Formation of finite antiferromagnetic clusters and the effect of electronic phase separation in Pr0.5Ca0.5Mn0.975Al0.025O3

Real-space imaging of magnetic phase transformation in single crystalline Sm-Ca-Sr based manganite compound

Low-temperature-high-magnetic field magnetic force microscopy studies on colossal magnetoresistance material Sm_0.5Ca_0.25Sr_0.25MnO₃have been carried out. These measurements provide real-space visualization of antiferromagnetic-ferromagnetic (AFM-FM) transition on sub-micron length scale and explain the presence of AFM-FM transition in the temperature-dependent magnetization measurements, but the absence of corresponding metal-insulator transition in temperature-dependent resistivity measurements at the low magnetic field. Distribution of transition temperature over the scanned area indicates towards the quench disorder broadening of the first-order magnetic phase transition. It shows that the length scale of chemical inhomogeneity extends over several micrometers.

Crossover of positive and negative magnetoconductance in composites of nanosilica glass containing dual transition metal oxides

A facile sol-gel approach to prepare composites of nanosilica glass containing dual transition metal oxides with compositions xCoO·(20 - x)NiO·80SiO2 comprising x values 5 (NC-1), 10 (NC-2) and 15 (NC-3) within hexagonal pores of SBA-15 template has been demonstrated. The synergistic effect of dual transition metal oxide ions on MD properties and crossover of positive and negative magnetoconductance phenomena were observed in these nanocomposite systems. The physical origin of magnetoconductance switching is explained based on the factors: nanoconfinement effect, wave-function shrinkage and spin polarized electron hopping. DFT calculations were performed to understand the structural correlation of the nanoconfined system. The static (dc) and dynamic (ac) responses of magnetization revealed the spin-glass behaviour of the investigated samples. Both scaling law and Vogel-Fulcher law provide a satisfactory fit to our experimental results which are considered as a salient feature of the spin-glass system. Our studies indicate the possibilities of fabricating magnetically controlled multifunctional devices.

An automated setup to measure the linear and nonlinear magnetic ac-susceptibility down to 4 K with higher accuracy

An automated stepper motor controlled ac-susceptibility setup has been developed to measure the phase resolved linear and nonlinear magnetic ac-susceptibilities of a material in the temperature range of 4 K-300 K with a frequency range of 0.1 Hz-1.5 kHz. A maximum dc-field of ±150 Oe can be superimposed with a maximum ac-field of 100 Oe in the same coil by using a homemade ac-dc superimposing circuit. The induced voltage in the detection coil is measured by a lock-in amplifier, and temperature is controlled by a temperature controller. The very common offset voltage drifting problem is resolved by implementing a two-point measurement technique at every temperature, field, and frequency using a stepper motor. Operation of the stepper motor is controlled by a homemade computer programmable driver circuit. Sensitivity of the setup is obtained around ∼10^-7 emu, and relative accuracy of the measurement is much better than 0.1%. Higher harmonics can be measured with a maximum noise level of ±15 nV throughout the temperature, field, and frequency range.

Unusual ferromagnetism enhancement in ferromagnetically optimal manganite La0.7-yCa0.3+yMn1-yRuyO3 (0≤y<0.3): the role of Mn-Ru t2g super-exchange

The e_g-orbital double-exchange mechanism as the core of physics of colossal magnetoresistance (CMR) manganites is well known, which usually covers up the role of super-exchange at the t_2g-orbitals. The role of the double-exchange mechanism is maximized in La_0.7Ca_0.3MnO₃, leading to the concurrent metal-insulator transition and ferromagnetic transition as well as CMR effect. In this work, by a set of synchronous Ru-substitution and Ca-substitution experiments on La_0.7–yCa_0.3+yMn_1–yRu_yO₃, we demonstrate that the optimal ferromagnetism in La_0.7Ca_0.3MnO₃ can be further enhanced. It is also found that the metal-insulator transition and magnetic transition can be separately modulated. By well-designed experimental schemes with which the Mn³⁺-Mn⁴⁺ double-exchange is damaged as weakly as possible, it is revealed that this ferromagnetism enhancement is attributed to the Mn-Ru t_2g ferromagnetic super-exchange. The present work allows a platform on which the electro-transport and magnetism of rare-earth manganites can be controlled by means of the t_2g-orbital physics of strongly correlated transition metal oxides.

