Diluted Magnetic Oxide Semiconductors

A) Co:TiO₂ B) Origin of Ferromagnetism in Mn-Zn-O C) Co:HfO₂ D) Co:SnO₂
E) La_0.5Sr_0.5Ti_1-xCo_xO_3-d F) Transition Metals in CuO₂

Multiferroic

A) GaFeO₃

Wideband gap semiconductors

A) High Temperature Compatible Capping of SiC for dopant activation B) AlN/SiC
C) AlN MEMS and NEMS Resonators for RF Communications D) ZnO and MgZnO

1) Diluted Magnetic Oxide Semiconductors

Ferromagnetism in diluted magnetic semiconductors (DMS) has been a subject of great scientific and technological interest for the past few years due to its implications for spintronics, advanced magneto-optics and sensor applications. The materials challenge is to magnetize functional non-magnetic materials by introducing dilute concentrations of magnetic species, thereby harnessing the benefits of a magnetic response without significantly affecting the desirable physical properties of the non-magnetic host. In Mn:GaAs, the desired spin-related properties were found, but the Curie temperatures were cryogenic. Therefore, there is a need for investigation of DMS systems exhibiting ferromagnetic at above room temperature.

Prof. Venkatesan's group has observed above room-temperature ferromagnetism in many oxide-based DMS systems, and this has brought significant interest to this materials systems. However, after the unrestrained excitement of the early exploratory research in this area, many materials issues related to the oxide-based systems remain unsolved and the intrinsic DMS character in such systems continues to be questioned. Various projected DMS materials studied by this group are described below.

A) Ferromagnetism and DMS Behaviour in Co Doped TiO₂

Recently, oxide DMS systems have shown ferromagnetism above room temperature. One promising oxide is Ti_1-xM_xO₂ (M: magnetic dopant). However, evidence shows that in the anatase Co:TiO₂ system, clustering of cobalt atoms occurs above a certain doping level ~2% and it is believed that the observed high-temperature ferromagnetism in such samples is manifested in these clusters [1]. Under specific growth and annealing conditions, samples without any obvious clusters have also been shown to exhibit ferromagnetism with a T_c close to 700 K [1]. However, whether the ferromagnetism in this system is carrier-induced or extrinsic still remains an unresolved issue. In this context, studies of the Hall effect, optical magnetic circular dichroism O-MCD, and electric-field effect measurements have been suggested to be the clarifying experimental windows. We have performed detailed studies regarding the anomalous Hall effect and electric field effect in the Co:TiO₂ system. The results and conclusions of these studies are reported below.

Coexistence of Anomalous Hall Effect (AHE) and Superparamagnetism

We grew thin films of anatase and rutile Ti_1-xCo_xO_2-d (x=0, 0.01, 0.02) via pulsed laser deposition onto LaAlO₃ substrates (for anatase films) and R-Al₂O₃ substrates (for rutile films) at the substrate heater temperature of 700°C. Interestingly, highly reduced rutile films exhibited AHE as shown in Figure 1 [Shinde et al. PRL (2004)]. The magnitude of AHE was found to decrease with decreasing the carrier concentration. Thus, it may be tempting to interpret this data as related to the intrinsic DMS nature of highly reduced Co:TiO₂ films. However, our careful studies of magnetic measurement revealed that this indeed is not the case [Shinde et al. PRL (2004)].

Figure 1: Hall resistivity as a function of magnetic field for highly reduced Co:TiO₂ film.

The magnetic hyteresis loop data at room temperature and low temperature as well as temperature dependence of magnetization under filed cooled and zero field cooled conditions suggest the presence of magnetic nano particles in these films.

The M-H data for films with lower (x = 0.01) cobalt concentration in the highly reduced films (Fig. 3) show further evidence for the presence of single domain particles as well as superparamagnetism. A large H_C at 5 K, a rapid decrease of H_C and M_R , and their disappearance above ~250 K [insets (b) and (c)] can be clearly noted. These features indicate the occurrence of superparamagnetism with a blocking temperature (T_B) of 250 K. The temperature dependence of M-H curves for Ti_0.98Co_0.02O_2-d and Ti_0.96Co_0.04O_2-d films is similar to that of Ti_0.99Co_0.01O_2-d except with T_B higher than 380 K [insets (b) and (c)], which could not be measured with SQUID.

From the measured T_B, we estimate the particle diameter, D, to be ~7 nm corresponding to T_B = 250 K for the Ti_0.99Co_0.01O_2-d film. For Ti_0.98Co_0.02O_2-dfilm, a higher T_B suggests a particle size ~8–10 nm. The TEM data for the Ti_0.98Co_0.02O_2-d film (Fig. 4) enabled a direct observation of clustering and cluster size in the film. From electron diffraction pattern, these clusters were identified as cobalt metal clusters. Interestingly, these clusters are located at the film-substrate interface. The particles are about 9–10 nm in diameter and therefore are expected to show superparamagnetism.

From the above results, we concluded that the AHE can not be used as an unambiguous test of the intrinsic nature of diluted magnetic semiconductor (DMS) without a detailed microscopic characterization of the sample.

Percolative Ferromagnetism

Taking a clue from the previous observation [1] that the clusters formed in Co:TiO₂ films with higher doping concentration dissolve in TiO₂ matrix after 900^oC annealing, we grew films at higher substrate temperature (875^oC). Through detailed TEM and EELS characterization we confirmed that such films do not show any evidence of obvious clustering (Fig. 5 a,b & c)(bottom left column).

