Giant magnetocaloric effect near the curie temperature in the Sm0.6Sr0.4MnO3 manganite

A method is proposed for the calculation of the magnetocaloric effect from simultaneous measurements of thermal expansion and magnetostriction made in different regimes (adiabatic and isothermal). The magnitude of the magnetocaloric effect for Sm_0.6Sr_0.4MnO₃ is estimated. It is found that near the Curie temperature T _C it passes through a maximum to reach a giant value ΔT=4.6 K for ΔB=0.84 T. In addition, in the neighborhood of T _C, we observed colossal magnetoresistance Δρ/ρ = [ρ(H) ? ρ(0)]/ρ(0) = 72% in a weak magnetic field of 0.84 T, a giant negative volume magnetostriction ω=?5×10^?4 in a field of the same strength, and a large change in the sample volume ΔV/V ≈ 0.1%.     

Peculiarities of the magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Sm<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

Manganites of the Sm1?xSr_xMnO₃ system (x=0.33, 0.4, and 0.45) possess giant negative values of the magnetoresistance Δρ/ρ and the volume magnetostriction ω near the Curie temperature T_C. In the compound with x=0.33, the isotherms of Δρ/ρ, ω, and magnetization σ exhibit smooth variation and do not reach saturation up to maximum magnetic field strengths (120 kOe) studied (according to the neutron diffraction data, this substance comprises a ferromagnetic (FM) matrix with distributed clusters of a layered antiferromagnetic (AFM) structure of the A type). In the compounds with x=0.4 and 0.45 containing, besides the FM matrix and A-type AFM phase, a charge-ordered AFM phase of the CE type (thermally stable to higher temperatures as compared to the A-type AFM and the FM phases), the same isotherms measured at T ≥ T_C show a jumplike increase in the interval of field strengths between H_c1 and H_c2 and then reach saturation. In the interval H_c1 > H > H_c2, the σ, ω, and Δρ/ρ values exhibit a metastable behavior. At temperatures above T_C, the anisotropic magnetostriction changes sign, which is indicative of rearrangements in the crystal structure. The giant values of ω and Δρ/ρ observed at T ≥ T_C for all compounds, together with excess (relative to the linear) thermal expansion and a maximum on the ρ(T) curve, are explained by the phenomenon of electron phase separation caused by a strong s-d exchange. The giant values of magnetoresistance and volume magnetostriction (with ω reaching ～10^?3) are attributed to an increase in the volume of the FM phase induced by the applied magnetic field. In the compound with x=0.33, this increase proceeds smoothly as the FM phase grows through the FM layers in the A-type AFM phase. In the compounds with x=0.4 and 0.45, the FM phase volume increases at the expense of the charge-ordered CE-type AFM structure (in which spins of the neighboring manganese ions possess an AFM order). The jumps observed on the σ(H) curves, whereby the magnetization σ reaches ～70% of the value at T=1.5 K, are indicative of a threshold character of the charge-ordered phase transition to the FM state. Thus, the giant values of ω and Δρ/ρ are inherent in the FM state, appearing as a result of the magnetic-field-induced transition of the charge-ordered phase to the FM state, rather than being caused by melting of this phase.     

Peculiarities of magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu_0.55Sr_0.45MnO₃ ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=10⁶ Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field H_C1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and H_C1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρ _H ?ρ_H=0)/ρ _H is positive for H<H_C1 and passes through a maximum at 41 K, where Δρ/ρ = 6%. For H>H_C1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×10⁵ % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10^?4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu_0.55Sr_0.45MnO₃, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ～3% of the sample volume is occupied by the FM phase; ～67%, by the CE-type AFM phase; and ～30%, by the A-type AFM phase.     

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.     

