Introduction on Magnetic Sensing and Spin Electronics
DRF/IRAMIS/SPEC/LNO, CEA CNRS Paris Saclay, 91191 Gif sur Yvette Cedex, France
This introductory chapter provides the basic knowledge of magnetism and spin electronics, which will help the reader to understand the contents of the book. Then, after a brief introduction to magnetic fields, some bases of magnetic sensing and spin electronics are proposed. The last part of the chapter provides definitions that are useful for understanding spin electronics applications. More in-depth information can be found [1,2]. A number of books have been published on nanomagnetism , spin electronics [4,5], GMR , and spin dynamics , where each particular topic is discussed in detail.
1.1 Magnetic Fields
Magnetism and magnetic field are known since thousands of years. First magnetic sensors were compass made of magnetite stones in China during the Han dynasty rule and later used by sailors to navigate. Today, magnetic objects, such as fridge magnets, are used as ornaments or for health purpose. In parallel, electricity is associated with electrons flowing in conductors and its use in domestic applications. Rotating magnetic fields seen by a coil is today the major source of electricity and, inversely, current in a coil produces magnetic fields like in MRI devices. The fundamental reason is that both are, in fact, identical depending on the reference frame taken. This has been highlighted by the well-known Maxwell equations that link electric fields and magnetic fields, one being the derivative of the other.
In parallel to the enormous importance of electricity in our life, electromagnetism has a fundamental property that justifies the billions of magnetic sensors and antennas produced each year: it is the only long-range interaction that we can create, modify, and detect. This long-range interaction property takes various forms. Light is an electromagnetic wave. Radiofrequency transmissions used for radio, TV, or mobiles are electromagnetic waves at lower frequencies. Static or low-frequency magnetic fields are the extremely low or zero frequency aspect of the same interaction.
1.1.2 Magnetic Field, Magnetic Induction, and Units
Historically, the magnetic field has been described by two different quantities. The first one is the field created by a magnet that has been called , the magnetic field intensity. The second one is the field created by a current that has been called , the magnetic induction.
It took some time to reconcile the two quantities that are proportional in the vacuum.
Magnetic field intensity H is given in A/m or in Oersted and magnetic field induction is given in Tesla or in Gauss. They are related by the following relation:
where is the magnetization of the material at the point where the field is measured. In the presence of vacuum or in nonmagnetic materials that quantity is 0. is a constant equal to
A/m is not a very useful quantity for a common comparison, and now nearly everybody is using Tesla or Gauss as a unit both for magnetic field intensity and induction. In this book, we will follow the same use knowing that this is just a commodity.
The relationship between these quantities is given in Table 1.1 .
Table 1.1 Main fields units.
Quantity Designation Unit Link Magnetic field intensity H A/m (MKS) In vacuum
. Oe: Oersted (CGS) In vacuum
1Oe=1G Magnetic field induction B T : Tesla (MKS) In vacuum