Magnetism
A. K. Majumdar
1

History of Magnetism
Diamagnetism
Paramagnetism
Ferromagnetism
Weiss’ “molecular field” theory
Heisenberg’s theory
Bloch’s spin-wave theory
Band theory of ferromagnetism
Spin glass (1972)
R & D in Magnetic Materials after 1973
Magnetoresistance (MR)
Ordinary or normal magnetoresistance (OMR)
Anisotropic magnetoresistance (AMR) in a ferromagnet
Giant magnetoresistance (GMR)
Hall effect in ferromagnets
Fe-Cr multilayers: GMR and Hall effect
Applications
Nano-magnetism
Superparamagnetism
Examples: Ni nano/TiN
Exchange bias
Tunnel magnetoresistance (TMR)
Colossal magnetoresistance (CMR)
2

History of Magnetism
The study of magnetism started with the discovery of the
Lodestone (or Magnetite Fe
Greece & China.
3
O
4
) around 500-800 B.C. in
Lodestones attract pieces of iron and the attraction can only be stopped by placing between them an iron plate, which acts as a Magnetic Shield.
The directional property of the Lodestone was utilized to design
“Compass”, which was invented around 100 A.D. in China.
Theoretical understanding of magnetism came only in the 19 th century along with some basic applications .
3

Diamagnetism
The weakest manifestation of magnetism is Diamagnetism
⇒
Change of orbital moment of electrons due to applied magnetic field.
The relevant parameter which quantifies the strength of magnetism is the Magnetic Susceptibility,
χχχχ
, which is defined as
χ
= Lt
H
→
0
(dM/dH).
Diamagnetism arises from two basic laws of Physics, viz.,
Faraday’s law & Lenz’s law:
An electron moves around a nucleus in a circle of radius, r.
A magnetic field H is applied. The induced electric field, E(r), generated during the change is
E(r) 2 π r = -d/dt (H π r 2 )/c or, E(r) = - (r/2c) dH/dt.
4

Diamagnetism
This E(r) produces a force & hence a torque = – e E(r) r
= dL/dt = (e r 2 /2c) dH/dt, (e > 0).
The extra
∆
L(L was already there for the orbital motion) due to the turning of the field = (e r 2 /2c) H.
The corresponding moment
µ
= - (e/2mc) ∆ L = - (e 2 r 2 /4mc 2 ) H, r 2 = x 2 + y 2 .
For N atoms per unit volume with atomic no. Z, the Magnetic Susceptibility
χχχχ
= - (Ne 2 Z<r 2 > av
) /6m c 2 .
QM treatment also produces the same answer!!!
χ is negative
≈
- 10 -6 in cgs units in the case of typical diamagnetic materials. In
SI units it is - 4 π x 10 -6 .
5

Diamagnetism
The Hamiltonian of a charged particle in a magnetic field B is
H = (p – eA/c) 2 /2m + e φ , A = vector pot, φ = scalar pot, valid for both classical & QM.
K.E. is not dependent on B, so it is unlikely that A enters H. But p = p kin
+ p field
= mV + eA/c (Kittel, App. G, P: 654)
& K.E. = (m V ) 2 /2m = (p – eA/c) 2 /2m, where B =
∇
X A.
So, B-field-dependent part of H is ieh/4
π mc[
∇
. A+ A.
∇
] + e 2 A 2 /2mc 2 .(1)
6

Diamagnetism
If B is uniform and II z-axis, A
X
= - ½ y B, A
Y
= ½ x B, and A
Z
= 0.
So, (1) becomes H =(e/2mc) B [ ih/2
π
(x
∂
/
∂ y - y
∂
/
∂ x)] + e 2 B 2 /8mc 2 (x 2 + y 2 ).
L
Z
↓ ↓
⇒
Orbital PM; E’ = e 2 B 2 /12mc 2 <r 2 > by 1st order perturbation theory.
M = -
∂
E’/
∂
B = - (Ne 2 Z<r 2 > av
)/6mc 2 B for N atoms/volume with atomic no. Z.
Since B = H for non-magnetic materials
χ
= Lt
H
→
0
(dM/dH) = - (Ne 2 Z<r 2 >av )/6mc 2 independent of T.
7

Diamagnetism
M(T) shows negative and temperature independent magnetization.
Meissner effect in a superconductor:
χ as high as – 1 below T
C perfect diamagnetism to make B = 0 inside (flux expulsion) below H
C1
.
Substrates of present day magnetic sensors are mostly diamagnetic like Si, sapphire, etc.
8

Paramagnetism
Atoms/molecules in solids/liquids with odd no of electrons(S
≠
0): free Na atoms with partly filled inner shells, compounds like molecular oxygen and organic biradicals, metals (Pauli), etc. contribute to electron paramagnetism.
Free atoms/ions with partly filled inner shells, e.g., Mn 2+ , Gd 3+ , U 4+ show ionic paramagnetism.
A collection of magnetic moments, m , interact with external magnetic field H :
Interaction energy U = - m . H.
Magnetization results from the orientation of the magnetic moments but thermal disorders disturb this orientation.
9

Paramagnetism
The energy levels, according to quantum mechanics, is given by
10

Paramagnetism
1.0
0.8
0.6
0.4
0.2
S=3/2 FOR Cr3++
In this <m> vs. H/T plot paramagnetic saturation is observed only at very high H & low
T, i.e., x >>1 when coth x
→
< m >
→
Ng µ
B
S
→
1 and
At 4 K & 1tesla, <m> ~ 14 % of its saturation value.
For ordinary temperatures like 300
K & 1 T field, x <<1.
Then coth x
→
1/x + x/3 and so
0.0
0 2 4 6 8
Magnetic Field/Temperature(B/T)
10
< m
>→

 g
B
S ( S
3 k
B
T
+
1 )


H Ng µ
B
.
Thus <m> varies linearly with field.
< m > = Ng
µµµµ
B
S [coth X- 1/X] = L(X) = Langevin function when S
→ ∞ with X = Sx.
11

Paramagnetism
So the Magnetic Susceptibility,
at temperature T is
= ( N g 2
B
2 S(S+1)/3k
B
)*1/T
= C/T , where N = No. of ions/vol, g is the Lande factor ,
B is the Bohr magneton and C is the Curie constant.
This is the famous
Curie law for paramagnets.
Some applications:
To obtain temperatures lower than 4.2 K, paramagnetic salts like,
CeMgNO
3
, CrKSO
4
, etc.
, are kept in an isothermal 4.2 K bath
& a field of, say, 1 T applied.
>>> Entropy of the system decreases & heat goes out. Finally the magnetic field is removed adiabatically.
>>> Lattice temperature drops to mK range.
Similarly microkelvin is obtained using nuclear demagnetization.
12

