Antiferromagnetic materials could represent the future of spintronic applications thanks to the numerous interesting features they combine: they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics and are capable of generating large magneto-transport effects Intense research efforts over the past decade have been invested in unraveling spin transport properties in antiferromagnetic materials Whether spin transport can be used to drive the antiferromagnetic order and how subsequent variations can be detected are some of the thrilling challenges currently being addressed Antiferromagnetic spintronics started out with studies on spin transfer, and has undergone a definite revival in the last few years with the publication of pioneering articles on the use of spin-orbit interactions in antiferromagnets This paradigm shift offers possibilities for radically new concepts for spin manipulation in electronics Central to these endeavors are the need for predictive models, relevant disruptive materials and new experimental designs This paper reviews the most prominent spintronic effects described based on theoretical and experimental analysis of antiferromagnetic materials It also details some of the remaining bottlenecks and suggests possible avenues for future research

Antiferromagnetic spintronics

Due to finite size effects, such as the high surface-to-volume ratio and different crystal structures, magnetic nanoparticles are found to exhibit interesting and considerably different magnetic properties than those found in their corresponding bulk materials. These nanoparticles can be synthesized in several ways (e.g., chemical and physical) with controllable sizes enabling their comparison to biological organisms from cells (10–100 μm), viruses, genes, down to proteins (3–50 nm). The optimization of the nanoparticles’ size, size distribution, agglomeration, coating, and shapes along with their unique magnetic properties prompted the application of nanoparticles of this type in diverse fields. Biomedicine is one of these fields where intensive research is currently being conducted. In this review, we will discuss the magnetic properties of nanoparticles which are directly related to their applications in biomedicine. We will focus mainly on surface effects and ferrite nanoparticles, and on one diagnostic application of magnetic nanoparticles as magnetic resonance imaging contrast agents.

https://www.mdpi.com/1422-0067/14/11/21266/pdf

Magnetic Nanoparticles: Surface Effects and Properties Related to Biomedicine Applications

Manipulating a stubborn magnet Spintronics is an alternative to conventional electronics, based on using the electron's spin rather than its charge. Spintronic devices, such as magnetic memory, have traditionally used ferromagnetic materials to encode the 1's and 0's of the binary code. A weakness of this approach—that strong magnetic fields can erase the encoded information—could be avoided by using antiferromagnets instead of ferromagnets. But manipulating the magnetic ordering of antiferromagnets is tricky. Now, Wadley et al. have found a way (see the Perspective by Marrows). Running currents along specific directions in the thin films of the antiferromagnetic compound CuMnAs reoriented the magnetic domains in the material. Science, this issue p. 587; see also p. 558 Transport and optical measurements are used to demonstrate the switching of domains in the antiferromagnetic compound CuMnAs. [Also see Perspective by Marrows] Antiferromagnets are hard to control by external magnetic fields because of the alternating directions of magnetic moments on individual atoms and the resulting zero net magnetization. However, relativistic quantum mechanics allows for generating current-induced internal fields whose sign alternates with the periodicity of the antiferromagnetic lattice. Using these fields, which couple strongly to the antiferromagnetic order, we demonstrate room-temperature electrical switching between stable configurations in antiferromagnetic CuMnAs thin-film devices by applied current with magnitudes of order 106 ampere per square centimeter. Electrical writing is combined in our solid-state memory with electrical readout and the stored magnetic state is insensitive to and produces no external magnetic field perturbations, which illustrates the unique merits of antiferromagnets for spintronics.

/pdf/electrical-switching-of-an-antiferromagnet-2bcb2cox7l.pdf

Electrical switching of an antiferromagnet

https://dr.ntu.edu.sg/bitstream/10356/146755/2/Published_Spintronics%20based%20random%20access_a%20review.pdf

Spintronics based random access memory: a review

Atomistic modelling of magnetic materials provides unprecedented detail about the underlying physical processes that govern their macroscopic properties, and allows the simulation of complex effects such as surface anisotropy, ultrafast laser-induced spin dynamics, exchange bias, and microstructural effects. Here we present the key methods used in atomistic spin models which are then applied to a range of magnetic problems. We detail the parallelization strategies used which enable the routine simulation of extended systems with full atomistic resolution.

