Magnetic recording device capable of stabilizing oscillations of high frequency assisted element

1. A magnetic disk device comprising:

a magnetic disk having a recording layer;

a recording head that applies a perpendicular recording magnetic field to the recording layer; and

a controller that controls the recording head, wherein

the recording head includes:

a main pole that generates the perpendicular recording magnetic field at a distal end portion;

a return pole having a distal end portion facing the distal end portion of the main pole via a write gap;

a coil that excites a magnetic flux in a magnetic circuit formed by the main pole and the return pole;

a high frequency assist element that is disposed in the write gap, oscillates a high frequency wave by an application of a bias voltage, and assists magnetization of the magnetic disk using the high frequency wave; and

a bias voltage supply source that supplies the bias voltage to be applied to the high frequency assist element through the main pole and the return pole,

the controller applies a bias voltage for measurement to the high frequency assist element according to a setting instruction of the bias voltage to measure a conduction current, calculates a resistance value in a supply path of the bias voltage from a relationship between the measured current and the bias voltage for measurement, and changes the bias voltage applied at the time of data recording based on the calculated resistance value.

2. The magnetic disk device of claim 1 , wherein

the controller changes the bias voltage applied at the time of data recording to a lower value as the resistance value is higher.

3. The magnetic disk device of claim 2 , wherein

assuming that the resistance value is R and a reference value of the bias voltage at which an oscillation characteristic of the high frequency assist element is stabilized is Vs,

the controller changes a bias voltage Vset applied at the time of data recording within a range of

R/|R− 10|× Vs or more,

R/|R− 30|× Vs or less.

4. The magnetic disk device of claim 2 , wherein

assuming that an average value of the resistance value R of the recording head is Rave, an incidental resistivity at that time is α, and a maximum value of a ratio β to the average value Rave of the resistance value R of the recording head is βmax,

the controller changes a ratio of a maximum value to an average value of a bias voltage Vset applied at the time of data recording to

βmax/(βmax−0.4)×0.33 or more,

βmax/(βmax−0.67)×0.6 or less.

5. The magnetic disk device of claim 2 , wherein assuming that an average value of the resistance value R of the recording head is Rave, an incidental resistivity at that time is α, and a minimum value of a ratio β to the average value Rave of the resistance value R of the recording head is βmin,

the controller changes a ratio of a minimum value to an average value of a bias voltage Vset applied at the time of data recording to

βmin/|βmin−0.4|×0.33 or more,

βmin/|βmin−0.67|×0.6 or less.

6. The magnetic disk device of claim 1 , wherein

the recording head further comprises a temperature sensor that measures an internal temperature; and

the controller changes the bias voltage applied at the time of data recording to a higher value as the temperature measured by the temperature sensor is higher.

7. The magnetic disk device of claim 6 , wherein

assuming that the resistance value observed under an environment of a measured temperature Th is R(Th), and the resistance value observed under an environment of a reference temperature Tr is R(Tr) under an environment where the measured temperature Th of the temperature sensor is equal to or higher than the reference temperature Tr, the controller changes the bias voltage applied at the time of data recording so that a ratio Vset(Th)/Vset(Tr) of a bias voltage Vset(Th) applied at the time of data recording under the environment of the measured temperature Th to a bias voltage Vset(Tr) applied when data is recorded under the environment of the reference temperature Tr becomes 1 or more and R(Th)/R(Tr) or less.

8. The magnetic disk device of claim 6 , wherein

in an environment where a measured temperature TI of the temperature sensor is equal to or lower than a reference temperature Tr, the controller changes the bias voltage applied at the time of data recording so that a ratio Vset(Tl)/Vset(Tr) of a bias voltage Vset(Tl) applied at the time of data recording under an environment of the measured temperature TI to a bias voltage Vset(Tr) applied under an environment of the reference temperature Tr becomes R(Tl)/R(Tr) or more and 1 or less, assuming that the resistance value observed under the environment of the temperature Tl is R(Tl), and the resistance value observed under the environment of the reference temperature Tr is R(Tr).