Paramagnetism
Fascinating magnetic properties, also quite complex in nature:
Ce58: [Xe] 4f 2 5d 0 6s 2 , Ce 3+ = [Xe] 4f 1 5d 0 since 6s 2 and 4f 1 removed
Yb70: [Xe] 4f 14 5d 0 6s 2 , Yb 3+
Note: [Xe] = [Kr] 4d 10 5s 2 5p 6 .
=
[Xe] 4f 13 5d 0 since 6s 2 and 4f 1 removed
In trivalent ions the outermost shells are identical 5s 2 5p 6 like neutral Xe.
In La (just before RE) 4f is empty, Ce +++ has one 4f electron, this number increases to 13 for Yb and 4f 14 at Lu, the radii contracting from 1.11
Å (Ce) to 0.94 Å (Yb) → Lanthanide Contraction.
The number of 4f electrons compacted in the inner shell is what determines their magnetic properties. The atoms have a (2J+1)-fold degenerate ground state which is lifted by a magnetic field.
In a Curie PM
χ
=
= g
[
NJ
J
(
J
+
3
κ
(
J
+
1
) ]
1
B
T
) g 2
µ
2
B
=
C
T
=
Np
3
κ
2
µ
B
T
2
β
, p = effective Bohr Magnetron number
, g = Lande factor =
1
+
J ( J
+
1 )
+
2
S
J
(
(
S
J
+
+
1 )
1 )
−
L ( L
+
1 )
‘p’ calculated from above for the RE
3
+ ions using Hund’s rule agrees very well with experimental values except for Eu
3
+
& Sm 3
+
.
13

Paramagnetism
Formulated by Friedrich Hund, a German Physicist around 1927.
The ground state of an ion is characterized by:
1. Maximum value of the total spin S allowed by Pauli’s exclusion principle.
2. Maximum value of the total orbital angular momentum L consistent with the total value of S, hence PEP.
3. The value of the total angular momentum J is equal to when the shell is less than half full & L
S
L
S when the shell is more than half full.
When it is just half full, the first rule gives L = 0 and so J = S.
14

Paramagnetism
Explanation 1 st rule: It’s origin is Pauli’s exclusion principle and
Coulomb repulsion between electrons.
↑↑ are separated more and hence have less positive P.E. than for
↑↓ electrons. Therefore, all electrons tend to become ↑↑ giving maximum S.
Explanation of 2 nd rule: Best approached by model calculations.
Example of rules 1 & 2: Mn
2
+
Mn 3 d
5
4 s
2
, Mn
2
+
3d
5
(half- filled 3d sub shell).
All 5 spins can be ║ to each other if each electron occupies a different orbital and there are exactly 5 orbitals characterized by orbital angular quantum nos. m l
= -2, -1, 0, 1, 2. One expects S = 5/2 and so
∑ m l
= 0.
Therefore L is 0 as observed.
15

Paramagnetism
Explanation of 3 rd rule:
Consequence of the sign of the spin-orbit interaction. For a single electron, energy is lowest when S is antiparallel to L (L.S = - ve). But the low energy pairs m l and m s are progressively used up as one adds electrons to the shell. By PEP, when the shell is more than half full the state of lowest energy necessarily has the S ║ L.
Examples of rule 3:
Ce 3+ = [Xe] 4f 1 5s 2 5p 6 since 6s 2 and 4f 1 removed. Similarly Pr 3+ = [Xe]
4f 2 5s 2 have 4f 3
5p 6 . Nd 3+ , Pm 3+ , Sm 3+ , Eu 3+ ,Gd 3+ ,Tb 3+ , Dy 3+ , Ho 3+ ,Er 3+ to 4f 13 . Take Ce 3+
,Tm 3+ ,Yb 3+
: It has one 4f electron, an f electron has l = 3, s = ½, 4f shell is less than half full (full shell has 14 electrons), by third rule = J =

L – S

= L - 1/2 = 5/2.
Spectroscopic notation
2 s
+
1
L
J
2
F
5 / 2
[L = 0, 1, 2, 3, 4, 5, 6 are S, P, D, F, G, H, I].
Pr
3
+ has 2 ‘4f’ electrons: s = 1, l = 3. Both cannot have m l so max. L is not 6 but 5. J =

L – S

= 5 -1 = 4.
⇒
= 3 (PEP),
3 H
4
16

Paramagnetism
Nd
3
+ has 3 ‘4f’ electrons: s = 3/2, L = 6 , J = 6 - 3/2 = 9/2
⇒ 4
I
9 / 2
Pm
3
+ has 4 ‘4f’ electrons: s = 2, L = 6, J = L - S = 4
⇒ 5
I
4
Exactly ½ full 4f shell: : has 7 ’4f’ electrons; s = 7/2, L = 0, J = 7/2
⇒
8
S
7 / 2
4f shell is more than half full: Ho
3
+ has 10 ‘4f’ electrons: 7 will be
,
3 will be
. S = 2, L = 6 [3 2 1 0 -1 -2 -3] ; J = 6 + 2 = 8
⇒ 5
I
8 and so on.
Note: 4f shell is well within the inner core (localized) surrounded by 5s 2 , 5p 6 , and
6s 2 & so almost unaffected by crystal field (CF).
3d transition element ions, being in the outermost shell, are affected by strongly inhomogeneous electric field, called the “crystal field” (CF) of neighboring ions in a real crystal. L-S coupling breaks, so states are not specified by J. (2L + 1) degenerate levels for the free ions may split by the CF and L is often quenched.
“p” calculated from J gives total disagreement with experiments.
17

Paramagnetism
Details: If E is towards a fixed nucleus, classically all L
X are constants (fixed plane for a central force). In QM L
Z
& L 2
, L are
Y
, L
Z constants of motion but in a non-central field (as in a crystal) the orbital plane is not fixed & the components of L are not constants & may be even zero on the average. If <L
Z
> = 0, L is said to be quenched.
In an orthorhombic crystal, say, the neighboring charges produce about the nucleus a potential V = Ax 2 + By 2 – (A + B)z 2 satisfying Laplace equation & the crystal symmetry. For L =1, the orbital moment of all 3 energy levels have <L
Z
> = 0. The CF splits the degenerate level with separation >> what the B-field does. For cubic symmetry, there is no quadratic term in V & so p electron levels will remain triply degenerate unless there is a spontaneous displacement of the magnetic ion, called
Jahn-Teller effect , which lifts the degeneracy and lowers its energy.
Mn 3+ has a large JT effect in manganites which produces Colossal
Magnetoresistance (CMR).
For details see C. Kittel, ISSP (7 th Ed.) P. 425-429; Ashcroft & Mermin, P. 655-659.
18

