We are mainly interested in the study of dilute magnetic semiconductors, materials that are key to the endeavors of spintronics to combine logic and storage functionnalities in a single componant.
In particular, we study the control of magnetization in GaMnAs and GaMnAsP (epitaxied at the Laboratoire de Photonique et Nanostructures by A. Lemaître) by different stimuli : inductive, electrical, optical, or acoustic.
Electric excitation of surface acoustic waves using interdigitated transducers, collab. J.Y.-Duquesne :
1) experimental evidence of ferromagnetic resonance driven by a 550 MHz surface acoustic wave (SAW) in out-of-plane magnetized GaMnAsP [PRB14]
|[click to enlarge] : (a) Experimental geometry for a resonant coupling of the acoustic wave to the magnetization. (b) Changes of acoustic attenuation of a surface acoustic wave through interaction with a GaMnAsP layer. The resonance field is indeed the one for which the precession frequency is equal to the SAW frequency. (c) Principle of SAW-induced precessionnal switching. [PRB16prec]|
|[click to enlarge] : Domain nucleation assisted by an acoustic wave (T=30K, P=8.9 W) : hysteresis cycles with and without SAW, and corresponding Kerr images (690 x 924 µm²). [PRB16nuc]|
4) Experimental determination of the surface acoustic wave amplitude - collab. B. Croset (INSP) et L. Largeau (C2N)
The amplitude of the acoustic wave is a crucial parameter for magnetization switching. We have therefore developed two complementary methods to estimate it :
|[click to enlarge] : (a) All electrical approach [JAP17]. (b) X-ray diffraction approach. The acoustic wave induces the appearance of satellite diffraction peaks whose amplitude increases with the excitation power. Once correctly modeled, it yields the amplitude of the surface displacement. [JAC16] (c) A perfect agreement is obtained between the two techniques.|
all-optical (picosecond acoustics,collab. B. Perrin, E. Peronne) : evidence of acousto-magneto-optical effects induced by a picosecond acoustic strain wave in thin layers of GaMnAsP [PRB10], which led to part of a sub-study on acoustic soliton characterization [PRB17-soliton].
The idea is to manipulate magnetization on time scales much shorter than those allowed by magnetic fields, and a lot more locally. For this we use a « pump-probe », time-resolved Kerr experiment relying on a femto-second laser. This allows us to follow the magnetization dynamics over a few ns after the pump pulse has triggered the magnetization precession.
1) Excitation of standing spin waves
When several spin-waves are excited, the spin stiffness constant may be extracted from the separation between frequencies [APL15].
|[click to enlarge] : Typical dynamical signal obtained by time-resolved Kerr effect in GaMnAsP. The experimental curve can be decomposed into 2 oscillating signals of different frequencies (T=12 K) [APL15].|
2) Detection of spin-waves : a magneto-optical illusion
The fundamental mode has a uniform amplitude across the layer thickness. That of the first excited mode is sinusoidal, and averages out to zero. It should therefore not give any magneto-optical signal. We have evidence that it is the optical phase shift felt by the light in the layer that allows it to be observed in the end. A consequence of this is the surprising experimental observation that the two modes seem to be counter-rotating [PRB17-sw].
|[click to enlarge] : (a) Different modes that can be excited. (b) Dynamical reconstruction of the spin waves trajectory. A magneto-optical illusion is responsable for the apparent rotation sense.|
3) Quantitative estimate of the steady-state thermal gradient
Studying linear magnetic dichroism hysteresis cycles versus temperature and pump fluence yields a quantitative determination of the stationnary temperature increase, and of its spatial gradient [JAP16].
|[click to enlarge] : (a) Radial profile of the temperature rise induced by the pump (fluence 17.5 µJ/cm²) : data (symbols) and model taking into account a thermal contact resistance (full line) . (b) Analytical calculation of the radial and depth thermal profile [JAP16].|
Current-driven domain wall motion is investigated experimentally in in-plane magnetized GaMnAs tracks. The wall dynamics is found to differ in two important ways with respect to perpendicularly magnetized GaMnAs or GaMnAsP : the wall mobilities are up to ten times higher and the walls move in the same direction as the hole current. We demonstrate that these observations cannot be explained by spin orbit field torques (Rashba and Dresselhaus types) but are consistent with non-adiabatic spin transfer torque driven by the strong spin-orbit coupling of GaMnAs. This mechanism opens the way to domain wall motion driven by intrinsic (bulk) rather than interface spin-orbit interaction as in ultrathin ferromagnet/heavy metal multilayers [PRB17-dwp].
