Enhanced perpendicular magnetic anisotropy in Fe/Mn bilayers by incorporating ultrathin ferromagnetic underlayer through magnetic proximity effect
Bo-Yao Wang,1Chieh-Chen Chiu,2Wen-Chin Lin,2,a)and Minn-Tsong Lin1,3,b)
1Department of Physics, National Taiwan University, 106 Taipei, Taiwan
2Department of Physics, National Taiwan Normal University, 116 Taipei, Taiwan
3Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
(Received 7 June 2013; accepted 5 July 2013; published online 23 July 2013)
The perpendicular magnetic anisotropy(PMA) was shown to be established in ferromagnetic (FM)/
fcc-Mn bilayers through the FM-antiferromagnetic (AFM) exchange coupling. We demonstrate here that such PMA can be further enhanced by incorporating an ultrathin Fe film as an underlayer. In a series of Fe/Mn bilayers, hysteresis loop measurement shows that the thickness of top Fe layer with PMA can be extended to a thicker range while an ultrathin Fe underlayer is inserted. Such enhancement of PMA is attributed to an increase of AFM ordering on the Mn film originated from the magnetic proximity effect with the Fe underlayer.VC 2013 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4816478]
As the size of the magnetic bits is reduced for magnetic devices gaining the higher storage capacity, maintaining high thermal stability of the magnetic storage layer for nonvolatility and low switching field for reasonable power consumption becomes important issues. Utilizing the perpen- dicular magnetic anisotropy (PMA) and spin-transfer torque (STT) effect for reading and writing the magnetic logic states in magnetic devices is expected to be path way for achieving these goals.1–4To stabilize the PMA of the magnetic storage layer in STT devices, current approach often applies the interface anisotropy between the ferromagnetic (FM) layer and non-magnetic layer.3–6A conventional way for gaining the higher thermal stability of PMA is through a fabrication of multilayer structure. However, this approach results in a thicker magnetic storage layer, which could lead to a higher STT threshold and a drawback of increasing power consumption.3,5,7
It has been reported recently that the antiferromagnetic (AFM) fcc-Mn thin film can lead to PMA on adjacent FM layer through the AFM-FM exchange coupling.8–10 Although the AFM-FM exchange coupling is known to be established at interface, the strength is determined by the magnetic ordering of whole AFM layer through the finite size effect.11Therefore, a stable PMA could be achieved by increasing the thickness of AFM layer in the same FM/AFM bilayer, and the overall thickness required for perpendicular magnetic layer could be much reduced as compared with the current multilayer structure. However, similar to other well- known AFM-induced phenomenon like exchange bias field, the magnitude of PMA in FM/Mn bilayers is determined by the magnetic ordering of the AFM Mn layer characterized by the magnetic ordering temperature (TOrdering). For the bulk fcc-Mn antiferromagnet, the TOrdering is about 540 K.12,13 The TOrderingcould become much lower as the thickness of fcc-Mn layer is reduced into the ultrathin regime. From the
aspect of future application in perpendicular-based magnetic devices, it is important to search for an approach that can enhance the TOrdering of the fcc-Mn thin film. According to prior reports,14–16the magnetic ordering of a magnetic ultra- thin film is possible to be enhanced by an another magnetic layer with the higher TOrderingthrough a direct exchange cou- pling, namely magnetic proximity effect. In FM/AFM bilayer, such effect is also known to enhance the TOrderingof AFM layer or improve the long range order for exchange bias field.17–19However, the influence of magnetic proximity effect on the magnetic ordering of the fcc-Mn thin film as well as the strength of the induced PMA is still unknown.
In this letter, we report that the PMA of Fe/Mn bilayer can further be enhanced by incorporating an ultrathin Fe film as an underlayer. Our hysteresis loop measurement shows that the thickness of top Fe layer with PMA can be extended to a thicker range while an ultrathin Fe underlayer is inserted. Such enhancement of PMA is attributed to an increase of AFM ordering (or TOrdering) on the Mn film origi- nated from the magnetic proximity effect with the Fe under- layer. This finding demonstrates a possible approach to increase the thermal stability of a low dimensional antiferro- magnet for promoting the PMA.
Samples of Fe/Mn and Fe/Mn/Fe ultrathin films were prepared and investigated in-situ in an ultrahigh-vacuum (UHV) NTU-NSRRC Nanomagnetism Preparation Chamber with a base pressure of 2 1010Torr. The Cu3Au(001) single-crystal substrates with 0.1 miscut were cleaned by cycles of 2 keV Arþion sputtering and annealed at 765 K for 5 min and at 645 K for 30 min to obtain a well-ordered crys- talline structure. The Fe and Mn thin films were deposited with thermal evaporation guns (Omicron, EFM-3) at 300 K.