Figure 6 (Right panel). Magnetic hysteresis loops and the coercivity as a function of cobalt concentration. The hysteresis loops for film with cobalt concentration of (a) 7%, (b) 4%, (c) 2%, (d) 0.75%, (e) 0.5%, and (f) 0.25%. The inset of (a) shows the hysteresis loop of the 7% doped film on expanded scale. The coercivity and saturation magnetizations obtained from these loops is plotted in (Fig. 7 (top)) as a function of cobalt concentration.

Figure 7. Magnetization (M) and resistivity (r ) data for the Co:TiO₂ films as a function of cobalt concentration. (a) Magnetization decreases with decrease in cobalt concentration. (b) Plot of temperature dependent resistivity for different cobalt concentration in TiO₂. Right panel shows the dependence of activation energy and RT resitivity on Co conentrations.

Interestingly, these films exhibit lower saturation magnetization as compared to ~1.3 m_B/Co for those grown at 700^oC. Ferromagnetism is observed for the films with Co concentration as low as 0.25%. Figure 7(a)(top) shows magnetization as a function of Co concentration. Not only the magnetization systematically varies with Co concentration but also the shape of hysteresis loops is drastically changed [Fig. 7]. With the decreasing doping concentration, the coercivity follows a behavior expected for percolation system.

Given the highly insulating nature of the samples no itinerant electron based picture is feasible, and therefore the RKKY-type scenario discussed extensively in the context of GaMnAs magnetization simply does not apply here. Two other physical pictures which could conform naturally to the attendant insulating magnetic state may thus be relevant. One is the magnetic polaron percolation picture discussed in the context of strongly insulating DMS materials [4], and the other is the defect (F-center) state percolation model discussed by Coey et al. [3] in the context of the large ordered moment in Fe doped SnO₂ system. In brief, we observe that under the new high temperature growth conditions, Co distributes uniformly in the anatase TiO₂ matrix at low cobalt concentrations. Our data and computer simulation suggest that F-center based polaron formation and their percolation are responsible for FM [Shinde et al., cond-mat/0505265 (2005)].

In the polaron percolation picture [4], each local magnetic moment (the local moment bound to carriers) is the effective magnetic polaron and the whole system is a collection of a random distribution of these polarons. At T_c, the bound magnetic polarons form a percolating path leading to global ferromagnetism. (The magnetic percolation does not imply transport percolation since each polaron is strongly localized). This is schematically describe in below figure.

Electric Field effect

We have performed the effect of electric field on the magnetic properties of these films. The FET device structure used in these experiments was as shown in Figure 8. The sharp interfaces and very good structural quality of the device was observed during X-ray diffraction and TEM studies [Zhao et al., PRL 94, 126601 (2005)].

Fig. 9 shows the magnetic hysteresis loops of the Co:TiO₂ layer measured after PZT poling. The saturation magnetization of the Co:TiO₂ layer is seen to change between the two states (100 and 85 memu) for the +ve and –ve poling of PZT. The M_S value as a function of applied voltage is plotted in the lower inset in Fig. 9(a). It is very clear that M_S of Co:TiO₂ can be switched with a difference of about 15% by switching the polarization states of PZT. In Fig. 9 (b) we show the data of Fig. 9(a) on an expanded H-scale to bring out two significant points: (a) the loops are ferromagnetic with well-defined coercivity value, and (b) the coercive field is also modulated reversibly with positive and negative poling.

When the PZT is positively poled, its polarization points down inducing electron accumulation in the Co:TiO₂ layer towards the interface, while negative poling has an opposite effect on the distribution. In the presence of two interfaces for the Co:TiO₂ channel (PZT/ Co:TiO₂ and Co:TiO₂/SRO) and a long field penetration depth in insulating Co:TiO₂, the quantitative aspects of the effective degree of charge modulation/injection/depletion will be controlled by the corresponding band offsets. The polarization charge obtained from the inset of Fig. 9(a) is ~60 mC/cm², which corresponds to a surface density of ~3.5 x 10¹⁴ charges/cm². Since the sample is highly insulating, an equivalent of this charge will be induced or depleted as a function of polarity over the film volume across its thickness. This corresponds to about 3% minimum average modulation of carrier density. Within the framework of possible percolative mechanisms of ferromagnetism in diluted insulating system, this magnitude of modulation of carrier density is quite significant. Indeed, in the case of mixed-valent manganites wherein such percolation picture is known to be operative within the framework of electronic phase separation model, large electric field induced modulation effects have been demonstrated.

[1] S.R. Shinde et al., Phys. Rev. B 67 , 115211 (2003).

[2] Efrat Shimshoni and Assa Auerbach, Phys. Rev. B 55 , 9817 (1997).

[3] J. M. D. Coey, A. P. Douvalis, C. B. Fitzgerald, and M. Venkatesan, Appl. Phys. Lett. 84 , 1332 (2004).

[4] A. J. Kaminski and S. Das Sarma, Phys. Rev. Lett. 88 , 247202 (2002)

Publications:

Co-occurrence of superparamagnetism and anomalous Hall effect in highly reduced cobalt-doped rutile TiO_2-d films, S.R. Shinde, S.B. Ogale, J.S. Higgins, H. Zheng, A.J. Millis, V.N. Kulkarni, R. Ramesh, R.L. Greene, and T. Venkatesan Phys. Rev. Lett. 92, 166601 (2004).

