Spin-echo EPR spin-probe measurement of the microsecond-range magnetic field fluctuations near the surface of crystals of the nanomagnet Mn12-Ac

Electron spin-echo detected EPR (ED-EPR) at the X-band, using the spin probe N-methylphenazinium-tetracyanoquinodimethane (NMP-TCNQ), enabled us to detect fluctuations of the local magnetic field close to the surface of a single crystal of [Mn12O12(CH3COO)16(H2O)4]·2CH3COOH·4H2O (Mn12-Ac) in its superparamagnetic regime. The ED-EPR signal from NMP-TCNQ is unexpectedly broad in comparison to standard sharp and intense spectrum detected by continuous wave EPR (CW-EPR). This broadened peak represents inhomogeneous broadening, whose homogenous component line width, δ, was related to its phase memory time TM as δ∼1/TM∼1/T2; TM was found to be in the microsecond range around 15 K. The temperature dependence of TM follows that of the bulk magnetization fluctuations. The Arrhenius activation energy for these probe detected fluctuations is ∼45 K, which is distinctly lower than that (∼60 K) for the bulk magnetization reversal for Mn12-Ac, implicating a significant role for surface effects.