Magnetic Field Sensing Ferrite Magnets As A Cost-Effective Solution for Scarce Rare Earths

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TDK-Micronas GmbH

FAULHABER Drive Systems

Wuxi InfiMotion Technology Co., Ltd.

TDK sets new standards in magnetic field sensing by eliminating the use of rare earths. An alternative is ferrite magnets. They are cost-efficient and also environmentally friendly. In use, they impress with performance and temperature stability.

Karsten Köhler is Manager Business Development at TDK-Micronas.

In today's technological landscape, geopolitical tensions play an increasingly decisive role, jeopardizing the supply chain for rare earths as they are the centerpiece of many electronic products. Faced with the risk of production halts, companies in the electronics industry are desperately seeking solutions. TDK demonstrates a groundbreaking approach in magnetic field sensing that eliminates the use of rare earths.

In modern automation and control systems, magnetic field sensing is indispensable. Whether it involves the precise determination of position, speed, rotational speed, or torque, the performance and precision of these sensors are crucial for their reliable operation in devices.

A complete magnetic measurement system consists of two main components: the magnet, also called the target, and the sensor, such as a Hall sensor. This detects the magnetic field and converts it into a measurable voltage. The magnet is precisely positioned on the moving component, such as a motor shaft or a control pedal. The sensor is installed in the exact geometric relationship to it to measure the precise movement and condition of the component.

Pure Iron is Simply Unsuitable for Many Applications

Since the magnet represents a foreign object on the component, it makes sense for the magnet to be as small as possible. Nevertheless, it should generate a strong field so that the sensor receives a strong and low-noise signal. This is where rare earths come into play. Basically, magnetism arises when oppositely charged structural elements in a metal, also called dipoles, are evenly aligned, forming a north and a south pole. The most common magnetizable metal is iron. However, iron alone creates only weakly pronounced magnetic fields and loses its magnetic properties, especially under the influence of heat. This phenomenon is known as thermal demagnetization. Since many technical applications take place in heat-intensive environments, such as in motors, pure iron is simply unsuitable as a target material.

Here, rare earth metals come into play: a group of 17 metals from the 3rd subgroup of the periodic table, most of them led by lanthanum. Among these metals, neodymium is particularly well-known, while terbium as a heavy rare earth and samarium as well as dysprosium are intermediately positioned. These elements provide the necessary stability and strength even at elevated temperatures. This is a critical property for the next generation of high-performance magnets.

Geopolitics And the Problem With Rare Earths

What initially sounds like theoretical chemistry has practical implications for magnet production: The electron structure of rare earths stabilizes the bipolar structure in magnetizable metals. Two key advantages result from this:

• Increasing coercivity: The field strength per unit volume can be increased so that smaller magnet sizes lead to the desired magnetic fields.
• Improved temperature stability: Magnets can operate reliably even at higher temperatures without losing their magnetic properties.

These properties have made magnets with rare earths a technical standard that is recognized worldwide. However, the global deposits of these raw materials are geologically limited and concentrated in specific regions, which leads to supply shortages during times of geopolitical tensions. Unfortunately, the situation is currently significantly affecting industrial production.

Ferrite Magnets As Alternatives to Rare Earths

Alternatives such as ferrite magnets and electromagnets offer potential possibilities. Ferrite magnets, produced through sintering processes, exhibit stable crystal structures with excellent field strength and thermal stability. Electromagnets, on the other hand, are unsuitable for moving components because their power supply would be too complex. Ferrites therefore present themselves as the preferred substitute for rare earth magnets.

Even in times when the supply situation was more favorable, ferrites were preferred due to their cost efficiency and lower environmental impact. In light of current developments, many companies, especially in the automotive industry, are working closely with sensor manufacturers to replace rare earth metals with ferrites. Existing sensory developments for ferrite magnets facilitate smooth implementation in this regard.

For example, the existing product families HAL 35xy and HAL 39xy from TDK-Micronas have already been optimized in their design phase for use with ferrite magnets. These products are high-performance position sensors that can detect movements in all three spatial directions. Image 2 schematically shows a typical setup in an end-of-shaft position, meaning for attachment at the end of a shaft.

The key factor is the high sensitivity of the sensors, meaning the ratio between output voltage (signal) and the magnetic field strength to be measured, specified in mV/mT. The image illustrates the signal progression using three horizontal Hall elements, which in this case are used for angle measurement. It becomes clear that the HAL 39xy already provides a low-noise, easily evaluable signal progression for magnets with a field strength of 20 mT. This field strength can be well represented with ferrite magnets (Image 3).

The Performance of A Ferrite Magnet

Ferrite magnets generate a weaker magnetic field and are more prone to thermal demagnetization, which affects measurement accuracy. Error magnitudes are therefore crucial for signal quality. The image shows the performance of the HAL 39xy based on errors in the output angle signal depending on the air gap, meaning the distance between the sensor and the magnet surface. It can be seen that the resulting error (top yellow line) as the sum of various error sources remains within the specified maximum value of only 1° deviation.

It is particularly noteworthy that the total error already takes into account the thermal stress during the lifespan (determined according to the Automotive AEC-Q100 Grade 0 temperature profile), which includes the critical thermal behavior of ferrite magnets. How does the HAL 39xy achieve this? Two main features are crucial for its performance with ferrite magnets:

• 6ZD silicon structure: The chip's silicon structure integrates six Hall cells in a specific geometry. This configuration captures the entire magnetic field generated by the ferritic magnet. Test series with automotive customers demonstrate that this arrangement delivers significantly better results than sensors with fewer Hall cells. 
• Internal temperature compensation: The chip contains temperature compensation that compensates for the magnetic system behavior and thus minimizes the impact on the overall error.

Benchmark tests show that the HAL 39xy, at 10 mT—only half the typical field strength mentioned above—exhibits the lowest interference field impact. This makes the sensor ideal for ferrite magnets. The sensor is available in standard packages such as SOIC-8, simplifying replacement during a switch to ferrite magnets. TDK supports customers in quickly transitioning to ferrite systems and avoiding production downtimes.

All sensor families from Micronas benefit from TDK's technology pool. The described advantages of the HAL 39xy and HAL 35xy can also be applied to other sensors, such as the HAL 302x for e-motor commutation applications and the HAL 15xy switches. In light of geopolitical changes in supply chains, TDK helps mitigate negative impacts through advanced technological solutions. (heh)

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