Magnetization and magneto resistance in Fe-ion-implanted Cu and Ag thin films

EL.SEVIER

Journal

Magnetization

of Magnetism

and Magnetic

Materials

173 (1997) 230-240

and magnetoresistance in Fe-ion-implanted Cu and Ag thin films

M.M. Pereira de Azevedo”, J.B. Sousaa**, J.A. Mendes”, B.G. Almeida”, M.S. Rogalski”, Yu.G. Pogorelov”, I. Bibicub, L.M. Redondo”, M.F. da Silva”, C.M. Jesusd, J.G. Marquesd, J.C. Soaresd aIFIMUP. Physics Department. Faculty q/Sciences. University ofPorte.Rua do Camp0 Alegre, 687. 4150 Porte. Portugal b Institute of Atomic Ph.vsics. P.O. Box MG-06. Bucharest. Romania ‘Institute of Nuclear Technology, P-2680 Sacartk Portugal d CFNUL. Unitwsiy of Lisbon, 1699 Lisbon. Portugal Received

14 April 1997; received

in revised form 16 June 1997

Abstract Diluted granular films of CupFe and Ag-Fe (iron content < 2%) were produced using 57Fe ion implantation on Cu(Ag) films previously grown by laser ablation. Conversion electron MGssbauer spectroscopy shows that the implanted Fe forms either very small clusters (up to a few atoms) or large iron a-phase particles. These structural characteristics directly reflect on the magnetization, which exhibits ferromagnetic-like behaviour at room temperature (due to large clusters) superimposed by a significant paramagnetic contribution at low temperatures due to the small clusters. We observe deviations from strict superparamagnetic behaviour due to non-negligible local anisotropy effects at low temperatures and low fields. The Kondo effect is particularly enhanced in the Cu-Fe films which have higher concentration of isolated Fe atoms and small sizes clusters. The magnetoresistivity Ap/p of our films is dominated (for 0 d &f < 15 T) by a linear term in H, attributed to GMR-like effect from spin-dependent scattering when an electron passes between adjacent large and small clusters. At low fields we observe instead Ap/p x Hz, due to the usual GMR effect between large clusters, during the alignment of their easy axes. The relevant physical differences (structural, magnetic and magnetoresistive) observed in our ion-implanted diluted Fe films, with respect to the concentrated granular films, are critically analysed.

pACS:

39.10. + j; 76.80. + y; 75.50. + Bb; 72.90. + Y

Keywords:

Magnetoresistance;

*Corresponding

author.

Implantation;

Massbauer

spectroscopy;

FeCu; FeAg; Granular

alloy

Tel.: + 351 2 608 2656: fax: + 351 2 608 2679; e-mail: [email protected]

0304-8853/97/$17.00 ICI 1997 Elsevier Science B.V. All rights reserved PII SO304-8853(97)00207-2

M.M. Pereira de Azevedo et al. / Journal of Magnetism and Magnetic Materials 173 (1997) 230 240

233

Table 1 M6ssbauer parameters of the implanted Fe phases Sample FeCu FeAg

IS (mm/s) 0.25 - 0.13 0.5 0.36 0.08

QS (mm/s)

Relative areas (%)

Fe-phases

-

85 15 46 37 17%

Isolated Fe atoms Large Fe clusters Isolated Fe atoms Small Fe clusters Large Fe clusters

1.1 -

I

3.0x10 ~

2.0x10"

l'0xl0 ~ &

I

I

I

I

I

I

1

FeCu

-----o---T--4.5 K --a--- T=10 K T=77 K T=300 K

2-"

.e 0.0

t~ ID

eta0 C~

3.oxm~ FeAg

-1.0xl0" 2 0xl0~" ~

O

~

K

_2.0x10-4 I

o0

-3.0x10 ~ -5

0xl0~

I

I

I

I

-4

-3

-2

-1

0

0

1

,

i

t

,

2

i

,

i

2 3 IIoH(T)

3

,

i

4

4

,

,

5

5

t~0H(T) Fig. 2. Magnetization curves at various temperatures for the Fe/Cu film. Inset: Magnetization curves at 4.5 and 77 K for the FeAg film.

