Ι What Is the Demagnetization Curve?

When selecting a permanent magnet material, most engineers start—and often end—with the datasheet. We look at the residual induction (remanence) Br to gauge strength and Maximum Energy Product (BH)max to gauge efficiency. While these numbers are critical, they only tell a fraction of the story. They represent the magnet in an ideal, static state.

However, in real-world applications—especially in permanent magnet motors, sensors, or magnetic couplings—magnets operate in harsh environments. They face high temperatures and strong opposing magnetic fields (demagnetizing fields) that constantly threaten to demagnetize them.

To understand how a magnet will behave under this stress, the datasheet is not enough. You need the demagnetization curve.

Hysteresis Loops

Demagnetization Curve

The Second Quadrant

Technically, the demagnetization curve is the second quadrant of the complete magnetic hysteresis loop.

While the full loop represents the complete “life cycle” of a magnet (from magnetization to demagnetization and re-magnetization), the second quadrant represents its working state.

Once a permanent magnet is fully magnetized (saturated) and removed from the magnetizer, it enters this second quadrant. Whether it is sitting on a shelf fighting its own self-demagnetization (due to its shape) or embedded in a rotor fighting the stator’s opposing field, the magnet is actively resisting demagnetization.

The Battle Between B and H

The curve is essentially a graph of a battle between two forces. By understanding the axes, you can read the “score” of this battle:

The Y-Axis (Vertical): Magnetic induction (B) or polarization (J).This represents the magnet’s strength. It shows how much magnetic flux the material is generating. The higher the point on this axis, the stronger the magnet.
The X-Axis (Horizontal): Magnetic field strength (H).This represents the external attack. In the second quadrant, this value is negative, representing a demagnetizing field (such as the opposing field in a motor’s air gap). The further left you go on this axis, the stronger the opposing force becomes.

The curve itself describes the trade-off: As the opposing field ( $H$ ) increases, how much strength (B) does the magnet lose?

A “perfect” permanent magnet would maintain its strength regardless of the opposing field (a flat horizontal line). In reality, every magnet eventually succumbs. The shape of this curve tells you exactly when and how that failure will happen.

The Map of the Second Quadrant: Normal vs. Intrinsic Curves

When you open a detailed test report, you won’t just see one line; you will typically see two curves diverging from the same Br point.

When you analyze a comprehensive test report, you won’t see a single line. Instead, you will see two distinct demagnetization curves diverging from the same Br point. Understanding the difference between these two is critical for any application, from heavy-duty motors to precision sensors.

The Normal Curve (B-curve): The “Performance Line”

The Normal Curve represents the magnetic flux density (B) that actually exits the magnet to interact with the external circuit. For design engineers, this line defines your device’s output:

In permanent magnet motors, the normal curve determines the air-gap flux density and, consequently, the torque output.
In magnetic sensors (e.g., Hall effect sensors), this curve dictates the signal strength. If the operating point on the normal curve drops too low, the sensor triggers a false reading or fails to detect the target entirely.
In magnetic couplings, this curve defines the maximum static holding force or breakaway torque.

The Intrinsic Curve (J-curve): The “Survival Line”

The intrinsic curve (often labeled J or M) represents the material’s internal polarization—its inherent physical resistance to being demagnetized. While it contributes to the flux output, its primary role is defense. This is your “survival line”:

In permanent magnet motors, this curve tells you if the magnet can withstand the severe opposing fields caused by start-up currents or short circuits without suffering irreversible loss.
In magnetic couplings, if the load exceeds the torque limit and the coupling “slips” (the two magnet rotors rotate against each other), the magnets face massive repelling forces. A strong intrinsic curve ensures the magnets survive this slippage event without becoming dead weight.

In benign environments, focusing on the normal curve is often sufficient. However, in dynamic applications requiring high reliability, the intrinsic curve is the only barrier between a temporary performance dip and catastrophic system failure.

Interpreting the Demagnetization Curve: Br, Coercivity, and (BH)max

To accurately predict performance, we must understand the four key metrics that define the shape of the demagnetization curve. These parameters—known as the “four major indices”—are the cornerstones of every permanent magnet datasheet.

Br (Remanence): The “Muscle”

Remanence (Br) measures the magnetic flux density remaining in the material after saturation.

Significance: It represents the magnet’s raw strength. A higher Br means a higher potential magnetic flux density flowing into your system.
Application: In permanent magnet motors, Br is the primary driver of torque density. For magnetic sensors, it determines the strength of the field at a given distance (air-gap), directly affecting signal reliability.

Hcb (Normal Coercivity): The “External Resistance”

Normal coercivity (Hcb) marks the point on the normal demagnetization curve where the magnetic flux density ( $B$ ) drops to zero.

Significance: While Hcb indicates when the magnet stops outputting flux to the external magnetic circuit, it can be misleading. A zero reading here does not mean the magnet is fully demagnetized internally.
Application: It helps engineers gauge resistance to weak stray fields in static applications like holding magnets or latches.

