Magnetic, Transport and electron magnetic resonance studies of nanomanganite Nd 0.67 Sr 0.33 MnO 3 S . S . Rao Department of Physics, Indian Institute of Science, Bangalore, India. ssrao@physics.iisc.ernet.in
Introduction to Rare Earth Manganites A 3+ 1-x B 2+ x Mn 3+ 1-x Mn 4+ x O 2- General Formula : 3 A : Rare earth Ion La 3+ ,Pr 3+ ,Nd 3+ Ca 2+ ,Pb 2+ ,Sr 2+ B : Divalent Ion Phenomena Exhibited by the Manganites • Colossal Magnetoresistance (CMR) • MI transition concurrent with FM-PM transition • Charge Ordering , Orbital Ordering • Phase Separation Department of Physics,Indian Institute of Science,Bangalore.
Perovskite Structure A or B : Body Centre (purple) Mn : Corners (gray) O : Midpoints of the edges (green & blue) Department of Physics,Indian Institute of Science,Bangalore.
Electronic Configuration Mn 3+ Mn 4+ JT Splitting e g e g 1.5 eV d 4 d 3 Δ = 2 eV Hund ’ s coupling 3 eV t 2g t 2g Hole Doping : Doping of Divalent ion in AMnO 3 introduces Mn 4+ Electron Doping : Doping of trivalent ion in BMnO 3 introduces Mn 3+ Department of Physics,Indian Institute of Science,Bangalore.
Nanomanganites - Properties-Importance • Magnetic recording, magnetic data storage and magnetic field sensors etc … • Tuning of intrinsic colossal magneto resistance (CMR) with the particle size leads to intergranular magneto resistance (IMR) which is due to the spin polarized tunneling between the neighbouring grains. IMR can be increased by decreasing the grain size. • Reduction of saturation magnetization with the particle size due to the enhancement of outer layer (shell) thickness as the particle size decreases. • In nano range, each grain consists of core and shell. Core exhibits the properties similar to the bulk and the outer shell consists of oxygen faults, vacancies and dangling bonds. Department of Physics,Indian Institute of Science,Bangalore.
• Magnetic Calorific Effect (MCE) reduces with the surface to volume ratio. Core shows the first order magnetic phase transition and the shell shows the second order. The nano crystal exhibits the second order phase transition by hiding the intrinsic behaviour. • Exhibiting the superparamagnetic behaviour, surface spin glass behaviour, large coerceivities and improved low field magneto resistance (LFMR) as compared to their corresponding bulk values. • Tuning of magnetic phase transitions with the particle size. • Increase in resistivity with the decrease of particle size. Department of Physics,Indian Institute of Science,Bangalore.
Motivation • The above mentioned properties are addressed only for the limited number of nanomanganite systems (LCMO, LSMO) and are not studied in other systems. The transport and magnetic properties of this system (NSMO) are studied for the first time in our report. • There are very few EMR reports on nanomanganite systems which gives the information about the interaction mechanisms, spin-orbit couplings, nanoscopic phase separations and magnetic phase transitions. Department of Physics,Indian Institute of Science,Bangalore.
Nd 1-x Sr x MnO 3 phase diagram Department of Physics,Indian Institute of Science,Bangalore.
Experimental details: • Sample preparation - Sol-gel method • X-ray diffraction (XRD) to know the phase purity and Transmission electron microscopy (TEM) was used to measure the grain size and it ’ s distribution. • Resistivity measurements were done both in the presence (7T) and in the absence of magnetic field down to liquid nitrogen temperature from room temperature to study the transport properties. • AC susceptibility measurements were performed from room temperature down to 77K to study the magnetic phase transitions. • Electron Magnetic Resonance measurements were performed from 10K to 300K to study the spin dynamics. Department of Physics,Indian Institute of Science,Bangalore.
Results: N S M O -1 1 NSMO-8 Intensity Mean grain size 20nm N S M O -8 0.2 µm 0.5 µm 0 2 0 4 0 6 0 8 0 1 0 0 T w o T h e ta (in d e g re e s ) NSMO-11 F ig 1 Mean grain XRD micrograph size 35nm Unit cell: orthorhombic, a = 5.45 A o , b = 5.43 A o , c = 7.71 A o, , space group is P B NM. TEM micrographs Bulk values: a = 5.46 A o , b = 5.45 A o , c = 7.73 A o Department of Physics,Indian Institute of Science,Bangalore.
Effect of grain size: 30 • With the sintering temperature, 260 28 grain size increases. 26 250 T P 24 • With the decrease in grain size, <S>nm T C 240 22 T(K) T c (ferromagnetic-paramagnetic 20 230 transition temperature) 18 <S> increases. 16 220 14 • As the grain size decreases, T p 800 850 900 950 1000 1050 1100 o C) Sintering Temperature( (metal-insulator transition Fig 5 temperature) decreases. Department of Physics,Indian Institute of Science,Bangalore.
