The principles of several of the most popular macroscopic magnetic measurement techniques are discussed for the vibrating sample magnetometer, SQUID magnetometer, ac susceptometer, ac force magnetometer, and torque magnetometers. These types of instruments are available commercially as stand-alone computer-controlled systems, Selected examples of recent measurements on thin films, small particles, and clusters of magnetic ions are presented.

Ferromagnetic Heusler alloys such as NiMnSb may have unique applications to magnetoresistive devices because of their predicted half-metallic band-structure (i.e., 100% spin-polarized at the Fermi level). However, the high temperatures (>500°C) used to date for the synthesis of the ordered alloy in bulk or thin-film form is incompatible with the reliable fabrication of nanoscale multilayer structures from this material. We report on the growth of high-quality polycrystalline thin films, 200–1000Å thick, of Clb-structured NiMnSb by RF-magnetron sputtering of a composite NiMnSb target onto glass and silicon substrates at temperatures as low as 250°C. We have established that substrate temperature, deposition rate and argon gas pressures all play a critical role in obtaining the Clb-structured phase by direct deposition. Optimal conditions result in films whose properties, including lattice parameter, saturation magnetization, resistivity and magneto-optical Kerr rotation spectrum are identical to those of bulk NiMnSb. Additionally, coercive fields as low as a few oersteds make these films compatible with low-field applications.

We have grown superlattices based on the Clb Heusler alloys PtMnSb, CuMnSb and NiMnSb between 200–500°C on A12O3 (0001). X-ray diffraction (XRD) indicates (111) oriented ordered structures for growth at 300°C. Higher deposition temperature leads to interdiffusion, loss of the multilayer structure and appearance of extra phases. Growth at 200°C slightly reduces the intermixing but also reduces the quality of the crystal structure. For PtMnSb/CuMnSb, we found an enhancement of the saturation magnetization compared to equivalent PtMnSb single layer films and a CuMnSb spacer thickness dependence of the squareness of the M(H) 100ps suggestive of interlayer coupling. Short periodicity NiMnSb/PtMnSb superlattices show an in-plane magnetic easy axis, but correction for shape anisotropy indicates a tendency for perpendicular anisotropy.

Annealing of CoAV(110) ultrathin films causes rather strong changes of film morphology and magnetic properties, which has been studied by scanning tunneling microscopy (STM) and alternating-current magneto-optical Kerr effect (ac-MOKE). Annealing above 500 K of room-temperature grown 2 monolayers (ML) thick Co/W(110) films leads to the formation of islands and a decrease of the Curie temperature 7C. At 4 ML, on the other hand, a network of interconnected islands is formed upon annealing, resulting in an enhanced TC. While the increase of TC at 4 ML is explained by an increase of the local film thickness, the anomalous TC behavior at 2 ML is assigned to the occurrence of superparamagnetism.

Granular thin films consisting of cobalt nanocrystals encapsulated in graphite-like carbon were fabricated by co-deposition of cobalt and carbon with subsequent annealing. The gram size and the crystal structure of the Co-C films depended on the substrate temperature, the carbon concentration, and the annealing temperature. The film deposited with 36 at.% carbon at 200°C consisted of crystalline carbide and hep cobalt, which transformed into hep cobalt and graphite-like carbon by annealing at ≥300 °C. The as-deposited film with a carbon of 46 at.% had an amorphous-like phase and grain sizes of ≤10 nm. By annealing at ≥300°, the amorphous-like phase transformed into cobalt grains with a random stacking structure encapsulated in graphite-like carbon, and the initial size of the grains was unchanged. The saturation magnetization and the in-plane coercivity of these films were also reported.

Layer-by-layer molecular self-assembly of nanosized magnetite (Fe3O4) particles and polyimide precursor molecules into multilayer ultrathin films has been accomplished on single crystal silicon and quartz substrates. This process involves the alternate dipping of a substrate into an aqueous solution of cationic polymer which coats on nanoscale Fe3O4 particles as a stabilizer followed by dipping into an aqueous solution of anionie polyimide precursor (polyamic acid salt, PAATEA). The growth process and the structure have been characterized using UV-vis spectroscopy, FT-IR, and scanning electron microscopy. The results demonstrate that well-ordered uniform monolayer and multilayer magnetic films have been formed on silicon and silica surfaces.

The synthesis of ferromagnetic particles of the size of several nm (10… 100nm) imbedded in semiconductors is of particular interest for device applications such as magnetic sensors, switches or memories based on integrated semiconductor devices. We have investigated the formation as well as the structural and magnetic properties of MnAs nanomagnets (size ≈ 20nm) in low temperature grown GaAs (LT-GaAs). The MnAs nanomagnets which show ferromagnetism at room temperature have been synthesized using ion implantation with Mn and subsequent rapid thermal annealing (RTA). To characterize these nanomagnets, we present SQUID measurements (superconducting quantum interference device), TEM analysis (transmission electron microscopy) and EDX analysis (electron beam induced x-ray fluorescence).