/pdf/atomistic-spin-model-simulations-of-magnetic-nanomaterials-31j7wcva63.pdf

Atomistic spin model simulations of magnetic nanomaterials.

A new paradigm for exchange bias in polycrystalline thin films

We report on a theoretical framework for magnetic hyperthermia where the amount of heat generated by nanoparticles can be understood when both the physical and hydrodynamic size distributions are known accurately. The model is validated by studying the magnetic, colloidal and heating properties of magnetite/maghemite nanoparticles of different sizes dispersed in solvents of varying viscosity. We show that heating arising due to susceptibility losses can be neglected with hysteresis loss being the dominant mechanism. We show that it is crucial to measure the specific absorption rate of samples only when embedded in a solid matrix to avoid heating by stirring. However the data shows that distributions of both size and anisotropy must be included in theoretical models.

https://iopscience.iop.org/article/10.1088/0022-3727/46/31/312001/pdf

Mechanisms of hyperthermia in magnetic nanoparticles

A method for the measurement of the anisotropy constant of the antiferromagnet (AF) KAF in exchange biased systems has been developed. This has been achieved by measurement of the median blocking temperature ⟨TB⟩ of a CoFe∕IrMn bilayer. In thermal activation-free conditions, this is the temperature at which equal volumes of the AF are oriented in opposite senses. Hence, for a grain size dependent model, the critical volume for thermal activation at this point is equal to the median volume of the grain size distribution. A value of (5.5±0.5)×106erg∕cc has been obtained at room temperature for a 4nm thick IrMn layer.

Measurement of the anisotropy constant of antiferromagnets in metallic polycrystalline exchange biased systems

In this work we present a new interpretation of the role of the antiferromagnetic (AF) grain size in polycrystalline exchange bias thin films. It is found that at a finite temperature AF grains with sizes below a given critical volume VC do not contribute to the loop displacement because they are not thermally stable. There is a second critical volume VSET above which the AF grains cannot be set due to their anisotropy energy being too large. Therefore, only grains in the window between VC and VSET contribute to the loop displacement. Using this interpretation we can explain both the increase and the decrease in the exchange field with the AF grain size and the layer thickness.

Antiferromagnetic grain volume effects in metallic polycrystalline exchange bias systems

We report an enhancement of the average blocking temperature in IrMn/CoFe exchange bias systems due to an increase in the anisotropy constant of the IrMn. This is related to a (111) texture of IrMn parallel to the interface. Si /Seed (5 nm)/IrMn (10 nm)/CoFe (3 nm)/Ta (10 nm) were prepared by dc sputtering. Cu, Ru, and NiCr seed layers were used. X-ray diffraction studies involving both thetas/2thetas and grazing incidence scans revealed a strong (111) texture of the IrMn parallel to the interface for the samples with a NiCr seed layer. Cu and Ru seed layers did not cause a marked texture. Thermal activation measurements found an enhancement of the average blocking temperature from 367 K for the sample with Cu seed layer to 477 K for the sample with a NiCr seed. The anisotropy constant was found to increase from (2.8plusmn0.2) times 106 erg/cm3 for the sample with a Cu seed layer to (3.3 plusmn 0.4) times 107 erg/cm3 for the sample with a NiCr seed layer. We find that the increase in the anisotropy constant leads to an enhanced blocking temperature.

Gonzalo Vallejo-Fernandez

Papers

A new paradigm for exchange bias in polycrystalline thin films

Mechanisms of hyperthermia in magnetic nanoparticles

Measurement of the anisotropy constant of antiferromagnets in metallic polycrystalline exchange biased systems

Antiferromagnetic grain volume effects in metallic polycrystalline exchange bias systems

Texture Effects in IrMn/CoFe Exchange Bias Systems