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Journal of Alloys and Compounds

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Scrutinising magnetic disorder through metastable 3d- and 4f-nanostructuredalloys

L. Fernández Barquína,∗, D. Alba Veneroa, C. Echevarria-Boneta, R. García Calderóna, D.P. Rojasa,b,J. Rodríguez-Carvajal c, Q.A. Pankhurstd

a CITIMAC, Universidad de Cantabria, Santander 39005, Spainb Dpto. Física, Universidad Carlos III de Madrid, Leganés 28911, Spainc Lab. Leon Brillouin, CE-Saclay, Gif-sur-Yvette 91191, France & Inst. Laue-Langevin, BP156, Grenoble 38042, Franced Royal Inst. Great Britain, Davy Faraday Res. Lab., London W1S 4BS, United Kingdom

a r t i c l e i n f o

Article history:Received 9 July 2011Received in revised form15 November 2011Accepted 21 November 2011Available online 9 December 2011

Keywords:Magnetic materialsNanostructured alloysRietveld refinementAC-susceptibility

a b s t r a c t

Mechanical milling of magnetic alloys 3d-(Fe) and 4f-based (Tb) results in the promotion of magneticdisorder. In Fe22Cu14Ag64 the nanoscopic arrangement has been revealed by a very comprehensive multi-pattern Rietveld analysis using a combination of four sets of data including X-ray and neutron diffraction.The nanostructure is formed by Fe61Cu39 particles of D = 4.6 nm with a collinear ferromagnetic structurein a metallic matrix constituted by D = 11.9 nm Ag nanoparticles. The creation of an ensemble of nanopar-ticles is favoured by the immiscible character of the starting metals. The nanostructured alloy presentsa magnetic transition at around 160 K which cannot be considered as a pure single-domain blocking butaffected by interparticle dipolar interactions. The latter is confirmed by the spin dynamics, displaying acritical slowing down of the AC-susceptibility and a definite peak of the non-linear susceptibility. Whenthe matrix is alternatively formed by Au nanoparticles in the Fe14Au86 (D = 77 nm) and Fe10Cu10Au80

(D = 35 nm) alloys, the magnetic response resembles that of a reentrant state as the milling time is notenough to reduce the particle size, triggering ferromagnetic interparticle coupling enhanced by a mul-tidomain magnetic structure. In milled 4f-alloys formed by miscible Tb and Al, as TbAl2, the productionprocess promotes the magnetic disorder modifying the pure ferromagnetic order of the bulk alloy andsimultaneously the existence of particles (D = 20 nm) which are affected by the structural disorder.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

During the last decades, the investigation on complex magneticstructures has been a steady focus of magnetism [1]. Among thosestructures, some of the most intriguing and attractive are related tothe understanding of a spin-disordered arrangement, usually foundat low temperatures in many alloys. Some decades ago, the frozenspin disorder was found mostly in magnetic elements diluted in ametallic host forming a bulk alloy (for example, CuMn). The combi-nation of the oscillatory sign of the indirect exchange (RKKY) whenvarying atomic distances with the randomness of the site of themagnetic spins gives rise to a frustrated (disordered) spin structurebelow a temperature called the freezing temperature. Such a spinarrangement was named as the spin glass (SG) phase [2]. Increas-ing the concentration of the magnetic element favours a slightlymodified behaviour resulting in a cluster spin glass or reentrantspin glass. Since the burst of nanoscience, there has been found a

∗ Corresponding author.E-mail address: barquinl@unican.es (L. Fernández Barquín).

parallelism between magnetic nanoparticles in different metallicmatrices and the mentioned more simple spin glasses. Effectively,the other similar magnetic state is found typically in ensemblesof magnetic nanoparticles randomly dispersed in an insulatingmatrix. The magnetic nanoparticles display a uniaxial anisotropywith two equivalent spin configurations (0–180 deg) separated byan energy barrier; the height of such a barrier is controlled by theanisotropy K and the volume V of the particle (Ea = KV). Depend-ing on the thermal energy compared to such an anisotropy energy,the ensemble of magnetic moments in the particle can either beblocked at random configurations when kBT < Ea or flipping overbetween the spin up–down directions, when the thermal energyovercomes the barrier. For higher temperatures, the magnetic sys-tem is considered in the superparamagnetic (SPM) regime [3].

