Magnetic properties of Ni and Ni–Cu nanoparticles

Phys. Status Solidi B, 1–5 (2012) / DOI 10.1002/pssb.201248152 p s sb

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basic solid state physics

Magnetic properties of Ni and Ni–Cunanoparticles

Vladimir Leontyev*

A. Baikov Institute of Metallurgy and Material Science, Institution of Russian Academy of Sciences (RAS),

Leninsky Av. 49, Moscow, Russia

Received 8 April 2012, revised 25 August 2012, accepted 3 September 2012

Published online 3 October 2012

Keywords adsorption, magnetization, nanoparticles, Ni–Cu solid solution

* e-mail [email protected], Phone: þ74991359465, Fax: þ74991358730

Nanoparticles of pure Ni and Ni–Cu solid solution were

synthesized by the sol–gel method at 400–600 8C. The particle

sizes and values of specific surface of powders were dependent

on the synthesis temperature. The magnetization of the obtained

powders was studied using a high-temperature magnetic

balance. The Curie temperature for particles of pure nickel

did not depend on their size but the specific magnetization at

300 K decreased linearly with increasing specific surface area

of particles. For particles of the Ni–Cu binary solid solutions,

the Curie temperature and values of magnetization increased

with decreasing particle size for particles with diameters up to

values less than 100 nm. The amount of adsorbed copper on the

particles surface, the change of nickel concentration in the

volume of the particles and the Curie temperatures, depending

on the Cu (Ni) concentration in a volume of particles, were

calculated using the Gibbs equation. The calculations made in

the framework of a perfect solution showed qualitative

agreement between experimental and calculated values of the

Curie temperature for the system Ni–Cu.

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Metals and alloys in a high-dispersionstate often have physical and chemical properties that differsignificantly from the properties of materials in the bulkstate. These properties include: electrical, mechanical,optical, and others [1]. The magnetic properties ofnanoparticles of metals and alloys are of particular interest.This is because the magnetic nanoparticles may findapplication in various fields of engineering. Nickel andcopper have unlimited mutual solubility in the wholetemperature range. The magnetic properties of nickel–copper alloys are well known [2, 5]. The Curie temperature(Tc) of the bulk Ni–Cu alloys decreases linearly withincreasing concentrations of copper and at a concentrationof 12.5 at.% Cu it value is 210 8C (Fig. 1).

In this paper the thermomagnetic properties of nano-particles of metallic nickel and nickel–copper alloy withdifferent size of particles are discussed from the standpointof classical physical chemistry of surface phenomena.

2 Experimental The sol–gel method has been chosenas a method for producing metal particles. The ultradispersed powder mixture of Cu and Ni hydroxides (or onlynickel hydroxide) with a given mole ratio Ni/Cu¼ 87.5/12.5

was prepared at the first stage. For this purpose, an 200 mlaqueous solution of Ni, Cu chlorides with the concentration1 mol/L after ultrasonic mixing for 5 min was mixed with aconcentrated solution of Na2CO3(1l) at room temperatureand with vigorous stirring. Deposition of oxides was carriedout with stirring for 20–30 min, and then the pulp was treatedwith ultrasound for 10 min. The resulting precipitateseparated from the solution by filtration. The precipitatedried at a temperature of 80 8C. The dried residue was groundmechanically. The resulting precipitate hydroxides wasloaded into a quartz crucible and heated to 400–600 8C in atube-type furnace under a flow of hydrogen for 30 min. Toprepare metal powders with different average particles sizethe hydrogen reduction was carried out at differenttemperatures in the range 400–1000 8C. Then the preparedparticles were cooled to room temperature in hydrogen at arate of 5–10 8C/min. The obtained powders were washedfrom Na2CO3 and NaCl with distilled water. The size of theresulting metal particles was determined by the value ofspecific surface area with the using Tristar 3000 (by the BETmethod) and monitored by microscopic studies. Using thevalues of specific surface and assuming that the particleswere spherical, we calculated the average radius of the


2 V. Leontyev: Magnetic properties of Ni and Ni–Cu nanoparticlesp

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Figure 1 Phase diagram Ni–Cu [2].Figure 2 (online color at: www.pss-b.com) Temperature depend-ences of the magnetization of the pure Ni particles of the differentsize. Line (1) Ni-bulk, (2) r¼ 56 nm, (3) r¼ 9 nm, (4) r¼ 7 nm(r is the average radius of the particles calculated from values ofspecific surface).

particles by the equation: r¼ 3/Sr, where r is the density ofNi, S is the specific surface. The magnetization of theobtained powders was studied using a high-temperaturemagnetic balance. For these experiments, we used athermogravimetric instrument, Netzsch STA-409 pc Luxxequipped with a magnetic attachment. At a constant heatingrate the temperature of the sample was recorded with 1 8Caccuracy. The heating rate in all experiments was 10 8C/min.To eliminate the experimental error associated with thechange in the gas density during the heating process, wemeasured the change of the weight of each sample within amagnetic field (experimental curve) and without it (correc-tion curve) under identical conditions (heating rate, gas flowrate). The final result was obtained from the differencebetween the experimental curve and correction curve usingthe instrument software. A sample of the powder was placedin a crucible with cover (Al2O3 cylinder with a 5-mmdiameter and a 4-mm height). A crucible was completelyfilled with ferromagnetic powder. The mass of the sample inall experiments was close to 100 mg. The accuracy of weightmeasurements was 0.2 mg. Calibration was carried out on themagnetization of pure nickel (plate thickness 0.5 mm). Thevalue of the magnetic field in all experiments was around1 mT.

