1

Computational Magnetism research at York

Academic staff and collaborators

Prof Roy Chantrell (spin models, ab-initio calculations) and collaboration with Dr Irene d’Amico and Dr. Matt Probert

Postdoctoral Research Fellows

Dr Richard Evans (FEMTOSPIN)

Dr Ondrej Hovorka (Marie Curie Fellow)

Dr. Ramon Cuadrado (FANTOMAS ITN)

Dr Unai Atxitia (FANTOMAS ITN)

Funding

from EPSRC, EU, Seagate Technology, ASTC.

2

Postgraduate Students

PhD Students: Tom Ostler (models of ultrafast reversal) Joe Barker (Excitations and ultrafast reversal) Matt Ellis (Ultrafast magnetism and HAMR) James McHugh (FEMTOSPIN) – joint supervision with I d’A Cecilia Aas (ab-initio calculations of FePt and FeRh) Weijia Fan (large-scale atomistic models) Matthew Hodgson (models of DMS) – joint supervision with

I d’A Phanwadee Churemart (spin torque) Jenefried Gay (Spin torque) – joint supervision with I d’A Lewis Atkinson (Models of Heat Assisted Magnetic

Recording)

3

Facilities and collaboration

Facilities 256 node computer

cluster 100 node cluster for

statistical calculations.

High memory nodes for DFT calculations

GPU support Peak perfermance28

TFlops

Collaborations Academic

ICMM, Madrid Univ. Konstanz BME, Budapest Univ. Nijmegen Drexel University Nanogune, San Sebastian National Taiwan University Akita prefectural Univ., Japan Tongji University, Shanghai National University of

Singapore Industry

Seagate Technology ASTC

4

Overview of interests Pioneering the development of Atomistic spin models of

the properties of nanostructured magnetic materials. This development is vital – standard theories cannot

cope with complex nanostructures or elevated temperatures.

MD models of nanostructures and interfaces Current work is aimed at bridging the link between ab-

initio calculations and spin models. Studies of transport processes and spin torque. Investigation of the properties of magnetic materials,

especially those related to magnetic recording. Studies of ultrafast, laser-induced magnetisation process

especially the reversal of magnetisation direction by circularly polarised laser light.

Theory and applications of nanoparticle magnetism

VAMPIRE

Multilayers are also possible, eg hard-soft coupled layers

Intermixing of layers included Spin glass could also be added using RKKY interaction

• Could be used to calculate inter-granular exchange

Understanding Inter-granular Exchange

Assuming some kind of spin-glass with RKKY interactions, gives rise to a degree of coupling between grains

• Coupling should increase with density -Possibly oscillatory? Could use 2 Grains to calculate inter-grain exchange at low

temperatures• Initialise both grains spin up and equilibrate• Switch direction of one grain and equilibrate• Calculate DF between these two states gives 2 x J

For high temperature grains could flip• Needs Constrained Monte Carlo to fix directions• CMC could also be used to calculate DF(q)

7

Transient dynamics in GdFeCo by XMCD(I. Radu et. al., Nature, 472, 205 (2011))

Experiments (left) in good agreement with atomistic model calculations (right)

8

Ultrafast magnetism

Atomistic model calculations agree well with the experimental timescale for demagnetisation.

Demagnetisation in less than 1 picosecond! New reversal mechanism predicted with a reversal time of

around 500 femtoseconds – 2 orders of magnitude faster than conventional techniques

0.0 0.5 1.0 1.5 2.00.710

0.715

0.720

0.725

0.730

0.735

0.740

0.745

0.750

Magnetisation

Time (ps)

Experiments on Ni (Beaurepaire et al PRL 76 4250 (1996))

Atomistic model Calculations for peak temperature of 375K

9

Heat Assisted Magnetic Recording (HAMR)

HAMR is proposed as a technique for Ultra High Density recording. Heating reduces the field necessary to write the information. It is necessary to heat through the magnetic ordering temperature. This requires atomistic simulation Above is the first simulation of recording at the atomic level. The grains are 6nm FePt and the recording density is 1Tbit/sq in.

The colours indicate the direction of magnetisation of each atom.

10

Damping channels; experiments

Rare Earth doping increases the macroscopic damping constant

Radu et al investigated the relationship between demagnetisation time and the macroscopic damping parameter (FMR)

11

Radu et al PRL 102, 117201 (2009)

Experimental demagnetisation times increase with damping! Consistent with spin model if energy transfer predominantly via

the FM spins No effect of Gd (isotropic). ‘dominant fast relaxation process is slowed down by adding slow

relaxing impurities.’ (Radu et al) Complex energy transfer channels

12

Model for Gd and Ho Ho given l=0.5 due to strong

spin/orbit coupling Gd assigned low damping l=0.05

13

In complex materials it is evidently important not to expect a single damping parameter but to consider the energy transfer channelrelevant to the technique and timescale of the measurement.

14

Transient dynamics in GdFeCo by XMCDI. Radu et al Nature, 472, 205 (2011)

Gd and Fe sublattices exhibit different dynamics

15

Note there is a transient FM state at around 400fs. Results in reversal of the magnetisation Initial experiments were carried out in a reversing

field of 0.2T As part of a systematic investigation we found that

reversal occurred in the absence of an applied field.

16

‘Magic reversal’

17

Sequence of heat pulses

Zero applied field, linearly polarised light GdFeCo is apparently a heat-driven bistable magnetic

system Far too weird to publish without experimental

verification, so ….

18

Physical origin - ? Latest work; LLB equation of ferrimagnet. This equation describes the mutual relaxation of

sublattices. The analysis of this equation shows that the sign of the

relaxation can be inverted at high temperature leading to a polarisation of one sub-lattice by the other.

This is accompanied by the instability of a linear reversal mode and the transfer of the angular momentum from linear to transverse motion.

This induces ultra-fast precession occurring in the exchange field and consequent switching of the TM spins into the transient FM state.

As the temperature decreases the TM establishes a stable structure which forces the RE to flip due to the exchange field

19

Recent results – Fe Rh (Joe Barker

Spin model reproduces metamagnetic transition

No magnetovolume effect

20

21

Ultrafast heat pulse

No magnetovolume effect No (explicit) spin fluctuations on Rh Driven by temperature dependence of 4-spin term Parametric study – commencing comparison with

ab-initio results (BME)

22

FePt surface, Cecilia Aas, Laszlo Szunyogh

Currently mapping onto spin model for atomistic calculations

23

Fe/FePt/Fe (CJA, LS)

Again, mapping to spin model