Colossal Magnetoresistance of Mex

Mn1-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

Contents

IntroductionColossal Magnetoresistance (CMR)

LaMnO3

MexMn1-xS (Me = Fe, Cr)

Motivation

Experimental method

Results and Discussion

Summary

Colossal Magnetoresistance (CMR)

introduction

There are 2 types of CMR.

1. On applied external magnetic field for sample, electrical resistance noticeably increase.

2. On applied external magnetic field for sample and doped hole, electrical resistance noticeably decrease and phase change from paramagnetic to ferromagnetic.

At hard disk, Fringing field (漏れ磁場 ) is very low, so disk head must be magnetic-field-sensitive.

At this paper, they focus on this CMR.

LaMnO3

introduction

Crystal structure

Perovskite structure

electron state

3d4 (Mn3+)

energy state of 3d electron of Mn3+

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

eg electrons are strongly hybridized with the oxygen 2p states, so eg electrons are conduction electron.

Manganese oxides with the cubic perovskite structure.

LaMnO3

introduction

Mott Insulator ： Spins are alternately up and down. When coulomb repulsion for two electrons in a Mn is large, Electrons cannot move freely.

LaMnO3 is Mott insulator.

Antiferromagnetic insulator

Carriers are doped in eg orbital.

Substitute divalent(2 価 ) cation (Sr2+, Ca2+ etc.) for trivalent(3 価 ) cation (La3+)Electrical conduction is

provided.For example, La1-xSrxMnO3

doped cation Mn3+ and Mn4+ are present by doping cation.

La1-xSrxMnO3

introduction

Spins are paralleled in different orbital by Hund’s rule.

(a) t2g electrons (localized spins) are paralleled :

Double-exchange interaction

eg electron can transfer to Mn4+, because energy is not changed if any eg electron is exist which site.

(b) t2g electrons (localized spins) are antiparalleled : eg electron cannot transfer to Mn4+, because to transfer results in a loss of energy of Hund’s rule.

Paralleled t2g electrons work ferromagnetic interaction.

This is double-exchange interaction.

La1-xSrxMnO3

introduction

External magnetic field aligns the t2g spins and reduces the carrier scattering by the local spins.

T dependence of resistivity for single crystal of (LaxSr1-x)3Mn2O7. The resistivity with the current parallel aband perpendicular (c).

t2g spins are leant.

The formula of transfer integral (t) is this.

The critical ferromagnetic transition temperature

Tc :

※A steep rise of the magnetization observed around 130 K.

ferromagnetic phase is stabilized for high temperature.

MexMn1-xS (Me = Fe, Cr)introduction

Manganese sulfides with the rock salt structure.

Mn

S

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

Crystal structure

Rock salt structure

The features of MexMn1-xS

① O → S :

② NaCl structure :

It increases electron orbital overlap.

Rock salt structure is core of Perovskite structure.

MnS (not doping) is Mott insulator.

(NaCl structure)

Motivationintroduction

To observe Colossal magnetoresistance of non-perovskite MexMn1-xS (Me = Fe, Cr)

To observe magnetic order of MexMn1-xS (Me = Fe, Cr)

Experimental method

sample : MexMn1-xS (Me = Fe, Cr) ← polycrystalx of Fe is 0.29. x of Cr is 0.5

measurement

・ Electrical resistance measurement :

Temperature range 4.2~300 K

Magnetic field H = 0, 2, 10, 30, and 50 kOe

・Magnetic properties : measured on vibrating-coil magnetometer with superconducting solenoid at 30 kOe

Temperature range 77~300 K

・ X-ray structural analysis :

monochromatic Cu Kradiation

Temperature range 100~300 K

※10 kOe = 1 T

Temperature-dependent (a) lattice parameter, (b) magnetization, and (c) resistivity of Fe0.29Mn0.71S

According to X-ray data, the Fe0.29Mn0.71S undergoes a structural transition at Ts ~ 147 K.

The lattice parameter is independent of temperature at 120~140 K.

Lattice distortion is occurred.

The conduction in the samples is of the semiconductor type with thermal hysteresis.

Result Fe0.29Mn0.71S

Similar transition is observed in MnS.

Rhombohedral distortion

Result Fe0.29Mn0.71S

Temperature curves for the magnetoresistance H of Fe0.29Mn0.71S at 10, 30, and 50 kOe.

At 30 kOe, the negative magnetoresistance H reaches -450 % at ~50 K.

At 50 kOe, this value is -87 %.

It’s found that FexMn1-xS have a colossal magnetoresistance .

Result Cr0.5Mn0.5S

Temperature curves for the magnetization (a) and magnetoresistance (b) of Cr0.5Mn0.5

S at 30 kOe.

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

At 30 kOe, the negative magnetoresistance H reaches -25 % at ~4.2 K.

The transition to the negative magnetoresistance occurs in the range of magnetic transition.

It’s same behavior of La1-xSrxMnO3

It’s found that CrxMn1-xS have a colossal magnetoresistance .

The nature is caused by Jahn-Teller effect.

Discussion

Electrical and magnetic properties of MexMn1-xS are similar to those observed for La1-xSrxMnO3.

analogous point

① the cubic lattice is distorted in the range of appeared CMR.

② LaMnO3 and MnS (not doping) are Mott insulator.

③ AF semiconductor-FM metal are produced in cation-substituted LaMnO3 ( or MnS).

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

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

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

Summary

Colossal magnetoresistance is observed in non-perovskite MexMn1-xS (Me = Fe, Cr).

This MexMn1-xS sample has little data. So, this sample must be given results and should be understood the magnetism mechanisms.

My study MexMn1-xS

The less interspace of octahedral structure, the more increase electron transfer. MexMn1-xS (Me = Fe, Cr) may appear metallic state. One way is ion substitution.

Other way ?

apply hydrostatic pressure

For FexMn1-xS, x = 0.29 is critical concentration of metal transition.

FexMn1-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 in MexMn1-xS (Me = Fe, Cr) by pressure.