Exchange bias and magnetothermal properties in [email protected] nanocomposites

Journal of Magnetism and Magnetic Materials 324 (2012) 3503–3507 Contents lists available at SciVerse ScienceDirect Journal of Magnetism and Magneti...

396KB Sizes 0 Downloads 13 Views

Journal of Magnetism and Magnetic Materials 324 (2012) 3503–3507

Contents lists available at SciVerse ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Exchange bias and magnetothermal properties in [email protected] nanocomposites S. Laureti a, D. Peddis a,n, L. Del Bianco a,b, A.M. Testa a, G. Varvaro a, E. Agostinelli a, C. Binns c, S. Baker c, M. Qureshi c,d, D. Fiorani a a

ISM-CNR, Area della Ricerca, Via salaria Km 29,500, P.B. 10-00016 Monterotondo Scalo, Roma, Italy Dipartimento di Fisica, Universita’ di Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy c Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK d Department of Physics, Hazara University, Mansehra, Pakistan b

a r t i c l e i n f o

abstract

Available online 27 February 2012

We have studied the Exchange Bias (EB) effect in nanocomposite films consisting of Fe nanoparticles (mean size  1.9 nm) embedded in an antiferromagnetic Mn matrix. They were produced by codeposition through a gas aggregation cluster source and molecular beam epitaxy and have different Fe volume filling fractions (2.2% and 24.8%). The exchange field, higher in the sample with higher Fe concentration (at T ¼5 K, Hex  460 Oe for 24.8% and  310 Oe for 2.2% ), in both the samples decreases with increasing T, finally disappearing at T  40 K. The EB properties have been studied in conjunction with results on the thermal dependence of the magnetic coercivity, zero-field-cooled and field-cooled magnetization and thermoremanence. The different Fe content strongly affects the magnetothermal properties, featuring superparamagnetic relaxation in the diluted sample and a reentrant ferromagnettype transition in the concentrated one. Hence, the EB properties of the two samples have been discussed in consideration of such peculiarities of the magnetic behavior and highlighting the role of the Mn matrix. & 2012 Elsevier B.V. All rights reserved.

Keywords: Nanoparticle Exchange bias [email protected] nanocomposite AFM/FM interface

1. Introduction Nanostructured magnetic materials characterized by the coexistence of different exchange-coupled magnetic phases are among the most interesting systems engineered during the last decades for both applications and fundamental studies [1,2]. In particular, when a ferromagnetic phase (FM) is in close contact with an antiferromagnetic phase (AFM), the Exchange Bias (EB) phenomenon may be observed, depending on both intrinsic and extrinsic properties of the two materials as well as on the interface features. Exchange Bias refers to the shift of the hysteresis loop observed when the sample is cooled in a magnetic field through the Ne´el temperature of the AFM material [3]. The shift is taken as a measure of the exchange field, Hex. The effect was explained as due to an induced unidirectional anisotropy (exchange anisotropy) arising from the exchange interaction at the FM/AFM interface. In nanocomposite systems, consisting of FM nanoparticles dispersed in an AFM matrix, the exchange anisotropy contribution increases the energy barriers for magnetization reversal [1,2]. This has been indicated as a promising route to enhance the magnetization stability against demagnetizing effects (reversing field or temperature) in nanopatterned magnetic devices,

n

Corresponding author. Tel.: þ39 0 706754373. E-mail address: [email protected] (D. Peddis).

0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.02.076

including recording media. For instance, it was found out that Co nanoparticles, with mean size 1.6 nm, embedded in an AFM Mn matrix were definitely more stable against thermal fluctuations than in a diamagnetic Ag matrix [4,5]. Therefore, there is a growing interest in the study of FM/AFM nanocomposite, also in order to elucidate how the EB properties are affected by the disorder at the clusters boundary, the chemical and structural properties of the matrix, the packing density [6–8]. In this paper, we have studied the EB effect and the magnetothermal behavior, between 5 and 300 K, in composite samples of Fe nanoparticles embedded in a Mn matrix. In particular, two samples have been investigated, differing for the Fe concentration (volume filling fraction VFF ¼2.2% and 24.8%). We will show that, with varying temperature, the different Fe contents determine the onset of peculiar magnetic regimes in the two samples and we will discuss how and to what extent the EB mechanism is affected by such different magnetothermal evolution.

