What Is a Permalloy?
A Permalloy is a type of nickel-iron alloy, especially one with a nickel content of 35-80%.
The name Permalloy is common, but the official name defined by the Japanese Industrial Standard JIS C 2531 is an iron-nickel soft magnetic material. A Permalloy has a low coercivity and high permeability and has the characteristics of a high magnetic shielding effect and a high magnetic focusing effect.
Another property is that it can exhibit high magnetization when a fine magnetic field is applied, and it also increases the impedance in AC circuits.
Uses of Permalloys
Permalloys are used to prevent magnetic leakage from magnetic heads installed in magnetic recording devices such as TVs, PCs, videotapes, and hard disks. The aforementioned properties make permalloys suitable as a magnetic shielding material.
In addition, biomagnetic measurement, a next-generation diagnostic method that has been attracting attention in recent years, requires the measurement of extremely weak magnetic fields and must be shielded from the effects of environmental magnetic fields. Therefore, a magnetically shielded room with permalloy magnetic shielding has been installed to prevent the influence of environmental magnetic fields.
Principle of Permalloys
Permalloys are a type of nickel-iron alloy with a nickel content of 35-80%, but raw permalloys do not have a very high magnetic permeability. Permalloys undergo a process called “magnetic annealing” and “strain relief burning.”
1. Magnetic Annealing
Magnetic annealing is a heat treatment to remove oxide films, etc., which prevents the movement of magnetic domains where the magnetic moment of the atoms in permalloys are aligned. This removal of impurities allows the external magnetic domains to move.
The removal of these impurities promotes the movement of magnetic walls and rotation of magnetic domains when an external magnetic field is applied, thereby improving soft magnetic properties. The magnetic permeability of permalloys after magnetic annealing is about 100 times higher than that of permalloys before magnetic annealing.
2. Strain-Relief Burning
Straining and burning is a process performed at lower temperatures than magnetic annealing to remove residual stress by recrystallization. The purpose is to make it easier to process. It is also possible to achieve even higher magnetic permeability by adding molybdenum, copper, or chromium.
Other Information on Permalloys
1. Main Types of Permalloys and Magnetic Properties
There are several types of permalloys, which are used for different purposes. Two of the most commonly used are Permalloy B (PB) and Permalloy C (PC), where PB is a binary alloy of iron and nickel and PC is a multi-alloy of iron, nickel, molybdenum (Mo) and copper (Cu).
In magnetic materials, the higher the saturation magnetization Bs, which indicates the absolute value of magnetic force, the more suitable the material is for magnetic shielding in strong magnetic fields. On the other hand, the larger the magnetic permeability μ (the larger the maximum permeability near saturation magnetization Bs), the more suitable it is for magnetic shielding in weak magnetic fields because it can respond to changes in weak magnetic fields.
In this case, the maximum permeability of PB and PC are 50,000 and 180,000, respectively, and the saturation magnetization Bs is 1.55T for PB and 1.72T for PC. In other words, PB with large saturation magnetization is suitable for shielding in strong magnetic fields, while PC with large permeability is suitable for shielding in weak magnetic fields.
2. Practical examples of Permalloy Cores
In addition to its function as a magnetic shield as described above, permalloys also have a function as a core that detects weak magnetic fields increases the magnetic flux for output, and is used as a core for current sensors and transformers. A current sensor is a sensor for measuring electric current. When current flows through a conductor, magnetic flux is induced in the core, and the magnitude of the magnetic field is used to measure the current value.
A transformer is a device for converting voltage and insulating between circuits; it consists of an input coil and an output coil wound independently on a single core, and when current flows in the input coil, voltage is output to the output coil by the nature of electromagnetic induction. The use of Permalloys, which have high magnetic permeability, makes it possible to downsize the transformer.
3. Workability of Permalloys
Permalloys are flexibly deformable and have excellent workability. Like other metals, it can be processed by bending, cutting, pressing, and punching. However, nickel alloys, of which permalloys are a part, are representative of materials that are generally considered difficult to cut. For this reason, cutting permalloys requires a high level of technical skill.
Permalloys are widely used in magnetic shields, measuring instruments, magnetic heads, audio equipment, communication cables, etc., and are processed and used in a variety of shapes, including cylindrical, plate, ring, wire, and foil shapes, depending on the application.