Journal of Magnetism and Magnetic Materials 69 (1987) 237-246 North-Holland, Amsterdam

237

MAGNETIZATION AND MAGNETIC AFTEREFFECT IN TEXTURED Ni/Cu COMPOSITIONALLY-MODULATED ALLOYS

U. ATZMONY *, L.J. SWARTZENDRUBER, L.H. BENNETT, M.P. DARIEL *, D. LASHMORE

National Bureau of Standardr, Gaithersburg IUD 20899, USA

M. RUBINSTEIN and P. LUBITZ

Naval Research Laboratory, Washington, DC 20375-5000, USA

Received 12 May 1987; in revised form 15 June 1987

The magnetic properties of Ni/Cu compositionally-modulated alloys with [lOO], [110] and [ill] textures were measured by magnetometry and ferromagnetic resonance. These alloys were found to exhibit a pronounced magnetic aftereffect.

1. Introduction

Studies of composition-modulated alloys (CMA), especially [l] those of Cu/Ni, have been extensively carried out in the last several years, as is evident from a number of reviews [2-61. The reasons for this interest lie in their unusual prop- erties which includes elastic and magnetic phe- nomena. The Cu-Ni system is of special interest as it provides a good model system for magnetic studies. The constituents share the same crystal structure, exhibit continuous solid solution over the entire concentration range and only one of them, Ni, exhibits ferromagnetic properties. The magnetic properties of the bulk alloy system de- pend on concentration and those of the CMA depend on the modulation wavelength, A, and the slab thicknesses, d,, and d,i (h = d,, + d,i).

The magnetic properties of the CMA also de- pend on the orientation of the superlattice com- pared to its crystallographic orientation. Most of the samples that were previously reported were limited to either [ill] or to mixed textures [1,7-lo]. Recently, Xiao and Chien [ll] reported a sample with a [lOO] modulation orientation. In the past,

l Permanent address: Nuclear Research Center-Negev, Beer- Sheva, Israel.

the majority of these materials were prepared by vacuum evaporation or by sputtering. An altema- tive technique to obtain these materials, which historically is earlier [12], utilizes electrodeposi- tion. This technique has some advantages and together with recent developments it might be- come the preferred one. There is a strong tendency of the electrodeposited materials to adopt the tex- ture of the substrate. By using oriented single crystals of copper as substrates, Lashmore and Dariel [13] fabricated samples of CMA of copper and nickel layers displaying strong texture. The electrodeposition of binary copper nickel CMA from a single electrolyte has been described by Tenth and White [14], Ogden [15] and Yahalom and Zadok [16,17]. The method used in this study is a modified form of the method described by Yahalom and Zadok [17]. The modifications used have been described in detail by Lashmore and Dariel [13]. The samples were prepared by pulsed galvanostatic deposition from a single electrolyte which contained the two constituent elements, the concentration of copper in the electrolyte being 1% of that of the nickel. The copper was deposited at 0.3 mA/cm2 and the nickel at 12 to 10 mA/cm2 current density. Typically, the overall thickness was 15 to 30 pm. The details of the sample preparation are given elsewhere [13].

030~8853/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

238 U. Atzmony et al. / Ni/Cu compositionally-modulated alloys

In this work, magnetic properties of three such electrodeposited CMA samples, one for each main cubic crystal axis, were investigated utilizing vibrating sample magnetometry (VSM), supercon- ducting magnetometry (SQUID) and ferromag- netic resonance (FMR). A time-dependent magne- tization was discovered and is discussed below.

2. Experimental methods

Three textured samples, one for each of the principal cubic orientations were measured. As an example, fig. 1 shows the 220 peak of the [llO]- textured sample with its satellite peaks which re- veal the superlattice structure. This, and similar figures for the [ill] and [lOO] textured samples, are shown and described in detail elsewhere [13]. The values of the modulation wavelength (X), slab thickness (dNi), total thickness and texture quality are summarized in table 1. The texture quality was evaluated by comparing the relative intensities of the different lines in a-219 X-ray pattern to those of a theoretical random polycrystal.

