GROWTH AND CHARACTERIZATION OF EPITAXIAL THIN HETEROSTRUCTURES OF FERROMAGNETIC/ANTIFERROMAGNETIC SrRuO₃/Sr₂YRuO₆

R.A. PRICE, M.K. LEE, C.B. EOM

Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708

, M.S. RZCHOWSKI

Department of Physics, University of Wisconsin-Madison, Madison, WI

ABSTRACT

We have grown epitaxial thin films of antiferromagnetic ruthenate Sr₂YRuO₆ on miscut (001) SrTiO₃ by 90° off-axis sputtering. Sr₂YRuO₆ is a unique material that allows us to grow epitaxial ferromagnetic/antiferromagnetic heterostructures. Antiferromagnetic Sr₂YRuO₆ has the same pseudo-cubic perovskite crystal structure as the ferromagnetic conductive oxide SrRuO₃. The Sr₂YRuO₆ perovskite crystal structure has Y and pentavalent Ru located on the octahedral sites and the pseudo-cubic lattice parameter of 4.08�. The Neel temperature of bulk Sr₂YRuO₆ is known to be 26K. Four-circle X-ray diffraction analysis revealed the Sr₂YRuO₆ films are purely (110) normal to the substrate with two 90° domains in the plane. We have also grown epitaxial heterostructures of SrRuO₃/Sr₂YRuO₆. These bilayers permit detailed studies of the magnetic exchange bias phenomena at these interfaces, including the role of uncompensated spins thought to arise from interface roughness. Magnetization measurements on the SrRuO₃/Sr₂YRuO₆ heterostructures show a shifting of the hysteresis loop, indicating exchange bias. Such exchange-biased interfaces are important for electrode pinning in magnetic tunnel junctions.

INTRODUCTION

Sr₂YRuO₆ is an interesting material among oxides with ordered perovskite structure because of its antiferromagetic character as a bulk material. The successful growth of antiferromagnetic Sr₂YRuO₆ thin films makes possible the fabrication of a unique ferromagnetic/antiferromagnetic bilayer. This bilayer system allows the detailed study of interface interactions and the exchange bias effect in a high quality ferromagnetic/antiferromagnetic bilayer system. Battle et al. have reported that bulk Sr₂YRuO₆ exhibits Type I antiferromagnetic ordering with a Neel temperature of 26K. In Type I antiferromagnetic ordering the nearest neighbor ions are antiferromagnetically coupled to the central ion and the next-nearest neighbors are ferromagnetically coupled. From their results, they concluded that bulk Sr₂YRuO₆ has a magnetic structure consisting of ferromagnetic (001) sheets that are coupled antiferromagnetically along the [001] direction with dipolar interactions causing the spins to be in the (001) planes, as seen in Figure 1, and a ordered magnetic moment of 1.85m_B per Ru⁵⁺ ion. [1] Pentavalent Ru

Fig. 1

Type I antiferromagnetic magnetic

ordering in bulk Sr₂YRuO₆. (001)

ferromagnetic sheets are coupled

antiferromagnetically along [001].

and Y are located on the octahedral sites of the perovskite Sr₂YRuO₆ crystal structure which has orthorhombic unit cell parameters a = 5.752�, b = 5.773�, and c = 8.158�. [2] Using these parameters, the pseudocubic lattice parameter is calculated to be 4.08Å. Further studies show that bulk Sr₂YRuO₆ is not a metallic conductor and has an electrical resistivity of 5.9×10⁴ W-cm at 300K. [3]

The reasons for choosing the conductive oxide SrRuO₃ for the ferromagnetic layer in the bilayer system are two-fold. SrRuO₃ thin films have a pseudocubic perovskite crystal structure and pseudocubic lattice parameter 3.905�, similar to Sr₂YRuO₆. Also, structure and crystal quality studies have shown that single domain in-plane single crystal growth of SrRuO₃ is possible on the smooth surface of SrTiO₃. Uniform growth can occur because of the small unit cell step height, 4Å, of SrTiO₃. [4] Therefore, SrRuO₃ has the potential for providing a good growth surface for Sr₂YRuO₆ film growth with a very uniform interface between the Sr₂YRuO₆ and SrRuO₃ layers. A high quality bilayer allows unprecedented control and optimization of exchange bias, an important phenomena in modern magnetic technology.

EXPERIMENT

The Sr₂YRuO₆ films were deposited on (001) SrRuO₃ substrates, miscut 0°, 0.8°, and 2° towards [100], by 90° off-axis sputtering using a stoichiometric target. The films were deposited at an operating pressure of 200mTorr (60%Ar/40%O₂) and temperature of 600°C. The samples were cooled to room temperature at an oxygen pressure of 300 Torr. The thickness of the films is approximately 1000�. Thicker films, approximately 3000�, were deposited on the same substrate materials and under the same conditions except for an increased operating temperature of 700°C. The temperature was increased in an attempt to improve crystalline quality, but similar results for the two different films showed that this temperature change was not very significant. X-ray diffraction analysis, q-2q scans and f scans, was used to characterize crystallographic orientation and domain structure and sensitive magnetic measurements were made with the SQUID magnetometer.

