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The effects of Cr3+ ions doping on the magnetic and magnetocaloric properties of manganites La0.5Sr0.5Mn1−xCrxO3 (x=0.05, 0.1, 0.15, and 0.2) have been investigated. The magnetization measurements show that a ferromagnetic cluster behavior is induced by Cr3+ dopants and the paramagnetic to ferromagnetic transition temperature TC decreases from 319 to 251 K with increasing Cr3+ content from 0.05 to 0.2. The positive slope of the Arrott plots near TC indicates a second order phase transition. The magnetic entropy change (−ΔSM) is estimated from the isothermal magnetization measurements, and the maximum of the magnetic entropy change (−ΔSmaxM) near TC shows a power law dependence of magnetic field: −ΔSmaxM = m ( μ 0 H ) n with n being close to the predicted value ~2/3. The experimental results suggest that the magnetic entropy change and TC can be tuned by changing the Cr3+ ions doping level, which offers an effective method for finding an optimal doping level for magnetic refrigeration near room temperature.

Pulsed high magnetic fields up to 52 T have been used in the systematic investigation of the magnetic properties of manganites Nd 0.5 Sr 0.5 Mn 1− x Cu x O 3 (0≤ x ≤0.15). The Cu-doping dependent first-order metamagnetic transitions are observed below the charge ordering temperature, which is ascribed to both Cu-doping and field-induced collapse of the charge ordering with antiferromagnetic phase. Based on the magnetization and electrical transport measurements, a three-dimensional phase diagram with coordinate axis of temperature, magnetic field, and doping level has been obtained, in which the critical fields of the metamagnetic transitions increase with the increase in Cu content and decrease with increasing temperature. The experimental results confirm that Mn-site substitution with Cu destroys the Mn 3+ –O 2− –Mn 4+ bridges and weakens the double exchange interaction between Mn 3+ and Mn 4+ ions, which shows an obvious tuning effect on the metamagnetic transition under the external magnetic field.

The structural, magnetic, and dielectric properties of the Y1−xHoxFe0.5Cr0.5O3 (x=0, 0.05, 0.1, 0.3, and 0.5) compounds have been investigated. Rietveld refinement of the XRD patterns shows that the compounds possess orthorhombic perovskite structure. The dual magnetization reversal is observed in the samples with x=0.05 and 0.1, and it vanishes when x≥0.3. Ferromagnetic-like behavior with large coercive fields is observed in all Ho3+ doped YFe0.5Cr0.5O3 samples, indicating a doping induced metamagnetic behavior. This abnormal magnetization behavior can be explained by the antiparallel magnetic coupling between the Ho3+ and the canted Cr3+/Fe3+ moments, as well as the Ho–O–Ho magnetic interaction. The dielectric behavior in the frequency range from 100 Hz to 10 MHz is investigated. The low doped samples (x=0, 0.05, and 0.1) exhibit relaxation-like dielectric behavior and colossal dielectric constant in a wide temperature and frequency range. The dual magnetization reversal under low magnetic field makes these materials attractive candidates for the magnetic dual sensor devices.

The temperature-induced dual magnetization reversal and field-induced spin reorientation transition are observed in Y 0.95 Ho 0.05 Fe 0.5 Cr 0.5 O 3 . The Ho 3+ dopant improves the sensitivity of the magnetization switching and weak ferromagnetism of YFe 0.5 Cr 0.5 O 3 . Based on the experimental results, we suggest that the ferrimagnetic ground state and the magnetization reversal mainly result from the antiferromagnetic coupling of weak ferromagnetic moments of Fe–O–Fe and Cr–O–Cr with that of Fe–O–Cr and Ho–O–Fe/Cr interactions. The fact that the magnetization reversal of Y 0.95 Ho 0.05 Fe 0.5 Cr 0.5 O 3 under a lower magnetic field makes it attractive candidates for the magnetic dual sensor devices.

