Magnetic stability of ultrathin FeRh films

Abstract

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...

Modeling the thickness dependence of the magnetic phase transition temperature in thin FeRh films

“…The curve shows an almost linear decrease initially with little change between the 7.5-and the 10-nm samples as the film thickness is increased. In comparison to that of the work by Han et al [22] the trend is reversed. Our aim is to attempt to understand if this increase in the phase transition temperature can be explained by the difference between the two substrates (Si/SiO/MgO buffer and MgO, respectively) rather than arising from the reduction in the thickness of FeRh itself, for example, due to loss of coordination at the surface.…”

“…In 2013, Han et al [22] demonstrated the thickness dependence of the phase transition temperature in FeRh deposited on Si/SiO wafers with a MgO buffer layer. They showed that as the thickness of FeRh in those samples decreased the phase transition temperature also decreased.…”

Antiferromagnetic spintronics

“…From Kohn et al, 2013 Ir 20 Mn 80 690 (Petti et al, 2013;Frangou et al, 2016) (Han et al, 2013;Saidl et al, 2016) a…”

Higher-order exchange interactions leading to metamagnetism in FeRh

“…A limited system size restricts the maximum extent of nucleated domains of each ordering and hence the size of the interface region. Experimental observations of small FeRh nanoparticles [20] and thin films [21] have also shown a large increase in the thermal hysteresis and even the suppression of the AFM phase [22]. 3.48 .…”