2. Experimental [email protected] nanocomposites were prepared in the form of film (  200 nm thickness) by co-deposition of the Fe nanoparticles and Mn matrix using a gas aggregation cluster source and a molecular beam epitaxy (MBE) source [9]. A buffer and a capping layer of Ag were deposited (from an MBE source) in order to protect the films

3504

S. Laureti et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3503–3507

800

0.30

600 400

0.20 M (emu/cm3)

Ion current (nA)

0.25

0.15 0.10

ZFC FC

200 0

200

-200

0.05

0

-400

0.00

-200

-600

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

(nm)

3. Results and discussion Previous studies on systems produced by the same co-deposition technique showed that if the magnetic nanoparticles are dispersed in an immiscible matrix ([email protected]), they retain the same size distribution as before the deposition whereas, in the case of a miscible matrix, some alloying occurs ([email protected]) [5]. As for our samples, some information in this respect are inferred by the values of the saturation magnetization MS, obtained by analyzing the ZFC hysteresis loops at T¼5 K (Fig. 2 full symbols), in which the value of the magnetic moment measured by SQUID has been normalized to the total Fe volume: by extrapolating to H¼0 the high-field linear part of the curves, so as to subtract the nonsaturating contribution from the AFM phase, MS values of about 600 emu/cm3 and 1000 emu/cm3 are obtained for [email protected]_2.2 and [email protected]_24.8, respectively. Hence, [email protected]_2.2 has lower MS and,

-2

-1

0

1

2

3

-3

-2

-1

0

1

2

3

1500

Fig. 1. Particle diameter distribution as measured in situ by an axially mounted quadrupole filter.

1000 ZFC M (emu/cm3)

against oxidation after removal from the deposition chamber. Deposition rates of the Fe clusters (particles) and Mn matrix material at the substrate were measured by means of a quartz crystal thickness monitor to control the VFF of particles within the film that is the rate between the equivalent thickness of the deposited particles and the thickness of the film. By choosing the deposition rates appropriately two samples with VVF ¼2.2% and 24.8% have been prepared, labeled [email protected]_2.2 and [email protected]_24.8 respectively. The cluster source produces a log-normal distribution of cluster sizes, as measured in situ by an axially mounted quadrupole filter (Fig. 1). The log-normal fit indicates mean particle diameter of (1.970.1) nm. Magnetic measurements were carried out by a SQUID magnetometer. The magnetization vs. temperature T was measured according to zero field cooled (ZFC) and field cooled (FC) protocols: the sample was cooled from 300 down to 5 K in a zero magnetic field; then a static magnetic field Happl was applied and the magnetization MZFC was measured on warming up to 300 K; finally the sample was cooled down to 5 K under the same Happl and the magnetization MFC was measured in the meanwhile. The thermoremanent magnetization TRM was measured by cooling the samples from 300 to 5 K in an applied field, then the field was turned off and the remanent magnetization was measured on warming up. The EB properties have been studied by measuring FC hysteresis loops at different T between 5 and 60 K after cooling down the samples from 300 K, under an external field of 10 kOe.

-3

-800

500

FC

0 1000

-500

500

-1000

-500

0

-1000

-1500 -20

-10

0 H (kOe)

10

20

Fig. 2. Hysteresis loop measured in ZFC (full symbols) and in FC (empty symbols) for samples: (a) [email protected]_2.2 and (b) [email protected]_24.8. The insets are enlarged views of the central regions of the loops.

