--- title: "Why an iron crystal but not iron piece magnetizes" subject: "Electrical Engineering Material" chapter: 4 key: "semester_3/electrical_engineering_material:iron-crystal-magnetic-field" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/iron-crystal-magnetic-field site: BCT Engineering --- # Why an iron crystal but not iron piece magnetizes Electrical Engineering Material · Chapter 4: Magnetic Materials - Asked in: 2070 Chaitra, 6 marks - Video: [MAGNETS: How Do They Work?](https://www.youtube.com/watch?v=hFAOXdXZ5TM) — minutephysics - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/iron-crystal-magnetic-field - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4 ## Question A crystal of iron created magnetic field around it but a piece of iron doesn't why? ## Short answer A small iron single crystal can be a single domain with all moments aligned, creating an external field. Ordinary bulk iron is polycrystalline, splitting into many randomly oriented domains whose magnetization cancels, so no net external field appears. ## Answer The difference is explained by the **domain theory** of ferromagnetism. #### Why a (small) iron crystal shows a magnetic field - In iron, the strong **exchange interaction** aligns the spins of neighbouring atoms parallel, giving **spontaneous magnetization** (about 2.2 Bohr magnetons per atom). - A small, perfect single crystal of iron, below a critical size, can exist as a **single domain** (or with its domains mostly along one easy axis). All atomic moments point the same way, so the moments add up and the crystal has poles and produces an external magnetic field around it, like a tiny bar magnet. #### Why an ordinary piece of iron does not 1. **Domain formation**: a bulk piece would have a very large magnetostatic energy if it were one domain. To lower this energy it splits into many domains with **closure domains** at the ends, so the flux is closed inside the material. 2. **Random orientation**: an ordinary piece of iron is **polycrystalline**: it has many grains, each with its own crystal axes and many domains. The magnetization directions of all these domains are random. 3. **Cancellation**: the vector sum of the magnetization of all domains is zero, so no free poles appear at the surface and no external field is produced, even though each domain is saturated inside. ```text Small crystal Piece of iron (single domain) (many random domains) +-------------+ +--+--+--+--+ | --> --> --> | |^ |->|v |<-| | --> --> --> | +--+--+--+--+ +-------------+ |<-|v |^ |->| S N +--+--+--+--+ external field net M = 0, no field ``` #### Making the piece magnetic If the piece of iron is placed in an external field, favourable domains grow by domain wall motion and others rotate, so the domains line up and the piece becomes a magnet. Soft iron loses this alignment when the field is removed, which is why it is used as an electromagnet core. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why can a small iron crystal produce an external field? A. It can be a single domain whose aligned moments add up and create poles B. It contains no atomic moments C. Its moments are random D. Its domains are all closure domains Answer: A. Exchange aligns spins, and a single domain has free poles. (Key point: Small crystal) 2. Why does a bulk piece of iron split into many domains? A. To increase exchange energy B. To reduce the large magnetostatic energy of a single domain C. To eliminate exchange interaction D. To raise saturation Answer: B. Multiple domains with closure domains close the flux inside. (Key point: Bulk iron domains) 3. Why does an ordinary iron piece show no external field? A. Each domain is unmagnetized B. Iron has no spontaneous magnetization C. Domain magnetizations point randomly and their vector sum is zero D. Domains are all parallel Answer: C. Each domain is saturated, but the sum cancels. (Key point: Net field) 4. How does an external field make a piece of iron a magnet? A. Atoms acquire new moments B. Walls disappear permanently in all materials C. Closure domains grow D. Favourable domains grow by wall motion and others rotate, aligning the domains Answer: D. Soft iron loses the alignment when the field is removed. 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