Most of the VSM measurements were carried out at room temperature, with some measure- ments at 86 K. The samples were mounted either parallel or perpendicular to the magnetic field. For every sample, hysteresis loops with amplitude of 2900 Oe and of 200 Oe were recorded for applied magnetic field both parallel and per-

4.0 -j

3.8 -

3.6 -

3.4 -

3.2 - satellite

3.0 -

2.8 -

2.6, n , . , . , . ( . , . I

66 70 72 74 76 76 60

Angle (two Theta), degrees

Fig. 1. X-ray diffraction pattern of [110] Cu/Ni superlattice showing the satellites around the 220 peak.

Table 1 Values of the superlattice parameters for Cu/Ni CMA

[loo] [ill] [llO]

Modulation wavelength, h (nm) 5.3 5.5 3.6 Ni slab thickness, dNi (nm) 3.0 3.1 2.1 Total thickness (pm) 25 25 25 Texture quality fair h&h moderate

pendicular to the plane of the deposited layers. The dependence of the magnetic moment at 1000 and 3000 Oe on the angle of the direction of the magnetic field within the plane of the sample and that of the angle between the magnetic field and the perpendicular to the sample plane were also measured. Magnetic aftereffects were measured for all the samples with the magnetic field parallel to the sample plane. These effects were observed by recording the time dependence of the magnetic moment after being aligned at 2500 Oe and abruptly lowering the field to a value of HZ ( - 200 Oe <HZ c 200 Oe). The time range for most of the measurements was 20 s. For some of them it was extended to 450 s.

The magnetic moments of the samples were measured in a SQUID magnetometer as a func- tion of temperature, with a constant applied field of 20 kOe. The SQUID was also used to obtain low-temperature, in-plane magnetization as a function of magnetic field.

Ferromagnetic resonance (FMR) spectra of the oriented layered structures were observed at room temperature at 9.26 and 34.78 GHz using a con- ventional microwave spectrometer. Rotation mea- surements were made as a function of the direc- tion of the applied field in the sample plane.

3. Results

3.1. Magnetic properties

The temperature dependences of 4nM in a constant 20 kOe applied field is shown in fig. 2. Low temperature magnetization data taken with the SQUID show that all three samples have considerable remanence for applied fields in the

U. Atzmony et al. / Ni/Cu compositionally-moduIated alloys 239

1.1

1

0.9

ii r

T

& O.9

0.7

0.0

0.5

-I m=voQ 0

Q

+ WI

0 [llll

Q [Ilo] 1

m Q p P

Q I

Q

0 50 120 150

l’EMPElWlJRE, K

200 240 280

Fig. 2. Normalii temperature dependences of 4-M at 20 kOe applied field, taken in a SQUID magnetometer.

sample plane, but require several kOe for com- tion at low temperatures, about 3/4 of the bulk plete saturation (see fig. 3). These features are Ni magnetization, but has the most rapid reduo consistent with the considerable magnetocrystal- tion of magnetization with increasing temperature. line anisotropy of Ni at low temperatures. The The [lOO] and [ill] samples have lower magnetiza- [llO] oriented sample has the largest magnetiza- tions at 300 K and below (see table 2).

4.5

4.4

4.3

4.2

4.1

4

3.6

3.5

3.7

5.5

3.5

3.4

3.3

5.2

3.1

3

2.9

2.8

2.7

0 10 20 30 40

MMNEflC !XLO. kOa

Fig. 3. Magnetization vs. applied field at 10 K, taken in a SQUID magnetometer.

240 U. Atzmony et al. / Ni/Cu compositionally-modulated alloys

Table 2 Magnetization of textured, layered structures

Temp.

WI WI Pill WI

4W (kG) 10 2.2 3.2 4.5

4nM, (kG) 300 1.6 2.4 2.1

M/M(bulk) 300 0.27 0.40 0.46

4qM” * (kG) 300 1.50 2.23 2.55

4nM L * (kG) 300 1.30 1.77 1.91

HI, We) 300 4.2 5.6 5.7

ff, (04 300 13 33 25

4 (W 86 100

* 2.9 kOe.

