Yuchen Zhou

Current induced noise in CPP spin valves

In this paper, an experimental investigation of current-induced noise in current perpendicular-to-plane spin-valve heads is reported. It is found that the sense current yields excitations of coherent spin waves as well as substantial 1/f-like noise when current exceeds a critical value. The noise is magnetic in nature. The criticality and current directional dependence of the noise excitation clearly show that the spin transfer is the cause.

Thermally excited ferromagnetic resonance as diagnostic tool for spin valve heads

Thermally excited ferromagnetic resonance (TEFMR) in a spin valve head gives rise to mag-noise while its power spectral density directly reflects the magnetic field distribution in the sensing layer as well as the corresponding micromagnetic states. In this article, we report an experimental study on utilization of TEFMR spectral characteristics for spin valve head diagnosis and characterization. The study suggests that the variation of the micromagnetic states in the PM layer at the junctions is one of the most important factors causing the head-to-head variations of the recording performance, especially the heads from the same wafer.

Perpendicular write-head remanence characterization using a contact scanning recording tester

In this study, an experimental method for characterizing perpendicular write-head pole-tip remanence using a contact scanning recording tester is presented. The head remanence is characterized by imaging the written patterns generated by perpendicular write heads on dc-saturated perpendicular media. Write-head footprints are recorded with pulsed unidirectional currents while the head is stationary, followed by scanning the head in various directions at current-off state. Sufficient head remanent field yields written traces during the current-off scan. Projection of traces in multiple scanning directions enables pinpointing of the pole-tip remanence locations. It is found that the traces written by the remanent field appear only when pulse current amplitudes exceed a certain threshold value. The strength of remanent field also shows a dependence on current pulse polarity. The “stiffness” of the remanence is characterized by the amplitude of the opposite write current needed to eliminate the remanent field.

Experimental observations of thermally excited ferromagnetic resonance and mag-noise spectra in spin valve heads

Thermally excited ferromagnetic resonance (FMR) gives rise to so-called mag-noise in the spin valve read heads used in disk drives. In this article, an experimental method of measuring the mag-noise power spectrum density (PSD) in a frequency range of 2.5–6 GHz is presented. The thermally excited FMR modes in the spin valve and the corresponding resonant frequencies have been observed. It is found that the resonance frequencies occur in the range from 3 to 5 GHz in today’s spin valve heads at the quiescent state. Multiple resonance peaks were observed for some spin valve read heads while others only show a single resonance peak. An external field was applied to the spin valve heads to study the field dependence of the FMR spectrum. The mag-noise level at low frequencies is at the similar level of the head’s Johnson noise for heads of track width W=0.3 μm and 50 Ω resistance. It is suggested that the thermally excited FMR PSD could be used as a rapid diagnostic tool for spin valve heads.

Effect of pole-tip size on perpendicular recording head remanence

To aid understanding the physical mechanisms responsible for the perpendicular write head remanent field and thereby improve the write head design, we studied the remanence of single-pole perpendicular recording heads with trailing shield by using a contact scanning recording tester. We used two different pole-tip size heads in the study. We found that the remanence areas of the heads tested usually locate at the trailing edge corners of the pole tip. We also found that the remanence areas of both types of heads are also the head field easy-rising areas when the heads are driven by low write currents. Additionally, the current level that leads to remanence is the same for both types of heads. However, the remanence of the small pole-tip heads can be demagnetized by a much lower amplitude reverse current than that of the large pole-tip heads.

Dependence of the pole-tip remanence on the medium magnetization state underneath the trailing shield of a perpendicular write head

Understanding the physical mechanisms responsible for the write head pole-tip remanence is critical for making perpendicular recording a practically viable technology. In this paper, one of these mechanisms is explored in detail by using a contact scanning recording tester. The pole-tip remanence of unlaminated perpendicular write heads is studied by varying the magnetization states underneath the trailing shield. It is found that the net medium magnetization underneath the trailing shield acts effectively as magnetomotive force in the head-soft underlayer magnetic flux circuit. A clear correlation between the pole-tip remanence and the medium magnetization state is demonstrated and characterized. The medium magnetization facilitates the head remanence effectively, resulting in significant erasure after write. The stiffness of the remanence shows a clear dependence on the current amplitude of the prior writing. The large physical dimension of the trailing shield makes the effect of medium magnetization si...

Experimental Study of Perpendicular Write Heads at Data Rates Beyond 2 Gb/s

A modified contact tester utilizing RF probes is used to test perpendicular write heads at slider level with frequencies up to 3.3 GHz. The setup enables demonstration of the recording dynamic effects at extremely high frequency. A study using single pulse writing at deep sub-nanosecond pulse widths has been performed as a function of write current amplitude. The sub-nanosecond writes show clear current polarity dependence. AC erasure performed at different frequencies and current amplitudes indicates possible head magnetization dynamics that may cause the ac field degradation and disappearance at very high current switching frequencies. For some head, the ac erasure at certain driving frequencies is able to lock the head magnetization into remanence state instead of demagnetizing it

Experimental study of overwrite mechanism in perpendicular recording

In this paper, we present an experimental study of the overwrite mechanism in perpendicular recording, utilizing a scanning contact recording tester. It is found when writing new data over existing old data in a Stoner-Wohlfarth like perpendicular medium, the on-track remanence level (the regions between two adjacent transitions) of the new transitions is modulated by the old data pattern. This is because of the relatively poorer effective write field in the middle of the pole-tip footprint. It is argued that the demagnetization field from the residual magnetization pattern underneath the middle of the pole-tip region significantly affects the writing at the trailing edge, resulting in the observed modulation The phenomenon becomes less pronounced when the magnetic reversal becomes less dependent on the field angle.

Experimental study of overwrite modulation in perpendicular magnetic recording

Perpendicular magnetic recording is regarded as next generation recording technology for storage industry. However, overwrite modulation in perpendicular recording was not thoroughly studied. In longitudinal recording, old information modulates the newly recorded transitions through in-plane field arising from write field leading edge transitions. In this study, experimental perpendicular recording overwrite tests are performed on several different perpendicular media. It shows that modulation in perpendicular recording comes directly from demagnetization field of previously recorded bits. It also shows that write head effective switching field profile significantly affects the modulation behavior.

Pole-tip size effect on perpendicular recording head remanence

A contact scanning recording tester system is utilized to study perpendicular recording head remanence. Remanence behavior between large and small pole-tip single-pole perpendicular write heads are experimentally compared. Both tip sizes show asymmetry between current directions, i.e. one polarity write current creates more significant remanent field than the other polarity. It is found that the current driven head field easy-rising areas are identical to the pole-tip remanence areas, which further indicates the head structure dependence of the perpendicular recording head remanence.