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Electric field induced changes in the coercivity of a thin-film ferromagnet
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 J. Phys. D: Appl. Phys. 44 305001 (http://iopscience.iop.org/0022-3727/44/30/305001) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 44 (2011) 305001 (5pp)
doi:10.1088/0022-3727/44/30/305001
Electric field induced changes in the coercivity of a thin-film ferromagnet C Fowley, K Rode, K Oguz, H Kurt and J M D Coey CRANN and School of Physics, Trinity College Dublin, Ireland
Received 23 March 2011, in final form 14 June 2011 Published 7 July 2011 Online at stacks.iop.org/JPhysD/44/305001 Abstract Data are presented which indicate a modification of magnetic anisotropy in the MgO/CoFeB/Pd and MgO/CoFeB/Pt systems, using electric fields of order 500 MV m−1 (0.5 V nm−1 ) applied across a thermally grown SiO2 as a gate dielectric. The effect is most prominent at low temperature (12 K) and is manifested as a small change in coercivity. The sign of the effect depends on the choice of both capping layer and annealing temperature. The results suggest that both interfaces play a role in the appearance of perpendicular magnetic anisotropy in these thin-film stacks, and not just the interface with MgO. (Some figures in this article are in colour only in the electronic version)
is thin enough to give a high interface/volume ratio the charging can cause a significant change in the band filling and hence modify the magnetization and the anisotropy. It seems that a capacitor structure should allow simple control over the band filling. Early experiments on L10 ordered alloys [1] were explained using a band-filling model [8]. Calculation of anisotropy energy as a function of band filling, n, show that the addition or subtraction of electrons can modify the anisotropy energy and the magnetic easy axis. It is not however possible to influence the bulk material due to the screening effect in metals. We, therefore, have a requirement for the ferromagnetic material to be thin. The spin-dependent screening length calculated by Zhang [9] was shown to be of order 0.1 nm, which can be larger than a typical Thomas–Fermi screening length (calculated to be less than 0.1 nm). As a result the electric field can alter the spin polarization at the ferromagnet surface. An accumulated moment at the interface builds up due to the fact that the electronic potentials seen by spin-up and spindown electrons are different. Niranjan et al later proposed that the spin-dependent screening effect would lead to changes in magnetocrystalline anisotropy at the interface [10]. Nakamura et al calculated the magnetocrystalline anisotropy of [0 0 1]oriented Fe monolayers and found that the application of an electric field can modify the density of states around the Fermi level [11]. Later they showed that the hybridization of Fe and O at the Fe/MgO interface promotes perpendicular magnetic anisotropy (PMA) at this interface [12]. Calculations also suggest that the size and sign of the effect of interface charging is highly dependent on the ferromagnetic material [8, 13].
1. Introduction Magnetism in transition metals arises from spin splitting of partly filled 3d-orbitals. The number of 3d conduction electrons in the conduction band determines the magnetic properties of Fe, Co and Ni and their alloys. The ability to significantly influence the band filling with an electric field would open a new pathway to influence the magnetic properties of thin-film materials. This could pave the way to faster, low-power devices that use electric fields to manipulate magnetically stored information (either alone or with a combination of other switching techniques). Following the first results by Weisheit et al on the modification of the anisotropy in FePt and FePd thin films, there have been several reports which use electric fields to modify the magnetic anisotropy in thin film metallic ferromagnets [1–7]. These experiments all involve a capacitor-like structure where one of the plates is an ultra-thin ferromagnet. In a parallel plate capacitor, the charge per unit area on the plates is given by q = εV /d, where ε is the permittivity of the dielectric, V the applied voltage and d is the thickness of the dielectric material between the two plates. When one plate of the capacitor is a ferromagnet, charging the capacitor will charge the ferromagnet/dielectric interface. The amount of charge added to one atom of the ferromagnetic material at the interface is then εS V (1) q= d where S is the surface area for one atom, which can be calculated from the lattice constant. Provided the ferromagnet 0022-3727/11/305001+05$33.00
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J. Phys. D: Appl. Phys. 44 (2011) 305001
C Fowley et al
device structures are studied in this report, the thickness in parentheses are in nanometres: n++ Si/SiO2 (200)/MgO (1.4)/CoFeB (0.6)/Pd (5), and n++ Si/SiO2 (200)/MgO (2)/CoFeB (1)/Pt (3). Both samples show a dependence of PMA on the CoFeB thickness, but only the above two samples were patterned and subjected to an electric field. An MgO seed layer was required for PMA in both systems. Since we would see a maximum change in the Hall signal close to the crossover between perpendicular and in-plane anisotropy the largest thickness of CoFeB which would still give PMA was chosen. The samples were patterned into 2 µm Hall bars by UV lithography and argon ion milling. EHE measurements were conducted in an electromagnet with the applied magnetic field perpendicular to the sample surface at room temperature and at 12 K. The MgO/CoFeB/Pt sample was annealed under high vacuum at 350 ◦ C in a perpendicular field of 800 mT for 1 h. The electric field was applied between the highly doped silicon and the metal layers. A positive (negative) voltage implies addition (removal) of electrons to the dielectric/FM interface. The dielectric breakdown of the 200 nm SiO2 layer was above 100 V, implying that we can access electric fields in the region of 500 MV m−1 (or 0.5 V nm−1 ). During the application of the applied electric field the leakage current was below 1 nA, taking the resistance of the multilayer stack to be in the k range, the power dissipated within the sample due to the fact that leakage is below 10−15 W. Using (1) we calculate that ±100 V corresponds to ± 0.007 electrons per magnetic atom.
