The Curve That Tells You Everything: BH Curves for Non-Engineers

Every magnet datasheet has one. It's the little squiggly graph tucked into the corner, usually labeled B-H curve or demagnetization curve, and if you're not an engineer it looks like something you're allowed to skip. I skipped it for a long time too. Then I started poking around on our Magnetic Material Selection page, clicked into Neodymium IU, and found a bunch of links that led to charts like:

QT Magnetic Solutions has published B-H curves for most materials we deal in. They can be found by clicking into the different materials on our Magnetic Material Selection page and then selecting a grade. We'll dive deeper into the four numbers at the bottom and the importance of that knee bend in each of the curves.

Now, I have a math degree and a computer science degree, so I'm no stranger to charts and numbers, but this was... a lot. So here's the tutorial of B-H curves I wish someone had given me before perusing these. For a complete background on who I am and why this post exists, head over to the Introduction to Alisa Learns about Magnets.

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💡 TLDR: What we're covering

• B-H Curve Basics: A magnet's B-H curve plots the applied magnetic field (H) against the produced magnetic field (B), focusing on the second quadrant to model real-world operating behavior.  

• The Four Metrics: Remanence (Br) defines maximum ideal strength, Coercivity (Hc or Hcb) measures resistance to opposing fields, Intrinsic Coercivity (Hci or Hcj) measures tolerance to heat and mechanical stress, and BHmax reflects peak energy efficiency.  

• Preventing Failure: Pushing a magnet below the curve's "knee" causes permanent demagnetization. Because heat lowers the entire curve, components must be evaluated using their specific load line (permeance coefficient) at maximum operating temperature, not just at room temperature. 

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What is a B-H curve?

A B-H curve plots two things against each other: H, the external magnetic field being applied to the magnet, and B, the magnetic field the magnet is actually producing in response. Engineers usually only care about one quarter of the full curve, the second quadrant, because that's the region that describes how the magnet behaves when it's installed and doing real work in your product, being pushed against by its own circuit rather than an outside magnetizer.

Think of it less as an abstract chart and more as a report card. It tells you three things:

1. How strong the magnet is at its best

2. How hard it is to weaken, and

3. Exactly how far you can push it before it stops being a magnet and becomes a very expensive paperweight.

The Four Numbers That Matter

The four numbers excerpted from the N35H above.

• Remanence (Br): This is the magnet's maximum strength under ideal conditions; fully magnetized, no opposing field pushing back. It's the "how strong is this thing at its absolute best" number, similar to pulling a slingshot back as far as it goes. It's a useful headline figure, but it's not what you'll actually see in your assembly, because your assembly is never the ideal case.

• Coercivity (Hc or Hcb): This measures how much reverse magnetic field it takes to knock the magnet's usable field down to zero. Higher Hc means the magnet is more stubborn about staying magnetized when something is actively working against it.

• Intrinsic coercivity (Hci or Hcj): This is the number I now check first, and it's the one that gets skipped most often by people reading a datasheet quickly. Hci tells you how resistant the magnet is to heat and stress specifically, not just an opposing field, but the everyday conditions of running hot inside a motor housing or getting knocked around during assembly. For anything that lives near a heat source, Hci matters more than Br.

• Maximum energy product (BHmax): This is the single number most often used to compare magnet grades against each other; it's where the "35" in N35 and the "52" in N52 come from. It marks the point on the curve where the product of B and H is largest. In plain terms, the magnet's most efficient operating point, where it delivers the most magnetic energy for its size. In practice, your finished assembly rarely operates exactly at BHmax; it's a benchmark for comparison, not a promise of real-world performance.

The Knee and Why It's the Part that Actually Bites You

Traversing beyond the knee is NOT the bees' knees.

Follow the curve down and at some point it stops being a nice straight line and bends sharply. That bend is called the knee, and it's the most consequential feature on the entire graph. Above the knee, small changes in operating conditions cause small, recoverable changes in the magnet's field. Cross below the knee, and the magnet suffers irreversible demagnetization; it doesn't spring back when conditions return to normal. It's permanently weaker, and the only fix is re-magnetizing or replacing the part.

Where your magnet actually sits on this curve during operation is determined by its load line, also called the permeance coefficient: a line whose slope depends on the magnet's shape and whatever magnetic material surrounds it. The point where the load line crosses the B-H curve is called the operating point, and that's the number that actually describes your part, not Br and not BHmax.

The yellow arrows are pointing at the operating point at 140°C for permeance coefficients of 2.0 and 5.0.

The practical takeaway: a magnet specified with plenty of margin at room temperature can get pushed dangerously close to its knee by heat alone. Temperature drags the whole curve down; remanence drops, and coercivity can fall off sharply as things get hot. Standard NdFeB grades are typically rated for somewhere around 80-150°C depending on grade, while SmCo variants commonly tolerate up to roughly 300°C (with some specialized high-temperature grades rated higher still), which is exactly why we reach for SmCo in high-temperature applications even though NdFeB is stronger at room temperature. Design a part using only the room-temperature curve, and you may be closer to the knee at operating temperature than the datasheet made it look.

What to Actually Do With This

You don't need to become a magnetics engineer to use a B-H curve well. You need three habits:

• Ask for the curve at your actual operating temperature, not just at 20°C.

• Check Hci, not just Br, if your application runs hot or takes mechanical shock.

• Make sure whoever is designing your magnetic circuit has calculated the load line for your specific geometry, so you know where on that curve your part will really be living: comfortably above the knee, not gambling on it.

Using B-H curves well is the difference between a magnet that quietly does its job for ten years and one that mysteriously loses half its strength six months after launch.

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Still need experts to guide you through the characteristics of your specific assembly? Contact QT Magnetic Solutions who've been looking at these for over three decades. I'll just be here comparing Samarium Cobalt (SmCo) B-H curves with Neodymium.