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Unusual magnetic behavior in a chiral-based magnetic memory device
Journal of Magnetism and Magnetic Materials 398 (2016) 259–263
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Unusual magnetic behavior in a chiral-based magnetic memory device Oren Ben-Dor a, Shira Yochelis a, Israel Felner b,n, Yossi Paltiel a a b
Department of Applied Physics, Center of Nanoscience and Nanotechnology, Hebrew University, Jerusalem 91904, Israel “Racah” Institute of Physics, Hebrew University, Jerusalem 91904, Israel
art ic l e i nf o
a b s t r a c t
Article history: Received 29 June 2015 Received in revised form 18 August 2015 Accepted 2 September 2015 Available online 12 September 2015
In recent years chiral molecules were found to act as efficient spin filters. Using a multilayer structure with chiral molecules magnetic memory was realized. Observed rare magnetic phenomena in a chiralbased magnetic memory device was reported by O-Ben Dor et. al in Nature Commun, 4, 2256 (2013). This multi-layered device is built from α-helix L-polyalanine (AHPA-L) adsorbed on gold, Al2O3 (7 nm) and Ni (30 nm) layers. It was shown that certain temperature range the FC branch crosses the magnetic peak (at 55 K) observed in the ZFC curve thus ZFC4FC. We show here that in another similar multi-layered material, at low applied field, the ZFC curve lies above the FC one up to 70 K. The two features have the same origin and the crucial necessary components to exhibit them are: AHPA-L and 30 nm Ni layered thick. Similar effects were also reported in sulfur doped amorphous carbon. A comparison between the two systems and the ingredients for these peculiar observations is discussed. & 2015 Elsevier B.V. All rights reserved.
Keywords: Unusual magnetic properties Multi-layered device Chiral magnetic memory
1. Introduction Thermal irreversibility in dc magnetic measurements at constant applied magnetic field (H) is a well-known common phenomenon. It is readily found in ferromagnetic (FM), antiferromagnetic (AFM) and spin-glasses as well as in superconducting materials. The magnetic zero-field-cooled curves (ZFC) defined for samples which are first cooled (at H¼0) to the designated temperature and then H is applied, whereas the magnetic field-cooled (FC) branches measure samples which were cooled under H to low temperatures. In all standard cases the ZFC curves lie below curves (FC 4ZFC) up to a typical characteristic temperatures which represent critical transitions corresponding to the various physical states. The unusual magnetic behavior where the FC branches cross the ZFC curves (ZFC 4FC) has been recently observed in two unrelated materials: (i) inhomogeneous commercial and fabricated amorphous carbon powders synthesized with sulfur (a-CS) which exhibit pronounced peaks in their ZFC curves at 50–80 K only. These peaks are believed to be an intrinsic property of the a-CS powders. Around the peak position, the ZFC curves are much higher than the FC branches, thus at a certain temperature range ZFC4FC. This complex behavior is actually irreproducible and disappears in the second ZFC run [1]. (ii) In a chiral-based magnetic memory device its structure and electrical properties are n
Corresponding author. E-mail address: [email protected] (I. Felner).
http://dx.doi.org/10.1016/j.jmmm.2015.09.005 0304-8853/& 2015 Elsevier B.V. All rights reserved.
reported in Ref. [2]. Fig. 1 shows that the main components of this device are α-helix L-polyalanine (AHPA-L) adsorbed on gold, Al2O3 (7 nm) and Ni (30 nm) layers. Here again, the FC branch, crosses the peak around 50 K obtained in the ZFC curve only. Similar behavior was also observed very recently in carbon double nano tubes [3]. The anomalous behavior of ZFC 4FC was observed in several ferri-magnetic systems such as: Sr0.8La0.2Ti0.9Co0.1O3 [4], HoMnO3 [5], NiMnPd thin films [6] and in microporous carbon [7]. None of these cases are applicable to a-CS (or to the Au/(AHPA-L)/Ni device) as discussed in Ref. [1]. We are not aware of any theoretical model which explains these peaks appearances, thus the question of their origin is still open. In contrast to the a-CS powders which are inhomogeneous, where scattered results are obtained on different samples taken from the same source, the composition and layers thickness of Au/ (AHPA-L)/Ni multi-layer samples, are well known and can easily be reproduced. In order to explore the unusual phenomenon described above we present here an extensive magnetic studies on controlled compositions of the multi-layered Au/(AHPA-L)/Ni systems shown in Fig. 1. For the sake of clarity, we first present data obtained on (i) a multi-layered sample without the organic AHPA-L component (assigned as blank). (ii) Next we show data obtained on a sample with AHPA-L layers but with a Ni layer thickness of 7 nm (assigned as Ni7). In both samples the “normal” behavior of FC 4 ZFC is observed. (iii) The seemingly rare magnetic features of ZFC 4FC are observed only in samples with AHPA-L and 30 nm Ni layers
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Fig. 1. Schematic drawing of the chiral-based magnetic (Ni 30 nm) memory device. The gold upper and lower contacts and the thin 30 nm ferromagnetic Ni are represented in yellow and in transparent purple respectively. The thin Al2O3 layer above the AHPA-L chiral molecules is in red. The thermal SiO2 (on top of Si) is displayed in dark blue and on the PECVD SiO2 is in light blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(assigned as Ni30).