Table 1. Summary of different systems where electric field induced anisotropy changes are observed. The sign of the anisotropy change produced by negatively charging the interface of the ferromagnet and the insulator is given in the second column. The magnitude of the coercivity change from the study is given in the third column. The measurement temperature is given in the fourth column. References are given in the fifth column. System
Keff Hc
T
Ref.
2 nm L10 FePt 2 nm L10 FePd Au/Fe (0.58 ML)/MgO Au/Fe80 Co20 (0.48 ML) /MgO CoCrPt MgO/Co40 Fe40 B20 (0.6 nm)/Pd Ta/Co40 Fe40 B20 (1.16 nm)/MgO MgO/Co40 Fe40 B20 (1 nm)/Pt
↓ ↑ ↓ ↓ ↓ ↑ ↓ ↓
300 K 300 K 300 K 300 K 300 K 12 K 300 K 12 K
[1] [1] [2] [3] [4] [5] [6] This work
4.5% 1% — 5% 30% 0.7% 40% 1%
The set of available experimental data, summarized in table 1, is still small. Weisheit et al experimentally demonstrated a controllable change of coercivity in L10 ordered FePt and FePd thin films by applying an electric field in an electrochemical cell [1]. Maruyama et al demonstrated control of magnetic anisotropy in Fe upon an application of an electric field in a solid state capacitor structure [2], and more recently in FeCo [3]. All of these studies were conducted using optical techniques, primarily by magnetooptical Kerr effect (MOKE) [1–3] but also Brillouin light scattering [14]. The effect of an electric field has also been reported on magnetization dynamics [15], through the observation of the distortion of the spin polarization of the secondary electrons emitted after the interaction of a thin metallic ferromagnetic film with a synchrotron produced electron bunch. An electric field effect was also reported in CoCrPt [4], where an increased signal-to-noise ratio during read back of magnetically encoded information was attributed to a local decrease of coercivity during the writing process. The measured change in coercivity or anisotropy was not quantified by any magnetic measurements. We previously reported a small electric field effect in MgO/CoFeB/Pd using the extraordinary Hall effect (EHE) [5]. Endo et al observed electric field driven changes in Ta/CoFeB/MgO [6]. Even more recently an effect was reported in CoPd thin films [7]. These calculations and experiments show that an electric field applied to the surface of a ferromagnetic metal can impact both the magnetization and the magnetocrystalline anisotropy at the interface. Here, we expand our previous report on MgO/CoFeB/Pd and compare it with new observations of an electric field effect in the MgO/CoFeB/Pt system.
3. Results 3.1. MgO/CoFeB/Pd First we present data on the as-grown MgO (1.4)/CoFeB (0.6)/Pd (5) system. We previously reported on the appearance of PMA, the stabilization of coercivity at low temperature and noted a small electric field effect in this system, PMA only appears within a narrow thickness range [5]. At room temperature we observe no coercivity when the applied magnetic field is perpendicular to the sample surface and the CoFeB layer is most likely superparamagnetic. The 12 K coercivity of this system is 140 mT. We applied electric fields of ±250 MV m−1 across the SiO2 layer. A plot of low temperature coercivity as a function of applied electric field appears in figure 2. Several Hall loops (5) were recorded at each fixed value of applied electric field. We confirm that the coercivity is modified by the applied electric field. It is increased at positive voltages and decreased at negative voltages. Adding electrons at the CoFeB/MgO interface increases the coercivity. We grew several nominally identical samples with Pd capping and, although the coercivity varied from sample to sample, the same effect of an applied electric field was observed in each case. The result is similar to that obtained with L10 FePd [1]. Upon annealing of this sample, the low temperature coercivity exceeded the limits of our measurement setup and therefore cannot be discussed here. This is most likely due to alloy formation with Pd [17].
