Suppression of Coercivity in Nanoscale Graded Magnetic Materials

We investigate temperature-dependent magnetization reversal of $\mathrm{Co}\mathrm{Ru}$ graded films, in which a predefined depth-dependent exchange-coupling strength J follows a V-shaped profile. Magnetometry reveals an extended temperature range below the Curie temperature $T_C$ where the reversal of the magnetization M is not accompanied by the conventionally occurring hysteresis, in stark contrast with homogeneous $\mathrm{Cr}\mathrm{Ru}$ reference films. This is caused by the temperature-driven paramagnetic (PM)-ferromagnetic (FM) phase transition, which does not occur in the entirety of the graded material but only in well-defined nanoscopic regions at any given temperature, enabling the creation of two internal PM/FM interfaces that assist the external magnetic field in reversing the magnetization of the FM graded sample region. Hysteretic reversal is recovered at sufficiently low sample temperatures or by using graded structures with very steep J gradients, so that no PM/FM interfaces form inside the exchange-coupled layer material that could influence the magnetization reversal process. Our findings open interesting material design options, and we envision wide application potential, since we succeed in engineering a temperature gap between the temperature dependent magnetization and hysteresis onsets, a capability of interest for any technology benefitting from field-free magnetization switching.

Figure 1

(a) Schematic of the layer growth sequence. (b) $\mathrm{Ru}$ content and the associated effective Curie temperature ${\hat{T}}_C$ depth profile for the two extreme cases of graded ${\mathrm{Co}}_{1\text{−}x(z)}{\mathrm{Ru}}_{x(z)}$ layers 10 nm (s = 7.4%) and 150 nm (s = 0.5%) thick. (c) XRD θ/2θ scans for samples with different s. Each scan is normalized to the intensity of its $\mathrm{Ag}$ (220) peaks and vertically offset by a constant value to permit comparison.

Figure 2

(a) and (b) M(H) measurements along the easy axes for two different samples with s = 7.4% (a) and s = 1.2% (b), both measured at T/T_C(s) = 0.85. Here, the M data are normalized to their values at µ₀H = 40 mT. (c) and (d) coercive field H_C versus temperature for s = 7.4% (c) and s = 1.2% (d). (e) and (f) temperature dependence of the magnetization M for samples with s = 7.4% (e) and s = 1.2% (f). The M(T) data are measured in the presence of an applied field µ₀H = 4 mT along the easy magnetization axis.

Figure 3

Temperature dependence of the coercive field µ₀H_C for ${\mathrm{Co}}_{0.653}{\mathrm{Ru}}_{0.347}$ homogeneous (a), and graded samples with s = 7.4% (b), 3.7% (c), 1.8% (d), 0.8% (e), and 0.5% (f). The highlighted light green regions correspond to the temperature range where the samples are paramagnetic, whereas the yellow and green dashed vertical lines in (d)–(f) define the $\mathrm{\Delta }T$ range of anhysteretic behavior below $T_C$.

Figure 4

(a) $T_C$ values of our graded structures as a function of s, together with the $T_C$ values of the ${\mathrm{Co}}_{0.675}{\mathrm{Ru}}_{0.325}$ (blue dashed line) and ${\mathrm{Co}}_{0.63}{\mathrm{Ru}}_{0.37}$ (gold dashed line) uniform samples. (b) s dependence of the $\mathrm{\Delta }T/T_C$ temperature range. Inset I schematically shows the magnetization depth profile below T_C for systems with high s values, whereas inset II illustrates the magnetization depth profile with the two characteristic PM sample regions that form below $T_C$ for samples with low s. Inset III displays an exemplar µ₀H_C (T) for s ≤ 2.5% and the corresponding temperature ranges where the graded samples adopt the magnetization profiles shown in I (low temperature region) and in II (high temperature region).