Crystal Structure Manipulation of the Exchange Bias in an Antiferromagnetic Film

Abstract

Exchange bias is one of the most extensively studied phenomena in magnetism, since it exerts a unidirectional anisotropy to a ferromagnet (FM) when coupled to an antiferromagnet (AFM) and the control of the exchange bias is therefore very important for technological applications, such as magnetic random access memory and giant magnetoresistance sensors. In this letter, we report the crystal structure manipulation of the exchange bias in epitaxial hcp Cr₂O₃ films. By epitaxially growing twined oriented Cr₂O₃ thin films, of which the c axis and spins of the Cr atoms lie in the film plane, we demonstrate that the exchange bias between Cr₂O₃ and an adjacent permalloy layer is tuned to in-plane from out-of-plane that has been observed in oriented Cr₂O₃ films. This is owing to the collinear exchange coupling between the spins of the Cr atoms and the adjacent FM layer. Such a highly anisotropic exchange bias phenomenon is not possible in polycrystalline films.

Introduction

Exchange bias refers to the shift of the magnetic hysteresis loop of a ferromagnetic (FM) layer away from the zero magnetic field, resulting from the exchange interaction from an antiferromagnet (AFM) layer^1,2,3,4. In practical applications such as magnetic random access memory and giant magnetoresistance sensors, etc.^5,6, the exchange bias is used to “pin” the FM magnetization from switching in small magnetic fields so that the FM layer could serve as a fixed reference layer. In previous studies, it has been established that the exchange bias hinges on the spin orientations of the surface magnetic atoms in the AFM layer^{7,8,9,10,11,12,13}. Since the direction of the spin orientations of the surface magnetic atoms is associated with the crystal structure of the AFM layer, the highly anisotropic exchange bias could be simply manipulated by the crystal orientation design. An ideal candidate AFM material to achieve this objective is the single crystalline Cr₂O₃ films with a hexagonal close packed (hcp) structure, of which the spin orientations of the Cr atoms are uniquely directed along the c axis and could dictate the direction of the exchange bias^7,14,15.

In this letter, we report the manipulation of the exchange bias by controlling the surface spin configuration via crystal orientation design. By epitaxially growing oriented Cr₂O₃ films on (001) oriented rutile TiO₂ substrates, we force the c axis to lie in the film plane, which results in a strong in-plane exchange bias between the Cr₂O₃ film and an adjacent permalloy (Py) layer, whilst the perpendicular exchange bias is completely suppressed. The perpendicular exchange bias was previously shown in (0001) oriented Cr₂O₃ films^{1,2,3,4,7,15,16}. Our results along with the previous studies demonstrate crystal structure manipulations of the exchange bias based on the collinear exchange coupling between the spins of the Cr atoms and the adjacent FM layer.

Results and Discussion

The oriented Cr₂O₃ films are grown on the oriented rutile TiO₂ substrates via laser molecular beam epitaxy (LMBE) with a base pressure of 2 × 10⁻⁸ mbar (see methods for details). Fig. 1a,b show the in-situ reflection high-energy electron diffraction (RHEED) characterization of the oriented TiO₂ substrate’s surface viewed from the and directions. After the initial growth of 3 nm Cr₂O₃, the RHEED pattern of the TiO₂ substrate disappears and that of Cr₂O₃ starts to appear, as shown in Fig. 1c,d. As the thickness of Cr₂O₃ film increases, its RHEED pattern becomes brighter. Fig. 1e–h show the RHEED patterns of 10 nm and 27 nm Cr₂O₃ films, respectively. To be noted, four satellite RHEED spots are observed around each main diffraction spot viewed from the direction of the TiO₂ substrates (Fig. 1e,g).

Figure 1: In-situ RHEED characterization for the oriented Cr₂O₃ films grown on (001) oriented rutile TiO₂ substrates.

(a,b) The RHEED patterns of the rutile TiO₂ substrate viewed from and directions prior to the Cr₂O₃ growth. (c–h) The RHEED patterns of 3 nm (c,d), 10 nm (e,f), and 27 nm (g,h) Cr₂O₃ films, respectively. The figures in the left/right column are RHEED patterns viewed from TiO₂ /[110] direction.

