TEM Imaging of Magnetic Transition in FeRh Thin Films

One material of great interest is Equi-atomic FeRh. It undergoes a magnetostructural transition from an antiferromagnetic (AF) to a ferromagnetic (FM) phase between 75–105 °C. It provides huge potential for exploitation of their domain wall (DW) motion in spintronic devices as it can present phase co-existence separated by DWs above room temperature.

DPC image of a planar FeRh thin film heated in situ to 150 °C with the direction of magnetisation depicted in the colour wheel (inset). Measurements of magnetic deflection are taken across the line and average profile along the arrows and boxed region (black). The SEM image (inset) shows the FeRh FIB lamella positionedon aWildfireD6 chip.

Figure 1. DPC image of a planar FeRh thin film heated in situ to 150 °C with the direction of magnetisation depicted in the colour wheel (inset). Measurements of magnetic deflection are taken across the line and average profile along the arrows and boxed region (black). The SEM image (inset) shows the FeRh FIB lamella positionedon a Wildfire D6 chip.

Figure 1 demonstrates ~ 1 nm spatial resolution imaging of magnetic induction within nanostructured thin films as a function of applied electric and magnetic fields. This is enabled with the scanning transmission electron microscopy (STEM) technique of differential phase contrast (DPC).

Purpose

  • To improve the area bit density in magnetic data storage materials and facilitate effective control over the DW movement to read/write information.
  • To properly understand the localized dynamic evolution of the domain nucleation, growth stages and DW movement that are associated with the magnetostructural transition in FeRh.

Challenges

  • Using FIB to prepare planar FeRh thin films on a MEMS chip
    • Avoid exposing the FeRh film to the Ga+ beam during transfer and final thinning on the MEMS chip
    • Achieving good-quality lamella with uniform thickness across the entire planar thin film
    • Achieving good electrical contact with biasing lines
  • Avoid diffraction contrast during DPC imaging of magnetic domains
  • Heating and imaging DW motion while applying current pulses

Results

1. Sample Preparation

A MgO substrate or NiAl buffer layer on GaAs substrate, which were mechanically back-polished from ~ 500 μm until ~ 50 μm thick, were used to grow a series of planar FeRh thin film samples. These samples were ion-milled within the FIB until they were ~ 1 μm thick before being transferred using a micromanipulator onto the MEMS Nano- Chip.

Progressively lower ion-beam currents were used to perform the final thinning on the Nano- Chip and low energy ions at 5 kV were used to complete the final polishing.

The associated manuscript provides a detailed description of the sample preparation:
Almeida, T. P. et al. Preparation of high-quality planar FeRh thin films for in situ TEM investigations. Journal of Physics: Conference Series, 903, 012022 (2017).

2. In Situ Heating and Quantitative Measurements from DWs

The FeRh planar thin films were heated in situ within the TEM using the Wildfire system. This provided thermal energy to induce the magnetostructural transition from the AF to FM phase. As shown in Figure 2, this allowed for the DWs to be driven into a parallel configuration through sample tilting and the magnetic field of the objective lens.

DPC images of DWs at (a) 20 °C; and during in situ heating to (b) 80 °C; (c) 120 °C; and (d) 300 °C. (e) Saturation induction measured from DWs as a function of temperature.

Figure 2. DPC images of DWs at (a) 20 °C; and during in situ heating to (b) 80 °C; (c) 120 °C; and (d) 300 °C. (e) Saturation induction measured from DWs as a function of temperature.

The black-boxed region in Figure 2 shows direct measurements of magnetic deflection from the DWs. This was as a function of temperature in Figure 2a-d, which allowed for the calculation of the saturation induction and quantitative charting of magnetic transition in Figure 2e.

3. In Situ Biasing and Current-induced DW Movement

Synergistic in situ heating of the FeRh thin film (Figure 3a) and application of current pulses (Figure 3 b&c) is allowed by combining the Lightning D9+ system and Source Measure Unit from Keithley. In situ heating allowed the magnetostructural transition into the FM phase, which then induced the formation of magnetic domains. As shown in Figure 3a, this is demonstrated by an intricate system of DWs. Movement into a magnetic state with DWs laying in normal to the direction of the application was promoted through the subsequent application of current pulses (Figure 3b&c).

(a-c) DPC imaging of DWs in a planar FeRh thin film transferred onto a Lightning D9+ TEM Nano-Chip heated to (a) 150ºC, after application of 500μs current pulses (CPs) of (b) 950 mV and and (c) 1020mV in the arrowed direction. The direction ofmagnetization in (a-c) is depicted in the colour wheel in (a) (inset).

Figure 3. (a-c) DPC imaging of DWs in a planar FeRh thin film transferred onto a Lightning D9+ TEM Nano-Chip heated to (a) 150 ºC, after application of 500 μs current pulses (CPs) of (b) 950 mV and and (c) 1020 mV in the arrowed direction. The direction of magnetization in (a-c) is depicted in the colour wheel in (a) (inset).

4. Key Message

  • These results provide crucial insight into the intricate details of the magnetostructural transition from the AF to FM phase in equiatomic FeRh thin film systems.
  • Quantitative charting of the magnetic transition during in situ heating was enabled with direct measurements from SWs on a very localized scale and high level of detail.
  • It is possible to make a direct comparison with how the materials would behave on the nanoscale in a functional device by performing biasing &/or heating in situ within the TEM, together with visualizing the magnetism using DPC imaging.

This information has been sourced, reviewed and adapted from materials provided by DENSsolutions.

For more information on this source, please visit DENSsolutions.

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