# Damping constant estimation in magnetoresistive readers

## Abstract

The damping constant is a key design parameter in magnetic reader design. Its value can be derived from bulk or sheet film ferromagnetic resonance (FMR) line width. However, dynamics of nanodevices is usually defined by presence of non-uniform modes. It triggers new damping mechanisms and produces stronger damping than expected from traditional FMR. This work proposes a device-level technique for damping evaluation, based on time-domain analysis of thermally excited stochastic oscillations. The signal is collected using a high bandwidth oscilloscope, by direct probing of a biased reader. Recorded waveforms may contain different noise signals, but free layer FMR is usually a dominating one. The autocorrelation function is a reflection of the damped oscillation curve, averaging out stochastic contributions. The damped oscillator formula is fitted to autocorrelation data, producing resonance frequency and damping constant values. Restricting lag range allows for mitigation of the impact of other phenomena (e.g., reader instability) on the damping constant. For a micromagnetically modeled reader, the technique proves to be much more accurate than the stochastic FMR line width approach. Application to actual reader waveforms yields a damping constant of ∼0.03.

- Authors:

- Recording Heads Group, Seagate Technology, Bloomington, Minnesota 55435 (United States)

- Publication Date:

- OSTI Identifier:
- 22410153

- Resource Type:
- Journal Article

- Journal Name:
- Journal of Applied Physics

- Additional Journal Information:
- Journal Volume: 117; Journal Issue: 17; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0021-8979

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; DAMPING; FERROMAGNETIC RESONANCE; FILMS; LINE WIDTHS; MAGNETORESISTANCE; OSCILLATIONS; OSCILLATORS; PROBES; REFLECTION; STOCHASTIC PROCESSES; WAVE FORMS

### Citation Formats

```
Stankiewicz, Andrzej, E-mail: Andrzej.Stankiewicz@seagate.com, and Hernandez, Stephanie.
```*Damping constant estimation in magnetoresistive readers*. United States: N. p., 2015.
Web. doi:10.1063/1.4916498.

```
Stankiewicz, Andrzej, E-mail: Andrzej.Stankiewicz@seagate.com, & Hernandez, Stephanie.
```*Damping constant estimation in magnetoresistive readers*. United States. doi:10.1063/1.4916498.

```
Stankiewicz, Andrzej, E-mail: Andrzej.Stankiewicz@seagate.com, and Hernandez, Stephanie. Thu .
"Damping constant estimation in magnetoresistive readers". United States. doi:10.1063/1.4916498.
```

```
@article{osti_22410153,
```

title = {Damping constant estimation in magnetoresistive readers},

author = {Stankiewicz, Andrzej, E-mail: Andrzej.Stankiewicz@seagate.com and Hernandez, Stephanie},

abstractNote = {The damping constant is a key design parameter in magnetic reader design. Its value can be derived from bulk or sheet film ferromagnetic resonance (FMR) line width. However, dynamics of nanodevices is usually defined by presence of non-uniform modes. It triggers new damping mechanisms and produces stronger damping than expected from traditional FMR. This work proposes a device-level technique for damping evaluation, based on time-domain analysis of thermally excited stochastic oscillations. The signal is collected using a high bandwidth oscilloscope, by direct probing of a biased reader. Recorded waveforms may contain different noise signals, but free layer FMR is usually a dominating one. The autocorrelation function is a reflection of the damped oscillation curve, averaging out stochastic contributions. The damped oscillator formula is fitted to autocorrelation data, producing resonance frequency and damping constant values. Restricting lag range allows for mitigation of the impact of other phenomena (e.g., reader instability) on the damping constant. For a micromagnetically modeled reader, the technique proves to be much more accurate than the stochastic FMR line width approach. Application to actual reader waveforms yields a damping constant of ∼0.03.},

doi = {10.1063/1.4916498},

journal = {Journal of Applied Physics},

issn = {0021-8979},

number = 17,

volume = 117,

place = {United States},

year = {2015},

month = {5}

}