Effect of the substrate crystalline structure on the magnetic, electrical, and crystallographic characteristics of La0.35Nd0.35Sr0.3MnO3 epitaxial films

A study has been made of the electrical resistivity ρ, magnetoresistance Δρ/ρ, and magnetization of La_0.35Nd_0.35Sr_0.3MnO₃ epitaxial films on ZrO₂(Y₂O₃), SrTiO₃, LaAlO₃, and MgO substrates. The first film can exist in four equivalent crystallographic orientations in the sample plane, while the other three have only one orientation. The maxima of ρ and Δρ/ρ of the first film are broadened considerably in the vicinity of the Curie point T _C compared to the three others, the magnitude of ρ itself being larger by 1.5 orders of magnitude, and a large negative magnetoresistance (|Δρ/ρ| ～ 10% in a field of 8.4 kOe) is observed at temperatures 80≤T≤200 K. In all films, the magnetic moment per molecule at 5 K is ～46% smaller than the pure spin value, due to the existence of magnetically disordered regions. The larger value of ρ of the film deposited on ZrO₂(Y₂O₃) is due to the electrical resistance of the boundaries separating regions with different crystallographic orientations, and the magnetoresistance is associated with polarized carriers tunneling through the boundaries coinciding with domain walls. The low-temperature magnetoresistance in fields above technical saturation is caused by the strong p-d exchange coupling within spin-ordered regions.     

Specific features of magnetoresistance during the antiferromagnet—paramagnet transition in Tm1 − x Yb x B12

The transverse magnetoresistance Δρ/ρ(H, T) of Tm_{1 ? x} Yb _x B₁₂ single crystals is studied in the ytterbium concentration range corresponding to the antiferromagnet-paramagnet transition in a magnetic field up to 80 kOe at low temperatures. A magnetic H-T phase diagram is constructed for the antiferromagnetic state of substitutional Tm_{1 ? x} Yb _x B₁₂ solid solutions with x ≤ 0.1. The contributions to the magnetoresistance in the antiferromagnetic and paramagnetic phases of the dodecaborides under study are separated. Along with negative quadratic magnetoresistance -Δρ/ρ ∝ H ₂, the magnetically ordered phase of these compounds is found to have component Δρ/ρ ∝ H that linearly changes in a magnetic field. The negative contribution to the magnetoresistance of Tm_{1 ? x} Yb _x B₁₂ is analyzed in terms of the Yosida model for a local magnetic susceptibility.     

Magnetoresistance and magnetic phase transitions in a Pr11B6 antiferromagnet

In isotopically pure praseodymium hexaboride (Pr¹¹B₆) single-crystal samples, the transverse magnetoresistance Δρ/ρ has been measured in a temperature range of 2–20 K in magnetic fields up to 80 kOe. The field and angular dependences Δρ/ρ(H, ?, T ₀) reveal a new magnetic phase in the AFM state of Pr¹¹B₆ which is observable only for the external magnetic field orientation in a narrow angular range near H ‖ 〈110〉. The data remove the previous contradictions in the Pr11B6 magnetic phase diagram representation and can be explained under the assumption that the spin-polarized regions (ferrons) are involved in the formation of the complex magnetic structure in the Pr¹¹B₆ AFM state in the 5d band in the vicinity of the rare-earth ions.     

Zero-bias tunneling anomaly in a two-dimensional electron system with disorder

The temperature dependence of a zero-bias anomaly in the tunneling conductance of an Al/δ-GaAs tunneling structure with a two-dimensional electron density in the δ-layer of 3.5 × 10¹² cm^?2 has been investigated. It has been shown that the respective drop Δρ(?, T) in the tunneling density of states ρ near the Fermi level E _F of the two-dimensional electron system depends logarithmically on the energy ? within the range of 2.7kT < |?| < ?/τ, where ? is measured with respect to E _F and τ is the momentum relaxation time of two-dimensional electrons. It has been found that the drop depth Δρ(0, T)/ρ is also proportional to ln(kT/?₀) in the temperature range T = 0.1–20 K and saturates below 0.1 K.     

The forced magnetostriction of nickel at 4.2 K

The forced volume magnetostriction of polycrystalline nickel at 4.2 K has been determined with a relative accuracy of 2 × 10^?2. Combining our result with previous data on the forced magnetostriction, we derive for the forced magnetostriction constants: h'₀ = (40 ± 1) × 10^?8T^?1, h'₁ = (-95 ± 2) × 10^?8T^?1, h'₂ = (-19 ± 2) × 10 ^?8T^?1.     