|[click to enlarge] : (a) Two track configurations : current flowing parallel (perpendicular) to the easy axis. The resulting effective spin-orbit field (hollow arrow) then lies perpendicular (parallel) to the magnetization direction. (b) Current-driven domain-wall velocity : 2µm wide C// track, under field (open symbols) and 10µm wide C┴ track without field (closed symbols). (c) Three consecutive current pulses applied to a 2µm wide C// track observed under longitudinal Kerr microscopy (B=11 G, T=40K, J=24.5 GA/m²) [PRB17-dwp].|
2) Propagation under field
very high domain-wall propagation velocities (500 m/s) in in-plane magnetized GaMnAs layers [PRB12]
|[click to enlarge] : Domain-wall propagation under field (in-situ micro-coil) in a 50nm out-of-plane magnetized GaMnAs layer. Longitudinal Kerr effect microscopy [PRB12]|
collaboration with the University of Latvia and the Laboratoire de Physique des Solides (Orsay)
domain-wall propagation in the flow regime in GaMnAs layers magnetized perpendicularly [PRB08]
3) Determination of micromagnetic parameters using Kerr microscopy
evidence of a slight enhancement of the exchange constant in GaMnAsP following the introduction of Phosphorus [PRB10_]
development of different methods to determine the micromagnetic parameters (exchange constant, domain-wall width) in GaMnAs/GaMnAsP using Kerr microscopy[PRB07]
Main experimental technics :
We collaborate with different groups on theoretical or experimental aspects :
A. Cebers, University of Latvia : theoretical studies on resonance phenomena in magnetic domain-walls [PRB13]
Teams of M. Maaref at the Institut Préparatoire aux Études Scientifiques et Technologiques in La Marsa, and K. Boujdaria at the Faculté des Sciences de Bizerte (Tunisia) : magnetic characterization of GaMnAs(P) layers [JMMM13],[JMMM15], and k.p method calculations of the magnetic parameters of GaMnAs(P) [PRB13] , [JAP12]
University of Liège : high field characterization of FePt nanostructures
Recent publications :
[PRB17-sw] Counter-rotating standing spin-waves : a magneto-optical illusion , S. Shihab, L. Thevenard, A. Lemaître, Catherine Gourdon, Physical Review B 95 144411 (2017)
[PRB-dwp] Spin transfer and spin-orbit torques in in-plane magnetized (Ga,Mn)As tracks, L. Thevenard, B. Boutigny, N. Güsken, L. Becerra, C. Ulysse, S. Shihab, A. Lemaître, J.-V. Kim, V. Jeudy, C. Gourdon, Physical Review B 95 054422 (2017)
[PRB17-soliton] Acoustic solitons : A robust tool to investigate the generation and the detection of ultrafast acoustic waves, E. Péronne, N. Chuecos, L. Thevenard, and Bernard Perrin, Physical Review B 95 064306 (2017)
[JAP17] Vector network analyzer measurement of the amplitude of an electrically excited surface acoustic wave and validation by x-ray diffraction, I. Camara, B. Croset, L. Largeau, P. Rovillain, L. Thevenard, J.-Y. Duquesne, Journal of Applied Physics 121 044503 (2017)]
[JAC16] Laboratory X-ray characterization of a surface acoustic wave on GaAs : the critical role of instrumental convolution, L. Largeau, I. Camara, J.-Y. Duquesne, C. Gourdon, P. Rovillain, L. Thevenard, B. Croset, Journal of Applied Crystallography 49 2073 (2016)
[PRB16prec] Precessional magnetization switching induced by a surface acoustic wave, L. Thevenard, I. S. Camara,S. Majrab, M. Bernard, P. Rovillain, A. Lemaître, C. Gourdon, and J.-Y. Duquesne, Physical Review B 93 134430 (2016)
[JAP15] Stationary thermal gradient induced by ultrafast laser excitation in a ferromagnetic layer, S. Shihab, L. Thevenard, A. Lemaître, C. Gourdon, J.-Y. Duquesne, J. Appl. Phys. 119 153904 (2016)
[PRB16nuc] Strong reduction of the coercivity by a surface acoustic wave in an out-of-plane magnetized epilayer, L. Thevenard, I. S. Camara, J.-Y. Prieur, P. Rovillain, A. Lemaître, C. Gourdon, and J.-Y. Duquesne, Physical Review B 93, 140405(2016)
[JMMM15] Optimizing magneto-optical effects in the ferromagnetic semiconductor GaMnAs, H. Riahi, L. Thevenard, M. Maaref, B. Gallas, A. Lemaître, C. Gourdon, Journal of Magnetism and Magnetic Materials 395, 340 (2015)
[APL15] Systematic study of the spin stiffness dependence on Phosphorus alloying in the ferromagnetic semiconductor (Ga,Mn)As , S. Shihab, H. Riahi, L. Thevenard, H. J. Von Bardeleben, A. Lemaître, C. Gourdon, Appl. Phys. Lett. 106 142408 (2015)