for both samples, the estimated MS values are well below that of bulk Fe (1714 emu/cm3) [10]. This magnetization reduction suggests that the alloying of Fe and Mn has occurred in both samples and to a larger extent in the diluted one. Structural investigations on the [email protected] system prepared by the same technique [11] reveal that, in samples with low Fe content, the fcc Cu matrix induces the formation of the fcc Fe phase and bcc Fe appears only for VFFZ24.8%. This switch from fcc to bcc Fe has been considered to mark the Fe percolation threshold [11] and, actually, this is the reason why we have produced a sample with this same amount of Fe also for the present study. Hence, a general tendency may be observed according to which a configuration of high structural coherence is favored in diluted samples, leading to an enhanced degree of alloying in the [email protected] system and to the formation of fcc Fe in immiscible [email protected] On the contrary, at higher Fe content, starting from VFF¼24.8%, the stabilization of the bcc Fe phase is promoted both in [email protected] and in [email protected], as the higher magnetization value of [email protected]_24.8 suggests. Further structural investigations are in progress on the [email protected] samples to confirm such description. In Fig. 2, the ZFC and FC hysteresis loops measured at T¼5 K on [email protected]_2.2 and [email protected]_24.8 are shown. The FC loops appear horizontally shifted to the left, revealing the presence of EB effect. The shift is accompanied by a coercivity enhancement, as generally observed in nanogranular systems [7,12]. The exchange field parameter is defined as Hex ¼  (HC þ þHC )/2

S. Laureti et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3503–3507

[MFC-MZFC] (emu/cm3)

25

MZFC-FC (emu/cm3)

50

40

20 15 10 5 0

30

0

100

150 T (K)

200

200 T (K)

300

20

800 700 MZFC-FC (emu/cm3)

and the FC coercivity HC FC ¼ (HC þ  HC )/2, where HC þ and HC are the points where the loop intercepts the positive and negative H axis. At T¼5 K, Hex  310 Oe in [email protected]_2.2 and  460 Oe in [email protected]_24.8 and the ratio HC  FC/Hex is 0.4 and 0.5, respectively. Moreover the increase in coercivity after FC with comparison to the ZFC value (HC  ZFC) is of 58% for [email protected]_2.2 and of  125% for [email protected]_24.8. Thus, a stronger EB effect is found in the sample with higher Fe content [6]. However, in both samples, Hex decreases with increasing T and vanishes at T  40 K and HC  FC decreases accordingly (Fig. 3). In Fig. 4, the temperature dependence of MZFC and MFC measured in Happl ¼300 Oe on the [email protected] samples is shown. The diluted sample (Fig. 4a) exhibits the typical behavior of an assembly of superparamagnetic particles whose magnetic moments block with reducing T, according to the distribution of their magnetic anisotropy energy barriers. With decreasing T, the difference between MZFC and MFC strongly increases below T 40 K, as seen in the inset of Fig. 4a, where [MFC MZFC] vs. T is plotted. The splitting temperature between the MFC and MZFC branches is associated to the blocking of the FM elements with the highest anisotropy barrier, whereas the peak temperature, T¼35 K, in MZFC is associated to the mean blocking temperature, which conventionally marks the passage to the blocked state for the whole array. A very different behavior is observed in [email protected]_ 24.8 (Fig. 4b). For 90 KoTo300 K, the MZFC and MFC branches are superposed and have an almost constant trend; then, below T 90 K, MZFC shows an abrupt fall, whereas MFC does not change substantially. To elucidate the magnetothermal behavior of this second sample, in Fig. 5a we show the TRM vs. T (the field applied on cooling was 300 Oe), together with its derivative curve and,

3505

600 500 400 300 200 100

700

500

0 600 500

300

400 300

200

HC_FC (Oe)

Hex (Oe)

400

200 100

100 0

0

1200

500

1000

Hex (Oe)

800 300 600 200

400

100

HC_FC (Oe)

400

200

0

0 0

10

20

30 40 T (K)

50

60

Fig. 3. Hex (empty symbols) and HC  FC (full symbols) vs. T for samples (a) [email protected]_2.2 and (b) [email protected]_24.8.

50

100

250

300

Fig. 4. MZFC (full symbols) and MFC (empty symbols) vs. T in (a) [email protected]_2.2 and (b) [email protected]_24.8. Inset of (a): difference between MFC and MZFC in [email protected]_2.2.

