Specific heat of Sm0.55Sr0.45MnO3 manganite in magnetic fields up to 15 T: An anomalous critical behavior of the ferromagnet in magnetic field and the observation of a tricritical point

The specific heat of Sm_0.55Sr_0.45MnO₃ manganite is measured in the vicinity of the ferromagnetic phase transition in strong magnetic fields up to 15 T. An anomalous critical behavior of the Sm_0.55Sr_0.45MnO₃ ferromagnet in magnetic field is predicted and experimentally observed. The anomalous behavior manifests itself in that, in magnetic fields up to 4 T, the field favors fluctuations and the specific-heat anomaly increases with the field and sharpens, becoming λ-shaped. In fields above 4 T, the behavior becomes classical: the field suppresses the fluctuations and the specific-heat peak is smeared out. The hysteresis of the transition temperature measured in the heating and cooling runs is about 15 K in zero magnetic field. As the field increases, it narrows gradually and vanishes completely when the field reaches 4 T. The results of the experiments are explained in terms of the competition between the hysteresis and the fluctuations of the magnetic order parameter. The H-T phase diagram of Sm_0.55Sr_0.45MnO₃ certainly indicates that, at 4 T, Sm_0.55Sr_0.45MnO₃ has a tricritical point, at which the ferromagnetic and paramagnetic phases of Sm_0.55Sr_0.45MnO₃ are leveled.     

Metamagnetic phase transitions induced by static and pulsed fields in (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics

It has been found that the magnetic susceptibility of (Sm_0.5Gd_0.5)_0.55Sr_0.45MnO₃ ceramic samples in zero external magnetic field exhibits a sharp peak near the temperature of 48.5 K with a small temperature hysteresis that does not depend on the frequency of measurements and is characteristic of the phase transition to an antiferromagnetic state with a long-range charge orbital ordering, which is accompanied by an increase in the magnetic susceptibility with a decrease in the temperature. The magnetization isotherms in static and pulsed magnetic fields at temperatures below 60 K demonstrate the occurrence of an irreversible metamagnetic transition to a homogeneous ferromagnetic state with a critical transition field independent of the measurement temperature, which, apparently, is associated with the destruction of the insulating state with a long-range charge ordering. In the temperature range 60 K ?? T ?? 150 K, the ceramic samples undergo a magnetic-field-induced reversible phase transition to the ferromagnetic state, which is similar to the metamagnetic transition in the low-temperature phase and is caused by the destruction of local charge/orbital correlations. With an increase in the temperature, the critical transition fields increase almost linearly and the field hysteresis disappears. Near the critical fields of magnetic phase transitions, small ultra-narrow magnetization steps have been revealed in pulsed fields with a high rate of change in the magnetic field of ??400 kOe/??s.     

Relation between giant volume magnetostriction and colossal magnetoresistance near the Curie temperature of Sm0.55Sr0.45MnO3

The magnetic, transport, and elastic properties of Sm_0.55Sr_0.45MnO₃ have been established to be interrelated. At the Curie point, one observes a large volume compression ΔV/V≈0.1%, a sharp minimum in the temperature dependence of negative volume magnetostriction ω(T), and a maximum in the temperature dependence of the electrical resistivity. Giant negative volume magnetostriction ω=?5×10^?4 has been found in a magnetic field H=0.9 T, which is accompanied by a colossal negative magnetoresistance of 44% in the same field. The results obtained are discussed in terms of a model of electronic phase separation.     

Ultrafast photoinduced insulator-ferromagnet transition in the perovskite manganite Gd0.55Sr0.45MnO3

We have investigated the ultrafast spin and charge dynamics in the course of a photoinduced phase transition from an insulator with short-range charge order and orbital order (OO) to a ferromagnetic metal in perovskite-type Gd0.55Sr0.45MnO3. Transient reflectivity changes suggest that the metallic state is formed just after the photoirradiation and decays within approximately 1 ps. The magnetization, however, increases with the time constant of 0.5 ps and decays in approximately 10 ps. The relatively slow increase of the magnetization is attributable to the magnetic-field-induced alignment of ferromagnetic domains in the initially produced metallic state and its slow decay to the partial recovery of the OO.     

