Anomalous damping dependence of the switching time in Fe/FePt bilayer recording media

Gilbert damping plays a significant role in magnetic reversal processes, and it determines the timescale of the switching. Here we investigate the properties of exchange-coupled composite media and the dependence of the switching time on the damping constant of the soft layer using atomistic spin dynamics. For a bilayer Fe/FePt medium, we find an anomalous increase of the switching time with increasing soft layer damping constant. The reversal occurs via a high-temperature exchange spring, and we show that the increase in the switching time is related to a corresponding increase in the time to establish the exchange spring. This phenomenon is delicately balanced in that the switching time increase occurs only in fields close to the coercivity.

Figure 1

Calculated temperature-dependent magnetization for Fe and FePt using Monte Carlo simulations. The solid lines represent the fitting using the following expression: $M\left(T\right)=M_0{[1-{(T/T_C)}^{\alpha }]}^{\beta }$, which gives a Curie temperature of 1050 K for Fe and 700 K for FePt.

Figure 2

The temperature dependence of the coercive field for ECC media. Graph (a) illustrates a series of hysteresis loops for ECC media at different temperatures below the Curie point for an interface exchange constant of $J_{\text{int}}=1\times {10}^{-21}\mathrm{J}$. In graph (b) the coercivity has been extracted from the temperature-dependent hysteresis loops for three different values of the interface exchange constant. The solid lines are fits using Eq. (5) for each interface exchange constant.

Figure 3

Plot of the magnetization for each atomic layer. The total height of the ECC media grain is 10 nm, which contains 60 atomic layers. The first 30 layers correspond to FePt, and the layers between 30 and 60 correspond to Fe. The applied field varies from 20 to $-20$ T representing the first part of the hysteresis loop.

Figure 4

Calculated evolution of $z$-components of reduced magnetization during the reversal mechanism for different interface exchange coupling strength (a) $J_{\text{int}}=5\times {10}^{-21}\mathrm{J}$, (b) $J_{\text{int}}=3\times {10}^{-21}\mathrm{J}$, and (c) $J_{\text{int}}=2\times {10}^{-21}\mathrm{J}$ with a variation of damping of the soft layer (Fe). The damping constant of the hard phase (FePt) has been kept fixed at 0.1.

Figure 5

Calculated evolution of the transverse component of the reduced magnetization of the Fe layer as $S_{\text{transverse}}^{\text{Fe}}={\displaystyle \frac{1}{N}}{\sum }_i^N\sqrt[]{{\left(S_{ix}^{\text{Fe}}\right)}^2+{\left(S_{iy}^{\text{Fe}}\right)}^2}$ during the reversal mechanism for different interface exchange coupling strength (a) $J_{\text{int}}=5\times {10}^{-21}\mathrm{J}$, (b) $J_{\text{int}}=3\times {10}^{-21}\mathrm{J}$, and (c) $J_{\text{int}}=2\times {10}^{-21}\mathrm{J}$ with a variation of damping of the soft layer (Fe). The damping constant of the hard phase (FePt) has been kept fixed at 0.1.

Figure 6

The magnetization profile during the reversal mechanism for two different exchange couplings and two different Gilbert damping values. The system contains 60 atomic layers; the first 30 layers $\approx 5$ nm correspond to FePt, and the top layers correspond to Fe. The colors indicate the value of the $z$-component of magnetization. Here red indicates a positive value of reduced magnetization on the $z$ direction, blue indicates a negative value, and white suggests a plane orientation of the reduced magnetization.

Figure 7

The dependence of reversal time as a function of damping parameter for different cases: (a) low field near the coercivity, $H_{\text{applied}}=1$ T, and (b) higher fields.