Kumar Srinivasan

Sub-6-nm grain size control in polycrystalline thin films using synthetic nucleation layer

A key objective in the design of recording media for hard disk drives is to tailor the grain size. A fourfold increase in the recording density from the current levels demands that the mean grain diameter be reduced from 7 to about 5.5nm, and distribution to below 15%. Traditional techniques involving dopants can realize small grain sizes but also deteriorate the structural and magnetic properties. In this study, the utility of an extremely thin synthetic nucleation layer in reducing the grain size and distribution of the recording media is demonstrated, with significant implications for other applications too.

Observation of microwave-assisted magnetization reversal in perpendicular recording media

This letter reports microwave-assisted magnetization reversal (MAMR) in a 700-Gbit/in2 perpendicular media sample. The microwave fields were applied by placing a coplanar waveguide on the media sample and feeding it with narrow microwave pulses. The switching states of the media grains were measured by magnetic force microscopy. For microwaves with a frequency close to the ferromagnetic resonance (FMR) frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.

Microstructure and magnetic properties of CoCrPt–SiO2 perpendicular recording media with synthetic nucleation layers

Thermally stable, highly textured CoCrPt–SiO2 perpendicular magnetic recording media with ∼6nm size exchange decoupled grains and tight grain size distribution were prepared on synthetic nucleation layers. The media structure consisted of 14nm thick CoCrPt–SiO2 recording layer on one Ru intermediate layer. For these films, insertion of a synthetic nucleation layer directly under the recording layer was seen to reduce the grain sizes from 6.7to6.2nm and grain size distribution from 17% to 14%. Significant reduction in the intergranular exchange coupling interactions could be achieved while improving magnetic properties. The crystallographic texture of the recording layer remained strongly (00⋅2) oriented with the dispersion Δθ50 below 3.7°. Isothermal remanence magnetization and thermal stability behavior were studied. A model that describes the role of the synthetic nucleation layer in affecting the grain growth and magnetic properties of the recording layer is presented to understand these observations.

Antiferromagnetic iridium manganese based intermediate layers for perpendicular magnetic recording media

Current generation CoCrPt oxide based perpendicular magnetic recording media use two Ru-based intermediate layers in order to grow crystallographically textured, and magnetically isolated granular media. In this work, the advantage of replacing the Ru grain isolation layer with antiferromagnetic IrMn is demonstrated. Media samples using 7.5nm thick IrMn intermediate layers show perpendicular texture with dispersion below 4°, coercivity of over 4000Oe alongside magnetic exchange decoupling, average grain sizes of 6nm with distributions under 14%, and thermal stability factor of 88. The IrMn layer may also help to stabilize the recording layer grains against thermal instability effects.

Characterization of Coupled Novel Magnetic Multilayers With Anomalous Hall Effect

We investigated the switching behavior of magnetic thin films with coupled in-plane and perpendicular anisotropy. The in-plane layer is made up of 20 A¿ of Co while the perpendicular layer consists of multiple Co/Pd bilayers. The coupling strength is moderated by a Cu intermediate layer. The method of Anomalous Hall effect was chosen because of its ability to distinguish the in-plane and normal-to-plane magnetizations in a single measurement. We discovered that without the Cu decoupling layer, the magnetization in the Co layer was induced to point out of the plane by the Co/Pd multilayer only to point back into the plane by an external in-plane field. As the coupling is reduced by increasing the Cu layer thickness, the magnetization in the Co layer behaves more independently with respect to the Co/Pd layer.

Anomalous Hall effect measurement of novel magnetic multilayers

We have used the method of anomalous Hall effect (AHE) to characterize magnetic thin film structures consisting of both perpendicular (normal-to-plane) and longitudinal (in-plane) anisotropies. AHE enables the simultaneous extraction of the perpendicular and in-plane magnetizations. The method is used to obtain the hysteresis loops and study the switching behavior of [Co/Pd] multilayers coupled to different seed layers. Results show that Cu seed layer helps to reduce the switching field distribution (SFD) and increase the coercivity while Co underlayer with an in-plane anisotropy helps to reduce the coercivity and increase the SFD. Furthermore, the magnetization in the Co in-plane layer is found to be pointing out of plane possibly due to exchange coupling with the [Co/Pd] multilayers.