3 Results and discussion Electron-microscopic stu-dies have established that the powders are polydispersesystem with a particle size of 1–200 nm. The shape of the

Table 1 Characteristics of the prepared metallic ferromagnetic pow

prep. temp., 8C (�5 8C) particle composition specifi

400 Ni 48.5500 Ni 36.8600 Ni 61000 Ni <0.1,400 Ni–12.5 at.% Cu 42.6500 Ni–12.5 at.% Cu 17.4600 Ni–12.5 at.% Cu 14.71000 Ni–12.5 at.% Cu <0.1,

aCalculated from the values of specific surface.


particles was close to spherical. This fact indirectly confirmsthe important role of surface phenomena in the process offormation of particles. The characteristics of the obtainedmetal powders are given in Table 1.

The results of thermomagnetic studies of the powders areshown in Figs. 2 and 3.

Figure 2 shows the variation of the magnetization duringthe heating of the pure nickel particles. The Tc of pure nickelparticles did not depend on particle size, but the magnetiza-tion at 300 K decreased with increasing specific surface ofpowders and reducing of the particles size (Fig. 4). For thesmallest particles of pure nickel, the temperature dependenceof the magnetization corresponded to the Curie–Weiss lawonly near the Curie temperature.

In Fig. 3 we can see the change in the shape of themagnetization curves with increasing particle size of thesolid solution. In this case there was no abrupt demagnetiza-tion near the Tc as it was for this composition of bulk samplesof the Ni–Cu solid solution (Fig. 2, Tc¼ 210 8C) and bulk Ni(Figs. 2 and 3, Tc¼ 255 8C). The temperature range of thedemagnetization increased with decreasing particles size.Values of the Tc and magnetization at room temperatureincreased up to that typical for pure nickel values with thedecrease in particles radius less than 10 nm (Fig. 4).

ders.

c surface, m2/g (�0.1 m2/g) average particle radiusa, nm

7.09.256.2

bulk –7.919.422.9

bulk –

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Figure 3 (online color at: www.pss-b.com) Temperature depend-ence of the magnetization of the Ni–Cu solid solution particlesof different size r (radius): line (3) r¼ 23 nm, (4) r¼ 19 nm,(5) r¼ 8 nm. For comparison, the figure shows the temperaturedependence of the magnetization of a bulk sample of pure nickel(line 1) and a bulk sample of the solid solution of the same compo-sition as the particles (line 2).

Figure 4 The dependence of the magnetization at 300 K with theparticle size for pure Ni and solid solution Ni–12.5 at.% Cu.

Figure 5 Modelof theeffectof thecopperadsorptiononthesurfaceof small particles on the volume composition and magneticproperties. Cb

Ni, CsNi are the Ni concentrations in the volume of the

big and small particles, rb and rs are the radii of the big and smallparticles, Tb

c and Tsc are the Curie temperatures of the big and small

particles.

Extending the temperature range of the demagnetization canbe explained by the high dispersion of these powders. Thesmallest particles provided high magnetization at roomtemperature and a high value of Tc. Larger particles had a lowmagnetization at room temperature and a low value of Tc, asfor the bulk sample of the solid solution. Because theexperimental curves of Ni–Cu alloys had an S-type shape theexperimental value of Curie temperatures corresponding tothe mean radius of powder particles can be determined as thetemperature at which the modulus of the derivative of themagnetization on temperature jdM/dTj has a maximumvalue.

Assuming that the magnetization and Tc of the particlesof the solid solution are determined by the volumeconcentration of nickel [2, 5], and with decreasing particlesize the fraction of atoms located on the surface increases itcan be assumed that with decreasing particle size, theconcentration of nickel in the central part of the particlesincreases and the particle surface enriches with copper. Thus,

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the obtained size dependence of the magnetization indicatesthat the effective composition of the particles (in terms of theimpact on the value of specific magnetization) is shifted fromits mean value to the value for pure nickel. According to theNi–Cu phase diagram (Fig. 1), there is only one phase – asolid solution (at 400–600 8C). Therefore, the formation ofmetal particles in the high-temperature reduction processwas accompanied by the creation of an adsorption enrichedwith copper layer on the surface of the particles (Fig. 5). Thisprocess occured at temperatures of 400–600 8C. The Cuconcentration in the volume of the particle was changedcompared with the average composition of the particle. Theextent of this change depended on the specific surface ofparticles, and in the case of a spherical particle depended onits radius. Since the heat of physical adsorption is alwayspositive, increasing the synthesis temperature of the particles(with subsequent quenching) must reduce the value ofadsorption on the particles surface. The reduction inconcentration of copper in the volume of particles increasedthe Curie temperature [3] as compared to that for bulksamples with the same average chemical composition. Thenumber of excess copper atoms on the surface of the particlecan be estimated on the basis of the Gibbs equation:

G¼� ðCCu=RTÞds=dCCu;

where T is the temperature of formation of particles (the

temperature of hydrogen reduction of nickel and copperoxides), CCu is the concentration of solute (Cu), s is thesurface tension.