The growth of the thin films was monitored by medium- energy electron diffraction (MEED), in which the oscillation of specular intensity provided the information for a precise control of the film thickness.20 The crystalline structure of films was characterized by low-energy electron diffraction (LEED) and LEED-I/V curves.8,21 A finding of invariant
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b)Electronic mail: [email protected]
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APPLIED PHYSICS LETTERS 103, 042407 (2013)
structure for the top Fe layer or Mn layer upon an insertion of ultrathin Fe underlayer suggests its unconcerned relation to the variation of magnetic properties.22The magnetic hys- teresis loops of thin films were measured on the basis of the magneto-optical Kerr effect (MOKE) in both the longitudi- nal and polar geometries at various temperatures.
Figure 1(a) shows the hysteresis loops of the 6 ML Fe/Mn bilayers with a variation of Mn film thickness (tMn).
The magnetic anisotropy of the Fe film aligns in-plane direc- tion as the Mn film at low coverage and then changes to out-of-plane direction as tMn was increased. This presents a phenomenon of spin-reorientation transition (SRT) from in- plane to out-of-plane direction, which is consistent with previous report.8On the other hand, Fig.1(b)shows the hys- teresis loops of the Fe/6 ML Mn bilayers with a variation of top Fe film thickness (ttopFe). Inversely, an increase of the ttopFe leads to change of the magnetic anisotropy from out-of-plane to in-plane direction. This SRT is attributed to an increased contribution of the in-plane oriented shape anisotropy from a thicker FM film.
For those Fe/Mn bilayers with in-plane magnetic anisot- ropy, however, an insertion of 1.5–2.5 ML ultrathin Fe underlayer leads to the presence of PMA, as displayed in Fig. 2(a). According to previous reports23,24 and also our measurement, ultrathin Fe film (tFe< 3 ML) grown on Cu3Au(001) exhibits PMA owing to interface anisotropy with substrate. However, in our measurements, 5 ML Mn/1.5–2.5 ML Fe/Cu3Au(001) films show absence of mag- netic hysteresis loops even though the temperature was decreased to 190 K (not shown in figures). Such behavior could be attributed to a strong coercivity enhancement effect caused by the AFM Mn layer, which is usually significant while the thickness of FM layer in FM/AFM bilayer is decreased.25Thus, the hysteresis loops presented in Fig.2(a) are concluded to be mainly contributed by the top Fe layer.
To realize a general magnetic feature of the Fe/Mn bilayer upon an insertion of ultrathin Fe underlayer, we compare the magnetic hysteresis loops between Fe/Mn bilayers and Fe/
Mn/2.5 ML Fe trilayers as functions of thickness for the top Fe film and Mn layer. Figures2(b)and2(c)display the mag- netic easy axis phase diagram of the Fe/Mn bilayers and
Fe/Mn/Fe trilayers, respectively. As compared with Fe/Mn bilayers, the SRT boundary of Fe/Mn/2.5 ML Fe trilayers is further shifted to the higher value of ttopFe by 2 ML. This indi- cates that an insertion of ultrathin Fe underlayer can in gen- eral lead to enhancement of PMA on the top Fe layer.
To clarify the origin behind the enhanced PMA in Fe/Mn upon an insertion of ultrathin Fe underlayer, the crys- talline structure of top Fe layer and Mn layer has been exam- ined by LEED I/V with kinetic approach.22 Since the crystalline structure of the top Fe film and Mn layer is nearly invariant while the ultrathin Fe underlayer is inserted, its effect on the enhancement of the PMA in Fe/Mn/Fe trilayers can therefore be excluded.22The origin is much possible to be associated with a variation of magnetic ordering on Mn layer affected by the magnetic coupling from the Fe under- layer. To confirm this point, we compare the TOrderingof the Mn film in Fe/Mn bilayer and Fe/Mn/Fe trilayer. Despite a difficulty of directly measuring the TOrdering of an AFM ultrathin film, the “effective” TOrdering can in general be probed through monitoring the threshold temperature of the AFM-induced phenomenon like the enhancement of Hcin a FM/AFM bilayer.17,18 Figure 3 shows the hysteresis loops and the corresponding Hcof the perpendicularly magnetic 6 ML Fe/4.5 Mn bilayer and 6 ML Fe/4.5 ML Mn/2.5 ML Fe trilayer as functions of temperature. In the former case, the Hcbehaves only weak enhancement from 25 Oe to 50 Oe as the temperature decreases from 196 K to 186 K. For the latter case, however, the Hcis significantly enhanced by 80 times from 10 Oe to 800 Oe as the temperature decreases from 210 K to 196 K. Such a huge Hc enhancement in the latter case suggests an enhanced AFM ordering for the Mn layer incorporated with ultrathin Fe underlayer. According to the temperature-dependent Hc curves displayed in Fig. 3, the TOrderingof the Mn layers can be estimated. In the present
FIG. 1. Magnetic hysteresis loops of (a) 6 ML Fe/n ML Mn bilayers as a function of the Mn film thickness and (b) m ML Fe/6 ML Mn bilayers as a function of thickness of top Fe layer, measured at 195 K.