2. Experimental details The multi-layered materials were grown on a standard intrinsic Si substrate. A 820 nm SiO2 layer was thermally grown on top of the Si. Using conventional optical lithographic methods 15 nm Cr and 150 nm Gold contacts were deposited creating the lower contacts pattern. Next, a 500 nm SiO2 layer was deposited by Plasma-enhanced chemical vapor deposition (PECVD) followed by an optical lithography patterning of the molecular effective adsorption areas. These areas were etched using a chemical wet etch procedure thus opening holes in the SiO2 PECVD layer and creating 40 μm 50 μm areas in the lower Gold contacts. AHPA-L molecules were then chemisorbed using a self-assembly monolayer adsorption method. Samples were immersed in absolute ethanol under inert environmental conditions for 20 min then dried in nitrogen and immediately placed in a 1 mM AHPA-L in ethanol solution. Samples were taken out after 3 h in solution and then dried. A thin Al2O3 layer was deposited in a two step evaporation process of 4 nm and later 3 nm followed by 7 nm or 30 nm Ni evaporation and 150 nm Au continuous evaporations. DC Magnetization (M) measurements at various applied magnetic fields (H) in the temperature interval 5 KoT o200 K, as well as at different isothermal M versus H up to 50 kOe, have been performed using a commercial (Quantum Design) superconducting quantum interference device (SQUID) magnetometer. Prior to recording the ZFC curves, the SQUID magnetometer was always adjusted to be in a “true” H ¼0 state. The temperature dependence of the FC and the ZFC plots were taken via warming the samples. All multi-layered samples were measured parallel and perpendicular to the layer structure.
Fig. 2. ZFC and FC M(T) plots for sample without the organic component (blank) for H parallel to the layers measured at 500 Oe. The inset shows similar M(T) plots measured at 2.5 kOe.
(TB) of super-paramagnetic nano-particle Ni grains (not necessary all of them). Below 40 K, the FC curve exhibits a typical paramagnetic (PM) shape and adheres closely to the Curie–Weiss (CW) law: χ(T)¼ χ0 þC/(T θ), where χ ¼(M/H), χ0 is the temperature independent part, C is the Curie constant, and θ is the CW temperature. The PM parameters extracted are: χ0 ¼3.54 10 5 emu /g Oe, C ¼4.14 10 6 emu K/g Oe and θ ¼6.0(1) K. Since the only magnetic element in this device is Ni, both the peaks and the PM nature are attributed to it. The low C value applies to 1 2 ppm of PM Ni particles embedded in the Ni layers. The isothermal magnetization M(H) curve measured at 5 K (shown in Fig. 3) first increases up to 4 kOe and then becomes negative due to the diamagnetic substrate and the rest of the deposited layers. The experimental M(H) curve can be fitted as: (i) M (H ) = MS − χd H, where, MS ¼ 5.5 10 4 emu is the spontaneous FM magnetization of the Ni layers and – χdH is the linear intrinsic diamagnetic susceptibility. For 6.6.30 nm thick Ni layers the calculated MS is: 5.3 10 4 emu, a value which is in perfect agreement with the observed one. (ii) Multi-layers (with AHPA-L) and 7 nm Ni (Ni7). The M (T) measurements at 0.5, 1 and 2.5 kOe performed on Ni7 sample, are very similar to Fig. 2 and Fig. 4 shows the data obtained at 1 kOe for H parallel and perpendicular to the layers. Generally speaking the two curves are similar to each other, but the M values
3. Experimental results (i) The blank sample (30 nm Ni without AHPA-L). Fig. 2 shows the ZFC and FC temperature dependence of the magnetization (M (T)) of the blank material measured at H ¼500 Oe parallel to layers, in which a broad peak in the ZFC curve around 54 K is observed. This peak is barely visible in the FC branch. However, at H ¼2.5 kOe (Fig. 2 inset) the peak is observable in both branches. Probably the bifurcation around 210 K is the blocking temperature
Fig. 3. Experimental M(H) (black) and calculated – χdH (red) at 5 K for the blank material. Ms (in blue) presented is the calculated spontaneous FM magnetization of the Ni layers (see (i)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
O. Ben-Dor et al. / Journal of Magnetism and Magnetic Materials 398 (2016) 259–263
Fig.4. ZFC and FC M(T) plots measured at 1 kOe for sample with 7 nm Ni thick (Ni7) layers for H parallel and perpendicular to the layers. The inset in the upper plot shows the M(H) curve measured at 5 K.
obtained in the parallel direction are higher than those of the perpendicular one. That means that the hard axis of the magnetization is along the parallel direction (see below). This is also due to pronounced different demagnetization factors (not calculated) of the two orientations. A bump around 50 K is observed in the ZFC branch of the parallel direction. Because of the lower Ni layers thickness TB is around 120 K. At 5 K, the M(H) plot for the parallel direction (not shown) is similar to that presented in Fig. 3. Whereas for H perpendicular to planes (see inset) the curve is almost linear and negative down to very low fields, confirming that the easy axis of the Ni layers is in this orientation. This is in contrast to other thin Ni-based multi-layered systems in which the easy axis lies in the Ni planes [8]. (iii) Multi-layers (with AHPA-L) and 30 nm Ni (Ni30). The main topic of the present paper is to extend the peculiar magnetic behavior (ZFC 4FC) of the multi-layered Ni30 material presented in Ref. [2]. In what follows, we describe the magnetic data collected on a second Ni30 sample grown under the same procedure, for H perpendicular and parallel to the Ni planes. We definitely prove that the rare behavior occurs for the parallel (the hard) direction only. (a) H perpendicular. Figs 5 and 6 show the M(T) measured at 500 Oe and M(H) plots measured at 5, 80 and 155 K for H perpendicular to the Ni30 layers. The two ZFC and FC curves also
Fig. 5. ZFC and FC M(T) plots measured at 500 Oe for a sample with 30 nm Ni and H perpendicular to the layers. The inset compares between isothermal M(H) curves measured at 5 K for parallel and perpendicular directions.