Hall effect in cobalt-doped TiO_2-d, J.S. Higgins, S. R. Shinde, S.B. Ogale, T. Venkatesan, and R.L. Greene, Physical Review B 69, 073201 (2004).

Optical band-edge shift of anatase Ti_1-x Co_xO_2-d , J.R. Simpson, H.D.Drew, S.R. Shinde, R.J. Choudhary, S.B. Ogale, and T. Venkatesan, Phys. Rev. B 69, 193205 (2004).

Ferromagnetism in laser deposited anatase Ti_1-xCo_xO_2-d films, S. R. Shinde, S. B. Ogale, S. Das Sarma, J. R. Simpson, H. D. Drew, S. E. Lofland and C. Lanci, J. P. Buban, N. D. Browning, V. N. Kulkarni, J. Higgins, R. P. Sharma, R. L. Greene, and T. Venkatesan, Phys. Rev. B 67, 115211 (2003)

Temperature-dependent optical studies of Ti_1-xCo_xO₂, Guha S, Ghosh K, Keeth J G, Ogale S B, Shinde S R, Simpson J R, Drew H D, Venkatesan T. , Appl. Phys. Lett. 83, 3296 (2003)

Comparative x-ray absorption spectroscopy study of Co-doped SnO₂ and TiO₂, A. Lussier, J. Dvorak, Y.U. Idzerda, S. B. Ogale, S. R. Shinde, R.J. Choudary, and T. Venkatesan, J. Appl. Phys. 95, 7190 (2004)

Channeling and resonant backscattering investigations of Co doped diluted magnetic oxide films prepared by pulsed laser deposition, V.N. Kulkarni, S.R. Shinde, Y.G. Zhao, R.J. Choudhary, S.B. Ogale, R.L. Greene, and T. Venkatesan, Nucl. Instru. & Meth. B 219-20, 902 (2004).

XAS Characterization of Growth Parameter Effects for Pulsed Laser Deposited
Co_xTi_1-xO_2-d Films, A. Lussier, J. Dvorak, and Y.U. Idzerda, S. R. Shinde, S.B. Ogale, and T. Venkatesan, Physica Scripta, accepted for publication (2004)

Electric Field Effect in Diluted Magnetic Insulator Anatase Co:TiO₂, T. Zhao, S. R. Shinde, S. B. Ogale, H. Zheng, T. Venkatesan, R. Ramesh, and S. Das Sarma, Phys. Rev. Lett. 94, 126601 (2005).

Percolative Ferromagnetism in Anatase Co:TiO₂, S. R. Shinde, S. B. Ogale, Abhijit S. Ogale, S.J. Welz, A. Lussier, Darshan C. Kundaliya, H. Zheng, S. Dhar, M.S.R. Rao, R. Ramesh, Y.U. Idzerda, N.D. Browning and T. Venkatesan, Cond-mat/0505265 (2005).

Cobalt Site Symmetry and Oxygen Vacancies in Ferromagnetic Cobalt-Doped Oxides, A. Lussier, J. Dvorak, Y. U. Idzerda, S. R. Shinde, S. B. Ogale, and T. Venkatesan, J. Appl. Phys., submitted (2004)

B) Origin of Ferromagnetism in Mn-Zn-O System

Diluted magnetic semiconductors (DMS) have attracted considerable attention in the past few years in view of their projected potential for the development of novel magneto-opto-electronics [1-2]. The recent discovery of ferromagnetism above room temperature in low temperature processed MnO₂:ZnO has generated significant interest [3], because the corresponding results are in sharp contrast to all previous works on the same but high temperature processed system.

Realization of high temperature ferromagnetism in dilutely magnetic-impurity-doped functional wide bandgap oxide semiconductor such as ZnO is undoubtedly a major development if the ferromagnetism is unambiguously established to be intrinsic (carrier-induced) in nature. While Sharma et al. [3] have argued in favor of such intrinsic mechanism based on their interesting observations using various techniques, we felt it necessary to examine three new aspects to validate the claim or otherwise, namely, a) temperature dependent chemical reaction kinetics between manganese and zinc oxides, b) high temperature magnetization behavior, and c) low temperature interdiffusion of Mn and Zn oxides.

Employing suitably designed bulk and film studies, we demonstrate that the ferromagnetism in this system originates in a metastable phase rather than via carrier induced interaction between separated Mn atoms in ZnO. Thermo-Gravimetric Analysis (TGA) brings out the role of oxygen loss and phase changes during low temperature processing. The corresponding results for pure MnO₂ and 2% MnO₂ + 98% ZnO mixture presented in terms of percent weight loss are summarized in Fig. 1.

Figure 1. TGA data for MnO₂ (red) and ZnO + 2% MnO₂ powder mixture (blue) shown as weight loss %. The data for the mixture are slightly up-shifted as explained in the text. The weight levels for total conversion of MnO₂ into other stoichiometric forms are indicated by horizontal dashed lines.