Present research lines are supported by diverse synthesis routesspanning the production of a rich variety of compounds. Such apossibility has led to systems which cannot be purely classifiedwithin any of the ideal SG or SPM cases introduced above andthe absence of a clear cut definition has been vastly investigated.Recently, and seizing Mössbauer results previously discussed ingoethite [4], a new general term supermagnetism has been put

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Table 1Results obtained by multipattern Rietveld refinements of four diffractograms taken with X-ray (Cu-K˛ and Mo-K˛) and neutron diffraction (3T2 and G4.2) for the Fe22Cu14Ag64

sample.

Phase a(A) Site Occupation (x, y, z) D (nm) �(%) Radiation RB

2.892(1) 4.6(1) 1.10(1)

3T2 12.46Im–3m Fe(2a) 0.01271 (0,0,0) G4.2 10.71Fe61Cu39 Cu(2a) 0.00812 (0,0,0) Mo-K˛ 9.04

Cu-K˛ 29.36

4.059(1) Ag(4a) 0.02803 (0,0,0) 11.9(2) 1.5(3)

3T2 10.03Fm–3m G4.2 9.77Ag Mo-K˛ 5.67

Cu-K˛ 15.59

Magnetic2.892(1)

Fe3+ 0.61 (0,0,0)4.6(1) 1.10(1)

3T2 14.68Im–3mFe61Cu39 Fe3+ 0.61 (1/2,1/2,1/2) G4.2 10.04

forward to define this somehow intermediate spin-disordered sys-tems [5,6]. In this context, there could exist two main magnetic spinnanostructures: a super spin glass (SSG) state with nanoparticlesat random, strongly coupled by dipolar interactions and superfer-romagnetism (SFM) in which there is a direct magnetic couplingamong particles. Examples of the supermagnetic state have beenfound in Fe-oxides [4], milled FeAlCu [7], discontinuous magneticmultilayers [5] and very recently, thin films of FeAg [8], amongothers.

Once the mainframe to classify diverse magnetically disorderedcompounds has been introduced, it is obvious that mechanicalmilling (MM) is an excellent route to produce those intriguingmaterials. An enormous advantage of the MM is the possibility toproduce large amounts of sample. This asset contrasts with sophis-ticated sputtering, epitaxy or even some chemical routes, whichaltogether provide a small mass yield. Significantly, the advantageof counting with an unlimited sample quantity is not only thatit is easier to get an unambiguous high response when using themacroscopic techniques, but also applying some magnetic micro-scopic techniques (Mössbauer or neutron diffraction, for instance)of tremendous relevance in Magnetism.

Here we are presenting a thorough analysis of macro and micro-scopic results in Fe22Cu14Ag64, with the use of AC-susceptibility andX-ray and neutron diffraction, respectively. This alloy belongs toa family in which we have found a disordered behaviour relatedto the presence of magnetic nanoparticles. This constitutes themain focus of the present work. Previously there were detailedstudies of other FeCuAg MM alloys [9–11]. The new alloy herepresented is intermediate among those two alloys and allows toreach a complete characterisation of the compositional phase dia-gram. In addition, we will complete our investigation observingthe effect induced by the change of the diamagnetic matrix, thatis, by changing from Ag to Au in the alloy. Thus complementaryresults in Fe14Au86 and Fe10Cu10Au80 are also presented. Finally, toprovide a complete picture, results of milled TbAl2 are shown. Wewill analyse what modifications are found between milled alloysof 3d- (Fe-based) and 4f-character (Tb-based), the latter showingan increased disorder respect to the bulk form (unmilled) with aferromagnetic behaviour that should be modified in the milling[12].

