# US 10,319,400 B1 · Calculating a heater power that compensates for laser induced writer protrusion for a heat-assisted magnetic recording device > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 10,319,400 B1 - Document ID: US10319400B1 - Status: Granted - Kind: B1 - Application number: 16/104,214 - Filed: 2018-08-17 - Granted: 2019-06-11 - Earliest claimed: 2018-08-17 - Assignees: Seagate Technology LLC - Applicants: Seagate Technology LLC - Inventors: Zhen Wei; Hua Liu - Web page: https://patentgrep.com/patent/US10319400B1 ## CPC - G11B 5/607: {using thermal means} - G11B 20/1816: {Testing} - 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 A method includes moving a heat-assisted magnetic recording (HAMR) slider relative to a magnetic recording medium. The slider comprises a writer, a writer heater, and a near-field transducer (NFT). For each of a plurality of different head-to media spacings a test tone is written to a track of the medium, the test tone is read and a Discrete Fourier Transform (DFT) of an amplitude of the read test tone is captured. A first DFT curve is generated at a beginning of writing the test tones. A second DFT curve is generated at a saturated state of writing the test tones. An amount of horizontal shift between the first and second DFT curves is computed. The amount of horizontal shift corresponding to writer heater power required to compensate for NFT clearance offset due to laser induced writer protrusion. ## Claims ### Claim 1 A method, comprising: moving a heat-assisted magnetic recording (HAMR) slider relative to a magnetic recording medium, the slider comprising a writer, a writer heater, and a near-field transducer (NFT); for each of a plurality of different head-to-medium spacings: writing a test tone to a track of the medium; and reading the test tone and capturing a Discrete Fourier Transform (DFT) of an amplitude of the read test tone; generating a first DFT curve at a beginning of writing the test tones; generating a second DFT curve at a saturated state of writing the test tones; and computing an amount of horizontal shift between the first and second DFT curves, the amount of horizontal shift corresponding to writer heater power required to compensate for NFT clearance offset due to laser induced writer protrusion. ### Claim 2 (depends on 1) The method of claim 1 further comprising adjusting the writer heater power based on the computed amount of horizontal shift. ### Claim 3 (depends on 1) The method of claim 1 wherein the plurality of head-to-media spacings are in a range of about 0 nm to about 6 nm. ### Claim 4 (depends on 1) The method of claim 1, wherein computing an amount of horizontal shift comprises determining a difference in writer heater power used to achieve a same head-to media spacing for the first DFT curve and the average DFT curve. ### Claim 5 (depends on 1) The method of claim 1 further comprising erasing the track before writing the test tone to the track. ### Claim 6 (depends on 1) The method of claim 1, wherein generating a second DFT curve at the saturated state of writing the test tones comprises generating the second DFT at a last sector of the track. ### Claim 7 (depends on 1) The method of claim 1, wherein the first and second DFT curves are captured over one revolution. ### Claim 8 (depends on 1) The method of claim 1, wherein for each head-to-medium spacing a preheat level is set to be substantially the same as a write-heat level. ### Claim 9 (depends on 1) The method of claim 1, wherein the horizontal shift includes local laser induced writer protrusion (LLIWP). ### Claim 10 (depends on 1) The method of claim 1, wherein: the laser induced writer protrusion comprises a first region having a first thermal time constant and a second region having a second thermal time constant; the second region is closer to the medium than the first region; and measuring comprises measuring the magnitude of at least the second region. ### Claim 11 An apparatus comprising: a slider movable relative to a magnetic recording medium, the slider comprising a writer, a heater, a near-field transducer, and an optical waveguide for communicating light from a laser diode to the near-field transducer; and a controller coupled to the slider and configured to: for each of a plurality of different head-to-medium spacings: write a test tone to a track of the medium; and read the test tone and capture a Discrete Fourier Transform (DFT) of an amplitude of the read test tone; generate a first DFT curve at a beginning of writing the test tones; generate a second DFT curve at a saturated state of writing the test tones; and compute an amount of horizontal shift between the first and second DFT curves, the amount of horizontal shift corresponding to writer heater power required to compensate for NFT clearance offset due to laser induced writer protrusion. ### Claim 12 (depends on 11) The apparatus of claim 11 wherein the controller is further configured to adjust the writer heater power based on the computed amount of horizontal shift. ### Claim 13 (depends on 11) The method of claim 11 wherein the plurality of head-to-media spacings are in a range of about 0 nm to about 6 nm. ### Claim 14 (depends on 11) The method of claim 11, wherein the controller is configured to determine a difference in writer heater power used to achieve a same head-to media spacing for the first DFT curve and the average DFT curve. ### Claim 15 (depends on 11) The apparatus of claim 11, wherein the controller is further configured to erase the track before writing the test tone to the track. ### Claim 16 (depends on 11) The apparatus of claim 11, wherein generating a second DFT curve at the saturated state of writing the test tones comprises generating the second DFT at a last sector of the track. ### Claim 17 (depends on 11) The apparatus of claim 11, wherein the first and second DFT curves are captured over one revolution. ### Claim 18 (depends on 11) The apparatus of claim 11, wherein for each head-to-medium spacing a preheat level is set to be substantially the same as a write-heat level. ### Claim 19 (depends on 11) The apparatus of claim 11, wherein the horizontal shift includes local laser induced writer protrusion (LLIWP). ### Claim 20 (depends on 11) The apparatus of claim 11, wherein: the laser induced writer protrusion comprises a first region having a first thermal time constant and a second region having a second thermal time constant; the second region is closer to the medium than the first region; and measuring comprises measuring the magnitude of at least the second region.