Publications by Roger Ward

Controlling spin-dependent tunneling by bandgap tuning in epitaxial rocksalt MgZnO films

Scientific Reports Nature Publishing Group 4 (2014) 1-8

D Li, S-G Wang, R Ward, T Hesjedal, X-G Zhang, A Kohn, E Amsellem, G Yang, J Liu, J Jiang, H-X Wei, X Han

Widespread application of magnetic tunnel junctions (MTJs) for information storage has so far been limited by the complicated interplay between tunnel magnetoresistance (TMR) ratio and the product of resistance and junction area (RA). An intricate connection exists between TMR ratio, $RA$ value and the bandgap and crystal structure of the barrier, a connection that must be unravelled to optimise device performance and enable further applications to be developed. Here, we demonstrate a novel method to tailor the bandgap of an ultrathin, epitaxial Zn-doped MgO tunnel barrier with rocksalt structure. This structure is attractive due to its good Δ_1 spin filtering effect, and we show that MTJs based on tunable MgZnO barriers allow effective balancing of TMR ratio and $RA$ value. In this way spin-dependent transport properties can be controlled, a key challenge for the development of spintronic devices.

A simple approach for characterization of spring-type superlattice magnets using magneto-optical Kerr effect

MATERIALS LETTERS 116 (2014) 143-145

K Wang, RCC Ward, PAJ de Groot

In-plane uniaxial magnetic anisotropy induced by anisotropic strain relaxation in high lattice-mismatched Dy/Sc superlattices


L Benito, C Ballesteros, RCC Ward

Ultrafast optical parametric pumping of magnetization reorientation and precessional dynamics in DyFe2/YFe2 exchange springs.

Physical review letters 113 (2014) 067601-

LR Shelford, Y Liu, U Al-Jarah, PAJ de Groot, GJ Bowden, RCC Ward, RJ Hicken

The magnetization dynamics of a wound [DyFe(2)(20  Å)/YFe(2)(80  Å)](×40) exchange spring multilayer have been explored in optical pump probe experiments. Ultrafast optical heating was used to modify the magnetic parameters of the multilayer, while the time resolved magneto-optical Kerr effect was used to probe its response. Although the probe signal is dominated by precession and winding of the exchange spring within the soft YFe(2) layer, reorientation of the DyFe(2) hard-layer magnetization is detected on time scales less than 100 ps. Micromagnetic simulations reproduce the main features of the experimental data and indicate a dramatic optically induced reduction of the hard-layer anisotropy. The results establish the feasibility of switching a spring system by means of parametric excitation.

Direct evidence of the anisotropy of magnetization in rare-earth metals and rare-earth/Fe-2 alloys

PHYSICAL REVIEW B 90 (2014) ARTN 054407

L Benito, K Dumesnil, RCC Ward

Malleability of uranium: Manipulating the charge-density wave in epitaxial films

PHYSICAL REVIEW B 89 (2014) ARTN 245101

R Springell, RCC Ward, J Bouchet, J Chivall, D Wermeille, PS Normile, S Langridge, SW Zochowski, GH Lander

Positive magnetoresistance induced by fan-type phases in a spin-spiral magnet

PHYSICAL REVIEW B 88 (2013) ARTN 224407

L Benito, RCC Ward, MG Blamire

Structure of epitaxial L1(0)-FePt/MgO perpendicular magnetic tunnel junctions


A Kohn, N Tal, A Elkayam, A Kovacs, D Li, S Wang, S Ghannadzadeh, T Hesjedal, RCC Ward

Switching the in-plane easy axis by ion implantation in rare earth based magnetic films.

Journal of physics. Condensed matter : an Institute of Physics journal 25 (2013) 086002-

AR Buckingham, D Wang, GBG Stenning, GJ Bowden, I Nandhakumar, RCC Ward, PAJ de Groot

Ar(+) ions have been implanted into Laves phase epitaxial thin films of YFe(2) and DyFe(2). Magneto-optical Kerr effect and vibrating sample magnetometry experiments show that the easy and hard axes of magnetization in both materials rotate through an in-plane angle of 90°, whilst the strength of the magnetic anisotropy remains unaltered. This is supported by OOMMF computational modelling. Atomic force microscopy confirms that the film roughness is not affected by implanted ions. X-ray diffraction data show that the lattice parameter expands upon ion implantation, corresponding to a release of strain throughout the entire film following implantation with a critical fluence of 10(17) Ar(+) ions cm(-2). The anisotropy of the films is linked to the strain and from these data it is concluded that the source of anisotropy alters from one where magnetoelastic and magnetocrystalline effects compete to one which is governed solely by magnetocrystalline effects. The ability to locally tune the source of magnetic anisotropy without affecting the film surface and without inducing or eliminating anisotropy could be important in the fabrication of high density magnetic data storage media, spintronic devices and magneto-optical materials.

