Strain-Modulated Exchange-Spring Magnetic Behavior in Amorphous Tb-Fe Thin Films

This paper studies the room-temperature exchange-spring magnetic behavior of amorphous TbFe films subjected to an applied strain. The cross-sectional composition measurement of the sputter-deposited TbFe film shows a compositional gradient through the thickness. The gradient is near the compensation composition of amorphous TbFe film producing a Tb-dominant region and a Fe-dominant region. The as-deposited film shows a two-step switching behavior with a negative coercive field, while an applied compressive (or tensile) strain eliminates (or enhances) the two-step switching behavior. The strain influence is attributed to the TbFe composition gradient and relatively large magnetoelastic property of the Tb-dominant region as compared to the Fe-dominant one.

Figure 1

Atomic composition variation of Tb and Fe as a function of film thickness. Amorphous TbFe film ranges from $0.09\mu \mathrm{m}$ (bottom) to $0.38\mu \mathrm{m}$ (top). A dashed vertical line marks the compensation composition of TbFe film, and arrow schematics represent the Tb- and Fe-dominated regions based on the compensation composition.

Figure 2

$M\text{−}H$ curves for TbFe film. (a) $H$ field is swept between $\pm 2\mathrm{T}$ but truncated to show $\pm 1200\mathrm{Oe}$. Two switching steps ($S1$ and $S2$) are marked on a sweep-down curve. Arrow schematics show the Tb and Fe spin configuration at the corresponding points ($P1$–$P3$) in a sweep-down curve. (b) Sweep-up curves from four different $H$ fields ($-200$, $-400$, $-700$, and $-900\mathrm{Oe}$) of minor loops. Sweep-down curves, which are identical to the one in (a), are not shown. Thicker and thinner arrows are marked to show different trajectories. (Inset) $M\text{−}H$ loop of the TbFe thin film with a magnetic field applied normal to the film plane.

Figure 3

MOKE $M\text{−}H$ curves measured at three different mechanical strains, 0, $-820$, and $+540\mu \epsilon$ (a)–(c). Insets in (b) and (c) indicate the four-point bending fixture used to apply mechanical strain. The first and second switching-field [see $H_{S1}$ and $H_{S2}$ marked in (a)] variations as a function of mechanical strain (d). The arrow illustrations show the change of the exchange-coupling state depending on mechanical strain.