The proximity of the physical properties of thecomponents, crystal lattice, the density, and electronicstructure of atoms, especially the phase diagram (with awide homogeneity range of solid and liquid states) permitconsideration of copper in nickel as a perfect solution andcalculations of the s and ds/dCCu of the solid solutions on thebasis of the classic theory of perfect binary solutions can be


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Table 2 Values of the physical properties used for calculation [6].

property Ni(solvent)

Cu(solute)

atomic mass, g/mol 58.69 63.54Tc, C 355 –density, g/cm3 8.96 8.9Tmelting, 8C 1455 1085surface tension at Tmelting, erg/cm2 1750 1250temperature coefficient ofsurface tension, mJ/m2 K

�0.34 �0.24

Figure 6 (online color at: www.pss-b.com) Calculated depend-ence of copper concentration in the volume of the particle on its size(Ni–12.5 at.% Cu).
carried out [3, 4]:
� 20

s¼sNi � ZRT ln½1þ ðB� 1Þ�NCu;

where sNi, sCu are the surface tension of Ni and Cu, Z is thenumber of adsorption sites (the number of atoms per unitsurface area), R is the universal gas constant, T is thetemperature, K, B ¼ expfðsNi � sCuÞ=ZRTg; NCu is theatomic fraction of Cu in solution. Hence, in accordance withthe Gibbs equation of the amount of impurity component(Cu) adsorbed per unit surface of the particles of can bedetermined by the expression:

GCu¼ZðB� 1ÞNCu

1þ ðB� 1ÞNCu

; g atom=cm2:

Figure 7 The calculated and experimental values of Curie temper-ature with size of the particle (Ni–12.5 at.% Cu).

In the proposed model (Fig. 5) the increase of the particleformation temperature must reduce the amount of adsorbedCu on the particle surface. For the calculations we used thephysical values listed in Table 2.

It should be noted that the appearance of excess(compared with the average composition) of copper on theparticle surface increases the concentration of the maincomponent (Ni) in the volume of the particle. The degree ofchange depends on the specific surface area of particles.Using the value of the average concentration of componentsin the particle, and the material-balance equation with theGibbs equation, we can determine the change of copper (andnickel) concentration in the volume of the particlesdepending on its diameter. The calculations assumed thatthe volume of the particle had uniform composition. Theresults of the calculations (average composition of particleswas 12.5 at.% Cu) at different temperatures are shown inFig. 6.

Changing the volume concentrations of copper andnickel with decreasing particle size leads to a change in theCurie temperature of the particle. Using the knowndependence of Tc on the composition of bulk samples ofalloys of nickel–copper (Fig. 1), we can estimate thedependence of the Curie temperature with the particle size(Fig. 7).

Figures 6 and 7 show that the volume composition of theparticles and the Curie temperature change slowly withtemperature, but they change dramatically with decreasingof the particle with radius to less than 50 nm. The

12 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

experimental Curie temperatures corresponding to the meanradius of powder particles were determined as the tempera-ture at which the modulus of the derivative of themagnetization on temperature jdM/dTj had a maximumvalue.

4 Conclusions The experimentally determined Curietemperatures for particles of pure nickel do not depend ontheir size and the specific magnetization at 300 K decreaseslinearly with increasing specific surface area of particles.The Curie temperature and magnetization increase withdecreasing particle size of the Ni–Cu binary solid solutionparticles with diameter less than 100 nm. Experimental dataand theoretical calculations indicate that such effects can beexplained in terms of the classical theory of adsorption andthe theory of perfect binary solutions. We have shown astrong influence of surface phenomena on the nanostructureand physical properties of nanoparticles of solid solutions.

Acknowledgements This work was supported by thePresidium of the Russian Academy of Sciences (program no. 8)and the Russian Foundation for Basic Research (project no.11-08-01109-a).

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References

[1] C. Q. Sun, Prog. Solid State Chem. 35, 1–159 (2007).[2] Th. B. Massalski (ed.), Binary Alloy Phase Diagrams (ASM

International, Materials Park, OH, 1987).[3] E. A. Guggenheim and N. K. Adam, Proc. R. Soc. A139, 216

(1933).

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[4] B. S. Bokstein, M. I. Mendelev, and D. J. Srolovitz, Thermo-dynamics and Kinetics in Materials Science (Oxford Univer-sity Press, Oxford, 2005).

[5] H. Kadomatsu and H. Fujiwara, Solid State Commun. 29(3),255–258 (1979).

[6] I. K. Kikoin (ed.), Tables of Physical Quantities: A Handbook(Atomizdat, Moscow, 1976).


Magnetic properties of Ni and Ni–Cu nanoparticles

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