Rare Elements in Magnetic Materials

WPI Advanced Institute Materials Research Tohoku University

Terunobu Miyazaki

JST Japan-EU workshop, Nov. 22, 2011

My Research Experience• 1972-1975 : hcp Ni, fcc Co, Fe-Co-Ni• 1975-1985 : Rapidly quenched amorphous materials,

Sndust (Fe-Al-Si) alloys• 1985-1991 : Amorphous thin films (Gd-Co, ・・・），Spin-

glass (Fe-based alloy), Magnetiresistance (Fe,Co,Ni)• 1991-1993 : Tunnel magnetoresistance, Soft magnetic

materials, Multilayer films, Kerr effect of alloy films• 1993-2007 :Tunnel magnetoresistance, (Magnetism of

organic materials), (Permanent magnet), HDD, MRAM• 2007- : MRAM materials, Organic/In-organic hybrid

materials

Price of the Elements used for Magnetic Materials

Elements Price ($/kg)B 0.65N

Mg 2.7 (2005)Al 2.2 (2010)Si 2.49V 16Cr 7.6Mn 0.003Fe 0.06Co 67Ni 37Zn 3.2

Elements Price ($/kg)Ga 530Ge 1240Ru 1400 (2011)Pd 11500Ir 37000 (2011)Pt 42100Nd 45→460Sm 250Gd 140Tb 800→4900Dy 150→3800

At 2007

レアメタルニュース(発行元:アルム出版社)2010 年 03 月 24 日p01 抜粋

Dy

Nd

Nd

Dy

Tb

June 2010～August 2011

2005～2010

Price of rare earth elements

： used for magnetic materials

Classification of magnetic materials

• Spintronics materials : Fe-Co-B, Heusler alloys (Co2MnSi), Pt-Mn, Ir-Mn, Co-Pt, Fe-Pt, CoCrPt , Mn-Ga

• Hard magnetic materials : NdFeB+Dy, SmCo, FeSmN

• Soft magnetic materials : Fe-Ni, Permendur (CoFe), Fe-based amorphous alloys, Sendust (Fe-Al-Si)Except oxides

Staff of Spintronics Materials Group and Collaboration

Materials physics in AIMR

TOSHIBA

Dept. Appl. Phys RIEC

T. Miyazaki S. Mizukami

T. Kubota Q. Ma

Our Target

Finding a new material which will be used as the electrodes of MTJ in high density MRAM

Ms < 300 Gauss

K⊥ = 1x107 erg/cc

α < 0.01

TMR ratio > 100 %

TMRdevice

MOS-FET

Large magnetic friction

(Large write current)

Low magnetic frictionLarge perpendicular magnetic anisotropy

Thermal fluctuation(Memory is lost）

Spintronics Materials for Gbit STT-MRAM

K⊥ > 10 Merg/cc α < 0.01

- 5 0 - 2 5 0 2 5 5 0- 3 0 0

- 2 0 0

- 1 0 0

0

1 0 0

2 0 0

3 0 0

I n - P l a n e

P e r p e n d i c u l a r

M (

emu/

cc)

H ( k O e )

Wu et al, APL 94, 122503 (2009), Mizukami et al, PRL 106, 117201 (2011), Kubota et al, APEX 4, 043002 (2011)

Large ( K⊥ =1－2x107

erg/cc ) perpendicular anisotropy

Small damping constant

α＝0.01－0.02

TMR ratioWe must improve this point

0 20 40 60 80-60

-50

-40

-30

-20

-10

0

Kerr

sign

al (a

rb. u

nit)

Delay time (ps)Delay time (ps)

0.83

0.430.28

0.12 (mJ/cm2)Pump laser fluence

Ker

r si

gnal

（ar

b. u

nit）

Typical magnetic and transport properties for Mn-Ga thin film (green Material) developed in our group

Perpendicular magnetic anisotropy, K⊥ > 10 Merg/cc

D. Weller et al. IEEE Trans Mag. 36 10 (2000)

FePdCo3Pt

CoPt

Co5SmFePt

Fe14Nd2B

CoCrPt

Noble or Rare-earth metals are crucial ?

Mn-Ga alloys exhibit large-K⊥ as well as low-α

10-2 10-1 100 101

10-2

10-1

α o

r αef

f.