Influence of the magnetic and structural heterogeneities on the thermopower,magnetothermpower, and spontaneous generation of electric voltage in the manganite Sm0.55Sr0.45MnO3

The thermopower α and magnetothermopower Δα/α have been studied on single-crystal Sm_0.55Sr_0.45MnO₃ samples consisting of three types of magnetic clusters (ferromagnetic clusters with the Curie temperature T _C = 134 K, the A-type antiferromagnetic clusters with the Néel temperature T _NA ≥ T _C, and the CE-type antiferromagnetic clusters with T _NCE = 240 K). The temperature dependences of α and Δα/α have extremes in the vicinity of T _NCE, namely, a wide maximum in curve α(T) and a sharp minimum in curve{Δα/α}(T). The negative magnetothermopower in the minimum has a giant value of 50% in the magnetic field of 13.2 kOe. The thermopower is shown to be mainly due to the existence of the CE-type antiferromagnetic clusters, in which there is a charge-orbital ordering that displaces the oxygen atoms. The changed crystal lattice inside these clusters changes the value of the thermopower inside them. This thermopower influences the voltage drop across the sample during measuring the thermopower and, thus, the effective value of α of the whole sample. The application of a magnetic field near T _NCE accelerating the destruction of the CE-type antiferromagnetic order causes the sharp decrease in the thermopower of the whole sample. This implies that the CE-type antiferromagnetic clusters with the charge-orbital ordering make main contribution to the thermopower of the whole sample.     

Fluctuations of the order parameter in R 0.55Sr0.45MnO3 manganites near the metal-insulator phase transition

The magnetic phase transformations induced by changes of the composition, external magnetic field strength, and temperature in manganites with a nearly half-filled conduction band in the vicinity of the metal-insulator phase transition have been investigated experimentally. It has been found that the substitution of rare-earth ions (Sm) for Nd ions with a larger ionic radius in R _0.55Sr_0.45MnO₃ manganites leads to a linear decrease in the Curie temperature T _C from 270 to 130 K and a transformation of the second-order ferromagnetic (FM) phase transition into a first-order phase transition. The results of measurements of the alternating-current (ac) magnetic susceptibility in the (Nd_{1 ? y} Sm _y )_0.55Sr_0.45MnO₃ system indicate the existence of a Griffiths-like phase in samples with a samarium concentration y > 0.5 in the temperature range T _C < T < T* (where T* ～ 220 K). For samples with y > 0.5, the magnetization isotherms at temperatures above T _C exhibit specific features in the form of reversible metamagnetic phase transitions associated with strong fluctuations of the short-range ferromagnetic order in the system of Mn spins in the high-temperature Griffiths phase consisting of ferromagnetic clusters. According to the results of measurements of the ac magnetic susceptibility in the (Sm_{1 ? y} Gd _y )_0.55Sr_0.45MnO₃ system for a gadolinium concentration y = 0.5, there is an antiferromagnetic (AFM) phase with an unusually low critical temperature of the spin ordering T _N ? 48.5 K. An increase in the external static magnetic field at 4.2 K leads to an irreversible induction of the ferromagnetic phase, which is stable in the temperature range 4.2–60 K. In the temperature range 60 K < T < 150 K, there exists a high-temperature Griffiths-like phase consisting of clusters (correlations) with a local charge/orbital ordering. The metastable antiferromagnetic structure is retained in samples with gadolinium concentrations y = 0.6 and 0.7, but it is destroyed with a further increase in the gadolinium concentration upon the transition to the spin-glass state. The magnetization isotherm obtained with variations in the external static magnetic field in the field range ±70 kOe at 4.2 K and the temperature dependence of the ac-magnetic susceptibility χ suggest that, in the Gd_0.55Sr_0.45MnO₃ ceramics, there is a mixed two-phase low-temperature state consisting of the quantum Griffiths phase with a characteristic divergence of χ(T) near T = 0, which was embedded in the spin-glass matrix with the spin “freezing” temperature T _G ? 42 K. The low-temperature state with quantum fluctuations exists in the (Sm_{1 ? y} Gd _y )_0.55Sr_0.45MnO₃ system for y ≥ 0.5.     

