CORT MAGNETS

Alnico Magnet Stability: From Lab Research to Industrial Breakthrough

Alnico Magnet Stability: From Lab Research to Industrial Breakthrough

Assorted alnico magnet components with "ALNICO" branding and metallic finishes

Introduction: "Temperature King" in Permanent Magnetic Materials

In modern industrial precision instruments, such as aerospace navigation systems and medical sensors, the stability of permanent magnets is crucial. It directly affects the reliability and lifespan of the equipment. Alnico Magnet is known for its great temperature stability. This makes it a top choice for high-temperature and long-term use. CORT Magnets is dedicated to studying permanent magnetic materials. Our goal is to offer safe and high-quality magnets to people around the world.

Why Focus on Magnetic Stability?

The “stability” of permanent magnets means they can keep their magnetic properties. This is true even with internal and external interference after they are magnetized. When equipment faces magnetic loss due to extreme temperatures or long use, it can cause serious problems. This may include inaccurate precision instruments and lower motor efficiency. By comparing the core parameters of mainstream permanent magnetic materials, the paper highlights the irreplaceability of Alnico Magnet:

Table 1: Comparison of Temperature Performance of Permanent Magnetic Materials

Permanent Magnetic MaterialCurie Temperature Tc (℃)Remanence Br Temperature Coefficient (10⁻²/℃)Coercivity Hcj Temperature Coefficient (10⁻²/℃)Upper Limit of Service Temperature (℃)
AlNiCo5750~890-0.02-0.03525
Ferrite450-0.200.40300
SmCo5725-0.04-0.30250
NdFeB310-0.126-0.60150

From the data, Alnico magnets perform better in high temperatures and loss control. Their upper service temperature limit is 525℃, much higher than NdFeB‘s 150℃. The irreversible loss for Alnico is only -2.1% at +500℃. In contrast, NdFeB has lost 65% at +200℃. However, previously limited by testing equipment, the magnetic stability of Alnico Magnet over time has lacked accurate characterization.

Solving the Problem: Innovative Testing Methods and Experimental Design

To fill this gap, the domestic team built a non-destructive magnetoelectric performance testing device for hard magnetic materials, enabling high-precision dynamic monitoring of magnetic properties. On this basis, a multi-dimensional experimental scheme was designed:

1. Screening of Thermal-Cold Cycle Stabilization Processes

Through small temperature rise experiments, we chose four key processes from a wide range of temperatures. These cover extreme temperature scenarios
• Process A: -70℃~130℃ (conventional ambient temperature fluctuation)
• Process B: -70℃~190℃ (medium-high temperature working condition)
• Process C: -196℃~130℃ (deep cold + normal temperature)
• Process D: -196℃~190℃ (deep cold + high temperature limit)

2. Microstructure Characterization Methods

To reveal the nature of performance changes, the study employed multi-scale analysis techniques:
• XRD: Analyzing phase composition and crystal structure
• EBSD: Observing grain orientation and texture distribution
• TEM: Characterizing nanostructured precipitated phases (α₁, α₂ phases)
• MFM (Magnetic Force Microscopy): Visualizing magnetic domain morphology evolution

Mysteries of the Microcosm: Structure and Magnetic Domain Evolution of Alnico Magnet

Various shapes of alnico magnet including cylindrical, U-shaped, and rectangular designs

The magnetic properties of Alnico Magnet originate from its unique microstructure. TEM and EBSD observations revealed:

1. Crystal Structure Foundation

Alnico Magnet has a body-centered cubic (BCC) structure with crystal orientation along the <100> direction and a columnar crystal spatial configuration. This highly ordered structure is the source of its magnetic anisotropy—the <100> direction is precisely the easy magnetization axis of the alloy, ensuring high remanence characteristics.

2. Influence of Temperature and Time on Magnetic Domains

MFM observations showed an important pattern. Higher temperatures or longer times make the magnetic domains grow larger. This growth leads to less order and uniformity (Figure 1).
• Temperature effect: As the temperature rises from room temperature to 190℃, the size of magnetic domains grows. It increases from about 500nm to 1.2μm. At the same time, the clarity of domain walls decreases by 30%
• Time effect: After aging at 130℃ for 1000 hours, the size of the magnetic domains increases by 45%. Local areas also show “magnetic domain merging.”
This change is linked to the weakening of magnetic properties. Magnetic domain disorder lowers remanence Br. At the same time, less resistance to domain wall movement increases the loss of coercivity Hcj.

