Magnetoresistance and Hall effect of chromium ... - Brown University

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Department of Physics, Brown University, Providence, Rhode Island 02912 ..... Rev. Lett. 59, 2788 1987. 11 H. Brändle, D. Weller, S. S. P. Parkin, J. C. Scott, ...

JOURNAL OF APPLIED PHYSICS

VOLUME 85, NUMBER 8

15 APRIL 1999

Magnetoresistance and Hall effect of chromium dioxide epitaxial thin films X. W. Li Department of Physics, Brown University, Providence, Rhode Island 02912

A. Gupta, T. R. McGuire, and P. R. Duncombe IBM T. J. Watson Research Center, Yorktown Heights, New York 10598

Gang Xiao Department of Physics, Brown University, Providence, Rhode Island 02912

Epitaxial CrO2 thin films have been grown on TiO2(100) and Al2O3(0001) substrates by atmospheric pressure chemical vapor deposition. The films have a Curie temperature (T c ) of around 393 K with the ones grown on TiO2 exhibiting in-plane uniaxial magnetic anisotropy. They also display metallic characteristics, with room temperature resistivity of about 285 mV cm, dropping by about two orders of magnitude upon cooling down to 5 K. Magnetoresistance ~MR! properties of the films have been measured with the magnetic field in the plane. For a field of 40 kOe, a positive transverse MR of about 25% at 5 K and a negative MR of about 7% at near T c have been observed. In addition, Hall resistivity has been measured with magnetic field up to 40 kOe. A positive ordinary Hall effect is found at low temperatures, indicating the conduction carriers are holes. © 1999 American Institute of Physics. @S0021-8979~99!25908-7#

I. INTRODUCTION

zone where it decomposes on the substrates to form CrO2 with evolution of O2. The source zone temperature and the oxygen flow rate are fixed at 260 °C and 100 cc/min, respectively. We have found that over a range of substrate temperature around 400 °C, single phase CrO2 films can be grown epitaxially on TiO2 and Al2O3 substrates. The film thickness, determined by Rutherford backscattering spectroscopy ~RBS! after deposition, has been varied from 400 to 8000 Å. Ex situ RHEED analysis along with x-ray diffraction and RBS channeling measurements have been used for structural characterization of the films. Standard four-probe dc method has been used for the resistivity measurements, whereas the van der Pauw method is used for the Hall measurements. The magnetic measurements have been carried out using a superconducting quantum interference device ~SQUID! magnetometer.

Chromium dioxide (CrO2) is a metallic ferromagnetic oxide that has been widely used as a particulate recording medium in magnetic tapes.1 It has a rutile structure with a tetragonal unit cell ~a5b54.419 Å and c52.912 Å!2 consisting of two formula units. The chromium ions are in the Cr14 state with the electronic configuration @ Ar# 3d 2 with a magnetic moment of 2 m B per ion. Theoretical calculations have predicted CrO2 to be half metallic, with almost complete spin polarization at the Fermi level.3–5 This makes it attractive for fabricating tunnel junction devices with enhanced magnetoresistance ~MR!. Indeed, there have been a number of recent reports of large low-field MR associated with intergranular transport of spin-polarized electrons in polycrystalline films and powder compacts of CrO2. 6–8 Since CrO2 is a metastable phase, it normally has to be synthesized at high oxygen pressures. This has proved to be an impediment for the growth of high-quality thin films because traditional thin film growth techniques usually operate at atmospheric or much lower pressures. Based on some previous work reported in the literature,9–11 we have successfully grown epitaxial thin films of CrO2 by chemical vapor deposition at atmospheric pressure. In this article, we briefly describe the growth and characteristics of the films, along with their magnetic and transport properties.

III. RESULTS AND DISCUSSION

We have investigated the crystalline quality of the CrO2 films grown on TiO2~100! and Al2O3(0001) substrates using reflection high-energy electron diffraction ~RHEED!. Figure 1 shows ex situ RHEED patterns obtained with the electron beam aligned along two orthogonal zone axes directions for the two substrates. The diffraction patterns observed for the film grown on TiO2 are sharp and streaky with radial Kikuchi lines indicative of an well-ordered and flat surface. The patterns for the film grown on Al2O3 are also quite sharp, but somewhat spotty indicative of three-dimensional ~3D! growth. We have confirmed that the CrO2^ 010& (b) and ^ 001& (c) axes are aligned with the respective axes of the TiO2 substrate, whereas the film grown on Al2O3 has multiple domains with b axis of CrO2 aligned along three ¯ 0 & directions of the Al2O3. X-ray u –2u equivalents ^ 112

II. EXPERIMENT

Chromium dioxide thin films have been grown on TiO2(100) and Al2O3(0001) single crystal substrates by chemical vapor deposition using CrO3 as a precursor. The deposition reactor consists of a two-zone furnace and a quartz tube, in which the precursor powder is placed in a quartz boat in the source zone and the substrates placed on a susceptor in the reaction zone. Oxygen is used as a carrier gas for the sublimed CrO3 to be transported to the reaction 0021-8979/99/85(8)/5585/3/$15.00

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© 1999 American Institute of Physics

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J. Appl. Phys., Vol. 85, No. 8, 15 April 1999

FIG. 1. Ex situ RHEED patterns of CrO2 thin films on TiO2(100) substrate ~top! and on Al2O3(0001) substrate ~bottom! along two-zone axis directions.

scans show exclusively the even-ordered (h00) CrO2 peaks along with the substrate reflections, with no evidence of Cr2O3 or any other phases. Ion channeling measurements of the film grown on TiO2(100) have confirmed the epitaxial nature of the growth, with a ratio of 1.7% between aligned and random backscattering from 1.0 meV 4He1 ions suggesting a very high degree of structural perfection. Figure 2 displays the magnetic properties for a 6500 Å CrO2 film grown on TiO2(100) substrate along the b-axis and c-axis directions. The film has a Curie temperature (T c ) around 393 K and exhibits magnetic saturation along the easy c-axis direction at a field not much larger than the coercive field (H c ), which is around 15 Oe at 5 K. The b axis

FIG. 2. Magnetic hysteresis loops of a CrO2 thin film grown on TiO2(100) at T55 K along the b-axis and c-axis directions. The top two insets show the low- and high-field range magnetization, respectively. The spontaneous magnetization as a function of temperature is shown as the bottom inset.