Dynamic response in La0.55Ca0.45MnO3: temperature and field dependence

We report AC susceptibility measurements on bulk La_0.55Ca_0.45MnO₃ as functions of temperature, DC field and frequency ( f<1500 Hz). Both the in-phase (χ′) and out-of-phase (χ″) parts of the response are studied. It is observed that the in-phase part goes through a maximum that is removed on the application of moderate DC fields. The temperature and field dependence of χ″ are consistent with the picture of an inhomogeneous transition proceeding through a growth of clusters. DC fields suppress both the components with the strongest effects being at or close to T_c. The out of phase response is completely suppressed by fields of the order of a few hundred Gauss. The effect of the DC field is discussed in terms of the suppression of spin fluctuations close to T_c, and to the changes in coercivity for lower temperatures.     

Magnetic properties of erbium iron garnet in high magnetic fields up to 150kOe

The magnetic properties of single crystals of erbium iron garnet (ErIG) were studied in applied fields up to 150kOe between 1.4 and 300K. At low temperature, the macroscopic easy direction of the bulk magnetization is [100]; below the compensation temperature (80±2K), the magnetization presents non-linear field evolution. On the assumption of an isolated ground doublet, the anisotropy constantsK _i (i=1,2) of ErIG are given byK _i (Er)+K _i (YIG); theK _i are calculated as a function of theG andg tensor components. It is worthwhile noting that theK _i (Er) are strongly temperature dependent; so at low temperature the anisotropy of the garnet is determined by the rare earth ions, while in the 50 K regionK ₁(Er) becomes comparable toK ₁(YIG) with the opposite sign which results in a very weak anisotropy of the garnet. Above 50 K,K ₁(YIG) is predominant and the Fe³⁺ ions determine the garnet anisotropy.     

Magnetic properties of erbium iron garnet in high magnetic fields up to 150kOe

The magnetic properties of single crystals of erbium iron garnet (ErIG) were studied in applied fields up to 150kOe between 1.4 and 300K. At low temperature, the macroscopic easy direction of the bulk magnetization is [100]; below the compensation temperature (80±2K), the magnetization presents non-linear field evolution. On the assumption of an isolated ground doublet, the anisotropy constantsK _i (i=1,2) of ErIG are given byK _i (Er)+K _i (YIG); theK _i are calculated as a function of theG andg tensor components. It is worthwhile noting that theK _i (Er) are strongly temperature dependent; so at low temperature the anisotropy of the garnet is determined by the rare earth ions, while in the 50 K regionK ₁(Er) becomes comparable toK ₁(YIG) with the opposite sign which results in a very weak anisotropy of the garnet. Above 50 K,K ₁(YIG) is predominant and the Fe³⁺ ions determine the garnet anisotropy.     

La0.55Ca0.45MnO3的电子密度分布变温X射线衍射测量

运用X射线衍射(XRD)分析技术,对钙钛矿型La_0.55Ca_0.45MnO₃化合物的空间结构进行了变温测量,用Rietveld方法对XRD实验数据进行了精修拟合.随着温度的降低,实验上观测到衍射峰的劈裂,暗示该化合物内部晶体结构发生了相变.磁化曲线测量表明,对应晶体结构相变的温度,化合物的磁性也发生由铁磁性向反铁磁性的转变.运用Rietica软件对化合物的XRD数据进行了处理,得到了La_0.55Ca_{0.45< 关键词： 锰氧化物 相变 电子密度分布 磁结构}     

Ag复合对La2/3(Ca0.45Sr0.55)1/3MnO3/xAg体系电输运和室温磁电阻效应影响

文中采用甘氨酸-硝酸盐法分别制备了La2/3(Ca0.45Sr0.55)1/3MnO3/xAg纳米复合材料(x=0;0.1;0.2;0.3).通过X射线衍射、扫描电子显微镜和磁电阻效应测试,对合成产物的结构及性能进行表征.结果表明,随Ag复合量增加,样品均为正交钙钛矿结构,低场室温磁电阻效应增强,电阻率减小.     