in Fig. 5b, HC  ZFC measured at different T. The analysis of these curves allows different magnetic regimes to be distinguished. Going from 300 to 90 K (high-T regime), i.e. the T range where MZFC and MFC vs. T are superposed (Fig. 4), HC  ZFC and TRM increases slightly (at T¼300 K, HC  ZFC  20 Oe and it does not exceed 100 Oe in this T range). Then, between 90 and 40 K (intermediate regime), both HC  ZFC and TRM exhibit a rapid increase (at T¼40 K, HC  ZFC  300 Oe). In the TRM derivative curve this regime is identified by the presence of a well-defined peak. Finally, a low-T regime, below T40 K, is marked by the change of slope in the TRM curve and especially by the onset of the EB effect, as described above (Fig. 3). Actually, no clear evidence of this lowT regime is provided by the HC  ZFC curve that keeps increasing up to  10 K, reaching the value of  515 Oe. No substantial increase in HC  ZFC is experienced with further reducing T down to 5 K and a small peak. The whole of the experimental results allows providing the following explanation for the magnetothermal and EB properties of the investigated samples centered at 10 K characterizes the TRM derivative curve. Let us address first the [email protected]_24.8 sample that exhibits a more complex behavior. The trend of MZFC  FC vs. T (Fig. 3) strongly recalls that of a reentrant ferromagnet [13,14], passing from a high temperature FM regime, above T 90 K, to a lowtemperature disordered frozen state below T  90 K. A drop in MZFC is observed below T 80 K. Such temperatures are close to

3506

S. Laureti et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3503–3507

9 8

500

7

400

6 5

300 4 200

3

100

2

-[d(TRM)/dT] (emu/cm3 K)

TRM (emu/cm3)

600

1

0

0 600

HC_ZFC (Oe)

500 400 300 200 100 0 0

50

100

150 T (K)

200

250

300

Fig. 5. (a) TRM (full symbols) and derivative curve  [dMTRM/dT] (empty symbol) vs. T for [email protected] _24.8; (b) HC  ZFC vs. T for [email protected] _24.8. The vertical dotted lines have been added for schematic purposes, see text for explanation.

the Ne´el temperature of Mn in the bulk state [10]. In addition to the finite size effect, leading to a reduction of TN, it should be considered that, due to the interdiffusion of Fe and Mn that is expected to involve mainly the Fe/Mn interface region, a distribution of TN values around that of bulk Mn is likely to exist. In the high-T range (90 oTr300 K), the results indicate that a FM regime, characterized by low HC  ZFC, is established. This implies that the FM inclusions (probably consisting of a bcc Fe core and a FM FeMn shell) that are dispersed in the paramagnetic Mn matrix are exchange-interacting and give rise to a soft FM network extending throughout the sample. This also reveals that, similarly to the case of the [email protected] system [11], the Fe percolation threshold is reached at VFF ¼24.8%, actually. With reducing T below TN, the Mn matrix starts to play a relevant role since it interacts with the soft FM network, affecting its magnetic behavior. In particular, with decreasing T within the intermediate regime (40 KoT o90 K), an increase in both TRM and HC  ZFC (Fig. 5) and a fall of the ZFC susceptibility (Fig. 4b) together with the appearance of magnetic irreversibility (i.e., difference between MFC and MZFC in Fig. 4b) are experienced. These effects are a consequence of the fact that the exchange interaction with the AFM Mn matrix starts to compete with the exchange coupling among the FM elements in determining the orientation of the magnetic moments. This tends to decouple the FM elements and, thus, to break up the soft FM network, resulting in a magnetic hardening of the whole sample. Therefore, with reducing T, the magnetic moments of the FM inclusions tend to align along local randomly oriented anisotropy easy axes, stemming out from the interplay between the crystalline anisotropy, the interparticle exchange and dipolar interactions