The crystalline structural properties of these Cr₂O₃ films are further characterized by x-ray diffraction (XRD). The θ–2θ scans of the rutile TiO₂ substrate, 10 nm, 20 nm, and 27 nm Cr₂O₃ films are shown in Fig. 2a. The peak at 2θ of ~63 degrees corresponds to the peak of the TiO₂ substrates. For the 10 nm Cr₂O₃ film, a peak at 2θ of ~65 degrees is observed, which corresponds to the peak of the Cr₂O₃ crystal. As the thickness of Cr₂O₃ increases, the intensity of the peak at ~65 degrees becomes stronger. We note that for the 27 nm Cr₂O₃ film, only peak is detectable for the whole scan range (see supplementary information; Fig. S1), indicating good crystalline properties of the oriented Cr₂O₃ thin films. The surface morphology is characterized by atomic force microscopy (AFM). The root-mean-square (RMS) roughness is 0.09 nm for the rutile TiO₂ substrate after annealing in the chamber (Fig. 2b). After the growth of 13 nm Cr₂O₃ film, the RMS roughness increases to 0.28 nm (Fig. 2c), indicating that the surface of the epitaxial Cr₂O₃ films is quite smooth.

Figure 2: The crystalline structure and surface roughness of the oriented Cr₂O₃ films.

(a) X-ray diffraction measurement of the rutile TiO₂ substrate and the 10 nm, 20 nm, and 27 nm Cr₂O₃ films, respectively. (b,c) AFM images of a typical TiO₂ substrate and a 13 nm Cr₂O₃ film.

The epitaxial growth of the oriented Cr₂O₃ films on TiO₂ is quite interesting, given the fact that Cr₂O₃ and TiO₂ belong to totally different space groups. Cr₂O₃ has a hexagonal crystal structure, which belongs to the group¹⁷, while rutile TiO₂ has a cubic structure, which belongs to the P4₂ group¹⁸. However, the c lattice constant of Cr₂O₃ is 13.599 Å, and the a lattice constant of TiO₂ is 4.584 Å, which results in a coincidental anion alignment with a lattice mismatch of only ~1.1% . Hence, the Cr₂O₃ films could be grown with the c axis lying in-plane and parallel to a or b axis of the TiO₂ substrates (Fig. 3a). As the TiO₂ crystal’s ab plane has four-fold symmetry, which could result in four-fold in-plane rotational symmetry of the crystalline structure of the Cr₂O₃ thin films. To investigate this, HRTEM is used to characterize the interfacial structure properties between Cr₂O₃ and TiO₂ viewed from the direction of the TiO₂ substrate. As shown in Fig. 3b, a sharp interface with TiO₂ is observed, as indicated by the pink dashed line. For Cr₂O₃, two distinct zones of the crystalline structures are observed, one of which is denoted as zone , and the other one is denoted as zone . The boundary of these two zones is marked by a yellow dashed line. For zone [0001], the c axis of Cr₂O₃ is parallel to the direction of the TiO₂ substrate, and the six-fold symmetry pattern of the basal plane can be identified. For zone , the c axis is parallel to the direction of the TiO₂ substrate. The crystal orientation of Cr₂O₃ films has been shown to be highly associated to the substrate crystalline structures. For example, preferential oriented growth of Cr₂O₃ is achieved on oriented Al₂O₃, Co, and oriented Cu and oriented Cr₂O₃ is grown on Fe films^15,19,20. The growth mode of Cr₂O₃ film in our study is similar to that observed in an earlier report for the oriented Fe₂O₃ films grown on rutile .TiO₂ substrates²¹.

Figure 3: Crystalline structure of the oriented Cr₂O₃ film by HRTEM.

(a) The schematic drawing showing the atomic interface between Cr₂O₃ and TiO₂. The spins of the Cr are parallel to the film plane, indicated by the red arrows. (b) HRTEM characterization. The pink dashed line indicates the interface between Cr₂O₃ and TiO₂, and the yellow dashed line indicates the crystalline boundary between the zones and for the oriented Cr₂O₃ film. In the zone , the [0001] directions of Cr₂O₃ is parallel to the , or of the TiO₂ substrate. Whilst in the zone [0001], the direction of Cr₂O₃ is parallel to the [100], or of the TiO₂ substrate.