High dose iron implantation into silicon substrates has been performed with a metal vapor vacuum arc ion source to doses ranging from 5×1016 to 2×1017 cm'2 at various beam current densities. The magnetoresistance (MR) effects in these implanted granular layers were studied at temperatures from 15K to 300K. A positive MR effect, i.e., an increase in the resistance at the presence of a magnetic field, was observed at temperatures lower than about 40K in samples prepared under appropriate implantation conditions. The magnitude of the MR effect, defined as ΔR/Ro ≡ (R(H)-Ro)/Ro where R(H) and Ro denote respectively the resistance value at a magnetic field intensity H and that at zero field, was found to depend not only on the implantation dose but also on the beam current density. This is attributed to the beam heating effect during implantation which affects the formation of the microstructures. The ratio δR/Ro was found to attain high values larger than 400% for some samples at low temperatures. The dependence of the MR effects on temperature, implantation dose, and beam current density will be presented and discussed in conjunction with results of transmission electron microscopy.

Magnetic position sensors are transducers whose output is an electrical signal that is a function of a mechanical motion. They consist of a permanent magnet, a magnetic field sensor, and a moving magnetic circuit. Being contactless, they do not wear out. Unlike optical systems, they are impervious to contamination. With the proliferation of microprocessor control in many on-board automotive applications, magnetic position sensing becomes a crucial function, found e.g. in cam and crankshaft sensors used for ignition timing and engine misfire detection, in brushless electrical motors, in wheel speed sensors and in other applications.

This paper first reviews the requirements put on the magnetic field sensors in these applications. Five main types of field sensors are used: Si Hall sensors with integrated amplifiers, GaAs Hall sensors, InSb-based magnetoresistors, anisotropie magnetoresistors (AMR), and metal-multilayer magnetoresistors (GMR). Magnetoresistors are almost always used as differential pairs, either in Wheatstone bridges or with matched constant current sources. The two characteristics of each type of field sensors that matter most are their sensitivity to magnetic field, and the smallest amount of field modulation they can detect, i.e. their resolution, which is mainly limited by the drift of the outputs with temperature. These parameters are measured and reported here on the 5 types of sensors; the work is an update of a 1993 review by the author.

We observe spin-dependent transmission of hot electrons through the ferromagnetic metallic layer of a Schottky diode. A 25% spin-polarized electron beam is produced in vacuum by a GaAs photocathode under illumination with circularly polarized light. The photoemitted electrons are then injected from the vacuum into the sample: a 3.5 nm-thick Fe layer deposited on a n-doped GaAs substrate. We measure a transmission coefficient of the order of a few 10-4. Using an incident energy above the Fermi level equal to 5 eV, a 5% asymmetry in the transmitted current is observed by changing the spin-polarization of the incident electrons (i.e. the light polarization) and/or reversing the Fe-layer magnetization. This corresponds to a transmission spin-dependence of 20% for a 100% spin-polarized incident beam.

In granular NiFe-SiO2 films which display giant Hall effect, extraordinary Hall resistivity ρxys, normal Hall resistivity ρxyo, and magnetoresistivity δρ, were all found to follow power lav dependencies on resistivity ρ. Namely, -ρxys ∼ ρ0.81, -ρxyo ∼ ρ0.64 and -δρ ∼ ρ1.15. We propose that the presence of nanometer sized particles in the percolating conduction channels gives rise to I giant enhancement for both ordinary and extraordinary Hall effect in magnetic metal-insulator nanocomposite films. This physical picture describes qualitatively well the general features of GHE, and its correlation with resistivity and magnetoresistivity.

Applications of Zalar rotation and field emission Auger (FE-AES) analysis in various magnetic hard disc problems will be presented. These applications include thin layer structure characterization, low concentration contaminant (0.1–0.5 atomic percent) detection at deeper interfaces (greater than 1000Å), and small particle (-∼1000Å) contaminant identification.

The magnetic interaction of magneto-optical multilayer films have been analyzed using the stress mechanism of coercivity and the single ion model of magnetic anisotropy. The relation between the magnetic properties of the multilayer films and the dielectric film thickness has been explained.

We introduce an interface selective structure, composed of a spin-valve on top of which a thick nonmagnetic back layer is deposited as a straightforward experimental tool to measure the GMR induced by ultrathin magnetic layers. The interface selectivity of spin-dependent scattering is evidenced by calculations and illustrated in both Co/Cu/Co and Ni80Fe20/Cu/Ni80Fe20 spin-valves by an almost discontinuous behavior in the GMR ratio. The temperture dependence of the extremely short length scale associated with this discontinuity is discussed in relation to the structure of ultrathin ferromagnetic layers.

