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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