2. Experimental methods Samples were deposited by magnetron sputtering on highly doped Si wafers with 200 nm of thermally grown silicon dioxide. All growth rates were calibrated by x-ray reflectivity thickness measurements and were below 0.02 nm s−1 . The detection mechanism was the EHE [16]. The stack structure and measurement geometry is shown in figure 1. The following 2
J. Phys. D: Appl. Phys. 44 (2011) 305001
C Fowley et al
Figure 1. (a) Device structure for electric field measurements, the ferromagnet (FM) and highly doped (n++) Si form a capacitor across the SiO2 dielectric layer. For the systems presented here we use a thin MgO buffer layer between the FM and the SiO2 . (b) EHE measurement geometry, bias is applied between the metal layers and the highly doped silicon. The magnetic field is applied perpendicular to the film plane.
Figure 2. Coercivity as a function of applied electric field for MgO (1.4)/CoFeB (0.6)/Pd (5). Application of negative (positive) electric field decreases (increases) the coercivity. The inset shows the low temperature EHE loops.
Figure 3. Room temperature Hall resistance loops as function of applied electric field for MgO (2)/CoFeB (1)/Pt (3) annealed at 350 ◦ C. Positive (black) fields appear to decrease the PMA, whereas negative (red) fields appear to stabilize it. The magnetic field and electric field are both applied perpendicular to the film plane.
3.2. MgO/CoFeB/Pt Next we present data on MgO (2)/CoFeB (1)/Pt (3) annealed at 350 ◦ C, which is known to exhibit PMA [18]. We applied electric fields of ±500 MV m−1 across the SiO2 insulating layer. Again several Hall loops were recorded at each applied electric field. The room temperature coercivity is only about 0.4 mT. The results obtained with the electric field applied at room temperature are shown in figure 3; it appears that the squareness of the EHE loop is modified upon the application of an electric field. As in the case of the Pd capped sample in the previous section we performed measurements at low temperatures. At 12 K the coercivity increases to 100 mT. The coercivity detected by EHE as a function of applied electric field at 12 K is shown in figure 4(a). The EHE loops from which the coercivity is determined are shown in the inset of figure 4(a) and the switch at positive magnetic fields in figure 4(b), which clearly shows the variation of coercivity as a function of applied electric field. The slope has opposite sign to that of figure 2. For the unannealed sample we found no detectable change in the Hall effect response. However, this is a consequence of the extraordinary Hall effect which is sensitive to the magnetization component perpendicular to the film plane.
4. Discussion Our primary observation is a change in coercivity. Since coercivity is usually proportional to the anisotropy field, µ0 Ha = 2Keff /Ms [19], an increase in coercivity may be explained by an increase in anisotropy or by a decrease in magnetization. Assuming that the magnetization of CoFeB increases (decreases) linearly with the number of electrons at the Fermi energy, we infer that the magnetization should change by a maximum of 0.1%. Hence the observed change in coercivity of about 1% cannot be accounted for by a decrease (increase) in magnetization. Also, the maximum saturated Hall resistance does not change to within 0.05% in figure 4, which indicates that Mz does not change within the same range. From this we may deduce that the electric field most likely acts to change the anisotropy Keff , rather than the magnetization. We find that in both systems, as the interface is charged, the coercivity is modified. The magnitude of the effect seen here is of order 1%, in agreement with previous reports [1, 2, 6]. The sign of the change is however opposite in the CoFeB/Pd and CoFeB/Pt systems. For the case of MgO/CoFeB/Pd the application of positive electric fields results in a increase in 3
J. Phys. D: Appl. Phys. 44 (2011) 305001
C Fowley et al
suggested to stabilize PMA in MgO/CoFeB systems [6, 18], as in the case of AlOx /Co/Pt [20]. Here, however, the only oxygen-transition metal bonds that could form in our CoFeB/Pd system would be due to dangling oxygen bonds on the MgO surface during growth. Recently, x-ray photoelectron spectroscopy studies of the MgO/CoFeB/Pd system shows that Pd is likely to be alloying with Co to produce the strong PMA upon high-temperature annealing [17]. The fact that we obtain stable PMA and observe an effect of the applied electric field for the as-grown MgO/CoFeB/Pd samples seems to indicate that alloying at high temperatures is not the only origin of PMA in this system. It is useful to point out that the MgO/CoFeB/Pd system only exhibits coercivity at low temperature. We argued previously that this is due to formation of superparamagnetic grains [5]. If this is the case, the Pd cap layer may fill the gaps between the grains and make the CoFeB/Pd interface accessible to the electric field within the screening length calculated by Zhang [9]. It is therefore possible that in the unannealed case the CoFeB/Pd interface is sufficiently close to the MgO to allow an appreciable change with applied electric field. Likewise, high-temperature annealing in the CoFeB/Pt case may allow the formation of Pt rich areas close to the MgO interface which may be affected with applied electric field. The results presented here however cannot be used to rule out a contribution of the MgO/CoFeB interface to the anisotropy since the MgO layer is required for the appearance of PMA in both systems. It has been reported that the MgO/CoFeB interface alone can produce strong PMA [6]. In order to investigate the effect of the MgO layer further we also investigated the MgO/CoFeB/Ta system. However, we succeeded in stabilizing PMA in the CoFeB/Ta system only when MgO was a capping layer but not when it was grown on an MgO seed layer directly on SiO2 . The experimental treatment we employ, since we use the SiO2 as the gate dielectric, can only be applied to ferromagnets exhibiting PMA when grown on insulating buffer. Therefore, we were unable to apply an electric field in the CoFeB/Ta case.
