Magnetic orientation in advanced recording media

Chureemart, J.; Chureemart, P.; Evans, Richard F. L.; Chantrell, R.W.; Grady, K. O’

doi:10.1088/0022-3727/44/45/455002

Cited by 10 publications

(14 citation statements)

References 11 publications

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“…The hcp-to-bcc (or α-to-β) transition in Ti, Zr and Hf is a martensitic phase transformation that has been thoroughly investigated both from an experimental [2,4] and a theoretical [5][6][7][8] perspective. Recently Hennig et al [5] developed and used a classical potential of the modified embedded atom method (MEAM) [9] to accurately reproduce the phase boundary between the hcp and bcc structure in Ti.…”

mentioning

confidence: 99%

Temperature-driven α-to-β phase transformation in Ti, Zr and Hf from first-principles theory combined with lattice dynamics

Souvatzis

Arapan²,

Eriksson³

et al. 2011

EPL

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Lattice dynamical methods used to predict phase transformations in crystals typically deal with harmonic phonon spectra and are therefore not applicable in important situations where one of the competing crystal structures is unstable in the harmonic approximation, such as the bcc structure involved in the hcp to bcc martensitic phase transformation in Ti, Zr and Hf. Here we present an expression for the free energy that does not suffer from such shortcomings, and we show by self consistent ab initio lattice dynamical calculations (SCAILD), that the critical temperature for the hcp to bcc phase transformation in Ti, Zr and Hf, can be effectively calculated from the free energy difference between the two phases. This opens up the possibility to study quantitatively, from first principles theory, temperature induced phase transitions.Martensitic phase transformations are common, both in alloys frequently used in industry, such as shape memory alloys [1], and in the the elemental group 3 to 4 transition metals [2], not to mention martensitic transformation in iron and iron-based alloys, a crucial phenomenon for metallurgy [3]. Thus there exists a substantial interest both from an industrial, applied and an academic point of view to develop accurate and effective methods to understand and even predict martensitic phasetransformations.The hcp to bcc (or α to β) transition in Ti, Zr and Hf is a martensitic phase transformation that has been thoroughly investigated both from an experimental [2,4] and theoretical [5][6][7][8] perspective. Recently Hennings et al developed and used a classical potential of the modified embedded atom method (MEAM) [9] to accurately reproduce the phase boundary between the hcp and bcc structure in Ti. However, there is up to this date a lack of first principles theoretical studies made of the martensitic hcp to bcc phase-transformation in Ti, Zr and Hf. The problem is that anharmonic effects in lattice dynamics [10] are of crucial importance for finite-temperature structural phase transitions, and their quantitative firstprinciple treatment is a real challenge.A straightforward calculation using DFT molecular dynamics (DFT-MD) [11] should in principle be able to reproduce the bcc to hcp phase transformation in Ti and similar materials, since DFT-MD implicitly include anharmonic effects. However, DFT-MD is a computationally very demanding task which makes its use problematic. Instead we will here exploit the method of self consistent ab initio lattice dynamical calculations (SCAILD) [12]. Here, we further develop this method in order to be able to calculate thermodynamic properties, such as structural free energy difference (before we were restricted by the calculations of temperature-dependent phonon frequencies only [12][13][14][15][16]). Since the SCAILD scheme is a constrained sampling method, in that it only samples the lattice dynamical phase-space along the normal mode directions of commensurate phonons [12,13], the SCAILD calculations are much faster and, thus, much more practical t...

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confidence: 99%

Temperature-driven α-to-β phase transformation in Ti, Zr and Hf from first-principles theory combined with lattice dynamics

Souvatzis

Arapan²,

Eriksson³

et al. 2011

EPL

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“…In Park et al [15], they studied the effect of roughness on the electrical performance of MTJs; however, they did not account for magnetic properties. In Chureemart et al [21], they performed an atomistic investigation of the magnetic properties of MTJ (e.g. damping constant); however, roughness was not taken into account [21].…”

Section: Introductionmentioning

confidence: 99%

“…damping constant); however, roughness was not taken into account [21]. In the current study, we established a new approach to the atomistic simulation of MTJs, which accounts for the effects of roughness [22] as well as the damping constant [21] when addressing switching performances in STT-MRAM.…”

Section: Introductionmentioning

confidence: 99%

Interface imperfection effects on spin transfer torque switching: an atomistic approach

Ramesh¹,

Cheng²,

Ku³

et al. 2022

J. Phys. D: Appl. Phys.

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The further commercialization of spintronic memory devices depends on the development of methods by which to assess performance. This paper presents an approach to the atomistic investigation of switching performance in spin transfer torque (STT) magneto-resistive random access memory (MRAM) devices with the use of interface imperfection model. Switching simulation in the nanosecond regime was made possible under this model, and we first time demonstrate that switching time is inversely proportional to interface imperfection (i.e. roughness). In investigating the damping of CoFeB/MgO films, we analyzed the effective damping constant , which cannot be accurately predicted for ferromagnetic layers of less than 2 nm using existing micromagnetic models. The proposed model includes a roughness parameter, which has nearly no effect on the effective damping constant in films of >2nm, but a profound effect in films of <2nm, reaching a 27% decrease in a 1.0 nm CoFeB film. Our finding is supported by the experimental data of classic references. We expect that these results will prove valuable in magnetic simulation and research on MRAM with ultrathin films.

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“…Recently, the angular variation of coercivity technique [17][18][19] was proposed to measure the easy axis distribution of PRM via comparison with calculations based on the Stoner-Wohlfarth model 20 which is performed under the assumption that each grain reverses via coherent rotation. This is a promising technique due to the fact that the complicating effects of the demagnetizing field vanish at the coercivity.…”

Section: Introductionmentioning

confidence: 99%

Model of advanced recording media: The angular dependence of the coercivity including the effect of exchange interaction

Chureemart

Chantrell

2016

Journal of Applied Physics

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We use a micromagnetic model based on the kinetic Monte Carlo (kMC) approach to investigate theoretically the magnetic properties of advanced recording media. The model is employed to examine the impact of the magnetostatic and exchange interaction between grains of realistic perpendicular recording media on the angular-dependent coercivity since the exchange field between grains is an important factor in recording performance. The micromagnetic model allows to take the easy axis distribution and the exchange interaction between grains into account. The results confirm the importance of exchange interaction since the variation of coercivity with angle between the applied field and the orientation of easy axis which is perpendicular to the film plane, (θ) is seen to broaden with decreasing exchange field. We show that a two-stage fitting procedure involving the separate determination of the exchange field and easy axis dispersion provides a useful tool for the characterization of media for perpendicular recording and heat assisted recording. We find excellent agreement between previous experimental results and the simulations including exchange interactions leading to estimates of the exchange coupling and easy axis dispersion.

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Magnetic orientation in advanced recording media

Cited by 10 publications

References 11 publications

Temperature-driven α-to-β phase transformation in Ti, Zr and Hf from first-principles theory combined with lattice dynamics

Temperature-driven α-to-β phase transformation in Ti, Zr and Hf from first-principles theory combined with lattice dynamics

Interface imperfection effects on spin transfer torque switching: an atomistic approach

Model of advanced recording media: The angular dependence of the coercivity including the effect of exchange interaction

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