soft　layers　are　5　nm　and　15　nm,　respectively.　Moreover,　the　calculated　angular　distributions　in　the　thickness　direction　for　the　magnetic　moments　are　similar.　Nevertheless,　the　calculated　nucleation　and　pinning　fields　as　well　as　the　energy　products　by　3D　OOMMF　are　systematically　smaller　than　those　given　by　the　1D　model,　due　mainly　to　the　stray　fields　at　the　corners　of　the　films.　These　demagnetization　fields　help　the　magnetic　moments　at　the　corners　to　deviate　from　the　previous　saturation　state　and　facilitate　the　nucleation.　Such　an　effect　enhances　as　LS　increases.　When　the　thicknesses　of　hard　and　soft　layers　are　10　nm　and　20　nm,　respectively,　the　pinning　field　difference　is　as　large　as　30%,　while　the　nucleation　fields　have　opposite　signs,