Coherent magnetization precession in ferromagnetic ( Ga,Mn )As induced by picosecond acoustic pulses

1. s-polarized 800 nm 200 fs GaAs 100 mm 200-nm-(Ga,Mn)As probe pump j strain pulse B Al film 100 nm Coherent magnetization precession in ferromagnetic (Ga,Mn)As induced by picosecond acoustic pulses Margaret Dobrowolska, University of Notre Dame, DMR 0603752 Description of achievement: High crystal quality of semiconductors allows one to implement tools for fast manipulation of their electrical and optical properties. Our work extends this to manipulation of spin by sub-THz picosecond acoustic pulses in the ferromagnetic semiconductor (Ga,Mn)As, i.e., we have succeeded in ultrafast control of magnetism in this material by high-frequency sound. Access to such control is obtained through the strong sensitivity of magnetic properties of (Ga,Mn)As to magneto-crystalline anisotropy. Methodology: Fast control of magnetization was obtained by injecting an ultrashort high-amplitude acoustic wavepacket into a ferromagnetic (Ga,Mn)As layer. Illustration of results in figure on the right: (a) Schematic of pump-probe experiments with strain pulses. (b) Spatial shape of strain pulse εzz injected into the GaAs substrate. (c) Temporal evolution of relative layer thickness of the (Ga,Mn)As layer created by strain pulse. (d) Changes of strain-pulse-induced Kerr rotation angle at various magnetic fields; and (e) at B=0.8 kOe. The thick solid curve in (e) is the calculated DMz(t)/M. Inset in (e): pump excitation opposite to the probe spot (thin line) and displaced by 100 mm (thick line), showing that the effect is caused by the strain pulse (the diameters of pump and probe spots are 100 mm and 50 mm, respectively). (a) (d) (b) (e) (c) A.V. Scherbakov et al., PRL (accepted).

2. DMR 06-03752: Electron spin Effects in Semiconductor Nanostructures Margaret Dobrowolska, University of Notre Dame, DMR 0603752 Societal Impact: Our group has been continually active as a resource of materials for other groups. We are currently interacting with at least 20 other institutions by providing them with magnetic semiconductor specimens for their research. The understanding of these materials and their heterostructures, as well as of devices based upon them developed in our laboratory, is thus automatically of benefit to our collaborators (including graduate students in their Ph.D. research), who depend not only on the specimens that we provide, but on the intellectual input from our group in the form of characterization and general understanding of the properties of these materials. Education: Two undergraduate students: Ms. Megan Kirkendall(Kenyon College.)and Mr. Kirk Post (Notre Dame); and two grad students (Mr. Y.-J. Cho and Mr. J. Leiner) contributed to this work. Ms. Kirkendall was an REU student in the summer of ‘09, and Mr. Post was a Notre Dame student supported by Notre Dame Funds. Mr. Cho has now graduated and is a post-doc at Paul Drude Institute, and Mr. Leineris a continuing graduate student. In addition, the Ph.D. research of Mr. D. Y. Shin and Mr. S. J. Chung -- bothgraduate students at Korea University, Seoul, Korea -- has directly benefited from this program.

3. Observation of Antiferromagnetic Interlayer Exchange Coupling in GaMnAs/GaAs:Be/GaMnAsTrilayer Geometry Margaret Dobrowolska, University of Notre Dame, DMR 0603752 • Motivation and Impact: • The nature of interlayer exchange coupling (IEC) in GaMnAs-based multilayers still remains unresolved. It is extremely important to establish whether the IEC between adjacent GaMnAs layers is antiferromagnetic (AF) or ferromagnetic (FM), since manipulation of such IEC can serve as the basis for a wide range of devices. A study of two GaMnAs layers separated by a non-magnetic GaAs spacer is particularly useful, since such a trilayer enables one to “zoom • in” on the specific properties of GaMnAs that determine the IEC. • Central Achievement: • Direct observation of AF IEC in a GaMnAs-based trilayer. • Approaches used: • Magnetization, Magnetotransport and Neutron scattering. • Specific Results: • Temperature dependence of remnant magnetization shows clear signs of AF IEC in the GaMnAs/GaAs:Be/GaMnAstrilayer (top figure). • Polarized neutron reflectometry (PNR) also shows unambiguous evidence for the existence of AF IEC in the trilayer (bottom figure). • Magnetotransport measurements show tunneling magneto-resistance in the trilayer with AF coupling, clearly indicating potential for device applications. [001] GaMnAs 8.6nm GaBeAs 4.3nm GaMnAs 17.2nm J. Leiner et al. PRB (submitted)

4. Broader Impact Margaret Dobrowolska, University of Notre Dame, DMR 0603752 Education: Two undergraduate students, Mr. Kevin Miller (St. Bonaventure U.)and Miss KrystynaTraudt(Notre Dame); and two Notre Dame graduate students, Mr. R. Chakarvorty and Mr. Z. Ge, contributed to this work. Mr. Miller was an REU student in the Summer of ‘07, and Miss Traudt worked on this project for her Undergraduate Thesis Project. Mr. Ge has now received his Ph.D. degree and is a post-doc at Princeton. Mr. Chakarvorty is presently writing up his Ph.D. dissertation, and is expected to graduate in the Fall of ’08. Two new grad students will continue this work starting in the Fall of ’08. Education: Two undergraduate students: Ms. Megan Kirkendall(Kenyon College.)and Mr. Kirk Post (Notre Dame); and two grad students (Mr. Y.-J. Cho and Mr. J. Leiner) contributed to this work. Ms. Kirkendall was an REU student in the summer of ‘09, and Mr. Post was a Notre Dame student supported by Notre Dame Funds. Mr. Cho has now graduated and is a post-doc at Paul Drude Institute, and Mr. Leineris a continuing graduate student. In addition, the Ph.D. research of Mr. D. Y. Shin and Mr. S. J. Chung -- bothgraduate students at Korea University, Seoul, Korea -- has directly benefited from this program. Societal Impact: Our group has been continually active as a resource of materials for other groups. We are currently interacting with at least 25 other institutions by providing them with magnetic semiconductor specimens for their research. The understanding of these materials and their heterostructures, as well as of devices based upon them developed in our laboratory, is thus automatically of benefit to our collaborators (including graduate students in their Ph.D. research), who depend not only on the specimens that we provide, but on the intellectual input from our group in the form of characterization and general understanding of the properties of these materials. Societal Impact: Our group has been continually active as a resource of materials for other groups. We are currently interacting with at least 20 other institutions by providing them with magnetic semiconductor specimens for their research. The understanding of these materials and their heterostructures, as well as of devices based upon them developed in our laboratory, is thus automatically of benefit to our collaborators (including graduate students in their Ph.D. research), who depend not only on the specimens that we provide, but on the intellectual input from our group in the form of characterization and general understanding of the properties of these materials. One of our graduate students, Jonathan Leiner, conducted his research at Korea University in the Summer of 2009 as NSF-EAPSI Fellow.