Performance of switch between exchange bias and coercivity: Influences of antiferromagnetic anisotropy and exchange coupling

doi:10.1016/j.jmst.2021.12.053

Abstract

Abstract:

The study on temperature dependence of exchange bias field and coercivity is crucial to solving the writing/reading dilemma in magnetic recording. Motivated by recent experimental findings, a complete switch between exchange bias field and coercivity with temperature is proposed, and the performance, characterized by average switching temperature (T_S) and switching temperature width (ΔT_W), controlled by antiferromagnetic anisotropy (K_AF) and exchange coupling (J_AF) constants is studied based on a Monte-Carlo simulation. The results show that a linear relationship between T_S and K_AF is established when K_AF is above a critical value, while T_S is weakly influenced by J_AF. On the contrary, ΔT_W is insensitive to K_AF, while strongly depends on J_AF. Besides overcoming thermal energy, the increase of K_AF for a small J_AF guarantees the completely frozen states in the antiferromagnetic layers during magnetizing at higher temperature, below which the exchange bias field exists with a negligible coercivity. Otherwise, for a large J_AF, the uncompensated antiferromagnetic magnetization behavior during the ferromagnetic magnetization reversal becomes complicated, and the switching process in the low temperature range depends on the irreversibility of uncompensated antiferromagnetic magnetization reversal during magnetizing, while in the high temperature range mainly influenced by the field-cooling process, resulting in a large ΔT_W. This work provides an opportunity to control/optimize the performance of the temperature-induced switch between unidirectional and uniaxial symmetries through precisely tuning K_AF and/or J_AF to meet different application demands in the next generation information technology.

Key words: Magnetic recording, Ferromagnet/antiferromagnet multilayer, Exchange-bias/coercivity switch, Temperature dependenceAntiferromagnetic anisotropy, Antiferromagnetic exchange coupling, Monte-Carlo simulation

Ruijun Li, Fan Zhang, Yong Hu. Performance of switch between exchange bias and coercivity: Influences of antiferromagnetic anisotropy and exchange coupling[J]. J. Mater. Sci. Technol., 2022, 120: 186-195.

Figures/Tables 6

Fig. 1. Hysteresis loops at selected temperatures after field cooling for different AFM anisotropy constants (KAF), where dashed and solid lines are used to indicate the behaviors of loop widening and shift with the decrease of temperature. Inset shows the schematic illustration of the model structure, where two 2-monolayer FM layers (green) are sandwiched into three 4-monolayer AFM layers (red), and the directions of easy axis (eK) and magnetic field (H) are also labelled.

Fig. 2. (a-c) Trained (loop index n = 2, open) and untrained (n = 1, solid) exchange bias field (HE) and coercivity (HC) as a function of temperature for different AFM anisotropy constants (KAF), where cyan shadow areas indicate the switching temperature width (ΔTW). (d) Average switching temperature (TS, open symbols) and ΔTW (bars) as a function of KAF in the trained and untrained cases, where symbols and bars are simulation data and lines are fit results by using the formula shown in (d).

Fig. 3. Hysteresis loops at selected temperatures after field cooling for different AFM exchange constants (JAF).

Fig. 4. Exchange bias field (HE) and coercivity (HC) as a function of temperature for the AFM exchange constant (a) JAF = -1.2 and (b) -2.0 meV, where cyan shadow areas indicate the switching temperature width (ΔTW). (c) Average switching temperature (TS, solid symbols) and ∆TW (bars) as a function of JAF, where the bar length is equal to ∆TW, indicated by arrows.

Fig. 5. (a, b) Coercive field and (c, d) AFM magnetization at FM/AFM interface (MAF) at descending (HL) and ascending (HR) branches as a function of temperature for selected AFM exchange constants (JAF), where cyan shadow areas indicate the switching temperature width (ΔTW), and the temperatures of the dashed lines are used to discuss the relationship between HL, HR and MAF. The dashed curves in (a, b) also show the HL and HR results in the single FM layers. (e) Schematic illustrations of the spin configurations at FM (upper magenta arrows)/AFM (lower red and blue arrows) interface for specific MAF values numbered in (c, d).

Fig. 6. (a-c) Hysteresis loops at different angles (α) between easy-axis and magnetic field directions for selected AFM anisotropy (KAF) and exchange (JAF) constants at 10 K. (d-f) Exchange bias field (HE) and coercivity (HC) as a function of α.

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