2. Experimental

Fe22Cu14Ag64, Fe14Au86, Fe10Cu10Au80 and TbAl2 alloys have been produced inlarge quantities (at least 2 g) in a planetary miller. In the Fe-based alloys we havestarted using Fe powder with sizes below 10 �m (Alfa Aesar), Ag, Au with 2 and3.5 �m (Aldrich Chemical Co.), and Cu with spherical particles of −40 mesh (AldrichChemical Co.). These were milled in 125 ml WC containers sealed under a protectiveAr-atmosphere in a glove box. The ball/powder mass ratio is 12:1 and the rotationspeed 200 rpm. The total milling process lasted 70 h with 1 h stop to avoid recrys-tallisation due to the thermal energy in the metallic containers. An EDX analysisconfirmed the homogeneity and sample composition. It is here worth to remind

the great immiscibility between Fe Cu, Ag and Au. The process to achieve a mag-netically disordered and nanosized alloy TbAl2 was slightly different based in ourprevious experience [12]. There, the milling process has been extended to 150 hfrom a starting pellet of the bulk TbAl2 molten in an arc furnace.

A common characterisation for all the alloys is the use of X-ray diffraction whichwas recorded on a Bruker D8-Advance diffractometer at room temperature with Cu-K˛ (� = 1.5418 A) and/or Mo-K (� = 0.7107 A). In the magnetic characterisation wehave used a PPMS magnetometer to record AC-susceptibility, real (�′) and complex(�′′) components, at frequencies between � = 10 Hz and 10 kHz and oscillating fieldsh in the 0.1–10 Oe range. As commented above, we will focus on the FeCuAg alloy forwhich neutron diffraction experiments have also been performed at the Lab. LeonBrillouin in two instruments employing different wavelengths � = 2.342 A at G4.2(24 h/pattern) and � = 1.225 A at 3T2 (72 h/pattern).

3. Results and discussion

The analysis will be firstly focused on the FeCuAg alloy. Initially,we will be showing the microstructure of the alloy, crucial to under-stand the magnetic disorder and describing in detail our completeprocedure. The latter presents some relevant features, only dis-cussed in some recent reports analysing nanostructures [11,13,14].Diffraction patterns have been analysed fitting the experimentaldata (least-squares) to a calculated pattern following the Rietveldmethod, implemented in the FULLPROF suite [15]. In our case wehave simultaneously used four different patterns combining the X-ray data and neutron data altogether. This has probed to be usefulto provide quantitative results in a less magnetically concentratedFeCuAg milled alloy [11]. The crucial point to bear in mind is theneed of getting physically meaningful parameters for the nano-structure, especially those related with the size (D) and strain (�)of the alloys after milling. This is especially difficult in the crys-tallographic combination of Fe and Ag as the Fe–Im–3m structurepresents all its peaks in the vicinities of the fcc Ag (Fm–3m) peaks. Inaddition, the use of neutron data allows to record the magnetic sig-nal of the magnetic nanoparticles. The profile of the peaks has beentaken as a Thompson–Cox–Hastings with Lorentzian and Gaussiancomponents.

If we use the instrumental broadening for the peaks separately,then V and W parameters can been considered zero and the restof the factors are related to the strain (isotropic U and X; DST:anisotropic) or to the sizes (isotropic Y and IG; F(SZ), anisotropic), allof them fitting variables implemented in the Rietveld refinement[15]. In short, it is feasible to use a combination of patterns provid-ing resolution (large wavelength) and peak intensity over a largenumber of angles (short wavelength) in a procedure known as mul-tipattern Rietveld. The instrumental broadening has been obtainedusing calibrated standards. The experimental data and Rietveldfitting are shown in Fig. 1 and the results have been inserted inTable 1 following a standard presentation. Comparing the resultswith other MM alloys of FeCuAg [16], it is deduced that the pat-terns here analysed are similar to that of pure Ag also after a 70 hmilling period, that is to say, a fcc structure with broad peaks. The

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Fig. 1. Multipattern Rietveld refinement of Fe22Cu22Ag22 using simultaneously fourdifferent patterns taken with X-ray (� = 1.5418 A and � = 0.7107 A) and neutrondiffraction (� = 2.342 A and � = 1.225 A). For the latter, a collinear ferromagneticphase (Mag) has been included as the spectra were taken at RT (below Tc). Numericalresults have been inserted in Table 1.

lattice parameter is a = 4.059(1) A for Fe22Cu14Ag64. This is smallerthan Ag milled in similar conditions (a ∼ 4.07 A) and merely shorterthan a = 4.062(1) A for Fe13Cu10Ag77 [11].