The hysteresis loops of the three samples for fields of 2900 and 200 Oe parallel to the sample planes are shown in figs. 4 and 5, respectively. The magnetic moments are given in emu/g(Ni) as calculated from the relative Ni slab thickness. The values of 47rM at 2900 Oe and room temperature, and of the lower limits of the coercive forces, H,, are given in table 3. The highest coercive force was obtained for the [ill] texture and the smallest for the [lOO] one. Since the slab thickness, as well as the wavelength, of the [lOO] sample were nearly

Table 3 Magnetic aftereffect of textured, layered structures

WI 11111 [W 300K 300K 300K 86 K

A max MWMW W)l) 1.04 0.56 0.77 1.60

&, (Ge) 20 40 25 120

We) 22 25 22 50

the same as those of the [ill] sample (see table l), the difference in H, appears to arise from the difference in crystalline orientation. The value of A4 was the largest for the [llO] texture and the smallest for the [loo] one. None of these values revealed any substantial dependence on the direc- tion of the magnetic field within the plane of the samples. It is therefore concluded that the texture of the samples is exhibited by preferred orienta- tion of the relevant cubic axis perpendicular to the layers whereas essentially complete isotropy is ob- tained within the planes of the layers. This means that the samples are multicrystals with one axis preferentially oriented perpendicular to the de- posited plane (and parallel to the relevant cubic axis of the substrate).

-1 1 3

WONElK FlEiD, kOa

Fig. 4. Magnetization vs. applied field for three textured Cu/Ni compositionally modulated alloys, taken in a VSM at room temperature.

U. Atzmony et al. / Ni/Cu compositional&modulated alloys 241

16

10

5

0

-5

-10

I -20 ( - I , I I I I I

-200 -100 0 100 200

h4MNEilCFlEL0, Oe

Fig. 5. Same as fig. 4, but with smaller applied fields (i.e., minor loops).

Hysteresis loops for both the parallel and per- sotropy fields (determined as the field at which pendicular configurations for the [llO] sample are the magnetization reaches its saturation), HK, were shown in fig. 6. Similar results were obtained for roughly estimated by extrapolating both magneti- the other two samples. The values of the ani- z&ion lines to their intersection points. These

Fig. 6. Magnetization vs. applied field for [110] textured Cu/Ni compositionally modulated alloy, taken in a VSM at room

temperature with the field parallel to, and perpendicular to, the plane of the foil.

242 U. Atzmony et al. / Ni/Cu compositionally-modulated alloys

values, together with the magnetic moments per g(Ni) at 2900 Oe for the perpendicular configura- tion are also given in table 2. From these values, it is concluded that the room-temperature magnetic surface anisotropy (MSA) is the greatest for the [llO] and [ill] orientations and the smallest for the [lOO]. At 5 K, Xiao and Chien found [ll] that the MSA of the [ill] was bigger than that of the

WW Ferromagnetic resonance (FMR) spectra were

observed at 9.26 GHz with the applied field either in, or perpendicular to, the sample plane, and at 34.78 GI-Iz as a function of orientation within the sample plane. The observed FMR linewidths are about 1 kOe at the lower frequency, and at least several hundred Oe greater at the higher frequency. An increase in width of at least 10 Oe/GHz is consistent with the broadening caused by mag- netic losses, but the additional width must arise form inhomogeneous broadening, especially that related to non-uniform strains. These linewidths are somewhat narrower than those seen in our earlier polycrystalline samples made by elec- trodeposition [18], but are still a factor of four greater than seen in pure Ni bulk or fihns [19,20]. Since our present samples consist of a large num- ber of layers and since the quality of the layers is seen to decrease somewhat with the number of layers, it can be expected that the observed line- widths are not the narrowest that can be produced by this method.

Rotation of the applied field direction in the sample plane results in resonance position shifts of several hundred Oe but these shifts do not have the symmetry expected for the sample planes ex- posed. This is not surprising since Laue X-ray diffraction spots could not be seen in these sam- ples either. The observed shifts may be a result of slight texturing or non-isotropic stress distribu- tions, e.g. as might be caused by bending of the sample.

