Ya. MukovskiiYa. Mukovskii, A. Pestun, A. Pestun, , D.ShulyatevD.Shulyatev Moscow State Institute of Steel and Alloys, Moscow, 119049, RussiaMoscow State Institute of Steel and Alloys, Moscow, 119049, Russia

Wei Li, H.P. Kunkel, X Z Zhou, G.WilliamsWei Li, H.P. Kunkel, X Z Zhou, G.WilliamsUniversity of Manitoba, Winnipeg, MB, R3T2N2, CanadaUniversity of Manitoba, Winnipeg, MB, R3T2N2, Canada

ANOMALOUS MAGNETIC BEHAVIOR ANOMALOUS MAGNETIC BEHAVIOR IN LaIN La

1-x1-xAAxxMnOMnO33 (A =Ca, Ba) (A =Ca, Ba)

SINGLE CRYSTALSSINGLE CRYSTALS

Introduction

Similarity and difference in manganites behavior.

Many aspects of the response of the manganite perovskites still remain unresolved, including the interrelationship between the metal–insulator and the paramagnetic to ferromagnetic (PFT) transitions. A related question that has also emerged concerns the order of the magnetic phase transition, particularly the role played by disorder.

In some papers beginning from Imry and Ma (PRL 35, 1399 (1975)) and Aharony and Pytte (PRL 45, 1583 (1980)) it was shown that various types

of randomness destroy magnetic long-range order and lead into glassy state (also Burgy et al. PRL 87, 277202 (2001)).

In some recent studies of this question for manganites (e.g. Rivadulla, Rivas, Goodenough (Phys.Rev.B 70, 172410 (2004))

in La1−xCaxMnO3 regions with 2 order (x<0.25) and 1 order

magnetic transitions (0.25<x<0.4) were observed.

H/M vs. M2 plots for various compositions around TC and Tf showing the change in the sign of the slope. Tf is defined from the minimum in the M/T measured at low field. (After F. Rivadulla, J. Rivas, J. B. Goodenough, Phys.Rev.B 70, 172410 (2004))

the magnetic transition out of the range 0.275 < x < 0.43 is not a true phase transition, but only a change in the relative volume fractions of the fluctuations that compete to develop below a certain temperature, Tf,,

(H=0,T) will saturate when the correlation length becomes comparable to a cluster size L.

Plan of the Talk

1. Introduction

2. Coincident 1 and 2 order magnetic transitions in La0.73Ca0.27MnO3

3. Magnetic behavior of La0.73Ba0.27MnO3

4. Summary

Our detailed magnetization and susceptibility data on single crystal La0.73Ca0.27MnO3 reveal a not previously predicted and hence unexpected result—a combination of characteristics associated with both first-order and second-order transitions simultaneously: namely, metamagnetic behavior in magnetic isotherms occurring coincidentally with a ‘crossover’ line in the field and temperature dependent susceptibility.

Single crystal La0.73Ba0.27MnO3 demonstrated drastically different behavior - second-order transition into Heisenberg ferromagnetic, - but with strange feature at low temperature.

La0.73Ca0.27MnO3

(a) The zero-field ac susceptibility measured on warming and on cooling. (b) The temperature dependence of the coercive field Hc. (c) The metamagnetic field plotted against temperature.

(d) The metamagnetic boundary as in (c) and the crossover line (▲) (the line of χ(Ha,T) maxima) plotted against temperature.

The coercivity HC values do not exceed 10 Oe at any temperature

Magnetization isotherms, showing limited hysteresis, for temperatures increasing from 234 K (top) to 245 K in 1 K steps..

La0.73Ca0.27MnO3

H (kOe)

The S-shaped character of isotherms for T > 237 K (metamagnetic transition)

=1.75

La0.73Ca0.27MnO3

(a) The ac susceptibility χ(Ha, T), measured in applied fields Ha increasing from 600Oe (top) to 2000 Oe;

the locus of these maxima—the crossover line—is shown by the dashed curve; (b) the susceptibility maxima (a test of equation (2)) against internal field Hi = H,-NM

(c) the (reduced) peak temperature (a test of equation (1)) against internal field, (d) the susceptibility maxima against reduced peak temperature (a test of equation

(3)). The solid lines show Heisenberg model exponents for comparison.

(1)

(2)

(3)

The observed behavior is a characteristic signature of a second-order/continuous phase transition.

= 1.384

= 4.8

La0.73Ba0.27MnO3

Magnetization isotherms at selected temperatures below 100 K.Below 50 K the magnetization in the same field range—a qualitative measure of the spontaneous magnetization—falls. No evidence of a metamagnetic transition

Magnetization isotherms between 240 and 251 K (in 1 K steps). Inset: the zero-field ac susceptibility

Modified Arrott plots [M1/ vs (H/M)] using Heisenberg model exponents

for a selection of magnetic isotherms at temperatures of 242 (top), and 248 K (bottom), step 1 K.

