1678 IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4,JULY 2001 Analysis of Random Telegraph Noise in Spin Valve Heads with Ultra-Thin Free Layers Jing Zhang, Ningjia Zhu, Yiming Huai, Anil Prabhakar, Paul Rana, Dave Seagle, and Marcos Lederman Abstract—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 quanti- fied by the flatness of RTN spectra and is correlated with the nor- malized 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. Index Terms—Lifetime, random telegraph noise, spin valve heads, thermally activated domain instability. I. INTRODUCTION T HIN free layers (FL) are demanded for ultra-high density spin valve (SV) read heads to maintain amplitude sensi- tivity at sub-micron read trackwidth. It is well known that thin recording media have an adverse impact on long-term stability due to thermally activated signal decay [1]. This concern can be extended to the case of SV heads with ultra-thin free layers due to reduced , where is the energy barrier and is the thermal energy. However, magnetic moment reversal oc- curs much faster compared to years for media, since of free layers is much smaller. This spontaneous moment reversal manifests itself as random telegraph noise (RTN) in SV heads [2], [3] and poses a serious threat for reader instability. It is of both practical and academic importance to understand RTN in SV heads with ultra-thin free layers. This paper for the first time explores the impact of the free layer thickness on RTN. II. EXPERIMENTAL RESULTS AND ANALYSIS A. Time Domain Characterization SV heads with ultra-thin free layer and synthetic pinned layer [4] were used for RTN analysis. Bias current dependence of noise amplitude and asymmetry was measured on a Guzik spinstand tester with a head flying over DC erased media. For a stable head, noise amplitude increases slightly with due to Joule heating and asymmetry remains near zero. For an unstable head showing RTN, noise amplitude peaks at a certain bias cur- rent with a corresponding asymmetry swing in a pre-determined Manuscript received October 13, 2000. The authors are with the Read-Rite Corporation, Fremont, CA 94539 USA (e-mail: jing.zhang@readrite.com). Publisher Item Identifier S 0018-9464(01)07300-9. Fig. 1. Bias current dependence of RTN amplitude and asymmetry. Notice the RTN amplitude peak and the corresponding asymmetry swing. Fig. 2. Scope traces show that the lifetime of each meta-stable state is modulated by the bias current. The duration of the scope trace is 1 s. direction, as shown in Fig. 1. The amplitude asymmetry is cal- culated as , where and is the pos- itive and negative noise amplitude, respectively. Scope traces were captured with a Lecroy digital oscilloscope (Fig. 2). Heads toggle between two meta-stable states and the lifetime of each state is modulated by the bias current [3]. The amplitude asym- metry is a manifestation of lifetime asymmetry. Positive and negative asymmetry actually corresponds to positive and nega- tive RTN spikes, respectively. The average lifetime of two meta- stable states is approximately equal when RTN amplitude peaks while amplitude asymmetry is near zero. Since the amplitude is measured through a peak detector with a finite time constant, it is an indication of frequency and duration of noise spikes. Quasistatic test (QST) reveals large Barkhausen jumps in transfer curves at low fields for heads showing RTN, consistent with reported data [5]. at which RTN peaks might show reversible shift after magnetic and thermal stress, for example, continuous write operation. More detailed time domain analysis was reported elsewhere [6]. RTN in SV heads is attributed to thermally activated free layer fluctuations [2], [3]. Data reported in this paper was taken at room temperature. It was known that the lifetime shortens at elevated temperature [3]. When bias current sweeps, free layer 0018–9464/01$10.00 © 2001 IEEE