Very interestingly, in the presence of ZnO the oxygen loss begins at a temperature as low as 450-500 K, which may signify Zn incorporation into Mn 4+ sites in MnO₂ causing oxygen release for charge neutrality or catalytical role of ZnO in forming interface phases of lower manganese oxides. Also, the downward jump occurs at a somewhat lower temperature near 780 K (507ºC). Importantly, the weight level stabilizes just below Mn₃O₄ instead of at the Mn₂O₃ level as in pure MnO₂ case. Remarkably, a second downward jump is noted near 980 K which also happens to be the temperature T* at which the magnetization disappears, as discussed in Figure 2. This is followed by a slow uptake of oxygen and weight gain up to ~ 1220 K followed by a considerable weight loss again. This confirms that T* is not really the Curie temperature, but a temperature at which the magnetic phase transforms into a non-magnetic phase, possibly releasing some oxidizable manganese phase causing oxygen uptake.

Figure 2. Plot of 300 K Magnetization (M) Vs applied magnetic field (H) for low temperature processed 2 at % MnO₂:ZnO bulk compound. The * on Mn in m B/Mn indicates that the Mn content is taken to be the quantity introduced in making the sample. The inset shows Magnetization as a function of temperature above 300 K.

Figure 3. XRD pattern for ZnO (black), sintered 2 at% MnO₂:ZnO (olive) mixture, and unsintered 2 at % MnO₂:ZnO (red) mixture. The intensity is plotted on log scale.

X-ray diffraction (XRD) data suggest that MnO₂ grains remain mostly unaffected, implying interdiffusion and ferromagnetism only in the surface shell as shown in Figure 3. The ferromagnetism persists up to ~980 K and further heating transforms the metastable phase and kills ferromagnetism.

Study of interface diffusion and reaction between thin film bilayers of Mn and Zn oxides shows that a uniform solution of Mn in ZnO does not form under low temperature processing ; instead a metastable ferromagnetic phase develops by Zn diffusion into Mn-oxide as shown in Figure 4 by Rutherford backscattering spectrometry and X-ray diffraction.

Figure 4. a, XRD patterns for as-grown (black) and annealed (red) thin film bilayer sample of ZnO (700 Å):Mn₃O₄ (600 Å), where Mn₃O₄ grew on ZnO due to epitaxial relationship. ZnO was grown epitaxially on c-Al₂O₃ at 700ºC in 5mTorr oxygen, followed by manganese oxide at 500ºC in 400 mTorr oxygen. Inset compares the spectra on an expanded scale. After annealing treatment, the peaks corresponding to Mn₃O₄ are seen to have shifted to higher angles while those corresponding to ZnO to lower angles, although the latter with smaller magnitude. This is highlighted in the inset. Such shifts can occur due to dopant incorporation by interdiffusion or stress caused by a thin interface phase with different lattice parameters. If such phase is polycrystalline, however, it will not appear in the film x-ray diffraction. b, Rutherford backscattering (RBS) spectra recorded along random and axially aligned directions for the case of Mn₃O₄ (170 Å) grown at 500ºC on epitaxial ZnO film (3650 Å) (black), grown on c-axis single crystal sapphire substrate. The data for the same bilayer annealed for 3 hrs (red), 9 hrs (blue) and 18 hrs (olive) are also shown. Dashed curve is a simulated spectrum showing what one would expect if all Mn was uniformly dispersed in ZnO.

Finally, direct low temperature film growth of Zn incorporated Mn-oxide by pulsed laser deposition shows ferromagnetism at low Zn concentration for an optimum oxygen pressure during growth. In Fig. 5 we show the XRD and magnetization data for Zn incorporated manganese oxide films grown on single crystal c-axis sapphire at 500ºC using a high temperature sintered Zn_xMn_3-xO₄ target. This attempt was made to explore the possibility of incorporating and stabilizing Zn directly into the manganese oxide during low temperature growth, instead of via diffusion as discussed earlier, and to confirm thereby that Zn incorporation in Manganese oxide can lead to ferromagnetism under appropriate conditions. In the 2% Zn incorporated film grown at 400 mTorr ferromagnetism was observed and the corresponding XRD (black) showed presence of Mn₂O₃ and Mn₃O₄ phases. When the 2% Zn incorporated film was grown at lower pressure of 100 mTorr, only Mn₃O₄ peaks (Olive) were noted but no ferromagnetism was observed. In the case of 4% Zn incorporated film grown at 400 mTorr, ferromagnetism was observed and once again Mn₂O₃ peaks were seen in addition to the Mn₃O₄ peaks (red). When 10% Zn incorporated film was grown at different oxygen pressures over a range from 100 mTorr to 1 Torr (a representative case shown in Fig. 5), only Mn₃O₄ peaks were seen and no ferromagnetic signal was observed (blue). These data together tell us that increase in Zn concentration and/or lowering of oxygen pressure lead to Mn₃O₄ (with Zn) without ferromagnetism. This implies that Zn incorporated Mn₃O₄ is not a likely candidate for the high temperature ferromagnetic phase. Indeed, substitutional incorporation of substantial Zn in Mn₃O₄ spinel is known to lead to low temperature ferrimagnetism, same as that of Mn₃O₄ but with lower Curie temperature.

