# US 10,482,908 B2 · Performance monitoring of a near-field transducer of a heat-assisted magnetic recording slider > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 10,482,908 B2 - Document ID: US10482908B2 - Status: Granted - Kind: B2 - Application number: 16/193,189 - Filed: 2018-11-16 - Published: 2019-04-04 - Granted: 2019-11-19 - Earliest claimed: 2017-05-15 - Assignees: Seagate Technology LLC - Applicants: Seagate Technology LLC - Inventors: Riyan Mendonsa; Jon D. Trantham; James Dillon Kiely; Peng Li; Joshua Ward Christensen - Web page: https://patentgrep.com/patent/US10482908B2 ## CPC - G11B 5/455: Arrangements for functional testing of heads; Measuring arrangements for heads - G11B 5/6029: {Measurement using values derived from the data signal read from the disk} - G11B 5/6088: {Optical waveguide in or on flying head} - G11B 7/126: Circuits, methods or arrangements for laser control or stabilisation - G11B 2005/0021: {Thermally assisted recording using an auxiliary energy source for heating the recording layer locally to assist the magnetization reversal} - G11B: INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER (recording measured values in a way that does not require playback through a transducer G01D9/00; recording or playback apparatus using mechanically marked tape, e.g. punched paper tape, or using unit records, e.g. punched or magnetically marked cards G06K; transferring data from one type of record carrier to another G06K1/18; circuits for coupling output of reproducer to radio receiver H04B1/20; gramophone pick-ups or like acoustic electromechanical transducers or circuits therefor H04R) ## Abstract An apparatus comprises a laser diode configured to generate light during a write operation. A slider comprises a near-field transducer (NFT) and an optical waveguide. The slider is configured for heat-assisted magnetic recording and to communicate the light to the NFT via the waveguide. A writer heater of the slider is configured to receive power during the write operation. A thermal sensor is situated at or near an air bearing surface of the slider. The thermal sensor is configured to produce a sensor signal in response to sensing back-heating from the medium while the NFT generates heat during a write operation. Circuitry, coupled to the thermal sensor, is configured to compare the sensor signal to a threshold and generate an output signal indicative of degradation of NFT performance in response to the sensor signal exceeding the threshold. ## Claims ### Claim 1 A method, comprising: moving a slider configured for heat-assisted magnetic recording relative to and spaced apart from a magnetic recording medium, the slider comprising a writer, a writer heater, a thermal sensor, and a near-field transducer (NFT), the sensor situated on the slider so as to be substantially insensitive to heat conducted from the NFT to the sensor; measuring, while the NFT generates heat when writing to the medium, back-heating from the medium not confounded by heat from the NFT using the thermal sensor; and generating an output signal indicative of NFT performance degradation in response to the back-heating measurements. ### Claim 2 (depends on 1) The method of claim 1, comprising: comparing back-heating measurements made by the thermal sensor to a threshold; and generating the output signal indicative of NFT performance degradation in response to a back-heating measurement exceeding the threshold. ### Claim 3 (depends on 2) The method of claim 2, wherein the threshold is based on a back-heating measurement made during manufacture of a hard disk drive that incorporates the slider and the medium. ### Claim 4 (depends on 1) The method of claim 1, comprising taking corrective action in response to the output signal, wherein the corrective action comprises an action to further characterize performance of the NFT. ### Claim 5 (depends on 1) The method of claim 1, wherein the slider has a fly height of less than about 10 nm when the back-heating measurements are made. ### Claim 6 (depends on 1) The method of claim 1, wherein measuring the back-heating comprises measuring a DC response of the thermal sensor. ### Claim 7 (depends on 1) The method of claim 1, comprising supplying power to the writer heater to cause the slider to move toward the medium, wherein: writing to the medium comprises supplying a modulated current to a laser diode of or near the slider to produce modulated light that is communicated to the NFT; and measuring the back-heating comprises measuring an AC response of the thermal sensor at a frequency of the modulated current or a subharmonic of the frequency of the modulated current. ### Claim 8 (depends on 7) The method of claim 7, wherein: supplying power comprises supplying power having a constant magnitude to the writer heater; and supplying the modulated current comprises supplying the modulated current to the laser diode having a mean amplitude that is constant. ### Claim 9 (depends on 7) The method of claim 7, wherein: supplying power comprises supplying power having a constant magnitude to the writer heater; and supplying the modulated current comprises supplying the modulated current to the laser diode having a mean amplitude that varies. ### Claim 10 (depends on 7) The method of claim 7, wherein: supplying power comprises supplying power having a magnitude that varies to the writer heater; and supplying the modulated current comprises supplying the modulated current to the laser diode having a constant mean amplitude. ### Claim 11 An apparatus, comprising: a laser diode configured to generate light during a write operation; a slider comprising a near-field transducer (NFT) and an optical waveguide, the slider configured for heat-assisted magnetic recording and to communicate the light to the NFT via the waveguide; a writer heater of the slider configured to receive power during the write operation; a thermal sensor situated on the slider so as to be substantially insensitive to heat conducted from the NFT to the sensor, the thermal sensor configured to produce a sensor signal in response to sensing back-heating from a magnetic recording medium while the slider is spaced apart from the medium and the NFT generates heat during a write operation; and circuitry coupled to the thermal sensor, the circuitry configured to measure back-heating from the medium not confounded by heat from the NFT using the sensor signal and generate an output signal indicative of NFT performance degradation in response to the back-heating measurements. ### Claim 12 (depends on 11) The apparatus of claim 11, wherein the circuitry is configured to compare the sensor signal to a threshold and generate the output signal indicative of degradation of NFT performance in response to the sensor signal exceeding the threshold. ### Claim 13 (depends on 11) The apparatus of claim 11, wherein the threshold is based on a back-heating measurement made during manufacture of a hard disk drive that incorporates the apparatus. ### Claim 14 (depends on 11) The apparatus of claim 11, comprising a processor configured to take corrective action in response to the output signal, wherein the corrective action comprises an action to further characterize performance of the NFT. ### Claim 15 (depends on 11) The apparatus of claim 11, wherein the thermal sensor is situated at or near an air bearing surface of the slider and configured to produce the sensor signal at a slider fly height of less than about 10 nm. ### Claim 16 (depends on 11) The apparatus of claim 11, wherein the sensor signal is a DC signal measured by the circuitry. ### Claim 17 (depends on 11) The apparatus of claim 11, wherein the sensor signal is an AC signal measured by the circuitry at a frequency of modulated current received by the laser diode or a subharmonic of the frequency of the modulated current. ### Claim 18 (depends on 17) The apparatus of claim 17, wherein: the power received by the writer heater has a constant magnitude; and modulated current received by the laser diode has a constant mean amplitude. ### Claim 19 (depends on 17) The apparatus of claim 17, wherein: the power received by the writer heater has a constant magnitude; and modulated current received by the laser diode has a mean amplitude that varies. ### Claim 20 (depends on 17) The apparatus of claim 17, wherein: the power received by the writer heater has a magnitude that varies; and modulated current received by the laser diode has a constant mean amplitude.