Magnetic Properties of Bulk Metallic Glasses

European Research and Training Network ”Ductile BMG Composites”

Magnetic Properties of Bulk Metallic Glasses Paola Tiberto INRIM, Torino

Outline •

Magnetic Materials

Overview Applications •

Bulk Metallic Glasses

•

Hard and soft compositions

•

Nanocomposite Magnets

•

Recent Experimental data

•

Conclusions

Magnetic Effects of Electrons -- Domains • Permanent magnetism is an atomic effect due to electron spin. In atoms with two or more electrons, the electrons are usually arranged in pairs with their spins oppositely aligned → NOT MAGNETIC • If the spin does not pair → ferromagnetic materials

m = −( e / 2me )⋅ L

m =i⋅A

-e v i i

L

me

Domains • Large groups of atoms in which the spins are aligned are called domains • When an external field is applied, the domains that are aligned with the field tend to grow at the expense of the others. material becomes magnetized

Domains • (a)

Random alignment shows an unmagnetized material • (b) When an external magnetic field is applied, the domains aligned parallel to external magnetic field grow

Ferromagnetic materials Hysteresis loop

H=0 H = Hmax

B= µ0(H + M) = µ0H + J H=0

Types of Ferromagnetic Materials •Two categories: a)Soft magnetic materials •i.e. Fe: easily magnetized If the external field is removed, magnetism disappears

Hc < 10 A/m b) Hard magnetic materials • Co and Ni: difficult to magnetize –They tend to retain their magnetism

Permanent magnets Hc > 100 A/m

Hysteresis Loops SOFT magnetic materials A small amount of dissipated energy in repeated Reversing magnetisation

HARD magnetic materials Retains a large fraction of the saturation field when H is removed

M

Transformers Motor cores

permanent magnets, memory devices Magnetic recording

Ferromagnetic Materials Magnetic materials are widely used in modern devices for their ability to produce or to amplify a magnetic field in the outer space. •SOFT magnetic

•HARD magnetic

easy to magnetise

difficult to demagnetise

provide great amplification of magnetic field produced by electric current in a coil

provide a source of magnetic field without power supply

Ferromagnetic materials

Soft Magnetic materials • Shape of the M,H (or B,H) curve affected by magnetic anisotropy K: K magnetic properties depend on the direction in which they are measured. • K is exploited in the design of most magnetic materials of commercial importance. Soft and extra-soft magnetic properties: low values of the magnetic anisotropy (K in the range of few ten J/m3 and less, i.e. Fe-Ni alloys). Vanishing anisotropy can be obtained in amorphous and nanocrystalline alloys, because the structural order in these materials is extended over limited distances, from few atomic spacings to few nanometers.

Soft Magnetic materials High saturation magnetisation Low magnetocrystalline anisotropy Low coercive field

High magnetic response Easy and fast attain of saturation Easy magnetic domain wall movement (homogeneous material without difects, inclusions, stress …)

High Curie Temperature High electrical resistance

Magnetic stability at higher temperatures Minimise magnetic losses due to Eddy currents

Non-expensive

For application in devices on large scale

Soft Magnetic materials Composition: values of saturation magnetization Ms, the magnetic anisotropy constants K and the magnetostriction constants magnetization process related to the material structure (e.g. crystallographic texture, grain size,lattice defects, etc.). Proper choice of composition and suitable metallurgical and thermal treatments allow to obtain extra soft magnets (Hc≈ 0.1 A/m and µ0 ≈ 106). A number of additional properties, like thermal and structural stability, stress sensitivity of the magnetic parameters, mechanical properties and machinability, thermal conductivity have to be considered. The final acceptance of a material in applications will result from a cost-benefit evaluation of all these properties.

Soft Magnetic materials Composition

µmax

Hc (A/m)

Js (T)

----------------------------------------------------------------------------------------------------------Fe100 3-50⋅⋅103 1-100 2.16 Fe NO Fe-Si

Fe(>96)-Si( δ

Conclusions • Magnetic properties similar to the one achieved in

materials conventionally exploited in applications • preparation geometries;

in

one-step

process

in

different

• miniaturization opportunities for magnetic cores or inductive components and could be used successfully in making transformers, dc-dc and dc-ac converters, magnetic heads, etc. • have more degrees of freedom to tailor magnetic properties due to the flexibility in composition, shape, and dimensions.

Heat flow

EXO

DSC: as-prepared •Significant amount of an amorphous phase is formed in both samples regardless of the quenching rate: strong exothermic signal (around 500 °C) due to a crystallisation process (see Fig. 1)

100 mW/g

AM master alloy

Tx = 531°C

CM Cone

300

350

Tx = 500°C 400

450

500

Temperature [°C]

550

•An additional exothermic signal is observed at lower temperature ⇒ growth of Nd precipitates? DSC traces of the Nd70Fe20Al10 as cast samples.

AM samples: master alloy ingots, through arc melting CM samples: cone-shaped ingots (diameter from 1 to 4 mm ), by copper mould casting

Effect of annealing H e a t in g r a t e : 4 0 K / m in

150

EXO

A ) C o p p e r M o u ld C o n e a s q u e n c h e d s a m p le s a m p le p r e t r e a t e d u p t o 4 9 0 ° C

100

Heat flux (mW/g)

50

Heat treatments: performed in the DSC cell at temperatures increasing from 200 to 500 °C (heating rate 40 K/min) with each step of 50°C.