Paramagnetism
Paramagnetic susceptibility of a free electron gas: There is a change in the occupation number of up & down spin electrons even in a non-magnetic metal when a magnetic field B is applied.
The magnetization is given by ( T << T
F
)
M = µ (N up
- N down
) = µ 2 D (
ε
F
∴
χ = 3 N µ
2 / 2
) B, D (
ε
F
) = 3N/2
ε
F
.
F
, independent of temperature, where D (
ε
F
) = Density of states at the Fermi level
ε
F
.
19

Ferromagnetism
Any theory of ferromagnetism has to explain: i) Existence of spontaneous magnetization M below T
C
(Paramagnetic to ferromagnetic transition temperature).
ii) Below T
C
, a small H
0 produces M
S from M ~ 0 at H = 0.
There are three major theories on ferromagnetism:
1. Weiss’ “molecular field” theory
2. Heisenberg’s theory
3. Bloch’s spin-wave theory (T << Tc).
20

Ferromagnetism
Weiss proposed that: i) Below T
C spontaneously magnetized domains, randomly oriented give M ~ 0 at H=0. A small H
0 produces domain growth with M || H
0
.
ii) A very strong “molecular field”, H atomic moments within a domain.
E of unknown origin aligns the
Taking alignment energy ~ thermal energy below Tc,
For Fe: H
E
= k
B
T
C
/
µ
~ 10 7 gauss ~ 10 3 T !!!
E d-d
~ [
µ
1
.
µ
2
- (
µ
1
.r
12
) (
µ
2
.r
12
)]/4
πε
0 r 3 .
Classical dipole-dipole interaction gives a field of ~ 0.1 T only
& is anisotropic but ferromagnetic anisotropy is only a second order effect. So ???
Weiss postulated that H
E
=
λ
M, where
λ is the molecular field parameter and M is the saturation magnetization.
21

Ferromagnetism
Curie theory of paramagnetism gave
M = [( N g 2
µ
B
2 S(S+1)/3k
B
)*1/T]*H
= (C/T)*H.
Replacing H by H
0
M = CH
0
/(T-C
λ
)
+
λ
M we get
= C/(T-T* where T*
C
C
),
= C
λ
= paramagnetic Curie temperature.
Putting g ~ 2, S = 1, M = 1700 emu/g one gets
λ
= 5000 & H
E
= 10 3 T for Fe.
This theory fails to explain
χ
(T) for
T < T*
C
.
22

Ferromagnetism
23

Ferromagnetism
24

Ferromagnetism
25

Ferromagnetism
Heisenberg’s Theory (Exchange effect)
Heisenberg found the origin of Weiss’ “molecular field” in the “quantum mechanical exchange effect”, which is basically electrical in nature.
Electron spins on the same or neighboring atoms tend to be coupled by the exchange effect – a consequence of Pauli’s Exclusion Principle. If u i and u j are the two wave-functions into which we “put 2 electrons”, there are two types of states that we can construct according to their having antiparallel or parallel spins. These are
Ψ (r
1
, r
2
) =1/
√
2 [u i
(r
1
) u j
(r
2
) ± u j
(r
1
) u i
(r
2
)] where
± correspond to space symmetric/antisymmetric (spin singlet, S = 0/spin triplet, S = 1) states.
Singlet: spin anisymmetric: S = 0: ( ↑↓ ↓↑ )/
√
2 , m = 0.
Triplet: spin symmetric: S = 1: ↑↑ m = 1; ↓↓
( ↑↓ + ↓↑ )/
√
2 , m = 0.
m = -1;
Also, if you exchange the electrons between the two states, i.e., interchange r
1 and r
2
, Ψ (-) changes sign but Ψ (+) remains the same. If the two electrons have the same spin( ║ ), they cannot occupy the same
r. Thus Ψ (-) = 0 if r
Coulomb energy e 2 / │ r
1
12
= r
2
. Now if you calculate the average of the
│ in these two states we find them different by
J ij
= ∫ u i
*
( r r
1
) u
* j
( r r
2
) r r i e
2
− r r j u j
( r r
1
) u i
( r r
2
) d r r
1 d r r
2 which is the “exchange integral”.
26

Ferromagnetism
27

Ferromagnetism
28

Ferromagnetism
29

Ferromagnetism
Heisenberg gave the exchange Hamiltonian the form which is isotropic.
H e
= r r
−
2 J ij
S i
.
S j
In the presence of a field H o
, the Hamiltonian becomes r r
H
= − g
µ
B
H
0
∑
S i
Z
−
2
∑
J ij
S i
.
S j i i i
,
≠ j j
Assuming nearest neighbour exchange interaction, J ij we get the Hamiltonian
= J & S i
H = - g
µ
B
H
0
NS - ½ {2JNZ(S) 2 } , where Z = no. of N.N.’s; N = no. of atoms/vol.
Simple algebra gives, replacing H
0 by H
0
+ H
E
(=
λ
M), using T
C
= C
λ
,
C= N g 2
µ
B
2 S(S+1)/3k
B
& M = Ng
µ
B
S in H = -M(H
0
+ ½ H
λλλλ
= (2JZ/Ng 2
µµµµ
B
2 ) & J = (3k
B
T
C
/2ZS(S+1))
E
),
~ 1.2 x 10 -2 eV for Fe with S = 1.
S
Taking S = ½ , one gets (k
B
T
C
/ ZJ) = 0.5 from mean-field theory
= 0.325
from Rushbrooke & Wood theory.
30

Ferromagnetism
In the ground state of a FM (at 0 K) the atoms at different lattice sites have their maximum z-component of spin, S j z = S .
As T increases the system is excited out of its ground state giving rise to sinusoidal “Spin Waves”, as shown below.
The amplitude of this wave at j th site is proportional to S-S j z .
The energy of these spin waves is quantized and the energy quantum is called a “magnon”.
31

Ferromagnetism
32

Ferromagnetism
, satisfying H
ψ
= E
ψ
,
↑
33

Ferromagnetism
The next 9 slides could be summarized as follows:
The spin raising and lowering operators S j
± = S j x ± i S j y operate on the eigenstates of the spin deviation operator
η j
. The spin raising operator raises the zcomponent of the spin and hence lowers the spin deviation and vice-versa. Then one defines Boson creation (a j
+ ) and annihilation (a j
) operators as well as the number operator (a j
+ a j
) in terms of S j
± and S j z . These relations are called Holstein-Primakoff transformation. Then one makes a transformation of S j
± and S j z in terms of magnon creation and annihilation operators, b k
+ and b k
. K is found from the periodic boundary condition. b k
+ b k is the magnon number operator with eigenvalue n k for the magnon state
K.
Finally, the energy needed to excite a magnon in the state K, in the case of a simple cubic crystal of nearest neighbor distance “a” is
(h/2
π
) ω k
= g
µ
B
H
0
+ 2 J S a 2 k 2 .
This is the magnon dispersion relation where E ~ k 2 like that of free electrons.
34