Figure 5 . In the 2% Zn incorporated film grown at 400 mTorr, ferromagnetism (inset 1) was observed and the corresponding XRD (black) showed presence of Mn₂O₃ and Mn₃O₄ phases. In the case of 4% Zn incorporated film grown at 400 mTorr, ferromagnetism was observed (inset 2) and Mn₂O₃ peaks were again seen in addition to the Mn₃O₄ peaks (red). When 10% Zn incorporated film was grown at 400 mTorr or at different pressures, only Mn₃O₄ peaks were seen (blue) and no ferromagnetic signal was observed. When 2% Zn incorporated film was grown at lower pressure of 100 mTorr (Olive), only Mn₃O₄ peaks were noted but no ferromagnetism was observed.

Appearance of Mn₂O₃ signatures in ferromagnetic film samples of Zn incorporated Mn-oxide favors the identification of Mn_2-xZn_xO_3-d phase with high temperature ferromagnetism and is completely consistent with the bulk and film data.

In conclusion, we show through a series of suitably designed bulk and thin film experiments that ferromagnetism in low temperature proceeded Mn-Zn-O system does not originate through a carrier induced mechanism between well separated and uniformly dispersed Mn atoms in ZnO matrix. Our data collectively suggest that an oxygen vacancy stabilized metastable phase, strongly indicated to be Mn_2-xZn_xO_3-d is responsible for the high temperature ferromagnetism.

[1]  Ohno, H. Making nonmagnetic semiconductors ferromagnetic. Science 281, 951-956 (1998).

[2] Wolf , S. A. et al. Spintronics: A spin-based electronics Vision for the future. Science 294 , 1488- 1495 (2001).

[3] Sharma, P. et al. Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO. Nature Materials 2 , 673-677 (2003).

Publication: Kundaliya et al., Nature Materials 3, 709 (2004)

C) Ferromagnetism in Hafnium Oxide

HfO₂ (Hafnia) is a wide band gap oxide high-k dielectric material (E_g ~4.5 eV). High thermal stability and low leakage current rate of hafnia makes it a potential replacement for conventional SiO₂ gates in metal-oxide-semiconductor (MOS) devices and devices for quantum computation [1]. We studied the structural and magnetic properties of dilutely (5 mol%) doped 3d elements (Co and Fe) in PLD grown HfO₂ high-k dielectric thin films for possible magneto-optic applications in view of the recent spur in research activity in new diluted magnetic semiconductor (DMS) systems [2-6].

We observed ferromagnetism at and above room temperature in Co and Fe doped HfO₂ thin films. Contrary to the recent report in Nature 430, 630 (2004), we do not see ferromagnetism in undoped HfO₂. We show that the ferromagnetism arises from a substitutional incorporation of the dopant. The concentration dependence of the magnetic moment (supported by simulation data) suggests a picture of percolative ferromagnetism involving a defect based exchange mechanism. In the context of intense research efforts on diluted magnetic semiconductor (DMS) oxides, this work threw new light on the understanding of ferromagnetism in insulating DMS. On the technology front, ferromagnetic high-k dielectric layers could provide a path for linking conventional CMOS with spintronic device ideas and should be of interest as spin filters in spin based quantum computing architectures.

X-ray diffraction (XRD) patterns for undoped and doped Hafnium Oxide. a, XRD patterns of undoped (black), 5mol% Co (blue) and Fe (red) doped hafnia films. b, A clear shift in the (002) peak of the hafnia phase indicates dopant incorporation. c, Note the narrowing of the FWHM rocking curves with doping.

High resolution Z-contrast TEM image of Co doped hafnia films showing epitaxial growth on YSZ. No cluster formation is seen. EELS line scans clearly show the presence of Co in an ionic state and the distribution of the dopant is homogeneous.

Cobalt Concentration dependence results in HfO₂ by Magnetic Force Microscopy (MFM) and Magnetization . Magnetic saturation (M_S) as a function of Co-concentration in Hf_0.95Co_xO₂ (0.03<x<0.05) films gown at 850^oC and 1x10^-6 torr. The decrease in M _S values with decrease in x, indicates a percolative magnetic behaviour. This scenario is well supported by the magnetic phase data obtained by magnetic force microscopy.

Magnetization and Magneto Optical Kerr Effect (MOKE) results of undoped and doped HfO₂. a, M-H curves obtained at room temperature on Hf_0.95Co_xO₂ (0.03<x<0.05) films gown at 850ºC and 1x10^-6 torr show the occurrence of ferromagnetism; b, No ferromagnetic hysteresis is seen in the undoped HfO₂ film deposited at the conditioned used in fig. a; c, M vs H/T curves of the Hf_0.95Co_0.05O₂ films do not show superparamagnetic behaviour; d, Magnetization vs time plot shows a robust ferromagnetic behaviour with t ~10⁵ s; In inset, MOKE study indicates ferromagnetism at room temperature in 5 mol% Co-doped HfO₂ film.

1. D.P. DiVincenzo, J. Appl. Phys. 85 , 4785 (1999).

2. H. Ohno , Science 281 , 951 (1998).

3 . A. Oiwa, Y. Mitsumori, R. Moriya, T. Slupinski, and H. Munekata , Phys. Rev.
Lett. 88 , 137202 (2002).

4. Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S.-Y. Koshihara, and H. Koinuma, Science 291 , 854 (2001).

5. S.R. Shinde, S.B. Ogale, S. Das Sarma, J.R. Simpson, H.D. Drew, S.E. Lofland, C.
Lanci, J.P. Gugan, N.D. Browning, V.N. Kulkarni, J. Higgins, R.P. Sharma, R.L.
Greene, and T. Venkatesan , Phys. Rev. B 67 , 115211 (2003).