Comparison of the FMR data with our VSM derived moments indicates that the magnetic ani- sotropy, other than that related to sample shape, is relatively small in these samples. In our earlier work [18] on less textured electrodeposited Cu/Ni CMA, we also found that the anisotropy was small in this thickness range, apparently because

of cancellation of bulk and surface terms. (It was observed [18] that the anisotropy changed sign from in-plane induced anisotropy in the thinnest films to the out-of-plane direction for thicker films.) The bulk term of several kOe is apparently caused by planar tensile strains of several percent induced in the Ni by the Cu layers, and favors the out-of-plane direction owing to the negative sign of the magnetostriction [19] in Ni, while the surface terms favor an in-plane magnetization. It is inter- esting to note that Xiao and Chien [ll] have, in contrast, a positive bulk term in their [lOO] sample at 5 K. Whether this difference is due to the temperature of observation or to less tensile strains in the vapor-deposited samples resulting from less coherent interfaces is not clear.

3.2. Time-dependent magnetic properties

The room-temperature magnetic aftereffect for the [ill] sample as a function of the natural logarithm of the time [In t] up to 200 s with a step field (H,) of - 35 Oe is shown, as an example, in fig. 7. The magnetic aftereffect is the tendency of the magnetization to change with time towards the initial magnetization curve. Thus the absolute value of the magnetization decreases for positive values of Hz and increases for negative values. Similar phenomena have been reported for thin iron films [21], for fine particles [22], and for spin glasses [23]. To the best of our knowledge, this is the first time that a magnetic aftereffect has been found in Ni thin films.

The slope, dM/d(ln t), is continually changing, as is evident in fig. 7. The initial part of the decay is attributed to the time needed for the external field to decrease and for any eddy currents to die out. A measurement of a bulk foil of Ni showed no time dependence beyond 7 s, hence the afteref- fect data up to this time were disregarded. The average slope of the lines between 7 and 20 s was calculated by linear least square fitting. These values depend on Hz and are different for the different samples. The values of A [ = ( - d&f/ d(ln t))] for the three samples as a function of H, are shown in fig. 8. The three sets of data exhibit maxima at values of H, which are nearly the same as the (negative) values of the coercive forces. For

U. Atzmony et al. / Ni/Cu compositionally-modulated alloys 243

12

11

10

a

8

7

6

5

4

3

2

1

0

-1

-2

0 2

LN(tlmr). we

Fig. 7. Magnetization vs. natural logarithm of the time after reducing the applied field from 2500 Oe to - 35 Oe (i.e., a small field in the opposite direction) for a [ill] textured Cu/Ni compositionally modulated alloy, taken in a VSM at room temperature.

one of these, fig. 9 compares the room temper- ature with a low temperature measurement. The change is consistent with the increased magnetiza-

tion, coercive force and anisotropy at the low temperature (table 3).

The solid lines in figs. 8 and 9 were fitted to

0.S

0.6

0.4

0.3

0.2

0.;

0

-100 -80 -00 -40 -20 0 20 40

He. 00

Fig. 8. The time dependence coefficient, A = ( - dM/d(ln t)), vs. H, for three textured Cu/Ni compositionally modulated alloys, taken in a VSM at room temperature. H2 is the applied field after the field has been stepped down from 2500 Oe. M is the

magnetization from curves such as shown in fig. 7, taken between 7 and 20 s.

244 (I. Atzmony et al. / Ni/Cu compositional&modulated alloys

-260 -220 -190 -140 -100 -60 -20 20 60

H2. Oa

Fig. 9. The time dependence coefficient, A = ( - dM/d(ln t)), vs. Hz for a [llO] textured Cu/Ni compositionally modulated alloy, taken in a VSM at room temperature and at 86 K. Hz is the applied field after the field has been stepped down from 2500 Oe. M is

the magnetization from curves such as shown in fig. 7, taken between 7 and 20 s.

where H,, is the value of H, for which A is a maximum. As noted above, H,, tends to have the value of the measured coercive force H,. (Note that H, is opposite in direction to the aligning field.) The best fit values of the parameters are given in table 3.