Ferromagnetism
35

Ferromagnetism
36

Ferromagnetism
37

Ferromagnetism
38

Ferromagnetism
39

Ferromagnetism
40

Ferromagnetism
41

Ferromagnetism
42

Ferromagnetism
43

Ferromagnetism
44

Ferromagnetism
D( ω ) d ω = g(k) d 3 k
45

Ferromagnetism
46

Ferromagnetism
47

Ferromagnetism
48

Ferromagnetism
49

Ferromagnetism
50

Using the spin-wave dispersion relation for a cubic system
(h/2
π
)
ω k
= g
µ
B
H
0
+ 2Jsa 2 k 2 , one calculates the number of magnons, n in thermal equilibrium at temperature T to be proportional to T 3/2 and hence
M(T) = M(0) [1+ A
Z
{3/2, T g
/T} T 3/2 + B
Z
{5/2,T g
/T}T 5/2 ].
This is the famous Bloch’s T 3/2 .
Spontaneous magnetization,
M
S of a ferromagnet as a
Similarly specific heat contribution of magnons is also function of reduced temperature T/T
C
.
~ T 3/2 .
Bloch’s T 3/2 law holds only for T/T
C
<< 1.
Both are in very good agreement with experiments.
51

Ferromagnetism
Hysteresis curve M(H)
M
S
M r
H
C
Permanent magnets:
Ferrites: low cost, classical industrial needs,
H
C
= 0.4 T.
NdFeB: miniaturization, actuators for read/ write heads, H
C
= 1.5 T, very high energy product = 300 kJ/m 3 .
AlNiCo & SmCo: Higher price/energy, used if irreplaceable.
52

Ferromagnetism
Soft magnetic materials
Used in channeling magnetic induction flux: Large M
S
& initial permeability µ
Minimum energy loss (area of M-H loop).
Examples: Fe-Si alloys for transformers, FeB amorphous alloys for tiny transformer cores, Ferrites for high frequency & low power applications, YIG in microwave range, Permalloys in AMR & many multilayer devices.
Stainless steel (non-magnetic ???): Very interesting magnetic phases from competing FM/AFM interactions.
53

The localized-moment theory breaks down in two aspects. The magnetic moment on each atom or ion should be the same in both the ferromagnetic and paramagnetic phases & its value should correspond to an integral number of
µ
B.
None are observed experimentally. Hence the need of a band theory or collective-electron theory. In Fe, Ni, and Co, the Fermi energy lies in a region of overlapping 3d and 4s bands as shown in Fig. 1. The rigid-band model assumes that the structures of the 3d and 4s bands do not change markedly across the 3d series and so any differences in electronic structure are caused entirely by changes in the Fermi energy (E
F
). This is supported by detailed band structure calculations.
Fig. 1: Schematic 3d and 4s densities of states in transition metals. The positions of the Fermi levels (E
F
) in
Zn, Cu, Ni, Co, Fe, and Mn are shown.
54

The exchange interaction shifts the energy of the 3d band for electrons with one spin direction relative to that with opposite spin. If E
F lies within the 3d band, then the displacement will lead to more electrons of the lower-energy spin direction and hence a spontaneous magnetic moment in the ground state (Fig. 2). The exchange splitting is negligible for the
4s electrons, but significant for 3d electrons.
In Ni, the displacement is so strong that one 3d sub-band is filled with 5 electrons, and the other contains all 0.54 holes. So the saturation magnetization of Ni is M
S
= 0.54N
µ
B
, where N is the total number of Ni atoms.
⇒ the magnetic moments of the 3d metals are not integral number of
µµµµ
B
!
In Cu, E
F lies above the 3d band. Since both the 3d subbands are filled, and the 4s band has no exchange-splitting, the numbers of up- and down-spin electrons are equal. Hence no magnetic moment.
Fig. 2: Schematic 3d and 4s upand down-spin densities of states in a transition metal with exchange interaction included.
In Zn, both the 3d and 4s bands are filled and hence no magnetic moment.
In lighter transition metals, Mn, Cr, etc., the exchange interaction is much weaker & ferromagnetism is not observed.
55

M
S
= 2.2
µµµµ
B M
S
= 1.7
µµµµ
B
M
S
= 0.6
µµµµ
B
M
S
= 0.0
µµµµ
B
56

The collective-electron and rigid-band models are further supported by the well-known plot known as the Slater-Pauling curve. They calculated the saturation magnetization as a continuous function of the number of 3d and 4s valence electrons per atom across the 3d series. They used the rigid-band model, and obtained a linear increase in saturation magnetization from Cr to Fe, then a linear decrease, reaching zero magnetization at an electron density between Ni and Cu. Their calculated values agree well with those measured for Fe, Co, and Ni, as well as Fe-Co, Co-Ni, and Ni-Cu alloys. The alloys on the right-hand side are strong ferromagnets. The slope of the branch on the right is −1 when the charge difference of the constituent atoms is small, Z ~ < 2. The multiple branches (on left) with slope ≈ 1, as expected for rigid bands, are for alloys of late 3d elements with early 3d elements for which the 3d-states lie well above the Fermi level of the ferromagnetic host & we have to invoke Friedel’s virtual bound states.
The average atomic moment is plotted against the number of valence (3d + 4s) electrons.
57

Magnetism
FERRIMAGNET FERROMAGNET ANTIFERROMAGNET
58

Spin glass t = 0 t = ∞
To distinguish between an antiferromagnet, a paramagnet and a spin glass.
Neutron scattering gives m = 0 but Q
≠≠≠≠
0 for T < T g
.
59

R & D in Magnetic Materials after 1973
•
The research on magnetism till late 20th century focused on its basic understanding and also on applications as soft and hard magnetic materials.
•
In 1980’s it was observed that surface and interface properties deviate considerably from those of the bulk. With the advent of novel techniques e-beam evaporation
Ion-beam sputtering
Magnetron sputtering
Molecular beam epitaxy (MBE) excellent magnetic thin films could be prepared with tailor-made properties which can not be obtained from bulk materials alone. Magnetic recording industry was also going for miniaturization and thin film technology fitted their requirement.
•
Then came, for the first time, the industrial application of electronic properties which depend on the spin of the electrons giving rise to the so-called Giant
Magnetoresistance(GMR), discovered in 1986.
•
Since early 80’s the Anisotropic Magnetoresistance effect (AMR) has been used in a variety of applications, especially in read heads in magnetic tapes and later on in hard-disk systems, gradually replacing the older inductive thin film heads. Now,
GMR recording heads offer a stiff competition to those using AMR effects.
60

Magnetoresistance
2
In low field LMR is positive and TMR is negative.
Negative MR after saturation due to quenching of spin-waves by magnetic field.
FAR is an inherent property of FM materials originating from spin-orbit interaction of conduction electrons with localized spins.
FAR(ferromagnetic anisotropy of resistivity)
= (
∆ρ
//s
-
∆ ρ
⊥ s
)/
ρ
//s
61