Hcj (Intrinsic Coercivity): The “Internal Endurance”

Intrinsic coercivity (Hcj) is the defining point on the intrinsic demagnetization curve. It indicates the opposing field strength needed to reduce the material’s internal polarization to zero.

Significance: This is the definitive measure of a magnet’s demagnetization resistance.
Application: In high-performance permanent magnet motors and magnetic couplings, the magnet faces intense opposing fields and high heat. A high Hcj ensures the magnet survives these harsh conditions without suffering irreversible demagnetization.

(BH)max (Maximum Energy Product): The “Efficiency”

(BH)max represents the peak energy density stored in the magnet, found at the optimal operating point on the demagnetization curve.

Significance: It combines both strength ( $Br$ ) and resistance ( $Hc$ ) into a single metric of efficiency.
Application: A higher Maximum Energy Product allows for device miniaturization. It enables designers to generate the same motor torque or holding force using a smaller volume of permanent magnet material.

The Thermal Reality: Temperature-Dependent Demagnetiztion Curves

While Br and HcJ tell you the magnet’s potential at room temperature (typically 20°C), they do not reveal how the material behaves when it heats up. In real-world applications like permanent magnet Motors, temperatures can easily soar above 100°C.

To predict performance under heat, you cannot rely on a single line. You need to analyze the temperature-dependent demagnetization curves.

What Are Temperature-dependent Curves?

On a comprehensive datasheet, you won’t just see a single line representing room temperature. Instead, you will see a series of nested curves—often referred to as a “family of curves”—labeled at increasing intervals like 20°C, 60°C, 100°C, and 150°C. As the temperature rises, the curve doesn’t just lower uniformly; it contracts in a specific way:

Temperature-dependent Demagnetization Curve

Strength Drops Slowly: The top of the curve (B) drops relatively slowly. Your motor might lose a little bit of torque, but it’s usually manageable.
Resistance Collapses Quickly: The left side of the curve ( $H_{cJ}$ ) retracts much faster. The “survival line” moves inward aggressively.

This means that a magnet that looks invincible at 20°C might become dangerously fragile at 120°C. By looking at the specific curve for your operating temperature, you can see exactly how much your “safety zone” has shrunk, preventing you from designing a system that works in the lab but fails in the field.

Comparative Analysis: Demagnetization Curves by Material Family

Demagnetization curves vary significantly by material. By overlaying the curves of the main commercial magnet families, we observe distinct geometric profiles. These shapes—ranging from the high, wide “square” loops of rare earths to the steep, curved descents of AlNiCo—are not merely visual differences; they represent fundamental physical characteristics, visualizing the inherent trade-off between raw magnetic strength and stability, that define the material’s resistance to demagnetization and its optimal application environment, ensuring your design can withstand the rigors of dynamic operation.

AlNiCo: The “Biased Specialist”

Curve Shape: Tall and narrow.
Visual: It has a Br that rivals NdFeB (very high), but the curve hugs the Y-axis tight and drops off almost immediately (very low Hcj).
Implication: It is extremely easy to demagnetize. The curve is non-linear and unstable against opposing fields.
Design Note: To “stand up,” AlNiCo magnets must be designed with a long shape (high aspect ratio).

Ferrite (Ceramic): The “Stable Workhorse”

Curve Shape: Low and curved (rounded)
Visual: Both its Br and Hcj are low to moderate compared to rare earths. The curve shows a gradual, rounded decline, rather than a sharp square shape or a straight line.
Implication: While not powerful, it is chemically stable (resists corrosion) and very cost-effective.
Design Note: Unlike other magnets that hate heat, ferrite hates the cold. At low temperatures, its $H_{cJ}$ drops, creating a risk of demagnetization in freezing environments.

Rare Earth (NdFeB & SmCo): The “Domineering Powerhouses”

Curve Shape: High squareness.
Visual: The curve stays horizontal for a long distance before dropping sharply. This “square” shape is the hallmark of modern high-performance materials.
Implication: This “squareness” allows these magnets to be made very thin (low Pc value) while still maintaining full magnetic strength.
Design Note: This characteristic is the fundamental reason why modern motors and electronics can be so miniaturized.

Material	Curve Shape	Br (Strength)	Hcj (Resistance)	Key Implication
NdFeB (Neodymium)	Square loop	Very high	High	Ideal for miniaturization. Note that its high resistance drops rapidly as temperature rises.
SmCo (Samarium Cobalt)	Square loop	High	High	The most stable curve. It maintains this “high” resistance even in extreme heat.
AlNiCo	Tall & narrow	High	Very low	Must be long/rod-shaped to hold magnetism. Very sensitive to demagnetizing fields.
Ferrite (Ceramic)	Linear	Low	Moderate	Cost-effective. Beware of demagnetization risks in low temperatures.

Need Help with Your Design?

At SDM Magnets, we provide comprehensive demagnetization data for every grade we manufacture. Whether you need to discuss a specific application or require raw data for magnetic simulation, contact our engineering team to request the full demagnetization curves.

Source Acknowledgment Some graphical data and technical illustrations regarding intrinsic curves in this article are referenced from Arnold Magnetics and LinkedIn. We acknowledge and appreciate their contribution to the technical community.