V替代Mn对La_(0.45)Ca_(0.55)MnO_3电荷有序相及自旋玻璃态的影响

用固相反应法制备了La0.45Ca0.55Mn1-xVxO3(x=0.00,0.10)多晶样品.通过X射线衍射谱、质量磁化强度-温度曲线、电子自旋共振谱,研究了V5+替代Mn3+/Mn4+对La0.45Ca0.55MnO3电荷有序相和自旋玻璃态的影响.实验结果表明,当x=0.10时,不仅母体的电荷有序相基本破坏,而且母体在40K左右出现的自旋玻璃态也被融化.电荷有序相被破坏的主要原因是用V5+替代Mn3+/Mn4+后,增加了Mn3+与Mn4+的比例,使铁磁双交换作用优于反铁磁超交换作用;自旋玻璃态的融化是由于V替代Mn后破坏了反铁磁背景下有少量铁磁成分的自旋玻璃态的形成条件.     

Structural and magnetic phase transitions in La0.9Sr0.1MnO3

A study is made of phase transitions in doped La_0.9Sr_0.1MnO₃ compounds using combined x-ray, electrical, and magnetic measurements. Structural phase transitions are observed accompanied by a change in the cell volume at temperatures of 100–110 K and 300–340 K. These structural changes are found to be related to different contributions of the rhombic Jahn-Teller Q ₂ mode to the formation of the crystal lattice. The structural transition at 100–110 K is accompanied by distinctive magnetic and electrical properties. The data are analyzed in detail. Fiz. Tverd. Tela (St. Petersburg) 41, 1064–1069 (June 1999)     

A位的Sm掺杂对La0.67Sr0.33MnO3体系磁电性质的影响

通过实验研究了La0.67-xSmxSr0.33MnO3(x=0.00,0.10,0.20,0.30)体系的M-T曲线、ESR曲线、红外光谱、拉曼光谱、ρ-T曲线和MR-T曲线. 实验结果表明:随着Sm掺杂的增加,体系从长程铁磁有序向自旋团簇玻璃态和反铁磁状态转变,Sm掺杂引起的磁结构变化和额外磁性耦合将导致CMR效应.     

Anomalous losses in a Bi 2212 vortex lattice in magnetic fields of 1–3 kOe

The results of an experimental investigation of the dependence of the dissipation in a vortex lattice on the magnetic field strength and orientation of a Bi 2212 crystal at a temperature of ~15 K are presented. A sharp peak, whose width depends on the orientation of the superconductor relative to the field, is observed in the dissipation. The results can be explained qualitatively by the appearance of a geometrical barrier that leads to a substantial redistribution of the magnetic field in the bulk of the sample. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 331–335 (10 March 1996)     

Thermopower and electrical resistivity of La0.875Sr0.125MnO3 single crystals under hydrostatic pressure

The thermopower α and the electrical resistivity ρ of La_1?x Sr_xMnO₃ single crystals with a strontium content x=0.125, which corresponds to the stoichiometric composition of the new charge-ordered (CO) phase, are measured in the temperature range 77–300 K at pressures up to 12 kbar. The dependence α(T) exhibits two maxima. The first low-temperature maximum is associated with the formation of the charge-ordered phase. The second high-temperature maximum is attributed to the O → O′ structural transition between the orthorhombic phases and the formation of ferromagnetic clusters. It is found that the phase transition observed at P>9.2 kbar is accompanied by a substantial shift of both maxima toward the low-temperature range.