3. Magnetization behaviour 3.1. Experimental results and preliminary analysis Fig. 2 shows typical magnetization curves recorded at various temperatures on the F e / C u granular film, in the as-deposited state. The most striking feature is the rapid field saturation of the magnetization at temperatures above 77 K, while

at low temperatures we observe an extra magnetization contribution, displaying a m u c h slower increase with applied magnetic field. The behaviour of our magnetization curves is consistent with a superposition of a contribution from relatively large ferromagnetic Fe clusters, which is easy to saturate and does not vary significantly with temperature, and a contribution from very small-size Fe clusters, which requires large saturation fields

234

M,M. Pereira de Azevedo et al. /Journal ol'Magnetism and Magnetic. Materials 173 (1997) 230-240

and increases rapidly at low temperature. The general shape of our magnetization curves has similarities with the M(H) curves obtained by Dieny et al. [-8] in silver-permalloy (Py) sputtered granular films. The magnetic hysteresis in our Fe/Cu granular film is very small at all measured temperatures. The coercive field is also relatively low, e.g. of the order of 100 Oe for T = 4.5 K. The low hysteresis in this film is compatible with a small magnetocrystalline anisotropy in the large clusters, and with the absence of magnetic domains within them. The smaller clusters behave independently with negligible magnetic anisotropy energy. At room temperature the magnetization saturates at a field It0H ~ 0.3 T, into a virtually constant value throughout our experimental field range up to 5 T. At such temperature only the large Fe-particles with ferromagnetic order inside and large magnetic moments are expected to contribute to the measured magnetization [8]. The virtual absence of a finite slope in the M(H) curve at high fields indicates that most of the remaining Fe-clusters have much smaller sizes and magnetic moments, displaying full paramagnetism at 300 K. This behaviour is consistent with the absence of magnetic splitting in the M6ssbauer spectra obtained at room temperature (Fig. 1). At room temperature the intrinsic cluster magnetic anisotropy energies are much smaller than kBT and the inter-cluster magnetic interactions are even weaker in our diluted (Fe) films. We can then make a better estimate of the average particle size of the large clusters, assuming that they are independent and free to rotate so as to give a magnetization contribution (M~) described by a Langevin function

l~.l~HT), MI(H, T) = N l l q L (tcB / (largeclusters), where L(x) = coth(x) - 1/x is the Langevin function and N1 is the number of clusters, with an average magnetic moment /~. We obtain /~1 12 600 ~tB which corresponds to about 5700 atoms per cluster with N~ of the order of 1012. This figure for the average cluster magnetic moment is of the same order of magnitude as that obtained from

similar fits made at 300 K in Co40Ag6o granular films prepared by sputtering (/~ ~ 13 000 ~tB) [9]. Assuming that such large clusters are internally saturated (I~1 independent of T), we can obtain the magnetization curves M2(H, T) corresponding to the contribution of the remaining (small) Fe clusters, by subtracting the above Langevin fit MI(H, T) for large clusters from the experimental magnetization at each temperature (Fig. 3):

Mz(H, T) = M(H, T) - M1(H, T) (small clusters). By fitting the magnetization curve Mz(H ) at T = 4 . 5 K with a Langevin function (inset of Fig. 3), we obtain an average magnetic moment of 10 ~t~, which corresponds to about five Fe atoms. Alternatively, if we use the experimental magnetization curve at 300 K to represent M1(H, T) down to low temperatures, we obtain an average magnetic moment of 9 ~tB at 4.5 K, corresponding to about 4.5 Fe atoms (see Fig. 3). Therefore, the large increase of the magnetization at low temperatures is essentially due to the contribution of small superparamagnetic clusters. A similar conclusion has been previously reached for the Co4oAg6o granular sputtered films E9], for which the low-temperature magnetization behaviour was attributed to clusters with about five cobalt atoms. In view of the smallness of such clusters, the use of a Langevin function is not exact (quantum effects are not included), and the above estimate only gives the right order of magnitude. SQUID magnetization measurements were also performed in the FeAg film, as shown in the inset of Fig. 2. A similar analysis of the magnetization data was performed for this film, indicating the dominance of large-size cluster effects above 77 K, with an average cluster magnetic moment lq ~ 13 500 la~, corresponding to about 6140 Fe atoms per cluster. For the FeAg film, the average magnetic moment of the small-size clusters, as estimated from M2(H) at 4.5 K is t~z ~ 11 ~t~ which corresponds to 5 Fe atoms.