Fe-Cr is a lattice matched pair : Exchange coupling of ferromagnetic Fe layers through Cr spacers gives rise to a negative giant magnetoresistance
(GMR) with the application of a magnetic field.
• RKKY interaction ~ cos(2k f r)/(2k f r) 3 .
• Established by experiments on light scattering by spin waves.
[ P. Grünberg et al., Phys. Rev. Lett. 57, 2442(1986).]
At low fields the interlayer antiferromgnetic coupling causes the spins in adjacent layers to be antiparallel and the resistance is high
At high fields the spins align with the field (saturating at H sat
) and the resistance is reduced.
Magnetoresistance is negative!
62

Giant Magnetoresistance (GMR)
Giant Magnetoresistance (GMR)
Magnetoresistance is defined by
MR
=
ρ
( H , T
ρ
)
−
( 0 , T
ρ
)
( 0 , T )
×
100 % .
(1)
Fe-Cr
[ Magnetic Multilayers and Giant Magnetoresistance, Ed. by Uwe Hartmann,
Springer Series in Surface Sciences, Vol. 37, Berlin (1999).] 63

GMR in Fe-Cr multilayers
Cr ( 50-t )Å
30 bi-layers of Fe/Cr Cr(t Å)
Fe(20 Å) bi-layer
Si/Cr(50Å)/[Fe(20Å)/Cr(tÅ)]
×
30/Cr((50-t)Å)
Varying Cr thickness t = 6, 8, 10, 12, and 14 Å
Fe/Cr multilayers prepared by ion-beam sputtering technique .
Ar and Xe ions were used.
Beam current 20 mA /30 mA and energy 900eV/1100eV.
64

GMR in Fe-Cr multilayers
GMR vs. H for 2 samples.
Negative GMR of 21 % at 10 K and 8 % at 300 K with H sat
Hardly any hysteresis
⇒ strong AF coupling.
≈
13 kOe.
65

Hall Effect
In ferromagnetic metals and alloys
ρ
H
= E y
/ j x
= R
0
B z
+ µ
0
R
S
M , where R
0
= ordinary Hall constant (OHC),
R
S
= extra-ordinary or spontaneous Hall constant (EHC), B = magnetic induction
µ
0
R s
M s
ρ
H and M = magnetization.
R
0
( Ordinary Hall constant ) :
Slope =
R
0
µ
0
M s
B
ρ
H vs. B of a ferromagnet.
In a 2-band model (as in Fe ) consisting of electrons and holes
R
0
= (
σ e
2 / n e e e
+
σ h
2 / n h e h
) / (
σ e
+
σ h
) 2 ,
Eq.(4) w here
σ
= conductivity, ne = charge density, e e
<
0.
Sign of R
0 determines relative conductivity. R
0
>
0 for Fe at 300 K.
Eq.(4) reduces to R
0
= 1/ ne for a single band.
66

S
a) Classical Smit asymmetric scattering(AS).
b) Non-classical transport (side-jump).
a) Classical Smit asymmetric scattering (AS)
[L. Berger, Phys. Rev. 177, 790(1969).]
Boltzmann Eq. is correct to the lowest order in h
E
F
( « 1 ) (
τ
= relaxation time), true for pure metals and dilute alloys at low temperatures.
R
S caused by AS of electrons by impurities in the presence of spin-orbit interaction in a ferromagnet .
Boltzmann Eq. d p
= dt r en E
+ r j
× r
B
−

 m e


[ ] j r
, where [s] = relaxation frequency tensor ~ impurity concentration.
Off-diagonal elements describe AS proportional to M.
Diagonal elements give Ohmic
ρ
.
Boundary condition: j y
= 0
⇒
ρ
H and ρ .
R
S
= a ρ .
67

b) Non-classical transport (side-jump mechanism) h
τ
E
F is not small
⇒
Concentrated and disordered alloys, high temperatures.
[R. Karplus and J. M. Luttinger, Phys. Rev. 95, 1154 (1954).]
Calculation: Free electron plane wave (e i kx ) is scattered by a short-range square well impurity potential
H
= h
2
−
2 m
∇
2
+
V ( r )
+
1
2 m
2 c
2


1 r
∂
V
∂ r


L
Z
S
Z with V(r) = 0 for r > R & V(r) = V
0 for r < R. Using Born approx. one finds a side-wise displacement of the wave-packet ∆ y ~ 0.1-0.2 nm (side-jump).
Transport theory for arbitrary
ω c
τ
= e B
τ
/ m,
ω c
= cyclotron frequency gives
ρ
H
= R
0
B z
+ µ
0
R
S
M
, where R
S
= b
ρ
2 .
Combining Eqs.(3) & (4) for the most general case one gets
R
S
= a
ρ
+ b
ρ
2 .
[L. Berger and G. Bergmann , in the Hall effect and its applications, edited by C. L.
Chien and C. R. Westgate (Plenum, New York , 1980), p.55 and references therein.]
68

Hall Effect t » 0 t » 0
δ y t = 0
Y t = 0
Y
t « 0 t « 0 a) b)
Skew scattering Side jump scattering
[L. Berger and G. Bergmann , in the Hall effect and its applications, edited by C. L. Chien and C. R. Westgate (Plenum, New York , 1980), p.55 and references therein.]
69

All the theories discussed earlier are valid for homogeneous ferromagnets.
Scaling law is valid only in the local limit; the mean free path λ « d, the layer thickness. It is invalid in the long mean free path limit; λ » d.
Zhang had shown the failure of scaling law in composite magneticnonmagnetic systems:
The standard boundary condition used for calculating ρ
H
, j y
(z) = 0, is not valid here for all z, but j y
(z), integrated over z, is zero.
The two-point local Hall conductivity is given by
σ yx
(z, z')
∝
(m λ
SO
M
Z
σ
CIP
(z, z'))/ τ (z), (3) where σ
CIP
(z, z') is the CIP two-point Ohmic conductivity.
To get σ yx
, integrate σ yx
(z, z') over z and z' and sum over the spin variables.
[S. Zhang , Phys. Rev. B 51 , 3632(1995).]
70

For homogeneous magnetic materials , σ
CIP and ρ yx
(z, z'), is proportional to τ (z) is simply proportional to the square of the ordinary resistivity ρ 2 .
For inhomogeneous magnetic systems:
σ yx is found in terms of average relaxation times ( of the components
τ ) and thickness (t)
σ yx
= λ
SO
M
Z
A t m
∑ s
(t m
+ t nm
τ m s / τ nm
) -1 . (4)
Thus Hall conductivity depends on the ratio of relaxation times &
ρ xy
~ ρ 2 is no more valid. Experiments give ‘n’ between ~ 1 to ~ 4.
Finally, Zhang had shown that ‘n’ could be < 2, > 2, or = 2 if MFP of only the magnetic layer or the non-magnetic layer or both but in
a fixed ratio, is temperature dependent.
71