6. S.B. Ogale, R. J. Choudhary, J.P. Buban, S.E. Lofland, S.R. Shinde, S.N. Kale,
V.N. Kulkarni, J. Higgins, C. Lanci, J.R. Simpson, N.D. Browning, S. Das Sarma, H.D.Drew, R.L. Greene, and T. Venkatesan , Phys. Rev. Lett. 91 , 077 205 (2003).

Publication: M.S.R Rao……….T. Venkatesan, Unpublished

D) Co:SnO₂ System

Occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn_1-xCo_xO_2-d (x<0.3). Interestingly, films of Sn_0.95Co_0.05O_2-d grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5 ± 0.5 m_B /Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

Publication: S.B. Ogale…….. T. Venkatesan, Phys. Rev. Lett. 91, 077205 (2003)

E) La_0.5Sr_0.5Ti_1-xCo_xO₃ System

Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite (La_0.5 Sr_0.5TiO_3-d. The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (<similar to 2%), but develops inhomogeneity at higher cobalt concentrations. The films range from being opaque metallic to transparent semiconducting depending on the oxygen pressure during growth and are yet ferromagnetic.

Magnetization vs temperature (M-T) curve for the La_0.5Sr_0.5Ti_0.98Co_0.015O_3-d film. The line represents five point averaged data. The inset shows hysteresis loop at 423 K.

Temperature dependence of resistivity for the La_0.5Sr_0.5Ti_0.985Co_0.015O_3-d films grown at (a) 10^-2, (b) 10^-4, and (c) 3×10^-6 Torr. The 300 and 5 K resistivity and the (002) lattice parameter measured at 300 K for the films grown at different oxygen pressures are shown in (d). The dependence of magnetization on resistivity at 5 K is plotted in (e). The carrier density is also shown for a few cases of interest. The magnetoresistance data are shown in (f).

Publication: Y.G. Zhao, S.R. Shinde, S.B. Ogale,……….T. Venkatesan, Appl. Phys. Lett. 83, 2199 (2003)

F) Cu₂O system

We explored the possibility of inducing ferromagnetism in cuprous oxide (Cu₂O) by dilute cobalt doping. In addition to doping with 5% Co, we have also examined cases of codoping with 0.5% Al, V, or Zn (each of which bears a different valence state) in an attempt to influence magnetism through possible carrier concentration and defect state changes. Cuprous oxide can be grown epitaxially on (001) MgO by pulsed laser deposition. This material is an insulator with a band gap of about 2 eV and room temperature resistivity in the range of 10²–10⁶ cm. This value is seen to be highly influenced by the deposition technique. Cuprous oxide is useful as an energy converter for solar cell applications and as a humidity and gas sensor material. It is an attractive material because it has advantages of nontoxicity, a high absorption coefficient, and low production cost.

Plot of magnetization as a function of the temperature for the Co-doped Cu₂O film without and with Al, V, and Zn codoping, measured from 4.2 to 300 K using a SQUID magnetometer.

Room temperature hysteresis for the Al codoped Co:Cu₂O sample. A well defined loop with coercivity of about 50 Oe signifies ferromagnetism.
   
Publication: S.N. Kale, S.B. Ogale,……….T. Venkatesan, Appl. Phys. Lett. 82, 2100 (2003).

2) Multiferroic

Multifunctional materials have attracted significant interest in recent years due to the coupling of their ferromagnetic and piezoelectric/ferroelectric properties which is of great interest from the fundamental as well as applied points of view. Multiferroics, having simultaneous ferroelectric and magnetic order, provide the powerful new functionality of cross responses between magnetic and electric field effects, such as the use of an applied electric field to control the magnetic state of a device and a magnetic field ( H ) to control the dielectric response and polarization. With an intrinsically non-centrosymmetric character, interesting linear and non-linear optical and magneto-optical phenomena are possible. GaFeO₃ (GFO) is an interesting example in this context which exhibits ferromagnetic and pyroelectric properties simultaneously. This material was first discovered by Remeika et al. (J. Appl. Phys. 31 (1960) 2635)

A) GaFeO₃

 Recently, magnetization-induced second harmonic generation (MSHG) and X-ray directional dichroism have been investigated in single crystal GFO (PRL 92, p. 47401 and p. 137401). Remarkably large effects have been found due to the intrinsically non-centro-symmetric nature of this crystallographic system. For building technologically viable device systems it is important to develop such multifunctional materials as crystalline thin films on different substrates, most desirably on Si. So, epitaxial thin films of gallium iron oxide (GaFeO₃) are grown on (001) silicon by pulsed laser deposition (PLD) using yttrium-stabilized zirconia (YSZ) buffer layer. The crystalline template buffer layer is in-situ PLD grown through the step of high temperature stripping of the intrinsic silicon surface oxide. The X-ray diffraction pattern shows c -axis orientation of YSZ and b -axis orientation of GaFeO₃ on Si (100) substrate (top left figure). The ferromagnetic transition temperature (T_c ~215 K) is in good agreement with the bulk data (top right figure). The films show a large nonlinear second harmonic Kerr rotation of ~15 degrees in the ferromagnetic state (bottom figure).

