# Presentation: 127205

## Thin Film Diluted Magnetic Semiconductors with Tetradymite StructureCtirad Uher				    DMR-0305221

- _References:_
-  C. Drasar, P. Lostak, and C. Uher, “Novel magnetic Semiconductors Based on Sb2Te3”, *Czech J. Physics* **97**, 103720 (2005).
-  Z. Zhou, Y.-J. Chien and C. Uher, “Thin Film Ferromagnetic Semiconductors Sb2-xVxTe3 with TC of 177K”, *Applied Physics Letters*, **87**, 112503 (2005).
-   Z. Zhou, M. Zabcik, P. Lostak, and C. Uher, “Magnetic and Transport Properties of Sb2-xFexTe3 (0<x<0.02)*Journal of Applied Phys*ics **99**, 043901 (2006).
-  Z. Zhou, C. Uher, M. Zabcik, and P. Lostak, “Carrier-mediated Ferromagnetism in Vanadium-Doped
-  (Sb1-xBix)2Te3 Solid Solutions”, *Applied Physics Letters* **88**, 192502 (2006).
-  J. S. Dyck, K. Ahilan, M. C. Aronson, C. Uher, and P. Lostak, “Substantial Pressure Effect on the   Resistivity and Curie Temperature for Diluted magnetic Semiconductor Sb2-xVxTe3”, *Physica Status Solidi (b)* **243**, 1862 (2006).
-  Z. Zhou, Y.-J. Chien, and C. Uher, “Thin Film Ferromagnetic Semiconductors Sb2-xCrxTe3 with a Curie Temperature up to 190K”, submitted for publication.

- Various single phase ferromagnetic films with tetradymite structure such as Sb2-xVxTe3 and Sb2-xCrxTe3 have been prepared using MBE technology and characterized for their magnetic and transport properties. High quality of films is confirmed by *in-situ* RHEED and x-ray diffraction, Fig.1. These semiconducting films display ferromagnetism with Curie temperature up to 190K, Fig.2. The Curie temperature is proportional to *xp**1/3* where *x* is the transition metal doping concentration and *p* is the concentration of holes. The dependence is consistent with the RKKY interaction scenario. The observed interplay among the carrier concentration, the density of transition metal ions, and magnetic interactions is very helpful in elucidating the origin of ferromagnetism in this family of diluted magnetic semiconductors and is an important issue for spintronics applications.

- Figure 1: (110) RHEED streak pattern taken during the growth of Bi2-xFexTe3 with x = 0.19.

- Figure 2: Temperature dependence of magnetization of several films of Sb2-xCrxTe3 . The upturns on the magnetization curves indicate the onset of long-range magnetic ordering (ferromagnetism) in the films.

- University of Michigan

**Thin Film Diluted Magnetic Semiconductors with Tetradymite Structure****Ctirad Uher**				    DMR-0305221

**Notes:**
Motivation of this work
The impetus for this project stems from our recent discovery of low temperature ferromagnetism in single crystals of narrow-bandgap semiconductor Sb2Te3 doped with very small concentrations of vanadium. Sb2Te3 possesses the tetradymite structure characterized by octahedral bonding and a van der Waals gap separating five atom layer lamella, Te(1)-Sb-Te(2)-Sb-Te(1), in the plane perpendicular to the trigonal axis. Sb2Te3 is diamagnetic, just as the other pure tetradymite semiconductors, and little is known about the magnetic properties upon doping the structure. 
In this project, we extend the study of the tetradymite-type diluted magnetic semiconductors to materials in their thin film form. The research involves the first study of doping tetradymite structures with transition metal impurities through the use of MBE growth. It is expected that the Curie temperature can be significantly enhanced due to the increase of vanadium concentration. Another important aspect of this research is the detailed characterization of the nature of the ferromagnetic state in these materials and the role played by the composition of the host matrix as well as of magnetic impurities.    

Research results
Ferromagnetic thin films of Sb2-xVxTe3 with x up to 0.35 have been successfully prepared on sapphire (0001) substrates by low temperature MBE growth. Films of Sb2-xVxTe3 display robust, out-of-plane ferromagnetic ordering that depends on the concentration of vanadium in the structure. The Curie temperature was determined from magnetization measurements and Arrott plots. Ferromagnetic order is manifested by hysteresis loops observed in magnetization, magnetoresistivity and the anomalous Hall effect. The highest Curie temperature we have achieved so far is 177 K in Sb1.65V0.35Te3. The linear dependence of the Curie temperature on the vanadium concentration suggests that even higher values of the magnetic ordering temperature may be obtained if one can further increase the concentration of vanadium. We have also observed a ferromagnetic transition in bulk single crystals of Cr-doped Sb2Te3 with the Curie temperature near 20K. Our work thus significantly extends the domain of DMSs and the results should have an important impact on the understanding of magnetism in low carrier density systems.