Magnetotransport and magnetoelastic effects in Co-doped La0.7Sr0.3MnO3 nanocrystalline perovskites

La_0.7Sr_0.3Mn_1−xCo_xO₃ (x=0, 0.05, 0.1) nanoparticles, prepared by sol-gel method, were studied by means of X-ray diffraction, transmission electron microscopy, resistivity, magnetoresistance, thermal expansion and magnetostriction measurements. Results show that partial substitution of Mn by Co leads to a reduction in lattice parameters, enhancement of resistivity and room temperature magnetoresistance MR, decrease of metal-insulator transition temperature T_MI and T_C, an increase in thermal expansion coefficient, volume magnetostriction and anisotropic magnetostriction. The latter increases about one order of magnitude with 10% Co substitution. In comparison with Mn ions, the Co ions possess higher anisotropy energy, larger magnetostriction effect, smaller ionic size and spin state transitions with increase in temperature and magnetic field; this suggests that Co substitution leads to double-exchange interaction weakening, resulting in suppression of ferromagnetic long-range order and metallic state and increase of magnetic anisotropy. Furthermore, our samples have a relatively lower T_MI and T_C, higher resistivity and MR, compared with the reported values for similar compounds with larger particle sizes. This is attributed to the nanometric grain size and spin-polarized tunneling between neighboring grains.     

Anomalies of magnetoresistance of compounds with atomic clusters RB12 (R = Ho, Er, Tm, Lu)

The magnetoresistance and magnetization of single-crystal samples of rare-earth dodecaborides RB₁₂ (R = Ho, Er, Tm, Lu) have been measured at low temperatures (1.8–35 K) in a magnetic field of up to 70 kOe. The effect of positive magnetoresistance that obeys the Kohler’s rule Δρ/ρ = f(ρ(0, 300 K)H/ρ(0, T)) is observed for the nonmagnetic metal LuB₁₂. In the magnetic dodecaborides HoB₁₂, ErB₁₂, and TmB₁₂, three characteristic regimes of the magnetoresistance behavior have been revealed: the positive magnetoresistance effect similar to the case of LuB₁₂ is observed at T > 25 K; in the range T _N ≤ T ≤ 15 K, the magnetoresistance becomes negative and depends quadratically on the external magnetic field; and, finally, upon the transition to the antiferromagnetic phase (T < T _N ), the positive magnetoresistance is again observed and its amplitude reaches 150% for HoB₁₂. It has been shown that the observed anomalies of negative magnetoresistance in the paramagnetic phase can be explained within the Yosida model of conduction electron scattering by localized magnetic moments. The performed analysis confirms the formation of spin-polaron states in the 5d band in the vicinity of rare-earth ions in paramagnetic and magnetically ordered phases of RB₁₂ and makes it possible to reveal a number of specific features in the transformation of the magnetic structure of the compounds under investigation.     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

Spin-polaron transport and magnetic phase diagram of iron monosilicide

The study of galvanomagnetic, magnetic, and magnetooptical characteristics of iron monosilicide in a wide range of temperatures (1.8–40 K) and magnetic fields (up to 120 kOe) has revealed the origin of the low-temperature sign reversal of the Hall coefficient in FeSi. It is shown that this effect is associated with an increase in the amplitude of the anomalous component of the Hall resistance ρ_H (the amplitude increases by more than five orders of magnitude with decreasing temperature in the range 1.8–20 K). The emergence of the anomalous contribution to ρ_H is attributed to the transition from the spin-polaron to coherent regime of electron density fluctuations in the vicinity of Fe centers and to the formation of nanosize ferromagnetic regions, i.e., ferrons (about 10 Å in diameter), in the FeSi matrix at T<T_C=15 K. An additional contribution to the Hall effect, which is observed near the temperature of sign reversal of ρ_H and is manifested as the second harmonic in the angular dependences ρ_H(?), cannot be explained in the framework of traditional phenomenological models. Analysis of magnetoresistance of FeSi in the spin-polaron and coherent spin fluctuation modes shows that the sign reversal of the ratio Δρ(H)/ρ accompanied by a transition from a positive (Δρ /ρ>0, T>T_m) to a negative (Δρ/ρ<0, T<T_m) magnetoresistance is observed in the immediate vicinity of the mictomagnetic phase boundary at T_m=7 K. The linear asymptotic form of the negative magnetoresistance Δρ/ρ ∝?H in weak magnetic fields up to 10 kOe is explained by the formation of magnetic nanoclusters from interacting ferrons in the mictomagnetic phase of FeSi at T<T_m. The results are used for constructing for the first time the low-temperature magnetic phase diagram of FeSi. The effects of exchange enhancement are estimated quantitatively and the effective parameters characterizing the electron subsystem in the paramagnetic (T>T_C), ferromagnetic (T_m<T< T_C), and mictomagnetic (T<T_m) phases are determined. Analysis of anomalies in the aggregate of transport, magnetic, and magnetooptical characteristics observed in the vicinity of H_m≈35 kOe at T<T_m leads to the conclusion that a new collinear magnetic phase with M∥H exists on the low-temperature phase diagram of iron monosilicide.     