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Fig. 6. Isothermal M(H) curves of Ni30 measured perpendicular to the Ni layers at 5, 80 and 155 K. Note the identical plots obtained at 80 and 155 K.
merge at 120(1) K and the peak at 54 K is observable in both branches. Similar to Fig. 2 the low temperature PM regime can be fitted with the CW relation and the extracted values obtained are: χ0 ¼ 1.45 10 5 emu/g Oe, C ¼1.24 10 5 emu K/g Oe and θ ¼1.2 (2) K. The deduced C value corresponds to 3–4 ppm of PM Ni particles. Here again the isothermal M(H) plots can be fitted as: M (H)¼MS χdH, and MS ¼5.78 10 4 emu (attributed to the FM saturated Ni layers) fits well with the measured value of the blank sample. Fig. 6 also shows the identical M(H) plots obtained at 80 and 155 K, temperatures which are below and above the blocking temperature. (b) H parallel. For the sake of clarity we show again (Fig. 7) the ZFC and FC curves measured at 500 Oe [2], where the FC curve crossing the ZFC one at peak around 55 K. The extracted data on a second Ni30 sample are presented in Figs. 8 and 9. The most striking effect (Fig. 8) is that below 70 K, the ZFC curve is above the FC one down to 5 K, whereas at higher temperatures (inset) the FC signals exceed the ZFC curves as expected, indicating that this behavior does not stem from any experimental failure. At 1 kOe (Fig. 9 inset) the two curves are very close to each other, but the ZFC 4 FC phenomenon is still observable. However at H ¼2.5 kOe the two curves merge at the peak position (55 K) and a normal FC 4ZFC behavior is obtained. The two peculiar magnetic phenomena (ZFC 4FC) observed in Figs. 7 and 8 have the same origin. The slightly discrepancy may be caused by a tiny difference in the layers thickness and/or by a different Ni particles size. Fig. 5 inset exhibits the isothermal M(H) curves of Ni30 at 5 K
Fig. 7. ZFC (black) and FC (red) of Ni30 material measured at 500 Oe [2]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. ZFC and FC M(T) plots of Ni30 at 500 Oe measured parallel to the Ni layers. The inset shows the same plots in an extended scale. Fig. 10. ZFC and FC plots of two a-CS powders measured at 100 Oe.
with sulfur (a-CS). This powder was obtained by heating a-C with sulfur at 400 K in a closed evacuated ampule and then cooling to ambient conditions [1]. The a-CS powder contains unavoidable 300 ppm of magnetite (Fe3O4) which is magnetically ordered up to 857 K. The product was divided into two parts and Fig. 10 depicts their magnetic behavior measured at 100 Oe. One part was measured as a “hot roll” just after the synthesis procedure (inset) and a second one two days later (main frame). Similar to Fig. 7, the ZFC curve of the “fresh” material crosses the FC branch, whereas the ZFC plot of the second part lies above the FC one up to 130 K. The difference between the two plots indicates inhomogeneity induced in the same batch. The FC branches of the two parts are qualitatively similar to each other, thus the major difference is depicted between the two ZFC branches only. Fig. 9. ZFC and FC M(T) plots of Ni30 2.5 kOe and 1 kOe (inset) measured parallel to the Ni layers.