Experimental data of NSMO material C T S a m p le C o m p o sitio n al S in terin g S M R % T T T(T P ) P C o C c o d e F o rm u la T e m p .( ) (n m ) --------------------------------------- (D eg reeK elv in) N S M O -8 N d 7 S r 3 M n O 8 0 0 2 1 5 2 6 0 4 5 1 5 4 5 0 .6 0 .3 3 N S M O -9 N d 7 S r 3 M n O 9 0 0 2 2 5 2 5 8 3 3 2 0 4 7 0 .6 0 .3 3 N S M O -1 0 N d 7 S r 3 M n O 1 0 0 0 2 4 0 2 5 3 1 3 2 5 4 4 0 .6 0 .3 3 N S M O -1 1 N d 7 S r 3 M n O 1 1 0 0 2 4 5 2 4 9 4 3 0 4 5 0 .6 0 .3 3 . Department of Physics,Indian Institute of Science,Bangalore.
T (T C Sample Compo Sintering T P (in T C (in Crystal MR% code sitional temperat Kelvin) Kelvi lite size - T P ) in formul ure (in n) S (nm) Kelvin o C) a NSMO8 Nd 0.67 Sr 800 215 268 45 15 45 0.33 MnO 3 NSMO9 Nd 0.67 Sr 900 225 258 33 20 47 0.33 MnO 3 NSMO Nd 0.67 Sr 1000 240 253 13 25 44 10 0.33 MnO 3 NSMO- Nd 0.67 Sr 1100 245 249 4 30 45 11 0.33 MnO 3
Electrical transport and MagnetoResistance (MR) • In high magnetic fields 40 0T resistivity decreases drastically N SM O -8 1T 35 3T at ferromagnetic to 5T 30 7T paramagnetic transition ( cm) 25 temperature (T C ). 20 • MR = ρ (H) - ρ (O) / ρ (O) 15 10 100 125 150 175 200 225 250 275 300 T(K) Fig3 Department of Physics,Indian Institute of Science,Bangalore.
Ferromagnetic Metallic region T(K) T(K) 90 120 150 180 210 240 270 300 90 120 150 180 210 240 270 300 24 • ρ = ρ 0 + ρ 2* T 2 45 0T NSMO-9 NSMO-8 40 20 7T 0T 7T 35 • ρ = ρ 0 + ρ 2.5 *T 2.5 ( cm) ( cm) 16 30 25 12 20 8 15 • ρ = ρ 0 + ρ 2* T 2 10 4 14 14 + ρ 4.5 *T 4.5 NSMO-10 NSMO-11 12 12 0T 0T 7T 7T 10 10 ( cm) ( cm) 8 8 6 6 4 4 2 90 120 150 180 210 240 270 300 90 120 150 180 210 240 270 300 T(K) T(K) Fig 6 Department of Physics,Indian Institute of Science,Bangalore.
Square of Linear Correlation Coefficient (R 2 ) = 0 + 2 T 2 + 4.5 T 4.5 = 0 + 2 T 2 = 0 + 2.5 T 2.5 Sample code NSMO-8 0.9910 0.9894 0.9993 NSMO-9 0.9977 0.9945 0.9992 NSMO-10 0.9961 0.9946 0.9993 NSMO-11 0.9931 0.9946 0.9993 ρ 0 = grain boundary resistivity ρ 2.5 = resistivity due to electron-electron scattering ρ 4.5 = resistivity due to electron-magnon scattering 2 ( cm K - ) 4.5 ( cm K -4. ) 0 ( cm) Sample Code 0T 7T 0T 7T 0T 7T 10.00 10 - 5.00 10 - 4.18 10 - 2.13 10 - 0 NSMO-8 8.75 4.09 4.40 10 - 2.20 10 - 9.97 10 - 8.39 10 - NSMO-9 5.44 1.81 1.81 10 - 1.80 10 - 2.94 10 - 7.64 10 - 2 NSMO-10 5.09 1.33 1.7 10 - 1.50 10 - 2.08 10 - 5.15 10 - 2 NSMO-11 4.88 0.89 Department of Physics,Indian Institute of Science,Bangalore.
From the above Transport studies in ferromagnetic metallic region, it is known that …… . • Grain boundary resistivity ( ρ 0 ) and the resistivity due to electron-electron scattering ( ρ 2 ) increase with the decrease of particle size and these values are larger than their bulk counterparts - size effect. • Resistivity due to electron-magnon scattering or spinwave scattering ( ρ 4.5 ) also decrease with the increase of particle size which may be due to the partial alignment of spins. • All the three parameters ( ρ 0 , ρ 2 , ρ 4.5 ) found to decrease with the increase of magnetic field attributed to the suppression of scattering mechanisms. Department of Physics,Indian Institute of Science,Bangalore.
Paramagnetic Insulating region Variable Range Hopping (VRH) model: T<Tp< Θ d/2 Mott ’ s Equation for 2.6 (a) VRH model is 2.8 σ = σ 0 exp (-T 0 /T) -1/4 3.0 ln( ) 3.2 σ 0 = pre factor 3.4 T o = 16 α 3 /K B N(E F ) NSMO-8 3.6 N(E F ) = density of states at 0.24 0.25 0.26 0.27 0.28 -1/4 (K -1/4 ) T the fermi level Fig 8 Department of Physics,Indian Institute of Science,Bangalore.
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