Correlation between the giant volume magnetostriction and softening of the crystal lattice in GMR manganites

Giant volume magnetostriction near the Curie temperature T _C has been revealed in single crystals and ceramics of the La_{1 ? x} A _xMnO₃ (A = Sr, Ba, Ca, Ag) and Re _{1 ? x} Sr_xMnO₃ (Re = Sm, Tb_0.25Nd_0.3, Eu_0.4Nd_0.15, Eu) compositions; its values were as high as ～10^?3 in a magnetic field of 8.2 kOe. The giant volume magnetostriction and giant magnetoresistance behave similarly near T _C: the temperature dependences of their moduli pass through a maximum, and their isotherms are similar. This phenomenon is explained by the presence of a magnetic two-phase state in these compositions, which is caused by the strong s-d exchange.     

Effect of the disorder parameter on the magnetic,electrical, galvanomagnetic,elastic, and magnetoelastic properties of <Emphasis Type="Italic">R</Emphasis><Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganites

This paper reports on a study of the magnetic, transport, magnetotransport, elastic, and magnetoelastic properties of the R_0.55Sr_0.45MnO₃ ceramics (R=Sm, Eu_0.40Nd_0.15, Tb_0.25Nd_0.30) with the same carrier concentration and identical tolerance factor but which differ in the cation disorder parameter σ². It was found that the Curie temperature T_C decreases linearly with increasing σ². An increase in σ² results in an increase in the maximum electrical resistivity and an increased jump in the temperature dependence of linear thermal expansion near T_C, as well as in a decrease in magnetoresistance and magnetostriction. For T>T_C, one observes an abrupt increase in magnetostriction, magnetization, and magnetoresistance in a critical FIeld H_C1 which grows with increasing temperature. The value of H_C1 determined at fixed T/T_C decreases with increasing σ².     

Magnetostrictions and Curie temperature measurements of (Fe–Co)91−xMo8Cu1Bx alloys with varying Co/Fe ratio

Amorphous rapidly quenched ribbons of (Fe–Co)₇₉Mo₈Cu₁B₁₂ and (Fe–Co)₇₆Mo₈Cu₁B₁₅ with the ratio of Co/Fe from 0 to 1 and 0 to 2, respectively, were prepared by planar flow casting. The dependence of Curie temperature T_C on Co/Fe ratio was determined from temperature dependencies of sample dilatation measured using a special dilatometer designed for these materials. Due to the presence of the invar effect, it was possible to measure the spontaneous volume magnetostriction in the temperature interval between 300 K and T_C, which is of the order of 10⁻³. Using special disc-shaped samples field dependencies of magnetostriction in parallel and perpendicular directions of the applied magnetic field were obtained by direct measurement. Subsequently, saturation magnetostriction and volume magnetostriction as well as forced magnetostriction were computed. Saturation magnetostriction λ_S increases with increasing Co/Fe ratio from 0 up to 15 and from 0 up to 17 ppm for both alloy systems, respectively, depending both on the Co/Fe ratio and on the shift of T_C with composition. After attaining the maximal value and further increase of the Co/Fe ratio the saturation magnetostriction decreases. Both alloy systems with ratio Co/Fe=0 exhibit T_C near room temperature and the system passes into paramagnetic state. T_C for higher Co/Fe ratios approaches the glass transition region. In paramagnetic state the field dependencies of magnetostriction are practically linear functions of applied field and approach saturation only for high-field values.     