Q1
An interplay between magnetic state and electrical transport
GMR !
Both magnetism & electrical transport are involved . Hall effect !
Q2.
72

H
R
5
73

9.0
7.5
6.0
4.5
3.0
1.5
F8 (30L)
Peak position
T = 300K
T = 4.2K
0.0
0 1 2 3 4
Field (tesla)
5 6 7
Hall resistivity (
ρ
H
) vs. applied magnetic field (
In Hall geometry B =
µ
0
[H applied
µ
0
H applied
).
+ (1-N) M], N=demagnetization factor.
Q 3
74

Magnetoresistance
0.00
-0.08
-0.16
F8 (30L)
T = 300K
-0.24
-0.32
0 1 2 3 4 5
T = 4.2K
6 7
Field (tesla)
MR ratio vs. applied magnetic field (
µ
0
H applied
).
75

Plots of R s
M, M and
ρ against applied magnetic field (
µ
0
H applied
All show saturation around 3 tesla.
).
76

1.5
1.2
0.9
F10(10L)
R s
~
ρ n n = 3.07 ± 0.04
0.6
0.3
F10(30L) n = 3.45 ± 0.11
Plot of ln R
0.0
5.6
s
5.8
6.0
6.2
ln
ρ
(at B = 0) vs. ln
ρ
(
ρ being the resistivity at B = 0).
The exponent is larger for samples having higher GMR !!!
Fits to R
S
= a
ρ
+ b
ρ n with a = 0 yield n much larger than 2 as found earlier in
Fe-Cr, Co-Cu, and Co-Ag films. [Y. Aoki et al., J. Magn. Magn. Mater. 126 , 448
(1993); T. Lucin ´ ski et al., J. Magn. Magn. Mater. 160 , 347 (1996); V. Korenivski et al., Phys. Rev. B 53 , R 11 938 (1996).]
Q 4.
77

Homogeneous ferromagnets like Bulk Fe
Inhomogeneous ferromagnets like Fe/Cr (GMR)
R s and Ohmic resistivity (
ρ
) have hardly any field dependence
Only Fe causes AHE, not Cr
Both R s
( and Ohmic resistivity
ρ
) are field dependent
In ln R s vs. ln
ρ plot, it does not matter whether one uses the resistivity(
ρ
) in zero
The antiferromagnetic coupling plays a crucial role in making both Extraordinary field or saturation field (the scaling law
Hall effect & GMR field dependent. remains unaltered).
↓ ↓ ↓
Since Rs should not have any field dependence (Eq. (2)), one could argue that the above field dependence is mainly due to the reduction in the current density through the FM layers while B-field switches the system gradually from antiparallel to parallel state.
Extraordinary Hall voltage at a given field depends on the current density passing through the FM layers.
78
GMR systems and homogeneous bulk systems should not be treated on equal footings !!!

1.6
1.2
0.8
0.4
0.0
Master plot for (Fe/Cr)
10
R s
~
ρ n n = 1.95 ± 0.03
5.4
Plot of ln R s
(B) vs. ln
ρ
(B).
2.0
1.5
Master plot for (Fe/Cr)
30
R s
~ ρ n n = 2.05 ± 0.03
F8
F10
F12
F14
Scaling exponent n =
1.95 for 10 bi-layer &
2.05 for 30 bi-layer series. Each plot has
4 samples x 4 fields x
12 temperatures =
192 points.
1.0
0.5
0.0
5.6
ln
ρ
5.8
6.0
5.4
5.7
ln
ρ
A realization of the charge confinement in a Quantum Well
6.0
F8
F10
F12
F14
6.3
Spin-polarized quantum well states are formed in the non-magnetic Cr spacer layer due to multiple reflections of electrons from the interfaces of adjacent magnetic Fe layers. They mediate the exchange coupling between the FM layers.
Majority electrons get gradually confined to the Cr spacer layer as H increases. This reduces the current in the Fe layers which, in turn, causes the decrease of both R s
&
ρ
.
79

Giant Magnetoresistance (GMR)
1973: Rare earth – transition metal film in magneto-optic recording.
1979: Thin film technology for heads in hard disks (both read and write processes) (IBM).
1991: AMR effect using permalloy films for sensors in HDD by IBM.
1997: GMR sensors in HDD by IBM.
Robotics and automotive sensors (e.g. in car).
Measuring electrical current in cables.
Pressure sensors (GMR in conjunction with magnetostrictive materials), Microphone.
Solid-state Compass systems.
Sensitive detection of magnetic field.
Magnetic recording and detection of landmines.
Currently both GMR and TMR are used for application in sensors and MRAMs
In contrast to AMR, GMR is isotropic & GMR effect is more robust than AMR.
80

Nano-magnetism
Nano-scale magnetism: Magnetic properties like T c
(Curie temperature), M s
(Saturation magnetization), and H c
(Coercive field) change as the size reduces to < 100 nm, due to higher surface /volume ratio.
Electrical transport properties are seriously affected by grain boundary scattering.
(A) (B) (C)
(A) Specific magnetization vs. temperature at 1.2 tesla.
(B) Specific magnetization vs. average diameter of Ni particles.
(C) Coercivity vs. average diameter of Ni particles.
[You-wei Du, Ming-xiang Xu, Jian Wu, Ying-bing Shi, and Huai-xian Lu,
81
J. Appl. Phys. 70, 5903 (1991).]

Nano-magnetism
A good medium must have high M r and H
C
.
In the year 2000 areal storage density of 65 Gbit/sq. inch was obtained in
CoCrPtTa deposited on Cr thin films/Cr
80
Mo
20 alloy.
To achieve still higher density like 400Gbit/sq. inch it is necessary to use patterned medium instead of a continuous one. This is an assembly of nano-scale magnetically independent dots, each dot representing one bit of information.
Some techniques are self-organization of nano-particles, nano-imprints, or local ion irradiation.
Problem of reducing magnetic particle size is the so-called
“Superparamagnetic limit”.
82

Superparamagnetism
There are several types of anisotropies, the most common is the
“magnetocrystalline” anisotropy caused by the spin-orbit interaction in a ferromagnet. It is easier to magnetize along certain crystallographic directions. This energy term is direction dependent although its magnitude is much less than that of the exchange term. It only dictates the direction of M.
Thus the axis of quantization z is always in a direction for which the anisotropy energy is minimum.
The energy of a ferromagnetic particle (many atoms) with uniaxial anisotropy constant K
1
(energy/vol.) and magnetic moment
µµµµ making an angle θ with H || z-axis is (1)
83

