Overcoming Size-Dependent Magnetic Thermal Stability via Atomical Coherence

Stabilization of ferromagnetic order in nanoscale magnets requires breaking time-reversal symmetry, a condition where the magnetic anisotropy energy ( K eff V ) surpasses the thermal fluctuation energy ( k B T ). However, further miniaturization of nanoscale magnets encounters the size-dependent superparamagnetic limit, where K eff V = k B T at the blocking temperature ( T B ). We realize concurrent enhancement of both magnetocrystalline and surface anisotropies in core–shell FePt@MnO nanoparticles via an atomically coherent interface, even with a 14% lattice mismatch between Fm3̅m FePt and MnO, unlike conventional methods that address each anisotropy individually. Ångström-scale alignment of magnetic Fe–Mn atoms at the interface boosts the effective magnetic anisotropy ( K eff ≈ 8.0 × 10 6 J/m 3 ) in 2 nm FePt core, a value approaching that of bulk L1 0 FePt (∼1.0 × 10 7 J/m 3 ), as evidenced by the anomalous vertical exchange-bias effect ( M EB = 1.3 emu/g). Consequently, the enhanced K eff yields a T B of 130 K, a 13-fold leap over bare FePt (10 K), which not only exceeds bulk MnO’s Néel temperature (118 K) but also extends magnetic stability to room temperature, thereby significantly broadening the operable temperature regime in ultrahigh-density recording and medical technologies.