M. Vopsaroiu

A new magnetic recording read head technology based on the magneto-electric effect

The existing magnetic recording read head technologies use one of the well-known magneto-resistance effects (i.e. anisotropic magneto-resistance, giant magneto-resistance or tunnelling magneto-resistance (TMR)) to read back the data from the magnetic recording medium. These are usually sophisticated devices that require a dc test current flowing through the sensor stack in order to measure its change in resistance (i.e. amplitude response signal) as a function of the fringing magnetic flux of the recorded bits, when the reader moves along the recorded track. In this paper, we propose the design of a new kind of highly sensitive read sensor for magnetic recording heads, which directly produces a voltage response without the need for a test current. This new design is based on the magneto-electric effect in laminated multiferroic materials. Such a magnetic read head is much simplified in terms of sensor construction (i.e. number of layers involved and horizontal biasing requirements) and has a range of potential advantages including similar sensitivity to that of the TMR heads, reduced power consumption, better thermal performances, excellent high frequency operation and reduced cost of production.

Deposition of polycrystalline thin films with controlled grain size

Difficulties in controlling the grain size and size distribution in polycrystalline thin films are a major obstacle in achieving efficient performance of thin film devices. In this paper we describe a sputtering technology that allows the control of the grain size and size distribution in sputtered films without the use of seed layers, substrate heating or additives. This is demonstrated for three different materials (Cr, NiFe and FeMn) via transmission electron microscopy imaging and grain size analysis performed using the cumulative percentage method. The mean grain size was controlled only via the sputtering rate. We show that higher sputtering rates promote the growth of larger grains. Similar trends were obtained in the standard deviation, which showed a clear reduction with the sputtering rate.

Preparation of high moment CoFe films with controlled grain size and coercivity

In this paper a preparation method for high moment CoFe thin films with soft magnetic properties is reported. A full control of coercivity in a series of 20-nm-thick CoFe films has been achieved without using seed layers, additives, or thermal annealing. The films were sputtered directly onto Si substrates and the coercivity was varied by changing the mean grain size in the sputtered films. The mean grain size was in turn controlled via the sputtering rate. A reduction in the coercivity has been observed from 120Oe for samples with a mean grain size larger than 17nm down to 12Oe for a sample with a mean grain size of 7.2nm. The results are in good agreement with the “random anisotropy model” relating the coercivity to the mean grain size in polycrystalline ferromagnetic films.

Probing the local strain-mediated magnetoelectric coupling in multiferroic nanocomposites by magnetic field-assisted piezoresponse force microscopy

The magnetoelectric effect that occurs in multiferroic materials is fully described by the magnetoelectric coupling coefficient induced either electrically or magnetically. This is rather well understood in bulk multiferroics, but it is not known whether the magnetoelectric coupling properties are retained at nanometre length scales in nanostructured multiferroics. The main challenges are related to measurement difficulties of the coupling at nanoscale, as well as the fabrication of suitable nano-multiferroic samples. Addressing these issues is an important prerequisite for the implementation of multiferroics in future nanoscale devices and sensors. In this paper we report on the local measurement of the magnetoelectric coefficient in bilayered ceramic nanocomposites from the variation in the longitudinal piezoelectric coefficient of the electrostrictive layer in the presence of a magnetic field. The experimental data were analyzed using a theoretical relationship linking the piezoelectric coefficient to the magneto-electric coupling coefficient. Our results confirm the presence of a measurable magnetoelectric coupling in bilayered nanocomposites constructed by a perovskite as the electrostrictive phase and two different ferrites (cubic spinel and hexagonal) as the magnetic phases. The reported experimental values as well as our theoretical approach are both in good agreement with previously published data for bulk and nanostructure magnetoelectric multiferroics.

Novel sputtering technology for grain-size control

In this paper, we present a description of a novel high-rate plasma sputtering system that allows the control of grain size in sputtered films. Additionally, the system has the advantage of a better utilization of the target material (around 80% to 90%) by eliminating the race track at the target as in conventional plasma magnetron sputtering systems. The potential and capabilities of this novel plasma sputtering device are demonstrated in this paper by the deposition of a number of different Cr thin films suitable for underlayers in thin-film media and for which we have performed a systematic X-ray and TEM analysis to determine the grain-size histograms, mean grain diameters, and their relationship to the sputtering processes.