Li et al.

FIG. 3. Resistivity of CrO2 films grown on TiO2(100) and Al2O3(0001) substrates. The current direction is along c axis for the CrO2 film on TiO2(100), and in an arbitrary direction for the CrO2 film on Al2O3(0001). The top and bottom insets show the MR for CrO2 films grown on TiO2(100) substrate at 380 and 5 K, respectively.

is a magnetic hard direction with anisotropy field (H k ) around 1400 Oe at 5 K. At 300 K, H c decreases to about 10 Oe, whereas H k is reduced to 1000 Oe. The resistivity, r, as a function of temperature for CrO2 films grown on TiO2(100) and Al2O3(0001) substrates is shown in Fig. 3. For the film on TiO2, r at room temperature along the c-axis direction is about 285 mV cm and decreases to about 2.5 mV cm at 5 K. The residual r at low temperatures is very sensitive to the structural disorder as suggested from the data of the CrO2 film grown on Al2O3(0001). Here a much higher residual r is observed presumably because of the growth of multiple domains. In the inset, we show the magnetoresistance for the H field ~0–40 kOe! in the plane and the current direction parallel to the c axis (I i c) at both 5 and 380 K for the CrO2 film grown on TiO2(100) substrate. At T55 K, the MR is positive with a H 2 field dependence, reaching a value of 25% in the transverse geometry when I i c and H i b. At T5380 K, a temperature somewhat lower than T c , the MR is negative and has a value of about 7% at H 540 kOe. The positive MR at low temperatures is attributed to the Lorentz force,12,13 with the MR being enhanced because of the relatively low residual resistivity of our sample. On the other hand, the negative MR observed near T c arises from suppression of the spin disorder scattering.13 For the films grown on Al2O3(0001), the MR is very small at low temperatures because of the high residual r, but have comparable values near T c . Figure 4 shows the Hall resistivity r H as a function of field H and temperature T of a CrO2 film. The Hall resistivity is measured by the van der Pauw method on a square film sample 0.530.5 cm2 and 3100 Å thick with leads attached at the four corners. The sign of the Hall voltage is determined by comparing the CrO2 with a nickel film as a standard. The average resistivity of the sample calculated using the van der Pauw method is about 4 mV cm at 5 K and goes up to 220

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Li et al.

J. Appl. Phys., Vol. 85, No. 8, 15 April 1999

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The films have been structurally characterized using ex situ RHEED, x-ray diffraction and ion channeling spectroscopy. A Curie temperature of around 393 K is observed for the films, with those grown on TiO2 exhibiting a large magnetocrystalline anisotropy. Transport measurements show that films on TiO2 have a resistivity drop of about two orders upon cooling down from room temperature to 5 K. The residual r is related to the structural disorder, with films grown on Al2O3 having a much smaller resistivity drop. Films with a low residual r exhibit large positive high-field MR at low temperatures. A negative MR peak is observed near T c resulting from suppression of spin disorder. Hall measurement indicates that the conduction carriers are holes, about 0.3 holes per formula unit. FIG. 4. Hall resistivity of a CrO2 thin film grown on TiO2(100) at various temperatures measured using the van der Pauw method.

mV cm at 300 K. Corresponding value of r H shown in Fig. 4 are small and positive at T55 K but at 300 K the Hall resistivity has the structure following the magnetization 4 p M and r H is now large and negative. The change of sign of r H as a function of T is most simply described in terms of competition between the ordinary Hall resistivity r o and extraordinary Hall resistivity r s due to scattering of conduction carriers, where at each H and T, r H 5 r o 1 r s . At T55 K, the Cr magnetic lattice is saturated and r s is small and the positive slope of r H vs H indicates that the current carriers are positive holes. With increasing temperature the contribution of r s increases with increasing r. The change in sign between r o and r s is not unusual; for example, Co and Fe as well as many alloys of these metals have differences in sign of r o and r s . 14,15 The equation, r H 5 r o 1 r s 5R o B1R s 4 p M defines the Hall coefficients R o and R s , where B5H14 p M . From Fig. 4 the data at T55 K gives R o 50.631025 m V cm/G and at T5300 K, R s 521.131023 m V cm/G, where at 300 K, the effect of R o is neglected. From R o at 5 K, the number of holes n is given by n51.0431022/cm3, about 0.3 holes per formula unit for CrO2. By obtaining careful measurement of 4 p M for various value of H and T, more detailed analysis can be made. It is possible that there is both hole and electron conduction in CrO2. IV. SUMMARY

In summary, we have successively grown CrO2 thin films epitaxially on TiO2(100) and Al2O3(0001) substrates.

ACKNOWLEDGMENTS

The authors thank W. J. Gallagher, J. C. Slonczewski, and S. S. P. Parkin for useful discussions. They are also grateful to T. H. Zabel for the RBS measurements. This work was supported in part by NSF Grant Nos. DMR-9414160 and DMR-9258306.

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