Superparamagnetism where V is the volume of the particle. Eq. (1) has two minima at θ = 0 and
π whose energies are and an energy barrier in between.
Assuming that M of the particles spend almost all their time along one of the minima, then the no. of particles jumping from min. 1 to min. 2 is a function of the barrier height ε m
– ε
1
, where ε m
= energy of the maximum. To get ε m
Put ∂ε / ∂θ = 0 = sin θ (2K
1
V cos θ +
µ
H)
84

Superparamagnetism sin θ = 0 gives two minima at θ = 0 and
π
.
The other solution gives the maximum.
Substituting cos θ in Eq. (1) one gets where
µµµµ
= V M and |M| =M
S
.
85

Superparamagnetism
The frequency of jump from min. 1 to min.2 is where
Similarly,
Converting to relaxation time for H = 0 gives
& K
1
V = Energy barrier .
Neel’s estimate of f
0 was 10 9 s -1 . Current values are ~ 10 10 s -1 .
86

Superparamagnetism
This table shows prominently the exponential behaviour of τ
(R). For, say, Ni τ increases by 5 orders of magnitude as R changes from 75 to 85 Å.
If τ >> τ exp
, experimental time scale, no change of M could be observed during τ exp
“ Stable ferromagnetism”.
If τ exp
~ 100 s (SQUID), 75 Å Ni particle will not show stable FM
but if the probe is Mossbauer (10 -8 s) it is indeed a FM.
If
τ
<<
τ exp
, M will flip many many times during
τ exp
& average M will be 0.
Thus there is a loss of “ Stable ferromagnetism” since the relaxation time,
τ
, is too small.
87

Paramagnetism
The final form of m along the direction of H comes out to be
< m
>= g
B
SB
S
 g
B
SH k
B
T
 where B
S
(x) is the Brillouin function
2 S
2 S
1 coth 

 ( 2 S
2 S
1 ) x 

1
2 S coth

 x
2 S

 where S is the spin of the ion.
< m > = g
B
[coth x- 1/x] = L(x) = Langevin function when S
.
88

Paramagnetism
S=3/2 FOR Cr3++
1.0
0.8
0.6
0.4
0.2
0.0
0 2 4 6 8
Magnetic Field/Temperature(B/T)
10
In this <m> vs. H/T plot paramagnetic saturation is observed only at very high
H & low T.
At 4 K & 1tesla, <m> ~ 14% of its saturation value.
For ordinary temperature like
300 K & 1 T field, x <<1 and
< m
>→

 g
B
S ( S
3 k
B
T
1 )


H
So <m> varies linearly with field.
89

Superparamagnetism
But in a field H , M will behave as a paramagnet as shown in <m> vs.
H/T plot of last page with easy saturation when all the particles align at a much lower field & higher temperature since S in Brillouin function
(~ SH/T) is now 10 3 - 10 4 . This is “Superparamagnetism”(SPM) ---
“super” meaning “very high” as in “Superconductor”.
Transition from stable FM to SPM shifts to smaller particle size when
T is decreased since τ is a function of V/T. The temperature, T which τ ~ τ exp
, is called the “Blocking temperature”. Above T
B at
B
, SPM with all <m> vs. H/T curves coalesce to one but no hysteresis. Below
T
B
, it is in a “blocked” (FM) state with hysteretic M r and H
C
. T
B moves to lower temperatures at higher fields due to the lowering of barrier height.
A single particle of only several Å dimension cannot be handled easily.
So, experiments are carried out with an ensemble of magnetic particles with a size distribution.
90

Superparamagnetism
Samples are single layers of Ni nanoparticles with non-conducting Al
2
O
3 on both sides deposited on both Si and Sapphire substrates using PLD technique.
M(H,T) measured using Quantum Design MPMS (SQUID magnetometer).
Diamagnetic contribution of substrate subtracted (typically χ = - 2 x 10 -4 emu/tesla).
4.0x10
-6
3.5x10
-6
3.0x10
-6
2.5x10
-6
2.0x10
-6
1.5x10
-6
1.0x10
-6
5.0x10
-7
0.0
0 50 100 150
T (K)
200 250 300
M (T) at H = 200 Oe for 6 nm Ni sample. Langevin/Brillouin function M = (cothx-1/x)well with
µ = 2700 µ
B where x = µ H/k
B
T.
91

Nano-magnetism
Five alternating layers of Ni ( nano dots ) and TiN ( metallic matrix ) were deposited on Si using PLD method:
Base pressure = 5
×
10 -7 Torr
Substrate temperature = 600 ˚C
Energy density and repeatition rate of the laser beam are 2J/cm 2 and 10Hz.
Si
STEM-Z image of Ni nanoparticles embedded in TiN metal matrix.
Chemical stability, hardness, acts as diffusion barrier for both Ni and Si, high electrical conductivity, grows as a buffer layer epitaxially on Si.
D. Kumar, H. Zhou, T. K. Nath, Alex V. Kvit, and J. Narayan, Appl. Phys. Lett. 79, 2817 (2001).
92

Nano-magnetism
Single layer epitaxial Ni nano dots on TiN/Si(100) template
Ni [220] || TiN[002]
Ni[110] || TiN[110]
Ni [002] || TiN[002]
Ni[110] || TiN[110]
Ni separation ≈ 10 nm
Triangular morphology with 17 nm base and
9 nm height
Some have rectangular morphology.
Thickness of TiN = 6
×
32 nm.
Ni particles grow epitaxially on TiN acting as a template which also grows epitaxially on Si.
Ni-nano/TiN is an ideal system for studying electrical transport in magnetic nano particles due to epitaxial growth and conducting nature of TiN .
93

Nano-magnetism
6x10
-10
Hall effect
Pure Ni-bulk
T = 300K
T=290K
5x10
-10
4x10
-10
3x10
-10
2x10
-10
1x10
-10
Hall effect
Ni-Nano/TiN
T=11K
T=25K
0
0 1 2 3 4 5
Field (tesla)
Hall resistivity (
ρ
H
) vs. applied magnetic field (
µ
0
H applied
).
11K
35K
60K
85K
105K
115K
135K
160K
185K
210K
235K
260K
290K
ρ
H is negative at all temperatures from 11 to 290 K.
In Hall geometry B = µ
0
[H applied
+ (1-N) M], N=demagnetization factor.
So, ρ
H
= R o
µ
0
H applied
+ R
S
µ
0
M,
94

Nano-magnetism
1.8
1.2
(10 K)
(100 K)
(300 K)
0.6
0.0
-0.6
-1.2
250 Mr
0.35
200
0.30
150
0.25
100
50
Hc
0 100 200 300
0.20
T (K)
-1.8
-3000 -2000 -1000 0
Field (tesla)
1000 2000 3000
Magnetization vs. applied field at 10, 100 and 300 K.
Inset: Coercivity & remanent magnetization vs. T.
[P. Khatua, T. K. Nath, and A. K. Majumdar, Phys. Rev. B 73 , 064408 (2006).]
95