Future plans
In the near future, the emphasis will be on increasing the vanadium concentration in Sb2-xVxTe3 thin films and establishing its solubility limits in thin films of Sb2Te3. This is a crucial step in order to enhance the Curie temperature. We will also explore the growth of Cr-doped Sb2Te3 thin films with the hope of extending the domain of ferromagnetism to much higher temperatures. The spin dynamics of these ferromagnetic thin films will be studied by ultrafast optical pump-probe method and they shall provide important information such as exchange interactions between the magnetic ions. Further tuning of the magnetic properties will be done by controlling the carrier density via doping. We also plan on starting the growth of thin films of Fe-doped Bi2Te3, the structure that might also display ferromagnetism.

## Thin Film Diluted Magnetic Semiconductors

**               ****with Tetradymite Structure****Ctirad Uher**                                          DMR-0305221

- University of Michigan

- The ferromagnetic state of the films is studied by magnetic measurements (susceptibility, hysteresis, Arrott plots), transport investigations (resistivity, magnetoresistance, anomalous Hall effect), and by pressure studies (Tc vs. pressure). Figure 3a shows robust hysteresis loops measured at 10K for a number of Cr-doped Sb2Te3 films. Equally well developed and closely matching the magnetization loops are hysteresis loops observed in the Hall effect, Fig.3b. The coercive fields scale with the concentration of Cr. Hysteresis can be detected to within a couple of degrees of Tc. The actual Curie temperature is determined via Arrott plots. The dependence of the Curie temperature on chromium concentration is shown in Fig.3c. The highest temperature so far exceeds 190K, the value that is comparable or even higher than the Curie temperature observed on Mn-doped films of GaAs. Doping with Cr causes an enhancement in the charge carrier density (holes) and the Curie temperature is proportional to *xp**1/3*, where *x* is the Cr ion content and *p* is the density of holes. We are exploring various other techniques (such as the influence of pressure and tunneling measurements) to elucidate the nature of magnetism in the tetradymite-type matrices of Sb2Te3 and Bi2Te3. Exploratory studies on the growth of Bi2Te3 films and on the influence of transition metal ions in the thin film matrix of Bi2Te3 are underway. Such studies are aided by our recent investigations of the magnetic ordering in vanadium-doped solid solutions of bulk crystals of (Sb1-xBix)2Te3.

- Figure 3: (a) Magnetization hysteresis loops obtained on films of Sb2-xCrxTe3 at 10K; (b) Magnetic field dependence of the Hall effect at 10K on the same films of Sb2-xCrxTe3; (c) Curie temperature as a function of chromium concentration.

## Thin Film Diluted Magnetic Semiconductors

**               ****with Tetradymite Structure****Ctirad Uher**                                          DMR-0305221

- University of Michigan

**Education:**
- This research is a thesis project of Y.-J. Chien, a fourth year student enrolled in our Applied Physics Graduate Program. I expect Mr. Chien to complete his Ph.D. in the forthcoming academic year. In addition, in the early stages of the project my then postdoc, Dr. Jeffrey S. Dyck, participated in the design and testing of our new MBE apparatus and associated hardware. Dr. Dyck is currently an Assistant Professor at the Physics Department of John Carroll’s University in Cleveland. Although not funded by this grant, my new postdoc, Dr. Xun Shi, is much interested in thin film growth of magnetic structures and is taking an active participation in the project. During the past two summers, I supported and much enjoyed the presence in my laboratory of two talented second year undergraduate students, Peter Landry (summer of 2004) and Kim Ruelle (summer of 2005). Both have now chosen physics as their major degree and both are part of our expanding cohort of undergraduate physics majors. In addition, during the Fall term I will be supporting another undergraduate student, Mr. Chris Lawrence.
**Outreach:**
- As Chair of the Department during 1994-2004 I organized and supported a series of public lectures called Saturday Morning Physics. These lectures are given by our postdocs (Fall term) and faculty (Winter term) and attract a wide and diverse audience (attendance in excess of 250 each Saturday morning, football season or not, spanning from middle school kids to senior citizens). These lectures highlight research work conducted in our department and my group is an active participant.
- The past two years, I taught Physics 160 Honors, an intro physics course for the most talented students our university attracts. As an integral part of the course, I introduced group projects on modern physics topics, among them spintronics. Students in groups of 4-5 students have 3 months to prepare a proper scientific report and then present their findings to the class. The topic of spintronics is one of the most successful presentations partly because the students have a chance to visit my laboratory and observe various stages of research on our thin film diluted magnetic semiconductors.
- I collaborate with chemists from the University of Pardubice in the Czech Republic (Prof. P. Lostak)—the collaboration that has led to the discovery of ferromagnetism in bulk crystals of transition metal-doped tetradymite-type semiconductors (e.g., V-doped Sb2Te3) and the work upon which this research project was originally based—and this interaction continues to play a very positive role in the training of material scientists in that country. During the past year I gave two colloquia on the topic of magnetic semiconductors in the Czech Republic (at the University of Pardubice and at the Institute of Inorganic Chemistry).