Antiferromagnetism in UO2 thin epitaxial films

PHYSICAL REVIEW B 88 (2013) ARTN 134426

Z Bao, R Springell, HC Walker, H Leiste, K Kuebel, R Prang, G Nisbet, S Langridge, RCC Ward, T Gouder, R Caciuffo, GH Lander

The antiferromagnetic structures of IrMn3 and their influence on exchange-bias

Scientific Reports Springer Nature 3 (2013) 2412

A Kohn, A Kovács, R Fan, GJ McIntyre, R Ward, JP Goff

We have determined the magnetic structures of single-crystal thin-films of IrMn3 for the crystallographic phases of chemically-ordered L12 and for chemically-disordered face-centred-cubic, which is the phase typically chosen for information-storage devices. For the chemically-ordered L12 thin-film, we find the same triangular magnetic structure as reported for the bulk material. We determine the magnetic structure of the chemically-disordered face-centred-cubic alloy for the first time, which differs from theoretical predictions, with magnetic moments tilted away from the crystal diagonals towards the face-planes. We study the influence of these two antiferromagnetic structures on the exchange-bias properties of an epitaxial body-centred-cubic Fe layer showing that magnetization reversal mechanism and bias-field in the ferromagnetic layer is altered significantly. We report a change of reversal mechanism from in-plane nucleation of 90° domain-walls when coupled to the newly reported cubic structure towards a rotational process, including an out-of-plane magnetization component when coupled to the L12 triangular structure.

Determination of the spin polarization of RFe2 (R = Dy, Er, Y) by point contact Andreev reflection


C Morrison, D Wang, GJ Bowden, RCC Ward, PAJ de Groot

Interface characterization of epitaxial Fe/MgO/Fe magnetic tunnel junctions.

Journal of nanoscience and nanotechnology 12 (2012) 1006-1023

SG Wang, RCC Ward, T Hesjedal, XG Zhang, C Wang, A Kohn, QL Ma, J Zhang, HF Liu, XF Han

Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect in epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs), measured magnetoresistance (MR) ratios of about 200% at room temperature (RT) have been reported in MgO-based epitaxial MTJs. Recently, a MR ratio of about 600% has been reported at RT in MgO-based MTJs prepared by magnetron sputtering, using amorphous CoFeB as the ferromagnetic electrode. These MTJs show great potential for application in spintronic devices. Fully epitaxial MTJs are excellent model systems that enhance our understanding of the spin-dependent tunneling process as the interface is well defined and can be fully characterized. Both theoretical calculations and experimental results clearly indicate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystal MgO barrier, especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE). Surface X-ray diffraction, Auger electron spectroscopy, X-ray absorption spectra, and X-ray magnetic circular dichroism techniques have been reported previously for interface characterization. However, no consistent viewpoint has been reached on the interfacial structures (such as FeO layer formation at the bottom Fe/MgO interface), and it is still an open issue. In this article, our recent studies on the interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and spin-dependent tunneling spectroscopy, will be presented.

Interlayer diffusion studies of a Laves phase exchange spring superlattice.

J Phys Condens Matter 23 (2011) 116001-

C Wang, A Kohn, SG Wang, RCC Ward

Rare earth Laves phase (RFe(2)) superlattice structures grown at different temperatures are studied using x-ray reflectivity (XRR), x-ray diffraction, and transmission electron microscopy. The optimized molecular beam epitaxy growth condition is matched with the XRR simulation, showing minimum diffusion/roughness at the interfaces. Electron microscopy characterization reveals that the epitaxial growth develops from initial 3D islands to a high quality superlattice structure. Under this optimum growth condition, chemical analysis by electron energy loss spectroscopy with high spatial resolution is used to study the interface. The analysis shows that the interface roughness is between 0.6 and 0.8 nm and there is no significant interlayer diffusion. The locally sharp interface found in this work explains the success of simple structural models in predicting the magnetic reversal behavior of Laves exchange spring superlattices.

Exchange-spring-driven spin flop in an ErFe2/YFe2 multilayer studied by x-ray magnetic circular dichroism

PHYSICAL REVIEW B 84 (2011) ARTN 104428

GBG Stenning, AR Buckingham, GJ Bowden, RCC Ward, G van der Laan, LR Shelford, F Maccherozzi, SS Dhesi, PAJ de Groot

New surprises "down below": Recent successes in the synthesis of actinide materials

MRS BULLETIN 35 (2010) 877-882

JL Sarrao, Y Haga, RCC Ward

Analysis of computational EELS modelling results for MgO-based systems

ULTRAMICROSCOPY 110 (2010) 1059-1069

CR Seabourne, AJ Scott, G Vaughan, R Brydson, S-G Wang, RCC Ward, C Wang, A Kohn, B Mendis, AK Petford-Long

Interplay of RKKY, Zeeman, and Dzyaloshinskii-Moriya interactions and the nonzero average spin chirality in Dy/Y multilayer structures

PHYSICAL REVIEW B 82 (2010) ARTN 195432

SV Grigoriev, D Lott, YO Chetverikov, ATD Gruenwald, RCC Ward, A Schreyer

Magnetic excitations in Dy/Y superlattices as seen via inelastic neutron scattering

PHYSICAL REVIEW B 82 (2010) ARTN 014426

ATD Gruenwald, AR Wildes, W Schmidt, EV Tartakovskaya, J Kwo, C Majkrzak, RCC Ward, A Schreyer

Structural characterization of interfaces in epitaxial Fe/MgO/Fe magnetic tunnel junctions by transmission electron microscopy

PHYSICAL REVIEW B 82 (2010) ARTN 024428

C Wang, A Kohn, SG Wang, LY Chang, S-Y Choi, AI Kirkland, AK Petford-Long, RCC Ward