M. L. Plumer

The Physics of Ultra-High-Density Magnetic Recording

1 Introduction to Micromagnetic Recording Physics.- 2 Microstructure of Longitudinal Media.- 3 Magnetization Dynamics and Thermal Fluctuations in Fine Grains and Films.- 4 Measurement of Dynamic Properties in Thin-Films.- 5 Thermal Effects in High-Density Recording Media.- 6 Dynamic Effects in High-Density Recording Media.- 7 Patterned Media.- 8 Perpendicular Recording Media.- 9 Self-Assembled Magnetic Nanoparticle Arrays.- 10 Theory of Magnetotransport for Magnetic Recording.- 11 Recording Head Design.

Measurements and modeling of soft underlayer materials for perpendicular magnetic recording

Measurements and modeling of soft magnetic underlayer (SUL) materials for perpendicular magnetic recording application are carried out. The process dependent magnetic properties of FeCoB, CoZrNb, and FeAlN SUL materials on glass and aluminum disk substrates are studied and correlated with spin-stand noise performance. The SUL-induced dc noise amplitude approaches the electronic noise floor for certain material combinations, e.g., FeCoB or CoZrNb on glass, when care is taken to relieve stress-induced perpendicular anisotropy by thermal annealing. Landau-Lifshitz-Gilbert micromagnetics, finite-element method calculations, and a micromagnetic recording model show that write field amplitude, write field gradient, and readback waveform are only slightly impacted by SUL moment in the 1-2 T range. Much more important are the head-to-SUL distance and the write head saturation moment. These results suggest that extremely high SUL moment may not be necessary, which can be leveraged to meet other key practical requirements such as corrosion resistance and manufacturability.

Wavevector and spin reorientation in MnSi

A mean-field theory of spin-density-wave wavevector and polarisation vector rotation induced by the application of a magnetic field to MnSi in its spin-density-wave phase, and a prediction of the associated spin reorientation phase transitions, is presented. The dependences of the Neel temperature and the homogeneous magnetisation on applied magnetic field strength and orientation are studied, as is the staggered susceptibility which diverges at the spin orientation phase transition.

Magnetic phase diagram of CuO via high-resolution ultrasonic velocity measurements.

High-resolution ultrasonic velocity measurements have been used to determine the temperature-magnetic-field phase diagram of the monoclinic multiferroic CuO. A new transition at T(N3)=230 K, corresponding to an intermediate state between the antiferromagnetic noncollinear spiral phase observed below T(N2)=229.3 K and the paramagnetic phase, is revealed. Anomalies associated with a first order transition to the commensurate collinear phase are also observed at T(N1)=213 K. For fields with B || b, a spin-flop transition is detected between 11 T-13 T at lower temperatures. Moreover, our analysis using a Landau-type free energy clearly reveals the necessity for an incommensurate collinear phase between the spiral and the paramagnetic phase. This model is also relevant to the phase diagrams of other monoclinic multiferroic systems.

High-frequency magnetic-force microscopy characterization of magnetic recording writer poles

Measurements of longitudinal writer poles at the air bearing surface were performed using high-resolution magnetic-force microscopy (MFM) with low coercivity tips. Two-dimensional MFM maps were obtained for various write currents. The modeling results indicate that the MFM maps are related more to the field than to its second derivative. Two techniques were used, dc MFM and high-frequency (HF) MFM. The results show that the HF-MFM technique can distinguish between different writer designs. The writers with the best high-frequency performance showed gradual decrease of the maximum MFM signal with frequency up to 1.5GHz.

Perpendicular media: alloy versus multilayer

Properties and performance for alloy and multilayer perpendicular recording media designs utilizing a soft magnetic underlayer are compared. Among samples considered here, grain size and grain size dispersion are more highly refined for alloy media deposited at high substrate temperature, and are beginning to approach those now available in longitudinal recording. Multilayer media made at ambient temperature typically sacrifice film density and surface smoothness for interface quality. Although microstructural development and the manufacturing process for multilayer media are less mature versus alloy, multilayer media remain attractive due to their high anisotropy potential and the ease with which H/sub n/ and H/sub c/ can be controlled. For thermally stable alloy media made on a pilot production sputtering machine, a spin-stand areal density of 61 Gb/in/sup 2/ has been demonstrated at 350 Mb/s data rate with an on-track bit-error-rate reference level of 1e-6. Using the same media, a working perpendicular drive has been demonstrated at 32 Gb/in/sup 2/ and 500-800 Mb/s data rate.

TRICRITICAL BEHAVIOR OF THE FRUSTRATED XY ANTIFERROMAGNET

Extensive histogram Monte-Carlo simulations of the XY antiferromagnet on a stacked triangular lattice reveal exponent estimates which strongly favor a scenario of mean-field tricritical behavior for the spin-order transition. The corresponding chiral-order transition occurs at the same temperature but appears to be decoupled from the spin-order. These results are relevant to a wide class of frustrated systems with planar-type order and serve to resolve a long-standing controversy regarding their criticality.

Micromagnetic simulations of interacting dipoles on an fcc lattice: application to nanoparticle assemblies.

Micromagnetic simulations are used to examine the effects of cubic and axial anisotropy, magnetostatic interactions and temperature on M-H loops for a collection of magnetic dipoles on fcc and sc lattices. We employ a simple model of interacting dipoles that represent single-domain particles in an attempt to explain recent experimental data on ordered arrays of magnetoferritin nanoparticles that demonstrate the crucial role of interactions between particles in an fcc lattice. Significant agreement between the simulation and experimental results is achieved, and the impact of intra-particle degrees of freedom and surface effects on thermal fluctuations is investigated.

An Ising-like model of stacking-sequence polytypism in ABX3 compounds

The crystallographic structure of a large class of ABX3 compounds can be characterised by the stacking sequences of face- and corner-sharing BX6-octahedra chains on a hexagonal lattice. Many different polytypes described by this type of stacking have been observed in these and related materials. An axial Ising-type model free energy that includes terms linear and cubic in the pseudo-spin variable is proposed to describe the possible phases which result from interactions between octahedra. The model is also shown to account for the increase in corner-sharing sequences with increasing pressure observed in, for example, CsBF3B=Mn, Fe, Co, Ni.

Angular dependence of giant magnetoresistance properties of exchange biased spin valves

Giant magnetoresistance (GMR) responses were studied as a function of applied measurement field angle in FeMn and NiMn pinned spin valves. As the measurement field varies away from the pinning field direction, the peak GMR ratio and the free layer coercivity decrease. The GMR curve changes from unsymmetrical to symmetrical with respect to the applied field polarity. The GMR responses from the component parallel to the applied field are proportional to the cosine of the angle between the pinning and the applied field. The results can be described well by a simple vector model, and an empirical method to measure the pinning field rotation is established.