In figs. 8 and 9, there is an apparent deviation between the data and the fit in the shoulders. To fit broader shoulders, a Lorentzian,

A =A,, r2

( H2 - H,,)' + r2 ’ (2)

was attempted. Although the shoulders are better fitted, the center fit is not as good as with eq. (1). It probably would be better to use a Gaussian distribution of Lorentzian lines [24], called a Voight profile [25], to fit the data.

A more complicated form of time dependence,

has been used for cobalt particles [22], where H, is the coercive force, HK is an effective value for the anisotropy field and u is the standard deviation of the reduced diameter of the particles. Eq. (3) appears to have been derived in an empirical manner, and the parameter, u, was not obtained from the actual distribution of particle sizes and it has no obvious meaning when applied to the thin films of this work. Nevertheless our results can be successfully fitted to this equation. The values of HK and u are strongly correlated (i.e. for every possible value of HK there is a value of u that yields nearly the same curve).

The differences between the fitted curves ob- tained according to eqs. (1) and (3) are negligible, and thus the experimental data cannot prefer one of them. (The data in ref. [13] could also be fitted, to the same degree of accuracy, to eq. (l).)

The magnetic aftereffect phenomena was also measured for one of the previously reported [25] polycrystalline layered samples and was found to be smaller and broader, as would be expected if the effect is texture dependent.

U. Aizmony et al. / Ni/Cu compositionally-modulated alloys 245

4. Discussion

The measurements reported in this paper are the first of magnetic properties of textured Cu/Ni compositionally modulated alloys prepared by electrodeposition. It is also the first report of magnetic properties of [llO] textured Cu/Ni CMA prepared by any technique.

The unexpected discovery of a magnetic aftereffect leads to a number of questions that will have to answered through further research: i) What is the mechanism? ii) Is the time dependence an intrinsic property of the texture? iii) Does the magnetic aftereffect depend on the method of preparation of the CMA?

As to the mechanism, the equation (eq. (3)) previously applied to superparamagnetic particles [22] fits the data quite well for each temperature, but has the wrong temperature dependence for this to suggest superparamagnetic behavior. In superparamagnetism, the effect of lowering the temperature is to “freeze-out” the relaxation. In contrast, in the Ni/Cu CMA, the relaxation is enhanced at the lower temperature as is evident in fig. 9. It is likely that the phenomenological nature of eqs. (l)-(3) permit them to be applied to differ- ing time-dependent phenomena. The meaning of the parameter u in eq. (3) for the CMA is not clear.

It is interesting to speculate that the time decay is due to spin glass behavior of the CMA, but what mechanism would permit Ni to be a high temperature spin glass is not evident. Further- more, the form of the time decay is not identical to the stretched exponential suggested [23] for a spin-glass system. Further tests are in progress to determine if Ni/Cu CMA has spin glass proper- ties.

Although the superlattice behavior of composi- tionally-modulated alloys is of special interest, CMA can also be viewed simply as repeated films (and interfaces) and as such yield larger signals than would be possible with a single film/inter- face. Then the magnetic aftereffect would be the same in a single Ni film between Cu plates. This can be tested by increasing the Cu layer thickness in the CMA. Though there is no previous report of a magnetic aftereffect in Ni, a time-dependent

effect has been observed [21] in thin iron films evaporated onto glass substrates. This effect, which was attributed to domain wall stabilization, in- volved discontinuous changes in magnetization on increasing field. No observations of changes oc- curring after reduction of fields was reported.

Theoretical studies [26-291 have suggested that there is a significant difference in the magnetiza- tion associated with a Ni interface layer in differ- ent crystal orientations. Since the interface struc- ture (interdiffusion, roughness, etc.) can also change with texture, further studies will be needed to sort out these effects.

References

Ill

PI

131

t41

151

WI

[71

181

PI

WY

WI WI

1131

iI41 1151

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