Sputter Growth and Characterization of Metamagnetic B2-ordered FeRh Epilayers

We have prepared many FeRh samples according to this protocol. In this section we show typical results obtained using the most common characterization procedures for a selection of representative samples. Results such as these are expected for samples in the thickness range 20-50 nm. Other methods we have used to characterize our material in more depth include X-ray magnetic circular dichroism²⁸, grazing incidence X-ray scattering²⁹, and polarized neutron reflectometry³⁰. We have also studied the effects of doping the alloy with Au²⁷. Further data on the properties that can be expected from this material can be found in those reports, and the references contained therein.

The structure of one of our epilayers is shown in detail in the transmission electron micrographs shown in Figure 1. The sample cross-section was prepared by the conventional dimpling and ion polishing technique- a standard specimen preparation method (see, for instance Williams and Carter³¹)- and observed using a 200 kV electron beam. The overall layer structure can be seen in Figure 1(a). In this case, a 30 nm FeRh film was epitaxially grown onto an MgO substrate, followed by a ~4 nm Cr layer and a ~1 nm thick Al layer. (The Cr layer was included here for a particular experiment and is not needed in general). The roughness of the FeRh/MgO and FeRh/Cr interfaces are 0.6 nm and 2.8 nm, respectively, as measured from the image. In Figure 1(b) a high resolution micrograph of the MgO/FeRh interface is shown. The epitaxial relationship as confirmed from selected area diffraction is FeRh[100](001)||MgO[110](001). The lattice matching across the interface demonstrates the high quality of the epitaxial growth. We do not show the data here but have used energy dispersive X-ray spectroscopy in the TEM to check the composition on a selection of samples: it has always been equiatomic to within the uncertainty of the measurement.

Χ-ray reflectometry data are shown in Figure 2 for a nominally 25 nm thick FeRh epilayer capped with a thin, polycrystalline layer of Al. The measurement was performed in a standard two-circle diffractometer in the Bragg-Brentano geometry, using Cu K_α radiation (λ = 0.1541 nm), with a Ni filter to attenuate the K_α radiation. The pronounced Kiessig fringes, which arise from the interference of X-ray beams that reflect from the various interfaces in the layer stack, indicate that those interfaces are smooth and well-correlated. The solid red line shows a fit to the data that has been performed using the GenX software³². The best fit parameters for the multilayer structure are shown in Table 1. The fact that a portion of the Al layer will have oxidized and self-passivated once the sample is exposed to the air is accounted for in the model.

Χ-ray diffraction data for the same sample are shown in Figure 3, collected on the same instrument. The (002) reflection of the MgO substrate is strong and sharp enough to just resolve the Cu K_α1and K_α2 lines. The FeRh layers shows both (001) and (002) reflections. There is some broadening due to the finite thickness of the epilayer and strain gradients. The (002) FeRh B2 peak is centered at 2θ = 61.3°±0.02, yielding an average out-of-plane lattice constant of 3.02±0.05 Å. It is possible to determine the chemical order parameter S of the FeRh B2 structure from the relative integrated intensities of these two peaks. This quantity is defined as S = r_Fe+r_Rh -1, where is the fraction of Fe(Rh) sites occupied by Fe(Rh) atoms³³. A brief inspection of the formula shows that when r_Fe = r_Rh = 1 and the structure is perfect, S = 1, whereas when r_Fe = r_Rh = 0.5, so that all the lattice sites are randomly occupied, S = 0. The reason is that when S = 0 the site-averaged structure is bcc, for which the structure factor forbids the (001) reflection, whereas when S = 1 the structure is primitive cubic, for which the (001) reflection is allowed. This means that in practical terms, , where and are the experimental and theoretical intensities of the (00I) Bragg reflection, respectively³³. For the calculation of the theoretical intensities the Debye-Waller factors from EXAFS measurements on FeRh were used³⁴. In this case, S = 0.855±0.001, typical for a sputtered thin film of this material.

The metamagnetic phase transition can be detected in several ways. Its presence indicates the correct equiatomic stoichiometry and B2-ordering of the lattice. The lattice expansion that accompanies the metamagnetic transition may be detected by the shift in the position of the Bragg peaks²⁷; however, this requires a diffractometer with a heater stage.