Interestingly, with the c axis lying in the film plane, the spin orientations of the Cr atoms also lie in-plane in these oriented Cr₂O₃ film, as schematically shown in Fig. 3a. This is very different from previously reported (0001) oriented Cr₂O₃ films grown on Al₂O₃ substrates, of which both the spin orientations and the exchanges bias are perpendicular to the films^7,15,16. To study the exchange bias of the epitaxial Cr₂O₃ films (see Methods for details), we deposit 10 nm Py on top of the oriented Cr₂O₃ films and measure the magnetic hysteresis loops by Magnetic Properties Measurement System (MPMS; Quantum Design) with both in-plane and out-of-plane magnetic fields at various temperatures (schematic drawings shown in Fig. 4a,b). The in-plane magnetic hysteresis loop are first measured. Prior to the measurement, the sample is cooled from 400 to 10 K in an in-plane magnetic field of 1000 Oe along the [100] direction of the TiO₂ substrate, which is much smaller than the spin-flop field of several Tesla for Cr₂O₃ reported previously²². By cooling through the blocking temperature, the magnetization direction of the Py sets the surface spin configurations of the Cr₂O₃ films. Then, we measure the magnetization of the Py as a function of the in-plane magnetic field along the [100] direction of the TiO₂ substrate (Fig. 4a) from 10 to 300 K. After subtracting a linear background which is mainly due to the diamagnetic response of the rutile TiO₂ substrate, the shifted magnetic hysteresis loops of Py are displayed in Fig. 4c. At 10 K, as the magnetic field ramps from negative to positive, a sharp jump in magnetic moment occurs at ~110 Oe, but the jump occurs at ~ −400 Oe on the return sweep. These two switching fields are labeled as H₁ and H₂, respectively, as indicated in the top panel of the Fig. 4c. The exchange bias field (H_B) is defined by the mean value of the H₁ and H₂, i.e. . As the temperature increases, the exchange bias field steadily decreases from the low temperature value.

Figure 4: The characterization of the exchange bias for the sample of TiO₂/Cr₂O₃ (13 nm)/Py (10 nm)/Al (20 nm).

(a,b) Schematic drawings of the sample structure and the measurement geometry for in-plane exchange bias and perpendicular exchange bias, respectively. (c) The magnetization hysteresis loops measured as a function of the in-plane magnetic field along the TiO₂ direction at 10 K, 20 K, 40 K, and 60 K, respectively. H₁ and H₂ indicate the coercive fields for the magnetization of Py and H_B indicates the exchange bias. (d) The magnetization curves measured as a function of the out-of-plane magnetic at 10 K, 20 K, 40 K, and 60 K, respectively.

To characterize the anisotropy of the exchange bias effect, magnetic hysteresis loops are measured with a magnetic field perpendicular to the films (Fig. 4b). The same field cooling procedure as with the in-plane magnetic fields is adopted. The out-of-plane magnetization curves measured at 10, 20, 40 and 60 K are shown in Fig. 4d. The symmetric magnetization hysteresis loops indicate a negligible perpendicular exchange, which is in stark contrast to the out-of-plane exchange bias observed in the oriented Cr₂O₃ films. The highly anisotropic exchange bias phenomenon can be attributed to the crystalline orientation difference of the Cr₂O₃ films. In hcp structures, the c-axis direction dictates the spin orientations of the magnetic atoms. In the oriented Cr₂O₃ films, the spin orientations of the Cr atoms are perpendicular to the films, whereas in the oriented Cr₂O₃ films, the spins of the Cr atoms lie in the film plane. These results further indicate the direct collinear exchange coupling between the spins of the Cr atoms and the adjacent Py layer.

Fig. 5a,b summarize the in-plane exchange bias field and in-plane conceive field (H_C), where , for the sample consisting of the 13 nm Cr₂O₃ films and 10 nm Py as a function of the temperature. As the temperature increases from 10 to 60 K, the in-plane exchange bias field (Fig. 5a, Blue dots) decreases quickly from ~ −150 Oe to almost 0 Oe, where the sign depends on the magnetic field direction during magnetic cooling. No exchange bias is observable at and above 60 K, which implies a blocking temperature (T_B) of ~60 K. An abrupt increase in Hc below 60 K is another property of exchange biased Py, which is due to the formation of the AFM order in this 13 nm Cr₂O₃ thin film². As there are two crystalline zones of oriented Cr₂O₃, as indicated in zones and , we also measure the exchange bias in the direction along the TiO₂ direction. Almost identical exchange biases are observed at each temperature (Fig. 5a, Green dots).