The development of crystallographic texture in thin film longitudinal recording media is discussed. Polycrystalline thin films may obtain their crystallographic texture by means of a nucleation process such as epitaxial nucleation on a polycrystalline underlayer or by means of a process involving a preferred growth direction. In this paper we will discuss various epitaxial nucleation textures that are obtained in media produced for magnetic recording. We will discuss the way that the underlayer controls the crystallographic texture of the magnetic layer, as well as methods used to control the texture of the underlayer itself. We give a brief overview of some of our recent findings in the growth of NiAl and FeAl films used for underlayers. Finally we will briefly discuss what we have called the tilted electron beam technique. In this technique selected area electron diffraction patterns are obtained at different angles of tilt and the development of arcs in the patterns is analyzed so as to determine the type and amount of crystallographic texture which is present in the films.

The high magnetic anisotropy and high coercivity of equiatomic CoPt thin films make them attractive as potential materials for magnetic recording applications. Magnetic coercivity (Hc) over 10 kOe has been measured in films in which the as-deposited fee phase has been partially transformed to the atomically ordered Ll0 phase. Very high Hc has been related to high volume fraction and small size of the Ll0 precipitates. A better understanding of the Ll0 phase formation and quantification of volume fraction is critical to optimizing the magnetic properties of this material.

As we have previously reported, an increase in Hc was observed with an increase in Ll0 volume fraction in 10 nm thick, equiatomic CoPt films. In our current investigation we have observed that, at anneal temperatures far from the order/disorder transition temperature, e.g. at T = 0.6 Tc, numerous, very fine Ll0 precipitates are seen, some of which cluster at the parent phase grain boundaries. At T = 0.85 Tc, very few, larger Ll0 regions were seen. As precipitates of the ordered phase grow to impingement, antiphase boundaries (APB) are formed. This is consistent with a heterogeneous nucleation and growth mechanism for the formation of the Ll0 phase throughout the temperature range studies.

The structure of 2.8 – 60nm thick films of CoCrTa sputtered onto a 50nm Cr underlayer at 250°C was studied using atomic resolution microscopy. The Cr grows with a (200) orientation and the CoCrTa grows epitaxially with the [0002] axis in the film plane. The thinnest films have a discontinuous microstructure with several CoCrTa nuclei forming on each Cr grain, with slight misalignment from the exact Pitsch-Schrader orientation relation. The CoCrTa grains coalesce as the film thickness exceeds lOnm and there is evidence for stacking faults at all film thicknesses. For most films the magnetic switching volumes are of similar size to the physical grain volume, suggesting that the entire grain volume participates in magnetic switching.

Diffraction contrast and high resolution imaging, which are generally used to visualize multilayers, fail when they are applied to study metallic multilayers (MLs) composed of elements with close atomic number. In such a case (i.e. Co/Cu multilayers) neither the absorption nor the structure can give enough contrast between elements to observe the details of the multilayer stacking. This is illustrated in Figure 1.a which shows a bright field image of (NiFe/Cu/Co/Cu)x8 multilayers deposited on Si. Thus, although the existence of a multilayer structure is clearly demonstrated by both small angle X-ray diffraction and electron diffraction ‘, this does not appear clearly in the images in diffraction contrast. In such systems, if the crystals in the layers are not in strong diffracting condition, absorption is the predominant cause of contrast, and since the constituent elements have extremely close Z, the absorption will be identical for all the layers leading to little or not contrast. However, the contrast can be enhanced by defocusing the objective lens of the microscope leading to the appearance of Fresnel fringes at the interface between the layers (Figure l.b). In this study we investigate the field of application of this contrast mode to extract both qualitative and quantitative information in metallic multilayers. We show that the fine details of the Fresnel fringes can be described quantitatively in order to assess the thickness and waviness of the layers, and the interface sharpness.

Thin multilayer films are of growing importance as magnetic sensors. One key parameter in their performance is the interlayer magnetic coupling field which can vary in both sign and strength. Experiments performed to explore the effects of sputter deposition methods on the coupling fields observed for Co/Cu multilayers and spin valves on single crystal substrates are described. Scanning Tunneling Microscopy is used to qualitatively understand the large differences in observed coupling fields.

We have studied the magnetic properties of the Co/Cu(110) system at 300K, using the magneto-optic Kerr effect. After a given deposition of Co in the thickness range 5ML<dCo<40ML, the M-H loop is observed to evolve continuously in time until the magnetic easy axis has switched 90° from the [001] to [1–10] direction. We attribute this behaviour to the reversal in sign of the effective in-plane uniaxial anisotropy constant, due to the adsorption of submonolayer quantities of a residual gas from the UHV environment. STM images reveal the growth of elongated Co island structures preferentially oriented along the [001] direction providing ‘step like’ edge sites for gas adsorption. We find the easy axis can be switched back to the [001] direction by depositing a submonolayer Cu overlayer, and that for relatively thick Co films (>15ML) the Cu overlayer can cause the magnetisation to take up intermediate angles. We analyse this behaviour using a simple model of the effective anisotropy energies.