Multiferroic magnetic recording read head technology for 1 Tbit/in.2 and beyond

Multiferroic (MF) materials are very promising candidates for new technologies and applications because they exhibit simultaneously multiple cooperative phenomena (i.e., magnetic, electric, and piezoeffects). The main feature of MF materials is the magnetoelectric (ME) effect, which can be used to engineer highly sensitive magnetic/electric sensors. In this paper, we discuss the requirements of a new kind of magnetic recording read head for 1Tbit∕in.2 recording densities, which is based on a MF structure. The MF sensor operates at room temperature via the strain mediated ME effect by producing a voltage signal in response to the magnetic field excitation from the recorded bits. We calculated the theoretical output from such a recording read head assuming a magnetic recording density of 1Tbit∕in.2. Our calculations demonstrate that the proposed read head technology could replace in the future the conventional magnetoresistive read heads bringing also a number of considerable advantages, as detailed in the ...

Experimental determination of the magnetoelectric coupling coefficient via piezoelectric measurements

Multiferroic materials exhibit a coupling between their intrinsic electric and magnetic fields known as magnetoelectric (ME) coupling. The ME coupling can be stress-mediated and is induced either magnetically or electrically. The linear ME coupling coefficient, α, gives a comprehensive description of this effect. However, the experimental determination of α is rather complex. In this paper, we report a simple and convenient experiment for the measurement of the magnetically induced ME coupling coefficient. Our experiment is based on the quasi-static piezoelectric coefficient measurement under applied magnetic field and can provide simultaneously the d33 piezo-coefficient of a multiferroic sample and the magnetically induced α coefficient as a function of both applied ac and dc magnetic fields.

Verified finite element simulation of multiferroic structures: Solutions for conducting and insulating systems

Composite multiferroics are an exciting class of engineered materials with a wide range of existing and potential applications. This paper describes an original finite element (FE) simulation for composite multiferroic devices. Detailed analysis is given on the flow of electric/magnetic fields between the composite regions with attention given to the conductivity of the magnetostrictive layers (piezoelectric assumed to be insulating). The simulation is verified against an existing FE code and against a theoretical analysis, also presented here. The issue of boundary and continuity conditions is discussed in detail and approximations made in both FE and analytical treatments are specified. We derive the I-V curves of the composite device when acting as a magnetic field detector, in both conducting and insulating cases. The magnetic field detector acts as a voltage source: we calculate its open-circuit voltage and internal impedance in each case.

Polarization dynamics and non-equilibrium switching processes in ferroelectrics

Time- and temperature-dependent effects are critical for the operation of non-volatile memories based on ferroelectrics. In this paper, we assume a domain nucleation process of the polarization reversal and we discuss the polarization dynamics in the framework of a non-equilibrium statistical model. This approach yields analytical expressions which can be used to explain a wide range of time- and temperature-dependent effects in ferroelectrics. Domain wall velocity derived in this work is consistent with a domain wall creep behavior in ferroelectrics. In the limiting case of para-electric equilibrium, the model yields the well-known Curie law. We also present experimental P-E loops data obtained for soft ferroelectrics at various temperatures. The experimental coercive fields at various temperatures are well predicted by the coercive field formula derived in our theory.

A new experimental design for noncontact giant magnetoresistance measurements using the magnetorefractive effect

Electrical magnetoresistance measurements of spin dependent materials or devices are very difficult without surface damaging or sample contamination, especially for thin-film giant magnetoresistance (GMR) multilayers. Moreover, the in situ determination of the GMR profile is almost impossible using electrical measurements. We propose a novel experimental design that allows the convenient measurement of magnetoresistance profiles using a noncontact method based on the magnetorefractive effect. This technique is applicable to metallic samples or devices showing magnetotransport properties and is also suitable for in situ measurements. The experiment involves infrared (IR) reflectivity measurements as a function of the applied magnetic field. By introducing IR optical fibers, the experimental setup has been substantially simplified while the need for costly optical components and time consuming alignments has been eliminated. Theoretical simulations of this experiment are also presented prior to the introduc...