Understanding the magnetostructural transition in epitaxial FeRh films

I16 is ideally suited to such studies given its flexible diffraction geometry and the well controlled sample environment. Using temperature dependent low angle X-ray reflectivity and grazing incidence diffraction it is possible to map out the atomic spacing of epitaxial FeRh thin films (~10-50nm) as a function of depth from the surface and the sample temperature. The low-angle X-ray reflectivity is shown in Figure 1(a) and was used to determine the structure of the thin film. At low angles one can extract the electron-density depth profile through the film, shown as the inset in Figure 1(a). High angle X-ray diffraction data are shown in Figure 1(b) where clear (001), (002), (003) and (004) reflection peaks of the highly chemically ordered FeRh structure phase are observed. The out-of-plane lattice constant at room temperature was calculated from the FeRh (00L) peak positions and has a value of 2.998 Å, and is in good agreement with the bulk value of 2.989 Å reported by Lommel.³ Figure 1(c) shows the out-of-plane lattice constant as a function of temperature, where a sharp structural transition is observed, consistent with previous bulk measurements. Figure 1(d) shows the in-plane lattice constant determined from the FeRh (202) reflection as a function of temperature across the magnetic transition. The in-plane lattice constant at room temperature was found to be slightly smaller than the bulk value indicating that the film is compressively strained in-plane by the MgO substrate. This lattice expansion is much smaller compared to that observed in the out of-plane direction, indicating that the strain from the substrate is restricting the in-plane structural expansion unlike the cubic volume expansion observed in the bulk. 

 

 

The bulk magnetization was studied using a SQUID magnetometer to compare with the structural behaviour. Figure 2 indicates that the FeRh has a bulk like transition at around 375 K with a temperature hysteresis of 10 K consistent with the lattice expansion shown in Figure 1. The insets are hysteresis loops taken at 300 and 400 K. Qualitatively, the 400 K loop shape is consistent with the sample being ferromagnetic while the 300 K loops can be described by a coexistence of both ferromagnetism and antiferromagnetism

To reconstruct the depth resolved magnetic profile we can combine the X-ray structural data with polarized neutron and magnetometry measurements as shown in Figure 3. For the magnetic structure there are several interesting features: above the transition temperature the FeRh near the top interface has a magnetic moment of 1.32±0.03µ_B per FeRh - slightly smaller than the rest of the FeRh film, which has a magnetic moment of 1.56±0.03µ_B per FeRh. This value is smaller than the bulk value which is 2µ_B. The reduced moment is to be expected as it was shown previously by van Driel et al.⁴ that thin films have a smaller saturated moment compared to the bulk. More detailed investigation has shown that approximately 80 Å of FeRh close to the MgO substrate remains robustly FM at room temperature with a magnetic moment of 0.08±0.03µ_B per FeRh. Similar to the substrate interface, the 58 Å of FeRh in contact with the MgO capping layer interface remains FM with a finite moment consistent with X-ray Magnetic Circular Dichroism⁵ measurements. The much smaller room temperature moment is due to the fact that within this region, as evident from our X-ray study, there is a significant change in composition and interdiffusion between the cap and top 100 Å of the FeRh film. Finally, away from the interface the FeRh thin film has no net magnetic moment at room temperature as expected in the bulk

 

 

In conclusion, the atomic structure of FeRh thin film across its magnetic transition was accurately determined using high and low angle X-ray scattering at I16. This important structural information correlates with the rather unusual magnetic structure for thin film FeRh, where the ferromagnetic FeRh at room temperate is confined within the top and bottom interfaces. The origin of the ferromagnetic moment in thin film is still,controversial; however our experiments have clearly shown that the room temperature ferromagnetic effect is completely confined close to the interfaces. 

 

The appealing and complex properties of FeRh in nanoscale form make this an ideal system to study with many challenges ahead to fully understand and control this fascinating material.  

 

 

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