From inspection of Table 1, the alloy in question presents a nano-structure with Fe(Cu) particles in solid solution with a D ∼ 4.6(1) nmand Ag particles with a larger size (>10 nm). This confirms thatthe energy associated to the production route is large enough toestablish a nanostructured arrangement with magnetic particleswithin the single-domain range which in the case for pure Fe isaround 18 nm [17]. Our particles are in fact formed by Fe61Cu39,much below the yet larger limit size for the single-domain. Sucha precise definition of the particle size is of vital importance toestablish the phenomena giving rise to the magnetic relaxation, andallowing a definite calculation of the effective anisotropy. Respectthe Ag matrix, it should retain the diamagnetic character althoughthe size is relevant as it allows to calculate the interparticle dis-tances among nanomagnetic particles. Considering a simple cubicpacking, the average distance between two neighbour particles d,is:

d = D

[(�

6fp

)1/3

− 1

](1)

where D is the mean diameter fp the volumic fraction (29%Fe22Cu14Ag64). Thus, d∼1 nm for Fe22Cu14Ag64 and thus the maxi-mum size contained in the cubic particle arrangement would give∼7 nm). This indicates that the real packing is lower and that surelythe Ag particles are deformed, with typical values for Ag around60% [18]). The other factor in the peak broadening is the strain,around 1%. This value is higher than that obtained for pure Fe

(〈�2〉1/2∼0.6 %), Cu (〈�2〉1/2∼0.39 %) or Ag 〈�2〉1/2∼0.39 %) [16],which within a Gaussian distribution is given by:

〈�2〉1/2 =√

2�

� (2)

In comparison to other similar alloys produced by MM, the strain

values are similar (or larger), as occurs in FeCu with: 〈�2〉1/2∼0.6 %

[19], in FeNi �∼ 0.5 % [20], in FeCr 〈�2〉1/2∼0.9 % [21] and lowerthan in alloys with a higher Fe-content within the FeCuAg sys-tem, (�∼ 5 %) [22]. Pure elements as nanocrystalline Cu obtained by

plastic deformation present extremely low values, 〈�2〉1/2∼0.05 %[23]. Hence MM naturally results in lattice deformations. These areexpected to be more important on the surface due to the bond mod-ification and in the grain boundary, reflected in magnetic terms inthe modification of the local magnetic anisotropy.

Neutron diagrams are scarce in granular nanoparticle systemsand are definitive to establish the magnetic structure of the alloy.Commonly, bcc Fe and related Fe-based alloys are expected topresent a ferromagnetic structure. In our two spectra (high- andlow-resolution) there are not additional reflections stemming fromantiferromagnetic contributions induced by disorder. Effectively,we can include a long-range collinear ferromagnetic phase with avery reasonable (standard) magnetic Bragg error RB (RB–Magn< 15 %)in Rietveld refinements. In this case the magnetic moment was afixed parameter � = 1 �B, which corresponds to the extrapolatedvalued of fcc Fe61Cu39 [24] and temperature-corrected. The excel-lent fit allows to establish that the random substitution of Fe atomsby Cu atoms does not affect the magnetic structure. This has alsobeen reported for INVAR alloys of FeCu (also milled) [25] andFeSi (Fm–3m) nanocrystals produced by the heating of amorphousprecursors FINEMET (FeNbCuSiB) [26]. The contribution of the mag-netic phase is small and the parameters coincide with the nuclearstructure of Fe(Cu) phase.

