We apply the first-order reversal curve (FORC) method, borrowed from studies of ferromagnetic materials, to the magneto-structural phase transition of Fe1+yTe. FORC measurements reveal two features in the hysteretic phase transition, even in samples where traditional temperature measurements display only a single transition. For Fe1.13Te, the influence of magnetic field suggests that the main feature is primarily structural while a smaller, slightly higher-temperature transition is magnetic in origin. By contrast Fe1.03Te has a single transition which shows a uniform response to magnetic field, indicating a stronger coupling of the magnetic and structural phase transitions. We also introduce uniaxial stress, which spreads the distribution width without changing the underlying energy barrier of the transformation. The work shows how FORC can help disentangle the roles of the magnetic and structural phase transitions in FeTe.

We describe a simple manganin foil resistance manometer for uniaxial stress measurements. The manometer functions at low pressures and over a range of temperatures. In this design no temperature seasoning is necessary, although the manometer must be prestressed to the upper end of the desired pressure range. The prestress pressure cannot be increased arbitrarily; irreversibility arising from shear stress limits its range. Attempting larger pressures yields irreproducible resistance measurements.Manganin has been used and studied extensively as a pressure manometer for decades, both for hydrostatic pressure and for shock waves. Its resistivity is linear up to high pressures, described by a piezoresistance coefficient ∆R RP . Near room temperature the resistivity has little temperature-dependence, due to a nearby maximum in the resistivity. Hence any temperature changes induced through compression do not interfere with the reading of a resistance manometer [1]. Also, while the resistivity is temperature-dependent, at fixed temperature it maintains its linearity in pressure [2].Typical commercial manganin gauges use wire coils or grids manufactured from very thin foil. Here we discuss a simple home-made manganin foil gauge for uniaxial stress measurements. Our work involves lower pressures than usual for manganin manometers, and we find that for our purposes various steps in setting up the manometer become unnecessary. A main advantage is the ease of making the gauge and adapting it to the geometry of the experiment.One such manometer, shown in Figure 1, is a 10 mm by 3 mm piece of manganin foil, 0.5 mm thick. The foil is inserted into a pressure column, with stress applied through a circular spacer to a 3 mm diameter region. Platinum wires for resistance measurements are spot-welded to the large top surface of the manganin foil, outside of the stressed region, making these connections very simple. Using a foil larger than the pressure column also helps in assembling the column; with a small piece it is more difficult to ascertain that the spacers and manometer are properly positioned and aligned. The manganin response is then used to determine the pressure on a sample located elsewhere in the pressure column. We use two different pressure setups for calibrating the manganin manometer and confirming that its behavior is reproducible. In one the stress is applied by tightening a screw; in the second it comes from pressurizing a helium bellows [3,4]. The latter setup is mounted on a dilution refrigerator, but in the work here we use it from room temperature down to liquid nitrogen temperature. Each apparatus has a piezoelectric in the pressure column to measure the applied stress. After calibration is complete we can transfer the manganin manometer to a different screw-operated pressure cell, which has too small a diameter to accommodate the piezoelectric sensors.In previous experiments manganin pressure gauges often require "seasoning" to give reproducible results. The treatment typically includes both annealing to ...