Balance of angular momentum and magnetization switching

Balance of angular momentum and magnetization switching Andrei !Kirilyuk Radboud University, Institute for Molecules and Materials, Nijmegen 1 Andre...
Author: Alberta Craig
3 downloads 0 Views 3MB Size
Balance of angular momentum and magnetization switching Andrei !Kirilyuk Radboud University, Institute for Molecules and Materials, Nijmegen

1

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Acknowledgments Nijmegen K. Vahaplar, I. Radu, S. Khorsand, M. Savoini, J. Mentink, 
 C.D. Stanciu, F. Hansteen, A.V. Kimel, M.I. Katsnelson, Th. Rasing Chiba A. Tsukamoto, A. Itoh BESSY, Berlin C. Stamm, T. Kachel, N. Pontius LCLS, Stanford C. Graves, A.H. Reid, J. Stöhr, A.O. Scherz, H.A. Dürr SLS, Villingen S.El Moussaoui, L. Le Guyader, F. Nolting York T.A. Ostler, J. Barker, R.F.L. Evans, R.W. Chantrell Madrid U. Atxitia, O. Chubykalo-Fesenko St.-Petersburg A.M. Kalashnikova, R.V. Pisarev Uppsala J. Hellsvik, O. Eriksson, Kiev D. Afanasiev, B.A. Ivanov 2

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Magnetic recording – searching for options

Other options: - Bit Patterned Media - Microwave-Assist - Two-Dimensional - .....

3

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Time-scales and stimuli in magnetism

Time (s)

4

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Magnetization dynamics N

B

energy gain: ! torque equation:

S Landau & Lifshitz, 1935

with damping:

5

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Consequence: conservation of angular momentum

Einstein – de Haas & Barnett effects A. Einstein & W.J. de Haas, Experimenteller Nachweis der Amperèschen
 Molekülströme, Verhandl. Deut. Phys. Ges. 17, 152 (1915) S.J. Barnett, Magnetization by rotation, Phys. Rev. 6, 239 (1915) 6

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

What laser does to a magnet?

Beaurepaire et al. PRL 76, 4250 (1996)

7

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Samples: ferrimagnetic alloys GdFeCo and similar

hysteresis domains

Magnetization

0.3

'up' 0.0

'down' -0.3 -3

0

3

Field (kOe)

‹#›

Andrei Kirilyuk, M-SNOWS – February 2012

Radboud University Nijmegen

Samples: ferrimagnetic alloys GdFeCo and similar TM TA RE (Gd)

MRE ATM

TC~500 K

M A ARE MTM

TM (FeCo) Temperature

gGd < gFeCo ‹#›

Andrei Kirilyuk, M-SNOWS – February 2012

Radboud University Nijmegen

Helicity dependence? Switching with a single
 40 fs laser pulse, circularly polarized is this Stanciu et al., PRL 99, 047601 (2007).

Faraday effect (refraction)? or magnetic circular dichroism 
 (absorption)?

10

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Helicity-dependent switching revised S. Khorsand et al, PRL 108, 127205 (2012)

beam-size determination, see J. M. Liu, Optics Letters 7, 196 (1982)

11

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

MCD vs Faraday effect

effective field from inverse Faraday effect

amount of energy absorbed in the sample per pulse stays constant

S. Khorsand et al, PRL 108, 127205 (2012)

12

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Absorbed amount of energy ⇒ demagnetization? Simple question: what happens if we demagnetize a ferrimagnetic structure with two inequivalent sublattices? Answer 1: they demagnetize at different rates o wrong answer, what about exchange?? Answer 2: because of the exchange, they demagnetize in exactly the same way

13

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

X-ray Magnetic Circular Dichroism below Tcomp

above Tcomp

Gd Mtot Fe

14

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Dynamics of sublattices Fe: 100±23 fs Radu et al., Nature 472, 205 (2011)

Gd: 427±102 fs

ferri-magnet turns ferro! 15

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Atomistic simulations localized atomistic spin model with a Heisenberg exchange for two sublattices exchange parameters (Fe-Fe, Gd-Gd, and Fe-Gd) obtained by fitting static MFe,Gd(T) dependencies. the usual stochastic term added to the effective field

! ! ! ! magnetic field can be present during the process Scubic et al, JPCM 20, 315203 (2008); Ostler et al, PRB 84, 024407 (2011)

16

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Simulation results

Radu et al., Nature 472, 205 (2011)

ferromagnetic state reproduced, as well as the reversal! 17

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

The role of the compensation point Intensity threshold

x(Gd)=24%

Coercive field Hc

Intensity threshold of
 helicity-dependent
 all-optical switching

x(Gd)=26%

x(Gd)=22%

It works in the broad vicinity 
 of the compensation 
 temperature

x(Gd)=20%

0

50

100

150

200

250

300

350

Temperature, K 18

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Longitudinal relaxation in multi-sublattice magnets from eqs. of motion based on Onsager relations, see V. G. Baryakhtar, JETP 87, 1501 (1984); 94, 196 (1988); Low Temp. Phys. 11, 1198 (1985).

dS1 dS 2

dt dt

= λe (H1 − H 2 )+ λ1 H1 = −λe (H1 − H 2 )+ λ2 H 2 exchange

where and

relativistic (usual damping)

Mentink et al., PRL 108, 057202 (2012)

19

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Temperature-dominated regime

λi >> λe

interaction with the environment dS1

dt

= λe (H1 − H 2 )+ λ1 H1

dt

= −λe (H1 − H 2 )+ λ2 H 2

dS 2

2α iγk BT λi ∝ µi

Bloch  relaxation

Dynamics  scales  with  magnetic  moment Brown 1963, Kubo 1970

small  magnetic  moments  change  faster   → less  angular  momentum  to  be  transferred 20

Andrei Kirilyuk, Belem – November 2014

Radboud University Nijmegen

Exchange-dominated regime

λi