9. A recording head controlling method for a magnetic disk device, when

a recording head that applies a perpendicular recording magnetic field to a recording layer of a magnetic disk comprises:

a main pole that generates the perpendicular recording magnetic field at a distal end portion;

a return pole having a distal end portion facing the distal end portion of the main pole via a write gap;

a coil that excites a magnetic flux in a magnetic circuit formed by the main pole and the return pole;

a high frequency assist element that is disposed in the write gap, oscillates a high frequency wave by an application of a bias voltage, and assists magnetization of the magnetic disk using the high frequency wave; and

a bias voltage supply source that supplies the bias voltage to be applied to the high frequency assist element through the main pole and the return pole,

the method comprising:

applying a bias voltage for measurement to the high frequency assist element according to a setting instruction of the bias voltage to measure a conduction current,

calculating a resistance value in a supply path of the bias voltage from a relationship between the measured current and the bias voltage for measurement, and

changing the bias voltage applied at the time of data recording based on the calculated resistance value.

10. The recording head controlling method for a magnetic disk device of claim 9 , wherein

the bias voltage applied at the time of data recording is changed to a lower value as the resistance value is higher.

11. The recording head controlling method for a magnetic disk device of claim 10 , wherein

assuming that the resistance value is R and a reference value of the bias voltage at which an oscillation characteristic of the high frequency assist element is stabilized is Vs, a bias voltage Vset applied at the time of data recording is changed to a range of

R/|R− 10|× Vs or more,

R/|R− 30|× Vs or less.

12. The recording head controlling method for magnetic disk device of claim 10 , wherein

assuming that an average value of the resistance value R of the recording head is Rave and an incidental resistivity at that time is α, and a maximum value of a ratio β to the average value Rave of the resistance value R of the recording head is βmax, a ratio of a maximum value to an average value of a bias voltage Vset applied at the time of data recording is changed to

βmax/(βmax−0.4)×0.33 or more,

βmax/(βmax−0.67)×0.6 or less.

13. The recording head controlling method for magnetic disk device of claim 10 , wherein

assuming that an average value of the resistance value R of the recording head is Rave, an incidental resistivity at that time is α, and a minimum value of a ratio β to the average value Rave of the resistance value R of the recording head is βmin, a ratio of a minimum value to an average value of a bias voltage Vset applied at the time of data recording is changed to

βmin/|βmin−0.4|×0.33 or more,

βmin/|βmin−0.67|×0.6 or less.

14. The recording head controlling method for magnetic disk device of claim 9 , wherein

when the recording head further includes a temperature sensor that measures an internal temperature,

the bias voltage applied at the time of data recording is changed to a higher value as the temperature measured by the temperature sensor is higher.

15. The recording head controlling method for a magnetic disk device of claim 14 , wherein

assuming that the resistance value observed under an environment of a measured temperature Th is R(Th), and the resistance value observed under an environment of a reference temperature Tr is R(Tr) under an environment where the measured temperature Th of the temperature sensor is equal to or higher than the reference temperature Tr, the bias voltage applied at the time of data recording is changed so that a ratio Vset(Th)/Vset(Tr) of a bias voltage Vset(Th) applied at the time of data recording under the environment of the measured temperature Th to a bias voltage Vset(Tr) applied when data is recorded under the environment of the reference temperature Tr becomes 1 or more and R(Th)/R(Tr) or less.

16. The recording head controlling method for a magnetic disk device of claim 14 , wherein

in an environment where a measured temperature TI of the temperature sensor is equal to or lower than a reference temperature Tr, the bias voltage applied at the time of data recording is changed so that a ratio Vset(Tl)/Vset(Tr) of a bias voltage Vset(Tl) applied at the time of data recording under an environment of the measured temperature TI to a bias voltage Vset(Tr) applied under an environment of the reference temperature Tr becomes R(Tl)/R(Tr) or more and 1 or less, assuming that the resistance value observed under the environment of the temperature Tl is R(Tl), and the resistance value observed under the environment of the reference temperature Tr is R(Tr).