High magnetic field phase diagram in electron-doped manganites La(0.4)Ca(0.6)Mn(1-y)Cr(y)O3

We report the charge-order to ferromagnetic phase transition induced by pulsed high magnetic field and impurity doping effects in manganites La_0.4Ca_0.6(Mn_1−yCr_y)O₃ (0 ≤ y ≤ 0.2). Significant charge-order suppression and ferromagnetic tendency upon the Cr³⁺-doping are evidenced, and three different ground states are identified, namely the charge-order state, the phase separated state, and the spin-glass like state. Phase diagram in the H-y plane at 4.2 K is determined by the high magnetic field study, in which the charge-order and ferromagnetic phase boundary is clearly figured out. The critical magnetic field for melting the charge-order phase of La_0.4Ca_0.6MnO₃ is revealed to reach up to 46 T at 4.2 K. Interestingly, distinct responses of the three states to the high magnetic field are observed, indicating the special physics regarding the charge order melting process in each state. The mechanism of the doping induced charge-order suppression and ferromagnetism promotion can be understood by the competition between the antiferromagnetic interaction of Cr-Mn and local enhancement of electron hopping by Cr³⁺.

Tuning the phase transition dynamics by variation of cooling field and metastable phase fraction in Al doped Pr0.5Ca0.5MnO3

We report the effect of field, temperature and thermal history on the time dependence in resistivity and magnetization in the phase separated state of Al doped Pr(0.5)Ca(0.5)MnO(3). The rate of time dependence in resistivity is much higher than that of magnetization and it exhibits a different cooling field dependence due to percolation effects. Our analysis shows that the time dependence in physical properties depends on the phase transition dynamics, which can be effectively tuned by variation of temperature, cooling field and metastable phase fraction. The phase transition dynamics can be broadly divided into the arrested and unarrested regimes, and in the arrested regime this dynamics is mainly determined by time taken in the growth of critical nuclei. An increase in cooling field and/or temperature shifts this dynamics from the arrested to unarrested regime, and in this regime, this dynamics is determined by the thermodynamically allowed rate of formation of critical nuclei, which in turn depends on the cooling field and available metastable phase fraction. At a given temperature, a decrease in metastable phase fraction shifts the crossover from arrested to unarrested regimes towards lower cooling field. It is rather significant that in spite of the metastable phase fraction calculated from resistivity being somewhat off that of magnetization, their cooling field dependence exhibits a striking similarity, which indicates that the dynamics in arrested and unarrested regimes are so different that it comes out vividly provided that the measurements are performed around the percolation threshold.

Coexistence of spin glass and antiferromagnetic orders in Ba3Fe2.15W0.85O8.72

Ba(3)Fe(2.15)W(0.85)O(8.72) has been grown as large single crystals using the floating-zone method, permitting very precise characterization of the nuclear and magnetic structures by neutron and synchrotron diffraction methods. The results of our structural investigation are combined with dc and ac magnetization and heat capacity measurements to give an unusually complete and detailed picture of a complex magnetic system. The compound crystallizes in the hexagonal perovskite structure (space group P6(3)/mmc) and reveals antiferromagnetic order below T(N) = 290 K. Frequency-dependent ac susceptibility and the presence of magnetic viscosity suggest the onset of a spin glass component in this material below T(f) = 60 K. These findings are discussed on the basis of detailed analysis of the crystalo-chemical properties, supported by ab initio (density functional theory) calculations.

Suppression of itinerant ferromagnetism and a spin-glass-like state in Zn substituted La0.67Ca0.33MnO3

We examine the magneto-transport, static and dynamic magnetic responses of the single phase glassy insulating compound La(0.67)Ca(0.33)Mn(1-x)Zn(x)O(3) (x = 0.10). This compound undergoes a field induced metal-insulator transition and exhibits colossal magnetoresistance for H ≥ 2 T. Many experimental features, such as a frequency dependent cusp in the in-phase component of linear ac susceptibility at a temperature T'(f), a broad maximum (at T(a)) in the ZFC cooled thermo-magnetization close to T'(f), large thermo-magnetic irreversibility below T(a), non-saturation of magnetization at 5 K even under 12 T with a characteristic S shape in the virgin curve, monotonic increase of the coercivity at low fields under ZFC condition as one approaches the T'(f), logarithmic relaxation of thermo-magnetization and ageing effects below T(a) and a characteristic maximum in the magnetic viscosity below the spin-glass transition temperature (T(g)) indicate a spin-glass-like state at low temperatures. This is further substantiated by the absence of second order non-linear ac susceptibility, a characteristic negative maximum at T(g) in third order non-linear ac susceptibility (χ'(3)), the critical divergence of χ'(3) as a function of temperature and ac probe field and an unusually large linear term in the low temperature specific heat.