Inset: conventional Arrott plots M2 vs H/M

La0.73Ba0.27MnO3

< t3*

< t3*

Oenear-neighbour three-dimensional Heisenberg model:

t=(T-TC)/TC , h~H/TC

The spontaneous magnetization Ms

plotted against temperature.

La0.73Ba0.27MnO3

The critical isotherm, which gives TC=245 K

=4.83±0.04=4.83±0.04

The initial susceptibility i plotted against temperature

A scaling plot of M/t vs H/t. 242.0 <T<247.5, 200<H<50 000 Oe The solid lines drawn in this inset represent the asymptotic forms of the scaling function.

La0.73Ba0.27MnO3

-3 -2 -1 0 1 2 3Energy (meV)

0

20

40

60

Q scan showing spin waves along the (001) direction in manganite single crystals.

a) A central quasielastic component to the fluctuation spectrum develops as TTC for La0.7Ca0.3MnO3 (J.W.Lynn, C.P.Adams, Y.M.Mukovskii, et al. J. Appl. Phys. 89, 6846,

2001)

b) NO quasielastic central peak for La0.8Ba0.2MnO3

(A.A.Arsenov, Ya.M.Mukovskii, J.W.Lynn et al. Phys.Stat.Sol.(a), 189, 673, 2002).

a) b)

(a) The zero-field ac susceptibility measured on warming following cooling at zero (dc) field. (b) The ac susceptibility measured on warming following cooling at zero (dc) field inprogressively increasing static (dc) applied fields of 200 Oe (top), and 1400 Oe (bottom).

La0.73Ba0.27MnO3

Arrott–Noakes plots (M2 versus H/M; with mean-field exponents) at temperatures of, sequentially, 50 K (top), and 230 K (bottom).Inset: data close to the Curie temperature plotted using the same equation, but with Heisenberg model exponents, at temperatures of, sequentially, 241 K (top), 250 K (bottom).

La0.73Ba0.27MnO3

La0.73Ba0.27MnO3

a) b)

(a) Plots of the reduced spontaneous magnetization (MS(T)/MS(0)) plotted against temperature. The solid line represents a fit to equation MS(T)/MS(0) = 1 − (NS)-1*( kBT/4πD)3/2*(3/2,kBT) between 60 and 140 K using D = 65.7 meV Å2 and = 0.45 meV; the dashed line extends thisfit below 60 K. (b) Spontaneous magnetization data below 30 K; the dashed line uses the D and values utilized in (a) scaled to MS(T)/MS(0) = 0.957(5); the solid line utilizes the same value for D as in (a) but with increased to 2.35 meV, while the dot–dashed line employs = 0.45 meV as in (a) but with D increased to 159 meV Å2.

Influence of radiation stimulated disorder (fast neutron irradiation with E ≥ 1 MeV, Trad ≈ 300 K, flux F = 2*1019 neutron/cm2) on temperature dependencies of AC susceptibility and electric resistance of single crystals of layered manganites La1.4Sr1.6Mn2O7 in magnetic field up to 13.6 T. It was observed that the disorder leads to suppression of the magnetic order (TC → 0) and to disappearing of metallic character of conductivity. In non magnetic state the CMR effect remains, and its value exceeds an original one. Also kinetics of the properties recovering under annealing was studied.The work was supported by the RFRB grant #02-02-16425 and ISTC grant #1859

The measured coercive field, HC(T). 

The magnetization, M(H, T), measured on warming following zero-field cooling in static applied fields.

La0.73Ba0.27MnO3

While the physical origin of the underlying magnetization processes in La0.73Ba0.27MnO3 may be questionable, the estimate for MS(0) = 0.96(NgμBS) is not. That this signifies an effective moment reduction is unequivocal; the specific mechanism leading to this moment reduction however cannot be identified definitively.

Spin canting - ?A spiral magnetic structure - ?Structural changes - ?Does not connect with technical processes (Hopkinson maximum, HC < 5 Oe).

Summary In La0.73Ca0.27MnO3 the transition at x = 0.27 displays features characteristic of both

continuous and discontinuous transitions that are—within experimental uncertainty—coincident.

This behaviour is fundamentally different from crossover effects from sequential second-order

to a first-order transition as T → Tc, where the first-order transition line would lie below that for the continuous transition, a situation for which the power-laws discussed above

would be expected to occur, as the transition is approached from higher reduced temperatures.

In La0.73Ba0.27MnO3 estimates of the spontaneous magnetization—supplemented by ac susceptibility data—indicate a spontaneous moment reduction below 60 K.

This reduction is not associated with a (further) structural phase change nor technical magnetic processes.

Spin-wave stiffness estimated with = 0 do not approach zero temperature monotonically.

Additional experiments investigating the microscopic/atomic spin arrangement in La1−xBaxMnO3 (0.2 ≤ x ≤ 0.3) at low temperature might be appropriate.