The role of interfaces in CoFe/IrMn exchange biased systems

A trilayer system consisting of an IrMn layer exchanged coupled to two CoFe layers of equal thickness has been studied. A single stage reversal was observed over a wide range of temperatures. Two bilayers with the same thicknesses of the pinning layer but different ferromagnetic thicknesses were also studied. By comparing the magnetic properties of these three stacks the effect of the interfacial area on the exchange field and the coercivity has been determined. We find that the interfacial area has a very minor effect on the exchange field H-ex and the blocking temperature (T-B) but causes a doubling of the coercivity (H-c). This indicates that H-c is dominated by the interface whereas the exchange bias is controlled by volumetric effects

Grain size effects in polycrystalline exchange biased systems

The capability of a novel sputtering technology known as HiTUS to produce thin films with controlled grain size and distribution has been evaluated. This has been achieved for different materials chosen due to their importance in industrial applications. The grain size of the deposited films was measured from TEM images in bright field mode. All the distributions were lognormal and over five hundred particles were measured to recompose each lognormal function. For a range of materials close control of grain size between 4nm and 30nm was achieved. The grain size was controlled through the deposition rate. This has allowed the investigation of the effect of the antiferromagnetic grain size on exchange bias without the use of underlayers, additives or substrate heating. It was found that the magnitude of the loop shift strongly correlated to the distribution of grain volumes in the antiferromagnetic layer. Using detailed measurements of the grain size distribution the features of exchange bias in these systems can be explained on the basis of a grain volume model with coherent rotation. For example using measurement protocols that ensure reproducibility of data has allowed the development of a new measurement technique for the anisotropy constant of metallic antiferromagnets. This is a key result since the anisotropy is used in all theoretical models of exchange bias as a fitting parameter. The anisotropy was determined only from experimental features and only its temperature dependence had to be assumed to be of the form K(OK)(1-T/TN)3 in order to calculate the values. For IrMn K(293K) =4.14xlO6erg/cc and for FeMn K(293K) = 1.36x106er

Magnetic and structural properties of antiferromagnetic Mn2VSi alloy films grown at elevated temperatures

80 nm thick polycrystalline Mn2VSi films have been deposited on silicon substrates with an 18 nm silver seed layer and a 3 nm aluminium capping layer using a sputtering system. The best quality film is obtained for 723 K growth. The Mn2VSi thin film is verified to be antiferromagnetic, where an exchange bias is found when a 3 nm ferromagnetic CoFe layer has been deposited on the top of the Mn2VSi layer. The exchange bias is measured to be 34 Oe at 100 K. The blocking and thermal activation temperature (TACT) of Mn2VSi is estimated to be below 100 K and within a range between 100 K and 448 K, respectively. These properties can be improved by substituting the constituent atoms with the other elements (e.g., Co and Al), suggesting a potential of Mn2VSi to be used as an antiferromagnet in a spintronic device

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

Effect of the distribution of anisotropy constants on the magnetic properties of iron oxide nanoparticles

The distribution of shape anisotropy constants in two colloids of iron oxide nanoparticles has been measured from the distribution of particle elongations within each system. The results are in good agreement with the values calculated from a temperature decay of remanence measurement. For a fluid with a saturation magnetisation of 420 emu/cc and an average particle elongation of ~1.3, the distribution of energy barriers is controlled by both the distribution of particle sizes and particle elongations. For a fluid with a saturation magnetisation of 320 emu/cc and a wide distribution of particle sizes, the energy barrier to reversal can be assumed to be controlled by the distribution of particle volumes. These results highlight the need to take into account the distribution of anisotropy constants when making predictions of the heating properties of assemblies of magnetic nanoparticles for hyperthermia applications

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

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

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