FIG. 2. (a) Magnetic hysteresis loops of 8 ML Fe/5 ML Mn/k ML Fe tri- layers as a function of Fe underlayer thickness measured at 195 K. (b) and (c) Magnetic easy axis phase diagrams of Fe/Mn bilayers and Fe/Mn/2.5 ML Fe trilayers, respectively, measured at 195 K. The dashed and solid lines in (b) and (c) show the SRT boundary of the Fe/Mn and Fe/Mn/2.5 ML Fe tri- layers, respectively, where the SRT boundary is shifted to the higher value of ttopFewhile 2.5 ML Fe film is included.
042407-2 Wang et al. Appl. Phys. Lett. 103, 042407 (2013)
work, we define 25–30 Oe as the reference value for the intrinsic Hc of the top 6 ML Fe layer. The TOrdering of the Mn layer can be obtained from the critical temperature that the Hccrosses over with the reference value. In the bottom of Fig. 3, the arrows display the TOrdering of the Mn films with different conditions. It is clear to find that the TOrdering
of the Mn film in Fe/4.5 ML Mn bilayer is enhanced from about 190 K to 208 K while the 2.5 ML Fe underlayer was incorporated. As referring previous report,24the Curie tem- perature (TC) of a 2.5 ML Fe ultrathin film is about 270 K.
This temperature is much higher than the present TOrdering of the 4.5 ML Mn films in Fe/Mn bilayers (190 K).
Therefore, it is very possible that the enhanced TOrderingof the Mn film in Fe/Mn/2.5 ML Fe trilayer is induced by a magnetic proximity effect with the 2.5 ML Fe underlayer.
Although the enhancement of TOrdering of Mn films and AFM-driven PMA may be limited by a use of 2.5 ML Fe film with TC only 270 K, these characteristics are expected to be much improved if a FM layer with higher TC is chosen.
On the other hand, for the FM/non-FM/FM trilayers, it is known that the magnetic anisotropy of the FM film could also be modulated by the long range exchange coupling across the top and bottom FM layers through the Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction or the quantum well states.26 In the case of the Fe/Mn/Fe tri- layers, however, the magnetic moments of the Mn film are known to be AFM ordered and highly localized.27,28 This could prohibit itself to be a passive medium transmitting the indirect exchange coupling.29 Thus, the effect of interlayer coupling that helps to sustain the PMA is expected to be minor.
Finally, the present work also helps to gain more knowl- edge on the nature of the magnetic moments of the AFM Mn layer which are responsible for the established PMA of adja- cent FM layer. Our result indicates that the PMA of Fe/Mn bilayer can be affected by the ultrathin Fe underlayer through the magnetic proximity effect. This finding confirms our pre- vious results that the established PMA of the top Fe film is not only contributed by the AFM-FM exchange coupling at interface but is also correlated with the magnetic ordering of whole AFM Mn layer.8
In summary, we have reported that an AFM-induced PMA of the Fe/Mn films can be promoted by including an ultrathin Fe film as an underlayer. The enhanced PMA is attributed to an increase of magnetic ordering of the Mn film originated from a magnetic proximity effect with the Fe underlayer. The success of this approach suggests a possibil- ity for promoting the thermal stability of a low dimensional antiferromagnet and would greatly extend our current knowl- edge on the control of magnetic properties of perpendicular magnetic devices composed of FM and AFM layers.
This work was supported by the National Science Council of Taiwan through Grant Nos. NSC 100-2120-M- 002-002 and NSC 99-2112-M-003-009-MY3.
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