Material and Layer Design to Overcome Writing Challenges in Bit-Patterned Media

In this paper, the problem of writability in bit-patterned media (BPM) for high areal density will be discussed. A new film structure is proposed, made of a composite in-plane and perpendicular anisotropy layers to improve writability and reduce the time of magnetization switching in BPM. To demonstrate the efficiency of an in-plane anisotropy layer in assisting the switching of the magnetization of the high perpendicular anisotropy recording layer, we use micromagnetic simulation to study magnetization reversals in BPM for 5 Tb/in2. Experiments have been carried out on patterned arrays of 60-nm-size dots made of [Co(0.3 nm)/Pd(0.8 nm)]x15 multilayer and Co(2 nm)/[Co(0.3 nm)/Pd(0.8 nm)]x15 composite structure. The mean switching field calculated from remanence magnetization curves shows a reduction of more than 50% from its initial value by adding a 2-nm-thick Co bottom layer with in-plane anisotropy. No difference in switching field distribution was observed in the two structures studied, indicating the merit of assisting the switching of high anisotropy patterned media by exchange coupling to an in-plane anisotropy layer.

Influence of synthetic nucleation layers on the microstructure, magnetic properties, and recording performance of CoCrPt–SiO2 perpendicular recording media

The performance of CoCrPt–SiO2 perpendicular recording media, which incorporate synthetic nucleation layers for inducing grain size control, was evaluated with regard to the crystallographic, microstructural, magnetic, interfacial, and recording properties. The media structure consisted of a 14 nm thick CoCrPt–SiO2 recording layer sputtered on two stacked Ru intermediate layers. When a submonolayer thick synthetic nucleation layer was inserted between the two Ru intermediate layers, the grain size and grain size dispersion were significantly reduced in both the upper Ru layer and the recording layer. An average grain size of 5.8 nm with 20% dispersion was achieved for the recording layer. The crystallographic growth of the Co (00·2) perpendicular texture remained unaffected with the Δθ50 at ∼4.4°; the coercivity was ∼2800 Oe and the coercivity squareness was ∼0.6. The roughness of the interface between the synthetic nucleation layer and the upper Ru intermediate layer did not significantly change, and the...

CoCrPt-oxide based perpendicular recording media with hybrid soft magnetic underlayers

Writability of magnetic recording media with high anisotropy constant (Ku) is one of the major challenges encountered in extremely high density recording. A possible avenue to improve writability in perpendicular magnetic recording is to reduce the spacing between the writing head and the soft magnetic underlayer (SUL) or head-to-keeper spacing (HKS). A hybrid layer structure for reducing the HKS between the SUL and the writing head is investigated. The proposed structure makes use of a crystalline SUL with in-plane anisotropy together with an amorphous SUL to form a hybrid SUL (H-SUL). The H-SUL design helps to achieve better surface smoothness, thinner HKS, and good c-axis dispersion for the recording layer while still maintaining small and exchange-decoupled grains. Micromagnetic simulation demonstrates that the H-SUL scheme helps in improving writing field effectiveness and increasing field gradient along down-track and cross-track directions.

Enhanced heteroepitaxial growth of CoCrPt–SiO2 perpendicular magnetic recording media on optimized Ru intermediate layers

The crystallographic growth, interfacial roughness, and magnetic properties of CoCrPt–SiO2 perpendicular magnetic recording media prepared on various types of Ru intermediate growth layers were systematically investigated based on high angle and omega offset x-ray diffraction scans, rocking curve scans, synchrotron radiation based grazing incidence reflectivity scans, and magneto-optical Kerr hysteresis loops. For samples that make use of one Ru growth layer, voltage bias applied on the Ru layer was seen to have two observable effects: (1) the dispersion in the Ru(00⋅2) perpendicular texture increased, but that of the Co(00⋅2) remained unchanged, leading to identical layered growth and (2) the in-plane a-lattice parameter of the Ru decreased leading to enhanced heteroepitaxy with the Co. There was no significant change in the Ru–Co interfacial roughness with changing the bias on the Ru layer. The bias effect can be used to optimize the design of the Ru intermediate layers. A scheme that makes use of two R...