and the particle/matrix exchange coupling: this causes the appearance of magnetic irreversibility below T 90 K (Fig. 4b). Hence, the passage from a FM regime to a frozen cluster-glass like state is determined, in agreement with the reentrant-type behavior in Fig. 4b. Passing to the EB properties, with reducing T from the high-T regime in a saturating cooling field the soft FM network, a particular spin configuration of the cluster-glass like state is selected at low T, which minimizes the complex mix of competing interactions inside the system, including the exchange coupling at the interface between the FM phase and the Mn matrix. At T  40 K, the onset of EB effect indicates that the effective magnetic anisotropy of the latter has become strong enough to firmly pin the moments of the FM elements, giving rise to unidirectional anisotropy. With further reducing T below  40 K (low-T regime), the EB effect increases more and more especially for Tr10 K, i.e. in correspondence with the small plateau in the HC  ZFC vs. T curve and with the small peak in the TRM derivative curve (Fig. 5), which may be ascribed to the onset of a completely frozen lowenergy state. As for the [email protected]_2.2 sample, the trend of MZFC  FC vs. T in Fig. 4a is consistent with an array of FM elements (very likely, they mainly consist of FM FeMn alloy) whose magnetic moments undergo superparamagnetic relaxation at high T and, with reducing T below  40 K, block independently. At T  40 K, Hex also appears (Fig. 3a) and it increases with decreasing T. This indicates that the small FM elements are stabilized against thermal fluctuations by the onset of the exchange anisotropy, arising from the interface exchange coupling between the FM inclusions and the Mn matrix. In other words, the FM elements are in the blocked state as long as the anisotropy of the Mn matrix succeeds in pinning the magnetic moment against thermally induced magnetization reversal. Above T 40 K thermal demagnetizing effects prevail and the FM elements become superparamagnetic. The final result is that Hex annihilates well below TN of the Mn matrix and, actually, TN is not seen to play any relevant role in the magnetothermal behavior of the diluted sample. In conclusions, the magnetic results allowed two different descriptions for the magnetothermal evolution in the [email protected] samples to be inferred. In spite of this, it has been found out that, in both the cases, the EB effect appears at T 40 K and it increases with reducing T. On the other hand, a higher value of Hex is reached at T¼ 5 K in [email protected]_24.8, with comparison to [email protected]_2.2. It should be remarked that, in both the descriptions, the key role in the EB mechanism is played by the anisotropy of the Mn phase, which, in the diluted sample, must be strong enough to overcome the thermal energy of the FM moments and, in the concentrated one, must pin the FM magnetic moments, in competition with the mix of interparticle magnetic interactions. Therefore, assuming a similar thermal dependence of the Mn anisotropy in the two samples, it appears reasonable that the onset of the EB effect could occur at approximately the same T in both of them. However, the higher value of Hex for the concentrated sample seems to indicate that, when a low-energy collective frozen state is established throughout the sample, the Mn phase possesses a higher effective anisotropy and thus it is able to exert a more effective pinning action on the FM moments.

References [1] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogues, Nature 423 (2003) 850. [2] A.E. Berkowitz, K. Takano, Journal of Magnetism and Magnetic Materials 200 (1999) 552. [3] W.H. Meiklejohn, C.P. Bean, Physical Review 105 (1957) 904.

S. Laureti et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3503–3507

[4] N. Domingo, D. Fiorani, A.M. Testa, C. Binns, S. Baker, J. Tejada, Journal of Physics D: Applied Physics 41 (2008) 134009. [5] C. Binns, N. Domingo, A.M. Testa, D. Fiorani, K.N. Trohidou, M. Vasilakaki, J.A. Balckman, A.M. Asaduzzaman, S. Baker, M. Roy, D. Peddis, Journal of Physics: Condensed Matter 22 (2010) 436005. [6] N. Domingo, A.M. Testa, D. Fiorani, C. Binns, S. Baker, J. Tejada, Journal of Magnetism and Magnetic Materials 316 (2007) 155. [7] O. Iglesias, A. Labarta, X. Batlle, Journal of Nanoscience and Nanotechnology 8 (2008) 2761. [8] J. Nogue´s, I.K. Schuller, Journal of Magnetism and Magnetic Materials 192 (1999) 203.

3507

[9] S.H. Baker, S.C. Thornton, K.W. Edmonds, M.J. Maher, C. Norris, C. Binns, The Review of Scientific Instruments 71 (2000) 3178. [10] J.M.D. Coey, Magnetism and Magnetic Materials, Edited by C.U. press, Cambridge, 2010. [11] S.H. Baker, A.M. Asaduzzaman, M. Roy, S.J. Gurman, C. Binns, J.A. Blackman, Y. Xie, Physical Review B 78 (2008) 014422. [12] L. Del Bianco, D. Fiorani, A.M. Testa, E. Bonetti, L. Signorini, Physical Review B 70 (2004) 052401. [13] A.H. Morrish, The Physical Principles of Magnetism, Wiley, New York, 1965. [14] S. Bedanta, W. Kleeman, Journal of Physics D: Applied Physics 42 (2009) 013001.