Spin-state transition in antiferromagnetic ${\text{Ni}}_{0.4}{\text{Mn}}_{0.6}$ films in Ni/NiMn/Ni trilayers on Cu(001)

The influence of the antiferromagnetic (AFM) spin structure of epitaxial ${\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}$ films on the magnetic properties of adjacent out-of-plane-magnetized ferromagnetic (FM) Ni layers in $\text{Ni}/{\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}/\text{Ni}$ trilayers on Cu(001) is investigated by magneto-optical Kerr-effect experiments. An AFM interlayer coupling between the two FM layers, which emerges below the AFM ordering temperature for an odd number of atomic layers of the antiferromagnet, suddenly disappears at a lower, AFM-thickness-dependent transition temperature. For even numbers of atomic layers of the antiferromagnet, a maximum of the coercivity is observed at a corresponding temperature. This result is interpreted as a transition of the AFM spin structure of the ${\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}$ layer that strongly affects the interlayer exchange coupling of the two FM layers by direct exchange through the AFM layer.

Figure 1

(a) Major magnetization loops of 15 ML Ni/15 ML ${\mathrm{Ni}}_{0.42}{\mathrm{Mn}}_{0.58}/15$ ML Ni/Cu(001) in the range of 40–460 K and (b) selected minor loops of the same system in the range of 325–445 K. Above a transition temperature $T_T\approx 325$ K, the loops indicate AFM coupling between the two FM layers. The coupling disappears above the antiferromagnetic ordering temperature of $\approx 435\text{K}$.

Figure 2

Magnetization loops at temperatures below and above the transition temperature $T_T$ for different AFM layer thicknesses. Antiparallel coupling is only observed if the number of atomic layers is odd. For layers containing an even number of atomic layers, only $H_C$ is enhanced when increasing the temperature across $T_T$.

Figure 3

(a) Coercivity $H_C$ for all samples and (b) offset of the minor loops from zero field $H_{\mathrm{shift}}$ for samples with odd AFM layer thicknesses as a function of temperature. The phase transition is accompanied by an increase of $H_C$ with increasing temperature, and the appearance of antiparallel coupling for odd-layer thicknesses of the AFM layer. $H_{\mathrm{shift}}$ is proportional to the coupling strength and is reduced for thicker AFM layers.

Figure 4

Blocking temperature for exchange bias $T_B$, transition temperature of the spin-state transition $T_T$, and antiferromagnetic ordering temperature $T_{AF}$ as a function of the ${\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}$ thickness. All characteristic temperatures increase in the same way. The exact compositions of the ${\mathrm{Ni}}_x{\mathrm{Mn}}_{1-x}$ layers are the same as listed in the legend of Fig. 3. Slight deviations from a smooth variation are possibly due to the slight variations in the composition of the AFM alloy.

Figure 5

Temperature dependence of the exchange bias field $H_{EB}$ of (a) the ${\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}/\text{Ni}$ bilayers before deposition of the top FM Ni layer, and (b) the $\text{Ni}/{\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}/\text{Ni}$ trilayers.