Thermally Induced Pulses in Magnetoresistive Heads

The thermal response of a magnetoresistive head is analyzed for frictional heating between the head surface and dust particles or other asperities on the recording medium surface during relative motion of head and medium. A theoretical model is presented showing that pulses are induced in the output of a magnetoresistive head as a result of this frictional heating. The model predicts the dependence of these thermal noise spikes on the thermal properties of the substrate and cover chip for the magnetoresistive head, the dimensions of the magnetoresistive stripe, the head-medium relative velocity, and the rate of frictional heat generation. Experimental verification of the theoretical model is obtained by scanning a focused laser beam across a head. Introduction Hunt has described a novel thin film transducer, using the magnetoresistive effect [ I ] for magnetic applications. Because the resistance of a magnetoresistive (MR) film is temperature dependent, the thermal response of an MR head to frictional heat sources results in noise in its output. Thermal noise spikes have been observed by Gorter and Potgiesser [ 2 ] in the output of an MR head used in a cassette recorder. In this paper, a theoretical model is given to predict the thermal response of an MR head to frictional heat sources. To test the predictions of the model, a laser beam was focused and scanned across an MR head (Fig. 1) to simulate a frictional heat source and the head output was measured. Model for thermal noise spikes The configuration studied is a thin (<500A) vertical stripe of MR material (such as an 83: 17 NiFe evaporated permalloy film) on a substrate with a cover chip “glued” on as shown in Fig. I . A constant current is passed through the MR stripe in the direction parallel to the magnetic easy axis and the voltage across the stripe is monitored. Frictional heut source Consider a frictional heat source such as a particle of dust or an asperity on the recording medium being dragged across the head surface by the moving medium. The temperatures of the substrate, magnetoresistive stripe, and cover chip rise because of this heat source. Since the MR stripe is thin it responds only to the temperature of the material surrounding it-acting like an ideal thermometer. By assuming that the “glue” holding the cover chip is negligibly thin, the temperature distribution in the substrate and cover chip can be approximated by the temperature distribution in a solid block of material having the same thermal properties as the substrate and cover chip material (referred to later as the host material). The temperature rise at a point in the magnetoresistive stripe can then be approximated as being equal to the calculated temperature rise at the equivalent position in the solid block. This approximate temperature rise can be used with the temperature coefficient of resistivity for the magnetoresistive material to compute a resistance rise for the stripe and the resulting voltage output. It has been shown [3] that for a point heat source of magnitude q moving at velocity u in the x direction across a plane surface (x-y plane, Fig. 1 ) of material with thermal conductivity k and thermal diffusivity a(where a = k / C , the value C being the heat capacity per unit volume), the temperature in the material at a point ( x , y , z ) at time t is given by T ( t , Y, 2) = q {exp L-45 + r ) /2a1) /25+r , ( 1 )