Colossal Magnetoresistance of Me x Mn 1-x S (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 Introduction Colossal Magnetoresistance (CMR) LaMnO 3 Me x Mn 1-x S (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.

LaMnO 3 introduction  Crystal structure Perovskite structure  electron state 3d 4 (Mn 3+ ) energy state of 3d electron of Mn 3+ t 2g electrons can be viewed as localized spins with S=3/2. e g electrons are strongly hybridized with the oxygen 2p states, so e g electrons are conduction electron. Manganese oxides with the cubic perovskite structure.

LaMnO 3 introduction Mott Insulator ： Spins are alternately up and down. When coulomb repulsion for two electrons in 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 価 ) cation (Sr 2+, Ca 2+ etc.) for trivalent (3 価 ) cation (La 3+ ) Electrical conduction is provided. For example, La 1-x Sr x MnO 3 doped cation Mn 3+ and Mn 4+ are present by doping cation.

La 1-x Sr x MnO 3 introduction Spins are paralleled in different orbital by Hund’s rule. (a) t 2g electrons (localized spins) are paralleled :  Double-exchange interaction e g electron can transfer to Mn 4+, because energy is not changed if any e g electron is exist which site. (b) t 2g electrons (localized spins) are antiparalleled :e g electron cannot transfer to Mn 4+, because to transfer results in a loss of energy of Hund’s rule. Paralleled t 2g electrons work ferromagnetic interaction. This is double-exchange interaction.

La 1-x Sr x MnO 3 introduction External magnetic field aligns the t 2g spins and reduces the carrier scattering by the local spins. T dependence of resistivity for single crystal of (La x Sr 1-x ) 3 Mn 2 O 7. The resistivity with the current parallel  ab  and perpendicular (  c ). t 2g spins are leant. The formula of transfer integral (t) is this. The critical ferromagnetic transition temperature T c : ※ A steep rise of the magnetization observed around 130 K. ferromagnetic phase is stabilized for high temperature.

Me x Mn 1-x S (Me = Fe, Cr) introduction Manganese sulfides with the rock salt structure. Mn S Mn 2+ ions are in the octahedral position s of the sulfur cubic lattice.  Crystal structure Rock salt structure The features of Me x Mn 1-x S 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)

Motivation introduction To observe Colossal magnetoresistance of non-perovskite Me x Mn 1-x S (Me = Fe, Cr) To observe magnetic order of Me x Mn 1-x S (Me = Fe, Cr)

Experimental method sample : Me x Mn 1-x S (Me = Fe, Cr) ← polycrystal x of Fe is 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 K  radiation Temperature range 100~300 K ※ 10 kOe = 1 T

Temperature-dependent (a) lattice parameter, (b) magnetization, and (c) resistivity of Fe 0.29 Mn 0.71 S According to X-ray data, the Fe 0.29 Mn 0.71 S undergoes a structural transition at T s ~ 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 Fe 0.29 Mn 0.71 S Similar transition is observed in MnS. Rhombohedral distortion

Result Fe 0.29 Mn 0.71 S Temperature curves for the magnetoresistance  H of Fe 0.29 Mn 0.71 S 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 Fe x Mn 1-x S have a colossal magnetoresistance.

Result Cr 0.5 Mn 0.5 S Temperature curves for the magnetization (a) and magnetoresistance (b) of Cr 0.5 Mn 0.5 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 La 1-x Sr x MnO 3 It’s found that Cr x Mn 1-x S have a colossal magnetoresistance. The nature is caused by Jahn- Teller effect.

Discussion Electrical and magnetic properties of Me x Mn 1-x S are similar to those observed for La 1-x Sr x MnO 3.  analogous point ① the cubic lattice is distorted in the range of appeared CMR. ② LaMnO 3 and MnS (not doping) are Mott insulator. ③ 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 Mössbauer data) G.V. Loseva et al., Phys, Solid States 25, 2142 (1983)

Summary Colossal magnetoresistance is observed in non-perovskite Me x Mn 1-x S (Me = Fe, Cr). This Me x Mn 1-x S sample has little data. So, this sample must be given results and should be understood the magnetism mechanisms.

My study Me x Mn 1-x S The less interspace of octahedral structure, the more increase electron transfer. Me x Mn 1-x S (Me = Fe, Cr) may appear metallic state. One way is ion substitution. Other way ? apply hydrostatic pressure For Fe x Mn 1-x S, x = 0.29 is critical concentration of metal transition. Fe x Mn 1-x S 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 Me x Mn 1-x S (Me = Fe, Cr) by pressure.