for both orientations. In contrast to the saturation achieved around 1.5 kOe in a perpendicular direction no saturation is observed up to 10 kOe in the parallel orientation. This clearly confirms, that the unusual ZFC 4FC behavior occurs in the parallel direction which is the hard axis of the magnetization. The magnetic behavior presented so far can be summarized as follows: (1) In the blank material as well as in Ni30 (both contain 30 nm Ni layers), a pronounce peak at 55 K appears in the M(T) curves in both orientations. It is reasonable to attribute this peak to the solidification of oxygen as observed in various adsorbed oxygen matrices [9]. However, we oppose this interpretation because (a) the peak amplitudes are a few orders of magnitude higher than that expected for oxygen. (b) The peak appears only in the ZFC curve (see Fig. 7). Solidification of oxygen should show up also in the FC branch. (2) In Ni30 only, the ZFC curve crosses or lies above the FC branch. The two unexplained phenomena have the same origin. (3) This peculiar magnetic behavior is obtained in the hard axis of the magnetization. (4) This behavior seems to originate from two major ingredients: the organic AHPA-L chiral chains and the thickness of the Ni layers. As a final point of interest, we present new similar magnetic behavior on commercial amorphous carbon powder (a-C) doped
4. Discussion Unusual magnetic features are observed in two different multilayered Ni30 samples which contain: Al2O3 (7 nm) Ni (30 nm) and α-helix L-polyalanine (AHPA-L) adsorbed on gold (Fig. 1). Samples without AHPA-L 7 and/or with thinner Ni layers (7 nm) do not show these phenomena. In contrast to the general trend where FC 4ZFC, in one Ni30 sample (Fig. 7) a pronounced peak at 55 K appears in the ZFC plots, and the FC curve crosses the ZFC one (ZFC4FC). In a second Ni30 sample the ZFC curve lies above the FC up 70 K (Fig. 8) and then behaves normally. Both are unique anisotropic observations seldom observed which appear only in the parallel direction (the hard axis), are intrinsic to the Ni layers and have the same origin. The slightly different magnetic observations are probably due to a tiny difference in the synthesis process which may affect the Ni layers thickness and/or the Ni particle size. This issue needs severe consideration. The speculations that experimental failures and/or adsorbed oxygen caused these phenomena have been ruled out. The salient features shown for Ni30 are also observed in commercial a-CS samples [1]. The same phenomenon exists also observed in carbon double nano tubes [3]. That means that these peculiar magnetic properties are not accidental, but rather are common intrinsic properties of Ni30 and a-CS materials. The bifurcation of the ZFC and FC branches at 120 or 210 K is well above the peak positions and probably represents the blocking temperature of the super-paramagnetic Ni particles. On the other hand Fig. 6 shows that the M(H) below and above TB are identical. That means the major part of the Ni particles are
O. Ben-Dor et al. / Journal of Magnetism and Magnetic Materials 398 (2016) 259–263
magnetically ordered at much higher temperatures and presumably only a small fraction of the Ni atoms cause this impressive thermal irreversibility [10]. Besides that, we also observed tiny amounts of the PM phases (around 1–4 ppm) features which are more pronounced at low temperatures. We discuss now (a) the peak observed in the ZFC branch in Ni30 (Fig. 7) and (b) peculiar behavior of ZFC4FC observed in Figs. 7 and 8. (a) So far no existing theoretical models can explain the appearance of this peak. However, as a wave-handing explanation, we may assume that structural disorder, or adsorbed foreign atoms in the fabrication process are responsible for a local magnetic state in the Ni layered structures which produces this peak. Alternatively, we may attribute the peak to a tiny migration of foreign atoms to the Ni surface caused by the external magnetic field. We may also assume that a combined effect of structural disorder, the presence of a foreign atom and the applied field, are behind the peak formation in Ni layers. (b) The main question remains concerning the peculiar behavior of the FC branch in Ni30 discussed above. We may assume that prior to applying the external field H, the Ni magnetic moments are randomly distributed over the entire volume. In the hard axis direction, low H, causes the Ni moments to flip along the field direction in a parallel FM manner up to 55 K. Above the peak position an antiparallel exchange coupling is more favored and an AFM or ferri-magnetic coupling is obtained. Since the magnetic ordering transition of the bulk Ni layers is above room temperature, the Ni ions remain in in the AFM state and as a result, in the following FC process their net magnetic moment is lower than at the beginning of the ZFC process. Note that at 2.5 kOe the system behaves “normally” and the FC 4ZFC type behavior is observed (Fig. 9). Alternatively, we may speculate that the Ni30 devices are in the so called - two-state system- separated by a certain energy barrier as discussed for a-CS in Ref. [1]. Similar results were already observed a-CS with powders as shown in Fig. 10. The common denominator between Ni layers and inhomogeneous a-CS is as follows. Both systems contain: (i) carbon (the major component in AHPA-L) (ii) a magnetic element Ni or magnetite in a-CS and (iii) sulfur. The mechanism that produces these magnetic anomalies in Ni30 and/or in a-CS samples remains still unclear. Whatever the explanation is, these
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results are challenging and definitely warrant further theoretical and experimental investigations. In conclusion, the unusual magnetic behavior of a chiral-based magnetic memory device reported in Ref. [2] was investigated. It is shown that at low applied magnetic fields a pronounced magnetic peak around 55 K appears in the ZFC curve and that at certain temperatures the FC branch crosses the ZFC one, thus ZFC 4FC. In another similar material the ZFC curve lies above the FC one up to 70 K. The multi-layered material are α-helix L-polyalanine (AHPAL) adsorbed on gold, Al2O3 (7 nm) and Ni (30 nm) layers. It is shown that only the materials which contain both AHPA-L and 30 nm of Ni layers exhibit these peculiar magnetic features. It is assumed that a combined effect of structural disorder and the presence of a foreign atom such as sulfur can be behind this peak formation. The current state of experiments does not allow us to suggest any consistent explanation to the ZFC 4FC phenomena presented here.
Acknowledgment This research is supported by the Volkswagen 88 367 Foundation and the Ministry of Industry, Israel.