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8

10

12

14

16

18

20

22

24

300250200150100500

Fe22Cu14Ag64 10000 Hz3000 Hz1000 Hz300 Hz100 Hz30 Hz10 Hz

' (10

-3 e

mu/

g)

T (K)

0

0.2

0.4

0.6

0.8

0 50 10 0 15 0 20 0 25 0 300

'' (1

0 e

mu/

g)

T (K)

Fig. 2. Temperature dependence of the real and imaginary (inset) componentsof the AC-susceptibility in Fe22Cu14Ag64 alloy at different frequencies. The broadmaximum around 160 K shifts up to higher temperatures with the increase of thefrequency.

The macroscopic magnetic behaviour of the Fe22Cu64Ag14 alloycan be accurately defined from the spin dynamics standpointby employing AC-susceptibility data. This type of measurementsallows the modification of the measuring time (frequency) ina sample keeping the same experimental conditions and theexperimental data are shown in Fig. 2. In the figure we show simul-taneously the real (�′) and the complex �′′ (inset) components foran oscillating field h = 10 Oe. A peak at T ∼= 160 K (blocking temper-ature, Tb) is found and the shift towards higher temperatures withincreasing frequency is very clear. The complex component followsthe same frequency trends but the magnitude of the �′′ increaseswhereas for the real susceptibility diminishes with the frequency.All those trends are familiar within the disordered magnets and inparticular the peak marks the blocking transition in an ideal SPMsystem (or the freezing temperature in SGs). However the posi-tion of the peak is larger than expected for 4–5 nm nanoparticles[9]. This is a clear sign of the existence of interactions. Also thepeak position is higher than in Fe13Cu10Ag77, which is a remarkableresult confirming than although the size of the particles is similar,the increased number of Fe(Cu) nanoparticles in the present alloyfavours the interparticle (dipolar) interactions. In the present casethe delta value is ı = (�Tf)/(Tf� log �) = 0.019(1), which is clearlylower than in SPM (expected 0.1–0.2) [3]. This simple estimationconfirms the existence of interactions as well. Given those signifi-cant interactions, it is plausible to include the nanostructured alloyin the SSG state (below the transition). In consequence, it is instruc-tive to observe if a critical slowing down process is present in thespin relaxation of the above the transition. This behaviour dependson the divergence of the spin correlation length. The characteristicrelaxation time is then given by [2]:

= 0

(T − T0

T0

)−zv(3)

The fit to our data gives the values of z� = 4(1) and T0 = 158 K forthe Fe22Cu14Ag64, which vary compared with z� = 9(1) and T0 = 84 Kfor the Fe13Cu10Ag77 [11]. These values indicate that the interac-tions are enough strong to enable a cooperative behaviour at thetransition, especially in the present alloy, in which the value of z� isclose to that for a conventional phase transition. This is in fact verysimilar to the excellent results reported for 5 nm FeC ferrofluids[28].

The non-linear behaviour of the AC-susceptibility has also beena tool to analyse the magnetic transition of the SG or SPM type.A detailed description of the involved equations has already been

-7

-6

-5

-4

-3

-2

-1

0

300250200150100500

h = 0.2 Oeh = 0.3 Oeh = 0.5 Oeh = 1 Oe

3' (10

-4em

u/g

Oe3 )

T (K)

Fig. 3. Non-linear term (�′3) of the AC-susceptibility vs. temperature in Fe22Cu14Ag64

alloy at different AC-magnetic fields (h). The inset details the oscillating field depen-dence of the minimum of �′

3(T).

discussed comparing several heterogeneous magnetic compounds[10]. Briefly, the non-linear susceptibility can be obtained throughDC-magnetisation measurements at different fields [27] or moredirectly with the aid of the lock-in detection of the anharmonicfrequency in AC-susceptibility [10]. The latter is a more directroute and has been employed here. The magnetisation can bethen expanded as a series of field-powers and the non-linearterm (�′

