colossal magnetoresistance

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Colossal Magnetoresistance ofMe xMn 1-xS (Me = Fe, Cr) Sulfides

G. A. Petrakovskii et al ., JETP Lett. 72 , 70 (2000)

Y. Morimoto et al ., Nature 380 , 141 (1996)

Shimizu-group

HANZAWA Akinori


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Contents

IntroductionColossal Magnetoresistance (CMR)

LaMnO 3

Me xMn1-xS (Me = Fe, Cr)

Motivation

Experimental method

Results and Discussion

Summary


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Colossal Magnetoresistance (CMR)

introduction

There are 2 types of CMR.

1. On applied external magnetic field forsample, electrical resistance noticeably

increase .

2. On applied external magnetic field forsample and doped hole, electricalresistance noticeably decrease andphase change from paramagnetic toferromagnetic.

At hard disk, Fringing field( ) is very low, sodisk head must bemagnetic-field-sensitive.

At this paper, theyfocus on this CMR.


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LaMnO 3

introduction

Crystal structurePerovskite structure

electron state

3d 4 (Mn 3+)

energy state of 3d electron of Mn 3+

t2g electrons can be viewed aslocalized spins with S=3/2.

e g electrons are strongly hybridizedwith the oxygen 2p states, so e g electrons are conduction electron .

Manganese oxides with thecubic perovskite structure.


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LaMnO 3

introduction

Mott Insulator Spins are alternately up anddown. When coulomb repulsion for two electronsin a Mn is large, Electrons cannot move freely.

LaMnO 3 is Mott insulator.

Antiferromagnetic insulator

Carriers are doped in e g orbital.

Substitute divalent (2 ) cation (Sr 2+ , Ca 2+ etc.)for trivalent (3 ) cation (La 3+)Electrical conduction is provided.

For example, La 1-xSr xMnO 3

doped cation Mn3+ and Mn 4+ are presentby doping cation.


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La 1-xSr xMnO 3

introduction

Spins are paralleled indifferent orbital by Hundsrule.

(a) t 2g electrons (localized spins) are paralleled :

Double-exchange interaction

e g electron can transfer to Mn 4+ , becauseenergy is not changed if any e g electron isexist which site.

(b) t 2g electrons (localized spins) are antiparalleled : e g electron cannot transfer to Mn 4+ , becauseto transfer results in a loss of energy ofHunds rule.

Paralleled t 2g electrons workferroma netic interaction.This is double-exchange

interaction.


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Me xMn1-xS (Me = Fe, Cr)

introduction

Manganese sulfides with therock salt structure.

Mn

S

Mn2+ ions are in the octahedral position s of the sulfur cubic lattice.

Crystal structureRock salt structure

The features of Me xMn1-xS

O S :

NaCl structure :

It increases electron orbitaloverlap.

Rock salt structure is coreof Perovskite structure.

MnS (not doping) is Mott insulator.

(NaCl structure)


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Motivationintroduction

To observe Colossal magnetoresistanceof non-perovskite Me xMn1-xS (Me = Fe, Cr)

To observe magnetic order of Me xMn1-xS(Me = Fe, Cr)


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Result Fe 0.29 Mn0.71 S

Temperature curves for themagnetoresistance dH of Fe 0.29 Mn0.71 Sat 10, 30, and 50 kOe.

At 30 kOe, the negativemagnetoresistance dH reaches -450 % at ~50 K.

At 50 kOe, this value is -87 %.

Its found that Fe xMn1-xS have a colossal magnetoresistance .


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Result Cr 0.5 Mn0.5 S

Temperature curves for the magnetization(a) and magnetoresistance (b) ofCr 0.5 Mn0.5 S at 30 kOe.

This sulfide undergoes the antiferromagnet-ferromagnet transition at 66K.

At 30 kOe, the negative

magnetoresistance dH reaches -25 % at ~4.2 K.

The transition to the negativemagnetoresistance occurs in the range ofmagnetic transition.

Its same behavior of La 1-x Sr xMnO 3

Its found that Cr xMn1-xS have a colossal magnetoresistance .

The nature is caused by Jahn-Teller effect.


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Discussion

Electrical and magnetic properties of Me xMn1-xS aresimilar to those observed for La 1-xSr xMnO 3.

analogous point

the cubic lattice is distorted in the range of appeared CMR.

LaMnO 3 and MnS (not doping) are Mott insulator.

AF semiconductor-FM metal are produced in cation-substituted

LaMnO 3 ( or MnS).

But, the mechanism of CMR effect still remains to be clarified.

The regions of antiferromagnetic semiconductor and ferromagnetic metal coexist.(by Mssbauer data)

G.V. Loseva et al ., Phys, Solid States 25, 2142 (1983)


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Summary

Colossal magnetoresistance is observed innon-perovskite Me xMn1-xS (Me = Fe, Cr).

This Me xMn1-xS sample has little data. So,this sample must be given results and shouldbe understood the magnetism mechanisms.


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My study Me xMn1-xS

The less interspace of octahedral structure, the moreincrease electron transfer. Me xMn1-xS (Me = Fe, Cr) mayappear metallic state. One way is ion substitution .

Otherway ?

apply hydrostatic pressure

For Fe xMn1-xS, x = 0.29 is critical concentration of metal transition.

FexMn

1-xS which its concentration is no more than x = 0.29 apply hydrostatic

pressure, the sample appear metallic state.

I anticipate that CMR will be observed inMe xMn1-xS (Me = Fe, Cr) by pressure.


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colossal magnetoresistance

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