Analyzer angle ( q ) dependence of the second harmonic light (a) at room temperature (above T_c ) and (b) at 100 K (below T_c ). q = 0 corresponds to p polarization and open (closed) circles correspond to a negative (positive) field of 3 kOe

Publication: D.C. Kundaliya, S.B. Ogale,........., T. Venkatesan, (J. Mag. Mag. Mat. to be published, 2005)

3) Wide Band Gap Semiconductors

A) High Temperature Compatible Capping of SiC for dopant activation

The activation of dopants in SiC is a major technological challenge, and needs to be annealed at 1600^oC and above temperatures to get the carriers available for the device operation. The major concern is the non-stoichiometry of the SiC surface after annealing and hence the need of capping layer that protects the SiC surface, non-reactive and can be easily removed after the annealing step. We have developed AlN based capping layer for 1600^oC annealing process. The SiC junction diode is shown after annealing at 1600^oC in conventional SiH 4 capped process which leaves the rough surface. On the other hand AlN capped device has the smooth coverage. We have developed multi-layer AlN/BN capping materials that will enable the process up to 1700^oC that can be completely etched of chemically after the process.

B) AlN based planar and side wall dielectric for SiC based devices

Our current research activities under this program are focused on the development of dielectrics for SiC devices, related processing and device fabrication to achieve ARL's goals on the power electronics. The major goal is to demonstrate high temperature power devices such as, power diodes, switches, and motor drive electronics that will operate for 10,000 hours at junction temperatures of 300°C and above.

AlN has been developed as the high temperature dielectric with the high breakdown strength and low leakage current layer on SiC based MIS deice. We also optimized the AlN covering on the side walls of the vertical SiC based device.

Low Leakage AlN/SiC M-I-S structure

AlN Passivation on SiC based device

C) AlN MEMS and NEMS Resonators for RF Communications

Advanced military and commercial systems need a wide range of microelectromechanical systems (MEMS) and related technologies for a variety of operations in harsh environment such as high temperatures, intense vibrations, erosive flow, corrosive media, and aerospace. Among these applications, RF MEMS in wireless communications is a driving force which can be employed in the next generation of wireless integrated micro-systems. AlN has been identified as a suitable candidate for achieving the higher resonant frequencies because of the lower density and higher Young's modulus as compared to PZT and ZnO (which are most often been used).

    
D) ZnO and MgZnO alloys for UV Detectors

Facility

Wide Bandgap Semiconductor Group

4) Electroresistance and Electronic Phase Separation in     
    Mixed-valent Mangar

Colossal magneto-resistance (CMR) - a huge decrease in resistivity by the application of magnetic field observed in mixed-valent manganites - has triggered intense scientific activity in recent years; yet, the mechanism of the effect is still not fully understood. One aspect of the phenomenon is a bulk metal to insulator transition tuned by changing spin correlations, but possibly involving other degrees of freedom also. Recent studies however suggest that the largest magnetoresistance in these systems is associated with spatial inhomogeneity related to multiphase coexistence. Multiphase coexistence generically causes a sensitivity of physical properties to external perturbations. We examine the sensitivity to external fields in the case of CMR manganites using PZT-ferroelectric or SrTiO 3 -dielectric based field effect configurations with La 0.7 Ca 0.3 MnO 3 (LCMO), Na 0.7 Sr 0.3 MnO 3 (NSMO), and La 0.7 Ba 0.3 MnO 3 (LBMO) channels, subjected to electric and magnetic fields, separately and in conjugation. We find that in our device, modest electric fields (~ 4 x 10^5 V/cm) cause very large changes (~75%) in resistivity in the case of LCMO, but the magnitude of the effect is much smaller (a few percent) in the case of NSMO and LBMO. We argue that these results support a percolative phase separation picture of transport in the LCMO film, with the insulating and metallic domains evenly balanced, while the NSMO and LBMO films are preponderantly insulating and metallic, respectively.

T. Wu†, S. B. Ogale*, J. E. Garrison, B. Nagaraj, Amlan Biswas, Z. Chen, R. L. Greene, R. Ramesh and T. Venkatesan, A. J. Millis, Phys. Rev. Lett. 86, 5998 (2001)

Dependence of resistivity of the LCMO channel on temperature, for field biasing with a PZT gate over an applied gate voltage range from +6 to -6 volts. Inset shows the device configuration.

Dependence of LCMO channel resistivity on temperature for the unbiased (A) and electric field-biased (B, 4x10^5 V/cm) channel, in the absence of magnetic field. The dependence (A) changes to (C), and (B) changes to (D) under a magnetic field of 6 T. The insets show MR Vs T in 6 T for the unbiased and E-field-biased CMR channel, and ER Vs T for the channel with and without a H field of 6 T.

5) Probing the Magnetism and Chemistry of a Burried Oxide Layer

Recent years have witnessed a tremendous growth of research in the field of magnetoelectronics, in view of its obvious potential for novel devices with entirely new capabilities. In this context, phenomena such as giant magnetoresistance (GMR), colossal magnetoresistance (CMR), spin-tunneling in junctions (STJ), and more recently spin coherence and spin phasing have attracted significant attention. The heterostructures/multilayers used in these studies generally involve ternary or quaternary alloys/compounds having magnetic, semiconducting, or superconducting properties, and the device action of interest generally occurs at the burried interface. It is thus of critical importance to know the properties of such an interface. Recetly, Idzerda and coworkers at NRL have developed a method to probe the burried interface, based on a combined use of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism(X-MCD). In our work, done in collaboration with Idzerda, we have used this method to elementally map the magnetic and chemical quality of the interface of La 0.7 Sr 0.3 MnO 3 (LSMO) covered by various overlayers. LSMO is characteristic of the class of Mn perovskites which display an extremely high degree of spin polarization, making them preferred candidates for magnetic-based devices which exploit spin polarized electron transport.

