Thermal stability of exchange bias systems based on MnN

At the present time there is a requirement to identify new antiferromagnetic alloys or compounds which might be suitable for the production of exchange bias systems. The phenomenon of exchange bias remains crucial for the operation of all read heads in hard disk drives and also has potential for use in magnetic random access memory (MRAM) systems. There is also an increasing interest in the use of antiferromagnets themselves in spintronic devices. Generally for applications the alloy IrMn is used, however given that Iridium is one of the rarest, and therefore most expensive elements on Earth, there is a search for alternative materials. In this paper we report on a study of the compound MnN in terms of its thermal stability. We have produced polycrystalline films of this compound with sub 10 nm grains and examined the thermal stability in layers of thicknesses of up to 30 nm. Using thermal activation studies we have determined a room temperature value of the anisotropy constant of this compound in a tetragonal structure of up to (6.3 ± 0.3) × 10 6 erg/cm 3 . The antiferromagnetic grains can be aligned by thermal annealing at an optimum temperature of 380 K. Above this temperature the magnetic properties deteriorate possibly due to nitrogen desorption

HAMR Media Based on Exchange Bias

In this work we describe an alternative strategy for the development of heat assisted magnetic recording (HAMR) 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 (0.8nm) layer of Co deposited beneath the IrMn layer. In this system the information is in effect stored in the antiferromagnetic (AF) 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

Measurement of the distribution of anisotropy constants in magnetic nanoparticles for hyperthermia applications

In this work we have applied theoretical calculations to new experimental measurements of the effect of the anisotropy distribution in magnetite nanoparticles which in turn controls hysteresis heating for hyperthermia applications. Good agreement between theory and experiment is reported where the theoretical calculation is based upon the detailed measurement of the particle elongation generally observed in the nanoparticles. The elongation has been measured from studies via transmission electron microscopy (TEM). We find that particle elongation is responsible for the anisotropy dispersion which can be obtained by analysis and fitting to a measurement of the temperature decay of remanence. A median value of the anisotropy constant of 1.5x105erg/cc was obtained. A very wide distribution of anisotropy constants is present with a Gaussian standard deviation of 1.5x105erg/cc. From our measurements, deviations in the value of the saturation magnetisation from particle to particle are most likely the main factor giving rise to this large distribution with 33% arising from the error in the measured elongation. The lower limit to the anisotropy constant of the nanoparticles is determined by the magnetocrystalline anisotropy of the material, 1.1x105erg/cc for magnetite which was studied in this work

A Comparative Measurement Technique for Magnetic Hyperthermia in Nanoparticle Suspensions

We describe a method for the determination of the heating power of magnetic nanoparticle colloids which have potential for application in the remedial treatment of malignant and non-malignant tumors. The method is based upon a comparison between the heating power observed when the colloid is exposed to a radio frequency magnetic field and that which is observed using a resistive electrical heater. A new design of measurement cell has been made which has the advantages of reducing or eliminating the effects of convection, ensuring the measurement is made in a magnetic field of known uniformity and that the heat losses in the system are constant and minimized under both magnetic and Joule heating

Magnetic Braille Using Ferrofluids

Braille provides an invaluable tactile reading system for the visually impaired. However, current braille keyboards and technology are external mechanical devices limited by their large form factor, high expense and long refresh rates. A magnetic touchpad in which braille dots are formed through a ferrofluid medium is a potential device that could promise to replace current braille technology by providing higher refresh rates, lower cost and give easy integration into current devices. In this report, work is shown towards developing a proof of concept magnetic braille touch pad, wherein a braille dot is formed using a ferrofluid by a controlled magnetic field produced by a small scale electromagnet. Attempted optimisation of braille dot formation is also undertaken, varying ferrofluid properties and electromagnet architecture to form accurate braille dots. Results show that magnetic braille touch pads are realistic devices that can be built and that there are clearly extensive opportunities for further research. In the future magnetic braille touch pads could be fully implemented into information technology devices for use by the visually impaired

Growth and Crystallisation of Ferromagnetic and Antiferromagnetic Fe2+xVyAl Heusler Alloy Films

We investigated growth, annealing conditions and magnetic properties of the Heusler alloy Fe2+xVyAl by means of X-ray diffraction, magnetic hysteresis and exchange-bias measurements. Ferromagnetic Heusler alloy films were obtained by sputtering Fe2VAl and Fe3VAl targets and performing post-growth annealing. The characteristic (220) Heusler alloy peaks were seen in the X-ray diffraction measurements and corresponding ferromagnetic behaviours were observed. In addition, antiferromagnetic Heusler alloy films were deposited by employing Al pegs on Fe3VAl sputtering targets. The deposited films had elemental ratios close to the predicted Fe2.5V0.5Al phase, and a 16 Oe exchange-bias was measured in a Fe2.5V0.5Al/Co60Fe40 system at 100 K

Risk and protective genetic variants in suicidal behaviour: association with SLC1A2, SLC1A3, 5-HTR1B &NTRK2 polymorphisms