Breakthroughs in Stabilization Processes

Aiming at the problem of magnetic domain evolution, the study systematically evaluated the stability improvement effects of thermal-cold cycling and aging treatment, leading to several key conclusions:

1. Thermal-Cold Cycling Treatment: Enhancing Phase Order and Magnetic Induction Intensity

After treatment with the four processes, the magnetic properties of the Alnico Magnet changed significantly:
• Magnetic domains and magnetic induction intensity: As the temperature difference increases, the size of the magnetic domains gets larger. Process D is 60% larger than Process A. The magnetic induction intensity also increases. The Br of Process D reaches 1.28T, which is an 8% increase compared to untreated samples.
  • Microscopic mechanism:
  • TEM analysis showed that thermal-cold cycling made the α₁ phase (ferromagnetic) more ordered.
  • It also increased the order of the α₂ phase (non-magnetic). This led to a more regular atomic arrangement at the phase interface. The order parameter S increased from 0.62 to 0.78. The grain size did not change significantly and stayed at 5 to 8 μm.

2. Aging Treatment: Promoting Element Diffusion and Grain Homogenization

After aging treatment at 480℃ for 4 hours:
• Element diffusion: Al and Ni atoms enriched in α₂ phase, Co and Fe migrated to α₁ phase, expanding the composition difference between the two phases by 15%;
• Microstructure: α₁ phase precipitated more uniformly, grains grew and size distribution standard deviation decreased from 1.2μm to 0.8μm;
• Magnetic domain optimization: Apparent magnetic domain size increased to 1.5μm, but order was restored through aging (uniformity improved by 25%).

3. Synergistic Effect of Composite Processes: Excellent Performance of Alnico8-1 Alloy

When aging + thermal-cold cycling composite treatment (aging for 4 hours followed by Process D cycling) was applied to different batches of Alnico Magnet, Alnico8-1 alloy showed optimal stability:
• After 1000 cycles at -196℃~190℃, magnetic irreversible loss was only 0.3%;
• After long-term storage (room temperature for 2 years), Br attenuation rate was <0.5%, far lower than 1.2%~1.8% of other batches.

Empirical Evidence of Long-Term Stability

To simulate actual service scenarios, the study conducted 10000-hour long-term stability tests, leading to two important findings:

1. Relationship Between Thermal-Cold Cycle Temperature Difference and Irreversible Loss

Under the same aging time, larger cycle temperature differences resulted in smaller magnetic irreversible losses (Figure 2). For example:
• Process A (temperature difference 200℃): 1.5% loss after 1000 hours
• Process D (temperature difference 386℃): only 0.4% loss after 1000 hours
This is because large temperature differences promote sufficient ordering of α₁/α₂ phases, forming a more stable magnetic domain structure.

2. "Positive Effect" of Storage Time

Contrary to common sense, longer storage time of magnetic steel results in smaller magnetic irreversible losses. Untreated samples showed 2.1% loss after 1 month of storage, while loss decreased to 0.8% after 1 year of storage. The study suggests this is due to slow release of residual stress within the alloy during natural aging, reducing relaxation of domain wall pinning points.

Industrial Application Value and Future Prospects

This research not only reveals the intrinsic mechanisms of Alnico Magnet’s magnetic stability but also provides clear guidance for industrial applications:

1. Process Optimization Recommendations

• Equipment for extreme environments (e.g., aero-engine sensors): Priority should be given to Process D (-196℃~190℃) + aging composite treatment, which can control magnetic performance attenuation within 0.5% per year;
• Conventional environment applications (e.g., automotive motors): Select Process B (-70℃~190℃) to ensure stability while reducing costs.

2. Material Selection Guide

Alnico8-1 alloy, with its optimal long-term stability, should be the first choice for high-precision, long-life equipment. Its application in medical CT magnets and oil logging instruments can extend equipment maintenance cycles by more than 3 times.

3. Future Research Directions

• Exploring the effect of lower temperatures (<-196℃) cycling on deep cold environment applications;
• Combining machine learning to predict magnetic property evolution under different processes, shortening process development cycles.

Conclusion

As the “evergreen tree” in permanent magnetic materials, Alnico Magnet’s potential is far from fully exploited. The research team’s study has injected new vitality into this classic material through correlation analysis between microstructure and macroscopic properties. Data on magnetic domains and phase structures are enhancing reliability in today’s industry. This data comes from lab MFM images and industrial sensors. This shows the value of materials science, “from microcosm to macrocosm.”
CORT Magnets is a Chinese company that focuses on research, production, and sales of magnetic materials. They offer complete solutions for magnetic materials to customers around the world. Currently, we have established long-term and stable cooperation with customers in over 30 countries globally. If you have any questions about magnets, feel free to contact us—we are happy to answer all your inquiries.
References: Liao Yaqin. Study on Magnetic Stability of Alnico Magnet Alloys [D]. Harbin Institute of Technology.
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Aaron Chen

I am Aaron Chen, the operations director of CORT MAGNET. CORT MAGNET is a manufacturer specializing in the research and development of magnetic materials. It has over 15 years of experience in the research and development of magnetic materials, as well as international sales experience. We are committed to providing customers with a one-stop solution for magnetic materials. Our products include neodymium iron boron magnets, ferrite magnets, and customized magnetic components.

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