Figure 4. (a) Electric field dependence, taken at 12 K, of the coercivity of the MgO (2)/CoFeB (1)/Pt (3) system annealed at 350 ◦ C. The inset shows full field EHE loops at various applied electric fields. Positive fields clearly decrease the magnetic anisotropy, whereas negative fields increase it. (b) Magnetic switching at positive magnetic field of the same system. The effect of an applied electric field on the switching is clear, with negative electric fields increasing the magnetic anisotropy. The magnetic field is applied perpendicular to the film plane.
5. Conclusion
the coercivity, whereas in MgO/CoFeB/Pt a positive electric field decreases it. For the case of MgO/CoFeB/Pt, the sign of the response at 12 K (figure 4(b)) is consistent with the room temperature data (figure 3). The change of sign depending on the use of Pt or Pd, is consistent with alloying of CoFeB with Pt or Pd as in [1] and [8]. However, since we are only able to modify the electron density at the CoFeB/MgO interface, the existence of an electric field effect in these systems suggests that the MgO interface is at least contributing to the PMA. The screening length calculated by Zhang [9] is of the order of 0.1– 0.2 nm for Fe and Co. Therefore, it is unlikely that the electric field modifies the CoFeB/Pd or CoFeB/Pt interface. However, if the PMA was solely due to the MgO/CoFeB interface alone then the effect should have the same sign regardless of capping layer. The formation of oxygen–transition metal bonds either during growth or during high-temperature annealing is
We have obtained reliable experimental evidence of an electric field effect, in both the unannealed MgO/CoFeB/Pd and the annealed MgO/CoFeB/Pt systems, using thermally grown SiO2 as a gate dielectric and EHE as a detection mechanism. The induced PMA in both systems is dependent on both the Pd or Pt layer thickness and annealing temperature. The sign of the electric field induced change depends on the use of Pt or Pd as a capping layer, and the sign is consistent with results from [1] which may indicate alloying of CoFeB with Pt and Pd. From the saturated Hall resistance we infer that the change in coercivity is likely to originate from a change in anisotropy rather than magnetization. The small screening length in metallic layers and the thickness of CoFeB point to the conclusion that the MgO interface could also have some influence on the PMA. The fact that we did not obtain PMA without the MgO buffer also points to the important role of the MgO/CoFeB interface. 4
J. Phys. D: Appl. Phys. 44 (2011) 305001
C Fowley et al
Although the effect is repeatable and reproducible, it is rather small in our case. The electric field strengths applied in other experiments, given in table 1, are the same order of magnitude (100 MV m−1 ). Other groups [1, 2, 6] have argued that this effect will have great technological impact. However the large changes in Hc are a result of the effective anisotropy, Keff , being close to zero. The induced change is calculated to be of the order 10 µJ m−2 while the interfacial anisotropy, which must overcome the volume anisotropy, is approximately 100 µJ m−2 [6]. This value is small compared with more traditional multilayers with strong PMA. Bi-layers of CoFeB/Pd, for example, exhibit an interfacial anisotropy over 300 µJ m−2 [5]. In Co/Ni multilayers, the interfacial anisotropy is close to 500 µJ m−2 [21]. For Co/Pd multilayers, it can be in excess of 700 µJ m−2 [22]. This finite size of the effect suggests that the application potential of such an electrically induced effect in metallic ferromagnets for magnetic memory and storage is limited to systems with total anisotropy very close to zero. In light of the current needs of magnetic storage, the reduction of the coercivity locally in a hard drive system would allow for easier switching without sacrificing thermal stability. Investigation of the electric field effect in alloy systems, such as CoFeB co-sputtered directly with Pt or Pd, might directly answer the question of the role of MgO. The investigation of electric field induced changes in anisotropy in insulating materials like magnetic oxides, may identify materials more suited for this kind of application.
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Acknowledgments This work was supported by Science Foundation Ireland (SFI) as part of the MANSE project Grant No SFI 05/IN/1850 and was conducted under the framework of the INSPIRE program, funded by the Irish Government’s Programme for Research in Third Level Institutions, Cycle 4, National Development Plan 2007-2013 and the EU FP7 programme IFOX.
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