<p>Abstract</p> <p>Background</p> <p>Suicidal behaviour is known to aggregate in families. Patients with psychiatric disorders are at higher risk for suicide attempts (SA), however protective and risk genetic variants for suicide appear to be independent of underlying psychiatric disorders. Here we investigate genetic variants in genes important for neurobiological pathways linked to suicidal behaviour and/or associated endophenotypes, for association with SA among patients with co-existing psychiatric illness. Selected gene-gene and gene-environment interactions were also tested.</p> <p>Methods</p> <p>DNA was obtained from bloods of 159 patients (76 suicide attempters and 83 non-attempters), who were profiled for DSM-IV Axis I psychiatric diagnosis. Twenty-eight single nucleotide polymorphisms (SNPs) from 18 candidate genes (<it>COMT, 5-HT2A, 5-HT1A, 5-HTR1B, TPH1, MAO-A, TPH2, DBH, CNR1, BDNF, ABCG1, GABRA5, GABRG2, GABRB2, SLC1A2, SLC1A3, NTRK2, CRHR1</it>) were genotyped. Genotyping was performed by KBioscience. Tests of association between genetic variants and SA were conducted using Chi squared and Armitage Trend tests. Binary logistical regression analyses were performed to evaluate the contribution of individual genetic variants to the prediction of SA, and to examine SNPs for potential gene-gene and gene-environment interactions.</p> <p>Results</p> <p>Our analysis identified 4 SNPs (rs4755404, rs2269272, rs6296 and rs1659400), which showed evidence of association with SA compared to a non-attempter control group. We provide evidence of a 3-locus gene-gene interaction, and a putative gene-environment interaction, whereby genetic variation at the <it>NTRK2 </it>locus may moderate the risk associated with history of childhood abuse.</p> <p>Conclusion</p> <p>Preliminary findings suggest that allelic variability in <it>SLC1A2/3, 5-HTR1B </it>and <it>NTRK2 </it>may be relevant to the underlying diathesis for suicidal acts.</p

Development of Antiferromagnetic Heusler Alloys for the Replacement of Iridium as a Critically Raw Material

As a platinum group metal, iridium (Ir) is the scarcest element on the earth but it has been widely used as an antiferromagnetic layer in magnetic recording, crucibles and spark plugs due to its high melting point. In magnetic recording, antiferromagnetic layers have been used to pin its neighbouring ferromagnetic layer in a spin-valve read head in a hard disk drive for example. Recently, antiferromagnetic layers have also been found to induce a spin-polarised electrical current. In these devices, the most commonly used antiferromagnet is an Ir-Mn alloy because of its corrosion resistance and the reliable magnetic pinning of adjacent ferromagnetic layers. It is therefore crucial to explore new antiferromagnetic materials without critical raw materials. In this review, recent research on new antiferromagnetic Heusler compounds and their exchange interactions along the plane normal is discussed. These new antiferromagnets are characterised by very sensitive magnetic and electrical measurement techniques recently developed to determine their characteristic temperatures together with atomic structural analysis. Mn-based alloys are found to be most promising based on their robustness against atomic disordering and large pinning strength up to 1.4 kOe, which is comparable with that for Ir-Mn. The search for new antiferromagnetic films and their characterisation are useful for further miniaturisation and development of spintronic devices in a sustainable manner

Distributions of easy axes and reversal processes in patterned MRAM arrays

The distribution of the easy-axes in an array of MRAM cells is a vital parameter to understand the switching and characteristics of the devices. By measuring the coercivity as a function of applied-field angle, and remaining close to the perpendicular orientation, a classic Stoner-Wohlfarth approximation has been applied to the resulting variation to determine the standard deviation, sigma , of a Gaussian distribution of the orientation of the easy-magnetisation directions. In this work we have compared MRAM arrays with nominal cells sizes of 20 nm and 60 nm and a range of free layer thicknesses. We have found that a smaller diameter cell will have a wider switching-field distribution with a standard deviation sigma = 9.5 deg. The MRAM arrays consist of pillars produced by etching a multilayer thin film. This value of is dominated by pillar uniformity and edge effects controlling the reversal, reinforcing the need for ever-improving etch processes. This is compared to larger pillars, with distributions as low as 5.5 deg . Furthermore we found that the distribution broadens from 5.5 to 8.5 deg with free layer thickness in larger pillars and that thinner films had a more uniform easy-axis orientation. For the 20 nm pillars the non-uniform size distribution of the pillars, with a large and unknown error in the free-layer volume, was highlighted as it was found that the activation volume for the reversal of the free layer 930 nm3 was larger than the nominal physical volume of the free layer. However for the 60 nm pillars, the activation volume was measured to be equal to one fifth of their physical volume. This implies that the smaller pillars effectively reverse as one entity while the larger pillars reverse via an incoherent mechanism of nucleation and propagation

Anisotropy in antiferromagnets

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 (TN) but, to have a stable AF configuration, it is necessary that the material have 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 is 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 the 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