Magnetic Recording Measurements

Magnetic Recording Measurements

Magnetic Recording Measurements Magnetic recording devices use materials that exhibit spontaneous magnetization in their operating temperature range: ...

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Magnetic Recording Measurements Magnetic recording devices use materials that exhibit spontaneous magnetization in their operating temperature range: ferromagnets and ferrimagnets (see Magnetic Recording Technologies: OŠerŠiew). The magnetic properties of these materials can be classed into two categories: properties that are determined by composition and crystal structure, denoted intrinsic; and properties that depend on microstructural factors such as grain size and shape in a polycrystal, or particle shape and orientation in a composite, denoted extrinsic. Both intrinsic and extrinsic magnetic properties are functions of environmental factors such as electric and magnetic fields, temperature, pressure, and atmosphere. The measurements described below depend on the scale on which they view the magnetic property to be measured. The microscopic scale focuses on a magnetic material as an arrangement of interacting atoms and interprets magnetic properties in terms of spin and orbital moments, energy levels, binding energies, spin–orbit, exchange, and superexchange interactions, etc. The mesoscopic scale averages these microscopic interactions over small homogeneous regions (grains, particles) comprising many atoms. Averages over many such (interacting) regions constitute the macroscopic properties of the magnetic material. An important consideration in measuring the magnetic properties of a material is its uniformity. Inhomogeneities may occur on the microscopic scale (point defects such as vacancies and interstitials, line and planar defects such as dislocations, or composition fluctuations in disordered alloys), on the mesoscopic scale (grain or particle clusters, voids, and segregations), and on the macroscopic scale, given the limits on quality control in fabrication.

These measurements are also affected by the rate at which the magnetic field is changing (viscosity and eddy current effects) and how long it takes to make the measurements. Indeed, the magnetization of all materials will eventually seek the lowest energy state, if we wait long enough at any nonzero temperature. In the absence of an applied field this is always a state of demagnetization with zero net magnetization when summed over the whole sample volume, because such a demagnetized state minimizes the magnetostatic or dipolar self-energy. We conclude that the sample’s coercivity, defined as the field that must be applied to a magnetized sample to reduce its magnetization to zero, is zero for infinite-time measurements! At the other end of the time scale we might try to coerce the spins in a magnetic material to change directions with a short field pulse. We are concerned here with the magnetic properties of electron spins but, as the word implies, spins are associated with angular momentum, and exhibit the (quantum mechanical) inertial effects of angular momentum. Moreover, spins are usually coupled to each other and to their surroundings, and thus spin dynamics exhibits spring-like inertial and damping effects. Put another way, spins do not change instantaneously but take time to alter their direction. In general, if the magnetization is to be changed a given amount (say from a magnetized to a demagnetized state) in a shorter time, this will require a larger externally applied field. Measured coercivities are larger for shorter applied field times. Measurements then need to be described by the applied field rates of change and are generally grouped in three regimes: slowly varying fields, quickly varying fields, and pulsed fields. The fields may be localized, uniform over large areas, or changing in space (field gradients). The measurements can interrogate a small region of a whole or an entire large sample. (See also Magnetic Measurements: Quasistatic and ac.)

1.1 Vibrating Sample Magnetometry 1. General Measurements In most instances, magnetic measurements described below are extrinsic measurements made by measuring a sample’s response to an externally applied magnetic field. The measurements can depend on the sample’s shape: the inhomogeneities and discontinuities of the magnetization in the sample’s interior and on its surface are sources of a magnetostatic demagnetizing field that depends on the sample’s shape and that affects the internal field. As the internal magnetization changes in response to applied fields, temperature, or time, the internal field will also change due to the changing demagnetizing fields. These fields can also be microstructure dependent because typical polycrystalline or particulate materials exhibit effects due to grain boundaries and particulate or grain orientation.

Developed in the late 1950s by S. Foner, the vibrating sample magnetometer (VSM) continues in widespread use for measuring magnetic properties of materials including thin films and particulate media. The basic technique involves a large-scale applied field, using an electromagnet to immerse an entire small sample in a nearly uniform field (Fig. 1). The field is swept slowly (usually less than mT s−") and the sample is moved (vibrated) in the applied field. As described previously, the magnetic sources associated with a sample’s geometry and magnetization cause internal magnetostatic fields and of course external fringing fields. Inductive pick-up coils are placed in close proximity to the sample, inside the applied field, and sensed synchronously with the motion of the sample. The assumption is made that the external fields are proportional to the net internal magnetization (modified 1