Tools/Techniques
Resonant torsion magnetometry
A cantilever holding the crystal is excited electrically at the lowest-resonance mode and small shifts in the resonant frequency, due to the magnetic response of the sample, are tracked as magnetic field temperature, and field orientation is changed.
This technique has a higher sensitivity because frequencies can be measured more precisely than amplitudes and it is a second derivative of the free energy hence a large discontinuous jump can be observed across the phase boundary in the magnetotropic coefficient
Electron Back-scattered Diffraction (EBSD)
Electron backscatter diffraction (EBSD) is a crystallographic characterization technique that is commonly used to determine the size and relative orientation of domains in polycrystalline materials. We make use of the fact that EBSD can provide crystallographic information on very small single crystals (on the order of 10s of nanograms) to clarify the crystal structure, orientation, and alignment of the crystals during our rotation studies.
SEM & Plasma focused ion beam (PFIB)
We use Plasma focused ion beam (PFIB) in conjunction with the Scanning electron microscope (SEM) to precisely cut a small piece of crystal from its bulk in a preferred and desired orientation. A PFIB can be employed for transport devices of various shapes, sizes, and requirements.
Torque magnetometry
Torque magnetometry works on the basic principle of the Wheatstone bridge Fig (a) and (b). R1 and R2 are external resistors with 475 Ohm resistances Under a balanced condition, 𝑉𝐴 = 𝑉𝐵, and the output signal VG is zero. The cantilever bends due to the torque exerted on the sample inside an applied magnetic field that results in a potential difference between VA and VB, hence a net signal is obtained. A Highly magnetized sample leads to more bending of the cantilever and more potential difference and hence a larger torque signal. The whole setup is rotated inside a magnetic field by using a rotator.
Four probe measurements
I use the four-probe method to measure the resistivity, magnetoresistance, and anisotropic magnetoresistance of a single crystal.
In a standard four-probe method, a current is applied along the outer contacts (1 & 4) and a floating potential is measured through the two inner (2 & 3 contacts).
As this method eliminates the effect of resistance between contact and sample therefore it is a more accurate way of measuring the resistivity of low-resistance samples.
Strain measurements on single crystals
We developed a strain device to produce a more homogenous strain in layered single crystals. This technique provides a more systematic route of changing single crystal dimensionality by gradually increasing the strain value. Figure (left) shows a homemade strain setup. The device consists of three piezo stacks (PZTs) where the middle PZT works as applying compressive strain whereas two end stacks work as applying tensile strain. Two copper plates right below the sample holders are used as strain sensors. This strain engineering technique has an advantage over the other traditional strain techniques like substrate and doping which are hard in terms of homogeneity and precise control of defects, respectively.