Figure 5: The exchange bias and the coercive field as a function of temperature for the sample TiO₂/Cr₂O₃ (13 nm)/Py (10 nm)/Al (20 nm).

(a) The exchange bias field as a function of the temperature for magnetic field along the TiO₂ , and directions, respectively. T_B indicates the blocking temperature, above which the exchange bias becomes zero. (b) The coercive field of the Py as a function of the temperature.

The measured T_B of 13 nm Cr₂O₃ is ~60 K, which is much lower compared to the value reported on (0001) oriented bulk Cr₂O₃ single crystals¹⁵. In antiferromagnetic films, it has been known that T_B is highly related to Neel temperature (T_N), and is usually slightly lower than the T_N. Both T_B and T_N increase as the AFM thickness increases due to finite-size effects^2,12,23,24. To obtain the T_B as a function of the thicknesses of the Cr₂O₃ thin films, the in-plane exchange bias for the samples consisting of 7, 10, 20 and 27 nm Cr₂O₃ films and 10 nm Py bilayer films are also measured. Fig. 6a,b show the exchange bias as a function of temperature for the bilayer structures consisting of Cr₂O₃ (7 nm)/Py (10 nm) and Cr₂O₃ (27 nm)/Py (10 nm), respectively. The blocking temperatures of these two structures are determined to be 40 K and 100 K. The blocking temperature increases as the thickness of the Cr₂O₃ films increases, as shown in Fig. 6c. For the 27 nm Cr₂O₃ film, the blocking temperature is only ~100 K, which is far below the T_B of bulk Cr₂O₃. One possible reason is the non-trivial finite size effects arising from the grain boundaries or oxygen defects in the Cr₂O₃²⁵.

Figure 6: The exchange bias and blocking temperatures for the Cr₂O₃ films of various thicknesses.

(a,b) The exchange bias as a function of the temperature for 7 nm and 27 nm Cr₂O₃ films, respectively. (c) The blocking temperature as a function of the Cr₂O₃ film thicknesses (t) for the samples TiO₂/Cr₂O₃ (t)/Py (10 nm)/Al (20 nm).

Conclusion

In summary, we have demonstrated the manipulation of the exchange bias of the Cr₂O₃ thin films by controlling the surface spin orientations of the Cr atoms via crystal orientation design. For the epitaxial growth of oriented Cr₂O₃ films, the spin configurations of Cr atoms give rise to only in-plane exchange bias at the interface between Py and the Cr₂O₃ thin films, while no perpendicular exchange bias is observed. Our results along with previous studies on (0001) oriented Cr₂O₃ films indicate the collinear exchange coupling between the spins of the Cr atoms and the adjacent FM layer.

Methods

Cr₂O₃ films growth

The oriented Cr₂O₃ films are grown on the (001) oriented rutile TiO₂ substrates via laser molecular beam epitaxy (LMBE) with a base pressure of 2 × 10⁻⁸ mbar. Prior to the Cr₂O₃ growth, the substrate temperature is increased to 350 °C with a rate of 20 °C/min in the chamber with an oxygen partial pressure of 0.08 mbar. Then the Cr₂O₃ film is deposited from a Cr₂O₃ target with a laser power of (8.0 ± 0.2) mJ and a frequency of 2.0 Hz. The thickness of the Cr₂O₃ thin film (t) is determined from the cross section high resolution transmission electron microscopy.

Exchange bias measurement

A 10 nm Py is grown on top of the oriented Cr₂O₃ films by RF magnetron sputtering with a growth rate of 0.02 Å/s. A capping layer of 20 nm aluminum is deposited prior to taking the samples out of this sputtering chamber to prevent oxidization of Py. Magnetic Properties Measurement System (MPMS; Quantum Design) is used to measure the magnetic hysteresis loops to determine the exchange bias.