3(T)) corresponding to the h3 term) can be detected (itis necessary the combination of sufficient quantity of sample andmagnetometer sensitivity). If a SG transition is expected, it can beobserved as a sharp peak in this non-linear (odd) component [29]. Inour measurements, there is a relatively broad peak at around 50 K.Thus, the system is behaving as a SG but the transition is wider dueto the presence of a distribution of particle sizes and consequentlya distribution of interparticle interactions. Another step in the anal-ysis is to measure �′

3(h). As it can be seen in Fig. 3, the �′3(T) value

decreases with the increase of h field. This kind of data permits theevaluation of the tendency of �′

3 when h tends to 0 (h ≥ 0.1 Oe, inpractice). Clearly, there is a divergence as it is expected in a SG,as it has been clearly reported in a comparison between SPM andSG in LixNi1−xO [30]. Summarising, the AC-susceptibility behaviourof the FeCuAg alloy presents characteristics of a SG alloy but ofnanostructure nature, and possibly a SSG state is the real situation.

The composition of the alloy can be easily altered and thusexchange Ag by Au to study eventual macroscopic magneticchanges. To cover such a possibility we have analysed Fe14Au86and Fe10Cu10Au80, using the same MM route. In Fig. 4, the ther-mal variation of the AC-susceptibility for both Au-based alloys isshown. Although the composition is similar to the FeCuAg abovediscussed, an evident modified response is found. For the Fe14Au86alloy, in the real susceptibility component, when decreasing thetemperature, there is a plateau followed by a decrease of the �′

value. For the complex component there is a distinct feature ataround 90 K, close to the decay in �′(T). A similar shape with alower transition temperature (∼=25 K, in the complex component)is observed in Fe10Cu10Au80. The XRD diffraction of both alloys (notshown) presents a major contribution from fcc Au peaks. An esti-mation of the values of D and �, through a Williamson–Hall analysisgives D = 77 nm and � = 2% for FeAu and D = 38 nm and � = 3% in thecase of FeCuAu. The relative large value of both D is responsible forthe observed magnetic changes. The particles are larger and strongdipolar interactions are expected and in addition the particles areno longer exclusively within the single-domain regime. Hence itis favoured a ferromagnetic-like behaviour and interparticle cou-pling, resulting in the flat plateau and absence of a Curie–Weiss

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0.0008

0.001

0.0012

0.0014

0.0016

5 10-6

1 10-5

1.5 10 -5

2 10-5

2.5 10 -5

Fe14

Au86

'(em

u/gO

e) ''(emu/gO

e)

0.005

0.0055

0.006

0.0065

0.007

0.0075

8 10-5

0.0001

0.00012

0.00014

300250200150100500

Fe10

Cu10

Au80

Temperature (K)

'(em

u/gO

e) ''(emu/gO

e)Fig. 4. Temperature variation of the real and imaginary components of AC-susceptibility in Fe14Au86 and Fe10Cu10Au80 alloys. It is observed a plateau followedby a decrease of the �′(T) value, and a maximum in �′′(T).

dependence for T > Tb, up to 300 K. It is then evident that the originfor the existence of these larger particles should be connected tothe metallurgic properties of Au and Ag. Probably a drastic varia-tion of the production conditions (especially the milling time) isneeded if a similar susceptibility behaviour to the FeCuAg alloy isto be studied.