3.2. Non-universal behaviour For superparamagnetic clusters with an arbitrary size distribution one expects a magnetization given by a superposition of the contributions from

M.M. Pereira de Azevedo et al. /Journal of Magnetism and Magnetic" Materials 173 (1997) 230--240 I

I

I

235

I

[

2.0x10~

& o 3"OxlO1

¢~

1.0xl0"

[]

p-0x,0~t

X

ff

0.0 i

,

I

I

,

I

2

3

, ~t'lH(Teslal 4

0.0 0

1

3

4

5

g0H(Tesla) Fig. 3. Langevin fit on the magnetization M2(H) at T = 4.5 K for the FeCu films. Inset: Magnetization curve M(H) at 4.5 K and the Langevin fit on the magnetization MI(H) at 300 K.

clusters with different sizes:

M(H, T) = fI~Fu(~BT)f(IJ) dtL, where f(/~) is a size-distribution function and F,(I~H/kBT) is the appropriate magnetization contribution of the different size clusters, e.g. a classical (Langevin) or quantum (Brillouin-like)function in the limits of large or small clusters, respectively. Under the above assumptions, M(H, T) is expected to be a universal function of HIT [10]. Such a behaviour, with all the M(H, T) experimental points falling into a common curve (when represented in terms of H/T), was reported in the data obtained in sputtered CoAg granular films [9]. In our ion-implanted FeCu granular films we do not observe such universal H/T dependence, as shown in Fig. 4. A discrepancy certainly arises if

the cluster magnetic moments are not strictly constant, a possibility which was not taken into account in the previous data analysis. Indeed, Fig. 2 shows that the 'saturation' magnetization arising from large-size clusters increases noticeably as we go from 300 to 77 K, implying p = I~(T)- This could be simply due to the enhancement of the cluster spontaneous magnetization caused by the reduction in spin thermal fluctuations. Other causes for discrepancies could arise from local magnetic anisotropic effects or dipolar interactions between neighbouring Fe clusters. However, it can be readily seen that for the small atomic iron concentration in our films ( < 2 % at the peak of the profile concentration), for which oTnly ~ 15% are large clusters, the characteristic random dipolar fields felt by each cluster from its neighbours are very small, within 3-10% of the anisotropy field H ~ 100 Oe. Hence, the interaction effects can be safely neglected in this

M.M. Pereira de Azevedo et al. /Journal of Magnetism and Magnetic Materials 173 (1997) 230 240

236

3.0x10

2.0x10-

/x

A

og

O A e-,

zx

~000 0

,0

o°

1.OxlO"

o A o []

O Oo

o

0.0

0.0

0.1

0.2

T:4K T--10K T=77K T=300K

0.3

0.4

0.5

po1-Iff (T/K)

Fig. 4. The non-universalbehaviour of the magnetizationwith applied field/temperaturefor the FeCu film at various temperatures. case, and only the anisotropy effects are considered below.

This difference in behaviour could be due to different structural/magnetic characteristics of the magnetic clusters, when produced by ion-implantation or by sputtering. Within the simplest model, we assume the existence of a cluster-shape magnetic anisotropy, with randomly oriented easy-axes for the different clusters, in zero external magnetic field, The anisotropy energy for each cluster has two equivalent minima related to the two opposite orientations of its magnetic moment along the easy -axis, separated by an energy barrier A. But these minima are no longer equivalent when a (small) external field H is applied, and the minimum corresponding to the magnetic moment opposite to H becomes metastable. Provided k~T is low compared to the energy barrier A between the two minima, the magnetization M(H) is mainly governed by the interplay between H and the local cluster magnetic anisotropy, described by a characteristic anisotropy field Ha. Such temperature-independent behaviour corresponds to the common magnetization curve as seen in Fig. 2. Since the barrier A diminishes with growing H, the departure from the common curve should occur for a particular value of the applied field H = H*, for which A(H*) ~ kBT. This explains qualitatively the increase of H* with decreasing temperature observed in our data [11].

3.3. Anisotropy effects at low fields 4. Transport properties

As can be seen in the M(H, T) data shown in Fig. 2, at any temperature (4.5, 10, 77 and 300 K) the magnetization experimental points fall into a common curve at low fields, within the experimental error, and such common behaviour persists up to progressively higher fields (H*) as the temperature decreases. For example, taking the M(H)curve at 4.5 K for reference, we have poll* = 0.1 T for the curve obtained at T = 300 K, 0.4 T at 77 K, and 0.7 T at 10 K. The existence of a common lowfield behaviour in M(H, T), irrespective of the temperature and of the dominant clusters at each temperature (within the region of magnetic fields used in our work), is a striking feature in our data. This effect was not observed in the M(H, T) data reported for granular concentrated CoAg sputtered films [9].