Low temperature thermal expansion and magnetostriction of TmCd

Thermal expansion measurements in various external magnetic fields confirm the recently found Jahn-Teller transition in TmCd at 3.16 K. The tetragonal strain ?₃ at T = 0 K is found to be 0.62 × 10^?3. From magnetostriction data we determine a coupling constant g₀² = 0.4 × 10^?3 K which is in good agreement with the value found from ultrasonic experiments. In order to explain our magnetostriction results in the cubic phase, g₀ has to be taken negative.     

Intercluster conduction in lightly doped La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La _{1 ? x} Ca _x MnO₃ polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature T _C. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ T _C is fitted by the relation logρ ～ T ^?1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.     

Relation between giant volume magnetostriction,colossal magnetoresistance,and crystal lattice softening in manganites La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>A<Subscript>y</Subscript>MnO<Subscript>3</Subscript> (A = Ca,Ag, Ba,Sr)

Giant volume magnetostriction (GVM) is detected near the Curie temperature T _C in La_1?x A_xMnO₃ single crystals (A = Ca, Sr, Ba, 0.1 ≤ x ≤ 0.3) and above T _C in La_1?x Ag_yMnO₃ (x = y = 0.15, 0.2 and x = 0.2, y = 0.1) ceramics (in the latter system, giant volume magnetostriction attains a value of 6.5 × 10^?4 in a magnetic field of 8.2 kOe). The behavior of GVM and colossal magnetoresistance (CMR) is found to be the same: both quantities have negative values, the temperature dependences of their absolute values pass through a peak, and the isotherms do not exhibit saturation up to the maximal measuring fields of 130 kOe. In compounds with compositions La_0.7Ba_0.3MnO₃ and La_0.85Ag_0.15MnO₃, GVM and CMR were observed at room temperatures (in a magnetic field of 8.2 kOe, GVM attains values of 2.54 × 10^?4 and 2 × 10^?4 and CMR is equal to 11.6 and 11.2%, respectively). Both phenomena are attributed to the presence of a magnetic (ferromagnetic-antiferromagnetic) two-phase state in these systems, which is associated with a strong s-d exchange. It is found that the maximum value of the GVM in single crystals of La_1?x A_xMnO₃ (A = Ba, Sr, Ca, Ag) depends on the radius R _A of cation A (it is the higher, the larger the difference \(|R_A - R_{LA^{3 + } } |\)). The only exception is the compound with A = Ag, in which the pattern is complicated by additional defectiveness. Local disorder in the La_1?x A_x sublattice, which is associated with the presence of cations with different radii, leads to a displacement of oxygen ions and to crystal lattice softening. The exchange s-d interactions in La_1?x A_xMnO₃ (A = Ca, Sr, Ba, Ag) are found to be comparable with electrostatic interactions ensuring the existence of the crystal; this facilitates manifestation of the GVM.     

Influence of the small substitution of Z=Ni, Cu, Cr, V for Fe on the magnetic, magnetocaloric, and magnetoelastic properties of LaFe11.4Si1.6

We have studied the magnetic, magnetocaloric, and magnetostriction properties of LaFe_11.4Si_1.6 and La(Fe_0.99Z_0.01)_11.4Si_1.6 (Z=Ni, Cu, Cr, V) compounds using magnetization and strain gauge techniques. It was found that substitution of 1% of the Fe by Z-elements results in an increase in the Curie temperature (T_C), and affects the magnetostriction and magnetocaloric properties of the parent compound, LaFe_11.4Si_1.6. A maximum shift in T_C of about 11 K, and significantly smaller hysteresis losses in the vicinity of T_C compared with those of the base compound, were found for Z=V. The maximum magnetovolume coupling constant was estimated to be n_dd≈2.7×10⁻³ (μ_B/Fe atom)⁻² for the parent compound. The changes in the volume magnetostriction, the magnetovolume coupling constant, and the magnetocaloric properties are strongly correlated with composition. The relative effects of the variation in cell parameters and electron concentration on the magnetostriction, T_C, and the magnetocaloric properties are discussed.