RESULTS

The Sr₂YRuO₆ films were deposited on (001) SrTiO₃ substrate samples that are miscut towards [100]. The rocking curve FWHM values ranging from 0.5507° to 0.6025° showed that the films had high crystalline quality. The out-of-plane lattice parameter of 4.05� is determined from the x-ray diffraction q-2q scan of Sr₂YRuO₆ film on 2° miscut (001) SrTiO₃ substrate, as shown in Figure 2, which shows the film peaks at 21.97° for the (110) reflection and at 44.80° for the (220) reflection. This value is similar to the pseudocubic lattice parameter of bulk Sr₂YRuO₆ (4.08Å). Therefore, incoherent growth occurs. Azimuthal x-ray scans of the off-axis (110) and (221) reflection indicate an epitaxial arrangement of two 90° domains in the film plane, as shown in Figure 3.

Magnetic measurements using a SQUID magnetometer were performed as a function of temperature and magnetic field. After grinding the backside of the SrTiO₃ substrate, the magnetization measurement at 5K showed a diamagnetic signal from the SrTiO₃ substrate that deviated slightly around 0 Oe. This result was different from the result of the measurement made prior to grinding the back surface, which showed weak ferromagnetic behavior. This indicates that there is some weak ferromagnetic component in the Sr₂YRuO₆ film, which is most likely due to defects and impurities in the film. Figure 4 shows this magnetization curve after subtracting out the diamagnetic background signal; only a very weak signal is apparent.

Fig. 2 Fig. 3

X-ray diffraction q–2q scan of X-ray diffraction off-axis f scan of

S_r2YRuO₆ thin film on 2°miscut the Sr₂YRuO₆ (002)° reflection.

(001) SrTiO₃.

Fig. 4

Magnetization curve for Sr₂YRuO₆

film on miscut (001) SrTiO₃ substrate.

The magnetic measurements made on the SrRuO₃/Sr₂YRuO₆ bilayer samples also indicate the antiferromagnetic nature of Sr₂YRuO₆ films. SrRuO₃/Sr₂YRuO₆ bilayers were grown on miscut (001) SrTiO3 by 90° off-axis sputtering with similar deposition conditions used for Sr₂YRuO₆ thin film growth. The two layers were not deposited in situ. X-ray diffraction analysis confirmed the epitaxial growth of Sr₂YRuO₆ on the SrRuO₃ film, as shown in Figure 5. Magnetization measurements made on these samples indicate exchange bias phenomena. The magnetization results show a shifting of the hysteresis loop to the left for the Sr₂YRuO₆/SrRuO₃ bilayer. The amount of offset of the hysteresis loop is temperature dependent and is best detected at 5K. Figure 5 compares the hysteresis loops for the 100� bilayer sample with the hysteresis loop of the 100� SrRuO₃ thin film. The results show a shifting of 1.02kOe to the left for the bilayer sample. The large coercive field of the SrRuO₃ layer makes the shifting of the hysteresis loop difficult to detect. The thicker bilayer sample has a smaller coercive field, approximately 0.75kOe at 5K, but the shifting of the hysteresis loop is also reduced and even more difficult to detect. The hysteresis loop shifting suggests that the SrRuO₃ film layer is being “pinned” by the Sr₂YRuO₆ upper film layer, a characteristic of a ferromagnetic/antiferromagnetic bilayer.

Fig. 5a Fig. 5b

X-ray diffraction q-2q scan of SrRuO₃/Sr₂YruO₆ X-ray diffraction off axis f scan of (a)

bilayer on 2° miscut (001) SrTiO₃ substrate. Sr₂YRuO₆ (002) reflection and (b)

SrRuO₃ with [110] normal orientation.

	H (Oe)

Fig. 6a Fig. 6b

The shifting of the hysteresis loop for The symmetry of the hysteresis loop for a

SrRuO₃/SrYRuO₆ bilayer sample at 5K 100� SrRuO₃ thin film on (001) SrTiO₃

can be detected in this magnetization substrate at 5K is seen in this magnetization

curve. curve. The field is applied parallel to the

[110] in plane.

CONCLUSIONS

We have grown films and SrRuO₃/Sr₂YRuO₆ bilayers on miscut (001) SrTiO₃ by 90° off-axis sputtering and studied their epitaxial arrangement and magnetic properties. These studies have shown that antiferromagnetic Sr₂YRuO₆ films with two 90° domains can be grown. The successful growth of the SrRuO₃/Sr₂YRuO₆ bilayer provides a new system for studying exchange bias phenomena and spin ordering at the interfaces of oxide materials.

ACKNOWLEDGMENTS

This work was supported by the NSF-REU Program, the David and Lucile Packard Fellowship, the NSF Young Investigator Award, and the NSF-DMR.

REFERENCES

1. P.D. Battle and W.J. Macklin, J. of Solid State Chem. 52, p. 138-145 (1984).

2. P.C. Donohue and E.L. McCann, Mat. Res. Bull. 12, pp.519-524 (1977).

3. R. Greatrex, N. Norman, M. Lal, and I. Fernandez, J. of Solid State Chem. 30, p. 137-148 (1979).

4. R.A. Rao, Q. Gan, C.B. Eom, Applied Physics Letters 71, p. 1171-1173 (1997).