Field dependence of temperature induced irreversible transformations of magnetic phases in Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) crystalline oxide

Glass-like arrest has recently been reported in various magnetic materials. As in structural glasses, the kinetics of a first order transformation is arrested while retaining the higher entropy phase as a non-ergodic state. We show visual mesoscopic evidence of the irreversible transformation of the arrested antiferromagnetic-insulating phase in Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) to its equilibrium ferromagnetic-metallic phase with an isothermal increase of magnetic field, similar to its iso-field transformation on warming. The magnetic field dependence of the non-equilibrium to equilibrium transformation temperature is shown to be governed by Le Chatelier's principle.

Selective substitution in orbital domains of a low doped manganite: an investigation from Griffiths phenomenon and modification of glassy features

An effort is made to study the contrast in magnetic behavior resulting from minimal disorder introduced by substitution of 2.5% Ga or Al in Mn site of La(0.9)Sr(0.1)MnO(3). It is considered that Ga or Al selectively create disorder within the orbital domains or on its walls, causing enhancement of Griffiths phase (GP) singularity for the former and disappearance of it in the latter case. It is shown that Ga replaces Mn(3+), which is considered to be concentrated within the domains, whereas Al replaces Mn(4+), which is segregated on the hole-rich walls, without causing any significant effect on structure or ferromagnetic transition temperatures. Thus, it is presumed that the effect of disorder created by Ga extends across the bulk of the domain having correlation over a similar length scale, resulting in enhancement of the GP phenomenon. In contrast, the effect of disorder created by Al remains restricted to the walls, resulting in the modification of the dynamics arising from the domain walls and suppresses the GP. Moreover, contrasting features are observed in the low temperature region of the compounds; a re-entrant spin-glass-like behavior is observed in the Ga-doped sample, while the observed characteristics for the Al-doped sample are ascribed only to modified domain wall dynamics with the absence of any glassy phase. Distinctive features in third-order susceptibility measurements reveal that the magnetic ground state of the entire series comprises of orbital domain states. These observations bring out the role of the nature of disorder on the GP phenomenon and also reconfirms the character of self-organization in low doped manganites.

Conversion of a glassy antiferromagnetic-insulating phase to an equilibrium ferromagnetic-metallic phase by devitrification and recrystallization in Al substituted Pr(0.5)Ca(0.5)MnO(3)

We show that Pr(0.5)Ca(0.5)MnO(3) with 2.5% Al substitution and La(0.5)Ca(0.5)MnO(3) (LCMO) exhibit qualitatively similar and visibly anomalous M-H curves at low temperature. Magnetic field causes a broad first order but irreversible antiferromagnetic (AF)-insulating (I) to ferromagnetic (FM)-metallic (M) transition in both and gives rise to a soft FM state. However, the low temperature equilibrium state of Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) (PCMAO) is FM-M whereas that of LCMO is AF-I. In both systems the respective equilibrium phase coexists with the other phase with contrasting order, which is not in equilibrium, and the cooling field can tune the fractions of the coexisting phases. It is shown earlier that the coexisting FM-M phase behaves like 'magnetic glass' in LCMO. Here we show from specially designed measurement protocols that the AF-I phase of PCMAO has all the characteristics of magnetic glassy states. It devitrifies on heating and also recrystallizes to an equilibrium FM-M phase after annealing. This glass-like AF-I phase also shows a similar intriguing feature observed in FM-M magnetic glassy state of LCMO, that when the starting coexisting fraction of glass is larger, successive annealing results in a larger fraction of the equilibrium phase. This similarity between two manganite systems with contrasting magnetic orders of respective glassy and equilibrium phases points to a possible universality.

Phase separation and the effect of quenched disorder in Pr(0.5)Sr(0.5)MnO(3)

The nature of phase separation in Pr(0.5)Sr(0.5)MnO(3) has been probed by linear, as well as nonlinear, magnetic susceptibilities and resistivity measurements across the second order paramagnetic to ferromagnetic transition (T(C)) and first order ferromagnetic to antiferromagnetic transition (T(N)). We found that the ferromagnetic (metallic) clusters, which form at T(C), continuously decrease their size with a decrease in temperature and coexist with non-ferromagnetic (insulating) clusters. These non-ferromagnetic clusters are identified to be antiferromagnetic. It is shown that they do not arise because of the superheating effect of the lower temperature first order transition. This reveals phase coexistence in manganite, around half-doping, encompassing two long-range order transitions. Substitution of quenched disorder (Ga) at Mn-sites promotes antiferromagnetism at the cost of ferromagnetism without adding any magnetic interaction or introducing any significant lattice distortion. Moreover, an increase in disorder decreases the ferromagnetic cluster size and with 7.5% Ga substitution cluster size reduces to the single-domain limit. Resistivity measurements also reveal the phase coexistence identified from the magnetic measurements. It is significant that, an increase in disorder up to 7.5% increases the resistivity of the low temperature antiferromagnetic phase by about four orders.