Kueff (Merg/cm3)

FePt

[Co/X]N(X=Ni,Pd,Pt)

CoCrPt MgO/CoFeB

Mn-Ga

GaMn

Mn

And also low-cost

40 45 50 55-3

-2

-1

0

1

2

3

Ku(

q) (m

eV/c

ell)

q (1/cell)

b

MnII : 2.48 µB

MnI : -3.09 µBMs = 306 emu/cc

Comparison Comparison between experiments and theorybetween experiments and theory

LMTO-ASA with LDA approximation

minority

majority

-6 -4 -2 0 2

-10

0

10

Den

sity

of s

tate

s（

1/eV

uni

t cel

l)

E-EF (eV)

S. Mizukami et al., Phys. Rev. Lett. 106, 117201 (2011).

K⊥ = 26 Merg/cc

Calc. including spin-orbit interaction

MnIIMnIGa

Electron number per cell

Ku

(meV

/uni

t cel

l)

Roughly consistent with exp. values

Mn 3

Ga

Small damping is only around EF, (between DOS peaks)

small

large

(still under investigation)

PossiblePossible story ?story ?

∆EξK2SO∝⊥

( )F2

2SO ED

∆Eξα ∝

Physics of Magnetic Materials Group (Yamagata University)

Prof Kato

・ Development technology for reducing Dyusage in a rare-earth magnet

METI-NEDO Project : Rare Metal Substitute Materials DevelopmentProject (H19－H23)

Yamagata Univ. + 2 Univ. +2 Institute + 4 Companies

・ Coercivity enhancement in bulk Nd-Fe-B by high-magnetic field process

・ Coercivity mechanism study by using model-interface sample made by thin-film process

Progress of energy product (BH)max

50

45

40

35

30

10 15 20 25 30

MRI, Speaker

Electric Vehicle(Dy 10%)

HDD, CDDigital Camera

ABS sensor

OA / FA motor

Servo motor

Air ConditioningRobot, Generator

Hc (kOe) required at 20 oC

(BH

) max

(MG

Oe)

Operating Temperature (℃)50 150 250

(Dy 0%)

(Dy 5%)

Natural Abundance : Dy << Nd

Main use and corresponding magnetic properties of Nd-Fe-B permanent magnet

Nd-Fe-B permanent magnet

Dy-free

Dy 10%

Tc =310˚C

How to increase the coercive force without Dy?

Single domain

Reverse domain

Uniform domain

Interface control

Disordered surface

Ⅰ．Reduction of particle size Ⅱ．Control of particle surface

Particle size (μm)

Hc(kOe) Multi-domain

0.3

90

Two approaches to increase Hc of Nd-Fe-B magnet without Dy

Hc ( Dy-free Nd-Fe-B ) ≒ 10 kOe

This value is approximately 10 % of the theoretically expected value

tNFB (nm)

702075

ΔHc= 10 kOe

without Nd overlayer

with Nd overlayer+ post-annealing

多underlayer

Nd2Fe14B single crystal Nd overlayer

Hc enhancement in Nd-Fe-B film by Nd capping

多underlayer

Nd2Fe14B single crystal

Dgrain(nm)

Hc

(kO

e)

t NdFeB(nm)

21

Hc versus size

size control

Sintered Magnets (Intermetallics)

Interface control

(YU)

0.01 0.1 1 10 100Grain Size D (μm)

Coe

rciv

ityH

c(k

Oe)

11

10

100

Any effect of mag. force by field gradient ?

Problems of Dy diffusion process

Effect of Strong Field Gradient on Hc in the grain boundary diffusion process

・ Max. diffusion depth : ３－5 mm

・ Strong Dy-content gradient from

surface to inside

*annealed at Ta=500˚C after diffusion

Cu 0.1% Nd-Fe-B

electric furnaceDy(3μm)

F ∝ H•dH/dz

18 T superconducting magnet

magnetic force:

Dy diffusion experiment in strong field gradient

Tdiffusion = 850˚C, 60 min

Dy (KDyS

< 0)

Nd (KNd > 0)

c Dy (KDy > 0)

土浦ら、固体物理14, 677 (2009), まてりあ 50, 389 (2011)

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

-2.0-1.5-1.0-0.5 0.0 0.5 1.0 1.5 2.0

M /

Ms

H / Hk

Dy layer = 10

31

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16 18 20

Hc /

Hk

Dy

J = 9.6 meV

J = 4.8 meV

How to get replaced materials ?

• Multilayer film• Fine particle• Hybrid• Nano-porous• Interface control• Topological control• ・・・

• Rapid quenching• Sputtering• MBE• Atomic layer deposition• High magnetic field

processing• ・・・

Morphology Change Technical Methods

We need beyond these changes

We must find further other new methods