This setup combined a high-resolution magneto-optic Kerr microscope and a flexible multifunctional probe station. Domain structures and magnetic dynamics in samples can be imaged at various test conditions, e.g., in-plane magnetic field, out of the plane magnetic field, DC/RF current. A control system and intelligent software are designed, allowing to apply the above signal simultaneously, with customer-defined timing logic; Kerr images can be acquired and processed automatically (e.g. subtracting background noise, drift correction, etc.). This system facilitates the experimental research and test of spintronic devices.
Components and specification
The Kerr microscope allows the microscopy of magnetic domains and magnetic dynamics in magnetic materials. Both the in-plane and out of plane component of magnetization can be imaged with the longitudinal and polar configuration, respectively.
- Spatial resolution: up to 450 nm compatible with DC/RF probes; 230 nm with oil-immersed objective lens (not compatible with probes);
- Field of view: 150μm @ 100X objective; 750μm @ 5X objective;
- Kerr image acquisition speed: up to 50 fps;
- Longitudinal and polar regime can be switched automatically with software control.
A magnetic probe station is designed and integrated with the Kerr microscope. The in-plane magnetic field, out of plane field and DC/RF signal current can be applied simultaneously, facilitating the magnetic dynamic observation under various conditions.
- Standard configuration:
- In-plane field: up to 0.7 T;
- Out of plane field: up to 0.2 T, and up to 0.3 T with an extended pole;
- Two pairs of DC/RF (0-40 GHz) probes compatible with 100X long work distance objective.
- Customized configuration:
- In-plane field: up to 1.4 T;
- Out of plane field: up to 1.5 T.
Multifunctional control system
1. Signal control
- Microsecond time synchronization between magnetic field/current/microwave, etc.
- Easy adjustment of the waveform, amplitude, frequency, relative delay and other parameters of the applied current/field.
2. Image processing
- Real-time elimination of background noise,
- Automatic correction of vibration and drift, etc.
3. Signal analysis
- Real-time display of the current and magnetic field.
- Hysteresis loop as a function of magnetic field/current through local or global Kerr image analysis.
Study the properties of magnetic materials
1. Quality test of magnetic materials
- MgO(sub)/Co/Pt samples: defects caused by lattice mismatch of MgO lattice and Co. Magnetic switching occurs around the defect under a weak magnetic field;
- Dendritic magnetic domains during magnetic reversal in poor-quality magnetic film.
- Uniform magnetic domain structure and smooth edges after domain wall motion in high-quality magnetic film.
2. Defect detection
Curvature of domain wall caused by pinning effect during movement. The position of these defects (surrounded by red circle) can be observed directly via high-resolution Kerr microscopy.
3. Damage detection of spintronic devices
Property degradation in the edge of spintronic devices caused by nanofabrication can be observed. In this example, the magnetic reversal occurs in the edge of the device under a weak magnetic field because of degradation of perpendicular anisotropy.
4. Global/local hysteresis loops
Thanks to its spatial resolution, the Kerr microscope can be used to extract the hysteresis loop based on global or local grayscale change of Kerr image. Notably, thanks to the multifunctional probe station, the variable of the hysteresis loop can either be the perpendicular/in-plane magnetic field or current. While scanning the loop, the corresponding Kerr image can be saved.
Local Magnetic parameter characterization
Kerr microscope has a set of methods to characterize the basic magnetic parameters. Compared with other methods, these parameters can be obtained in local area with high spatial resolution, allowing to detect the properties of non-uniform magnetic materials. For example, local properties can be tuned by irradiation, pressure control or optical excitation and the property change can be detected.
1. Saturation magnetization in local area
Magnetic domain walls (DW) repel when they are close to each other because of the dipole interaction. The saturation magnetization Ms in the local area can be extracted by measuring the DW distance under different magnetic fields. The method was first proposed and verified by Professor Nicolas Vernier at Paris-Saclay University (technical consultant of our company) in 2014. The test results are highly consistent with VSM measurement.
The strength of effective anisotropy field in local area can be obtained based on hysteresis loop scanning.
3. Exchange stiffness Aex
The labyrinth domain structure can be obtained after demagnetized by the custom-defined attenuate oscillating magnetic field. The domain width can be obtained by analyzing the Fourier transform of Kerr image. the Heisenberg’s exchange stiffness can be extracted [ref: M. Yamanouchi et al., IEEE Magn. Lett. 2, 3000304 (2011)].
4. Dzyaloshinskii-Moriya interaction (DMI)
The strength of DMI in magnetic thin films can be obtained by measuring the asymmetric expansion speed of the domain walls under the combined effect of in-plane and perpendicular magnetic field [A. Cao et al., Nanoscale 10, 12062 (2018)].
Study on magnetic domain wall dynamics
1. Domain wall motion velocity measurement under magnetic field, current or other excitations
Method: Applying a magnetic field/current pulse with an amplitude B and a duration t; taking two Kerr images before and after the pulse; the DW displacement d can be obtained by subtracting the two Kerr image; then the DW motion velocity can be calculated as v=d/t.
Note: the measurement of ultrafast domain wall motion requires an ultrashort magnetic field or current pulses. The mini coil provided by our system allows to get a magnetic field pulse with a microsecond rise time. The measurement of the domain wall speed up to 200 m/s is possible.
2. Observation of the magnetic domain wall tension effect
Magnetic bubbles can be created in tiny samples using ultra-fast microsecond magnetic field pulses. The spontaneous contraction of the magnetic bubble under the surface tension was observed using the high-resolution Kerr microscope [X. Zhang, N. Vernier et al. Phys. Rev. Appl. 9, 024032 (2018)].
3. The pinning effect of magnetic domain wall at Hall cross
Using magnetic field pulses, the position of the magnetic domain wall in the nanowire can be controlled precisely. We can observe the pinning effect of the magnetic domain wall at Hall cross and measure the depinning magnetic field [X. Zhang, N. Vernier et al. Phys. Rev. Appl. 9, 024032 (2018)].
Spin transport experiments
1. Magnetic domain wall motion driven by STT current
This system provides two pairs of probes and an arbitrary waveform generator. Square waves with a duration from 50 ns to seconds can be applied to the sample to induce the DW motion and the DW velocity can be measured precisely.
2. Magnetic domain wall motion under the combined effect of STT current and perpendicular magnetic field
Magnetic domain wall motion driven by pure current cannot be observed because of pinning effect or weak spin polarization ratio in some materials, e.g. some heavy metal/magnetic thin film. In this case, by applying a synchronized ultra-fast magnetic field pulse and current, the domain wall motion driven by the combined effect of perpendicular magnetic field and current can be observed. Some physical effects, such as the decay of spin polarization rate due to spin scattering is found.
3. Magnetic domain wall motion under the combined effect of current and in-plane magnetic field
Magnetization switching can be achieved under the combined effect of spin Hall current and in-plane magnetic field. This system allows to apply simultaneously the in-plane field and current and the SOT induced magnetic switching can be observed.