This paper presents magnetic properties of highly ordered ultrathin FeRh films deposited on Si/SiO wafers with MgO as a buffer layer. The antiferromagnetic to ferromagnetic (FM) transition is observed with a thickness as low as 3 nm. However, as the thickness decreases, the residual magnetization (M rs ) at low temperature increases and the amplitude of the transition decreases. In addition, the transition becomes much broader for the thinner films. This broadening is related to the grain size reduction in the thinner films. The temperature dependence of the magnetization of a highly ordered B2 FeRh film with a thickness of 10 nm was carefully measured as a function of field. The results show that the transition temperature decreases almost linearly with a rate of 0.93 K/kOe (heating) and 0.97 K/kOe (cooling) close to the value for the bulk samples, while M rs obtained at 100 K increases rapidly at low field and then linearly at a field larger than 10 kOe, which clearly demonstrates that an applied field would induce FM stabilization in ultrathin FeRh films. 3 For these applications, FeRh film is used to reduce the switching field of the storage layer through the exchange coupling between the ferromagnetic (FM) FeRh layer and the storage layer at elevated temperatures without sacrificing the thermal stability of the storage layer at ambient temperature, since the antiferromagnetic (AF) FeRh layer provides negligible exchange coupling to the storage layer. The major challenge is to prepare ultrathin FeRh films with a single AF phase at ambient temperature, which will convert to the FM phase at elevated temperatures, and vice versa. So far, a sharp AF-FM transition of FeRh film was only reported at a thickness above 14 nm. 4 For ultrathin films with a thickness of 10 nm or below, a large residual magnetization (M rs ) is generally observed. 5 The origin of this low temperature FM phase remains unclear. Fan et al.6 observed the existence of a FM phase in a region within 6-8 nm near the top and bottom interfaces of a FeRh film. Based on ab initio calculations, Lounis et al.7 found that a FM state is stable up to 9 atomic layers for Rh-terminated FeRh films. In addition, the AF structure would become unstable when the amount of the site-exchange defect density exceeds a threshold of 0.8%/f.u. 8 Furthermore, based on the phase diagram, 9 the AF to FM transition can be only achieved in the a 00 phase which is formed within a narrow Fe atomic concentration range from 45% to 51%. Due to slow diffusion rate of Rh, it is likely that a mixture of the FM Ferich a 0 phase and the paramagnetic Rh-rich c phase is formed in the film. In this paper, the magnetic stability of ultrathin (10 nm) FeRh films is examined. It is found that for such ultrathin FeRh films, the low temperature FM stabilization is sensitive to the film thickness and the applied magnetic field. An AF to FM transition is observed at a thickness as low as 3 nm, which is close to the FM stabilization thickness based on ab initio calculations for Rh-terminated fi...

Highly ordered B2 FeRh films with sharp magnetic transitions from the antiferromagnetic (AF) to ferromagnetic (FM) states were prepared on thermally oxidized Si wafers with thicknesses as low as 10 nm. It is found that the transition temperature increases as the thickness decreases from 80 nm to 15 nm, and then decreases from 15 nm to 10 nm. While the ratio of the residual magnetization to the maximum magnetization keeps nearly unchanged for the film thickness of 15 nm and larger, it increases significantly when the thickness is reduced to 10 nm. This residual magnetization was suppressed by slightly increasing the Rh atomic content in 10 nm thick FeRh films. Low-pressure deposition is found to play an important role in the stabilization of the AF phase. By depositing FeRh films at an extremely low pressure of 0.057 Pa, a residual magnetization as small as 13.5 emu/cc at 100 K was observed for a film with a nominal thickness of 10 nm deposited on Si wafer. This value was further reduced to 6 emu/cc when the film is deposited on MgO substrates due to much improved FeRh crystallinity. These results are in close agreement with theoretical predictions on defect and interface induced FM stabilization.

Applications using FeRh for controllable exchange coupling of two magnetic layers with in-plane and out-of-plane anisotropies require ultrathin (∼10 nm) films with pure antiferromagnetic (AF) FeRh α″ phase at room temperature (RT). However, it is also well known that the antiferromagnetic-ferromagnetic (AF-FM) transition of FeRh is sensitive to composition and deteriorates at low thicknesses. Hence, in this work, we study the composition-dependent phase ordering of co-sputtered FeRh thin films at ultrathin thicknesses of ∼10 nm. As the ultrathin films get richer in Rh, the appearance of α″ phase is typically characterized magnetically by a sudden drop in RT moment, and structurally by a slight decrease in degree of B2 chemical ordering with a sharp decrease in c-axis lattice constant. These observations are consistent with the FeRh phase diagram where FeRh abruptly enters the AF α″ phase once it becomes slightly disordered. Dependences of magnetic transition parameters on composition were also described. Moreover, higher sputtering powers possibly allow the formation of purer α″ phase with less γ-face centered cubic phase impurities. Consequently, a composition optimized 10 nm film shows a relatively low residual moment (13.5 emu/cc), thus suggesting good AF phase formation. In addition, correlation of the magnetic transition parameters with the crystal structural parameters reveal that the maximum rate of AF-FM transition (Rmax,h) and the corresponding total change in magnetization (ΔMh) interestingly shows linear dependence on the c-axis lattice constant, but would depart from this linearity under certain conditions, i.e., when grain sizes were large, crystallinity was improved and Fe content was high.

Absiract -The thermal decay of written bits in CoCrPt/CrTi thin film media for high density longitudinal magnetic recording is examined using a magnetoresistive head at room temperature. A decay in signal output over time is observed for the media with a thin magnetic layer less than about 14 nm thick and the media noise increases. The thermal stability factor Ku.V/k.T of these media is estimated to be less than about 100. The signal decay and noise increase are more serious at higher linear recording densities. These results are consistent with the results of a micromagnetic simulation taking into account the thermal aftereffects. These characteristics suggest that media with a large magnetic anisotropy constant Ku will be required to realize high storage densities of over 10 Gbhn'.

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