XMCD is an element-specific, magnetic spectroscopic tool where the difference in the absorption of left- and right-circular polarized photons is measured at the absorption edges of the relevant elements. Two total electron yield spectra, one with the incident light helicity (circular polarization) oriented parallel to the remnant magnetization of the sample and one anti-parallel, are collected as the X-ray energy is swept continuously through the L2,3 edge energies of a constituent element. The XAS is the average of these two recorded spectra, while the XMCD is the difference between the two. In XAS and XMCD, the excited electron probes the unfilled states above the fermi level to reveal local chemical (and, also from XMCD, magnetic) information about the element. All data reported here were collected at the NRL-NSLS Magnetic Circular Dichroism Facility located at beamline U4B of the National Synchrotron Light Source (NSLS). With our experimental configuration, the measurements are sensitive to the top 50-100 Å of the LSMO film. Based on the XAS and XMCD data thus obtained we have been able to establish that the deposition of YBa 2 Cu 3 O 7-ò on La 0.7 Sr 0.3 MnO 3 results in the diffusion of La away from the interfacial region, strongly modifying the interfacial electronic and magnetic properties. The results for other overlayers such as SrTiO 3 or LaAlO 3 are different.

In the XAS spectra shown in fig. 1 for YBa 2 Cu 3 O 7-ò on La 0.7 Sr 0.3 MnO 3 , there is a continuous energy shift of the L 3 peak, a narrowing in the energy distribution of the L 3 peak, and the development of a new feature at a fixed lower energy. Simultaneously, the XMCD intensity is observed to simply decrease with increasing YBCO coverage without any significant changes in the spectra. The variations in the XAS spectra with increased YBCO thickness are very similar to those for La 1-x Sr x MnO 3 as the composition is changed from La 0.7 Sr 0.3 MnO 3 to La 0.1 Sr 0.9 MnO 3 . This strongly suggests that the deposition of YBCO on LSMO initiates the elimination of La from the LSMO interfacial region, resulting in a more Sr rich alloy at the interface. This is well supported by the observed decrease in the XMCD intensity. Whereas La 0.7 Sr 0.3 MnO 3 is ferromagnetic, as the relative concentration of Sr is increased, the alloy becomes antiferromagnetic, no longer contributing to the XMCD signal. Therefore, although the XAS spectra (the average of the two helicity spectra, sensitive to the chemical state of the Mn) is continuously evolving with YBCO coverage, the XMCD spectra (the difference of the two helicity spectra, sensitive only to the ferromagnetic component of the Mn) simply diminishes in intensity without changing shape.

Figure 1: The Mn L-edge XAS (TOP) and XMCD (BOTTOM) spectra for various coverages of YBCO on LSMO. Included are spectra for 80 Å LNO/LSMO.

Figure 2: The Mn L-edge XAS and XMCD spectra for various coverages of STO (TOP) and LAO (BOTTOM) on LSMO.

The cation outdiffusion that we have identified here is not particular to the deposition of YBCO. In Figure 1, in addition to the YBCO coverage data, we have included the spectra for an 80 Å coverage of LNO (where similar interfacial degradation is observed). The LNO spectra match those for the evolving YBCO coverage quite well, suggesting that the interfacial disruption is not driven by the interfacial chemistry, but is most likely strain induced from lattice mismatch of the overlayer to the LSMO substrate.

Not all overlayer depositions result in interfacial degradation. The deposition of insulating STO and LAO on LSMO results in similar high quality epitaxial structures, but with very improved transport properties. A combined XAS and XMCD study of the interfacial development of these materials (shown in figure 2) show that no variations in the Mn L 2,3 edge XAS spectra occur and, for the STO overlayers, increased XMCD intensities are observed. The improved XMCD spectra indicates that the STO acts as an excellent passivation layer for LSMO and that the bare LSMO surface is slightly detrimentally affected by the in-air transfer between vacuum systems.

The elemental variation in interface distribution can also be quantified by X-ray resonant magnetic scattering (XRMS). XRMS is the angle dependent scattering of circular polarized X-rays, whose energy is tuned to the absorption edge of a magnetic element. Ydzerda and coworkers have shown that this method combines the element selectivity of X-ray resonant scattering with the magnetic contrast of XMCD and can be used to separately parameterize the magnetic and chemical roughness of buried interfaces. Such work on the oxide-oxide interfaces of our interest is currently in progress.

The work at the University of Maryland was supported by NSF MRSEC (Grant #DMR-96-32521) and ONR (Grant #N000149810092). The NRL work was supported by ONR. The Brookhaven National Laboratory is supported by DOE

Reference : The magnetism of a buried La0.7Sr0.3MnO3 interface., Stadler S, Idzerda YU, Chen Z, Ogale SB, Venkatesan T, Appl. Phys. Lett. 75, 3384 (1999).

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