- Email: [email protected]
Cu(111) trilayer systems grown by MBE
Journal of Magnetism and Magnetic Materials 148 (1995) 34-35
EWEYIER
NMR studies of “Co in Cu/@o/‘Cu( 111) trilayer systems grown by MBE T. Thomson ap*,
P.C.
Ridi
a, K.
BrW
‘, B.
Biideker b
a Dept. of Physics and Astronomy, University of St. Andrews, St. Andrew& Fife KY16 932, UK b Rlahr-Universitiit Bochum, Institut jiir Experimental Physik W, 4478Q Bochun~, Germany
trilayers grown by MBE with Co NMR using the s9Co nucleus has been measured for a series of Cu/Co/Cu(lll) thicknesses ranging from 25 to 178 A,. The CQ layer retains the Cu fee structuie up to a thickness of w 68 A at which point the hcp structure is favoured. This structural transition is not identical for each sample suggesting that the point at which the transition occurs depe?ds on the exact growth conditions including the thickness of the Cu unde$ayer. Our results suggest that for very thin (15 A) Cu underlayers the transition to hcp occurs at thicknesses less than 60 A.
Spin echo nuclear magnetic resonance (IWR) utilising is a well established technique for determining the structural phase of Co in bulk samples [1,2]. Three structural phases of Co are known, fee, hcp and bee. The bee phase is currently the subject of some interest [3-S], however the two well known structural phases of Co (fee and hcp) each have a characteristic NMR spectrum in multidomain materials with a sharp peak centred at 217.4 MHz for the fee phase at T = 4.2 K and a broader feature extending from 217 to 230 MHz for the hcp phase at the same temperature. This broad feature represents a range of atomic environments associated with the anisotropy of the hyperfine field in the kcp lattice. in this papc “9Co spin echo NMR is applied to MBE grown Cu/Co/Cu(lll) trilayer films, containing a single Co layer, to determine both the structural phase of Co in these materials and the thickness dependence of the phase. Trilayer systems are useful model systems for multilayer films as they eliminate the possibility of accumulated stacking errors during growth. vowever, as the amount of Co is small (0.1 kg) for the 2.5 A Cu/Co/Co(lll) trilayer the signal to noise ratio is poor for the thinnest sample even after extensive signal averaging. Four Cu/Co/Cu(lll) trilayers were grown by MBE at zochum with Co layer thicknesses of 25, 93, 102 and 17s A, and Cu layer thicknesses ranging from 1.5 to 42 A. Details of dhe growth techniques employed and X-ray thickness characterisation methods used are contained in the s9Co nucleus
Corresponding author. Fax: +44-334-463104; and.ac.uk. l
e-mail; tt@st
Ref. [6]. The NMR experiments were carried out at T = 4.2 K using a coherent spin echo spectrometer [7]. The echo, after two pulses separated by a time t, decays approximately as A(2t)
=A(O)
exp(-2:/r&
(1)
although a double exponential is needed to model the decay around the main peak 181. The spectra for all but the thinnest sample were corrected for the variation of T2 with frequency across the spectrum with A(O) plotted against frequency. T2 for the thinnest sample could not be accurately determined within reasonable experimental periods. However, an estimate of the variation of T2 with frequency was obtained from measurements of both thin film and powder fee Co samples. Our results show that in MBE grown Cu/Co/Cu(lll) films the Co initially adopts the fee phase, but with distribution of N 3-4 MHz (3-4 kG) in the effective magnetic field at the nucleus. As the thickness of the Co layer is increased it adopts a more hcp like character. Integrating the area under the fee and hcp peaks allows the ratio of the number of nuclei in each environment to be determined, and hence provides a quantitative measure of the film thickness at which the transition occurs. Fig. 1 %hows the NMR spectrum obtained for sample 2 with a 93 A Co layer. Table 1 summarises the results obtained for the four films investigated together with a fee powder calibration sample. Table 1 shows that the transition from fee to a more hcp like character occurs at H 60 A with the transition region spread over approximately 30 A. The shift in the frequencies at which the fee Co peak occurs in the three thicker films is typical of MBE grown Cu/Co(lll) supcr-
0304-8853/95/$09,50 0 1995 Elsevier Science B.V. All rights resewed SSDI 0304-8853(95)00138-7
T. Thomson et aL /Journal of Magnetism and Magnetic Materials 148 (1995) 34-35 m
Cu/Co(93A)/Cu
•
=t
~2
0
"'j, . . . . .
