Magnetization switching diagram of a perpendicular synthetic ferrimagnet CoFeB/Ta/CoFeB bilayer

Highlights

• Anisotropy, Zeeman and exchange energy determine sequence of magnetic transitions.

• Three temperature ranges manifest different shapes of the hysteresis loop.

• The critical transition fields are temperature dependent.

Abstract

Magnetic configurations in synthetic ferrimagnet CoFeB/Ta/CoFeB bilayer with strong perpendicular anisotropy have been systematically studied. Magnetization versus field hysteresis loop has been measured for different temperature ranging from 5 to 300 K. The applied field – temperature (H-T) magnetization switching diagram has been constructed by extracting the different switching fields as a function of temperature. This switching diagram can be well explained by considering the competition between energy barrier of layer’s magnetization reversal, interlayer exchange coupling, and Zeeman energy.

Introduction

Ferromagnetic layers exchange coupled antiferromagnetically known as Synthetic antiferromagnets (SAF) are currently used in technologically relevant systems such as magnetic random access memories (MRAMs) and read heads for magnetic data storage [1]. SAF using materials with perpendicular magnetic anisotropy (PMA) are now used to combine higher densities, thermal stability and sharp magnetic switching. It is also expected to be used to reduce the critical current density in Spin-Transfer-Torque (STT) MRAM. Since the prediction of large domain wall velocity [2] and the possibility of all Optical switching PMA-SAF [3], [4] are attracting a lot of attention. Multilayer devices based on CoFeB ferromagnetic layers separated by a thin nonmagnetic layer attract attention of specialists due to various possible fields of practical application. Metallic spintronics based on CoFeB multilayered structures rapidly gains new areas of its application in sensor industry [5], [6], biomedicine, and information technologies [7], [8]. Magnetic structure of CoFeB single layers have been studied in detail [9], [10], [11], [12], [13], [14]. Numerous reports on methods for inducing domain walls by electric current [15], [16], manipulating them [17], [18], [19], and reversing polarity of perpendicularly polarized nanosized single- and multilayer magnets without external magnetic field [20] by means of local electric fields [21], [22] and currents [23], [24], [25] can be found for single CoFeB magnets. However, magnetization reversal of multilayer magnetic structures remains a challenge due to a complicated game of anisotropy of layers [26], [27], [28], [29], [30], DMI interaction [31], [32], tensile strains [33], [34] and interlayer coupling [35], [36]. The magnetic configurations present in in-plane antiferromagnetically exchange coupled bilayers, as well as bilayers with PMA, have been studied in the past [37], [38], [39]. In the recent papers [35], [36] dependence of the exchange interaction value and sign on the thickness of non magnetic spacer was demonstrated. However, one of the unclear questions is determination of correspondence between the energy balance of key interactions (exchange coupling E_EX, Zeeman energy E_Z and barriers of reversal magnetization E_eff1 and E_eff2) and sequence of the magnetization switching under external magnetic field. Here we will demonstrate that sequence of the magnetization switching depends on the ratio between the exchange coupling and barrier of reversal magnetization.

Convenient diagram technique to predict sequence of switching of the bilayered devices depending on the ratio of layers magnetization and potential barriers of the reversal magnetization is proposed in our article. Sequence of the magnetization switching and shape of the hysteresis loop as well as prohibited and permitted transitions and their threshold magnetic fields controlled by competition between E_EX, E_Z, E_eff1 and E_eff2 values has been determined in our work. The experimental results are summarized in switching diagram of the magnetization states and transitions between different states depending on temperature T, applied field H and the thermal and magnetic prehistory in a perpendicularly magnetized antiferromagnetically exchange coupled CoFeB/Ta/CoFeB bilayer.