A new differential ac technique is described for rapid and continuous measurement of thermoelectric power of thin metallic ribbons over a wide temperature range. Sinusoidal heating of one thermocouple junction by means of a small resistive heater results in a measuring principle which eliminates spurious thermoelectric voltages in the lead wires and favorably benefits from ac signal processing. The resultant equilibrium temperature of the heated junction contains a constant term depending on thermal parameters of the system, and an oscillating term with a frequency-dependent amplitude. By choosing a suitable sample configuration no corrections are required for the thermal response of the system. By careful shielding of the measuring circuits and using modern lock-in amplifiers noise voltages can be reduced to less than 1 nV for measuring frequencies below 10 Hz. Measurements accurate to 1% are possible, using temperature differences smaller than 10 mK. The effectiveness of this technique is demonstrated by comparing our measurement of the thermoelectric power of an amorphous Fe80B20 alloy with measurements using conventional techniques and by studying the thermopower of an amorphous Fe90Zr10 alloy in the vicinity of its second-order magnetic phase transition.

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We develop the theoretical description of 3omega signals from the resistive Wollaston thermal probe (ThP) of a scanning thermal microscope (SThM) in terms of an equivalent low-pass filter. The normalized amplitude and phase frequency spectra are completely characterized by a single parameter, the crossover frequency f(c)(k) depending on the sample thermal conductivity k. The application concerns polycrystalline NiTi shape memory alloy microstructured by focused Ga ion beam milling and implantation. The calibration of the ThP combined with a novel two-step normalization procedure allowed quantitative exploitation of 3omega signal variations as small as -1.75% in amplitude and 0.60 degrees in phase upon heating the sample from room temperature to 100 degrees C. This corresponds to k increase of 23.9% that is consistent with the expected thermal conductivity variation due to martensite-austenite structural phase transition. To our knowledge this is for the first time that SThM 3omega phase information is used quantitatively as well. The static, calibrated 3omega measurements are complementary to 3omega SThM images of the patterned sample surface. The local SThM measurement of temperature-dependent thermal conductivity opens the possibility to imaging structural phase transitions at submicron scale.

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