-~j
Co fcc "~ - calibration J~
03
35
lattices and arises from the strain induced b y the slightly greater lattice constant ( 2 % ) o f fee Cu. The thinnest (25 ,~) film shows a somewhat different behaviour, here the fee peak frequency occurs close to the fcc powder calibration frequency. The width o f this peak is estimated b y correcting the spectrum with typical T2 data from fee Co. The results for the transition thickness between foe and hep are in agreement with the those obtained by Le D a n g et al. [9] for thick (1500 ,g,) C o / C u supeflattice f'dms. Analysis o f the peak associated with the hcp phase shows that the nuclei observed were probably parallel, rather than perpendicular, to the c-axis as the peak in the hep frequency occurs at 220 MHz, although the effect o f stacking faults in the fee phase cannot be ruled out. This indicates that the N M R signal arises principally from nuclei in a domain environment [2] rather than from nuclei in a domain wall environment.
o
,?,
References
i x = =
200
205
210
215
220
225
230
235
Frequency (MHz)
Fig. 1. 59Co NMR spectrum for Cu(34 A)/Co(93 A)/Cu(35 ~k) trilayer film at T = 4.2 K in zero applied field. Table 1 Parameters from NMR spectra of four C u / C o / C u trilayers Co Cu fee peak fee peak fce/hcp Sample thickness underlayer position width thickness (~k) (A) (MHz) (MHz) CA) 1 25 2 93 3 102 4 178 fee powder powder
15 34 15 23 -
217.2 215.2 215.3 215.6 217.4
~ 3.9 2.7 3.1 2.2 0.7
60/33 49/53 78/100 -
[1] R. Street, D.S. Rodbell, W.L. Roth, Phys. Rev. 121 (1961) 84. [2] M. Kawakami, I". Hihara, Y. K/~i, T. Wakiyama, J. Phys. See. Jn. 33 (1972) 1591. [3] D.J. Singh, Phys. Rev. B 45 (1992) 2258. [4] P.C. Riedi, T. Dumelow, M. Rubinstein, G.A. Prinz, S.B. Qadri, Phys. Rev. B 36 (1987) 4595. [5] J. Dekosier, E. Jedryka, C. M6ny, G. Langaueke, J. Magn. Magn. Mater. 121 (1993) 69. [6] P. B~deker, A. Abromeit, K. Br/Shl, P. Sonntag, N. Metoki, H. Zabel, Phys. Rev. B 47 (1993) 2353. [7] T. Dumelow, P.C. Riedi, Hyperfine Interactions 35 (1987) 1061. [8] T. Thomson, P.C. Riedi, D. Greig, Phys. Rev. B 50 (1994) 10319. [9] K. Le Dang, P. Veillet, P. Beauvillain, C. Chappett, Hui lie, FJ. Lamelas, C.H. Lee, R. Clarke, Phys. Rev. B 43 (1991) 13228.
EWEYIER
NMR studies of “Co in Cu/@o/‘Cu( 111) trilayer systems grown by MBE T. Thomson ap*,
P.C.
Ridi
a, K.
BrW
‘, B.
Biideker b
a Dept. of Physics and Astronomy, University of St. Andrews, St. Andrew& Fife KY16 932, UK b Rlahr-Universitiit Bochum, Institut jiir Experimental Physik W, 4478Q Bochun~, Germany
trilayers grown by MBE with Co NMR using the s9Co nucleus has been measured for a series of Cu/Co/Cu(lll) thicknesses ranging from 25 to 178 A,. The CQ layer retains the Cu fee structuie up to a thickness of w 68 A at which point the hcp structure is favoured. This structural transition is not identical for each sample suggesting that the point at which the transition occurs depe?ds on the exact growth conditions including the thickness of the Cu unde$ayer. Our results suggest that for very thin (15 A) Cu underlayers the transition to hcp occurs at thicknesses less than 60 A.