0

Tx = 461°C

150

B ) A r c m e lt e ld M a s t e r a llo y a s c a s t s a m p le

100

s a m p le p r e t r e a t e d

1) as-quenched: subjected to a DSC run up to (Tx-10)°C and then allowed to cool to room temperature.

u p to 5 2 0 °C

2) the same specimen was heated again up to 580°C to complete crystallisation.

50

0

Tx = 491°C 250

300

350

400

450

500

550

T e m p e ra t u re (° C )

Dashed line: first DSC run stopped at (Tx-10)°C; Continuous line second DSC run up to 580°C.

Magnetic behaviour : effect of annealing 300

• A reduction of Hc is observed in both samples, especially in the AM master alloy (relative decrease ≈ 24%).

Hc (kA/m)

250 200 150

• An increase of magnetisation particularly evident in the CM sample is observed. This effect can be related to variation in composition of the residual amorphous matrix induced by the thermal treatments and resulting from the segregation of Nd atoms in nanocrystalline form. form

AM master alloy CM sample

100 13,0

12,0


11,5

0,110 0,105 0,100

11,0

0,095

10,5 0,090 10,0

J (T)

M (emu/g)

12,5

• Hc reduction ⇒ size increase of the Nd precipitates responsible for the pinning mechanism of the domain walls and segregated from the matrix during the alloy solidification. Optimal dimensions of a nonnon-magnetic precipitate to obtain maximum hardening effect ≈ domain wall width [3]. A further increase beyond this size will cause a decrease in the pinning effect.

0,085

9,5 0 50 100 150 200 250 300 350 400 450 500

Annealing temperature (°C)

Annealing treatment induces a growth of the pre-existing Nd nanocrystals and a reduction of their effectiveness as pinning centres, being their size already above the optimal one.

Room-Temperature magnetic behaviour H (kOe) 0

5

B)

10 0,10

AM master alloy CM sample pure Nd

5

0,05

0

0,00

-5

-0,05

-500

0 H (kA/m)

500

-0,10 1000

0

5000

10000 0,10


0,05

5

0,00

0

-5

-10

-10 -1000

-10000 -5000

10

M (emu/g)

M (emu/g)

10

-5

-1000

J (T)

-10

J (T)

A)

H (Oe)

-0,05

Annealing temperature = 350 °C -500

0

500

-0,10 1000

H (kA/m)

Hc increases with the decrease of quenching rate • No saturation at Hmax ⇒ hysteresis loop can be decomposed into 2 terms: a) ferromagnetic contribution due to the amorphous phase and b) paramagnetic contribution due to the precipitated Nd.

•

Production techniques: ARC MELTING • 99.9% pure elements • Water-cooled copper plate • Ar atmosphere

COPPER MOULDING • Crushed portion of master alloy • melted in a quartz tube • ejected in a conical copper mould • Ar atmosphere

Nd70Fe20Al10 Planar Flow Casting

Ferromagnetic BMG • The optimization of soft or hard magnetic properties (e.g. by increasing the Fe content for increasing the saturation magnetization) also affects the mechanical properties of the material • the use of such glasses as magnetic parts in various devices is strongly related to their elastic and/or plastic response. • For practical use, it is desirable to exactly evaluate both the magnetic and the mechanical behavior, the modifications induced by (nano)crystalline inclusions, in order to finally reach a suitable compromise between magnetic and mechanical properties. • Mechanical properties of ferromagnetic BMGs and nanocomposites preliminarily investigated: fracture strength ≥ 3 GPa and the Young’s modulus moduli of up to 268 GPa n attained materials very attractive for applications.

Soft Magnetic materials

Magnetic behaviour/ RS ribbons •Faint coercivity: 4 kA/m •high initial susceptibility •No saturation

H [kA/m] [kA/m] H -1500 -1000 -1000

-500 -500

00

500 500

10001000 1500 0.06 0.06

RS ribbon ribbon 11 RS RS ribbon ribbon 22 RS

0.04 0.04

M [emu/g]

44 22

0.02 0.02

Langevin fit function of RS 1 anhysteretic curve

00

0.00 0.00

-2 -2

-0.02 -0.02

Langevin fit function of

-4 -4

RS 2 anhysteretic curve

00

   

-0.04 -0.04

30% µ1 ~ 1·10-16 emu

-0.06 -0.06

70% µ2 ~ 5·10-18 emu

-6 -6 -15000 -10000 -20000 -15000 -10000 -5000 -5000

 µ ⋅H M ∝ L  kB ⋅T

J [T]

66

5000 10000 1000015000 15000 5000 20000

H [Oe] [Oe] H

SUPERPARAMAGNET: •A collection of non-interacting magnetic moments disordered by thermal energy •Described by the Langevin Function (L)

Amorphous hard magnets ? Hard magnetic behaviour in the as-cast state for bulk amorphous samples; Hard magnetic behaviour for partially amorphous ribbons obtained at low speeds; Soft magnetic behaviour for fully amorphous ribbons spun at high speeds; Paramagnetic behaviour after heat treatment up to complete crystallisation STRONG DEPENDENCE OF COERCIVE FIELD ON QUENCHING CONDITIONS

Soft Magnetic materials Efficient flux multiplier in a large variety of devices, including transformers, generators, motors, to be used in the generation and distribution of electrical energy, and a wide array of apparatus, from household appliances to scientific equipment. High initial magnetic permeability and/or maximum • magnetic fast switching • electronics • magnetic recording and sensors • magnetic shielding