References [1] I. Felner, Mater. Res. Express 1 (2014) 016001. [2] O. Ben Dor, S. Yochelis, S.P. Mathew, R. Naaman, Y. Paltiel, Nat. Commun. 4 (2013) 2256. [3] J. Barzola-Quiquia, P. Esquinazi, M. Lindel, D. Spemann, M. Muallem, G. D. Nessim, Carbon 88 (2015) 16. [4] J. Yang, Y.P. Lee, B.W. Lee, Appl. Phys. Lett. 91 (2007) 052502. [5] Y. Oner, C.S. Lue, J.H. Ross Jr., K.D.D. Rathayka, D.G. Naugle, J. Appl. Phys. 89 (2001) 7044. [6] J. Imri, S.-K. Ma, Phys. Rev. Lett. 21 (1975) 1399. [7] D. Arcon, Z. Jaglicic, A. Zorko, A.V. Rode, A.G. Christy, N.R. Madsen, E.G. Gamaly, B. Luther-Davies, Phys. Rev. B. 74 (2006) 014438. [8] G. Kartopu, O. Yalçın, K.-L. Choy, R. Topkaya, S. Kazan, B. Aktaş, J. Appl. Phys. 109 (2011) 033909. [9] S. Bandow, T. Yamaguchi, S. Iijima, Chem. Phys. Lett. 401 (2005) 380. [10] M. Yoon, Y. Kim, Y.M. Kim, V. Volkov, H.J. Song, Y.J. Park, I.-W. Park, Mater. Chem. Phys. 91 (2005) 104.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Unusual magnetic behavior in a chiral-based magnetic memory device Oren Ben-Dor a, Shira Yochelis a, Israel Felner b,n, Yossi Paltiel a a b
Department of Applied Physics, Center of Nanoscience and Nanotechnology, Hebrew University, Jerusalem 91904, Israel “Racah” Institute of Physics, Hebrew University, Jerusalem 91904, Israel
art ic l e i nf o
a b s t r a c t
Article history: Received 29 June 2015 Received in revised form 18 August 2015 Accepted 2 September 2015 Available online 12 September 2015
In recent years chiral molecules were found to act as efficient spin filters. Using a multilayer structure with chiral molecules magnetic memory was realized. Observed rare magnetic phenomena in a chiralbased magnetic memory device was reported by O-Ben Dor et. al in Nature Commun, 4, 2256 (2013). This multi-layered device is built from α-helix L-polyalanine (AHPA-L) adsorbed on gold, Al2O3 (7 nm) and Ni (30 nm) layers. It was shown that certain temperature range the FC branch crosses the magnetic peak (at 55 K) observed in the ZFC curve thus ZFC4FC. We show here that in another similar multi-layered material, at low applied field, the ZFC curve lies above the FC one up to 70 K. The two features have the same origin and the crucial necessary components to exhibit them are: AHPA-L and 30 nm Ni layered thick. Similar effects were also reported in sulfur doped amorphous carbon. A comparison between the two systems and the ingredients for these peculiar observations is discussed. & 2015 Elsevier B.V. All rights reserved.
Keywords: Unusual magnetic properties Multi-layered device Chiral magnetic memory
1. Introduction Thermal irreversibility in dc magnetic measurements at constant applied magnetic field (H) is a well-known common phenomenon. It is readily found in ferromagnetic (FM), antiferromagnetic (AFM) and spin-glasses as well as in superconducting materials. The magnetic zero-field-cooled curves (ZFC) defined for samples which are first cooled (at H¼0) to the designated temperature and then H is applied, whereas the magnetic field-cooled (FC) branches measure samples which were cooled under H to low temperatures. In all standard cases the ZFC curves lie below curves (FC 4ZFC) up to a typical characteristic temperatures which represent critical transitions corresponding to the various physical states. The unusual magnetic behavior where the FC branches cross the ZFC curves (ZFC 4FC) has been recently observed in two unrelated materials: (i) inhomogeneous commercial and fabricated amorphous carbon powders synthesized with sulfur (a-CS) which exhibit pronounced peaks in their ZFC curves at 50–80 K only. These peaks are believed to be an intrinsic property of the a-CS powders. Around the peak position, the ZFC curves are much higher than the FC branches, thus at a certain temperature range ZFC4FC. This complex behavior is actually irreproducible and disappears in the second ZFC run [1]. (ii) In a chiral-based magnetic memory device its structure and electrical properties are n
Corresponding author. E-mail address: [email protected] (I. Felner).
http://dx.doi.org/10.1016/j.jmmm.2015.09.005 0304-8853/& 2015 Elsevier B.V. All rights reserved.
reported in Ref. [2]. Fig. 1 shows that the main components of this device are α-helix L-polyalanine (AHPA-L) adsorbed on gold, Al2O3 (7 nm) and Ni (30 nm) layers. Here again, the FC branch, crosses the peak around 50 K obtained in the ZFC curve only. Similar behavior was also observed very recently in carbon double nano tubes [3]. The anomalous behavior of ZFC 4FC was observed in several ferri-magnetic systems such as: Sr0.8La0.2Ti0.9Co0.1O3 [4], HoMnO3 [5], NiMnPd thin films [6] and in microporous carbon [7]. None of these cases are applicable to a-CS (or to the Au/(AHPA-L)/Ni device) as discussed in Ref. [1]. We are not aware of any theoretical model which explains these peaks appearances, thus the question of their origin is still open. In contrast to the a-CS powders which are inhomogeneous, where scattered results are obtained on different samples taken from the same source, the composition and layers thickness of Au/ (AHPA-L)/Ni multi-layer samples, are well known and can easily be reproduced. In order to explore the unusual phenomenon described above we present here an extensive magnetic studies on controlled compositions of the multi-layered Au/(AHPA-L)/Ni systems shown in Fig. 1. For the sake of clarity, we first present data obtained on (i) a multi-layered sample without the organic AHPA-L component (assigned as blank). (ii) Next we show data obtained on a sample with AHPA-L layers but with a Ni layer thickness of 7 nm (assigned as Ni7). In both samples the “normal” behavior of FC 4 ZFC is observed. (iii) The seemingly rare magnetic features of ZFC 4FC are observed only in samples with AHPA-L and 30 nm Ni layers
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Fig. 1. Schematic drawing of the chiral-based magnetic (Ni 30 nm) memory device. The gold upper and lower contacts and the thin 30 nm ferromagnetic Ni are represented in yellow and in transparent purple respectively. The thin Al2O3 layer above the AHPA-L chiral molecules is in red. The thermal SiO2 (on top of Si) is displayed in dark blue and on the PECVD SiO2 is in light blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(assigned as Ni30).
