Fridge magnets and compasses. These are two of the most well-known applications of magnetism. Some people may also be familiar with magnetic stripe cards and cassette tapes. However, many other exciting applications of magnetism exist that are often overlooked in high-school physics classes. Today, I will consider the example of magnetic recording and spintronics to create more awareness about the fascinating applications of magnetism. This discussion aims to motivate educators and students to place more focus on this topic and, more generally, the field of materials physics.
Note that this is a (slightly edited) reprint of an article that I wrote for the IEEE Magnetics Newsletter. It was published in the December 2023 issue and can be found under this link.
The introduction of magnetic recording into the commercial market can be traced back to the year 1956, when IBM rolled out the first commercial computer equipped with a magnetic hard disk drive (HDD), the IBM 305 RAMAC. Weighing over a ton, it consisted of fifty 24 inch (about 60 cm) disks and offered a storage capacity of about 5 megabytes of data. This implied an areal density of about 1 kilobit per square inch, which would result in a cost of 10 million dollars to store a mere gigabyte of data. In the following decades, magnetic recording underwent remarkable evolution. Presently, state-of-the-art HDDs (e.g., Ironwolf Pro by Seagate and Western Digital WD Gold) boast a capacity of about 20 TB and an areal density of one terabit per square inch, approximately a billion times higher than in 1956. Consequently, the cost of storing a gigabyte has plummeted to only US$0.02. Considering these substantial improvements, one may wonder what the key developments were that have shaped magnetic recording technology.
First, we need to understand that a magnetic HDD is a nonvolatile storage device that consists of glass- or aluminum-based circular platters that are coated with a thin layer of a magnetic material. The surface is divided into nanoscale regions, each representing a binary 0 or 1 (bits) encoded by the local magnetic orientation. These disks spin at high speeds, facilitating a read/write head that is mounted on an arm to access and determine the magnetic orientation (“read") or alter the configuration of a magnetic region (“write”). While the fundamental idea has remained the same since the IBM RAMAC era, the areal density has increased dramatically by scaling down the individual HDD components (e.g., by reducing the size of magnetic bits and the spacial extent of the read/write head).
This was accompanied by major conceptual improvements such as the transition from purely inductive reading (i.e., using a magnetic coil to pick up the orientation of the individual bits) to magnetoresistive reading, whereby changes in the magnetic field are detected through electrical resistance changes within a thin film that is part of the read head. A more mature version of these sensing devices is based on the so-called Giant Magnetoresistance (GMR) effect, discovered by Albert Fert and Peter Grünberg (Nobel Prize 2007). GMR occurs in multilayer stacks composed of ferromagnetic and nonmagnetic materials, leading to a more sensitive and accurate readout of magnetic bits. Further significant developments include perpendicular writing and more recently heat-assisted magnetic recording (HAMR). Perpendicular recording is a technology that arranges magnetic bits on a storage medium perpendicular to the surface, as opposed to the traditional horizontal orientation. This vertical alignment results in a reduced interference between neighboring bits, thereby allowing for higher storage densities; see Fig. 1.
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Figure 1. Longitudinal (top) vs. perpendicular (bottom) magnetic recording. Also shown is the inductive write head, which requires a different geometric design for both cases. Reproduced from https://en.wikipedia.org/wiki/Magnetic_storage under a Public Domain license. |
As of December 2023, the above-mentioned HAMR is currently being integrated into HDD mass production by Seagate, Western Digital, and potentially other competitors. The underlying idea is to use a laser that heats a small area of the disk surface prior to changing the magnetic orientation of the corresponding bit; see Fig. 2(a). As shown in Fig. 2(b), this very localized heating makes it easier to switch the magnetization (owing to a reduction of the coercivity) and thereby allows for more closely packed magnetic bits. Once the write head has flipped the magnetic orientation, the heated area cools down quickly, and the material retains the new magnetization state. The design is such that at room temperature no switching can occur. HAMR has been developed in order to counteract the effects of superparamagnetism, which has become increasingly relevant in the past years due to the shrinking of the magnetic bit size. Below a critical size, thermal fluctuations can cause the magnetization to randomly flip, leading to data loss in the HDD. Therefore, HAMR is making use of materials with magnetic regions that can only be switched at elevated temperatures.
