Kevin O'GRady

In this work we describe a new approach to magnetic hyperthermia based on hysteresis alone. This is achieved by encapsulating magnetite nanoparticles in polymer spheres to remove dynamic effects which cause unpredictable and uncontrollable heating. This also prevents aggregation of the nanoparticles. To generate significant hysteresis and hence heat, we have used nanoparticles grown by the standard coprecipitation process which are used as seeds and grown to a larger size by the addition of further Fe²⁺ salts in an alkaline solution and heating. We define this as a Controlled Growth Process (CGP). The CGP particles are found to have a much higher saturation magnetisation than particles produced by coprecipitation enhancing their magnetic shape anisotropy. Via magnetic measurements and complementary studies of the heat generated when the polymer spheres are exposed to an ac field, we show that the heat generated is due to hysteresis losses with potentially residual contributions from dynamic effects. At 111 kHz and a maximum applied field of 180 Oe, a specific absorption rate (SAR) value of ∼35 W/gFe was determined from calorimetry measurements. This value agrees within error with the losses calculated from the hysteresis loop. When the same particles are not encapsulated, significant viscous heating is present.

Recent rapid advancement in antiferromagnetic spintronics paves a new path for efficient computing with THz operation. To date, major studies have been performed with conventional metallic, e.g., Ir-Mn and Pt-Mn, and semiconducting, e.g., CuMnAs, antiferromagnets, which may suffer from their elemental criticality and high resistivity. In order to resolve these obstacles, new antiferromagnetic films are under intense development for device operation above room temperature. Here, we report the structural and magnetic properties of an antiferromagnetic Ni₂MnAl Heusler alloy with and without Fe and Co doping in thin film form, which has significant potential for device applications.

We report on an experimental study on an observed asymmetry in the magnetization of a sputtered polycrystalline Co–Fe/Ir–Mn exchange bias system. Such asymmetries have been predicted by numerical modeling but have never been measured in detail. In particular, we observe the magnetization asymmetry to increase with decreasing temperature down to 5.5 K. Given that the bulk of the antiferromagnetic grains is known to be stable below 300 K, this indicates that the asymmetry derives from interface spin effects at the ferromagnetic/antiferromagnetic interface. As the temperature is reduced, a peak in magnetization asymmetry is observed below 10 K. We also observed that when measured at low temperature there is an increase in the hysteresis loop coercivity and the well-known training effect on the loop shift in exchange bias systems. These effects both derive from the ferromagnetic/antiferromagnetic interface.

Due to the advent of antiferromagnetic (AF) spintronics, there is a burgeoning interest in AF materials for a wide range of potential and actual applications. Generally, AFs are characterized via the ordering at the Néel temperature (T_N), but to have a stable AF configuration, it is necessary that the material has a sufficient level of anisotropy so as to maintain the orientation of the given magnetic state fixed in one direction. Unlike the case for ferromagnets, there are little established data on the anisotropy of AFs and, in particular, its origins, other than it being magneto-crystalline, and those factors which control it. In this perspective article, these factors are reviewed in light of recent and established experimental data. The anisotropy can be found indirectly via the exchange bias phenomenon. This technique is reviewed and, in particular, the implications for the nature of the anisotropy that is measured and its distribution. Finally, a strategy is proposed that would allow for the development of AF materials with controlled anisotropy for future applications.

For solid-state magnetic memories and other potential devices based on giant magnetoresistance and tunneling magnetoresistance effects, it is invariably required that one ferromagnetic layer remains with its magnetization pinned in a given direction. The other ferromagnetic layer is then free to rotate creating the magnetoresistive effect. The fixing or pinning of one of the ferromagnetic layers can be achieved by using a material with a high coercivity or significantly higher anisotropy than the other layer. In conventional devices with in-plane anisotropy, this is achieved by the use of the exchange bias phenomenon where the pinned layer is coupled to an antiferromagnetic layer typically consisting of IrMn. In this paper, we consider strategies to achieve the exchange bias phenomena in materials with strong perpendicular orientation to the magnetization and the potential for achieving the required exchange bias. We report on successfully achieving this objective in a (Co/Pt)ₙ system. We have achieved values of loop shift approaching 700 Oe at room temperature and blocking temperatures in excess of 500 K.

In this work, we describe an alternative strategy for the development of heat assisted magnetic recording media. In our approach, the need for a storage material with a temperature dependent anisotropy and to provide a read out signal is separated so that each function can be optimised independently. This is achieved by the use of an exchange bias structure where a conventional CoCrPt-SiO2 recording layer is exchange biased to an underlayer of IrMn such that heating and cooling in the exchange field from the recording layer results in a shifted loop. This strategy requires the reorientation of the IrMn layer to allow coupling to the recording layer. This has been achieved by the use of an ultrathin (0.8 nm) layer of Co deposited beneath the IrMn layer. In this system, the information is in effect stored in the antiferromagnetic layer, and hence, there is no demagnetising field generated by the stored bits. A loop shift of 688 Oe has been achieved where both values of coercivity lie to one side of the origin and the information cannot be erased by a magnetic field.

The development of racetrack memory has been limited by the inability to produce domain wall pinning sites in the nanowires which pin domain walls reproducibly. Several techniques have been proposed to achieve the required domain wall pinning including the cutting of notches and the use of steps in ferromagnetic wires. These have had some limited success but are expensive to fabricate.

The pinning of domain walls in ferromagnetic (F) wires is one possible technique for the creation of a solid state magnetic memory. Such a system has been under consideration for some time but one of the main limitations is the control of, and non-uniformity of the domain wall pinning. Techniques such as the lithographic definition of notches and steps in the substrate have had some success in creating local pins but have the disadvantage of being expensive to fabricate and the reproducibility of the domain wall pinning strength is limited. In this letter, we report on an alternative strategy to create pins of reproducible strength using crossed ferromagnetic and antiferromagnetic (AF) wires such that exchange bias can be introduced at the crossing points. Such a system has the advantage of ease of fabrication and creating domain wall pins of controlled strength by varying the width of the AF wire. We have achieved domain wall pinning field strengths of up to 37 Oe in a system where the AF wire is deposited above the F wire which is comparable to the values achieved using notches.