2. Experimental details The multi-layered materials were grown on a standard intrinsic Si substrate. A 820 nm SiO2 layer was thermally grown on top of the Si. Using conventional optical lithographic methods 15 nm Cr and 150 nm Gold contacts were deposited creating the lower contacts pattern. Next, a 500 nm SiO2 layer was deposited by Plasma-enhanced chemical vapor deposition (PECVD) followed by an optical lithography patterning of the molecular effective adsorption areas. These areas were etched using a chemical wet etch procedure thus opening holes in the SiO2 PECVD layer and creating 40 μm 50 μm areas in the lower Gold contacts. AHPA-L molecules were then chemisorbed using a self-assembly monolayer adsorption method. Samples were immersed in absolute ethanol under inert environmental conditions for 20 min then dried in nitrogen and immediately placed in a 1 mM AHPA-L in ethanol solution. Samples were taken out after 3 h in solution and then dried. A thin Al2O3 layer was deposited in a two step evaporation process of 4 nm and later 3 nm followed by 7 nm or 30 nm Ni evaporation and 150 nm Au continuous evaporations. DC Magnetization (M) measurements at various applied magnetic fields (H) in the temperature interval 5 KoT o200 K, as well as at different isothermal M versus H up to 50 kOe, have been performed using a commercial (Quantum Design) superconducting quantum interference device (SQUID) magnetometer. Prior to recording the ZFC curves, the SQUID magnetometer was always adjusted to be in a “true” H ¼0 state. The temperature dependence of the FC and the ZFC plots were taken via warming the samples. All multi-layered samples were measured parallel and perpendicular to the layer structure.
Fig. 2. ZFC and FC M(T) plots for sample without the organic component (blank) for H parallel to the layers measured at 500 Oe. The inset shows similar M(T) plots measured at 2.5 kOe.
(TB) of super-paramagnetic nano-particle Ni grains (not necessary all of them). Below 40 K, the FC curve exhibits a typical paramagnetic (PM) shape and adheres closely to the Curie–Weiss (CW) law: χ(T)¼ χ0 þC/(T θ), where χ ¼(M/H), χ0 is the temperature independent part, C is the Curie constant, and θ is the CW temperature. The PM parameters extracted are: χ0 ¼3.54 10 5 emu /g Oe, C ¼4.14 10 6 emu K/g Oe and θ ¼6.0(1) K. Since the only magnetic element in this device is Ni, both the peaks and the PM nature are attributed to it. The low C value applies to 1 2 ppm of PM Ni particles embedded in the Ni layers. The isothermal magnetization M(H) curve measured at 5 K (shown in Fig. 3) first increases up to 4 kOe and then becomes negative due to the diamagnetic substrate and the rest of the deposited layers. The experimental M(H) curve can be fitted as: (i) M (H ) = MS − χd H, where, MS ¼ 5.5 10 4 emu is the spontaneous FM magnetization of the Ni layers and – χdH is the linear intrinsic diamagnetic susceptibility. For 6.6.30 nm thick Ni layers the calculated MS is: 5.3 10 4 emu, a value which is in perfect agreement with the observed one. (ii) Multi-layers (with AHPA-L) and 7 nm Ni (Ni7). The M (T) measurements at 0.5, 1 and 2.5 kOe performed on Ni7 sample, are very similar to Fig. 2 and Fig. 4 shows the data obtained at 1 kOe for H parallel and perpendicular to the layers. Generally speaking the two curves are similar to each other, but the M values
3. Experimental results (i) The blank sample (30 nm Ni without AHPA-L). Fig. 2 shows the ZFC and FC temperature dependence of the magnetization (M (T)) of the blank material measured at H ¼500 Oe parallel to layers, in which a broad peak in the ZFC curve around 54 K is observed. This peak is barely visible in the FC branch. However, at H ¼2.5 kOe (Fig. 2 inset) the peak is observable in both branches. Probably the bifurcation around 210 K is the blocking temperature
Fig. 3. Experimental M(H) (black) and calculated – χdH (red) at 5 K for the blank material. Ms (in blue) presented is the calculated spontaneous FM magnetization of the Ni layers (see (i)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig.4. ZFC and FC M(T) plots measured at 1 kOe for sample with 7 nm Ni thick (Ni7) layers for H parallel and perpendicular to the layers. The inset in the upper plot shows the M(H) curve measured at 5 K.
