Fernández González Anxo, Sapozhnikov Konstantin, Pal-Val Pavel, Kustov Sergey
Departament de Física, Universitat de les Illes Balears, Cra Valldemossa km 7.5, 07122 Palma de Mallorca, Spain.
Solid State Physics Division, Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia.
Materials (Basel). 2023 Mar 23;16(7):2547. doi: 10.3390/ma16072547.
Magnetic hysteresis is a manifestation of non-equilibrium state of magnetic domain walls trapped in local energy minima. Using two types of experiments we show that, after application of a magnetic field to a ferromagnet, acoustic oscillations excited in the latter can "equilibrate" metastable magnetic domain structure by triggering the motion of domain walls into more stable configurations. Single crystals of archetypal NiMnGa magnetic shape memory alloy in the cubic phase were used in the experiments. The magnetomechanical absorption of ultrasound versus strain amplitude was studied after step-like changes of a polarizing magnetic field. One-time hysteresis was observed in strain amplitude dependences of magnetomechanical internal friction after step-like variations of a polarizing field. We distinguish two ingredients of the strain amplitude hysteresis that are found in the ranges of linear and non-linear internal friction and show qualitatively different behavior for increasing and decreasing applied polarizing fields. The uncovered effect is interpreted in terms of three canonical magnetomechanical internal friction terms (microeddy, macroeddy and hysteretic) and attributed to "triggering" by acoustic oscillations of the irreversible motion of domain walls trapped in the metastable states. To confirm the suggested interpretation we determine the coercive field of magnetization hysteresis through the measurements of the reversible Villari effect. We show that the width of the hysteresis loops decreases when acoustic oscillations in the non-linear range of domain wall motion are excited in the crystal. The observed "equilibration" of the magnetic domain structure by acoustic oscillations is attributed to the periodic stress anisotropy field induced by oscillatory mechanical stress.
磁滞是被困在局部能量极小值处的磁畴壁非平衡态的一种表现。通过两种类型的实验我们表明，在对铁磁体施加磁场后，铁磁体中激发的声振荡可以通过触发畴壁运动到更稳定的构型来“平衡”亚稳态磁畴结构。实验中使用了立方相的典型NiMnGa磁性形状记忆合金单晶。在极化磁场发生阶跃式变化后，研究了超声的磁机械吸收与应变幅度的关系。在极化场发生阶跃式变化后，在磁机械内耗的应变幅度依赖性中观察到一次性磁滞。我们区分了在线性和非线性内耗范围内发现的应变幅度磁滞的两个成分，并表明对于增加和减小施加的极化场，它们表现出定性不同的行为。所发现的效应根据三个典型的磁机械内耗项（微涡旋、宏涡旋和滞后）进行解释，并归因于被困在亚稳态的畴壁不可逆运动的声振荡“触发”。为了证实所提出的解释，我们通过测量可逆维拉里效应来确定磁化磁滞的矫顽场。我们表明，当在晶体中激发畴壁运动非线性范围内的声振荡时，磁滞回线的宽度会减小。观察到的声振荡对磁畴结构的“平衡”归因于振荡机械应力引起的周期性应力各向异性场。