4.1. Electrical resistivity Fig. 5 shows the temperature dependence of the ideal electrical resistivity pi(T) = p(T) - Po for the FeCu and FeAg films, where the residual resistivity, Po, has values of 7.4 and 1.9 gf~ cm, respectively. The considerably higher Po value in the copper film indicates much larger impurity defect scattering in this ~ 1000 ,A thick film, most likely due to pointlike defects, which are very effective electron scatterers. For high temperatures T > 100 K, the resistivity exhibits an approximately linear increase with temperature, as expected for dominant electron phonon scattering. For temperatures below 100 K, p(T) deviates from this behaviour and goes through

M.M Pereira de Azevedo et al. / Journal of Magnetism and Magnetic Materials 173 (1997) 230 240

237

115

O O

0.4 ?

0

=k

20

&

'

30 InT'(K) 4)

'

.

Q_

~

FeCu

,

0

I

50

,

I

100

,

I

,

150

I

,

200

I

250

,

300

T(K) Fig. 5. The temperature dependence of the ideal electrical resistivity for the FeCu and FeAg film.

a shallow minimum at T* ~ 44 K in the FeCu film and through a much weaker minimum at T * ~ 15 K for the FeAg film. This anomaly at low temperatures is attributed to K o n d o electron scattering by isolated magnetic Fe atoms in the Cu(Ag) matrix, again indicating a higher concentration of isolated Fe atoms in the FeCu film. In fact, it is known that the K o n d o effect in dilute magnetic alloys produces an anomalous increase in p with decreasing temperature, according to the approximate dependence p ~ A - B In(T} [12]. It is interplay between this dependence and the usual increase of p due to electron-phonon scattering which produces a minimum in p near the Kondo temperature TK. Further insight into the low-temperature resistivity behaviour in the FeCu film (where the anomalous minimum is much larger) can be obtained if we subtract the phonon resistivity contribution Pph(T) from the experimental data. This has been

done taking a Debye temperature O = 315 K for the Cu matrix and using the Bloch-Griineisen formula to fit the experimental pi(T) data on its linear part at high temperatures. The inset of Fig. 5 displays the remaining resistivity, pi(T)- pph(T), as a function of ln(T) near the resistivity minimum Pro, indeed evidencing an approximately linear relation of p with ln(T).

4.2. Magnetoresistance A detailed study of the magnetoresistance from 4.5 up to 300 K has been made on the FeCu film, using magnetic fields up to 16 T. The magnetic field was applied in the plane of the film and perpendicular to the current. The magnetoresistance is here defined as Ap

p(H) - p ( H = O)

p

p ( H = O)

238

M.M. Pereira de Azevedo et al. / Journal of Magnetism and Magnetic Materials 173 (1997) 230-240

0.0 ~ "

0.ff

(b)

-0.2 ~- -0.4 -0.6

-0.1 ~

4.5K

~~

"oIt(T)

-0.2 •

-0.3

0

1

2

3

°o

"#'% 77K 4

5

].t0H(T) Fig. 6. (a) Magnetoresistance for the FeCu film taken at 100 K for magnetic fields up to 13 T. (b) Magnetoresistance at 4.5, 77 and 100 K for the FeCu film.

and was found to be always negative and independent of the field/current geometry, within the experimental resolution. Fig. 6a shows that for T ~> 100 K the magnetoresistance amplitude increases linearly with the applied magnetic field. This does not fit the usual GMR [1] and superparamagnetic behaviour [-13], for which Ap/p H z. An anomalous linear field behaviour of Ap/p was also recently reported for massive granular C usvCo13 samples [14] at sufficiently high temperatures (> 150 K). The linear increase of Ap/p with H in our ion-implanted FeCu films at 100 K occurs when the measured magnetization M(H) is nearly constant (above 1 T). Since the almost-saturated part of this magnetization is due to the contribution of large clusters, the rapid (linear) increase of Ap/p with H suggests that electron scattering by small clusters plays an important role here. A mechanism leading to linear field-dependent Ap/p was proposed by Hickey et al. [14], based on

the coexistence of large-size (blocked; ~tBiHa>

kBT) and small-size (superparamagnetic, SPM; ,uspHa "~ kBT) clusters at relevant ranges of temperatures. The probability of spin-dependent scattering, for an electron passing from a blocked to a SPM cluster (or vice versa), will be proportional to the thermal average of the SPM moment under the applied field, i.e. to the paramagnetic part (Mp) of the magnetization. As the latter is linear in H up to the highest measured magnetic fields, we obtain Ap/p oc Mp oc H. On the other hand, the contribution from spin-dependent scattering when an electron crosses two small clusters is proportional to (#H/kBT)2 and is negligible due to the smallness of kt for such clusters. However, for decreasing temperature, deviations from this linear behaviour are expected at low fields. Indeed, an approximate quadratic dependence of Ap/p with H is then observed at low fields, as shown in Fig. 6b, for the curves obtained at 77