[PRB14] Surface-acoustic-wave-driven ferromagnetic resonance in (Ga,Mn)(As,P) epilayers, L. Thevenard, C. Gourdon, J.Y. Prieur, H. J. von Bardeleben, S. Vincent, L. Becerra, L. Largeau, J.Y. Duquesne, Physical Review B 90, 094401 (2014)
[PRB13] Irreversible magnetization switching using surface acoustic waves, L. Thevenard, J.-Y. Duquesne, E. Peronne, H. J. von Bardeleben, H. Jaffres, S. Ruttala, J-M. George, A. Lemaître, and C. Gourdon, Physical Review B 87, 144402 (2013)
[JMMM13] Annealing effect on the magnetization reversal and Curie temperature in a GaMnAs layer, H. Riahi, W. Ouerghui, L. Thevenard, C. Gourdon, M.A. Maaref, A. Lemaître, O. Mauguin, C. Testelin, J. Magn. Mag. Mat. 342, 149 (2013)
[PRB13] Domain-wall flexing instability and propagation in thin ferromagnetic films, C. Gourdon, L. Thevenard, and S. Haghgoo, A. Cebers, Phys. Rev. B 88, 014428 (2013)
[PRB13] The influence of phosphorus content on magnetic anisotropy in ferromagnetic (Ga, Mn)(As,P)/GaAs thin films , M Yahyaoui, K Boujdaria, M Cubukcu, C Testelin and C Gourdon, J. Phys. : Condens. Matter 25 346001 (2013)
[APL12] Fast domain wall dynamics in MnAs / GaAs films Fast domain wall dynamics in MnAs / GaAs films, M. Tortarolo, L. Thevenard, H. J. von Bardeleben, M. Cubukcu, M. Eddrief, V. Etgens, C. Gourdon, Applied Physics Letters 101, 072408 (2012)
[PRB12] High domain wall velocities in in-plane magnetized (Ga,Mn)(As,P) layers, Thevenard, L., Hussain, S. von Bardeleben, H. Bernard, M. Lemaître, A. Gourdon, C., Physical Review B 85 064419 (2012)
[JAP12] The influence of the epitaxial strain on the magnetic anisotropy in ferromagnetic (Ga,Mn)(As,P)/GaAs thin films , M Yahyaoui, K Boujdaria, M Cubukcu, C Testelin and C Gourdon, J. App. Phys. 111 346001 (2012)
[PRB11] Domain wall propagation in ferromagnetic semiconductors : Beyond the one-dimensional model, L. Thevenard, C. Gourdon, S. Haghgoo, J-P. Adam, J. von Berdeleben, A. Lemaître, W. Schoch, A. Thiaville, Physical Review B 83, 245211 (2011)
[PRB10] Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P), L. Thevenard, E. Peronne, C. Gourdon, C. Testelin, M. Cubukcu, E. Charron, S. Vincent, A. Lemaître, and B. Perrin, Phys. Rev. B 82, 104422 (2010)
[PRB10_] Exchange constant and domain wall width in (Ga,Mn)(As,P) films with self-organization of magnetic domains, S. Haghgoo, M. Cubukcu, H. J. von Bardeleben, L. Thevenard, A. Lemaître, and C. Gourdon, Phys. Rev. B 82, 041301 (2010)
[PRB09] Unusual domain-wall motion in ferromagnetic semiconductor films with tetragonal anisotropy, C. Gourdon, V. Jeudy, A. Cēbers, A. Dourlat, Kh. Khazen, and A. Lemaître, Phys. Rev. B 80, 161202(R) (2009).
[PRB08] Field-Driven Domain Wall Dynamics in GaMnAs Films with Perpendicular Anisotropy, A. Dourlat, V. Jeudy, A. Lemaître, and C. Gourdon, Phys. Rev. B 78, 161303(R) (2008).
[Lemaître08] Strain control of the magnetic anisotropy in (Ga,M n) (As,P) ferromagnetic semiconductor layers, A. Lemaître, A. Miard, L. Travers, O. Mauguin, L. Largeau, C. Gourdon, V. Jeudy, M. Tran, and J.-M. George, Appl. Phys. Lett. 93, 021123 (2008).
[Gourdon07] Determination of the micromagnetic parameters in GaMnAs using domain theory, C. Gourdon, A. Dourlat, V. Jeudy, K. Khazen, H. J. von Bardeleben, L. Thevenard, and A. Lemaître, Phys. Rev. B 76, 241301(R) (2007).
[Dourlat07] Domain structure and magnetic anisotropy fluctuations in (Ga,Mn)As : Effect of annealing, A. Dourlat, V. Jeudy, C. Testelin, F. Bernardot, K. Khazen, C. Gourdon, L. Thevenard, L. Largeau, O. Mauguin, and A. Lemaître, J. Appl. Phys. 102, 023913 (2007).
[Dourlat08] Experimental determination of domain wall width and spin stiffness constant in ferromagnetic (Ga,Mn)As with perpendicular easy axis A. Dourlat, C. Gourdon, V. Jeudy,, K. Khazen, H.J. von Bardeleben, L. Thevenard, A. Lemaitre, Physica E 40 (2008) 1848–1850
[Dourlat07] Expansion and collapse of domains with reverse magnetization in GaMnAs epilayers with perpendicular magnetic easy axis A. Dourlat, C. Gourdon, V. Jeudy, C. Testelin, K. Khazen, J.L. Cantin, H.J. von Bardeleben, L. Thevenard, A. Lemaitre, IEEE Trans. Magn. 43, 3022 (2007).