Further intriguing topics include the integration of magnetic recording with artificial intelligence and machine learning, presenting a fascinating landscape for advanced algorithms in data retrieval and processing. The synergy of magnetic storage with neuromorphic (brain-inspired) computing holds the potential to reshape computing architectures. Additionally, the emergence of probabilistic computing introduces new dimensions and may provide an exciting middle ground between conventional and quantum computing. Here, so-called p-bits take on the binary values 0 and 1 with controlled probabilities and can be used for randomized algorithms. It has been proposed, for example, to realize stochastic p-bits by using magnetic tunnel junctions.
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Figure 2. Heat-assisted magnetic recording (HAMR). (a) Fundamental principle of HAMR, whereby a laser locally heats the magnetic medium through the excitation of surface plasmons in a near-field transducer (NFT). Note that the read sensor depicted here is based on the GMR effect. (b) Coercive field of the magnetic material is reduced through heating, which then allows the magnetic orientation to be switched (“write”) before the cooling process is completed. Reproduced under a CC BY-NC-ND 4.0 license from: Hsu and Victora, J. Magn. Magn. Mater. 563, 169973 (2022). |
Applications beyond traditional hard disk drives, such as three-dimensional magnetic architectures, spin waves-based logic, racetrack memories, and other novel concepts, may play an essential role in the technology's evolving trajectory in the field of data storage and information technology, but also for other applications such as magnetic sensing. Figure 3 contains numerous examples of three-dimensional nanomagnets that can be fabricated using two-photon direct-write optical lithography, chemical synthesis techniques, or focused electron beam induced deposition (FEBID). All of the above-mentioned ideas are pursued in the research field termed “spintronics.” In this field, unlike in conventional electronics, which relies solely on electron charge, the spin of an electron presents an additional degree of freedom and offers potential advantages such as increased energy efficiency and novel functionalities in devices.
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Figure 3. Different geometries of (a) three-dimensional and (b) two-dimensional nanomagnets, including a magnetic sphere, Möbius strip, cylindrical nanowire, and an antiferromagnetic superlattice. Reproduced under a CC BY 4.0 license from: Fernández-Pacheco et al., Nature Communications 8, 15756 (2017). |
In conclusion, our exploration of the development of magnetic recording has unveiled a rich history and transformative advancements in the underlying technology. From the humble beginnings of the IBM RAMAC in 1956 to the present state-of-the-art HDDs with 20 TB capacity, the evolution has been remarkable, significantly reducing the cost of storing a gigabyte to a mere $0.02. Key technological milestones, including the transition to magnetoresistive reading and the adoption of perpendicular writing and heat-assisted magnetic recording (HAMR), have paved the way for higher storage densities. As HAMR is actively being integrated into mass production, the field continues to evolve. Exploring innovative concepts like probabilistic computing and three-dimensional storage architectures further underscores the dynamic trajectory of data storage and information technology.
Should you have any comments, questions, or suggestions, please reach out to us by writing an email to lonskymartin@gmail.com. If you are interested in learning more about some of the above-mentioned topics, we provide a list of recommended articles in the following.
Recommended reading
• HAMR: Hsu and Victora, J. Magn. Magn. Mater. 563, 169973 (2022)
• 3D nanomagnetism: Fernández-Pacheco et al., Nature Communications 8, 15756 (2017)
• Spintronics: Hirohata et al., J. Magn. Magn. Mater. 509, 166711 (2020)
• Probabilistic computing: Chowdhury et al., IEEE Journal on Exploratory Solid-State Computational Devices and Circuits 9, 1 (2023)
• Spin waves-based logic: Mahmoud et al., J. Appl. Phys. 128, 161101 (2020)
• Neuromorphic computing: Grollier et al., Nature Electronics 3, 360 (2020)
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