IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Write heads for use in perpendicular recording applications were modeled using finite-element analysis. A comparative analysis of ring and single-pole write heads was conducted. The single-pole head design had a higher field at the media along with higher field gradients and lower head inductance. Both models included the effects of a soft underlayer in the media, which is used in perpendicular recording. The finite-element analysis was checked for accuracy using Ampere's law and the error in the calculated fields is estimated as less than 4%.

Department of Electrical Engineering

World University root node

IEEE Transactions on Magnetics

Finite-element analysis of ring and single-pole write heads for perpendicular recording

Random telegraph noise (RTN) in spin valve heads with ultra-thin free layers is analyzed in both time and frequency domain. RTN is characterized by random fluctuation between two meta-stable states and is attributed to thermally activated domain instability. Lifetime of each meta-stable state is changes with bias current, with both being equal when RTN amplitude peaks while asymmetry is near zero. The lifetime at equilibrium can be quantified by the flatness of RTN spectra and is correlated with the normalized peak area under RTN amplitude versus bias current curve. This area scales with the energy barrier associated with RTN. With the same RTN peak area, lifetime at equilibrium is shorter for heads with thinner free layers but otherwise the same structure. Impact on reader instability for ultra-high areal density recording is discussed.

Analysis of random telegraph noise in spin valve heads with ultra-thin free layers

The nonlinear algorithms that are a part of the TISEAN package were used to analyze the time domain response of complex nonlinear systems. This approach revealed the structure of the underlying attractor and helped differentiate between magnetization states exhibiting the same noise amplitude asymmetry.

Digests of the Intermag Conference

Time domain analysis of low frequency noise in giant magnetoresistive recording heads

Forward volume spin waves were excited in a yttrium–iron–garnet film using a microstrip delay line. Low frequency modulations in output power were captured on a digital oscilloscope for various combinations of microwave input frequency and power. Using the decrease in mutual information as a criterion, an embedding delay time was chosen for each time series. The method of false nearest neighbors was then used to calculate a minimal embedding dimension, D. Despite large changes in input frequency [formula omitted] and power [formula omitted] [formula omitted] for each time series, indicating a low dimensional system. Finally, the information dimension, d, was estimated by computing the distance from a collection of reference points to their nearest neighbors. The distribution for d was found to be bimodal. When correlated with the input microwave power and frequency, it was found that values of [formula omitted] occurred close to the auto-oscillation threshold in the vicinity of an even dipole gap. [formula omitted] was observed at higher power levels or at frequencies close to an odd dipole gap. © 2000, American Institute of Physics. All rights reserved.

Fulltext

Journal of Applied Physics

Information dimension analysis of chaotic forward volume spin waves in a yttrium–iron–garnet thin film

A nonlinear system theory is an alternative method to contemporary piecewise linear models in the estimation of flux reversal time in magnetic thin film write heads. A nonlinear system approach enables the measured pulsed and monotonic frequency response of severely-damped magnetic systems to be compared. The approach is applied to a magnetic head based on a Ni-Fe thin film.

Flux reversal time in thin film write heads: A nonlinear system theoretic approach

The excitation of large amplitude spin waves causes a shift in the resonance frequency of a ferrite resonator towards higher frequency values. Using this shift, a resonator can be designed to operate in a bistable mode where the history of the input power determines the output power. A theoretical calculation for a resonator with a linewidth of 1.5 MHz, measured at 10 GHz, predicts a discontinuous jump in output power by a factor of about 3.5 as the input power is increased to about 0.7 mW. The phenomenological model that describes the bistability also provides an explanation for the foldover of ferromagnetic resonance curves at high input power levels. An appropriate choice for the operating frequency ensures that the estimated input power required to observe the bistable behavior is below the predicted threshold power for spin-wave auto-oscillations at the main resonance.

Nonlinear microwave-magnetic resonator operated as a bistable device

The propagation of large-amplitude forward-volume magnetostatic waves (MSW’s) is studied in (Formula presented) and (Formula presented) thin films. The onset of auto-oscillations beyond the second Suhl instability influences the transmission characteristics of a MSW delay line. Global variations in transmitted power over a frequency-power input parameter space are monitored using density plots. There is a shift in the MSW passband with increasing input power that is explained by a decrease in demagnetizing field with an increase in dynamic magnetization. As the input power increases beyond a threshold value, we observe an increase in signal amplitude, heralding the onset of auto-oscillations. Fingers of auto-oscillation spanning an input range of 200 MHz and 10 dB are formed. The locations of these fingers are correlated with the locations of spin-wave resonance notches in the MSW passband. The auto-oscillation thresholds in the vicinity of a spin-wave resonance notch are 4-8 dB lower than elsewhere in the passband. A heuristic model based on a reduced group velocity near a notch can explain the lower threshold values. © 1998 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Auto-oscillation thresholds at the main resonance in ferrimagnetic films

Optical modulation with a bandwidth of about 3 GHz centered at 5.75 GHz was observed in a thin film of [BiLu]3Fe5O12<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00001.gif"><msub><mi mathvariant="normal">[BiLu]</mi><mrow><mn>3</mn></mrow></msub><msub><mi mathvariant="normal">Fe</mi><mrow><mn>5</mn></mrow></msub><msub><mi mathvariant="normal">O</mi><mrow><mn>12</mn></mrow></msub></math>&nbsp;using the magneto-optical interaction. A microstrip transducer was placed in contact with the film and TM polarized optical guided modes were excited beneath and parallel to the transducer. The TM→<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00002.gif"><mi>→</mi></math>TE mode conversion was measured as a function of the angle between an in-plane field, of up to 100 Oe, and the optical beam. The bandwidth of the modulation was limited by the microwave components used in the external circuit. The experiments qualitatively agree with a simple model that describes a modulation of the optical beam by a nonresonant precession of the magnetization about an equilibrium.