Single crystals of Sr 3 Cr 2 O 8 , Sr 3 Cr 1.9 V 0.1 O 8 and Sr 3 Cr 1.9 Fe 0.1 O 8 were grown using the floating zone technique, and their magnetization behaviors were systematically investigated under pulsed field up to 54 T at various temperatures ranging from 2 K to 20 K. Temperature-induced transition from singlet ground state to triplet state and magnetic-field-induced Bose–Einstein condensation were observed, of which the critical temperature and the critical field are correlative to ion-dopants. The analysis of the ground state and low-field magnetization behaviors indicates that the ion-dopants lead to an enhancement of the intradimer exchange J 0 , which changes the transition temperatures, and increases the critical fields H c1 . The magnetic Fe ion dopant makes the transition from singlet to triplet states more sensitive to the applied magnetic field, in clearly contrast to the situation of the nonmagnetic V 5+ dopant.

The magnetization behavior of Ti 4+ doped perovskite manganites La 0.5 Sr 0.5 Mn 1− x Ti x O 3 ( x =0.15, 0.175 and 0.2) was investigated. Experimental results show that Ti 4+ dopant suppresses the antiferromagnetic charge ordering and leads to a step-like magnetization behavior below 3 K. The step-like transitions and the critical magnetic fields are strongly dependent on the Ti doping level, magnetic field sweep rate and temperature. Above 3 K, the step-like transitions transform to broad ones. In addition, under pulsed high magnetic fields with an ultrafast field sweep rate of ~10 3 T/s, the sharp step-like transitions observed in the static magnetization measurements become smooth metamagnetic transition at low temperatures. This feature is correlated with the collapse of the balance between the magnetic energy and elastic energy in the phase separation system within a martensitic-like scenario.

In this study we report detailed magnetic property of the 4f-3d pseudo-quaternary Tb0.2Pr0.8(Fe0.4Co0.6)1.88C0.05 compound by detailed magnetization measurements. Very sharp magnetization jumps across the antiferromagnetic–ferromagnetic transition are observed below 3.0 K, and the number of jump-like transitions increases with decreasing temperature. The time-dependent magnetic relaxation, field sweep rate and cooling field dependence of magnetization jumps resemble the martensitic scenario. The number and occurrence of magnetization jumps are mainly determined by the competitions between the thermal fluctuation energy, elastic energy and Zeeman energy, and the field-induced antiferromagnetic to ferromagnetic phase transition at low temperatures is of first-order in nature.

The structural and magnetic properties of nonmagnetic Ga 3+ ion doped CuFe 1− x Ga x O 2 ( x =0, 0.02, 0.03, and 0.05) single crystal samples have been investigated. In pulsed high magnetic fields, the field-induced multi-step transitions were observed in all the samples. Compared with pure CuFeO 2 , the transition temperatures, critical magnetic fields decrease and the magnetic hysteresis of the doped samples become small, which may result from the partial release of the spin frustration and the changes of the magnetic coupling both inter- and intra-planes due to the Ga 3+ dopant. The magnetization measurements show an abnormal dilution behavior, especially in a lower temperature region, the magnetic moment was enhanced due to the nonmagnetic Ga 3+ ion doping, the enhancement becomes more obviously in the sample with the Ga 3+ doping level of x =0.03. These results may connected with the substitution of nonmagnetic Ga 3+ ions destroying the stability of ground state and affecting the stability of the ferroelectricity incommensurate phase. Based on the experimental results, a super-cell model and their magnetic diagram were assumed.

The structure and magnetic properties of non-magnetic Ga 3+ ion doped CuFe 1− x Ga x O 2 ( x =0, 0.03, and 0.05) single crystal samples were investigated. X-ray diffraction patterns analysis confirms that the samples are single-phase crystallizing. Doping effect on the magnetic behavior of the ground state and the field-induced spin flip/flop transitions were detected. The transition temperatures and critical magnetic fields of the spin flip/flop, as well as the magnetic hysteresis directly depend on the Ga 3+ doping level. Such doping effects may associate with the competition between dilution effect (partial release of spin frustration) and the induced local magnetic moment, which is the result of the changed magnetic coupling both inter- and intra-planes of Fe ions. Based on the experimental results, the effects of Ga 3+ doping on the spin flip/flop behavior and a detailed high field magnetic diagram were assumed.