_39

M.M. Pereira de Azevedo et al. /'Journal of Magnetism and Ma¢netic Materials 173 (1997) 230 240

and 100 K. We associate this effect with a particular set of large Fe-clusters, initially (when H = 0) blocked by local random anisotropy fields at low temperatures but sufficiently close to each other (well below the electron mean free path), so as to induce the G M R effect [1], associated with spindependent scattering when an electron moves from one large cluster to its neighbour. This effect readily saturates at fields H ~- 1 T, when the easy axes of the large clusters become completely aligned with the external field. We notice that the magnitude of the H 2 term increases as T is reduced, which is compatible with the fact that more clusters get blocked and also with the expected intrinsic increase of the G M R effect at decreasing temperature [I]. Slightly larger low-field/low-temperature G M R effects are observed in the FeAg films, as shown in Fig. 7, for measurements taken at 4.5 K. These resuits are consistent with the previous evidence from M6ssbauer spectra and magnetization data, which show presence of slightly larger clusters in the FeAg than in the FeCu granular films. Since both films have been prepared with the same ion-beam dose and impact energies, such differences on the cluster sizes are attributed to the intrinsic (physical/structural) differences between Cu and Ag matrices, in particular the degree of Fe immiscibility is greater in Ag than in Cu. An important point for the analysis of the transport properties in our Fe-implanted films is the non-uniformity of the Fe profile. Since the resistivity is considerably higher in the region of higher Fe-doping, and it is just this layer that mainly contributes to the magnetoresistance; the shunting effects due to the remaining regions cause the measured amplitude ( A p / p ) .... to be underestimated. compared to its intrinsic value ( A p / p ) i , t for an homogeneous 2% Fe-doped material. Although the shunting coefficient k {AlJ/[o)im/(Ap/PJme s cannot be estimated exactly from the available data, we expect k to be higher than the ratio between the doped-layer thickness to the film thickness, which is, respectively, - 2 . 7 for Cu and ~ 3.8 for the Ag film. Further work is in progress on FeAg ion-implanted films specially prepared, aiming to obtain more homogenous films and using higher Fe-doses. Preliminary results already obtained in these films =

0.00

-0.05

(2.

FeCu

-0.10

FeAg

-0.15

-0.20 0.0

h 0.3

,

i 0.6

, 0.9

~()H (T) Fig. 7. Magnetoresistancefor the FeCu and FeAglilln taken at 4.5 K for magnetic fields up to 1 T.

show much larger G M R effects than reported here for the low-dose irradiated films. A full report on the high-dose FeAg films will be reported in due course.

5. Conclusions The ion-implantation technique was used, for the first time to our knowledge, to prepare granular magnetic films of FeAg and FeCu. The combined study of the structural, magnetic and electron transport characteristics of these materials shows a number of similarities with reported results on concentrated granular films (CoAg and PyAg) prepared by sputtering. In particular, the cluster statistics seems to be dominated by two different particle sizes: the majority species corresponds to clusters with about 4-5 atoms, whereas the minority species corresponds to large clusters about 103 times bigger. The referred structural characteristics are

240

M.M Pereira de Azevedo et al. / Journal of Magnetism and Magnetic Materials 173 (1997) 230 240

clearly reflected on the magnetization, which exhibits ferromagnetic-like behaviour at room temperature (due to large clusters), added by a significant superparamagnetic contribution at low temperatures due to small clusters. Our diluted Fe-films also display relevant physical differences from the behaviour observed in concentrated granular films. First of all, the Kondo effect is clearly seen in our FeCu films, which have an higher content of isolated atoms and small-size clusters. Second, we observe deviations from strict superparamagnetic behaviour (which presumes a universal dependence on H/T), caused by incomplete cluster magnetic saturation at higher temperatures and by non-negligible local anisotropy effects at low temperatures and low applied fields. Large clusters have much less participation in transport processes than in the magnetization. Accordingly, the magnetoresistivity of our films is mainly governed by spin-disorder scattering by small magnetic moments which are far from saturation down to helium temperatures and up to the maximum experimental field of ~ 15 T. Incipient GMR-like behaviour, associated with electron scattering by sufficiently close large clusters, is only observed at low fields (~<1 T) and low temperatures.

Acknowledgements This work was supported by the project PRAXIS/3/3.1/FIS/21/94. Some of the authors

thank PRAXIS XXI for their grants, Grant BD/5346/94, Grant BCC/4803/95 and Grant BCC/6428/95.

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