Magnetic recording lies at the heart of many modern electronic devices. The density at which information can be stored is directy related to the size of the magnetic particles within the storage devices. In order to increase the recording density one must reduce the size of the magnetic particles. Unfortunately, there is a problem: the superparamagnetic limit. Below this size, the magnetization can change direction randomly, driven by thermal fluctuations. Heat assisted magnetic recording1 is a potential route to delay the onset of this limit and thereby increase the recording density. This is achieved through a high anisotropy material coupled to a low anisotropy, large moment material to form a so-called exchange spring. This spring state then lowers the magnetic field required to write the data whilst maintaining the high anisotropy which stabilizes the written data. FeRh is an ideal material for the soft, high moment component as it possesses a controllable ferromagnetic(FM)/anti-ferromagnetic(AF) phase transition just above room temperature. Local heating can drive the FeRh into the FM state during the write process which upon cooling then reverts to the AF state and the data is then securely stored in the high anisotropy layer.
Figure 1: I16 X-ray data. (a) The room-temperature low-angle X-ray reflectivity with the best chi-squared fit,(inset). The inset shows the scattering length density (SLD) which represents the electron density depth profile. (b) The high-angle x-ray diffraction data indicating the high degree of chemical ordering. (c) The out-of-plane lattice constant as a function of temperature. (d) The temperature-dependent measurement of the bulk in-plane lattice constant across the magnetic transition.
An interesting property of FeRh is that the magnetic transition is accompanied by a large (1%) change in the unit cell volume, suggesting a strong electron-phonon coupling. FeRh is also an attractive alloy given the possibilities for ultra fast magnetic switching on picoseconds timescales and magnetic refrigeration. Whilst the bulk behavior has been extensively studied2, its behavior in nanoscale films is not fully understood and it remains both an experimental and theoretical challenge. For example, magnetometry measurements on thin films suggest a ferromagnetic contribution even at room temperature where the bulk phase is antiferromagnetic. If one is to take advantage of the unusual transition it is essential that a detailed nanoscale description is provided. Clearly, given the change in volume and magnetism, both a detailed structural and magnetic study is required.
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.
Figure 2: The temperature dependence of the magnetization at an applied field of µ0H= 1 T clearly showing the AF-FM phase transition c.f. Figure 1. The insets show the hysteresis loop measured at 300 and 400K.
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±0.03µB per FeRh - slightly smaller than the rest of the FeRh film, which has a magnetic moment of 1.56±0.03µB per FeRh. This value is smaller than the bulk value which is 2µB. 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±0.03µB 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
Figue 3: The magnetic profiles for MgO/FeRh/MgO above and below the AF-FM transition temperature. The room-temperature magnetic profile is linked to the right hand ordinate.
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.
R. Fan, C. J. Kinane, T. R. Charlton, R. Dorner, M. Ali, M. A. de Vries, R. M. D. Brydson, C. H. Marrows, B. J. Hickey, D. A. Arena, B. K. Tanner, G. Nisbet, and S. Langridge, Ferromagnetism at the interfaces of antiferromagnetic FeRh epilayers Physical Review B 82, 184418 (2010)
- Thiele, J-U. et al. FeRh/FePt exchange spring films for thermally assisted magnetic recording media. Appl. Phys. Lett. 82 2859. (2003).
- Kouvel, J.S. and Hartelius, C, Anomalous Magnetic Moments and Transformations in the Ordered Alloy FeRh. J. Appl. Phys. 33, 1343 (1962).
- Lommel, J.., Magnetic and Electrical Properties of FeRh Thin Films. J. Appl. Phys. 37, 148 (1966).
- van Driel, J. et al., Compositional dependence of the giant magnetoresistance in FexRh1-x thin films. J. Appl. Phys. 85, 1026 (1999).
- Ding, Y. et al. Bulk and near-surface magnetic properties of FeRh thin films. J. Appl. Phys. 103, 07B515 (2008).