So far we have studied in detail the fine particle behaviourconnected to a disordered magnetic arrangement of magneticnanoparticles in FeCuAg. This is clearly modified with a relativelow increase of the size of the particles in FeAu-based alloys. Thescrutiny of the macroscopic magnetic properties can now be con-nected with a change of the atomic species from 3d- to 4f-alloys. Forthis analysis we can make use of Rare Earth alloys which by millingpresent a reduction of size and an acceptable distribution of parti-cle sizes [31]. In particular, attention is now paid to Rare Earth-Al2,which in bulk form is a ferromagnet. The temperature dependenceof the real and imaginary components of the AC-susceptibility atfrequencies of 1 and 5 kHz in 150 h TbAl2 milled alloy is presentedin Fig. 5 together with the XRD pattern taken with Cu-K (inset).The crystallographic structure of this alloy is cubic (Laves phase)Fd3m with a lattice parameter a = 7.8674(2) A. The milling inducesthe appearance of a nanostructure characterised by particles withD = 20 nm. The high-energy collisions during the milling processalso increase the strain value reaching � = 0.85%. The overall mag-netic behaviour is in accordance to that expected from the evolutionof these curves with the milling time, namely, a decrease of the fer-romagnetic contribution around 100 K, and the emergence of newpeaks at lower temperatures, as observed for other milled TbAl2alloys [12]. For 150 h of milling time, an intermediate situationbetween the domain wall dynamics of the bulk and the glassy prop-erties of the 300 h milled alloy would be expected. Effectively, theinterpretation of Fig. 5 suggests that the peak around 100 K canbe attributed to the ferromagnetic ordering, and the other lowertemperature ones at 45 K and 20 K are the result of the disorderpromoted by the MM, leading to a spin frozen state at low tempera-tures. On the other hand, the value of the relative shift per frequencydecade (ı = 0.011(1)), in the aforementioned alloy (150 h), is largerthan that reported for 300 h milled alloy (ı = 0.0026(2)). Clearly,the milling process favours the disorder of the magnetic interac-tions and, therefore, a behaviour closer to that observed in typical

0

0.002

0.004

0.006

0.008

0.01

0 10 0

5 10 -5

1 10 -4

1.5 10-4

2 10 -4

2.5 10 -4

3 10 -4

3.5 10-4

4 10 -4

120100806040200

' (emu/gOe) 1 kHz' (emu/gOe) 5 kHz

'' (emu/gOe) 1 kHz

'' (emu/gOe) 5 kHz

' (em

u/gO

e)

'' (emu/gO

e)

T (K)

20 40 60 80 1000

500

1 10

1.5 10

I (co

unts

)

2 (Degrees)

Fig. 5. Temperature dependence of the real and imaginary components of the AC-susceptibility in 150 h TbAl2 milled alloy at the frequencies of 1 and 5 kHz. In theinset, the milled TbAl2 X-ray diffractogram is depicted. A typical broadening of thepeaks in milled alloys is observed.

glassy magnets. Naturally, in this particular alloy there are two fac-tors acting together; one is the particle size effect and the other isthe existence of a crystallographic disordered grain boundary.

4. Conclusions

Disorder in magnetic metallic alloys can be easily induced bymechanical milling. The milling route enables the production oflarge quantities of nanostructured material easing the analysisof macroscopic measurements and allowing the use of relevantmicroscopic techniques from the structural/magnetic point of viewsuch as X-ray and especially neutron diffraction. The immisciblecharacter of starting powders of Fe, Cu, Ag and Au permits theproduction of binary and ternary alloys of FeAu and FeCuAg andFeCuAu. Our focus on FeCuAg has provided a detailed interpre-tation of the nanoscopic arrangement as an ensemble of Fe(Cu)nanoparticles embedded in an Ag matrix. These particles are single-domain (D ∼ 5 nm) and present a SSG behaviour as deduced fromthe existence of a peak in the AC-susceptibility at around 160 K. Thispeak relaxes following a critical slowing down which in turn is alsoreflected in a peak in the non-linear susceptibility, with a field-divergent behaviour. The nanostructured arrangement is largelymodified in the Au-based alloys for which 70 h of milling are notenough to produce single-domain particles and triggering stronginterparticle magnetic coupling in the form of a reentrant-like tran-sition. For miscible elements forming the TbAl2 the general pictureis slightly different. There, the 150 h milling gives rise to disorderedmagnetism as observed in the broad Tc and a progressive freezing atlower temperatures; the latter stemming from a disordered mag-netic domain structure. It is particularly appealing a further study ofthe effect of milling selecting a bulk antiferromagnetic alloy of sim-ilar Rare Earth nature and/or the eventual combination of 4f- and3d-elements within a single (milled) alloy, as for example TbFeAl.

Acknowledgement

We thank the CICYT MAT2008-06542 for financial support.

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