Spin echo nuclear magnetic resonance (IWR) utilising is a well established technique for determining the structural phase of Co in bulk samples [1,2]. Three structural phases of Co are known, fee, hcp and bee. The bee phase is currently the subject of some interest [3-S], however the two well known structural phases of Co (fee and hcp) each have a characteristic NMR spectrum in multidomain materials with a sharp peak centred at 217.4 MHz for the fee phase at T = 4.2 K and a broader feature extending from 217 to 230 MHz for the hcp phase at the same temperature. This broad feature represents a range of atomic environments associated with the anisotropy of the hyperfine field in the kcp lattice. in this papc “9Co spin echo NMR is applied to MBE grown Cu/Co/Cu(lll) trilayer films, containing a single Co layer, to determine both the structural phase of Co in these materials and the thickness dependence of the phase. Trilayer systems are useful model systems for multilayer films as they eliminate the possibility of accumulated stacking errors during growth. vowever, as the amount of Co is small (0.1 kg) for the 2.5 A Cu/Co/Co(lll) trilayer the signal to noise ratio is poor for the thinnest sample even after extensive signal averaging. Four Cu/Co/Cu(lll) trilayers were grown by MBE at zochum with Co layer thicknesses of 25, 93, 102 and 17s A, and Cu layer thicknesses ranging from 1.5 to 42 A. Details of dhe growth techniques employed and X-ray thickness characterisation methods used are contained in the s9Co nucleus
Corresponding author. Fax: +44-334-463104; and.ac.uk. l
e-mail; tt@st
Ref. [6]. The NMR experiments were carried out at T = 4.2 K using a coherent spin echo spectrometer [7]. The echo, after two pulses separated by a time t, decays approximately as A(2t)
=A(O)
exp(-2:/r&
(1)
although a double exponential is needed to model the decay around the main peak 181. The spectra for all but the thinnest sample were corrected for the variation of T2 with frequency across the spectrum with A(O) plotted against frequency. T2 for the thinnest sample could not be accurately determined within reasonable experimental periods. However, an estimate of the variation of T2 with frequency was obtained from measurements of both thin film and powder fee Co samples. Our results show that in MBE grown Cu/Co/Cu(lll) films the Co initially adopts the fee phase, but with distribution of N 3-4 MHz (3-4 kG) in the effective magnetic field at the nucleus. As the thickness of the Co layer is increased it adopts a more hcp like character. Integrating the area under the fee and hcp peaks allows the ratio of the number of nuclei in each environment to be determined, and hence provides a quantitative measure of the film thickness at which the transition occurs. Fig. 1 %hows the NMR spectrum obtained for sample 2 with a 93 A Co layer. Table 1 summarises the results obtained for the four films investigated together with a fee powder calibration sample. Table 1 shows that the transition from fee to a more hcp like character occurs at H 60 A with the transition region spread over approximately 30 A. The shift in the frequencies at which the fee Co peak occurs in the three thicker films is typical of MBE grown Cu/Co(lll) supcr-
0304-8853/95/$09,50 0 1995 Elsevier Science B.V. All rights resewed SSDI 0304-8853(95)00138-7
T. Thomson et aL /Journal of Magnetism and Magnetic Materials 148 (1995) 34-35 m
Cu/Co(93A)/Cu
•
=t
~2
0
"'j, . . . . .
-~j
Co fcc "~ - calibration J~
03
35
lattices and arises from the strain induced b y the slightly greater lattice constant ( 2 % ) o f fee Cu. The thinnest (25 ,~) film shows a somewhat different behaviour, here the fee peak frequency occurs close to the fcc powder calibration frequency. The width o f this peak is estimated b y correcting the spectrum with typical T2 data from fee Co. The results for the transition thickness between foe and hep are in agreement with the those obtained by Le D a n g et al. [9] for thick (1500 ,g,) C o / C u supeflattice f'dms. Analysis o f the peak associated with the hcp phase shows that the nuclei observed were probably parallel, rather than perpendicular, to the c-axis as the peak in the hep frequency occurs at 220 MHz, although the effect o f stacking faults in the fee phase cannot be ruled out. This indicates that the N M R signal arises principally from nuclei in a domain environment [2] rather than from nuclei in a domain wall environment.
o
,?,
References
i x = =
200
205
210
215
220
225
230
235
Frequency (MHz)
Fig. 1. 59Co NMR spectrum for Cu(34 A)/Co(93 A)/Cu(35 ~k) trilayer film at T = 4.2 K in zero applied field. Table 1 Parameters from NMR spectra of four C u / C o / C u trilayers Co Cu fee peak fee peak fce/hcp Sample thickness underlayer position width thickness (~k) (A) (MHz) (MHz) CA) 1 25 2 93 3 102 4 178 fee powder powder
15 34 15 23 -
217.2 215.2 215.3 215.6 217.4
~ 3.9 2.7 3.1 2.2 0.7
60/33 49/53 78/100 -
[1] R. Street, D.S. Rodbell, W.L. Roth, Phys. Rev. 121 (1961) 84. [2] M. Kawakami, I". Hihara, Y. K/~i, T. Wakiyama, J. Phys. See. Jn. 33 (1972) 1591. [3] D.J. Singh, Phys. Rev. B 45 (1992) 2258. [4] P.C. Riedi, T. Dumelow, M. Rubinstein, G.A. Prinz, S.B. Qadri, Phys. Rev. B 36 (1987) 4595. [5] J. Dekosier, E. Jedryka, C. M6ny, G. Langaueke, J. Magn. Magn. Mater. 121 (1993) 69. [6] P. B~deker, A. Abromeit, K. Br/Shl, P. Sonntag, N. Metoki, H. Zabel, Phys. Rev. B 47 (1993) 2353. [7] T. Dumelow, P.C. Riedi, Hyperfine Interactions 35 (1987) 1061. [8] T. Thomson, P.C. Riedi, D. Greig, Phys. Rev. B 50 (1994) 10319. [9] K. Le Dang, P. Veillet, P. Beauvillain, C. Chappett, Hui lie, FJ. Lamelas, C.H. Lee, R. Clarke, Phys. Rev. B 43 (1991) 13228.