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Magnetic particles in a hard drive are nanoscale grains embedded in an engineered thin-film recording layer on each platter—not loose particles moving around inside the drive. Their size and magnetic properties affect how much data a platter can hold and how reliably a write head can record it. Heat-assisted magnetic recording (HAMR) addresses a key tradeoff by briefly heating the recording spot so a strong, stable medium can be switched.
What are magnetic particles in a hard drive?
An HDD stores data as magnetic patterns on coated platter surfaces. The recording layer is a carefully engineered film containing tiny magnetic grains; a write head changes the grains’ magnetic orientation to represent recorded information. The grains are part of the platter medium, not free-floating particles inside the drive.
FePt, an iron-platinum alloy, is one material studied for high-density recording because its magnetic properties can help very small grains remain stable. That does not mean every hard drive—or every drive sold today—uses FePt. The cited work concerns particular experimental media and research approaches.
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To fit more bits on a platter, manufacturers need smaller recording regions and grains packed more closely together. Closer spacing can increase magnetic interactions between neighboring grains, while shrinking a grain can make its recorded magnetic state more vulnerable to thermal disturbance. The challenge is to retain a stable bit without making it too difficult for the write head to change.
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The magnetic-recording trilemma
A 2013 technical article from Sigma-Aldrich/Merck describes the linked constraints: smaller grains support higher density, but can lose thermal stability; raising magnetic anisotropy helps resist unwanted changes, but also makes a grain harder to switch during writing. These competing needs are often described as the magnetic-recording trilemma.
In a specific FePt-C experimental film, the article reported a mean particle diameter of 6.1 nm, a size dispersion of 1.8 nm, and coercivity of 3.7 T. It also reported a static-head recording pattern equivalent to 550 Gbpsi. Those are historical laboratory results for that medium, not specifications or performance measurements for current retail HDDs. Sigma-Aldrich/Merck’s 2013 FePt nanogranular films article.
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How does HAMR use FePt grains?
HAMR stands for heat-assisted magnetic recording. In Seagate’s manufacturer explanation, a laser-equipped recording head briefly heats a small target area of the medium while writing. At the elevated temperature, the magnetic field can switch a highly stable medium that would otherwise be difficult to write; as the spot cools, the written magnetic state is retained. Seagate’s HAMR explainer describes this approach and the density-related challenge of magnetic grains affecting their neighbors.
The point of localized heating is to assist switching only where data is being written, rather than making the entire platter permanently less stable. HAMR is a recording technology, not a synonym for every HDD, and the sources cited here do not establish which specific consumer models use FePt or HAMR.
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What density figures have researchers reported?
NIMS stated in a March 2024 release that conventional perpendicular magnetic recording (PMR) HDDs were at around 1.5 Tbit/in² in its account, and that HAMR can sustain up to 10 Tbit/in². The latter is a stated technology capability, not a measurement of a retail drive. NIMS’s 2024 release.
| Approach or result | What the cited source says | How to interpret it |
|---|---|---|
| Conventional PMR | Around 1.5 Tbit/in², as stated by NIMS in 2024 | A figure in the release’s account of the technology, not a specification for every PMR drive |
| HAMR | Up to 10 Tbit/in², as stated by NIMS in 2024 | A technology capability statement, not a retail-drive density measurement |
| FePt-C experimental medium | Static-head pattern equivalent to 550 Gbpsi, reported by Sigma-Aldrich/Merck in 2013 | A historical laboratory result for the specific experimental medium |
| Multilevel 3D recording principle | Above 10 Tbit/in² described as possible under a multilevel recording principle by NIMS in 2024 | A research direction; not an existing drive product or established commercial capability |
These values use different units and describe different kinds of claims, so they should not be treated as a direct consumer-drive comparison. The NIMS release’s above-10-Tbit/in² figure is a possibility associated with a multilevel recording principle, not evidence that such a drive is available.
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What is 3D or multilevel magnetic recording?
In a March 2024 report, NIMS, Seagate Technology, and Tohoku University described FePt/Ru/FePt multilayer films as a route toward three-dimensional recording. Rather than relying only on a single recording layer, the concept explores stacking magnetic layers and recording multiple levels.
The reported demonstration of the 3D recording principle used simulations based on a model that mimicked fabricated media. NIMS said practical media would still require reducing FePt grain size, improving grain orientation and anisotropy, and stacking more layers. The result is therefore a proof of principle, not a commercially available platter technology.
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- Built-In Magnetic Structure: SSD external hard drive adopts magnetic suction technology for firm and stable adsorption. The SSD can be tightly fixed on compatible devices to effectively prevent accidental slipping, dropping and displacement during use and movement, ensures continuous and smooth data transmission, and greatly reduces the risk of loss and damage.
- Type C Cable Included: External solid state drives comes with a Type C cable, no need to purchase extra accessories. The cable stores directly on the back, so it stays with your drive wherever you go. The supporting cable realizes fast file transmission, supports plug-and-play operation, and is widely compatible with multiple devices. It allows you to freely transmit, back up and store massive photos, videos, documents and work data anytime and anywhere.
- 2000MB/s Transmission Speed: Equipped with high-speed transmission technology, this portable SSD delivers read speed up to 2000MB/s and write speed up to 1800MB/s, greatly saving your file transfer and backup time. It supports ultra-large file batch transmission, no lag or long waiting time for HD videos, large-capacity games and massive document data. Effectively improve your work efficiency and bring a smooth high-speed using experience.
- 4K ProRes HDR Ready：Solid state drive is your great partner for iPhone 15/16/17 Pro/Pro Max. External ssd supports 4K ProRes HDR video recording—just connect this USB - C external drive to your iPhone. Record videos directly onto the drive no extra transfer needed. It fully meets the professional creation needs of vloggers, photographers and video editors, being an essential field recording storage accessory.
What is spin-torque HAMR?
A February 2025 NIMS report described work with Seagate on a spin-torque HAMR structure, with an MnPt layer beneath an FePt recording layer. NIMS said: “This structure achieved approximately 35% improvement in recording efficiency compared to conventional HAMR.” This percentage applies to the reported structure and comparison, not to HDD capacity or a general improvement in retail-drive performance. The report also says applying the method to FePt nanogranular media remains future work. NIMS’s February 2025 report.
What hard drive platters are made of
For this topic, the important point is the platter’s magnetic recording medium: an engineered thin film containing magnetic grains. The cited sources explain grain-level materials and structures, but do not provide a universal bill of materials for HDD platters. Composition can depend on the particular drive and media design, so it would be inaccurate to describe all platters as FePt or to infer a specific consumer model’s recording medium from these research results.
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