obtained in the parallel direction are higher than those of the perpendicular one. That means that the hard axis of the magnetization is along the parallel direction (see below). This is also due to pronounced different demagnetization factors (not calculated) of the two orientations. A bump around 50 K is observed in the ZFC branch of the parallel direction. Because of the lower Ni layers thickness TB is around 120 K. At 5 K, the M(H) plot for the parallel direction (not shown) is similar to that presented in Fig. 3. Whereas for H perpendicular to planes (see inset) the curve is almost linear and negative down to very low fields, confirming that the easy axis of the Ni layers is in this orientation. This is in contrast to other thin Ni-based multi-layered systems in which the easy axis lies in the Ni planes [8]. (iii) Multi-layers (with AHPA-L) and 30 nm Ni (Ni30). The main topic of the present paper is to extend the peculiar magnetic behavior (ZFC 4FC) of the multi-layered Ni30 material presented in Ref. [2]. In what follows, we describe the magnetic data collected on a second Ni30 sample grown under the same procedure, for H perpendicular and parallel to the Ni planes. We definitely prove that the rare behavior occurs for the parallel (the hard) direction only. (a) H perpendicular. Figs 5 and 6 show the M(T) measured at 500 Oe and M(H) plots measured at 5, 80 and 155 K for H perpendicular to the Ni30 layers. The two ZFC and FC curves also
Fig. 5. ZFC and FC M(T) plots measured at 500 Oe for a sample with 30 nm Ni and H perpendicular to the layers. The inset compares between isothermal M(H) curves measured at 5 K for parallel and perpendicular directions.
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Fig. 6. Isothermal M(H) curves of Ni30 measured perpendicular to the Ni layers at 5, 80 and 155 K. Note the identical plots obtained at 80 and 155 K.
merge at 120(1) K and the peak at 54 K is observable in both branches. Similar to Fig. 2 the low temperature PM regime can be fitted with the CW relation and the extracted values obtained are: χ0 ¼ 1.45 10 5 emu/g Oe, C ¼1.24 10 5 emu K/g Oe and θ ¼1.2 (2) K. The deduced C value corresponds to 3–4 ppm of PM Ni particles. Here again the isothermal M(H) plots can be fitted as: M (H)¼MS χdH, and MS ¼5.78 10 4 emu (attributed to the FM saturated Ni layers) fits well with the measured value of the blank sample. Fig. 6 also shows the identical M(H) plots obtained at 80 and 155 K, temperatures which are below and above the blocking temperature. (b) H parallel. For the sake of clarity we show again (Fig. 7) the ZFC and FC curves measured at 500 Oe [2], where the FC curve crossing the ZFC one at peak around 55 K. The extracted data on a second Ni30 sample are presented in Figs. 8 and 9. The most striking effect (Fig. 8) is that below 70 K, the ZFC curve is above the FC one down to 5 K, whereas at higher temperatures (inset) the FC signals exceed the ZFC curves as expected, indicating that this behavior does not stem from any experimental failure. At 1 kOe (Fig. 9 inset) the two curves are very close to each other, but the ZFC 4 FC phenomenon is still observable. However at H ¼2.5 kOe the two curves merge at the peak position (55 K) and a normal FC 4ZFC behavior is obtained. The two peculiar magnetic phenomena (ZFC 4FC) observed in Figs. 7 and 8 have the same origin. The slightly discrepancy may be caused by a tiny difference in the layers thickness and/or by a different Ni particles size. Fig. 5 inset exhibits the isothermal M(H) curves of Ni30 at 5 K
Fig. 7. ZFC (black) and FC (red) of Ni30 material measured at 500 Oe [2]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. ZFC and FC M(T) plots of Ni30 at 500 Oe measured parallel to the Ni layers. The inset shows the same plots in an extended scale. Fig. 10. ZFC and FC plots of two a-CS powders measured at 100 Oe.
with sulfur (a-CS). This powder was obtained by heating a-C with sulfur at 400 K in a closed evacuated ampule and then cooling to ambient conditions [1]. The a-CS powder contains unavoidable 300 ppm of magnetite (Fe3O4) which is magnetically ordered up to 857 K. The product was divided into two parts and Fig. 10 depicts their magnetic behavior measured at 100 Oe. One part was measured as a “hot roll” just after the synthesis procedure (inset) and a second one two days later (main frame). Similar to Fig. 7, the ZFC curve of the “fresh” material crosses the FC branch, whereas the ZFC plot of the second part lies above the FC one up to 130 K. The difference between the two plots indicates inhomogeneity induced in the same batch. The FC branches of the two parts are qualitatively similar to each other, thus the major difference is depicted between the two ZFC branches only. Fig. 9. ZFC and FC M(T) plots of Ni30 2.5 kOe and 1 kOe (inset) measured parallel to the Ni layers.