Electrical transport and magnetic properties of La0.5Sr0.5Mn1−xTixO3 (x = 0, 0.1, 0.15 and 0.2) have been investigated. Experimental results show that the different Ti4+ doping level has obviously different effect on electrical transport and magnetization properties. Especially, a martensitic transition was observed in higher Ti4+ doping levels (x = 0.15, 0.2). In which a doped Ti4+ ion substitutes a Mn4+ ion, with increasing in Ti4+ doping level from x = 0 to 0.2, the ratio of Mn3+/Mn4+ increases from 1 to 5/3, that is the coupling via Mn3+–O2−–Mn4+ double exchange increases leading to a localized ferromagnetic domain in the charge ordering of matrix of La0.5Sr0.5MnO3. With the increasing of magnetic field to a critical field, the localized magnetic moment was delocalized and rotated to parallel to an applied field showing a martensitic transition.

Magnetizations of the Fe doped Sr 3 Cr 2 O 8 single crystals Sr 3 Cr 2− x Fe x O 8 ( x =0, 0.05 and 0.1) were investigated in the temperature ranging from 2 to 300 K and the magnetic field ranging from 0 to 16 T. Based on the Curie–Weiss law and the Bleaney–Bowers model, the low field magnetization behaviors were analyzed. Experimental results show that the Fe 5+ dopant has influence on the intradimer interaction J 0 , the interdimer interaction J' , and the average change rate Δ χ / Δ T of spin-dimer susceptibility χ d i m e r of Sr 3 Cr 2− x Fe x O 8 . Especially, in this spin-dimer system, the Fe 5+ dopant can obviously enhance the magnetic sensitivity of singlet to triplet transition in Sr 3 Cr 2 O 8 .

Measurements of the isothermal magnetization on La0.27Nd0.4Ca0.33MnO3 have been performed in a field up to 7 T at various temperatures. A distinct field-induced metamagnetic transition is observed within a temperature region of 2–180 K, resulting from the coexistence of charge ordered (CO), antiferromagnetic (AFM) and ferromagnetic (FM) phases. A temperature and magnetic field dependent irreversible CO–FM transition appears in the lower temperature region. Accompanying the transition an obvious lattice strain was observed, indicating that the transition is related to the change of lattice strain due to strong spin–lattice coupling in La0.27Nd0.4Ca0.33MnO3. These properties make the doped manganites a good candidate for a magnetostriction sensor material.

The magnetic and dielectric polarization properties of the single crystal samples of CuFe1−xGaxO2 (x = 0 and 0.02) are investigated. Experimental results show that the magnetization and dielectric polarizations are anisotropy and coupled together. Compared with pure CuFeO2, in the case with the magnetic field parallel to the c axis, a field-induced phase transition with a hysteresis is clearly observed between the five-sublattice (5SL) and three-sublattice (3SL) phases. Specially, an obvious spontaneous dielectric polarization is observed in CuFe0.98Ga0.02O2 in a lower magnetic field region, indicating that the Ga doping has an effect on the enhancement of spontaneous dielectric polarization. Based on the dilution effect, change of exchange interaction, and partial release of the spin frustration due to the structural modulation of the Ga ion dopant, the origin of the magnetization, and spontaneous polarization characteristics are discussed and the complete dielectric polarization diagrams are assumed.

Magnetoelectric effects and their dynamic behavior in a CuFeO2 single crystal grown by the floating zone technique are investigated at low temperature in pulsed high magnetic fields. Experimental results show that the magnetization, dielectric polarization, and magnetoelectric coupling are anisotropic. In the magnetization, a field-induced multi-step-like transition with hysteresis is observed, in which the critical magnetic fields are independent of the field sweep rate. In the dielectric polarization, the field-induced dielectric polarization with hysteresis (or partial irreversible) is only observed in the incommensurate-noncollinear phase. In particular, no obvious spontaneous dielectric polarization is observed in the lower magnetic field regime. Using a pulsed high magnetic field with various magnetic field strength and field sweep rates, the coupling behavior between the magnetic moment and the dielectric polarization is investigated. Experimental results show that the dielectric polarization is weakly related to the field sweep rate. In the field-decreasing branch, the magnetic field is higher, the magnetic field region of the dielectric polarization remains is lower, indicating that the pulsed high magnetic field has an effect on the enhancement of the spontaneous dielectric polarization.