Wideband optical modulation via the magneto–optic interaction in a bismuth-lutetium-iron garnet film

A microstrip T-transducer excites forward volume magnetostatic waves (MSWs) in a [BiLu]3<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00001.gif"><msub><none>&nbsp;</none><mrow><mn>3</mn></mrow></msub></math>Fe5<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00002.gif"><msub><none>&nbsp;</none><mrow><mn>5</mn></mrow></msub></math>O12<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00003.gif"><msub><none>&nbsp;</none><mrow><mn>12</mn></mrow></msub></math>&nbsp;film. An optical beam is edge coupled into the film and the transverse MSW–optical interaction is observed at high microwave power levels. Using the coupled mode equations, we demonstrate that a T-transducer causes the intensity of the diffracted beam to be a product of terms involving the magneto-optic coupling coefficient (κ)<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00004.gif"><mi>(κ)</mi></math>&nbsp;and the phase mismatch between the optical guided modes and the MSWs. Taylor series expansions for κ<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00005.gif"><mi>κ</mi></math>&nbsp;and the MSW wave number (β)<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00006.gif"><mi>(β)</mi></math>&nbsp;are used to obtain an analytic expression for the MSW–optical interaction. The dependence on β<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00007.gif"><mi>β</mi></math>&nbsp;causes a shift in the interaction spectrum with increasing power. After correcting for this shift, the residual dependence on power is attributed to a quadratic correction to the dynamic magnetization which in turn affects κ<math xmlns="http://www.w3.org/1998/Math/MathML" overflow="scroll" altimg="eq-00008.gif"><mi>κ</mi></math>. We demonstrate how the quadratic dependence plays an important role in accurately determining an expansion for the MSW dispersion relation.

Quadratic power corrections to the dynamic magnetization using the transverse magnetostatic wave-optical interaction

Magnetostatic wave - optical interactions are studied in the transverse geometry using a [BiLu]3Fe5O12 film. Non-monotonic variations in the magneto - optic coupling coefficient are observed as we increase the input microwave power at a fixed frequency. Similar variations are also seen in the transmitted microwave signal, though the features appear at higher power levels. The non-monotonic behavior is due to a shift in the microwave passband with increasing power. Density plots are used to correlate the two sets of data over a 350 MHz region. The discrepancy in power levels associated with a feature in the line plots is a consequence of the different curvatures of the two density plots. © 1996 IEEE.

Variations in the magneto-optic coupling coefficient in a bismuth-lutetium-iron-garnet film

High power microwave experiments conducted on a rectangular yttrium-iron-garnet (YIG) film indicate a parabolic dependence of the square of the frequency of auto-oscillations on microwave signal amplitudes, an observation that is qualitatively consistent with prior theoretical predictions. Forward volume magnetostatic waves were excited using a microstrip transducer kept in contact with a YIG film placed in a constant external magnetic field (Hdc=3.48 kG). Variations in the input microwave power (10 dBm&lt;P&lt;25 dBm) and frequency (5.1 GHz&lt;f&lt;5.7 GHz) were used to locate and study auto-oscillations close to the Suhl instability at the main resonance. A novel method of viewing changes in the microwave passband using density plots enables us to study variations in the dipole gaps in the passband near the Suhl instability. A broadening of dipole gaps into fingerlike regions of weak transmission marks the onset of auto-oscillations beyond the instability threshold. These regions are associated with a variety of spin-wave dynamics ranging from simple auto-oscillations to auto-oscillations with period doubling and in some cases an abrupt transition to a turbulent wide-band power spectrum. The parabolic dependence of the auto-oscillation frequency persists despite a period-doubling bifurcation. © 1996 American Institute of Physics.

Variations in auto-oscillation frequency at the main resonance in rectangular yttrium-iron-garnet films

In the collinear interaction between magnetostatic waves and optical guided modes, the use of high microwave power is experimentally observed to broaden the range of microwave frequencies over which a significant optical mode conversion occurs. A mechanism contributing to this broadening is described that is based on a nonlinear shift in the wavelength of the magnetostatic waves at high power along with magnetic damping. Comparisons between previous experiments and our theoretical predictions show that this mechanism can explain some, but not all, of the changes observed in the optical spectrum at high microwave power levels.

Effects of high microwave power on collinear magnetostatic-optical wave interactions

Experiments have been conducted, and correlations are developed for the pressure drop and heat transfer coefficient for the tube and shell sides of a helical coil heat exchanger. In the tube side, the laminar friction factor and Nusselt numbers are represented as functions of Re√d/D, whereas in turbulent flow the results are correlated with Re·(d/D)2. The pressure drop and heat transfer values for the shell side are found to follow the classical Blasius and Dittus-Boelter type relations, while a strong dependence on the coil to tube diameter ratio is detected. The performance of the exchanger has been tested not only as simulated experimental exchanger but also as a waste heat recovery device for a 60 HP gas turbine. Excellent corroboration of the effectiveness-NTU relation has been observed between the simulation and in situ experiments.