Perhaps the most obvious method is to detect the appearance of the ferromagnetic moment as the sample is heated through T_T. This can be done using any temperature dependent magnetometer with sufficient sensitivity, for instance using the magneto-optical Kerr effect or a vibrating sample magnetometer. In Figure 4 we show the temperature dependence of the magnetization M, measured using a superconducting quantum interference device (SQUID) magnetometer. Measurements were made in the temperature range of 275-400 K with a temperature sweep rate of 2 K/min. The curve shown displays the anticipated AF → FM transition (heating) and FM → AF transition (cooling) with a 15 K thermal hysteresis. This measurement was made at high field (50 kOe) and yielded a transition temperature T_T ≈ 365 K. The transition temperature is field-dependent, as a higher magnetic field reduces the free energy of the FM phase with respect to the AF phase. Typically dT_T/dH ≈ 0.8 mK/Oe^{14,15, 27}. Note that the magnetic moment in the AF phase is not quite zero, but is a few tens of emu/cm³ when averaged over the volume of the entire sample. This moment resides in the near-interface regions of the FeRh epilayer, which remain ferromagnetic (albeit with a reduced magnetization) when the bulk of the sample transforms into the AF phase^{28, 30}.

A way to detect the transition that uses simpler equipment and is often used in our laboratory is to make an electron transport measurement. The simplest measurement is of the resistivity ρ of the film, since ρ in the FM phase is much less than in the AF phase^{35, 36, 20}. The temperature dependence of ρ for the same 25 nm FeRh epilayer for which X-ray data were shown is plotted in Figure 5, measured using a standard four-point probe method: spring-loaded, gold-plated pins were pressed on to the sample surface to make contact to the sample, which was mounted on a heater stage in a small custom vacuum chamber to prevent any sample oxidation when hot. A linear, metallic ρ(T) dependence is seen in both the AF and FM phases, but there is a marked drop in resistivity between the two. The hysteresis seen in Figure 5 is a clear fingerprint of the magnetostructural phase transition taking place and is a convenient method to measure the transition temperature, which is given by the minimum point in dρ/dT (shown in the inset of Figure 5). Another easily measured transport property, the Hall effect, can also be used to confirm the presence of the transition, as there is a large difference in the Hall coefficient between the two phases²⁰.

Figure 1. Transmission electron micrographs of an FeRh epilayer on an MgO substrate. (a) Image demonstrating the structure of the layer. The FeRh is 30 nm thick with a further ~4 nm Cr layer and ~1 nm Al cap deposited on top. The amorphous region at the top of the image is an epoxy resin used during cross-section sample preparation. (b) A high resolution image of the MgO FeRh interface. The epitaxial matching across the interface is seen here, and the associated relationship, as confirmed from selected area diffraction, is FeRh[100](001)||MgO[110](001). Click here to view larger figure

Figure 2. X-ray reflectometry spectrum from a 25 nm thick FeRh epilayer capped with polycrystalline Al. The solid line is a fit as described in the text, using the parameters given in Table 1. The inset shows the scattering length density profile associated with that set of fitting parameters. Click here to view larger figure

Figure 3. X-ray diffraction spectrum from a 25 nm thick FeRh epilayer capped with polycrystalline Al. The presence of the (001) FeRh peak indicates that B2 ordering has taken place. The chemical order parameter is S = 0.855±0.001, as determined using the method described in the text. Click here to view larger figure

Figure 4. Temperature dependence of magnetization M of a 50 nm thick FeRh epilayer capped with polycrystalline Al. These data were taken with a 50 kOe field applied in the film plane. The transition temperature T_T is seen to be ~365 K with a hysteresis width of about 15 K. Click here to view larger figure

Figure 5. Temperature dependence of the resistivity ρ of a 25 nm thick FeRh layer capped with Al. Inset is the derivative of ρ with respect to temperature T. The transition temperature T_T is seen to be 447 K on warming into the FM phase and 375 K on cooling into the AF phase. Click here to view larger figure

Table 1. Fitting parameters for the X-ray reflectivity spectrum shown in Figure 2, leading to the scattering length density profile shown in the inset of that Figure.