Exchange-biased Spin-valve structure
Ta
FeMn
Co
Cu
NiFe
Ta
Substrate
}
Antiferromagnetic
Ferromagnetic
FeMn providing local magnetic field to Co layer, and NiFe is magnetically soft.
GMR > 20 %.
Read heads for HDD detect fields ~ 10 Oe.
96

Tunnel Magnetoresistance (TMR)
Discovery of GMR effect in Fe/Cr superlattice in 1986 and giant tunnel magnetoresistance
(TMR) effect at room temperature (RT) in 1995 opened up a new field of science and technology called spintronics. The latter provides better understanding on the physics of spin-dependent transport in heterogeneous systems. Perhaps more significantly, such studies have contributed to new generations of magnetic devices for information storage and magnetic sensors.
A magnetic tunnel junction (MTJ), which consists of a thin insulating layer (a tunnel barrier) sandwiched between two ferromagnetic electrode layers, exhibits TMR due to spindependent electron tunneling. MTJ’s with an amorphous aluminium oxide (Al–O) tunnel barrier have shown magnetoresistance (MR) ratios up to about 70 % at RT and are currently used in magnetoresistive random access memory
(MRAM) and the read heads of hard disk drives. In 2004 MR ratios of about 200 % were obtained in MTJ’s with a singlecrystal MgO(001) barrier. Later CoFeB/MgO/CoFeB MTJs were also developed having MR ratios up to 500 % at RT.
97

Tunnel Magnetoresistance (TMR)
SPIN-POLARIZED TUNNELING
(a) Magnetizations in the two electrodes are aligned parallel (P).
(b) Magnetizations are aligned antiparallel (AP).
D
1
↑ and D
1
↓ ,respectively, denote the density of states at E
F for the majority-spin and minority-spin bands in electrode 1, and D
2
↑ and D
2
↓ are respectively those for electrode 2.
|D
↑
- D
↓
| >> than what is shown for high polarization materials!!!
↓ ↓
D
2 ↓
D
2 ↑
In an MTJ, the resistance of the junction depends on the relative orientation of the magnetization vectors M in the two electrodes.
When the M’s are parallel, tunneling probability is maximum because electrons from those states with a large density of states can tunnel into the same states in the other electrode. When the magnetization vectors are antiparallel, there will be a mismatch between the tunneling states on each side of the junction. This leads to a diminished tunneling probability, hence, a larger resistance.
98

Tunnel Magnetoresistance (TMR)
Assuming no spin-flip scattering, the MR ratio between the two configurations is given by,
(
∆
R/R
P
) = (R
AP
- R
P
)/R
P
= 2P 2 /(1-P 2 ), where R
P
/R
AP is the resistance for the P/AP configuration & P = (D
↑
– D
↓
)/ (D
↑
+ D
↓
) = spin-polarization parameter of the magnetic electrodes. A half-metal corresponds to P = 1.
Gang Mao, Gupta, el al. at Brown & IBM obtained below 200 Oe a
GMR of 250 % in MTJ’s with electrodes made of epitaxial films of doped half-metallic manganite La
0.67
Sr
033
MnO
3
(LSMO) and insulating barrier of SrTiO
3 using self-aligned lithographic process to pattern the junctions to micron size. They confirmed the spin-polarized tunneling as the active mechanism with P ~ 0.75. The low saturation field comes from the fact that manganites are magnetically soft, having a coercive field as small as 10 Oe. They have also made polycrystalline films with a large number of the grain boundaries and observed large MR at low fields.
Here the mechanism has been attributed to the spin-dependent scattering across the grain boundaries that serves to pin the magnetic domain walls.
99

Colossal Magnetoresistance (CMR)
Another class of materials, the rare-earth manganite oxides, La
1-x
D x
MnO
3
(D = Sr, Ca, etc.), show Colossal Magnetoresistance effect
(CMR) with a very large negative MR near their magnetic phase transition temperatures (T
C
) when subjected to a tesla-scale magnetic field.
The double-exchange model of Zener and a strong e-ph interaction from the Jahn-Teller splitting of Mn d levels explain most of their magnetotransport properties.
Though fundamentally interesting, the CMR effect, achieved only at large fields and below
300 K, poses severe technological challenges to potential applications in magnetoelectronic devices, where low field sensitivity is crucial.
100

Colossal Magnetoresistance (CMR)
101

Colossal Magnetoresistance (CMR) e g t
2g
The origin of CMR stems from the strong interplay among the electronic structure, e g magnetism, and lattice dynamics in manganites.
Doping of divalent Ca or Sr impurities into t
2g trivalent La sites create mixed valence states of
Mn 3+ (fraction: l-x) and Mn 4+ (fraction: x).
Mn 4+ (3d 3 ) has a localized spin of S = 3/2 from the low-lying t
2g
3 orbitals, whereas the e g obitals are empty. Mn 3+ (3d 4 ) has an extra electron in the e g orbital, which can hop into the neighboring Mn 4+ sites (Double
Exchange). The spin of this conducting electron is aligned with the local spin
(S = 3/2) in the t
2g
3 orbitals of Mn 3+ due to strong Hund's coupling. When the manganite becomes ferromagnetic, the electrons in the e g orbitals are fully spin-polarized. The band structure is such that all the conduction electrons are in the majority band. This kind of metal with empty minority band is generally called a half-metal and so manganites have naturally become a good candidate for the study of spin-polarized transport.
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Colossal Magnetoresistance (CMR)
Superexchange favors antiferromagnetism
Zener (1951) offered an explanation that remains at the core of our understanding of magnetic oxides. In doped manganese oxides, the two configurations
ψ
1
: Mn 3+ O
2
-2 Mn 4+ and
ψ
2
: Mn 4+ O
2
-2 Mn 3+ are degenerate and are connected by the so-called double-exchange matrix element. This matrix element arises via the transfer of an electron from Mn with transfer from O
2
-2 to Mn 4+ . The degeneracy of
ψ
1 and
ψ
3+
2 to the central O
2
-2 simultaneous
, a consequence of the two valencies of the Mn ions, makes this process fundamentally different from the above conventional superexchange. Because of strong Hund’s coupling, the transfer-matrix element has finite value only when the core spins of the Mn ions are aligned ferromagnetically. The coupling of degenerate states lifts the degeneracy, and the system resonates between
ψ
1 and
ψ
2 if the core spins are parallel, leading to a ferromagnetic, conducting ground state. The splitting of the degenerate levels is k
B
T
C conductivity to be s‘ x e 2 a h T
C and, using classical arguments, predicts the electrical
/T where a is the Mn-Mn distance and x, the Mn 4+ fraction.
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