for both orientations. In contrast to the saturation achieved around 1.5 kOe in a perpendicular direction no saturation is observed up to 10 kOe in the parallel orientation. This clearly confirms, that the unusual ZFC 4FC behavior occurs in the parallel direction which is the hard axis of the magnetization. The magnetic behavior presented so far can be summarized as follows: (1) In the blank material as well as in Ni30 (both contain 30 nm Ni layers), a pronounce peak at 55 K appears in the M(T) curves in both orientations. It is reasonable to attribute this peak to the solidification of oxygen as observed in various adsorbed oxygen matrices [9]. However, we oppose this interpretation because (a) the peak amplitudes are a few orders of magnitude higher than that expected for oxygen. (b) The peak appears only in the ZFC curve (see Fig. 7). Solidification of oxygen should show up also in the FC branch. (2) In Ni30 only, the ZFC curve crosses or lies above the FC branch. The two unexplained phenomena have the same origin. (3) This peculiar magnetic behavior is obtained in the hard axis of the magnetization. (4) This behavior seems to originate from two major ingredients: the organic AHPA-L chiral chains and the thickness of the Ni layers. As a final point of interest, we present new similar magnetic behavior on commercial amorphous carbon powder (a-C) doped
4. Discussion Unusual magnetic features are observed in two different multilayered Ni30 samples which contain: Al2O3 (7 nm) Ni (30 nm) and α-helix L-polyalanine (AHPA-L) adsorbed on gold (Fig. 1). Samples without AHPA-L 7 and/or with thinner Ni layers (7 nm) do not show these phenomena. In contrast to the general trend where FC 4ZFC, in one Ni30 sample (Fig. 7) a pronounced peak at 55 K appears in the ZFC plots, and the FC curve crosses the ZFC one (ZFC4FC). In a second Ni30 sample the ZFC curve lies above the FC up 70 K (Fig. 8) and then behaves normally. Both are unique anisotropic observations seldom observed which appear only in the parallel direction (the hard axis), are intrinsic to the Ni layers and have the same origin. The slightly different magnetic observations are probably due to a tiny difference in the synthesis process which may affect the Ni layers thickness and/or the Ni particle size. This issue needs severe consideration. The speculations that experimental failures and/or adsorbed oxygen caused these phenomena have been ruled out. The salient features shown for Ni30 are also observed in commercial a-CS samples [1]. The same phenomenon exists also observed in carbon double nano tubes [3]. That means that these peculiar magnetic properties are not accidental, but rather are common intrinsic properties of Ni30 and a-CS materials. The bifurcation of the ZFC and FC branches at 120 or 210 K is well above the peak positions and probably represents the blocking temperature of the super-paramagnetic Ni particles. On the other hand Fig. 6 shows that the M(H) below and above TB are identical. That means the major part of the Ni particles are
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magnetically ordered at much higher temperatures and presumably only a small fraction of the Ni atoms cause this impressive thermal irreversibility [10]. Besides that, we also observed tiny amounts of the PM phases (around 1–4 ppm) features which are more pronounced at low temperatures. We discuss now (a) the peak observed in the ZFC branch in Ni30 (Fig. 7) and (b) peculiar behavior of ZFC4FC observed in Figs. 7 and 8. (a) So far no existing theoretical models can explain the appearance of this peak. However, as a wave-handing explanation, we may assume that structural disorder, or adsorbed foreign atoms in the fabrication process are responsible for a local magnetic state in the Ni layered structures which produces this peak. Alternatively, we may attribute the peak to a tiny migration of foreign atoms to the Ni surface caused by the external magnetic field. We may also assume that a combined effect of structural disorder, the presence of a foreign atom and the applied field, are behind the peak formation in Ni layers. (b) The main question remains concerning the peculiar behavior of the FC branch in Ni30 discussed above. We may assume that prior to applying the external field H, the Ni magnetic moments are randomly distributed over the entire volume. In the hard axis direction, low H, causes the Ni moments to flip along the field direction in a parallel FM manner up to 55 K. Above the peak position an antiparallel exchange coupling is more favored and an AFM or ferri-magnetic coupling is obtained. Since the magnetic ordering transition of the bulk Ni layers is above room temperature, the Ni ions remain in in the AFM state and as a result, in the following FC process their net magnetic moment is lower than at the beginning of the ZFC process. Note that at 2.5 kOe the system behaves “normally” and the FC 4ZFC type behavior is observed (Fig. 9). Alternatively, we may speculate that the Ni30 devices are in the so called - two-state system- separated by a certain energy barrier as discussed for a-CS in Ref. [1]. Similar results were already observed a-CS with powders as shown in Fig. 10. The common denominator between Ni layers and inhomogeneous a-CS is as follows. Both systems contain: (i) carbon (the major component in AHPA-L) (ii) a magnetic element Ni or magnetite in a-CS and (iii) sulfur. The mechanism that produces these magnetic anomalies in Ni30 and/or in a-CS samples remains still unclear. Whatever the explanation is, these
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results are challenging and definitely warrant further theoretical and experimental investigations. In conclusion, the unusual magnetic behavior of a chiral-based magnetic memory device reported in Ref. [2] was investigated. It is shown that at low applied magnetic fields a pronounced magnetic peak around 55 K appears in the ZFC curve and that at certain temperatures the FC branch crosses the ZFC one, thus ZFC 4FC. In another similar material the ZFC curve lies above the FC one up to 70 K. The multi-layered material are α-helix L-polyalanine (AHPAL) adsorbed on gold, Al2O3 (7 nm) and Ni (30 nm) layers. It is shown that only the materials which contain both AHPA-L and 30 nm of Ni layers exhibit these peculiar magnetic features. It is assumed that a combined effect of structural disorder and the presence of a foreign atom such as sulfur can be behind this peak formation. The current state of experiments does not allow us to suggest any consistent explanation to the ZFC 4FC phenomena presented here.
Acknowledgment This research is supported by the Volkswagen 88 367 Foundation and the Ministry of Industry, Israel.
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